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Sample records for boundaries grain

  1. Grain Boundary Segregation in Metals

    CERN Document Server

    Lejcek, Pavel

    2010-01-01

    Grain boundaries are important structural components of polycrystalline materials used in the vast majority of technical applications. Because grain boundaries form a continuous network throughout such materials, their properties may limit their practical use. One of the serious phenomena which evoke these limitations is the grain boundary segregation of impurities. It results in the loss of grain boundary cohesion and consequently, in brittle fracture of the materials. The current book deals with fundamentals of grain boundary segregation in metallic materials and its relationship to the grain boundary structure, classification and other materials properties.

  2. GRAIN BOUNDARIES IN POLYPHASE CERAMICS

    OpenAIRE

    Clarke, D

    1985-01-01

    The majority of polyphase ceramics contain a residual glass phase at their grain boundaries. The stability of these phases, particularly at the two-grain boundaries, is of significance since they affect the properties of the material as a whole. Drawing analogies with soap films, the stability of a continuous intergranular phase is considered in terms of the balance between the capillarity and disjoining pressures. The individual components to the disjoining pressures are discussed. It is arg...

  3. Applied Thermodynamics: Grain Boundary Segregation

    OpenAIRE

    Pavel Lejček; Lei Zheng; Siegfried Hofmann; Mojmír Šob

    2014-01-01

    Chemical composition of interfaces—free surfaces and grain boundaries—is generally described by the Langmuir–McLean segregation isotherm controlled by Gibbs energy of segregation. Various components of the Gibbs energy of segregation, the standard and the excess ones as well as other thermodynamic state functions—enthalpy, entropy and volume—of interfacial segregation are derived and their physical meaning is elucidated. The importance of the thermodynamic state functions of grain boundary se...

  4. Applied Thermodynamics: Grain Boundary Segregation

    Directory of Open Access Journals (Sweden)

    Pavel Lejček

    2014-03-01

    Full Text Available Chemical composition of interfaces—free surfaces and grain boundaries—is generally described by the Langmuir–McLean segregation isotherm controlled by Gibbs energy of segregation. Various components of the Gibbs energy of segregation, the standard and the excess ones as well as other thermodynamic state functions—enthalpy, entropy and volume—of interfacial segregation are derived and their physical meaning is elucidated. The importance of the thermodynamic state functions of grain boundary segregation, their dependence on volume solid solubility, mutual solute–solute interaction and pressure effect in ferrous alloys is demonstrated.

  5. Determination of grain boundary geometry using TEM

    NARCIS (Netherlands)

    Jang, H.; Farkas, D.; Hosson, J.T.M. De

    1992-01-01

    An experimental method to obtain the grain boundary geometry using the transmission electron microscope is presented. The method allows Σ determination including grain boundary plane orientation. In order to determine the specialness of the grain boundary, three different criteria for maximum allowa

  6. Direct imaging of grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Gronsky, R.

    1979-09-01

    There are currently two types of microscopes which, in principle, are capable of imaging atom positions at grain boundaries. One, the field ion microscope (FIM), yields a projection of the specimen surface (approximately stereographic) by field ionization of an imaging gas at protruding atom sites, and provides topographic information in high-index pole regions which may be interpreted atom-by-atom. The other, a transmission electron microscope (TEM), yields a projection (approximately linear) of the entire specimen thickness by electron optical imaging, and provides atomic resolution detail throughout the illuminated area. In this paper, both methods are described and compared, using examples from practical materials systems.

  7. Determination of grain boundary geometry using TEM

    Energy Technology Data Exchange (ETDEWEB)

    Jang, H.; Farkas, D. (Department of Materials Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0237 (United States)); De Hosson, J.T.M. (Department of Applied Physics, University of Groningen, Nijenborgh 18, 9747 AG, Groningen (Netherlands))

    1992-07-01

    An experimental method to obtain the grain boundary geometry using the transmission electron microscope is presented. The method allows {Sigma} determination including grain boundary plane orientation. In order to determine the specialness of the grain boundary, three different criteria for maximum allowable deviations from exact CSL misorientations were examined. We tested these three criteria from a statistical distribution of grain boundary types in terms of {Sigma}. We compared grain boundary distributions from other studies in Ni{sub 3}Al and found discrepancies among them. It seems that the discrepancy came from the different criteria for special boundaries in {Sigma} determination and different experimental procedures they used. The statistical distribution of grain boundary plane orientations showed that low {Sigma} boundaries ({Sigma}{lt}11) were oriented to the plane of high density of coincident sites.

  8. Grain Boundary Engineering of Electrodeposited Thin Films

    DEFF Research Database (Denmark)

    Alimadadi, Hossein

    Grain boundary engineering aims for a deliberate manipulation of the grain boundary characteristics to improve the properties of polycrystalline materials. Despite the emergence of some successful industrial applications, the mechanism(s) by which the boundary specific properties can be improved...... to engineer new materials. In this study, one of the most widely used electrolytes for electrodeposition is chosen for the synthesis of nickel films and based on thorough characterization of the boundaries the potentials in grain boundary engineering are outlined. The internal structure of the nickel films...... of the favorable boundaries that break the network of general grain boundaries. Successful dedicated synthesis of a textured nickel film fulfilling the requirements of grain boundary engineered materials, suggests improved boundary specific properties. However, the textured nickel film shows fairly low...

  9. Superfluidity of grain boundaries and supersolid behavior.

    Science.gov (United States)

    Sasaki, S; Ishiguro, R; Caupin, F; Maris, H J; Balibar, S

    2006-08-25

    When two communicating vessels are filled to a different height with liquid, the two levels equilibrate because the liquid can flow. We have looked for such equilibration with solid (4)He. For crystals with no grain boundaries, we see no flow of mass, whereas for crystals containing several grain boundaries, we detect a mass flow. Our results suggest that the transport of mass is due to the superfluidity of grain boundaries. PMID:16873608

  10. Grain boundary strengthening in austenitic nitrogen steels

    International Nuclear Information System (INIS)

    The effect of nitrogen and carbon on the strengthening of the austenitic steel Cr18Ni16Mn10 by grain boundaries is studied. It is established in accordance with previous results that contrary to carbon nitrogen increases the coefficient k in the Hall-Petch equation markedly. Because of a pronounced planar slip induced by nitrogen and the absence of any noticeable segregation of nitrogen atoms at the grain boundaries, nitrogen austenite presents an excellent object for testing different existing models of grain boundary strengthening (pile-up, grain boundary dislocation sources, work hardening theories). Based on the analysis of available data and measurements of interaction between nitrogen (carbon) atoms and dislocations it is shown that the nitrogen effect can be attributed to a strong blocking of dislocation sources in grains adjacent to those where the slip started. (orig.)

  11. Transient solute drag in migrating grain boundaries

    International Nuclear Information System (INIS)

    Understanding the solute drag in migrating grain boundaries or interfaces has been a topic in materials research since Cahn's seminal paper in 1962. However, mostly steady-state solutions for solute segregation and drag in a migrating interface have been investigated. Here a new concept, based on the thermodynamic extremal principle, is introduced, which allows a detailed study of the transient processes in the migrating interface starting from a given initial configuration. The system is then described by two parameters, the first representing the amount of segregated solute in the grain boundary and the second the grain boundary position. Stability studies are performed using the perturbation concept. The model is demonstrated by simulations for a Fe-0.1 at.% Ni alloy taking different values for the grain boundary mobility and the driving force.

  12. GRAIN-BOUNDARY PRECIPITATION UNDER IRRADIATION IN DILUTE BINARY ALLOYS

    Institute of Scientific and Technical Information of China (English)

    S.H. Song; Z.X. Yuan; J. Liu; R.G.Faulkner

    2003-01-01

    Irradiation-induced grain boundary segregation of solute atoms frequently bring about grain boundary precipitation of a second phase because of its making the solubility limit of the solute surpassed at grain boundaries. Until now the kinetic models for irradiation-induced grain boundary precipitation have been sparse. For this reason, we have theoretically treated grain boundary precipitation under irradiation in dilute binary alloys. Predictions ofγ'-Ni3Si precipitation at grain boundaries ave made for a dilute Ni-Si alloy subjected to irradiation. It is demonstrated that grain boundary silicon segregation under irradiation may lead to grain boundaryγ'-Ni3 Si precipitation over a certain temperature range.

  13. SIMULATIVE INVESTIGATION OF GRAIN BOUNDARIES IN NiAl ALLOY

    Institute of Scientific and Technical Information of China (English)

    1999-01-01

    Using embedded atom potentials and molecular static relaxation method or Monte Carlo relaxation method, the grain boundary atomic structures of a series of [100], [110] and [111] symmetric tilt grain boundaries in NiAl with different geometrical indexes and compositions were studied. The grain boundary energies, grain boundary cohesive energies and point defects formation energies at the grain boundary were calculated. The results show that the grain boundaries of NiAl alloy have a periodic unit structure. Grain boundary energies of Al rich grain boundary is very high, but the cohesive energies are lower than those of others. The interaction of point defects and grain boundaries shows that grain boundaries incline to absorb point defects to decrease their orders and to relax the distortions caused by point defects. The boundary atomic structures relaxed by Monte Carlo method show that the disorder of them, grain boundary energies and cohesive energies increase with increasing temperature.

  14. Grain Boundary Effects in Solid Oxide Electrolytes

    Science.gov (United States)

    Ng, Mai

    Ion conducting ceramics are essential in applications such as solid oxide fuel cells and oxygen sensors. Traditional 8 mol% yttria-stabilized zirconia (8YSZ) solid oxide electrolytes operate at high temperatures (850°C-1000°C) to achieve high ionic conductivity (> 0.1 Scm-1 at 1000°C) by oxygen ion diffusion via vacancies. Operation at such temperatures requires high temperature electrode materials and shortens device lifetime due to interdiffusion and reactions at electrode/electrolyte interfaces. These concerns drive research in current systems and alternative materials to improve ionic conductivity at reduced operating temperatures. This research considers how grain size and grain boundary phases affect three electrolyte materials with different ion diffusion mechanisms. First, the conductivity of ultra-fine grained two-step sintered and large grained conventional sintered 8YSZ are compared to determine if enhanced ionic conductivity occurs supporting the theory that ion blocking impurities in grain boundaries are diluted with decreasing grain size. Second, apatite-type lanthanide silicates (Ln9.33(SiO4)6O2) which exhibit anisotropic interstitial oxygen diffusion at intermediate temperatures (400°C-800°C) are studied to determine whether grain boundaries detrimentally affect conductivity. Lastly, proton conducting La-monazite (LaPO4) is evaluated to determine the role of Sr-doping (up to 10% substitution of La with Sr) on grain size and conductivity as well as the effect of sintering in air or water vapor on the formation of intergranular phases rich in Sr and P. This research investigates grain boundary effects in three solid oxide electrolyte materials with the goal of understanding how grain boundaries affect ionic conductivity and the atomistic behavior governing these different diffusion mechanisms.

  15. Grain-boundary structures in hexagonal materials: Coincident and near coincident grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Farkas, D. (Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Materials Science and Engineering)

    1994-07-01

    Embedded atom method (EAM) simulations of the structure of grain boundaries in hexagonal metal, are presented. The simulations use recently developed interatomic potentials for Ti and Co. Structures were calculated for various symmetrical tilt boundaries with the [1,100] tilt axis. The structures obtained for both metals are very similar. The energies for the Co boundaries are higher than those for Ti by a factor of 2. The structural unit model was applied to the computed grain-boundary structures in these hexagonal materials. As in cubic materials, the structural unit model can describe a series of symmetrical tilt coincident boundaries. In addition, when the coincidence ratio in the grain-boundary plane varies with the c/a ratio, a structural unit-type model can describe the variation of grain-boundary structure with c/a ratio. This model is adequate for describing series of symmetrical tilt boundaries with the grain-boundary plane oriented perpendicular to a fixed crystallographic direction and varying c/a ratios. For the structures of the so-called near coincident boundaries that appear in these materials, it was concluded that near coincident boundaries behave similarly to exact coincidence boundaries if there is a coincident periodic structure in the grain-boundary plane. This may occur even without a three-dimensional (3-D) coincident site lattice.

  16. Grain Boundaries From Theory to Engineering

    CERN Document Server

    Priester, Louisette

    2013-01-01

    Grain boundaries are a main feature of crystalline materials. They play a key role in determining the properties of materials, especially when grain size decreases and even more so with the current improvements of  processing tools and methods that allow us to control various elements in a polycrystal. This book presents the theoretical basis of the study of  grain boundaries and aims to open up new lines of research in this area. The treatment is light on mathematical approaches while emphasizing practical examples; the issues they raise are discussed with reference to theories. The general approach of the book has two main goals: to lead the reader from the concept of ‘ideal’ to ‘real’ grain boundaries; to depart from established knowledge and address the opportunities emerging through "grain boundary engineering",  the control of morphological and crystallographic features that affect material properties. The book is divided in three parts:  I ‘From interganular order to disorder’ deals wit...

  17. Advantageous grain boundaries in iron pnictide superconductors

    Science.gov (United States)

    Katase, Takayoshi; Ishimaru, Yoshihiro; Tsukamoto, Akira; Hiramatsu, Hidenori; Kamiya, Toshio; Tanabe, Keiichi; Hosono, Hideo

    2011-01-01

    High critical temperature superconductors have zero power consumption and could be used to produce ideal electric power lines. The principal obstacle in fabricating superconducting wires and tapes is grain boundaries—the misalignment of crystalline orientations at grain boundaries, which is unavoidable for polycrystals, largely deteriorates critical current density. Here we report that high critical temperature iron pnictide superconductors have advantages over cuprates with respect to these grain boundary issues. The transport properties through well-defined bicrystal grain boundary junctions with various misorientation angles (θGB) were systematically investigated for cobalt-doped BaFe2As2 (BaFe2As2:Co) epitaxial films fabricated on bicrystal substrates. The critical current density through bicrystal grain boundary (JcBGB) remained high (>1 MA cm−2) and nearly constant up to a critical angle θc of ∼9°, which is substantially larger than the θc of ∼5° for YBa2Cu3O7–δ. Even at θGB>θc, the decay of JcBGB was much slower than that of YBa2Cu3O7–δ. PMID:21811238

  18. The influence of grain boundary structure on the penetration of gallium into aluminum grain boundaries

    Science.gov (United States)

    Hugo, Richard Charles

    1998-12-01

    Liquid Metal Embrittlement is a form of environmental embrittlement that dramatically reduces the fracture toughness of many metals and alloys. It occurs when surfaces of certain solid metals are wet by certain liquid metals. The Al-Ga system provides a remarkable example of intergranular attack. The Al-Ga equilibrium phase diagram reveals no intermetallic compounds and very limited mutual solubilities, which implies that interactions between Al and Ga should be minimal. Yet when liquid Ga wets the surface of an unstressed Al specimen, the Ga will penetrate the Al grain boundaries, replacing each boundary with a liquid layer. The driving force is generally considered to be the reduction in energy when a grain boundary is replaced by two Ga-Al interfaces. Once an Al sample has been penetrated by Ga, it fails at almost no load. In this dissertation, in-situ Transmission Electron Microscope (TEM) studies are presented that elucidate the physical nature of the Ga penetration front. Although many of the TEM specimens were bicrystals, all but one of the grain boundaries studied were "general" boundaries; that is, they were low symmetry boundaries with high-index rotation axes, and no low-index planes common to both grains. Since the atomic structure of these grain boundaries cannot be resolved experimentally, atomistic computer models were constructed to assist in interpreting TEM results. TEM observations indicated that the penetration front is a line defect, possessing a stress field that interacts with lattice dislocations. The penetration front was also observed to interact with structural variations within the grain boundary. Interactions with lattice dislocations were used to estimate the penetration front thickness. Penetration speeds were not found to be determined by grain boundary energy or grain boundary excess volume. Penetration speeds were, however, found to depend qualitatively on the presence of penetration barriers in the grain boundary.

  19. Interactions between Dislocations and Grain Boundaries

    NARCIS (Netherlands)

    Soer, Wouter Anthon

    2006-01-01

    Dislocations (line defects) and grain boundaries (planar defects) are two types of lattice defects that are crucial to the deformation behavior of metals. Permanent deformation of a crystalline material is microscopically associated with the nucleation and propagation of dislocations, and extensive

  20. Analytical model for intergrain expansion and cleavage: random grain boundaries

    International Nuclear Information System (INIS)

    A description of rigid-body grain boundary relaxation and cleavage in tungsten is performed using a pair-wise Morse interatomic potential in real and reciprocal spaces. Cleavage energies and grain boundary dilatation of random grain boundaries were formulated and computed using atomic layer interaction energies. These values were determined using a model for a relaxed random grain boundary that consists of rigid grains on either side of the boundary plane that are allowed to float to reach the equilibrium position. Expressions are given that describe in real space the energy of interatomic interaction on random grain boundaries with twist orientation. It was shown that grain-boundary expansion and cleavage energies of the most widespread random grain boundaries are mainly determined by grain boundary atomic density

  1. Micromechanical testing of stress corrosion cracking of individual grain boundaries

    International Nuclear Information System (INIS)

    Grain boundaries of different misorientation and chemistry have differing susceptibilities to stress corrosion cracking but carrying out mechanical tests on individual grain boundaries of known character has until now been very difficult. We present a method for manufacturing specimens containing a single grain boundary (in 304 stainless steel) using focused ion beam machining. A nanoindenter/atomic force microscope was used to drive controlled grain boundary crack growth under load in an active solution. Scanning electron microscopy examination confirmed the growth of the crack along the grain boundary.

  2. Grain Boundary Traction Signatures: Quantitative Predictors of Dislocation Emission

    Science.gov (United States)

    Li, Ruizhi; Chew, Huck Beng

    2016-08-01

    We introduce the notion of continuum-equivalent traction fields as local quantitative descriptors of the grain boundary interface. These traction-based descriptors are capable of predicting the critical stresses to trigger dislocation emissions from ductile ⟨110 ⟩ symmetrical-tilt nickel grain boundaries. We show that Shockley partials are emitted when the grain boundary tractions, in combination with external tensile loading, generate a resolved shear stress to cause dislocation slip. The relationship between the local grain boundary tractions and the grain boundary energy is established.

  3. Influence of Grain Boundary on Melting

    Institute of Scientific and Technical Information of China (English)

    王暾; 周富信; 刘曰武

    2001-01-01

    The temperature behaviour of an Al bicrystal with surfaces consisting of (110) and (111) crystals is simulated using molecular dynamics. The result shows that the (110) crystal losses its crystalline order at 820K, whereas the disorder does not propagate through the (111) crystal at this temperature. Instead, some disordered atoms are recrystallized into the (111) crystal and the initial grain boundary changes into a stable order-disorder interface. Thus, it was discovered that at a temperature near its melting point, the (111) crystal grew and obstructed the propagation of disorder. Such an obstruction is helpfulfor understanding melting.

  4. Electronic properties of grains and grain boundaries in graphene grown by chemical vapor deposition

    OpenAIRE

    Jauregui, Luis A.; Cao, Helin; Wu, Wei; Yu, Qingkai; Chen, Yong P.

    2011-01-01

    We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations, by ambient pressure CVD on polycrystalline Cu foils. We measure the electronic properties of such grains as well as of individual graphene grain boundaries, formed when two grains merged during the growth. The grain boundaries are visualized using Raman mapping of the D band intensity, and we show that individual boundaries between coalesced grains impede electrical transport in graphene a...

  5. Grain boundary structure in Ni{sub 3}Al

    Energy Technology Data Exchange (ETDEWEB)

    Farkas, D. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Materials Engineering

    1996-08-01

    The multiplicity of possible grain boundary structures was studied for the ordered compound Ni{sub 3}Al. Two symmetrical tilt boundaries were chosen for the detailed study corresponding to {Sigma}=3 left angle 110 right angle {l_brace}112{r_brace} and {Sigma}=9 left angle 110 right angle {l_brace}114{r_brace}. These boundaries were investigated considering possible variations of the local chemical composition and environment using atomistic computer simulation with EAM interatomic potentials. Many different grain boundary structures were found having very similar energies for the same orientation of the two crystals and the grain boundary plane. Possible transformations among these structures may result from the interaction of the boundaries with dislocations or antiphase boundaries. The role of the multiplicity of structures and these transformations in the grain boundary mechanical properties is discussed. In this paper, we explore the role that different possible grain boundary structures play in intermetallic alloys. (orig.)

  6. Atomistic aspects of crack propagation along high angle grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Farkas, D. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Materials Science and Engineering

    1997-12-31

    The author presents atomistic simulations of the crack tip configuration near a high angle {Sigma} = 5 [001](210) symmetrical tilt grain boundary in NiAl. The simulations were carried out using molecular statics and embedded atom (EAM) potentials. The cracks are stabilized near a Griffith condition involving the cohesive energy of the grain boundary. The atomistic configurations of the tip region are different in the presence of the high angle grain boundary than in the bulk. Three different configurations of the grain boundary were studied corresponding to different local compositions. It was found that in ordered NiAl, cracks along symmetrical tilt boundaries show a more brittle behavior for Al rich boundaries than for Ni-rich boundaries. Lattice trapping effects in grain boundary fracture were found to be more significant than in the bulk.

  7. Influence of Grain Boundary Properties and Orientation on Void Nucleation

    Energy Technology Data Exchange (ETDEWEB)

    Fensin, Saryu Jindal [Los Alamos National Lab. (LANL), Los Alamos, NM (United States). Center for Integrated Nanotechnologies (CINT)

    2016-03-01

    For ductile metals, dynamic fracture during shock loading is thought to occur through void nucleation, growth, and then coalescence that leads to material failure. Particularly for high purity metals, it has been observed by numerous investigators that, under incipient spall conditions, voids appear to heterogeneously nucleate at some grain boundaries, but not others. Several factors can affect the void nucleation stress at a grain boundary, such as grain boundary structure, orientation with respect to the loading direction, energy and excess volume, in addition to its interactions with dislocations. In this work, we focus on the influence of loading direction with respect to the grain boundary plane and grain boundary properties such as energy and excess volume on the stress required for void nucleation of a grain boundary, in copper from moleculardynamics simulations. Flyer plate simulations were carried out for four boundary types with different energies and excess volumes. These boundaries were chosen as model systems to represent various boundaries observed in “real” materials. Simulations indicate that there is no direct correlation between the void nucleation stress at a boundary and either its energy and excess volume. This result suggests that average properties of grain boundaries alone are not sufficient indicators of the spall strength of a boundary and perhaps local boundary properties need to be taken into account in order to predict its susceptibility to void nucleation for broad ranges of materials. We also present both experimental and simulation results corresponding to the affect of orientation on void nucleation.

  8. Randomized Grain Boundary Liquid Crystal Phase

    Science.gov (United States)

    Chen, D.; Wang, H.; Li, M.; Glaser, M.; Maclennan, J.; Clark, N.

    2012-02-01

    The formation of macroscopic, chiral domains, in the B4 and dark conglomerate phases, for example, is a feature of bent-core liquid crystals resulting from the interplay of chirality, molecular bend and molecular tilt. We report a new, chiral phase observed in a hockey stick-like liquid crystal molecule. This phase appears below a smectic A phase and cools to a crystal phase. TEM images of the free surface of the chiral phase show hundreds of randomly oriented smectic blocks several hundred nanometers in size, similar to those seen in the twist grain boundary (TGB) phase. However, in contrast to the TGB phase, these blocks are randomly oriented. The characteristic defects in this phase are revealed by freeze-fracture TEM images. We will show how these defects mediate the randomized orientation and discuss the intrinsic mechanism driving the formation of this phase. This work is supported by NSF MRSEC Grant DMR0820579 and NSF Grant DMR0606528.

  9. Atomically ordered solute segregation behaviour in an oxide grain boundary

    Science.gov (United States)

    Feng, Bin; Yokoi, Tatsuya; Kumamoto, Akihito; Yoshiya, Masato; Ikuhara, Yuichi; Shibata, Naoya

    2016-01-01

    Grain boundary segregation is a critical issue in materials science because it determines the properties of individual grain boundaries and thus governs the macroscopic properties of materials. Recent progress in electron microscopy has greatly improved our understanding of grain boundary segregation phenomena down to atomistic dimensions, but solute segregation is still extremely challenging to experimentally identify at the atomic scale. Here, we report direct observations of atomic-scale yttrium solute segregation behaviours in an yttria-stabilized-zirconia grain boundary using atomic-resolution energy-dispersive X-ray spectroscopy analysis. We found that yttrium solute atoms preferentially segregate to specific atomic sites at the core of the grain boundary, forming a unique chemically-ordered structure across the grain boundary. PMID:27004614

  10. Grain boundary and triple junction diffusion in nanocrystalline copper

    Energy Technology Data Exchange (ETDEWEB)

    Wegner, M., E-mail: m.wegner@uni-muenster.de; Leuthold, J.; Peterlechner, M.; Divinski, S. V., E-mail: divin@uni-muenster.de [Institut für Materialphysik, Universität Münster, Wilhelm-Klemm-Straße 10, D-48149, Münster (Germany); Song, X., E-mail: xysong@bjut.edu.cn [College of Materials Science and Engineering, Beijing University of Technology, 100124 Beijing (China); Wilde, G. [Institut für Materialphysik, Universität Münster, Wilhelm-Klemm-Straße 10, D-48149, Münster (Germany); Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, 200444 Shanghai (China)

    2014-09-07

    Grain boundary and triple junction diffusion in nanocrystalline Cu samples with grain sizes, 〈d〉, of ∼35 and ∼44 nm produced by spark plasma sintering were investigated by the radiotracer method using the {sup 63}Ni isotope. The measured diffusivities, D{sub eff}, are comparable with those determined previously for Ni grain boundary diffusion in well-annealed, high purity, coarse grained, polycrystalline copper, substantiating the absence of a grain size effect on the kinetic properties of grain boundaries in a nanocrystalline material at grain sizes d ≥ 35 nm. Simultaneously, the analysis predicts that if triple junction diffusion of Ni in Cu is enhanced with respect to the corresponding grain boundary diffusion rate, it is still less than 500⋅D{sub gb} within the temperature interval from 420 K to 470 K.

  11. Electronic Structure of a Disordered Grain Boundary in Graphene

    Science.gov (United States)

    Lambin, Ph.; Vancso, P.; Nemes-Incze, P.; Mark, G.; Biró, L. P.

    2013-05-01

    Grain boundaries are constitutional elements of graphene grown on a solid metallic surface by CVD. The electronic properties of computer models of grain boundaries in graphene have been investigated by tight-binding calculations and compared with available ab initio data and with recent experimental scanning tunneling spectroscopic measurements. It is shown that twofold coordinated atoms and non-hexagonal rings, both present in grain boundaries, give rise to specific features in the local density of states.

  12. Irradiation-induced grain growth and defect evolution in nanocrystalline zirconia with doped grain boundaries.

    Science.gov (United States)

    Dey, Sanchita; Mardinly, John; Wang, Yongqiang; Valdez, James A; Holesinger, Terry G; Uberuaga, Blas P; Ditto, Jeff J; Drazin, John W; Castro, Ricardo H R

    2016-06-22

    Grain boundaries are effective sinks for radiation-induced defects, ultimately impacting the radiation tolerance of nanocrystalline materials (dense materials with nanosized grains) against net defect accumulation. However, irradiation-induced grain growth leads to grain boundary area decrease, shortening potential benefits of nanostructures. A possible approach to mitigate this is the introduction of dopants to target a decrease in grain boundary mobility or a reduction in grain boundary energy to eliminate driving forces for grain growth (using similar strategies as to control thermal growth). Here we tested this concept in nanocrystalline zirconia doped with lanthanum. Although the dopant is observed to segregate to the grain boundaries, causing grain boundary energy decrease and promoting dragging forces for thermally activated boundary movement, irradiation induced grain growth could not be avoided under heavy ion irradiation, suggesting a different growth mechanism as compared to thermal growth. Furthermore, it is apparent that reducing the grain boundary energy reduced the effectiveness of the grain boundary as sinks, and the number of defects in the doped material is higher than in undoped (La-free) YSZ. PMID:27282392

  13. Grain boundary engineering to enhance thermal stability of electrodeposited nickel

    DEFF Research Database (Denmark)

    Alimadadi, Hossein

    , grain boundary engineering of electrodeposited nickel to achieve high population of coherent twin boundaries and, hence, higher thermal stability is a promising method to achieve simultaneous improvement in mechanical properties and thermal stability. This is of particular scientific and practical...

  14. Grain-boundary diffusion: structural effects, models and mechanisms

    Energy Technology Data Exchange (ETDEWEB)

    Peterson, N L

    1979-01-01

    Grain boundary diffusion phenomena were considered including: anisotropy, effect of orientation, crystallographic transformation, boundary type, dislocation dissociation, pressure, and isotope effects. Diffusivity is different for various boundaries. Dissociated dislocations and stacking faults are not efficient paths for grain boundary diffusion. Results suggest a vacancy mechanism along the dislocation core, and involves atomic jumps away from the back towards the dislocation as well as jumps along the core. Measurements were made on nickel and silver. (FS)

  15. Photoluminescence Imaging of Large-Grain CdTe for Grain Boundary Characterization

    Energy Technology Data Exchange (ETDEWEB)

    Johnston, Steve; Allende Motz, Alyssa; Reese, Matthew O.; Burst, James M.; Metzger, Wyatt K.

    2015-06-14

    In this work, we use photoluminescence (PL) imaging to characterize CdTe grain boundary recombination. We use a silicon megapixel camera and green (532 nm) laser diodes for excitation. A microscope objective lens system is used for high spatial resolution and a field of view down to 190 um x 190 um. PL images of large-grain (5 to 50 um) CdTe samples show grain boundary and grain interior features that vary with processing conditions. PL images of samples in the as-deposited state show distinct dark grain boundaries that suggest high excess carrier recombination. A CdCl2 treatment leads to PL images with very little distinction at the grain boundaries, which illustrates the grain boundary passivation properties. Other process conditions are also shown, along with comparisons of PL images to high spatial resolution time-resolved PL carrier lifetime maps.

  16. Grain-boundary melting: A Monte Carlo study

    DEFF Research Database (Denmark)

    Besold, Gerhard; Mouritsen, Ole G.

    1994-01-01

    Grain-boundary melting in a lattice-gas model of a bicrystal is studied by Monte Carlo simulation using the grand canonical ensemble. Well below the bulk melting temperature T(m), a disordered liquidlike layer gradually emerges at the grain boundary. Complete interfacial wetting can be observed...

  17. Diffusion behaviors of helium atoms at two Pd grain boundaries

    Institute of Scientific and Technical Information of China (English)

    XIA Ji-xing; HU Wang-yu; YANG Jian-yu; AO Bing-yun

    2006-01-01

    The diffusion behaviors of helium atoms at two symmetric grain boundaries (Σ5{210} and Σ3 {112}) of Pd were investigated using molecular dynamics simulations through an analytical embedded-atom method(MAEAM) model. The simulations demonstrate that the interstitial helium atoms are easily trapped at the grain boundaries and precipitated into clusters. Due to the closed-shell electronic configurations of both helium and palladium,Pd grain boundaries yield strong capability of retaining helium atoms. By calculating the mean square displacements(MSD) of an interstitial helium atom at the grain boundaries,the diffusion coefficients were determined,and the linear fits to Arrhenius relation. The diffusion activation energies of interstitial helium atom at these two Pd grain boundaries were also evaluated.

  18. Pipe and grain boundary diffusion of He in UO2

    Science.gov (United States)

    Galvin, C. O. T.; Cooper, M. W. D.; Fossati, P. C. M.; Stanek, C. R.; Grimes, R. W.; Andersson, D. A.

    2016-10-01

    Molecular dynamics simulations have been conducted to study the effects of dislocations and grain boundaries on He diffusion in \\text{U}{{\\text{O}}2} . Calculations were carried out for the {1 0 0}, {1 1 0} and {1 1 1} edge dislocations, the screw dislocation and Σ5, Σ13, Σ19 and Σ25 tilt grain boundaries. He diffusivity as a function of distance from the dislocation core and grain boundaries was investigated for the temperature range 2300-3000 K. An enhancement in diffusivity was predicted within 20 Å of the dislocations or grain boundaries. Further investigation showed that He diffusion in the edge dislocations follows anisotropic behaviour along the dislocation core, suggesting that pipe diffusion occurs. An Arrhenius plot of He diffusivity against the inverse of temperature was also presented and the activation energy calculated for each structure, as a function of distance from the dislocation or grain boundary.

  19. Grain boundary character distributions of coincidence site lattice boundaries in WC-Co composites with different WC grain sizes

    Energy Technology Data Exchange (ETDEWEB)

    Yuan, Xiaokun, E-mail: yuanxiaokun@bjut.edu.cn

    2013-12-05

    Highlights: •GBCDs of two kinds of CSL boundaries in cemented carbides are examined. •Carbide mean grain size is determinant in controlling the populations of CSL boundaries. •Rapid decrease of sigma2 boundaries occurs at higher speed than the coarsening of carbide grains. -- Abstract: The grain boundary character distributions of sigma2 and sigma13 boundaries were investigated in cemented carbide samples with different carbide grain sizes. Samples were prepared with average carbide grain sizes of about 0.5 μm, 1 μm and 2 μm, respectively. The electron backscattered diffraction measurements show that with the increase of the carbide grain size, populations of both sigma2 and sigma13 boundaries decrease; in the meantime, the sigma2 twist and sigma13 twist boundaries keep as the most common boundary types. The results suggest that the carbide grain size could be determinant in controlling the populations of coincidence site lattice boundaries in WC-Co composites.

  20. Grain boundary character distributions of coincidence site lattice boundaries in WC-Co composites with different WC grain sizes

    International Nuclear Information System (INIS)

    Highlights: •GBCDs of two kinds of CSL boundaries in cemented carbides are examined. •Carbide mean grain size is determinant in controlling the populations of CSL boundaries. •Rapid decrease of sigma2 boundaries occurs at higher speed than the coarsening of carbide grains. -- Abstract: The grain boundary character distributions of sigma2 and sigma13 boundaries were investigated in cemented carbide samples with different carbide grain sizes. Samples were prepared with average carbide grain sizes of about 0.5 μm, 1 μm and 2 μm, respectively. The electron backscattered diffraction measurements show that with the increase of the carbide grain size, populations of both sigma2 and sigma13 boundaries decrease; in the meantime, the sigma2 twist and sigma13 twist boundaries keep as the most common boundary types. The results suggest that the carbide grain size could be determinant in controlling the populations of coincidence site lattice boundaries in WC-Co composites

  1. A constitutive model of nanocrystalline metals based on competing grain boundary and grain interior deformation mechanisms

    KAUST Repository

    Gurses, Ercan

    2011-12-01

    In this work, a viscoplastic constitutive model for nanocrystalline metals is presented. The model is based on competing grain boundary and grain interior deformation mechanisms. In particular, inelastic deformations caused by grain boundary diffusion, grain boundary sliding and dislocation activities are considered. Effects of pressure on the grain boundary diffusion and sliding mechanisms are taken into account. Furthermore, the influence of grain size distribution on macroscopic response is studied. The model is shown to capture the fundamental mechanical characteristics of nanocrystalline metals. These include grain size dependence of the strength, i.e., both the traditional and the inverse Hall-Petch effects, the tension-compression asymmetry and the enhanced rate sensitivity. © 2011 Elsevier B.V. All rights reserved.

  2. Science at the interface : grain boundaries in nanocrystalline metals.

    Energy Technology Data Exchange (ETDEWEB)

    Rodriguez, Mark Andrew; Follstaedt, David Martin; Knapp, James Arthur; Brewer, Luke N.; Holm, Elizabeth Ann; Foiles, Stephen Martin; Hattar, Khalid M.; Clark, Blythe B.; Olmsted, David L.; Medlin, Douglas L.

    2009-09-01

    Interfaces are a critical determinant of the full range of materials properties, especially at the nanoscale. Computational and experimental methods developed a comprehensive understanding of nanograin evolution based on a fundamental understanding of internal interfaces in nanocrystalline nickel. It has recently been shown that nanocrystals with a bi-modal grain-size distribution possess a unique combination of high-strength, ductility and wear-resistance. We performed a combined experimental and theoretical investigation of the structure and motion of internal interfaces in nanograined metal and the resulting grain evolution. The properties of grain boundaries are computed for an unprecedented range of boundaries. The presence of roughening transitions in grain boundaries is explored and related to dramatic changes in boundary mobility. Experimental observations show that abnormal grain growth in nanograined materials is unlike conventional scale material in both the level of defects and the formation of unfavored phases. Molecular dynamics simulations address the origins of some of these phenomena.

  3. Electronic and plasmonic phenomena at graphene grain boundaries

    Science.gov (United States)

    Fei, Z.; Rodin, A. S.; Gannett, W.; Dai, S.; Regan, W.; Wagner, M.; Liu, M. K.; McLeod, A. S.; Dominguez, G.; Thiemens, M.; Castro Neto, Antonio H.; Keilmann, F.; Zettl, A.; Hillenbrand, R.; Fogler, M. M.; Basov, D. N.

    2013-11-01

    Graphene, a two-dimensional honeycomb lattice of carbon atoms of great interest in (opto)electronics and plasmonics, can be obtained by means of diverse fabrication techniques, among which chemical vapour deposition (CVD) is one of the most promising for technological applications. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries. However, the physical properties of these grain boundaries remain challenging to characterize directly and conveniently. Here we show that it is possible to visualize and investigate the grain boundaries in CVD-grown graphene using an infrared nano-imaging technique. We harness surface plasmons that are reflected and scattered by the graphene grain boundaries, thus causing plasmon interference. By recording and analysing the interference patterns, we can map grain boundaries for a large-area CVD graphene film and probe the electronic properties of individual grain boundaries. Quantitative analysis reveals that grain boundaries form electronic barriers that obstruct both electrical transport and plasmon propagation. The effective width of these barriers (~10-20 nm) depends on the electronic screening and is on the order of the Fermi wavelength of graphene. These results uncover a microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene, and suggest the possibility of using electronic barriers to realize tunable plasmon reflectors and phase retarders in future graphene-based plasmonic circuits.

  4. Electronic and plasmonic phenomena at graphene grain boundaries.

    Science.gov (United States)

    Fei, Z; Rodin, A S; Gannett, W; Dai, S; Regan, W; Wagner, M; Liu, M K; McLeod, A S; Dominguez, G; Thiemens, M; Castro Neto, Antonio H; Keilmann, F; Zettl, A; Hillenbrand, R; Fogler, M M; Basov, D N

    2013-11-01

    Graphene, a two-dimensional honeycomb lattice of carbon atoms of great interest in (opto)electronics and plasmonics, can be obtained by means of diverse fabrication techniques, among which chemical vapour deposition (CVD) is one of the most promising for technological applications. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries. However, the physical properties of these grain boundaries remain challenging to characterize directly and conveniently. Here we show that it is possible to visualize and investigate the grain boundaries in CVD-grown graphene using an infrared nano-imaging technique. We harness surface plasmons that are reflected and scattered by the graphene grain boundaries, thus causing plasmon interference. By recording and analysing the interference patterns, we can map grain boundaries for a large-area CVD graphene film and probe the electronic properties of individual grain boundaries. Quantitative analysis reveals that grain boundaries form electronic barriers that obstruct both electrical transport and plasmon propagation. The effective width of these barriers (∼10-20 nm) depends on the electronic screening and is on the order of the Fermi wavelength of graphene. These results uncover a microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene, and suggest the possibility of using electronic barriers to realize tunable plasmon reflectors and phase retarders in future graphene-based plasmonic circuits. PMID:24122082

  5. Influence of anisotropic grain boundary properties on the evolution of grain boundary character distribution during grain growth—a 2D level set study

    International Nuclear Information System (INIS)

    The present study elaborates on a 2D level set model of polycrystal microstructures that was recently established by adding the influence of anisotropic grain boundary energy and mobility on microstructure evolution. The new model is used to trace the evolution of grain boundary character distribution during grain growth. The employed level set formulation conveniently allows the grain boundary characteristics to be quantified in terms of coincidence site lattice (CSL) type per unit of grain boundary length, providing a measure of the distribution of such boundaries. In the model, both the mobility and energy of the grain boundaries are allowed to vary with misorientation. In addition, the influence of initial polycrystal texture is studied by comparing results obtained from a polycrystal with random initial texture against results from a polycrystal that initially has a cube texture. It is shown that the proposed level set formulation can readily incorporate anisotropic grain boundary properties and the simulation results further show that anisotropic grain boundary properties only have a minor influence on the evolution of CSL boundary distribution during grain growth. As anisotropic boundary properties are considered, the most prominent changes in the CSL distributions are an increase of general low-angle Σ1 boundaries as well as a more stable presence of Σ3 boundaries. The observations also hold for the case of an initially cube-textured polycrystal. The presence of this kind of texture has little influence over the evolution of the CSL distribution. Taking into consideration the anisotropy of grain boundary properties, grain growth alone does not seem to be sufficient to promote any significantly increased overall presence of CSL boundaries. (paper)

  6. Global Goss grain growth and grain boundary characteristics in magnetostrictive Galfenol sheets

    International Nuclear Information System (INIS)

    Single Goss grains were globally grown in magnetostrictive Galfenol thin sheets via an abnormal grain growth (AGG) process. The sample behaves like single crystal Galfenol, exhibiting large magnetostriction along the 〈100〉 axes. Small variations in surface energy conditions, which were governed by different flow rates of 0.5% H2S gas in argon during annealing, had a significant impact of the development of AGG. AGG with a fully developed Goss (011) grain over 95% of the sample surface is very reproducible and feasible for a broad range of annealing conditions. In addition, the 〈100〉 orientation of the single-crystal-like Galfenol sheet aligns exactly with the rolling direction, and produces magnetostriction values of ∼300 ppm. AGG often produces isolated grains inside Goss grains due to anisotropic properties of grain boundaries. To better understand island formation mechanisms, grain orientation and grain boundary characteristics of island grains in Goss-oriented Galfenol thin sheets were also investigated. We examined samples annealed either under an argon atmosphere or under a sulfur atmosphere, and characterized the observed island grain boundaries in terms of grain misorientation angles. Trends in measured and simulated data on misorientation angles indicate that the presence of (001) island grain boundaries with angles higher than 45° can be explained by the high energy grain boundary (HEGB) model, whereas (111) boundaries with intermediate angles (20°–45°) cannot. The role of low energy coincident site lattice (CSL) boundaries on AGG in both annealing cases was found to be negligible. (paper)

  7. Dynamical simulation of structural multiplicity in grain boundaries

    International Nuclear Information System (INIS)

    Work on a computer simulation study of a low-energy high-angle boundary structure which is not periodic have been recently reported. This result is of interest since grain boundary structures are usually assumed to have a periodicity corresponding to the appropriate coincidence site lattice (CSL) and many experimental observations of the structure of grain boundaries performed using conventional and high-resolution electron microscopy, electron diffraction and x-ray diffraction appear to support this work. However, this work, using empirical interatomic pair potentials and the relaxation method of molecular statics, have simulated a Σ = 5 36.870 (001) twist boundary and found a low energy structure having a larger repeat cell than the CSL and is composed of two different types of structural unit that are randomly distributed in the boundary plane. This result, which has been termed the multiplicity of grain boundary structures, has also been found in the simulation of tilt boundaries. The multiplicity phenomenon is of special interest in twist boundaries since it is used as a structural model to explain the x-ray scattering from a Σ = 5 boundary in gold. These scattering patterns had previously remained unexplained using stable structures that had simple CSL periodicity. Also, the effect of having a multiple number of low energy structural units coexisting in the grain boundary is of more general interest since it implies that the boundary structures may be quasi-periodic and, in some circumstances, may even result in a roughening of the boundary plane. This paper extends this work by showing, using molecular dynamics, that a multiplicity of structural units can actually nucleate spontaneously in a high-angle grain boundary at finite temperatures

  8. NANOSIZE EFFECT IN GRAIN BOUNDARY MIGRATION OF COPPER

    Institute of Scientific and Technical Information of China (English)

    L. Zhou; X.Q. Wei; N.G. Zhou; D.G. Li

    2004-01-01

    Molecular dynamics simulations of high temperature annealing of copper bicrystals have been carried out. The bicrystals have planar grain boundaries, and the gain size varies in nano range. An EAM (embedded atom method) potential of FS type is used for calculating the interatomic forces. The results show that in nanocrystalline copper, GB migration driven by inter-GB reaction can take place. A critical grain size is identified, below which the inter-GB reaction becomes strong enough to trigger GB motion, which accelerates rapidly and leads to annihilation of the grain boundaries. The critical size is found to be 16 atomic radii. A "through intermediate grain mechanism" is identified for the nano-grain boundary motion observed, which is never reported for GB migrations of conventional polycrystalline metals.

  9. Microscopic evidence of grain boundary moisture during granular salt reconsolidation

    Science.gov (United States)

    Mills, M. M.; Hansen, F.; Bauer, S. J.; Stormont, J.

    2015-12-01

    Very low permeability is a principal reason salt formations are considered viable hosts for disposal of nuclear waste and spent nuclear fuel. Granular salt is likely to be used as back-fill material and a seal system component. Salt formation pressures will promote reconsolidation of granular salt, eventually resulting in low permeabilities, comparable to native salt. Understanding the consolidation processes, dependent on the stress state, moisture availability and temperature, is important for demonstrating sealing functions and long-term repository performance. As granular salt consolidates, initial void reduction is achieved by brittle processes of grain rearrangement and cataclastic flow. At porosities less than 10%, grain boundary processes and crystal-plastic mechanisms govern further porosity reduction. When present, fluid assists in grain boundary processes and recrystallization. Fluid inclusions are typically found in abundance within bedded salt crystal structure and along grain boundaries, but are rarely observed internal to domal salt grains. We have observed fluid canals and evidence of moisture along grain boundaries in domal salt. In this research, we investigate grain boundary moisture in granular salt that has been reconsolidated under high temperatures to relatively low porosity. Mine-run salt from the Waste Isolation Pilot Plant and Avery Island was used to create cylindrical samples, vented and unvented, which were reconsolidated at 250°C and stresses to 20 MPa. Unvented reconsolidation retains essentially all the grain boundary moisture as found ubiquitously on scanning electron photomicrographs of consolidated samples which revealed an inhomogeneous distribution of canals from residual moisture. This contrasts significantly with the vented samples, which had virtually no grain boundary moisture after consolidation. Microstructural techniques include scanning electron, stereo-dynascopic, and optical microscopy. The observations will be used

  10. Towards effective analysis of large grain boundary data sets

    Science.gov (United States)

    Glowinski, K.; Morawiec, A.

    2015-04-01

    Grain boundaries affect properties of polycrystals. Novel experimental techniques for three-dimensional orientation mapping give new opportunities for studies of this influence. Large networks of boundaries can be analyzed based on all five ’macroscopic’ boundary parameters. We demonstrate benefits of applying two methods for improving these analyses. The fractions of geometrically special boundaries in ferrite are estimated based on ’approximate’ distances to the nearest special boundaries; by using these parameters, the times needed for processing boundary data sets are shortened. Moreover, grain-boundary distributions for nickel are obtained using kernel density estimation; this approach leads to distribution functions more accurate than those obtained based on partition of the space into bins.

  11. Grain boundary characterization in an X750 alloy

    Energy Technology Data Exchange (ETDEWEB)

    Kevin Fisher; Sebastien Teysseyre; Emmanuelle Marquis

    2012-11-01

    Grain boundary chemistry in an X750 Ni alloy was analyzed by atom probe tomography in an effort to clarify the possible roles of elemental segregation and carbide presence on the stress corrosion cracking behavior of Ni alloys. Two types of cracks are observed: straight cracks along twin boundaries and wavy cracks at general boundaries. It was found that carbides (M23C6 and TiC) are present at both twin and general boundaries, with comparable B and P segregation for all types of grain boundaries. Twin boundaries intercept ?’ precipitates while the general boundaries wave around the ?’ and carbide precipitates. Near a crack tip, oxidation takes place on the periphery of carbide precipitate.

  12. Electronic and plasmonic phenomena at graphene grain boundaries

    OpenAIRE

    Fei, Z.; A. S. Rodin; Gannett, W.; Dai, S; Regan, W.; Wagner, M.; Liu, M. K.; Mcleod, A. S.; Dominguez, G; Thiemens, M.; Castro Neto, Antonio H.; Keilmann, F.; Zettl, A.; Hillenbrand, R.; Fogler, M. M.

    2013-01-01

    Graphene, a two-dimensional honeycomb lattice of carbon atoms, is of great interest in (opto)electronics and plasmonics and can be obtained by means of diverse fabrication techniques, among which chemical vapor deposition (CVD) is one of the most promising for technological applications. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries. However, the physical properties of these grain boundaries remain challengin...

  13. Grain boundaries in ceramics and ceramic-metal interfaces

    Energy Technology Data Exchange (ETDEWEB)

    Clarke, D.R.; Wolf, D.

    1986-01-01

    Three interfaces exist: the crystal-crystal grain boundary in very pure single-phase ceramics, the crystal-glass-crystal grain boundary in most single-phase and polyphase ceramics, and the ceramic-metal interface. It is needed to correlate their structure and adhesion/failure. Methods for studying the bonding, interfacial structure, and fracture and adhesion are discussed, and recommendations are given. 42 refs. (DLC)

  14. On transport of helium grain boundaries during irradiation

    International Nuclear Information System (INIS)

    The rate of accumulation of helium at grain boundaries is one of the important parameters determining the integrity and lifetime of the structural components of a fusion reactor. A diffusion calculation is made of the flux of helium to a grain boundary. The flux is found to depend on the gas production rate, the width of the cavity denuded zone and the cavity sink strength in the grain interior. The calculated accumulation of helium is in good agreement with the measured gas content of grain boundaries in Al, PE16 and 316 stainless steel. The flux of helium to grain boundaries increases with helium generation rate but the increase is less than proportional to the generation rate. The loss of helium to grain boundaries during the nucleation of the bubbles within the grains has been estimated; no great loss is expected to occur. However, the loss would be considerably enhanced if any delay in bubble nucleation were to occur due to incubation effects. The role of material variables is found to be difficult to predict at present. (author)

  15. Atomic-scale structure of grain boundaries: Correlations to grain boundary properties

    International Nuclear Information System (INIS)

    It is generally believed that many properties of solid interfaces are ultimately determined by their structure and composition at the atomic level. We report here on work in two areas of grain boundary (GB) research in which structure-property correlations have been investigated recently. HREM observations in connection with computer modeling of GBs in fcc metals have given considerable insight into correlations between GB energy and atomic-scale GB structure. Efforts to understand and possibly control the supercurrent transport behavior across GBs in high-temperature superconductors require the combination of microstructure characterizations with investigations of electric transport properties. In both areas considerable progress is being made and has already lead to important insights concerning interfacial properties

  16. Annealing Twinning and the Nucleation of Recrystallization at Grain Boundaries

    DEFF Research Database (Denmark)

    Jones, A R.

    1981-01-01

    Experimental evidence is presented which shows that, in three different low stacking fault energy materials, annealing twins form at grain boundaries during the very early stages of recovery following deformation. These observations provide the basis for the suggestion that twinning at grain...

  17. Discovering the Role of Grain Boundary Complexions in Materials

    Energy Technology Data Exchange (ETDEWEB)

    Harmer, Martin P. [Lehigh Univ., Bethlehem, PA (United States)

    2015-03-19

    Grain boundaries are inherently an area of disorder in polycrystalline materials which define the transport and various other material properties. The relationship between the interfacial chemistry, structure and the material properties is not well understood. Among the various taxonomies for grain boundaries, Grain Boundary Complexion is a relatively new conceptual scheme that relates the structure and kinetic properties of grain boundaries. In this classification scheme, grain boundaries are considered to be distinct three dimensional (the thickness being considerably smaller as compared to the other two dimensions but nonetheless discernible) equilibrium thermodynamic phases abutted between two crystalline phases. The stability and structure of these interfacial phases are dictated by various thermodynamic variables such as temperature, stress (pressure), interfacial chemistry (chemical potential) and most importantly by the energies of the adjoining crystal surfaces. These phases are only stable within the constraint of the adjoining grains. Although these interfacial phases are not stable in bulk form, they can transform from one complexion to another as a function of various thermodynamic variables analogous to the behavior of bulk phases. Examples of different complexions have been reported in various publications. However, a systematic investigation exploring the existence of grain boundary complexions in material systems other than alumina remains to be done. Although the role of interfacial chemistry on grain boundary complexions in alumina has been addressed, a clear understanding of the underlying thermodynamics governing complexion formation is lacking. Finally, the effects of grain boundary complexions in bulk material properties are widely unknown. Factors above urge a thorough exploration of grain boundary complexions in a range of different materials systems The purpose of the current program is to verify the existence of grain boundary complexion

  18. The effects of grain size and grain boundary characteristics on the thermal conductivity of nanocrystalline diamond

    Science.gov (United States)

    Spiteri, David; Anaya, Julian; Kuball, Martin

    2016-02-01

    Molecular dynamics simulation was used to study the effects of each grain dimension and of grain boundary characteristics on the inter-grain thermal boundary resistance (TBR) and intragrain thermal conductivity of nanocrystalline diamond. The effect of the grain boundaries perpendicular to the heat flow was studied using a multiple slab configuration, which greatly reduced the artifacts associated with the heat source/sink. The TBR between the slabs was found to be more sensitive to the atomic arrangement at the boundary than to the tilt angle between the slabs. When the atomic arrangement at the interface was altered from the minimum energy configuration, the TBR increased by a factor of three, suggesting that a sub-optimal interface quality between the grains could play a large role in reducing the thermal conductivity of nanocrystalline diamond. The thermal conductivity between the boundaries was found to be similar to the bulk value, even when the boundaries were only 25 nm apart. The effect of grain boundaries parallel to the heat flow was found to have a large dependence on the microstructural details. Parallel boundaries which were 2 nm apart reduced the thermal conductivity of defect-free diamond by between one third and a factor of ten.

  19. Grain boundary engineering for structure materials of nuclear reactors

    Science.gov (United States)

    Tan, L.; Allen, T. R.; Busby, J. T.

    2013-10-01

    Grain boundary engineering (GBE), primarily implemented by thermomechanical processing, is an effective and economical method of enhancing the properties of polycrystalline materials. Among the factors affecting grain boundary character distribution, literature data showed definitive effect of grain size and texture. GBE is more effective for austenitic stainless steels and Ni-base alloys compared to other structural materials of nuclear reactors, such as refractory metals, ferritic and ferritic-martensitic steels, and Zr alloys. GBE has shown beneficial effects on improving the strength, creep strength, and resistance to stress corrosion cracking and oxidation of austenitic stainless steels and Ni-base alloys.

  20. New deformation model of grain boundary strengthening in polycrystalline metals

    International Nuclear Information System (INIS)

    A new model explaining grain boundary strengthening in polycrystalline metals and alloys by strain hardening due to localization of plastic deformation in narrow bands near grain boundaries is suggested. Occurrence of localized deformation is caused by different flow stresses in grains of different orientation. A new model takes into account the active role of stress concentrator, independence of the strengthening coefficient on deformation, influence of segregations. Successful use of the model suggested for explanation of rhenium effect in molybdenum and tungsten is alloys pointed out

  1. Analysis of Grain Boundary Character in a Fine-Grained Nickel-Based Superalloy 718

    Science.gov (United States)

    Araujo, L. S.; dos Santos, D. S.; Godet, S.; Dille, J.; Pinto, A. L.; de Almeida, L. H.

    2014-11-01

    In the current work, sheets of superalloy 718 were processed via thermomechanical route by hot and cold rolling, followed by annealing below the δ phase solvus temperature and precipitation hardening to optimum strength. Grain boundary character distribution throughout the processing was mapped via EBSD and its evolution discussed. The results show that it is possible to process the alloy to a fine grain size obtaining concomitantly a considerably high proportion of special boundaries Σ3, Σ9, and Σ27. The precipitation of δ phase presented a strong grain refining role, without significantly impairing the twinning mechanism and, consequently, the Σ3, Σ9, and Σ27 boundary formations.

  2. Theoretical progress in non-equilibrium grain-boundary segregation(II):Micro-mechanism of grain boundary anelastic relaxation and its analytical formula

    Institute of Scientific and Technical Information of China (English)

    2009-01-01

    Finding the internal-friction peak of grain boundary anelastic relaxation was one of the important breakthroughs in the study of internal friction in the last century.But the micro-mechanism of grain boundary anelastic relaxations is still obscure.Based on the observations of the grain boundary seg-regation or depletion of solute induced by an applied stress,the following micro-mechanism was suggested:grain-boundaries will work as sources to emit vacancies when a compressive stress is exerted on them and as sinks to absorb vacancies when a tensile stress is exerted,inducing grain-boundary depletion or segregation of solute,respectively.The equations of vacancy and solute con-centrations at grain boundaries were established under the equilibrium of grain-boundary anelastic relaxation.With these the kinetic equations were established for grain boundary segregation and depletion during the grain boundary relaxation progress.

  3. Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition

    Science.gov (United States)

    Yu, Qingkai; Jauregui, Luis A.; Wu, Wei; Colby, Robert; Tian, Jifa; Su, Zhihua; Cao, Helin; Liu, Zhihong; Pandey, Deepak; Wei, Dongguang; Chung, Ting Fung; Peng, Peng; Guisinger, Nathan P.; Stach, Eric A.; Bao, Jiming; Pei, Shin-Shem; Chen, Yong P.

    2011-06-01

    The strong interest in graphene has motivated the scalable production of high-quality graphene and graphene devices. As the large-scale graphene films synthesized so far are typically polycrystalline, it is important to characterize and control grain boundaries, generally believed to degrade graphene quality. Here we study single-crystal graphene grains synthesized by ambient chemical vapour deposition on polycrystalline Cu, and show how individual boundaries between coalescing grains affect graphene’s electronic properties. The graphene grains show no definite epitaxial relationship with the Cu substrate, and can cross Cu grain boundaries. The edges of these grains are found to be predominantly parallel to zigzag directions. We show that grain boundaries give a significant Raman ‘D’ peak, impede electrical transport, and induce prominent weak localization indicative of intervalley scattering in graphene. Finally, we demonstrate an approach using pre-patterned growth seeds to control graphene nucleation, opening a route towards scalable fabrication of single-crystal graphene devices without grain boundaries.

  4. Studies of Grain Boundaries in Materials Subjected to Diffusional Creep

    DEFF Research Database (Denmark)

    Nørbygaard, Thomas

    Grain boundaries in crystalline Cu(2%Ni) creep specimens have been studied by use of scanning and transmission electron microscopy in order to establish the mechanism of deformation. Creep rate measurements and dependencies were found to fit reasonably well with the model for diffusional creep......) with the activity displayed during diffusional creep testing. It was found that boundaries with low deviation from perfect Σ did not contribute macroscopically to the creep strain. A resist deposition procedure was examined to improve the reference surface grid so as to allow determination of the grain boundary...... plane by use of simple stereomicroscopy directly on the surface. The etched pattern deteriorated heav-ily during creep testing, supposedly because of dislocation creep, due to exces-sive creep stress. Grain boundaries have been studied and characterised by TEM providing an insight into the diversity...

  5. Carbon on Quartz Grain Boundaries: Continuous Films versus Isolated Plates

    Science.gov (United States)

    Price, J. D.; Watson, E. B.; Wark, D. A.

    2003-12-01

    Piston-cylinder experiments on quartzites containing a small amount of carbon were conducted at 1.0-1.4 GPa and 850-1500° C in order to assess the microstructure of graphite along grain boundaries in deep crustal materials. In one series of experiments, polished 3mm diameter single-crystal quartz discs were coated with ˜50 to 150 nm of evaporated carbon or 500 to 1000 nm of alcohol-based carbon paint. Stacks of these were subjected to high P-T conditions for durations ranging from 5 minutes to 10 days. Observations from our earlier experiments suggested that the coatings become discontinuous with time at high temperature. However, more recent observations show that coated disc boundaries contain a dark, interconnected material: those subjected to lower temperatures and shorter durations exhibited continuous films; those run at higher temperatures for longer durations contained thicker, yet still interconnected dendrite and plate structures. In contrast, relatively fine-grained synthetic quartzites produced at similar conditions typically do not contain continuous films. Quartz powder with an initial grain size between 75-150 μ m, coated with 30-50 nm of evaporated carbon, was subjected to 850-1300° C for durations ranging from 1 hour to 6 days. Only very short runs at low temperatures contained irregular boundaries still darkened by a connected film; longer duration and higher temperature quartzites exhibited texturally-equilibrated quartz grains accompanied by isolated small opaque carbon plates located along grain corners, edges, and grain boundaries. Identical features are seen in additional quartzite materials constructed in graphite cylinders using uncoated powdered silica glass or smaller quartz crystals (<22 μ m) taken to 1000° C and 1.4 GPa for 14 days. The results suggest that carbon may remain as a connected surface, at least metastably, on silicate mineral boundaries in the absence of grain boundary movement. With grain growth, carbon diffuses

  6. Simulation of grain boundary effect on characteristics of ZnO thin film transistor by considering the location and orientation of grain boundary

    Institute of Scientific and Technical Information of China (English)

    Zhou Yu-Ming; He Yi-Gang; Lu Ai-Xia; Wan Qing

    2009-01-01

    The grain boundaries (GBs) have a strong effect on the electric properties of ZnO thin film transistors (TFTs).A novel grain boundary model was developed to analyse the effect. The model was characterized with different angles between the orientation of the grain boundary and the channel direction. The potential barriers formed by the grain boundaries increase with the increase of the grain boundary angle,so the degradation of the transistor characteristics increases. When a grain boundary is close to the drain edge,the potential barrier height reduces,so the electric properties were improved.

  7. Pipe and grain boundary diffusion of He in UO2.

    Science.gov (United States)

    Galvin, C O T; Cooper, M W D; Fossati, P C M; Stanek, C R; Grimes, R W; Andersson, D A

    2016-10-12

    Molecular dynamics simulations have been conducted to study the effects of dislocations and grain boundaries on He diffusion in [Formula: see text]. Calculations were carried out for the {1 0 0}, {1 1 0} and {1 1 1} [Formula: see text] edge dislocations, the screw [Formula: see text] dislocation and Σ5, Σ13, Σ19 and Σ25 tilt grain boundaries. He diffusivity as a function of distance from the dislocation core and grain boundaries was investigated for the temperature range 2300-3000 K. An enhancement in diffusivity was predicted within 20 Å of the dislocations or grain boundaries. Further investigation showed that He diffusion in the edge dislocations follows anisotropic behaviour along the dislocation core, suggesting that pipe diffusion occurs. An Arrhenius plot of He diffusivity against the inverse of temperature was also presented and the activation energy calculated for each structure, as a function of distance from the dislocation or grain boundary. PMID:27537341

  8. Effect of grain boundary sliding on the toughness of ultrafine grain structure steel: A molecular dynamics simulation study

    Institute of Scientific and Technical Information of China (English)

    Xie Hong-Xian; Liu Bo; Yin Fu-Xing; Yu Tao

    2013-01-01

    Molecular dynamics simulations are carried out to investigate the mechanisms of low-temperature impact toughness of the ultrafine grain structure steel.The simulation results suggest that the sliding of the {001}/{ 11 0} type and { 110}/{ 111 } type grain boundary can improve the impact toughness.Then,the mechanism of grain boundary sliding is studied and it is found that the motion of dislocations along the grain boundary is the underlying cause of the grain boundary sliding.Finally,the sliding of the grain boundary is analyzed from the standpoint of the energy.We conclude that the measures which can increase the quantity of the {001}/{110} type and {110}/{111} type grain boundary and elongate the free gliding distance of dislocations along these grain boundaries will improve the low-temperature impact toughness of the ultrafine grain structure steel.

  9. The potential link between high angle grain boundary morphology and grain boundary deformation in a nickel-based superalloy

    Energy Technology Data Exchange (ETDEWEB)

    Carter, Jennifer L.W., E-mail: jennifer.w.carter@case.edu [Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 44321 (United States); Sosa, John M. [Center for Accelerated Maturation of Materials, The Ohio State University, Columbus, OH 44321 (United States); Shade, Paul A. [Air Force Research Laboratory, Materials & Manufacturing Directorate, AFRL/RXCM, Wright-Patterson AFB, Dayton, OH 45433 (United States); Fraser, Hamish L. [Center for Accelerated Maturation of Materials, The Ohio State University, Columbus, OH 44321 (United States); Uchic, Michael D. [Air Force Research Laboratory, Materials & Manufacturing Directorate, AFRL/RXCM, Wright-Patterson AFB, Dayton, OH 45433 (United States); Mills, Michael J. [Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 44321 (United States)

    2015-07-29

    Focused ion beam (FIB) based serial sectioning was utilized to characterize the morphology of two high angle grain boundaries (HAGB) in a nickel based superalloy, one that experienced grain boundary sliding (GBS) and the other experienced strain accumulation, during elevated temperature constant stress loading conditions. A custom script was utilized to serial section and collect ion-induced secondary electron images from the FIB-SEM system. The MATLAB based MIPAR{sup TM} software was utilized to align, segment and reconstruct 3D volumes from the sectioned images. Analysis of the 3D data indicates that the HAGB that exhibited GBS had microscale curvature that was planar in nature, and local serrations on the order of ±150 nm. In contrast, the HAGB that exhibited strain accumulation was not planar and had local serrations an order of magnitude greater than the other grain boundary. It is hypothesized that the serrations and the local grain boundary network are key factors in determining which grain boundaries experience GBS during creep deformation.

  10. Near-grain-boundary characterization by atomic force microscopy

    Energy Technology Data Exchange (ETDEWEB)

    Pramanick, A.K., E-mail: pramanick@nmlindia.org [MST Division, National Metallurgical Laboratory, Jamshedpur 831007 (India); Sinha, A. [MST Division, National Metallurgical Laboratory, Jamshedpur 831007 (India); Sastry, G.V.S. [Centre of Advanced Study, Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005 (India); Ghosh, R.N. [MST Division, National Metallurgical Laboratory, Jamshedpur 831007 (India)

    2009-05-15

    Characterization of near-grain boundary is carried out by atomic force microscopy (AFM). It has been observed to be the most suitable technique owing to its capability to investigate the surface at high resolution. Commercial purity-grade nickel processed under different conditions, viz., (i) cold-rolled and annealed and (ii) thermally etched condition without cold rolling, is considered in the present study. AFM crystallographic data match well with the standard data. Hence, it establishes two grain-boundary relations viz., plane matching and coincidence site lattice (CSL {Sigma}=9) relation for the two different sample conditions.

  11. Measurement and analysis of grain boundary grooving by volume diffusion

    Science.gov (United States)

    Hardy, S. C.; Mcfadden, G. B.; Coriell, S. R.; Voorhees, P. W.; Sekerka, R. F.

    1991-01-01

    Experimental measurements of isothermal grain boundary grooving by volume diffusion are carried out for Sn bicrystals in the Sn-Pb system near the eutectic temperature. The dimensions of the groove increase with a temporal exponent of 1/3, and measurement of the associated rate constant allows the determination of the product of the liquid diffusion coefficient D and the capillarity length Gamma associated with the interfacial free energy of the crystal-melt interface. The small-slope theory of Mullins is generalized to the entire range of dihedral angles by using a boundary integral formulation of the associated free boundary problem, and excellent agreement with experimental groove shapes is obtained. By using the diffusivity measured by Jordon and Hunt, the present measured values of Gamma are found to agree to within 5 percent with the values obtained from experiments by Gunduz and Hunt on grain boundary grooving in a temperature gradient.

  12. Grain boundary dynamics in ceramics superplasticity

    Directory of Open Access Journals (Sweden)

    Wakai, E.

    2001-04-01

    Full Text Available Superplasticity refers to an ability of polycrystalline solids to exhibit exceptionally large elongation in tension. The application of superplasticity makes it possible to fabricate ceramic components by superplastic forming (SPF, concurrent with diffusion bonding, and superplastic sinter-forging just like superplastic metals. Furthermore the superplastic deformation plays an important role in stress-assisted densification processes such as hot isostatic pressing (HIP and hot pressing (HP. The ceramics superplasticity has been one of intensive research fields in the last decade. Although most of reports are still limited to those of zirconia[1], new developments have been achieved in superplasticity of Si3N4 and SiC in recent years. It is clearly demonstrated that the superplasticity is one of the common natures of fine-grained ceramics and nanocrystalline ceramics at elevated temperatures.

    La superplaticidad se refiere a la capacidad que posee un sólido policristalino de presentar alargamientos excepcionalmente elevados en tracción. La aplicación de la superplasticidad hace posible la fabricación de componentes cerámicos por conformado superplástico, soldadura por difusión y forja-sinterizado superplástica, igual que en metales superplásticos. Además, la deformación superplástica tiene un rol importante en los procesos de densificación asistidos por tensiones, tales como la compactación isostática en caliente y el prensado en caliente. Las cerámicas superplásticas han sido uno de los campos donde se ha realizado una investigación más intensa en la última década. Aunque, la mayoría de los informes se limitan a la circonia[1] se han alcanzado nuevos desarrollos en superplasticidad de Si3N4 y SiC. Está claramente demostrado que la superplasticidad es una propiedad intrínseca de las cerámicas de pequeño tamaño de grano y de las cer

  13. A mechanistic study of impurity segregation at silicon grain boundaries

    International Nuclear Information System (INIS)

    The segregation behavior of carbon and oxygen atoms at various silicon grain boundaries was studied using a combination of atomistic simulation and analytical modeling. First, quasi-lattice Grand Canonical Monte Carlo simulations were used to compute segregation isotherms as a function of grain boundary type, impurity atom loading level, and temperature. Next, the atomistic results were employed to regress different analytical segregation models and extract thermodynamic and structural properties. The multilayer Brunauer–Emmett–Teller (BET) isotherm was found to quantitatively capture all the simulation conditions probed in this work, while simpler, single layer models such as the Langmuir-McLean model did not. Some of the BET parameters, namely, the binding free energy of the first adsorption layer and the impurity holding capacity of each layer, were tested for correlation with various measures of grain boundary structure and/or mechanical properties. It was found that certain measures of the atomistic stress distribution correlate strongly with the first-layer binding free energy for substitutional carbon atoms, while common grain boundary identifiers such as sigma value and energy density are not useful in this regard. Preliminary analysis of the more complex case of interstitial oxygen segregation showed that similar measures based on atomistic stress also may be useful here, but more systematic correlative studies are needed to develop a comprehensive picture

  14. Ab initio modelling of UN grain boundary interfaces

    Science.gov (United States)

    Kotomin, E. A.; Zhukovkii, Yu F.; Bocharov, D.; Gryaznov, D.

    2012-08-01

    The uranium mononitride (UN) is a material considered as a promising candidate for Generation-IV nuclear reactor fuels. Unfortunately, oxygen in air affects UN fuel performance and stability. Therefore, it is necessary to understand the mechanism of oxygen adsorption and further UN oxidation in the bulk and at surface. Recently, we performed a detailed study on oxygen interaction with UN surface using density functional theory (DFT) calculations. We were able to identify an atomistic mechanism of UN surface oxidation consisting of several important steps, starting with the oxygen molecule dissociation and finishing with oxygen atom incorporation into vacancies on the surface. However, in reality most of processes occur at the interfaces and on UN grain boundaries. In this study, we present the results of first DFT calculations on O behaviour inside UN grain boundaries performed using GGA exchange-correlation functional PW91 as implemented into the VASP computer code. We consider a simple interface (310)[001](36.8°) tilt grain boundary. The N vacancy formation energies and energies of O incorporation into pre-existing vacancies in the grain boundaries as well as O solution energies were compared with those obtained for the UN (001) and (110) surfaces

  15. A mechanistic study of impurity segregation at silicon grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Käshammer, Peter; Sinno, Talid, E-mail: talid@seas.upenn.edu [Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (United States)

    2015-09-07

    The segregation behavior of carbon and oxygen atoms at various silicon grain boundaries was studied using a combination of atomistic simulation and analytical modeling. First, quasi-lattice Grand Canonical Monte Carlo simulations were used to compute segregation isotherms as a function of grain boundary type, impurity atom loading level, and temperature. Next, the atomistic results were employed to regress different analytical segregation models and extract thermodynamic and structural properties. The multilayer Brunauer–Emmett–Teller (BET) isotherm was found to quantitatively capture all the simulation conditions probed in this work, while simpler, single layer models such as the Langmuir-McLean model did not. Some of the BET parameters, namely, the binding free energy of the first adsorption layer and the impurity holding capacity of each layer, were tested for correlation with various measures of grain boundary structure and/or mechanical properties. It was found that certain measures of the atomistic stress distribution correlate strongly with the first-layer binding free energy for substitutional carbon atoms, while common grain boundary identifiers such as sigma value and energy density are not useful in this regard. Preliminary analysis of the more complex case of interstitial oxygen segregation showed that similar measures based on atomistic stress also may be useful here, but more systematic correlative studies are needed to develop a comprehensive picture.

  16. The influence of grain boundary structure on diffusional creep

    DEFF Research Database (Denmark)

    Thorsen, Peter Anker; Bilde-Sørensen, Jørgen

    1999-01-01

    A Cu-2wt%Ni-alloy was deformed in tension in the diffusional creep regime (Nabarro-Herring creep). A periodic grid consisting of alumina was deposited on the surface of the creep specimen prior to creep. This makes it possible to separate the deformation caused by grain boundary sliding from...

  17. Chemistry of grain boundary environments in nanocrystalline Al 7075

    Energy Technology Data Exchange (ETDEWEB)

    Ferragut, Rafael, E-mail: rafael.ferragut@polimi.i [Physics Department, LNESS and CNISM, Politecnico di Milano, via Anzani 42, I-22100 Como (Italy); Liddicoat, Peter V. [Australian Key Centre for Microscopy and Microanalysis and ARC Centre of Excellence for Design in Light Metals, University of Sydney, NSW 2006 (Australia); Liao Xiaozhou [School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, NSW 2006 (Australia); Zhao, Yong-Hao; Lavernia, Enrique J. [Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616 (United States); Valiev, Ruslan Z. [Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, K. Marksa 12, Ufa 450000 (Russian Federation); Dupasquier, Alfredo [Physics Department, LNESS and CNISM, Politecnico di Milano, via Anzani 42, I-22100 Como (Italy); Ringer, Simon P. [Australian Key Centre for Microscopy and Microanalysis and ARC Centre of Excellence for Design in Light Metals, University of Sydney, NSW 2006 (Australia)

    2010-04-16

    Positron annihilation spectroscopy in two variants (coincidence Doppler-broadening CDB and lifetime spectroscopy LT) was used for the characterisation of severely deformed nanocrystalline 7075 Al alloy, with specific attention to the distribution of solute in the proximity of grain boundaries. The 7075 samples were deformed via the high pressure torsion (HPT) technique after solution treatment and quenching. The grain size at the end of the deformation was sub 100 nm. The deformed samples have undergone 3 months of natural ageing post-processing. CDB and LT measurements consistently indicate that the fraction of trapped positrons in these samples is near to 90%. The analysis of CDB data shows that the environment of the positron traps is enriched with solute up to 50 at.%, nearly evenly divided between Mg and transition metals (Zn and Cu). The CDB results indicate an enhancement of solute concentration at grain boundaries associated with HPT deformation.

  18. Probing grain boundary sink strength at the nanoscale: Energetics and length scales of vacancy and interstitial absorption by grain boundaries in α-Fe

    Energy Technology Data Exchange (ETDEWEB)

    Tschopp, Mark A.; Solanki, K. N.; Gao, Fei; Sun, Xin; Khaleel, Mohammad A.; Horstemeyer, Mark

    2012-02-10

    The energetics and length scales associated with the interaction between point defects (vacancies and self-interstitial atoms) and grain boundaries in bcc Fe was explored. Molecular statics simulations were used to generate a grain boundary structure database that contained {approx}170 grain boundaries with varying tilt and twist character. Then, vacancy and self-interstitial atom formation energies were calculated at all potential grain boundary sites within 15 {angstrom} of the boundary. The present results provide detailed information about the interaction energies of vacancies and self-interstitial atoms with symmetric tilt grain boundaries in iron and the length scales involved with absorption of these point defects by grain boundaries. Both low- and high-angle grain boundaries were effective sinks for point defects, with a few low-{Sigma} grain boundaries (e.g., the {Sigma}3{l_brace}112{r_brace} twin boundary) that have properties different from the rest. The formation energies depend on both the local atomic structure and the distance from the boundary center. Additionally, the effect of grain boundary energy, disorientation angle, and {Sigma} designation on the boundary sink strength was explored; the strongest correlation occurred between the grain boundary energy and the mean point defect formation energies. Based on point defect binding energies, interstitials have {approx}80% more grain boundary sites per area and {approx}300% greater site strength than vacancies. Last, the absorption length scale of point defects by grain boundaries is over a full lattice unit larger for interstitials than for vacancies (mean of 6-7 {angstrom} versus 10-11 {angstrom} for vacancies and interstitials, respectively).

  19. Probing grain boundary sink strength at the nanoscale: Energetics and length scales of vacancy and interstitial absorption by grain boundaries in α-Fe

    Science.gov (United States)

    Tschopp, M. A.; Solanki, K. N.; Gao, F.; Sun, X.; Khaleel, M. A.; Horstemeyer, M. F.

    2012-02-01

    The energetics and length scales associated with the interaction between point defects (vacancies and self-interstitial atoms) and grain boundaries in bcc Fe was explored. Molecular statics simulations were used to generate a grain boundary structure database that contained ≈170 grain boundaries with varying tilt and twist character. Then, vacancy and self-interstitial atom formation energies were calculated at all potential grain boundary sites within 15 Å of the boundary. The present results provide detailed information about the interaction energies of vacancies and self-interstitial atoms with symmetric tilt grain boundaries in iron and the length scales involved with absorption of these point defects by grain boundaries. Both low- and high-angle grain boundaries were effective sinks for point defects, with a few low-Σ grain boundaries (e.g., the Σ3{112} twin boundary) that have properties different from the rest. The formation energies depend on both the local atomic structure and the distance from the boundary center. Additionally, the effect of grain boundary energy, disorientation angle, and Σ designation on the boundary sink strength was explored; the strongest correlation occurred between the grain boundary energy and the mean point defect formation energies. Based on point defect binding energies, interstitials have ≈80% more grain boundary sites per area and ≈300% greater site strength than vacancies. Last, the absorption length scale of point defects by grain boundaries is over a full lattice unit larger for interstitials than for vacancies (mean of 6-7 Å versus 10-11 Å for vacancies and interstitials, respectively).

  20. Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers

    Science.gov (United States)

    Selli, Daniele; Boulfelfel, Salah Eddine; Schapotschnikow, Philipp; Donadio, Davide; Leoni, Stefano

    2016-02-01

    Thermoelectric materials are strategically valuable for sustainable development, as they allow for the generation of electrical energy from wasted heat. In recent years several strategies have demonstrated some efficiency in improving thermoelectric properties. Dopants affect carrier concentration, while thermal conductivity can be influenced by alloying and nanostructuring. Features at the nanoscale positively contribute to scattering phonons, however those with long mean free paths remain difficult to alter. Here we use the concept of hierarchical nano-grains to demonstrate thermal conductivity reduction in rocksalt lead chalcogenides. We demonstrate that grains can be obtained by taking advantage of the reconstructions along the phase transition path that connects the rocksalt structure to its high-pressure form. Since grain features naturally change as a function of size, they impact thermal conductivity over different length scales. To understand this effect we use a combination of advanced molecular dynamics techniques to engineer grains and to evaluate thermal conductivity in PbSe. By affecting grain morphologies only, i.e. at constant chemistry, two distinct effects emerge: the lattice thermal conductivity is significantly lowered with respect to the perfect crystal, and its temperature dependence is markedly suppressed. This is due to an increased scattering of low-frequency phonons by grain boundaries over different size scales. Along this line we propose a viable process to produce hierarchical thermoelectric materials by applying pressure via a mechanical load or a shockwave as a novel paradigm for material design.

  1. Improve sensitization and corrosion resistance of an Al-Mg alloy by optimization of grain boundaries

    Science.gov (United States)

    Yan, Jianfeng; Heckman, Nathan M.; Velasco, Leonardo; Hodge, Andrea M.

    2016-05-01

    The sensitization and subsequent intergranular corrosion of Al-5.3 wt.% Mg alloy has been shown to be an important factor in stress corrosion cracking of Al-Mg alloys. Understanding sensitization requires the review of grain boundary character on the precipitation process which can assist in developing and designing alloys with improved corrosion resistance. This study shows that the degree of precipitation in Al-Mg alloy is dependent on grain boundary misorientation angle, adjacent grain boundary planes and grain boundary types. The results show that the misorientation angle is the most important factor influencing precipitation in grain boundaries of the Al-Mg alloy. Low angle grain boundaries (≤15°) have better immunity to precipitation and grain boundary acid attack. High angle grain boundaries (>15°) are vulnerable to grain boundary acid attack. Grain boundaries with adjacent plane orientations near to {100} have potential for immunity to precipitation and grain boundary acid attack. This work shows that low Σ (Σ ≤ 29) coincident site lattice (CSL) grain boundaries have thinner β precipitates. Modified nitric acid mass loss test and polarization test demonstrated that the global corrosion resistance of sputtered Al-Mg alloy is enhanced. This may be attributed to the increased fractions of low Σ (Σ ≤ 29) CSL grain boundaries after sputtering.

  2. Grain boundary diffusion of Fe in ultrafine-grained nanocluster-strengthened ferritic steel

    International Nuclear Information System (INIS)

    Grain boundary diffusion of Fe in nanocluster-strengthened ferritic steel (Fe-14Cr-3W-0.4Ti-0.25Y2O3 in wt.%) has been investigated. The steel was produced by mechanical alloying followed by hot extrusion. The final grain size was ∼200 nm. The diffusivity of Fe was measured within the temperature range 423-820 K. The grain boundary penetration at lower temperatures revealed a specific time dependence, which indicates a residual interconnected porosity in the ferritic steel. In order to quantify the percolating porosity, conventional radiotracer (59Fe) diffusion measurements were combined with a study of room temperature penetration of liquid 110mAg solution to distinguish between solid-state diffusion along boundaries and penetration along the surface of interconnected cavities. The presence of porosity affected the diffusion process, introducing a hierarchy of internal interfaces. The grain boundary diffusion coefficient and the diffusivity along internal surfaces were determined in the so-called type C-C, C-B and B-B kinetic regimes of interface diffusion in a hierarchical microstructure. Using the residual activity method and a 65Zn tracer, the volume fraction of the percolating porosity was estimated to be 0.6%.

  3. Long time scale simulation of a grain boundary in copper

    DEFF Research Database (Denmark)

    Pedersen, A.; Henkelman, G.; Schiøtz, Jakob;

    2009-01-01

    A general, twisted and tilted, grain boundary in copper has been simulated using the adaptive kinetic Monte Carlo method to study the atomistic structure of the non-crystalline region and the mechanism of annealing events that occur at low temperature. The simulated time interval spanned 67 mu s at...... and a common neighbor analysis. Annealing events leading to lowering of the energy typically involved concerted displacement of several atoms-even as many as 10 atoms displaced by more than half an Angstrom. Increased local icosahedral ordering is observed in the boundary layer, but local HCP...... coordination was also observed. In the final low-energy configurations, the thickness of the region separating the crystalline grains corresponds to just one atomic layer, in good agreement with reported experimental observations. The simulated system consists of 1307 atoms and atomic interactions were...

  4. First principles modeling of grain boundaries in CdTe

    Science.gov (United States)

    Chan, Maria K. Y.; Sen, Fatih; Buurma, Christopher; Paulauskas, Tadas; Sun, Ce; Kim, Moon; Klie, Robert

    The role of extended defects is of significant interest for semiconductors, especially photovoltaics since energy conversion efficiencies are often affected by such defects. In particular, grain boundaries in CdTe photovoltaics are enigmatic since the achievable efficiencies of CdTe photovoltaics are higher in polycrystalline devices as compared to single crystalline devices. Yet, despite recent advances, the efficiency of poly-CdTe devices are still substantially below the theoretical maximum. We carry out an atomistic-level study using Scanning Transmission Electron Microscopy (STEM), together with first principles density functional theory (DFT) modeling, in order to understand the properties of specific bicrystals, i.e. artificial grain boundaries, constructed using wafer bonding. We discuss examples of bicrystals, including some involving large scale DFT calculations, and trends in defect and electronic properties. This work was funded by DOE SunShot BRIDGE program.

  5. Assessment of grain boundary segregation inducing intergranular fracture

    International Nuclear Information System (INIS)

    Intergranular fracture is often attributed to grain boundary embrittlement resulting from the segregation of some embrittling elements. Consequently, it is often associated with catastrophic premature brittle fracture, in particular with respect to the more common transgranular cleavage fracture. One of the embrittling elements known to promote intergranular fracture of reactor pressure vessel steels under irradiation is phosphorus, even without hardening. To better assess the effect of intergranular grain boundary segregation on the mechanical properties of reactor pressure vessel steels, an experimental program was carried out on an embrittled steel through a step cooling heat treatment. This reversible temper embrittlement treatment consists of gradually cooling the steel in the range 600-400 deg. C with increasing time resulting in phosphorus segregation to grain boundaries. Tensile, Charpy impact and fracture toughness tests were performed in both as received and step cooled condition, together with scanning electron microscopy examination to reveal the fracture mode. The results show that intergranular fracture is not as catastrophic as originally thought. Crack instability is clearly higher during transgranular cleavage than intergranular fracture. The interpretation of experimental results based on the Charpy impact test can be biased by the ductile fracture contribution. Experimental data including both Charpy impact and fracture toughness could be analyzed in a consistent way allowing for an improved interpretation. (authors)

  6. Grain boundary segregation and intergranular fracture in molybdenum

    International Nuclear Information System (INIS)

    The present paper reports results from a systematic study of segregation and intergranular embrittlement in binary molybdenum-oxygen and ternary molybdenum-oxygen-carbon alloys. The experiments were carried out on 'bamboo' specimens containing a series of identical single grain boundaries traversing their cross-sections. Measurements have been made of the activation energy for oxygen segregation to grain boundaries in the binary alloys. The influence of carbon additions on the level of oxygen segregation and the influence of oxygen segregation on the energy to fracture have been studied. Results from metallographic studies show the effects of segregation on fracture surface topography and dislocation surfaces immediately adjacent to the fracture surfaces. The thermodynamics of oxygen segregation to grain boundaries is considered and the role played by carbon in inhibiting segregation. The influence of segregation on the work of fracture is also considered and it is shown that the reduction in oxygen segregation resulting from the addition of carbon produces small increases in fracture energy. This increases the local stress to propagate a crack sufficiently to promote plastic deformation which blunts the crack tip; this reduces the stress concentration at the crack tip and the applied fracture stress and the work to fracture is thus increased. (author)

  7. Grain-boundary-induced melting in quenched polycrystalline monolayers

    Science.gov (United States)

    Deutschländer, Sven; Boitard, Charlotte; Maret, Georg; Keim, Peter

    2015-12-01

    Melting in two dimensions can successfully be explained with the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) scenario which describes the formation of the high-symmetry phase with the thermal activation of topological defects within an (ideally) infinite monodomain. With all state variables being well defined, it should hold also as freezing scenario where oppositely charged topological defects annihilate. The Kibble-Zurek mechanism, on the other hand, shows that spontaneous symmetry breaking alongside a continuous phase transition cannot support an infinite monodomain but leads to polycrystallinity. For any nonzero cooling rate, critical fluctuations will be frozen out in the vicinity of the transition temperature. This leads to domains with different director of the broken symmetry, separated by a defect structure, e.g., grain boundaries in crystalline systems. After instantaneously quenching a colloidal monolayer from a polycrystalline to the isotropic fluid state, we show that such grain boundaries increase the probability for the formation of dislocations. In addition, we determine the temporal decay of defect core energies during the first few Brownian times after the quench. Despite the fact that the KTHNY scenario describes a continuous phase transition and phase equilibrium does not exist, melting in polycrystalline samples starts at grain boundaries similar to first-order phase transitions.

  8. Denuded Zones, Diffusional Creep, and Grain Boundary Sliding

    Energy Technology Data Exchange (ETDEWEB)

    Wadsworth, J; Ruano, O A; Sherby, O D

    2001-06-27

    The appearance of denuded zones following low stress creep in particle-containing crystalline materials is both a microstructural prediction and observation often cited as irrefutable evidence for the Nabarro-Herring mechanism of diffusional creep. The denuded zones are predicted to be at grain boundaries that are orthogonal to the direction of the applied stress. Furthermore, their dimensions should account for the accumulated plastic flow. In the present paper, the evidence for such denuded zones is critically examined. These zones have been observed during creep of magnesium, aluminum, and nickel-base alloys. The investigation casts serious doubts on the apparently compelling evidence for the link between denuded zones and diffusional creep. Specifically, denuded zones are clearly observed under conditions that are explicitly not diffusional creep. Additionally, the denuded zones are often found in directions that are not orthogonal to the applied stress. Other mechanisms that can account for the observations of denuded zones are discussed. It is proposed that grain boundary sliding accommodated by slip is the rate-controlling process in the stress range where denuded zones have been observed. It is likely that the denuded zones are created by dissolution of precipitates at grain boundaries that are simultaneously sliding and migrating during creep.

  9. Design of zirconia grain boundaries——An exploration of the design of oxide ceramic grain boundaries

    Institute of Scientific and Technical Information of China (English)

    郭新; 袁润章

    1996-01-01

    The grain-boundary resistance of ZrO2 mainly consists of the contribution from the space-charge layers Rsc and that from the grain-boundary phase Rgbp The reduction of Rsc can be realized by the segregations of solutes with positive effective charge in the space-charge layers,while the reduction of Rgbp can be achieved by decreasing the amount of the grain-boundary phase,changing the morphology of the phase and forming grain-boundary phase with high conductivity.The best grain-boundary design effect can only be achieved by the joint design of the space-charge layers and the grain-boundary phase.

  10. Diffusive-to-ballistic transition in grain boundary motion studied by atomistic simulations

    International Nuclear Information System (INIS)

    An adapted simulation method is used to systematically study grain boundary motion at velocities and driving forces across more than five orders of magnitude. This analysis reveals that grain boundary migration can occur in two modes, depending upon the temperature (T) and applied driving force (P). At low P and T, grain boundary motion is diffusional, exhibiting the kinetics of a thermally activated system controlled by grain boundary self-diffusion. At high P and T, grain boundary migration exhibits the characteristic kinetic scaling behavior of a ballistic process. A rather broad transition range in both P and T lies between the regimes of diffusive and ballistic grain boundary motion, and is charted here in detail. The recognition and delineation of these two distinct modes of grain boundary migration also leads to the suggestion that many prior atomistic simulations might have probed a different kinetic regime of grain boundary motion (ballistic) as compared to that revealed in most experimental studies (diffusional).

  11. COBALT SEGREGATION IN CARBIDE GRAIN BOUNDARIES IN WC-Co COMPOSITES

    OpenAIRE

    Vicens, J.; Dubon, A.; Laval, J; Benjdir, M.; Nouet, G.

    1990-01-01

    The cobalt concentration at WC-WC grain boundaries in WC-Co composites was determined via X-ray energy dispersive analysis in STEM. Cobalt profiles were performed-across coincidence grain boundaries Σ2 and Σ5 with a (10(-1)0) prismatic plane. A small cobalt segregation has been detected when dislocations were imaged in the grain boundaries. The cobalt segregation value is compared to the segregation ratio obtained in low angle and general grain boundaries.

  12. EFFECT OF GRAIN BOUNDARY SEGREGATION OF ANTIMONY ON RELAXATION AT GRAIN BOUNDARIES IN SILICON-IRON ALLOYS

    OpenAIRE

    Iwasaki, Y.; Fujimoto, K

    1981-01-01

    A sharp grain boundary peak appears in both 2 and 3% silicon-iron alloys due to a substitutional solute of silicon. This peak is highly sensitive to the segregation of the third element of antimony and, contrary to orthodox solute peaks in binary and ternary alloys, largely decreases in magnitude on heating after a segregation treatment. The subsequent measurement on cooling returns the peak to the ordinary magnitude. As a function of annealing time at a temperature of segregation, the height...

  13. Bandgap tunability at single-layer molybdenum disulphide grain boundaries

    KAUST Repository

    Huang, Yu Li

    2015-02-17

    Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40±0.05 eV for single-layer, 2.10±0.05 eV for bilayer and 1.75±0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85±0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.

  14. Simulation on Grain Boundary Sliding during Superplastic Deformation Using Molecular Dynamics Method

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    Grain growth and grain boundary sliding are the two main superplastic deformation mechanisms. In the paper,simulation work is focused on the sliding of a ∑3 (111) symmetric twist coincidence grain boundary, a ∑13 (110)asymmetric tilt coincidence grain boundary, and a ∑3 (110) symmetric tilt coincidence grain boundary in Al, and the energies of grain boundary for each of equilibrium configurations are computed. An embedded atom method (EAM) potential was used to simulate the atomic interactions in a bicrystal containing more than 2000 atoms. At 0 K, the relationships between total potential energy and time steps for ∑3 (111) symmetric twist coincidence grain boundary and ∑3 (110) symmetric tilt coincidence grain boundary during sliding at 2 m/s represent the periodic characteristic. However, the relationship between total potential energy and time steps for ∑13 (110) asymmetric tilt coincidence grain boundary represents the damp surge characteristic. It is found that grain boundary sliding for ∑ 3 (110) symmetric tilt coincidence grain boundary is coupled with apparent grain boundary migration.

  15. Grain boundary structure and solute segregation in titanium-doped sapphire bicrystals

    Energy Technology Data Exchange (ETDEWEB)

    Taylor, Seth T.

    2002-05-17

    Solute segregation to ceramic grain boundaries governs material processing and microstructure evolution, and can strongly influence material properties critical to engineering performance. Understanding the evolution and implications of grain boundary chemistry is a vital component in the greater effort to engineer ceramics with controlled microstructures. This study examines solute segregation to engineered grain boundaries in titanium-doped sapphire (Al2O3) bicrystals, and explores relationships between grain boundary structure and chemistry at the nanometer scale using spectroscopic and imaging techniques in the transmission electron microscope (TEM). Results demonstrate dramatic changes in solute segregation stemming from small fluctuations in grain boundary plane and structure. Titanium and silicon solute species exhibit strong tendencies to segregate to non-basal and basal grain boundary planes, respectively. Evidence suggests that grain boundary faceting occurs in low-angle twis t boundaries to accommodate nonequilibrium solute segregation related to slow specimen cooling rates, while faceting of tilt grain boundaries often occurs to expose special planes of the coincidence site lattice (CSL). Moreover, quantitative analysis of grain boundary chemistry indicates preferential segregation of charged defects to grain boundary dislocations. These results offer direct proof that static dislocations in ionic materials can assume a net charge, and emphasize the importance of interactions between charged point, line, and planar defects in ionic materials. Efforts to understand grain boundary chemistry in terms of space charge theory, elastic misfit and nonequilibrium segregation are discussed for the Al2O3 system.

  16. Grain boundaries and glasses: birds of a feather

    Science.gov (United States)

    Zhang, Hao; Srolovitz, David; Douglas, Jack; Warren, James

    2010-03-01

    Polycrystalline materials can be viewed as composites of crystalline ``grains'' separated from one another by thin ``amorphous'' grain boundary (GB) regions. While GBs have been exhaustively investigated at low temperatures (T), where these regions are relatively ordered, much less is known about them at higher T where they exhibit structural disorder, and where characterization methods are limited. The time and spatial scales accessible to molecular dynamics (MD) simulation are appropriate for investigating the dynamical and structural properties of GB at elevated T and we exploit MD to explore basic aspects of GB dynamics as a function of T. It has long been hypothesized, based on the processing characteristics of polycrystalline materials, that GBs have features in common with glass-forming liquids. We find remarkable support for this suggestion, as evidenced by string-like collective motion, transient caging of atom motion, and non-Arrhenius T dependence of GB mobility. Evidently, the frustration caused by the inability of atoms in the GB region to simultaneously order with respect to competing grains is responsible for this similarity. The paradigm that grains are encapsulated by a ``frustrated fluid'' provides a powerful conceptual model of polycrystalline materials.

  17. Grain boundary sliding at high temperature deformation in cold-rolled ODS ferritic steels

    Energy Technology Data Exchange (ETDEWEB)

    Sugino, Yoshito, E-mail: y-sugino@eng.hokudai.ac.jp [Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, N13, W-8, Kita-ku, Sapporo 060-8628 (Japan); Ukai, Shigeharu [Materials Science and Engineering, Faculty of Engineering, Hokkaido University, N13, W-8, Kita-ku, Sapporo 060-8628 (Japan); Leng, Bin [Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, N13, W-8, Kita-ku, Sapporo 060-8628 (Japan); Oono, Naoko; Hayashi, Shigenari [Materials Science and Engineering, Faculty of Engineering, Hokkaido University, N13, W-8, Kita-ku, Sapporo 060-8628 (Japan); Kaito, Takeji; Ohtsuka, Satoshi [Advanced Nuclear System R and D Directorate, Japan Atomic Energy Agency (JAEA), 4002, Narita, Oarai, Ibaraki 311-1393 (Japan)

    2014-09-15

    A tensile test was performed at a loading direction perpendicular to elongated cold-rolled grains, and it confirmed the step of the scratched lines across the grain boundaries on the specimen surface, providing evidence for grain boundary sliding in oxide dispersion strengthened (ODS) steels. Dynamic recovery within the grains was also observed, and a simple model was constructed that consisted of grain boundary sliding and mismatch accommodation induced by vacancy flow. It was confirmed that such grain boundary sliding was suppressed in ODS steel relative to iron as a result of the pinning of the dislocation movement by the dispersed oxide particles.

  18. Investigation of grain boundary activity in nanocrystalline Al under an indenter by using a multiscale method

    Institute of Scientific and Technical Information of China (English)

    Shao Yu-Fei; Yang Xin; Zhao Xing; Wang Shao-Qing

    2012-01-01

    Grain boundary activity in nanocrystalline Al under an indenter is studied by using a multiscale method.It is found that grain boundaries and twin boundaries can be transformed into each other by emitting and absorbing dislocations.The transition processes might result in grain coarsening and refinement events.Dislocation reflection generated by a piece of stable grain boundary is also observed,because of the complex local atomic structure within the nanocrystalline Al.This implies that nanocrystalline metals might improve their internal structural stability with the help of some special local grain boundaries.

  19. Solute segregation on Σ3 and random grain boundaries in type 316L stainless steel

    International Nuclear Information System (INIS)

    Solute segregation and impurity segregation on random and Σ3 grain boundaries in a type 316L stainless steel were investigated by means of atom probe tomography (APT). Segregation of Mo, P, B, and C was observed on random grain boundaries, irrespective of grain boundary misorientation. Two-dimensional concentration maps along the grain boundary plane revealed that the concentrations of all segregated elements were not homogeneous and no co-segregation was observed. In contrast, no segregation was observed on Σ3 grain boundaries

  20. Quantitative high resolution electron microscopy of grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Campbell, G.H., King, W.E., Cohen, D., Carter, C.B.

    1996-12-12

    The {Sigma}11 (113)/[1{bar 1}0] symmetric tilt grain boundary has been characterized by high resolution transmission electron microscopy. The method by which the images are prepared for analysis is described. The statistics of the image data have been found to follow a normal distribution. The electron-optical imaging parameters used to acquire the image have been determined by nonlinear least-square image simulation optimization within the perfect crystal region of the micrograph. A similar image simulation optimization procedure is used to determine the atom positions which provide the best match between the experimental image and the image simulation.

  1. Grain boundaries and mechanical properties of nanocrystalline diamond films.

    Energy Technology Data Exchange (ETDEWEB)

    Busmann, H.-G.; Pageler, A.; Gruen, D. M.

    1999-08-06

    Phase-pure nanocrystalline diamond thin films grown from plasmas of a hydrogen-poor carbon argon gas mixture have been analyzed regarding their hardness and elastic moduli by means of a microindentor and a scanning acoustic microscope.The films are superhard and the moduli rival single crystal diamond. In addition, Raman spectroscopy with an excitation wavelength of 1064 nm shows a peak at 1438 l/cm and no peak above 1500 l/cm, and X-ray photoelectron spectroscopy a shake-up loss at 4.2 eV. This gives strong evidence for the existence of solitary double bonds in the films. The hardness and elasticity of the films then are explained by the assumption, that the solitary double bonds interconnect the nanocrystals in the films, leading to an intergrain boundary adhesion of similar strength as the intragrain diamond cohesion. The results are in good agreement with recent simulations of high-energy grain boundaries.

  2. Investigation of grain boundary migration in situ by synchrotron x-ray topography

    International Nuclear Information System (INIS)

    Grain boundary migration has been investigated in prestrained monocrystalline specimens of aluminum in situ, continuously and at temperatures ranging from 415 to 610 degrees C by synchrotron (polychromatic) x-ray topography (SXRT). In general, new (recrystallized) grains nucleate at prepositioned surface indentations and expand into the prestrained matrix, revealing complex evolution of crystallographic facets and occasional generation of (screw) dislocations in the wake of the moving boundaries. Analysis of corresponding migration rates for several faceted grain boundaries yields activation energies ranging from 56 to 125 kCal/mole, depending on the grain boundary character. It is concluded that grain boundary mobility is a sensitive function of grain boundary inclination, resulting in ultimate survival of low-mobility (faceted) inclinations as a natural consequence of growth selection. Advantages and disadvantages associated with measurement of grain boundary migration by SXRT are enumerated and correspondi

  3. Migration energy barriers of symmetric tilt grain boundaries in body-centered cubic metal Fe

    International Nuclear Information System (INIS)

    Graphical abstract: DFT calculated migration energy barrier (left) for symmetric grain boundary in metals is an essential physical property to measure the trend of grain boundary migration, in particular, in terms of the classical homogeneous nucleation model of GB dislocation/disconnection loops (right). - Migration energy barriers of two symmetric tilt grain boundaries in body-centered cubic metal Fe are obtained via first-principles calculations in combination with the nudged elastic band methods. Although the two grain boundaries show similar grain boundary energies, the migration energy barriers are different. Based on a homogeneous nucleation theory of grain-boundary dislocation loops, the calculated energy barrier provides a measure of intrinsic grain-boundary mobility and helps to evaluate effects due to vacancy and interstitial atoms such as carbon

  4. Properties of grain boundaries in BCC iron and iron-based alloys

    Energy Technology Data Exchange (ETDEWEB)

    Terentyev, D.; He, Xinfu

    2010-08-15

    The report contains a summary of work done within the collaboration established between SCK-CEN and CIEA, performed during the internship of Xinfu He (CIAE) in the period of September 2009 to June 2010. In this work, we have carried out an atomistic study addressing the properties of grain boundaries in BCC Fe and Fe-Cr alloys. Throughout this work we report on the structural and cohesive properties of grain boundaries; thermal stability; interaction of grain boundaries with He and diffusivity of He in the core of the grain boundaries; equilibrium segregation of Cr near the grain boundary zone; cleavage fracture of grain boundaries; influence of the Cr precipitates, voids and He bubbles on the structure and strength of grain boundaries.

  5. The influence of the grain boundary structure on diffusional creep

    Energy Technology Data Exchange (ETDEWEB)

    Thorsen, P.A

    1998-05-01

    An experiment was carried out to quantify the deformation in the diffusional creep domain. It was found that material had indisputably been deposited at grain boundaries in tension. A characterisation of 131 boundaries in terms of their misorientation was carried out and this was correlated to the observed deformation. Twin boundaries below a certain limit of deviation from an exact twin misorientation were totally inactive in the deformation. A large qualitative difference was found in the way general boundaries take part in the deformation. The experiments have taken place at Materials Research Department, Risoe National Laboratory at Roskilde. The present thesis has been submitted in partial fulfillment of the requirements for the Ph.D. degree in physics at the Niels Bohr Institute, University of Copenhagen. Besides the results of the creep experiment the thesis contains a description of the theoretical background to diffusional creep models. Also, the results from an investigation of helium bubble formation in an irradiated copper sample is included. (au) 7 tabs., 56 ills., 75 refs.

  6. Ostwald Ripening of Diffusion-Limited Small-Size Precipitates at Grain Boundaries

    OpenAIRE

    A.V. Koropov

    2012-01-01

    The paper describes a theoretical study of the Ostwald ripening of two-dimensional small-size precipitates of a newly formed phase at the grain boundary of finite thickness, taking into account the diffusion of impurity atoms from the grain interior to the grain boundary. The precipitate growth is believed to be limited by the impurity-atom diffusion in the grain boundary. The asymptotic time dependences are found for the average and critical precipitate radius, supersaturation of solid solut...

  7. Grain boundary and grain interior conduction in {gamma}'-Bi{sub 2}MoO{sub 6}

    Energy Technology Data Exchange (ETDEWEB)

    Vera, C.M.C. [Laboratorio de Peliculas Delgadas, Facultad de Ingenieria, Universidad de Buenos Aires, Paseo Colon 850, 1063 Buenos Aires (Argentina)]. E-mail: cvera@fi.uba.ar; Aragon, R. [Laboratorio de Peliculas Delgadas, Facultad de Ingenieria, Universidad de Buenos Aires, Paseo Colon 850, 1063 Buenos Aires (Argentina); CINSO, CONICET, CITEFA, Lasalle 4397, Villa Martelli, Buenos Aires (Argentina)

    2005-07-25

    Impedance spectroscopy of fine grained (<10 {mu}m) {gamma}'-Bi{sub 2}MoO{sub 6} samples, in the frequency range of 0.1 Hz-250 kHz, relevant to sensor applications, up to 800 deg. C, has been used to characterize grain boundary and grain interior contributions to conduction. Above 500 deg. C, the grain boundary contribution is no longer rate limiting and conduction is dominated by the grain interior component. The corresponding activation energies are 0.98 eV for grain boundary and 0.73 eV for grain interior components. The weak dependence of conductivity on oxygen partial pressure below 500 deg. C can be attributed to electrode-electrolyte interface phenomena, whereas the robust response to ethanol is commensurate with changes in intrinsic ionic conductivity.

  8. Multiferroic grain boundaries in oxygen-deficient ferroelectric lead titanate.

    Science.gov (United States)

    Shimada, Takahiro; Wang, Jie; Ueda, Taku; Uratani, Yoshitaka; Arisue, Kou; Mrovec, Matous; Elsässer, Christian; Kitamura, Takayuki

    2015-01-14

    Ultimately thin multiferroics arouse remarkable interest, motivated by the diverse utility of coexisting ferroelectric and (anti)ferromagnetic order parameters for novel functional device paradigms. However, the ferroic order is inevitably destroyed below a critical size of several nanometers. Here, we demonstrate a new path toward realization of atomically thin multiferroic monolayers while resolving a controversial origin for unexpected "dilute ferromagnetism" emerged in nanocrystals of nonmagnetic ferroelectrics PbTiO3. The state-of-the-art hybrid functional of Hartree-Fock and density functional theories successfully identifies the origin and underlying physics; oxygen vacancies interacting with grain boundaries (GBs) bring about (anti)ferromagnetism with localized spin moments at the neighboring Ti atoms. This is due to spin-polarized defect states with broken orbital symmetries at GBs. In addition, the energetics of oxygen vacancies indicates their self-assembling nature at GBs resulting in considerably high concentration, which convert the oxygen-deficient GBs into multiferroic monolayers due to their atomically thin interfacial structure. This synthetic concept that realizes multiferroic and multifunctional oxides in a monolayered geometry through the self-assembly of atomic defects and grain boundary engineering opens a new avenue for promising paradigms of novel functional devices. PMID:25485474

  9. Highly conductive grain boundaries in copper oxide thin films

    Science.gov (United States)

    Deuermeier, Jonas; Wardenga, Hans F.; Morasch, Jan; Siol, Sebastian; Nandy, Suman; Calmeiro, Tomás; Martins, Rodrigo; Klein, Andreas; Fortunato, Elvira

    2016-06-01

    High conductivity in the off-state and low field-effect mobility compared to bulk properties is widely observed in the p-type thin-film transistors of Cu2O, especially when processed at moderate temperature. This work presents results from in situ conductance measurements at thicknesses from sub-nm to around 250 nm with parallel X-ray photoelectron spectroscopy. An enhanced conductivity at low thickness is explained by the occurrence of Cu(II), which is segregated in the grain boundary and locally causes a conductivity similar to CuO, although the surface of the thick film has Cu2O stoichiometry. Since grains grow with an increasing film thickness, the effect of an apparent oxygen excess is most pronounced in vicinity to the substrate interface. Electrical properties of Cu2O grains are at least partially short-circuited by this effect. The study focuses on properties inherent to copper oxide, although interface effects cannot be ruled out. This non-destructive, bottom-up analysis reveals phenomena which are commonly not observable after device fabrication, but clearly dominate electrical properties of polycrystalline thin films.

  10. Grain shrinkage driven by surface and grain boundary energy in Ba{sub 5}Nb{sub 4}O{sub 15} powder

    Energy Technology Data Exchange (ETDEWEB)

    Tanaka, Hidehiko [Nano Ceramics Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan)]. E-mail: tanaka.hidehiko@nims.go.jp; Nakano, Hiromi [Electron Microscope Laboratory, Faculty of Science and Technology, Ryukoku University, 1-5 Yokotani Seta-Ooe, Otsu, Shiga 520-2194 (Japan); Suyama, Y. [Division for Research Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504 (Japan)

    2007-04-15

    The shrinkage and disappearance of small Ba{sub 5}Nb{sub 4}O{sub 15} grains in a large grain matrix at 1133 K observed by transmission electron microscopy (TEM), included grain volume and boundary shrinkage. Rate equations for these processes were formulated based on the concept that total excess free energy directly stimulates material transport in volume, surface, or grain boundary diffusion. Based on these equations, grain vanishing was simulated and it was found that volume diffusion combined with boundary diffusion occurred, and high grain boundary energy and low grain boundary diffusivity made grains vanish while maintaining a truncated spherical shape.

  11. 3D reconstruction of grains in polycrystalline materials using a tessellation model with curved grain boundaries

    Science.gov (United States)

    Šedivý, Ondřej; Brereton, Tim; Westhoff, Daniel; Polívka, Leoš; Beneš, Viktor; Schmidt, Volker; Jäger, Aleš

    2016-06-01

    A compact and tractable representation of the grain structure of a material is an extremely valuable tool when carrying out an empirical analysis of the material's microstructure. Tessellations have proven to be very good choices for such representations. Most widely used tessellation models have convex cells with planar boundaries. Recently, however, a new tessellation model - called the generalised balanced power diagram (GBPD) - has been developed that is very flexible and can incorporate features such as curved boundaries and non-convexity of cells. In order to use a GBPD to describe the grain structure observed in empirical image data, the parameters of the model must be chosen appropriately. This typically involves solving a difficult optimisation problem. In this paper, we describe a method for fitting GBPDs to tomographic image data. This method uses simulated annealing to solve a suitably chosen optimisation problem. We then apply this method to both artificial data and experimental 3D electron backscatter diffraction (3D EBSD) data obtained in order to study the properties of fine-grained materials with superplastic behaviour. The 3D EBSD data required new alignment and segmentation procedures, which we also briefly describe. Our numerical experiments demonstrate the effectiveness of the simulated annealing approach (compared to heuristic fitting methods) and show that GBPDs are able to describe the structures of polycrystalline materials very well.

  12. Energetic and kinetic considerations of grain boundary engineering of Ni(3)Al

    Science.gov (United States)

    Turi, Maria-Lynn

    1997-10-01

    Grain boundary design is a microstructural control technique which has demonstrated success in improving toughness, resistance to intergranular stress corrosion cracking and reducing tendency to intergranular fracture in several materials including nickel and nickel alloys. Driven by an energetic preference for twin boundaries and low Sigma grain boundaries, a twin limited structure in which 2/3 of the boundaries are special grain boundaries is possible. Twin formation is a complex function of energetic factors such as stacking fault energy, and kinetic factors such as grain boundary mobility both of which may be altered by segregation and alloying, as well as geometrical interactions between the grains. The effect of ordering on the development of microstructures during strain annealing has not been studied in detail. Nickel aluminide is a high temperature structural material which has not yet achieved its full market potential because of high susceptibility to intergranular fracture. Grain boundary design is a possible processing route which may reduce this problem. Simulation of grain boundaries in Ni and Nickel Aluminide have revealed that while average grain boundary energies are similar, Nisb3Al has a smaller energetic preference for twin and other low Sigma grain boundaries than Ni. An energetic criterion defining a special grain boundary has been developed and applied to Ni and Nisb3Al. Twin boundary energy was found to be significantly larger than for Ni. These results indicate that Nisb3Al should have a lower tendency for twinning. Strain annealing was successful in increasing the frequency of twin boundaries in Nisb3Al from a recrystallized value of 31% to 47% after three strain annealing treatments. Deformations in the range of 5% to 7%, annealing at 1050sp°C and anneal times of 15 minutes generated the best grain boundary character distributions, with the lowest low angle grain boundary frequency, lowest random boundary frequency and highest twin

  13. Grain boundary premelting and activated sintering in binary refractory alloys

    Science.gov (United States)

    Shi, Xiaomeng

    Quasi-liquid intergranular film (IGF) which has been widely observed in ceramic systems can persist into sub-solidus region whereby an analogy to Grain boundary (GB) premelting can be made. In this work, a grain boundary (GB) premelting/prewetting model in a metallic system was firstly built based on the Benedictus' model and computational thermodynamics, predicting that GB disordering can start at 60-85% of the bulk solidus temperatures in selected systems. This model quantitatively explains the long-standing mystery of subsolidus activated sintering in W-Pd, W-Ni, W-Co, W-Fe and W-Cu, and it has broad applications for understanding GB-controlled transport kinetics and physical properties. Furthermore, this study demonstrates the necessity of developing GB phase diagrams as a tool for materials design. Subsequently, Grain boundary (GB) wetting and prewetting in Ni-doped Mo are systematically evaluated via characterizing well-quenched specimens and thermodynamic modeling. In contrast to prior reports, the delta-NiMo phase does not wet Mo GBs in the solid state. In the solid-liquid two-phase region, the Ni-rich liquid wets Mo GBs completely. Furthermore, high-resolution transmission electron microscopy demonstrates that nanometer-thick quasi-liquid IGFs persist at GBs into the single-phase region where the bulk liquid phase is no longer stable; this is interpreted as a case of GB prewetting. An analytical thermodynamic model is developed and validated, and this model can be extended to other systems. Furthermore, the analytical model was refined based upon Beneditus' model with correction in determining interaction contribution of interfacial energy. A calculation-based GB phase diagram for Ni-Mo binary system was created and validated by comparing with GB diffusivities determined through a series of controlled sintering experiments. The dependence of GB diffusivity on doping level and temperature was examined and compared with model-predicted GB phase diagram. The

  14. Multiscale model of metal alloy oxidation at grain boundaries

    International Nuclear Information System (INIS)

    High temperature intergranular oxidation and corrosion of metal alloys is one of the primary causes of materials degradation in nuclear systems. In order to gain insights into grain boundary oxidation processes, a mesoscale metal alloy oxidation model is established by combining quantum Density Functional Theory (DFT) and mesoscopic Poisson-Nernst-Planck/classical DFT with predictions focused on Ni alloyed with either Cr or Al. Analysis of species and fluxes at steady-state conditions indicates that the oxidation process involves vacancy-mediated transport of Ni and the minor alloying element to the oxidation front and the formation of stable metal oxides. The simulations further demonstrate that the mechanism of oxidation for Ni-5Cr and Ni-4Al is qualitatively different. Intergranular oxidation of Ni-5Cr involves the selective oxidation of the minor element and not matrix Ni, due to slower diffusion of Ni relative to Cr in the alloy and due to the significantly smaller energy gain upon the formation of nickel oxide compared to that of Cr2O3. This essentially one-component oxidation process results in continuous oxide formation and a monotonic Cr vacancy distribution ahead of the oxidation front, peaking at alloy/oxide interface. In contrast, Ni and Al are both oxidized in Ni-4Al forming a mixed spinel NiAl2O4. Different diffusivities of Ni and Al give rise to a complex elemental distribution in the vicinity of the oxidation front. Slower diffusing Ni accumulates in the oxide and metal within 3 nm of the interface, while Al penetrates deeper into the oxide phase. Ni and Al are both depleted from the region 3–10 nm ahead of the oxidation front creating voids. The oxide microstructure is also different. Cr2O3 has a plate-like structure with 1.2–1.7 nm wide pores running along the grain boundary, while NiAl2O4 has 1.5 nm wide pores in the direction parallel to the grain boundary and 0.6 nm pores in the perpendicular direction providing an additional pathway for

  15. Segregation of solute elements at grain boundaries in an ultrafine grained Al-Zn-Mg-Cu alloy

    International Nuclear Information System (INIS)

    The solute segregation at grain boundaries (GBs) of an ultrafine grained (UFG) Al-Zn-Mg-Cu alloy processed by equal-channel angular pressing (ECAP) at 200 oC was characterised using three-dimensional atom probe. Mg and Cu segregate strongly to the grain boundaries. In contrast, Zn does not always show clear segregation and may even show depletion near the grain boundaries. Trace element Si selectively segregates at some GBs. An increase in the number of ECAP passes leads to a decrease in the grain size but an increase in solute segregation at the boundaries. The significant segregation of alloying elements at the boundaries of ultrafine-grained alloys implies that less solutes will be available in the matrix for precipitation with a decrease in the average grain size. -- Research Highlights: → Atom probe tomography has been employed successfully to reveal unique segregation of solutes at ultrafine grained material. → Mg and Cu elements segregated strongly at the grain boundary of an ultrafine grained Al-Zn-Mg-Cu alloy processed by 4-pass and 8-pass ECAP at 200 oC. Zn frequently depleted at GBs with a Zn depletion region of 7-15 nm in width on one or both sides of the GBs. Only a small fraction (3/13) of GBs were observed with a low level of Zn segregation where the combined Mg and Cu excess is over 3.1 atom/nm2. Si appeared selectively segregated at some of the GBs. → The increase in number of ECAP passes from 4 to 8 correlated with the increase in mean level segregation of Mg and Cu for both solute excess and peak concentration. → The change of plane normal of a grain boundary within 30o only leads to a slight change in the solute segregation level.

  16. Grain-boundary layering transitions and phonon engineering

    Science.gov (United States)

    Rickman, J. M.; Harmer, M. P.; Chan, H. M.

    2016-09-01

    We employ semi-grand canonical Monte Carlo simulation to investigate layering transitions at grain boundaries in a prototypical binary alloy. We demonstrate the existence of such transitions among various interfacial states and examine the role of elastic fields in dictating state equilibria. The results of these studies are summarized in the form of diagrams that highlight interfacial state coexistence in this system. Finally, we examine the impact of layering transitions on the phononic properties of the system, as given by the specific heat and, by extension, the thermal conductivity. Thus, it is suggested that by inducing interfacial layering transitions via changes in temperature or pressure, one can thereby engineer thermodynamic and transport properties in materials.

  17. Helium bubbles at grain boundaries of high-density 238PuO2 shards

    International Nuclear Information System (INIS)

    Hydroxide- and oxalate-base 238PuO2 shards that were sintered to high density at 1200 or 16000C and then aged for 6 months or more exhibited grain-boundary gas bubbles due to agglomeration of alpha-decay helium when heated to 12000C and above. Conditions for bubble formation depended markedly on shard microstructure; large-grained shards with few large residual sintering pores formed gas bubbles at lower temperatures than small-grained shards with many pores. This behavior was especially apparent in oxalate-base 238PuO2, in which small-grained shards resisted bubble formation to above 15000C; small-grained hydroxide-base shards with less internal porosity than oxalate-base shards formed bubbles at lower temperatures. Helium is apparently released from aged 238PuO2 shards at high temperatures by bulk diffusion within grains to the grain boundaries, where bubbles are formed and interconnect into networks which permit helium escape. It is postulated that helium is released by grain boundary diffusion at temperatures below thresholds for grain boundary bubble formation. Small grain size and high residual porosity within grains inhibit the formation of the grain-boundary gas bubbles by reducing the concentration of helium gas at the grain boundaries. (10 fig)

  18. Relationship between lattice mismatch and ionic conduction of grain boundary in YSZ

    Institute of Scientific and Technical Information of China (English)

    Fei Ye; Chunyu Yin; Dingrong Ou; Toshiyuki Mori

    2014-01-01

    The grain boundary plays an important role in the electrical behaviors of solid oxide electrolytes for solid state fuel cells. To reveal the relationship between the structure and the ionic conductivity of grain boundary, the conductive properties of {1 1 1} and {1 1 0} twist grain boundaries in 8 mol%yttria-stabilized zirconia have been examined. These boundaries have a series ofΣvalues defined by the coincident site lattice model. It has been found that the activation energy of{1 1 1}twist grain boundary increases and then decreases with theΣvalue, while that of the{1 1 0}boundary shows an opposite trend. It is suggested that the properties can reflect the balance of the effects of lattice mismatch on the diffusion ability of oxygen vacancies and the segregation of oxygen vacancies and Y3 þ ions. Therefore, the properties in polycrystalline electrolyte can be adjusted by controlling the grain boundary structures.

  19. The favourable large misorientation angle grain boundaries in graphene.

    Science.gov (United States)

    Zhang, Xiuyun; Xu, Ziwei; Yuan, Qinghong; Xin, John; Ding, Feng

    2015-12-21

    A grain boundary (GB) in graphene is a linear defect between two specifically oriented graphene edges, whose title angles are denoted as θ1 and θ2, respectively. Here we present a systematic theoretical study on the structure and stability of GBs in graphene as a function of the misorientation angle, Φ = (θ1-θ2) and the GB orientation in multi-crystalline graphene, which is denoted by Θ = (θ1 + θ2). It is surprising that although the number of disorders of the GB, i.e., the pentagon-heptagon pairs (5|7s), reaches the maximum at Φ∼ 30°, the GB formation energy versus the Φ curve reaches a local minimum. The subsequent M-shape of the Efvs. the Φ curve is due to the strong cancellation of the local strains around 5|7 pairs by the "head-to-tail" formation. This study successfully explains many previously observed experimental puzzles, such as the multimodal distribution of GBs and the abundance of GB misorientation angles of ∼30°. Besides, this study also showed that the formation energy of GBs is less sensitive to Θ, although the twin boundaries are slightly more stable than others. PMID:26568448

  20. Local and bulk melting of Cu at grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Luo, Shengnian [Los Alamos National Laboratory; Han, Li - Bo [USTC; An, Qi [USTC/CALTECH; Fu, Rong - Shan [USTC; Zheng, Lianqing [FSU

    2008-01-01

    We investigate gain boundary (GB) melting using molecular dynamics simulations on face-centered-cubic Cu bicrystals with symmetric {l_angle}110{r_angle} tilt grain boundaries. Two representative types of GBs are explored: {Sigma} = 11/(113)/50.48{sup o} (low GB energy) and {Sigma} = 27/(552)/148.41{sup o} (high GB energy). The temperature and temporal evolutions of the Cu bicrystals under stepped heating are characterized in terms of order parameters and diffusion coefficients, as ell as the nucleation and growth of melt. Within the GB region, continuous local melting precedes discontinuous bulk melting, while continuous solid state disordering may precede local melting. Premelting may occur for local melting but not for bulk melting. For {Sigma} = 11/(113)/50.48{sup o}, premelting of the GB region is negligible, and local melting occurs near the thermodynamic melting temperature. The GB region as a whole is superheated by about 13% before its bulk melting. In the case of {Sigma} = 27/(552)/148.41, considerable premelting is observed for local melting, while the bulk melting occurs with negligible superheating. The exact melting behavior of a general GB depends on the GB energy, but is likely bracketed within these two cases.

  1. The favourable large misorientation angle grain boundaries in graphene.

    Science.gov (United States)

    Zhang, Xiuyun; Xu, Ziwei; Yuan, Qinghong; Xin, John; Ding, Feng

    2015-12-21

    A grain boundary (GB) in graphene is a linear defect between two specifically oriented graphene edges, whose title angles are denoted as θ1 and θ2, respectively. Here we present a systematic theoretical study on the structure and stability of GBs in graphene as a function of the misorientation angle, Φ = (θ1-θ2) and the GB orientation in multi-crystalline graphene, which is denoted by Θ = (θ1 + θ2). It is surprising that although the number of disorders of the GB, i.e., the pentagon-heptagon pairs (5|7s), reaches the maximum at Φ∼ 30°, the GB formation energy versus the Φ curve reaches a local minimum. The subsequent M-shape of the Efvs. the Φ curve is due to the strong cancellation of the local strains around 5|7 pairs by the "head-to-tail" formation. This study successfully explains many previously observed experimental puzzles, such as the multimodal distribution of GBs and the abundance of GB misorientation angles of ∼30°. Besides, this study also showed that the formation energy of GBs is less sensitive to Θ, although the twin boundaries are slightly more stable than others.

  2. The function of the grain boundaries in embrittlement of corrosion-resistant steels

    International Nuclear Information System (INIS)

    The influence of processes proceeding at grain boundaries during heat treatment and grain size effect on ductility are considered for high-strength corrosion resistant steels [Fe-0.1%C-(13-15)%Cr-(5-6)%Ni-(1.5-3.7)%Mo-2%Cu-0.9%Si-0.2%Nb]. It is shown that to ensure high reliability of the steels it is necessary to obtain a fine-grained structure with no impurity segregations or phase precipitates along grain boundaries. The steel alloying with molybdenum in amounts of ∼ 2 mass % is stated to retard the grain boundary phase precipitation and to enhance the steel ductility

  3. THE INSTABILITY OF THE DIFFUSION-CONTROLLED GRAIN-BOUNDARY VOID IN STRESSED SOLID

    Institute of Scientific and Technical Information of China (English)

    王华; 李中华

    2003-01-01

    As atoms migrate along a void surface and grain-boundary, driven by various thermodynamic forces, the grain-boundary void changes its shape and volume. When the void changes its configuration, the free energy of the system also changes. In this article, the free energy is calculated for an evolving grain-boundary void filled with gas in a stressed solid. Then the instability conditions and the equilibrium shape of the void are determined as a function of the grain-boundary and surface energies, the void volume, the externally applied stresses, as well as the internal pressure built up by the gas filled in the void.

  4. Effect of thermal cycling on impurity grain boundary segregation in maraging steel

    International Nuclear Information System (INIS)

    The paper presents results of Auger spectroscopy of grain boundary elemental composition of maraging steel 11Cr10Ni2TiMo after typical heat treatment followed by thermal cycling. Specimens in the austenitic condition were subjected to aging at 550 deg. C and to cyclic heat treatment. Afterwards specimens were doped by hydrogen in an electrolytic cell in order to produce grain boundary brittleness. Fracture was performed by tensile loading in an ultrahigh vacuum chamber of a special Auger spectrometer. A noticeable phosphorus grain boundary segregation was observed after aging at 550 deg. C. A substantial decrease in grain boundary impurity segregation following thermal cycling has been observed

  5. Grain Boundary Engineering for Assessing Durability and Aging Issues with Nickel-Based Superalloys Project

    Data.gov (United States)

    National Aeronautics and Space Administration — The Grain Boundary Engineering (GBE) approach, successfully demonstrated in Phase I, that microstructural optimization provides a very significant improvement in...

  6. Grain boundaries in CdTe thin film solar cells: a review

    Science.gov (United States)

    Major, Jonathan D.

    2016-09-01

    The current state of knowledge on the impact of grain boundaries in CdTe solar cells is reviewed with emphasis being placed on working cell structures. The role of the chemical composition of grain boundaries as well as growth processes are discussed, along with characterisation techniques such as electron beam induced current and cathodoluminescence, which are capable of extracting information on a level of resolution comparable to the size of the grain boundaries. Work which attempts to relate grain boundaries to device efficiency is also assessed and gaps in the current knowledge are highlighted.

  7. Differentiation of Effects due to Grain and Grain Boundary Traps in Laser Annealed Poly-Si Thin Film Transistors

    Science.gov (United States)

    Armstrong, G.; Uppal, S.; Brotherton, S.; Ayres, J.

    1998-04-01

    A new physical model based on two dimensional simulations for high quality laser re-crystallised poly-Si thin film transistors is presented. It has been shown that to adequately explain the improved subthreshold slope and the lack of saturation of the output characteristics in these transistors, it is essential to distribute the density of defect states between traps in the grains alongside traps localised at grain boundaries. A double exponential density of states has been extracted for thin film transistors (TFTs) annealed at different excimer laser energies, using the field effect conductance method. By splitting the density of states between grain traps and grain boundary traps good fits to the output characteristics have been achieved. Lack of saturation is shown to be due to decrease in potential barrier at grain boundaries with increase in drain bias. At high gate voltages, however, evidence of a self-heating effect similar to that observed in silicon-on-insulator (SOI) transistors is apparent.

  8. Crystallographic Characteristics of Grain Boundaries in Dense Yttria-Stabilized Zirconia

    Energy Technology Data Exchange (ETDEWEB)

    Lam Helmick; Shen J. Dillon; Kirk Gerdes; Randall Gemmen; Gregory S. Rohrer; Sridhar Seetharaman; Paul A. Salvador

    2010-04-01

    Grain-boundary plane, misorientation angle, grain size, and grain-boundary energy distributions were quantified using electron backscatter diffraction data for dense polycrystalline yttria-stabilized zirconia, to understand interfacial crystallography in solid oxide fuel cells. Tape-cast samples were sintered at 14501C for 4 h and annealed for at least 100 h between 8001C and 16501C. Distributions obtained from both three-dimensional (3D) reconstructions and stereological analyses of 2D sections demonstrated that the (100) boundary planes {(111)} have relative areas larger {smaller} than expected in a random distribution, and that the boundary plane distribution is inversely correlated to the boundary energy distribution.

  9. Ostwald Ripening of Diffusion-Limited Small-Size Precipitates at Grain Boundaries

    Directory of Open Access Journals (Sweden)

    A.V. Koropov

    2012-10-01

    Full Text Available The paper describes a theoretical study of the Ostwald ripening of two-dimensional small-size precipitates of a newly formed phase at the grain boundary of finite thickness, taking into account the diffusion of impurity atoms from the grain interior to the grain boundary. The precipitate growth is believed to be limited by the impurity-atom diffusion in the grain boundary. The asymptotic time dependences are found for the average and critical precipitate radius, supersaturation of solid solution of impurity atoms in the grain boundary, precipitate size distribution function, precipitate density, and for the factor of grain boundary filling with precipitates. A discussion of the limits of validity of obtained results is given.

  10. Phase composition and structure of grain boundary of oversintered Y3Al5O12 ceramics

    Institute of Scientific and Technical Information of China (English)

    LI Chang-qing; ZUO Hong-bo; HAN Jie-cai; ZHANG Ming-fu; MENG Song-he; YAO Tai

    2006-01-01

    Phase composition and microstructures of grain boundary of oversintered yttrium aluminum garnet (Y3Al5O12, YAG) ceramics by vacuum sintering at 1 850 ℃ were investigated. For synthesizing YAG, grain boundary is a key factor for YAG ceramics. The morphology of grain boundary was observed by SEM, TEM and its composition was analyzed by EDS. It is identified that the grain boundary is composed of α-Al2O3 and yttrium aluminum perovskite (YAP, YAlO3) eutectics. At the edge of YAG crystal grain, YAG phase is decomposed into perovskite YAP and α-Al2O3 during high temperature sintering. Due to refractive indexes of YAP and α-Al2O3 phases in wide grain boundary are different from those of YAG, the transmittance of oversintered YAG ceramics is lower than that of YAG ceramics sintered at 1 750 ℃.

  11. The role of grain boundary sliding on creep deformation characteristics of discontinuous reinforced composites

    Energy Technology Data Exchange (ETDEWEB)

    Biner, S.B.

    1994-10-01

    In this study, the grain boundary sliding behavior in discontinuous reinforced composites is investigated numerically. Results indicate that the stress enhancement factor for the composite is much larger than the one observed for the matrix material. In the composite, the increase in the strain rates as a result of grain boundary sliding occurs in a wider stress range in comparison to the matrix. It is shown that the experimentally observed large scale triple point grain boundary cavitation in the composites could occur as a result of large grain rotations resulting from grain boundary sliding and evolution of triaxial stress state. Also, the observed larger creep exponent values or stress dependent creep exponent values for the composites may not be explained solely by the mechanism of grain boundary sliding.

  12. Strain induced grain boundary migration effects on grain growth of an austenitic stainless steel during static and metadynamic recrystallization

    International Nuclear Information System (INIS)

    Static and metadynamic recrystallization of an AISI 304L austenitic stainless steel was investigated at 1100 °C and 10− 2 s− 1 strain rate. The kinetics of recrystallization was determined through double hit compression tests. Two strain levels were selected for the first compression hit: εf = 0.15 for static recrystallization (SRX) and 0.25 for metadynamic recrystallization (MDRX). Both the as-deformed and the recrystallized microstructures were investigated through optical microscopy and electron back-scattered diffraction (EBSD) technique. During deformation, strain induced grain boundary migration appeared to be significant, producing a square-like grain boundary structure aligned along the directions of the maximum shear stresses in compression. EBSD analysis revealed to be as a fundamental technique that the dislocation density was distributed heterogeneously in the deformed grains. Grain growth driven by surface energy reduction was also investigated, finding that it was too slow to explain the experimental data. Based on microstructural results, it was concluded that saturation of the nucleation sites occurred in the first stages of recrystallization, while grain growth driven by strain induced grain boundary migration (SIGBM) dominated the subsequent stages. - Highlights: • Recrystallization behavior of a stainless steel was investigated at 1100 °C. • EBSD revealed that the dislocation density distribution was heterogeneous during deformation. • Saturation of nucleation sites occurred in the first stages of recrystallization. • Strain induced grain boundary migration (SIGBM) effects were significant. • Grain growth driven by SIGBM dominated the subsequent stages

  13. Anisotropic radiation-induced segregation in 316L austenitic stainless steel with grain boundary character

    International Nuclear Information System (INIS)

    Radiation-induced segregation (RIS) and subsequent depletion of chromium along grain boundaries has been shown to be an important factor in irradiation-assisted stress corrosion cracking in austenitic face-centered cubic (fcc)-based alloys used for nuclear energy systems. A full understanding of RIS requires examination of the effect of the grain boundary character on the segregation process. Understanding how specific grain boundary structures respond under irradiation would assist in developing or designing alloys that are more efficient at removing point defects, or reducing the overall rate of deleterious Cr segregation. This study shows that solute segregation is dependent not only on grain boundary misorientation, but also on the grain boundary plane, as highlighted by markedly different segregation behavior for the Σ3 incoherent and coherent grain boundaries. The link between RIS and atomistic modeling is also explored through molecular dynamic simulations of the interaction of vacancies at different grain boundary structures through defect energetics in a simple model system. A key insight from the coupled experimental RIS measurements and corresponding defect–grain boundary modeling is that grain boundary–vacancy formation energy may have a critical threshold value related to the major alloying elements’ solute segregation

  14. Electrical characterization of grain boundaries of CZTS thin films using conductive atomic force microscopy techniques

    Energy Technology Data Exchange (ETDEWEB)

    Muhunthan, N.; Singh, Om Pal [Compound Semiconductor Solar Cell, Physics of Energy Harvesting Division, New Delhi 110012 (India); Toutam, Vijaykumar, E-mail: toutamvk@nplindia.org [Quantum Phenomena and Applications Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012 (India); Singh, V.N., E-mail: singhvn@nplindia.org [Compound Semiconductor Solar Cell, Physics of Energy Harvesting Division, New Delhi 110012 (India)

    2015-10-15

    Graphical abstract: Experimental setup for conducting AFM (C-AFM). - Highlights: • Cu{sub 2}ZnSnS{sub 4} (CZTS) thin film was grown by reactive co-sputtering. • The electronic properties were probed using conducting atomic force microscope, scanning Kelvin probe microscopy and scanning capacitance microscopy. • C-AFM current flow mainly through grain boundaries rather than grain interiors. • SKPM indicated higher potential along the GBs compared to grain interiors. • The SCM explains that charge separation takes place at the interface of grain and grain boundary. - Abstract: Electrical characterization of grain boundaries (GB) of Cu-deficient CZTS (Copper Zinc Tin Sulfide) thin films was done using atomic force microscopic (AFM) techniques like Conductive atomic force microscopy (CAFM), Kelvin probe force microscopy (KPFM) and scanning capacitance microscopy (SCM). Absorbance spectroscopy was done for optical band gap calculations and Raman, XRD and EDS for structural and compositional characterization. Hall measurements were done for estimation of carrier mobility. CAFM and KPFM measurements showed that the currents flow mainly through grain boundaries (GB) rather than grain interiors. SCM results showed that charge separation mainly occurs at the interface of grain and grain boundaries and not all along the grain boundaries.

  15. Grain boundary resistance to amorphization of nanocrystalline silicon carbide

    Science.gov (United States)

    Chen, Dong; Gao, Fei; Liu, Bo

    2015-11-01

    Under the C displacement condition, we have used molecular dynamics simulation to examine the effects of grain boundaries (GBs) on the amorphization of nanocrystalline silicon carbide (nc-SiC) by point defect accumulation. The results show that the interstitials are preferentially absorbed and accumulated at GBs that provide the sinks for defect annihilation at low doses, but also driving force to initiate amorphization in the nc-SiC at higher doses. The majority of surviving defects are C interstitials, as either C-Si or C-C dumbbells. The concentration of defect clusters increases with increasing dose, and their distributions are mainly observed along the GBs. Especially these small clusters can subsequently coalesce and form amorphous domains at the GBs during the accumulation of carbon defects. A comparison between displacement amorphized nc-SiC and melt-quenched single crystal SiC shows the similar topological features. At a dose of 0.55 displacements per atom (dpa), the pair correlation function lacks long range order, demonstrating that the nc-SiC is fully amorphilized.

  16. Molecular dynamics simulations of point defects in plutonium grain boundaries

    Institute of Scientific and Technical Information of China (English)

    Ao Bing-Yun; Xia Ji-Xing; Chen Pi-Heng; Hu Wang-Yu; Wang Xiao-Lin

    2012-01-01

    A modified analytic embedded atom method (MAEAM) potential is constructed for fcc δ-Pu.Molecular dynamics (MD) simulations with the potential are performed to investigate the interactions between two symmetrical tilt grain boundaries (GBs) and point defects such as He atom,vacancy and self-interstitial atom (SIA) in Pu.The calculated results show that point defect formation energies are on average lower than those in the lattice but variations from site to site along the GBs are very remarkable.Both substitutional and interstitial He atoms are trapped at GBs.Interstitial He atom is more strongly bound at the GB core than the substitutional He atom.The binding energy of SIA at GB core is higher than those of He atom and vacancy.GB core can bind many He atoms and SIAs due mainly to the fact that it contains many vacancies.Compared with He atom and SIA,the vacancy far from GB core is difficult to diffuse into the core.The GBs can act as sinks and sources of He atoms and SIAs,which may be a reason for the swelling of Pu after a period of self-irradiation because of the higher concentration of vacancy in the bulk.

  17. Quantum Hall effect in polycrystalline CVD graphene: grain boundaries impact

    Science.gov (United States)

    Ribeiro-Palau, Rebeca; Lafont, Fabien; Schopfer, Felicien; Poirier, Wilfrid; Bouchiat, Vincent; Han, Zhen; Cresti, Alessandro; Cummings, Aron; Roche, Stephan

    2014-03-01

    It was demonstrated by Janssen et al. (New J. Phys. 2011) that graphene could surpass GaAs for quantum Hall resistance standards with an accuracy better than 10-10. Graphene should render possible the realization of a standard operating at T > 4 K and B graphene with outstanding electronic transport properties is required. We present measurements performed in large area Hall bars made of polycrystalline CVD graphene on Si/SiO2, with a carrier mobility of 0.6 T-1. Even at 20.2 T and 300 mK, the Hall resistance plateaus are insufficiently quantized at ν = +/- 2 and +/- 6 . This is due to a high dissipation manifested by a longitudinal resistance which does not drop to zero. We pointed out unusual power-law temperature dependencies of Rxx and an exponential magnetic field dependence. We do not observe the common thermally activated or VRH behaviors. This can be attributed to the grain boundaries in the sample that short-circuit the edge states, as supported by our numerical simulations. This reveals new and peculiar aspects of the quantum Hall effect in polycrystalline systems. Another unexpected feature is the observation of the ν = 0 and 1 states in such low mobility systems.

  18. Gettering effect in grain boundaries of multi-crystalline silicon

    Energy Technology Data Exchange (ETDEWEB)

    Nouri, H.; Bouaicha, M.; Ben Rabha, M.; Bessais, B. [Laboratoire de Photovoltaique, Centre de Recherches et des Technologies de l' Energie, Technopole de Borj-Cedria, BP 95, Hammam-Lif 2050 (Tunisia)

    2012-10-15

    In this work, we analyze the effect of three gettering procedures on the variation of the grain boundaries (GBs) defect density in multicrystalline silicon (mc-Si). The effective defect density (N{sup B}) was calculated using a theoretical model where we consider the potential barrier induced by the GB as being due to structural defects and impurities. Results are compared to those obtained from C-V measurements. The potential barrier was evaluated from the dark current-voltage (I-V) characteristic performed across the GB. In addition to the Rapid Thermal Annealing (RTA), we use aluminum (Al) in the first gettering procedure, in the second we use porous silicon (PS), whereas in the third one, we realize a chemical damage (grooving). Mc-Si wafers were annealed in an infrared furnace in the same conditions, at temperatures ranging from 600 C to 1000 C (copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  19. Simulations of thermal conductance across tilt grain boundaries in graphene

    Institute of Scientific and Technical Information of China (English)

    Peng Wang; Bo Gong; Qiong Feng; Hong-Tao Wang

    2012-01-01

    Non-equilibrium molecular dynamics (MD) method was performed to simulate the thermal transportation process in graphene nanoribbons (GNRs).A convenient way was conceived to introduce tilt grain boundaries (GBs) into the graphene lattice by repetitive removing C atom rows along certain directions.Comprehensive MD simulations reveal that larger-angle GBs are effective thermal barriers and substantially reduce the average thermal conductivity of GNRs.The GB thermal conductivity is ~ 10 W.m-1·K-1 for a bicrystal GNR with a misorientation of 21.8°,which is ~97% less than that of a prefect GNR with the same size.The total thermal resistance has a monotonic dependence on the density of the 5-7 defects along the GBs.A theoretical model is proposed to capture this relation and resolve the contributions by both the reduction in the phonon mean free path and the defect-induced thermal resistance.

  20. Disjoining potential and grain boundary premelting in binary alloys

    Science.gov (United States)

    Hickman, J.; Mishin, Y.

    2016-06-01

    Many grain boundaries (GBs) in crystalline materials develop highly disordered, liquidlike structures at high temperatures. In alloys, this premelting effect can be fueled by solute segregation and can occur at lower temperatures than in single-component systems. A premelted GB can be modeled by a thin liquid layer located between two solid-liquid interfaces interacting by a disjoining potential. We propose a single analytical form of the disjoining potential describing repulsive, attractive, and intermediate interactions. The potential predicts a variety of premelting scenarios, including thin-to-thick phase transitions. The potential is verified by atomistic computer simulations of premelting in three different GBs in Cu-Ag alloys employing a Monte Carlo technique with an embedded atom potential. The disjoining potential has been extracted from the simulations by analyzing GB width fluctuations. The simulations confirm all shapes of the disjoining potential predicted by the analytical model. One of the GBs was found to switch back and forth between two (thin and thick) states, confirming the existence of thin-to-thick phase transformations in this system. The proposed disjoining potential also predicts the possibility of a cascade of thin-to-thick transitions caused by compositional oscillations (patterning) near solid-liquid interfaces.

  1. Influence of Grain Boundary on Fatigue Behavior of Ni-base Bicrystals

    Institute of Scientific and Technical Information of China (English)

    2002-01-01

    The influence of the grain boundary on the fatigue behavior was studied by two three-point-bending (TPB) specimens.One TPB specimen was named Bicrystal 1, whose pre-crack was along the grain boundary and the applied loadparalleled to pre-crack direction, while the other TPB specimen was named Bicrystal 2, whose the pre-crack wasperpendicular to the grain boundary and the applied load paralleled also to the pre-crack. It was found that the rateof the fatigue crack growth of Bicrystal 1 was about a tenfold higher than that of Bicrystal 2. The fatigue behaviorof Bicrystal 2 specimens was dependent on the distance between the crack tip and grain boundary. The crack growthrate was highest when the crack tip was at a critical distance to the grain boundary, while the rate was the lowestwhen the crack tip reached grain boundary. After the crack was over the grain boundary, the crack growth rateincreased. The crystallographic finite element method was applied to analyze the stress and strain structure aheadof the crack, in order to reveal the above characteristics of the fatigue behavior. It is the grain boundary-inducedredistribution of stresses near the crack tip that induces the difference of fatigue behavior.

  2. Atomic scale study of grain boundary segregation before carbide nucleation in Ni–Cr–Fe Alloys

    International Nuclear Information System (INIS)

    Highlights: • Impurities segregated at grain boundaries were observed by atom probe tomography. • The comparison of segregation features in two Ni–Cr–Fe alloys was studied by APT. • C and Cr atoms co-segregated at grain boundaries before carbide precipitation. -- Abstract: Three dimensional chemical information concerning grain boundary segregation before carbide nucleation was characterized by atom probe tomography in two Ni–Cr–Fe alloys which were aged at 500 °C for 0.5 h after homogenizing treatment. B, C and Si atoms segregation at grain boundary in Alloy 690 was observed. B, C, N and P atoms segregation at grain boundary in 304 austenitic stainless steel was observed. C atoms co-segregation with Cr atoms at the grain boundaries both in Alloy 690 and 304 austenitic stainless steel was found, and its effect on the carbide nucleation was discussed. The amount of each segregated element at grain boundaries in the two Ni–Cr–Fe alloys were analyzed quantitatively. Comparison of the grain boundary segregation features of the two Ni–Cr–Fe alloys were carried out based on the experimental results

  3. RECENT THEORETICAL AND EXPERIMENTAL ADVANCES IN THE UNDERSTANDING OF GRAIN BOUNDARY MIGRATION

    OpenAIRE

    Bauer, Ch.

    1982-01-01

    The subject of grain boundary migration continues to command significant attention because of its relevance to many important technological processes. The purpose of this article is to review recent theoretical and experimental advances in the understanding of grain boundary migration in order to define the current state of present knowledge and identify important problems which require further investigation.

  4. Effect of grain boundary on the mechanical behaviors of irradiated metals: a review

    Science.gov (United States)

    Xiao, XiaZi; Chu, HaiJian; Duan, HuiLing

    2016-06-01

    The design of high irradiation-resistant materials is very important for the development of next-generation nuclear reactors. Grain boundaries acting as effective defect sinks are thought to be able to moderate the deterioration of mechanical behaviors of irradiated materials, and have drawn increasing attention in recent years. The study of the effect of grain boundaries on the mechanical behaviors of irradiated materials is a multi-scale problem. At the atomic level, grain boundaries can effectively affect the production and formation of irradiation-induced point defects in grain interiors, which leads to the change of density, size distribution and evolution of defect clusters at grain level. The change of microstructure would influence the macroscopic mechanical properties of the irradiated polycrystal. Here we give a brief review about the effect of grain boundaries on the mechanical behaviors of irradiated metals from three scales: microscopic scale, mesoscopic scale and macroscopic scale.

  5. Atomic structure of the Σ = 2 twist carbide grain boundary in WC-Co alloys

    Science.gov (United States)

    Lay, Sabine; Missiaen, Jean-Michel

    2013-04-01

    The Σ = 2 twist carbide grain boundary with a (10 ? 0) habit plane was investigated by high resolution transmission electron microscopy in a WC-Co alloy. The atomic structure at the boundary was determined by comparing experimental images and simulations. It corresponds to the boundary model with the lowest energy according to atomistic calculations. Periodic monolayer interfacial steps, compensating for the parametric misfit at the boundary were studied. The displacement field around the defects was simulated and corresponds to a dislocation with a mixed character and a Burgers vectors equal to 1/6⟨ ? 2 ? 3⟩ lying in the boundary plane. Another step with a larger height and connected to a stacking fault was analyzed. It likely arises from the interaction of a matrix dislocation with the boundary. The observations suggest that the migration of Σ = 2 grain boundaries can be induced by the glide of the monolayer steps along the grain boundary.

  6. The effects of grain boundary precipitates on cryogenic properties of aged 316-type stainless steels

    International Nuclear Information System (INIS)

    It is documented that sensitization in stainless steels results from the formation of grain boundary carbides that deplete the Cr in the vicinities of the grain boundaries. Sensitized austenitic stainless steels become brittle at cryogenic temperatures. Low carbon stainless steels are considered to be resistant to aging embrittlement. Our study of low carbon stainless steels demonstrates that aging at sensitization temperatures results in the formation of grain boundary intermetallic compounds or nitrides instead of carbides. The aging marginally change the 4 K yield strength, but decreases the 4 K stress intensity factor. The change of the yield strength is related to the pinning of the dislocations by solute atoms. The reduction of the stress intensity factor is attributed to the formation of the grain boundary precipitates. The sizes and amount of the grain boundary precipitate are so small that the 4 K crack growth rate at small ΔK is not affected.

  7. Effect of Quenching on the Grain Boundary Relaxation in PM2000 ODS Alloy

    Institute of Scientific and Technical Information of China (English)

    GAO Zhan-Yong; WU Jie; HAN Fu-Sheng

    2005-01-01

    @@ Grain boundary relaxation in a Fe-based ODS alloy is studied by internal friction measurements. It is found that a grain-boundary peak appears at a lower temperature in the quenched specimens than that in the annealed specimens. The activation energy of the peak is H = 2.82 ± 0.11 eV for the former while H = 2.53 ± 0.08 eV for the latter. In addition, a new relaxation peak is observed at the high temperature side of the grain boundary peak in the quenched specimens with an activation energy of 4.41 ± 0.25 eV. The height of the peak increases with increasing quenching temperature. The results suggest that both the shift of the grain-boundary peak and the appearance of the new peak are due to increasing vacancies by quenching that are favourable for the motion of the grain boundaries.

  8. Deformation characteristics of various grain boundary angles on AFM-based nanolithography using molecular dynamics

    International Nuclear Information System (INIS)

    Molecular dynamics simulations are performed to verify the deformation characteristics of grain boundaries on the AFM-based nanolithography. The model used has about 750,000 (Cu) atoms and is composed of two different crystal orientations. The grain boundaries are located in the center of model and have 45, 90, 135, and -135 degree angles in the xz-plane. The tool is made of rigid diamond-like carbon and is in the shape of the Berkovich indenter. The simulation has four different stages: relaxation, indentation, re-relaxation, and lithography. The simulation results reveal that the lithography deforms the grain boundary shape by the tool. The deformation of grain boundary's angle proceeds to minimize the total potential energy of whole system. Consequently, the grain boundary angle is changed about 90 degrees

  9. The Effect of Atomic Vacancies and Grain Boundaries on Mechanical Properties of GaN Nanowires

    Institute of Scientific and Technical Information of China (English)

    XIE Shi-Feng; CHEN Shang-Da; SOS Ai-Kah

    2011-01-01

    @@ Molecular dynamics simulations are carried out to investigate the influences of various defects on mechanical properties of wurtzite GaN nanowires by adopting the empirical Stillinger-Weber potential.Different types of vacancies and grain boundaries are considered and the uniaxial loading condition is implemented along the [001] direction.It is found that surface defects have less impact on Voung's moduli and critical stresses of GaN nanowires compared with random defects.The grain boundary normal to the axial direction of a nanowire would not significantly affect Young's moduli of nanowires.However, the inversion domain grain boundaries with and without wrong bonds would significantly lower Young's moduli of GaN nanowires.In addition, the inversion domain grain boundary affects the critical stress of GaN nanowires more than the grain boundary with interface normal to the axial direction of the nanowire.%Molecular dynamics simulations are carried out to investigate the influences of various defects on mechanical properties of wurtzite GaN nanowires by adopting the empirical Stillinger-Weber potential. Different types of vacancies and grain boundaries are considered and the uniaxial loading condition is implemented along the [001] direction. It is found that surface defects have less impact on Young's moduli and critical stresses of GaN nanowires compared with random defects. The grain boundary normal to the axial direction of a nanowire would not significantly affect Young's moduli of nanowires. However, the inversion domain grain boundaries with and without wrong bonds would significantly lower Young's moduli of GaN nanowires. In addition, the inversion domain grain boundary affects the critical stress of GaN nanowires more than the grain boundary with interface normal to the axial direction of the nanowire.

  10. Coincidence grain boundary and role of primary recrystallized grain growth on secondary recrystallization texture evolution in Fe-3%Si alloy

    Energy Technology Data Exchange (ETDEWEB)

    Yoshitomi, Y.; Takahashi, N. (Nippon Steel Corp., Kitakyushu (Japan). Yawata R D Lab.); Ushigami, Y.; Harase, J.; Nakayama, T.; Masui, H. (Nippon Steel Corp., Chiba (Japan). Steel Research Lab.)

    1994-08-01

    Secondary recrystallization behavior in the presence of AlN in Fe-3%Si alloy was investigated with special reference to the influence of primary recrystallized grain growth on secondary recrystallization texture. The more dominant grain growth was marked by the evolution of [110]<001> secondary recrystallized grains in the higher temperature range. In the case of smaller primary recrystallized grains, the [110]<227> secondary recrystallized grains were mainly evolved on annealing at the lower temperature range. The frequency of [Sigma]9 coincidence boundaries in relation to the [110]<001> texture component was higher than that of [Sigma]5 coincidence boundaries in relation to [110]<227> component. The mechanism of these evolutions of secondary recrystallization texture can be explained by the assumption that the [Sigma]5 coincidence boundaries are more mobile than the [Sigma]9 coincidence boundaries in the lower temperature range. The primary recrystallized grain growth is considered to have a role in determining what should be the secondary recrystallization temperature.

  11. Computer simulation of grain boundary self-diffusion in aluminum

    Energy Technology Data Exchange (ETDEWEB)

    Dragunov, Andrei S., E-mail: andrei.dragunov@aun.edu.ng [American University of Nigeria, Nigeria, Adamawa State, Yola Yola By-Pass 98 Lamido Zubairu Way (Nigeria); Weckman, A. V.; Demyanov, B. F. [Altai State Technical University, Russia, Altai Region, Barnaul (Russian Federation)

    2014-10-06

    In the work study the process of self-diffusion in symmetric tilt grain boundaries (GB) with the axes misorientation [100], [110] and [111]. The research was carried out by the methods of computer simulation The objects of the research are the three GB of common and special type for each axis misorientation. The angles of misorientation of the common GB is amounted to 10°, 30° and 50°. The simulation was performed by the method of molecular dynamics in the temperature range from 600 to 1000 K, with an interval of 50 K. For research on the direction jumps atoms were built tracks the movement of atoms in the process of self-diffusion. The calculations have shown, that for all of GB is characterized by pronounced anisotropy of the jumps at low temperatures (< 700K). At temperatures near to the melting point directions of the jumps are isotropic only for three GB (Θ=30°[100], Θ=50=[100] and Σ5(013)[100]). For other GB such as [100] and [110] remains priority direction of diffusion along the nuclei GB dislocations. Arrenius curves have from one to three linear plots with different tilt. Change the tilt of Arrenius dependences testifies to the change in the mechanism of self-diffusion. The parameters of grainboundary self-diffusion were determined The activation energy of grainboundary diffusion in 4–5 times lower than the energy of activation of a volume self-diffusion of aluminum (about 200 KJ/mol). The minimum value of activation energy has GB 10° with the axis misorientation [100] (10,15 KJ/mol), maximum (104.12 Kj/mol) - a special GB Σ11(113)

  12. Grain boundaries in graphene grown by chemical vapor deposition

    International Nuclear Information System (INIS)

    The scientific literature on grain boundaries (GBs) in graphene was reviewed. The review focuses mainly on the experimental findings on graphene grown by chemical vapor deposition (CVD) under a very wide range of experimental conditions (temperature, pressure hydrogen/hydrocarbon ratio, gas flow velocity and substrates). Differences were found in the GBs depending on the origin of graphene: in micro-mechanically cleaved graphene (produced using graphite originating from high-temperature, high-pressure synthesis), rows of non-hexagonal rings separating two perfect graphene crystallites are found more frequently, while in graphene produced by CVD—despite the very wide range of growth conditions used in different laboratories—GBs with more pronounced disorder are more frequent. In connection with the observed disorder, the stability of two-dimensional amorphous carbon is discussed and the growth conditions that may impact on the structure of the GBs are reviewed. The most frequently used methods for the atomic scale characterization of the GB structures, their possibilities and limitations and the alterations of the GBs in CVD graphene during the investigation (e.g. under e-beam irradiation) are discussed. The effects of GB disorder on electric and thermal transport are reviewed and the relatively scarce data available on the chemical properties of the GBs are summarized. GBs are complex enough nanoobjects so that it may be unlikely that two experimentally produced GBs of several microns in length could be completely identical in all of their atomic scale details. Despite this, certain generalized conclusions may be formulated, which may be helpful for experimentalists in interpreting the results and in planning new experiments, leading to a more systematic picture of GBs in CVD graphene. (paper)

  13. Atom probe study of grain boundary segregation in technically pure molybdenum

    International Nuclear Information System (INIS)

    Molybdenum, a metal with excellent physical, chemical and high-temperature properties, is an interesting material for applications in lighting-technology, high performance electronics, high temperature furnace construction and coating technology. However, its applicability as a structural material is limited because of the poor oxidation resistance at high temperatures and a brittle-to-ductile transition around room temperature, which is influenced by the grain size and the content of interstitial impurities at the grain boundaries. Due to the progress of the powder metallurgical production during the last decades, the amount of impurities in the current quality of molybdenum has become so small that surface sensitive techniques are not applicable anymore. Therefore, the atom probe, which allows the detection of small amounts of impurities as well as their location, seems to be a more suitable technique. However, a site-specific specimen preparation procedure for grain boundaries in refractory metals with a dual focused ion beam/scanning electron microscope is still required. The present investigation describes the development and successful application of such a site-specific preparation technique for grain boundaries in molybdenum, which is significantly improved by a combination with transmission electron microscopy. This complimentary technique helps to improve the visibility of grain boundaries during the last preparation steps and to evidence the presence of grain and subgrain boundaries without segregants in atom probe specimens. Furthermore, in industrially processed and recrystallized molybdenum sheets grain boundary segregation of oxygen, nitrogen and potassium is successfully detected close to segregated regions which are believed to be former sinter pores. - Highlights: • First study of grain boundary segregation in molybdenum by atom probe • Site-specific preparation technique by FIB and TEM successfully developed • Grain boundary segregation of

  14. Grain boundary sliding quantification by atomic force microscopy: contribution to high temperature embrittlement analyses

    International Nuclear Information System (INIS)

    We developed an original experimental method for measuring grain boundary sliding by atomic force microscopy. The analysed alloys were previously tested at high temperatures and low strain rates, for short time tests. We measured grain boundary sliding along his perpendicular component to the sample surface. The detection limit is of 10 nm. Thin and flat austenitic stainless steels and superalloys samples were tested at high temperature (360 C to 750 C), under ultra high vacuum atmosphere, with slow rate tensile tests and constant load tests. After short times tests, AFM characterization showed that grain boundary sliding could be activated at the loading, within amplitudes of several tens of nm. Furthermore, after short time tests, grain boundary sliding amplitudes are independent of the geometrical orientation of the boundary trace compared with loading direction. On the other hand crystallographic misorientation has a strong influence on boundaries propensity to slide. Intergranular segregation was also analyzed by AES for two alloys (304H austenitic stainless steel and X- 750 nickel-base alloy), previously submitted to high temperature slow strain rate tensile tests. AES analysis showed preferential segregation of S and P near 304H triple junctions. Whereas, AES analysis on X-750 alloy showed P segregation at the bottom of microvoids distributed on a micro-ductile grain facet and a strong segregation of S at the location of grain boundary precipitate. Our experimental methods allows the study of the correlation between two main phenomena contributing to high temperature embrittlement: grain boundary sliding and intergranular segregation embrittlement. Determining grain boundary sliding kinetics should confirm our method of step measure by AFM as a way to indicate high temperature embrittlement sensitivity of alloys in service conditions and also give information about the contribution of grain boundary sliding to stress corrosion cracking initiation and

  15. Grain boundary sliding and structure. Progress report, December 1, 1974--November 30, 1975

    International Nuclear Information System (INIS)

    Load relaxation experiments were performed in the region where the grain matrix plastic deformation is important on type 316 stainless steel and a zircaloy specimen, and the data are being analyzed. Anelastic deformation in pure aluminum was investigated. A dislocation glide-controlled anelastic component is identified which may be governed by the same laws as those for plastic deformation at low temperature and/or high plastic strain rate. The investigation of grain boundary sliding in type 316 stainless steel is being continued. Experimental data without the influence of thermal aging are successfully obtained. Load relaxation data on single crystal and polycrystal nickel specimens show that the constant hardness curves of these specimens belong to two different families. The growth of the grain boundary cavity under applied stress and irradiation has been analyzed. It is found that the normal stress at the grain boundary which is the driving force for atomic transport controlled cavity growth can be relaxed by grain boundary sliding

  16. High-strength chemical-vapor-deposited graphene and grain boundaries.

    Science.gov (United States)

    Lee, Gwan-Hyoung; Cooper, Ryan C; An, Sung Joo; Lee, Sunwoo; van der Zande, Arend; Petrone, Nicholas; Hammerberg, Alexandra G; Lee, Changgu; Crawford, Bryan; Oliver, Warren; Kysar, Jeffrey W; Hone, James

    2013-05-31

    Pristine graphene is the strongest material ever measured. However, large-area graphene films produced by means of chemical vapor deposition (CVD) are polycrystalline and thus contain grain boundaries that can potentially weaken the material. We combined structural characterization by means of transmission electron microscopy with nanoindentation in order to study the mechanical properties of CVD-graphene films with different grain sizes. We show that the elastic stiffness of CVD-graphene is identical to that of pristine graphene if postprocessing steps avoid damage or rippling. Its strength is only slightly reduced despite the existence of grain boundaries. Indentation tests directly on grain boundaries confirm that they are almost as strong as pristine. Graphene films consisting entirely of well-stitched grain boundaries can retain ultrahigh strength, which is critical for a large variety of applications, such as flexible electronics and strengthening components. PMID:23723231

  17. Linking stress-driven microstructural evolution in nanocrystalline aluminium with grain boundary doping of oxygen.

    Science.gov (United States)

    He, Mo-Rigen; Samudrala, Saritha K; Kim, Gyuseok; Felfer, Peter J; Breen, Andrew J; Cairney, Julie M; Gianola, Daniel S

    2016-01-01

    The large fraction of material residing at grain boundaries in nanocrystalline metals and alloys is responsible for their ultrahigh strength, but also undesirable microstructural instability under thermal and mechanical loads. However, the underlying mechanism of stress-driven microstructural evolution is still poorly understood and precludes rational alloy design. Here we combine quantitative in situ electron microscopy with three-dimensional atom-probe tomography to directly link the mechanics and kinetics of grain boundary migration in nanocrystalline Al films with the excess of O atoms at the boundaries. Site-specific nanoindentation leads to grain growth that is retarded by impurities, and enables quantification of the critical stress for the onset of grain boundary migration. Our results show that a critical excess of impurities is required to stabilize interfaces in nanocrystalline materials against mechanical driving forces, providing new insights to guide control of deformation mechanisms and tailoring of mechanical properties apart from grain size alone. PMID:27071458

  18. Order controlled dislocations and grain boundary mobility in Fe-Al-Cr alloys

    Energy Technology Data Exchange (ETDEWEB)

    Lambri, O.A., E-mail: olambri@fceia.unr.edu.ar [Instituto de Fisica Rosario, CONICET (Argentina); Laboratorio de Materiales, Facultad de Ciencias Exactas, Ingenieria y Agrimensura, Universidad Nacional de Rosario, Avda. Pellegrini 250, 2000 Rosario (Argentina); Perez-Landazabal, J.I.; Recarte, V. [Departamento de Fisica, Universidad Publica de Navarra, Campus de Arrosadia s/n, 31006 Pamplona (Spain); Cuello, G.J. [Institute Laue-Langevin, 6 Rue Jules Horowitz, B.P. 156, 38042 Grenoble Cedex 9 (France); Golovin, I.S. [Department of Physical Metallurgy of Non-Ferrous Metals of National Research Technological University ' MISiS' , Leninsky Ave. 4, 119049 Moscow (Russian Federation)

    2012-10-05

    Highlights: Black-Right-Pointing-Pointer The solute grain boundary relaxation in Fe-Al-Cr alloys was discovered. Black-Right-Pointing-Pointer The solute grain boundary peak appears after the order decreases by annealing. Black-Right-Pointing-Pointer In ordered alloys the mobility of dislocations and grain boundaries is reduced. Black-Right-Pointing-Pointer The decrease in order degree leads to the increase in damping. - Abstract: The dislocations and grain boundary mobility in elastic range of loading has been analysed by means of mechanical spectroscopy and neutron diffraction studies in Fe-25at.%Al-8at.%Cr and Fe-25at.%Al-25at.%Cr alloys, in relation to the order degree of the structure. If the alloys are in the ordered state (D0{sub 3} or B2), the mobility of dislocations and grain boundaries is markedly reduced promoting small values of damping. In contrast, when the ordering is suppressed by annealing at temperatures above 973 K, the mobility of dislocations and grain boundaries increases. This leads to an increase in overall damping. In particular, thermally activated damping peaks related to the solute grain boundary peaks due to presence of the aluminium and chromium atoms appear within the temperature interval from 900 to 1100 K, depending on the substitutional atomic content.

  19. Surface transition on ice induced by the formation of a grain boundary.

    Directory of Open Access Journals (Sweden)

    Christian Pedersen

    Full Text Available Interfaces between individual ice crystals, usually referred to as grain boundaries, play an important part in many processes in nature. Grain boundary properties are, for example, governing the sintering processes in snow and ice which transform a snowpack into a glacier. In the case of snow sintering, it has been assumed that there are no variations in surface roughness and surface melting, when considering the ice-air interface of an individual crystal. In contrast to that assumption, the present work suggests that there is an increased probability of molecular surface disorder in the vicinity of a grain boundary. The conclusion is based on the first detailed visualization of the formation of an ice grain boundary. The visualization is enabled by studying ice crystals growing into contact, at temperatures between -20°C and -15°C and pressures of 1-2 Torr, using Environmental Scanning Electron Microscopy. It is observed that the formation of a grain boundary induces a surface transition on the facets in contact. The transition does not propagate across facet edges. The surface transition is interpreted as the spreading of crystal dislocations away from the grain boundary. The observation constitutes a qualitatively new finding, and can potentially increase the understanding of specific processes in nature where ice grain boundaries are involved.

  20. Large-scale experimental and theoretical study of graphene grain boundary structures

    Science.gov (United States)

    Ophus, Colin; Shekhawat, Ashivni; Rasool, Haider; Zettl, Alex

    2015-11-01

    We have characterized the structure of 176 different single-layer graphene grain boundaries grown with chemical vapor deposition using >1000 experimental high-resolution transmission electron microscopy images using a semiautomated structure processing routine. We introduce an algorithm for generating grain boundary structures for a class of hexagonal two-dimensional materials and use this algorithm and molecular dynamics to simulate the structure of >79 000 linear graphene grain boundaries covering 4122 unique orientations distributed over the entire parameter space. The dislocation content and structural properties are extracted from all experimental and simulated boundaries, and various trends are explored. We find excellent agreement between the simulated and experimentally observed grain boundaries. Our analysis demonstrates the power of a statistically significant number of measurements as opposed to a small number of observations in atomic science.

  1. Strain induced grain boundary migration effects on grain growth of an austenitic stainless steel during static and metadynamic recrystallization

    Energy Technology Data Exchange (ETDEWEB)

    Paggi, A., E-mail: alpaggi@tenaris.com [Tenaris Dalmine R& D, Dalmine S.p.A., Piazza Caduti 6 Luglio 1944 n.1, 24044 Dalmine (Italy); Angella, G.; Donnini, R. [National Research Council (CNR), Institute for Energetics and Interphases (IENI), Via Roberto Cozzi 53, 20125 Milano (Italy)

    2015-09-15

    Static and metadynamic recrystallization of an AISI 304L austenitic stainless steel was investigated at 1100 °C and 10{sup −} {sup 2} s{sup −} {sup 1} strain rate. The kinetics of recrystallization was determined through double hit compression tests. Two strain levels were selected for the first compression hit: ε{sub f} = 0.15 for static recrystallization (SRX) and 0.25 for metadynamic recrystallization (MDRX). Both the as-deformed and the recrystallized microstructures were investigated through optical microscopy and electron back-scattered diffraction (EBSD) technique. During deformation, strain induced grain boundary migration appeared to be significant, producing a square-like grain boundary structure aligned along the directions of the maximum shear stresses in compression. EBSD analysis revealed to be as a fundamental technique that the dislocation density was distributed heterogeneously in the deformed grains. Grain growth driven by surface energy reduction was also investigated, finding that it was too slow to explain the experimental data. Based on microstructural results, it was concluded that saturation of the nucleation sites occurred in the first stages of recrystallization, while grain growth driven by strain induced grain boundary migration (SIGBM) dominated the subsequent stages. - Highlights: • Recrystallization behavior of a stainless steel was investigated at 1100 °C. • EBSD revealed that the dislocation density distribution was heterogeneous during deformation. • Saturation of nucleation sites occurred in the first stages of recrystallization. • Strain induced grain boundary migration (SIGBM) effects were significant. • Grain growth driven by SIGBM dominated the subsequent stages.

  2. Migration of grain boundaries and triple junctions in high-purity aluminum during annealing after slight cold rolling

    Energy Technology Data Exchange (ETDEWEB)

    Yin, Wenhong [Institute of Materials, Shanghai University, Shanghai 200072 (China); School of Mechanical Engineering, Shandong University of Technology, Zibo 255049 (China); Wang, Weiguo, E-mail: wang_weiguo@vip.163.com [Institute of Materials, Shanghai University, Shanghai 200072 (China); School of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350118 (China); Fang, Xiaoying [School of Mechanical Engineering, Shandong University of Technology, Zibo 255049 (China); Qin, Congxiang [Institute of Materials, Shanghai University, Shanghai 200072 (China); School of Mechanical Engineering, Shandong University of Technology, Zibo 255049 (China); Xing, Xiaoguang [School of Mechanical Engineering, Shandong University of Technology, Zibo 255049 (China)

    2015-09-15

    Grain orientations and grain boundary migrations near triple junctions in a high purity aluminum were analyzed by electron back scattered diffraction. The results indicate that there are good correlations between the Schmid factors or Taylor factors and the misorientation values of point to original point in grains near the triple junctions in a slightly deformed sample. Grains with higher Schmid factors or lower Taylor factors typically correspond to higher misorientation values near the triple junctions. In a subsequent annealing at 400 °C, both grain boundaries and triple junctions migrate, but the former leave ghost lines. During such migration, a grain boundary grows from the grain with lower Schmid factor (higher Taylor factor) into the grain with higher Schmid factor (lower Taylor factor). Usually, the amount of migration of a grain boundary is considerably greater than that of a triple junction, and the grain boundary becomes more curved after migration. These observations indicate that the triple junctions have drag effects on grain boundary migration. - Highlights: • Polycrystalline aluminum with fine grains about 30 μm were used. • Off-line in situ EBSD was used to identify TJs before and after annealing. • Grains with higher SFs have higher misorientation values near TJs after deformation. • Grain boundaries grow from hard grains into soft grains during annealing. • Triple junctions have drag effects on grain boundaries migration.

  3. Migration of grain boundaries and triple junctions in high-purity aluminum during annealing after slight cold rolling

    International Nuclear Information System (INIS)

    Grain orientations and grain boundary migrations near triple junctions in a high purity aluminum were analyzed by electron back scattered diffraction. The results indicate that there are good correlations between the Schmid factors or Taylor factors and the misorientation values of point to original point in grains near the triple junctions in a slightly deformed sample. Grains with higher Schmid factors or lower Taylor factors typically correspond to higher misorientation values near the triple junctions. In a subsequent annealing at 400 °C, both grain boundaries and triple junctions migrate, but the former leave ghost lines. During such migration, a grain boundary grows from the grain with lower Schmid factor (higher Taylor factor) into the grain with higher Schmid factor (lower Taylor factor). Usually, the amount of migration of a grain boundary is considerably greater than that of a triple junction, and the grain boundary becomes more curved after migration. These observations indicate that the triple junctions have drag effects on grain boundary migration. - Highlights: • Polycrystalline aluminum with fine grains about 30 μm were used. • Off-line in situ EBSD was used to identify TJs before and after annealing. • Grains with higher SFs have higher misorientation values near TJs after deformation. • Grain boundaries grow from hard grains into soft grains during annealing. • Triple junctions have drag effects on grain boundaries migration

  4. Phosphorus segregation behavior at the grain boundary in a Ti-IF steel after annealing

    International Nuclear Information System (INIS)

    The behavior of phosphorus grain boundary segregation is researched for an interstitial-free Ti steel which is cold rolled and recrystallized by annealing for different times at 810 oC. The results show that the segregation of phosphorus at the grain boundary is a non-equilibrium phenomenon. The critical segregation time is 120 s, when the segregation level is about 14 at.%. Its segregation mechanism is a solute atom 'atmosphere' that is formed around each dislocation and moves toward grain boundary.

  5. Non-equilibrium grain-boundary segregation of Bi in binary Ni(Bi) alloy

    International Nuclear Information System (INIS)

    The minimum ductility of Ni(Bi) alloy caused by isothermal annealing at 750 °C or 650 °C is determined through tensile tests at room temperature. Tensile samples with minimum ductility display intergranular facets. Strong grain-boundary segregation of Bi in nanocrystalline Ni–Bi layer is observed by atom probe tomography. The minimum ductility, intergranular facets and grain-boundary segregation of Bi confirm the non-equilibrium grain-boundary segregation of Bi in Ni(Bi) alloy for the first time

  6. Grain boundaries in Cu(In,Ga)(Se,S){sub 2} thin-film solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Rau, Uwe [Forschungszentrum Juelich, IEF5-Photovoltaik, Juelich (Germany); Taretto, Kurt [Universidad Nacional del Comahue, Dto. de Electrotecnia, Buenos Aires, Neuquen (Argentina); Siebentritt, Susanne [Universite du Luxembourg, Luxembourg (Luxembourg)

    2009-07-15

    The paper reviews the current status of the research on grain boundaries in polycrystalline Cu(In,Ga)(S,Se){sub 2} alloys used as absorber materials for thin-film solar cells. We discuss the different concepts that are available to explain the relatively low electronic activity of grain boundaries in these materials. Numerical simulations that have been undergone so far to model the polycrystalline solar cells are briefly summarized. In addition, we give an overview on the experiments that have been conducted so far to elucidate the structural, defect-chemical, and electronic properties of grain boundaries in Cu(In,Ga)(S,Se){sub 2} thin-films. (orig.)

  7. The influence of temperature and grain boundary volume on the resistivity of nanocrystalline nickel

    Energy Technology Data Exchange (ETDEWEB)

    Darnbrough, J. E., E-mail: J.E.Darnbrough@bristol.ac.uk; Flewitt, P. E. J. [Interface Analysis Centre, School of Physics, University of Bristol, H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol, BS8 1TL (United Kingdom); Roebuck, B. [National Physical Laboratory, Hampton Rd, Teddington, Middlesex, TW11 0LW (United Kingdom)

    2015-11-14

    The thermal stability and modes of recrystallisation of nanocrystalline nickel has been observed through a conduction-based non-destructive test. Resistivity measurements have been utilised to quantify grain boundary volume fraction and microstructure. This observation makes clear the distinction of the factors that contribute to resistivity and demonstrates that these contributions are related to microstructure, either directly or in-directly. In static systems, the contribution of ordered grains and low-order grain boundary atomic arrangements in small grained material has been measured and correlated with resistivity. Measurements of in-situ resistivity conducted at high temperature gives changes with time which are related to grain growth, during heat treatment. This shows that resistivity can be used as a technique for observing the microstructure and grain growth of small grained material.

  8. Negligible effect of grain boundaries on the thermal conductivity of rocks

    Science.gov (United States)

    Vandersande, J. W.; Pohl, R. O.

    1982-08-01

    The thermal conductivity of marble, gabbro, quartz-monzonite, basalt and of labradorite was measured between 0.3 and 80 K. In all cases, the phonon mean free paths ℓ¯ were found to increase with decreasing temperature, but even at the lowest temperatures, ℓ¯ was far smaller than the average grain sizes. This demonstrates that phonon scattering by grain boundaries has very little influence on the heat transport in these rocks. Evidence is presented that lamellae due to twinning or exsolution, rod-like large inclusions, and density fluctuations inside the grains mask the effect of grain boundaries.

  9. Micromechanical Modeling of Grain Boundaries Damage in a Copper Alloy Under Creep

    International Nuclear Information System (INIS)

    In order to include the processes on the scale of the grain structure into the description of the creep behaviour of polycrystalline materials, the damage development of a single grain boundary has been initially investigated in the present work. For this purpose, a special simulationmethod has been used, whose resolution procedure based on holomorphic functions. The mechanisms taken into account for the simulations include nucleation, growth by grain boundary diffusion, coalescence and shrinkage until complete sintering of grain boundary cavities. These studies have then been used to develop a simplified cavitation model, which describes the grain boundary damage by two state variables and the time-dependent development by a mechanism-oriented rate formulation. To include the influence of grain boundaries within continuum mechanical considerations of polycrystals, an interface model has been developed, that incorporates both damage according to the simplified cavitation model and grain boundary sliding in dependence of a phenomenological grain boundary viscosity. Furthermore a micromechanical model of a polycrystal has been developed that allows to include a material's grain structure into the simulation of the creep behaviour by means of finite element simulations. Thereby, the deformations of individual grains are expressed by a viscoplastic single crystal model and the grain boundaries are described by the proposed interface model. The grain structure is represented by a finite element model, in which the grain boundaries are modelled by cohesive elements. From the evaluation of experimental creep data, the micromechanical model of a polycrystal has been calibrated for a copper-antimony alloy at a temperature of 823 K. Thereby, the adjustment of the single crystal model has been carried out on the basis of creep rates of pure copper single crystal specimens. The experimental determination of grain boundary sliding and grain boundary porosity for coarse-grained

  10. Effects of low-frequency magnetic field on grain boundary segregation in horizontal direct chill casting of 2024 aluminum alloy

    Institute of Scientific and Technical Information of China (English)

    2005-01-01

    Effects of low frequency electromagnetic field on grain boundary segregation in horizontal direct chill (HDC)casting process was investigated experimentally. The grain boundary segregation and microstructures of the ingots,which manufactured by conventional HDC casting and low frequency electromagnetic HDC casting were compared.Results show that low frequency electromagnetic field significantly refines the microstructures and reduces grain boundary segregation. Decreasing electromagnetic frequency or increasing electromagnetic intensity has great effects in reducing grain boundary segregation. Meanwhile, the governing mechanisms were discussed.

  11. Influence of inhomogeneity of grain-boundary region of nanocrystalline materials on elastic properties

    Institute of Scientific and Technical Information of China (English)

    Stepanov; Y.; N.; Alymov; M.; I.

    2005-01-01

    Experimental data indicate that Young's modulus of materials decreases with the decreasing of the grain size. Obviously, the primary factor of this decrease is presence of grain-boundary region, which Young's modulus other than in the bulk of crystallites. There is a set of various expressions for calculation of Young's modulus of polycrystals, obtained under the assumption, that it is possible to consider a polycrystal as a composite consisting of a crystalline matrix and a intercrystalline layers (grain-boundary region). Calculations showed incorrectness of application of a majority of these expressions and a large error in the calculations for the nanocrystalline materials. By us, on the basis of the same assumptions, is also obtained analytical expression for calculating Young's modulus of materials with grain size more than 30 nm, which is more exact, than all others.It is necessary to consider under the calculation of effective Young's modulus nanocrystalline materials with grain size of less than 30nm, that grain-boundary region itself is not uniform. It is reliably established,that the triple joints of grain boundaries have a structure and properties, different from the structure and the properties of grain boundaries, which these joints connect. For nanocrystalline materials the volume fraction of the triple joints in the grain-boundary region can reach 50% and even more. Therefore assumption was made, that the nanocrystalline materials should be represented as consisting of three phases (triple joints,grain boundary between the triple joints and crystallite). On the basis of this idea is obtained analytical expression for calculating of Young's modulus nanocrystalline materials. The analysis shows that Young's modulus calculated by this analytical expression coordinated with the theory and the experiment.

  12. A Tool for Local Thickness Determination and Grain Boundary Characterization by CTEM and HRTEM Techniques.

    Science.gov (United States)

    Kiss, Ákos K; Rauch, Edgar F; Pécz, Béla; Szívós, János; Lábár, János L

    2015-04-01

    A new approach for measurement of local thickness and characterization of grain boundaries is presented. The method is embodied in a software tool that helps to find and set sample orientations useful for high-resolution transmission electron microscopic (HRTEM) examination of grain boundaries in polycrystalline thin films. The novelty is the simultaneous treatment of the two neighboring grains and orienting both grains and the boundary plane simultaneously. The same metric matrix-based formalism is used for all crystal systems. Input into the software tool includes orientation data for the grains in question, which is determined automatically for a large number of grains by the commercial ASTAR program. Grain boundaries suitable for HRTEM examination are automatically identified by our software tool. Individual boundaries are selected manually for detailed HRTEM examination from the automatically identified set. Goniometer settings needed to observe the selected boundary in HRTEM are advised by the software. Operation is demonstrated on examples from cubic and hexagonal crystal systems. PMID:25801740

  13. Molecular Dynamics Study of the Microstructure of the Near Grain Boundary Distortion Region in Small Grain Nano-NiAl Alloy

    Institute of Scientific and Technical Information of China (English)

    WANG Jing-Yang; WANG Xiao-Wei; J.Rifkin; LI Dou-Xing

    2001-01-01

    Using the molecular dynamics simulation method, the microstructure of the distortion region near the curved amorphous-like grain boundary in nano-NiAl alloy is studied. The results show that due to the internal elastic force of the high-energy grain boundary, a distortion layer exists between the grain and grain boundary. The lattice expansion and the decreasing structure factor are observed in this region. The stacking fault in samples with a grain size of 3.8 nm is clearly observed across the distortion region at the site very close to the grain. The inItuences of different grain sizes on average distortion degree and volume fractions of the distortion region, grain and grain boundary are also discussed.

  14. Grain boundary network control and its effect on intergranular corrosion resistance of Alloy 690

    Energy Technology Data Exchange (ETDEWEB)

    Shuang Xia; Hui Li; Chang Liang Hu; Ting Guang Lui; Bang Xin Zhou; Wen Jue Chen [Institute of Materials, Shanghai University, P.O. Box 269, 149 Yanchang Road (China)

    2011-07-01

    Grain boundary engineering was carried out in Alloy 690 tube sample and its effect on the intergranular corrosion resistance was investigated. Through small amount of deformation by cold drawing and short time annealing at high temperature, the proportion of low {Sigma} coincidence site lattice (CSL) grain boundaries of the Alloy 690 tube sample can be enhanced to about 75% which mainly were of {Sigma}3{sup n} (n = 1, 2, 3...) type. In this case, the grain boundary network (GBN) was featured by the formation of large grain-clusters produced by multiple twinning during recrystallization. All of the grains inside this kind of cluster had {Sigma}3{sup n} mutual mis-orientations, and hence all the boundaries inside the cluster were of {Sigma}3{sup n} type and form many interconnected {Sigma}3{sup n} type triple junctions. The weight losses due to grain dropping during intergranular corrosion for the samples with the modified GBN were much less than that with conventional microstructure. Based on the characterization by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) technique, it was shown that the large grain-cluster microstructure played a key role in enhancing the intergranular corrosion resistance: 1) the large grain-cluster can arrest the penetration of intergranular corrosion; 2) the large grain-cluster can protect the under layer microstructure. (authors)

  15. Experimental Characterization and Simulation of Slip Transfer at Grain Boundaries and Microstructurally-Sensitive Crack Propagation

    Science.gov (United States)

    Gupta, Vipul; Hochhalter, Jacob; Yamakov, Vesselin; Scott, Willard; Spear, Ashley; Smith, Stephen; Glaessgen, Edward

    2013-01-01

    A systematic study of crack tip interaction with grain boundaries is critical for improvement of multiscale modeling of microstructurally-sensitive fatigue crack propagation and for the computationally-assisted design of more durable materials. In this study, single, bi- and large-grain multi-crystal specimens of an aluminum-copper alloy are fabricated, characterized using electron backscattered diffraction (EBSD), and deformed under tensile loading and nano-indentation. 2D image correlation (IC) in an environmental scanning electron microscope (ESEM) is used to measure displacements near crack tips, grain boundaries and within grain interiors. The role of grain boundaries on slip transfer is examined using nano-indentation in combination with high-resolution EBSD. The use of detailed IC and EBSD-based experiments are discussed as they relate to crystal-plasticity finite element (CPFE) model calibration and validation.

  16. Melt growth of CdTe crystals and transmission electron microscopic investigations of their grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Sabinina, I.V.; Gutakovski, A.K.; Milenov, T.I.; Lyakh, N.N.; Sidorov, Y.G.; Gospodinov, M.M. (Inst. of Semiconductor Physics, Academy of Sciences of the USSR (Siberian Branch), Novosibirsk (USSR) Inst. of Solid State Physics, Bulgarian Academy of Sciences, Sofia (Bulgaria))

    1991-01-01

    Transmission electron microscopy investigations are carried out on CdTe crystals grown in quartz ampoules in a temperature region (1020-1091degC) near to the melting point of 1092degC, by travelling heater method in quasi-closed and in sealed (at 0.135 Pa) volume, and by the Bridgman method from nearly stoichiometric melts. An original method for preparation of CdTe thin foil is reported. Two types of grain boundaries are observed: high-angle misoriented grain boundaries (more than ten degrees misorientation between adjacent grains) and low-angle misoriented grain boundaries (less than one degree misorientation between adjacent sub-grain). Both dislocations with Burgers vector b=a/6<112> and b=a/2<110> are present. (orig.).

  17. Study of grain boundary tunneling in barium-titanate ceramic films

    CERN Document Server

    Wong, H; Poon, M C

    1999-01-01

    The temperature and the electric-field dependences of the current-voltage characteristics and the low-frequency noise of barium-titanate ceramic films are studied. An abnormal field dependence is observed in the resistivity of BaTiO sub 3 materials with a small average grain size. In addition, experiments show that the low-frequency noise behaviors are governed by grain-boundary tunneling at room temperature and by trapping-detrapping of grain-boundary states at temperatures above the Curie point. Physical models for the new observations are developed. Results suggest that grain-boundary tunneling of carriers is as important as the double Schottky barrier in the current conduction in BaTiO sub 3 materials with small grain sizes.

  18. Surface-Kinetics-Controlled Ostwald Ripening of Plane Precipitates at Grain Boundaries

    Directory of Open Access Journals (Sweden)

    A.V. Koropov

    2013-04-01

    Full Text Available Ostwald ripening (coarsening of plane precipitates was analytically analyzed at a grain boundary for the case when the precipitates growth is controlled by surface kinetics. Asymptotic characteristics of Ostwald ripening are found.

  19. Surface-Kinetics-Controlled Ostwald Ripening of Plane Precipitates at Grain Boundaries

    OpenAIRE

    A.V. Koropov

    2013-01-01

    Ostwald ripening (coarsening) of plane precipitates was analytically analyzed at a grain boundary for the case when the precipitates growth is controlled by surface kinetics. Asymptotic characteristics of Ostwald ripening are found.

  20. The role of vacancies in the mobility of dislocations and grain boundaries in magnesium

    Energy Technology Data Exchange (ETDEWEB)

    Lambri, O.A.; Lucioni, E.J. [Facultad de Ciencias Exactas, Ingenieria y Agrimensura, Instituto de Fisica Rosario, Universidad Nacional de Rosario - CONICET, Escuela de Ingenieria Electrica, Laboratorio de Materiales, Avda. Pellegrini 250, 2000 Rosario (Argentina); Massot, M.; Plazaola, F. [Elektrika eta Elektronika Saila, Zientzia eta Teknologia Fakultatea, Euskal Herriko Unibertsitatea, P.K. 644, 48080 Bilbao (Spain); Riehemann, W. [Institute of Materials Science and Technology, Clausthal University of Technology, Agricolastrasse 6, 38678 Clausthal-Zellerfeld (Germany); Garcia, J.A. [Departamento de Fisica Aplicada II, Facultad de Ciencias y Tecnologia, Universidad del Pais Vasco, Apdo. 644, 48080 Bilbao (Spain)

    2007-04-15

    Vacancy flux or supersaturation enhances grain-boundary mobility, but experimental evidence is not large and in many cases the role of vacancies is only inferred indirectly. We will show effectively in the present work the importance of the vacancy role in grain-boundary mobility in commercial pure and high-purity magnesium using mechanical spectroscopy, electrical resistivity and positron annihilation spectroscopy. It has been found that the mobility decrease of grain boundaries and dislocations is related to vacancy concentration reduction attained after the homogenisation treatment. Indeed, the largest vacancy concentration reduction is observed between 420 and 500 K. Unlocking grain boundaries and dislocations requires new vacancies, generated at temperatures above 500 K. In addition, a new damping peak related to vacancies was discovered at 490 K for an oscillating frequency of 1 Hz. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  1. DIFFUSION AND EQUILIBRIUM SEGREGATION IN GRAIN BOUNDARIES : EFFECTS OF LARGE SEGREGATION

    OpenAIRE

    Bernardini, J.; Cabane, F.

    1985-01-01

    From recent experiments in binary and ternary solid solutions, we discuss the determination of grain boundary diffusion coefficients, some structural aspects of diffusion and segregation and the influence of segregation on impurity diffusion.

  2. Phase-field-crystal study of grain boundary premelting and shearing in bcc iron

    OpenAIRE

    Adland, Ari; Karma, Alain; Spatschek, Robert; Buta, Dorel; Asta, Mark

    2012-01-01

    We use the phase-field-crystal (PFC) method to investigate the equilibrium premelting and nonequilibrium shearing behaviors of $[001]$ symmetric tilt grain boundaries (GBs) at high homologous temperature over the complete range of misorientation $0

  3. Grain Boundary Engineering the Mechanical Properties of Allvac 718Plus(Trademark) Superalloy

    Science.gov (United States)

    Gabb, Timothy P.; Telesman, Jack; Garg, Anita; Lin, Peter; Provenzano, virgil; Heard, Robert; Miller, Herbert M.

    2010-01-01

    Grain Boundary Engineering can enhance the population of structurally-ordered "low S" Coincidence Site Lattice (CSL) grain boundaries in the microstructure. In some alloys, these "special" grain boundaries have been reported to improve overall resistance to corrosion, oxidation, and creep resistance. Such improvements could be quite beneficial for superalloys, especially in conditions which encourage damage and cracking at grain boundaries. Therefore, the effects of GBE processing on high-temperature mechanical properties of the cast and wrought superalloy Allvac 718Plus (Allvac ATI) were screened. Bar sections were subjected to varied GBE processing, and then consistently heat treated, machined, and tested at 650 C. Creep, tensile stress relaxation, and dwell fatigue crack growth tests were performed. The influences of GBE processing on microstructure, mechanical properties, and associated failure modes are discussed.

  4. Molecular dynamics simulation of temperature profile in partially hydrogenated graphene and graphene with grain boundary.

    Science.gov (United States)

    Lotfi, Erfan; Neek-Amal, M; Elahi, M

    2015-11-01

    Temperature profile in graphene, graphene with grain boundary and vacancy defects and hydrogenated graphene with different percentage of H-atoms are determined using molecular dynamics simulation. We also obtained the temperature profile in a graphene nanoribbon containing two types of grain boundaries with different misorientation angles, θ=21.8° and θ=32.2°. We found that a temperature gap appears in the temperature profile of a graphene nanoribbon with a grain boundary at the middle. Moreover, we found that the temperature profile in the partially hydrogenated graphene varies with the percentage of hydrogens, i.e. the C:H ratio. Our results show that a grain boundary line in the graphene sheet can change the thermal transport through the system which might be useful for controlling thermal flow in nanostructured graphene.

  5. Chemomechanical Origin of Hydrogen Trapping at Grain Boundaries in fcc Metals.

    Science.gov (United States)

    Zhou, Xiao; Marchand, Daniel; McDowell, David L; Zhu, Ting; Song, Jun

    2016-02-19

    Hydrogen embrittlement of metals is widely observed, but its atomistic origins remain little understood and much debated. Combining a unique identification of interstitial sites through polyhedral tessellation and first-principles calculations, we study hydrogen adsorption at grain boundaries in a variety of face-centered cubic metals of Ni, Cu, γ-Fe, and Pd. We discover the chemomechanical origin of the variation of adsorption energetics for interstitial hydrogen at grain boundaries. A general chemomechanical formula is established to provide accurate assessments of hydrogen trapping and segregation energetics at grain boundaries, and it also offers direct explanations for certain experimental observations. The present study deepens our mechanistic understanding of the role of grain boundaries in hydrogen embrittlement and points to a viable path towards predictive microstructure engineering against hydrogen embrittlement in structural metals.

  6. APFIM Characterization of Grain Boundary Segregation in Titanium Carbide-Doped Molybdenum

    OpenAIRE

    Miller, M; Kurishita, H.

    1996-01-01

    The grain boundary segregation behavior of titanium and carbon have been characterized in two titanium carbide-doped molybdenum alloys. The matrix of these alloys was found to be significantly depleted in titanium, carbon, oxygen, and nitrogen. Both titanium oxycarbide and molydenum carbide precipitates were observed. The Gibbsian Interfacial Excess, Γ, determined from atom probe analyses revealed significant enrichments of carbon and nitrogen at the grain boundaries in both alloys. In atom p...

  7. Effect of the disorder in graphene grain boundaries: A wave packet dynamics study

    OpenAIRE

    Vancsó, Péter; Márk, Géza I.; Lambin, Philippe; Mayer, Alexandre; Hwang, Chanyong; Biró, László P.

    2013-01-01

    Chemical vapor deposition (CVD) on Cu foil is one of the most promising methods to produce graphene samples despite of introducing numerous grain boundaries into the perfect graphene lattice. A rich variety of GB structures can be realized experimentally by controlling the parameters in the CVD method. Grain boundaries contain non-hexagonal carbon rings (4,5,7,8 membered rings) and vacancies in various ratios and arrangements. Using wave packet dynamic (WPD) simulations and tight-binding elec...

  8. An improved method to identify grain boundary creep cavitation in 316H austenitic stainless steel.

    Science.gov (United States)

    Chen, B; Flewitt, P E J; Smith, D J; Jones, C P

    2011-04-01

    Inter-granular creep cavitation damage has been observed in an ex-service 316H austenitic stainless steel thick section weldment. Focused ion beam cross-section milling combined with ion channelling contrast imaging is used to identify the cavitation damage, which is usually associated with the grain boundary carbide precipitates in this material. The results demonstrate that this technique can identify, in particular, the early stage of grain boundary creep cavitation unambiguously in materials with complex phase constituents. PMID:21396524

  9. The vibrational spectrum of the atoms in the grain boundaries of nanocrystalline Pd

    Energy Technology Data Exchange (ETDEWEB)

    Stuhr, U. [Paul Scherrer Inst. (PSI), Villigen (Switzerland); Wipf, H.; Hahn, H. [Technische Hochschule Darmstadt (Germany); Natter, H.; Hemperlmann, R. [Universitaet des Saarlandes, Saarbruecken (Germany); Andersen, K. [Institut Max von Laue - Paul Langevin (ILL), 38 - Grenoble (France)

    1997-09-01

    The vibrational excitations of the atoms in nanocrystalline Pd was investigated by neutron-time-of-flight spectroscopy. Hydrogen was used as a probe for the vibrations in the grain boundaries. The separation between the H and Pd vibrations was done by spin analysis. The results show that in the grain boundary the density of states of low energy excitations ({<=}5 meV) is drastically increased. (author) 3 figs., 3 refs.

  10. Importance of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium thermoelectrics

    OpenAIRE

    Hua, Chengyun; Minnich, Austin J.

    2014-01-01

    Nanocrystalline silicon and silicon-germanium alloys are promising thermoelectric materials that have achieved substantially improved figure of merits compared to their bulk counterparts. This enhancement is typically attributed to a reduction in lattice thermal conductivity by phonon scattering at grain boundaries. However, further improvements are difficult to achieve because grain boundary scattering is poorly understood, with recent experimental observations suggesting that the phonon tra...

  11. Misoriented grain boundaries vicinal to the ? twin in nickel Part I: thermodynamics & temperature-dependent structure

    Science.gov (United States)

    O'Brien, Christopher J.; Medlin, Douglas L.; Foiles, Stephen M.

    2016-05-01

    Grain boundary-engineered materials are of immense interest for their corrosion resistance, fracture resistance and microstructural stability. This work contributes to a larger goal of understanding both the structure and thermodynamic properties of grain boundaries vicinal (within ?) to the ?? (coherent twin) boundary which is found in grain boundary-engineered materials. The misoriented boundaries vicinal to the twin show structural changes at elevated temperatures. In the case of nickel, this transition temperature is substantially below the melting point and at temperatures commonly reached during processing, making the existence of such boundaries very likely in applications. Thus, the thermodynamic stability of such features is thoroughly investigated in order to predict and fully understand the structure of boundaries vicinal to twins. Low misorientation angle grain boundaries (?) show distinct ? disconnections which accommodate misorientation in opposite senses. The two types of disconnection have differing low-temperature structures which show different temperature-dependent behaviours with one type undergoing a structural transition at approximately 600 K. At misorientation angles greater than approximately ?, the discrete disconnection nature is lost as the disconnections merge into one another. Free energy calculations demonstrate that these high-angle boundaries, which exhibit a transition from a planar to a faceted structure, are thermodynamically more stable in the faceted configuration.

  12. Observation of thermally etched grain boundaries with the FIB/TEM technique

    Energy Technology Data Exchange (ETDEWEB)

    Palizdar, Y., E-mail: y.palizdar@merc.ac.ir [Nanotechnology and advanced materials department, Materials and energy research centre (MERC), Karaj (Iran, Islamic Republic of); San Martin, D. [MATERALIA group, Department of Physical Metallurgy, (CENIM-CSIC), Centro Nacional de Investigaciones Metalúrgicas Av. Gregorio del Amo 8, 28040 Madrid (Spain); Ward, M.; Cochrane, R.C.; Brydson, R.; Scott, A.J. [Institute for Materials Research, SPEME, University of Leeds, Leeds LS2 9JT (United Kingdom)

    2013-10-15

    Thermal etching is a method which is able to reveal and characterize grain boundaries, twins or dislocation structures and determine parameters such as grain boundary energies, surface diffusivities or study phase transformations in steels, intermetallics or ceramic materials. This method relies on the preferential transfer of matter away from grain boundaries on a polished sample during heating at high temperatures in an inert/vacuum atmosphere. The evaporation/diffusion of atoms at high temperatures results in the formation of grooves at the intersections of the planes of grain/twin boundaries with the polished surface. This work describes how the combined use of Focussed Ion Beam and Transmission Electron Microscopy can be used to characterize not only the grooves and their profile with the surface, but also the grain boundary line below the groove, this method being complementary to the commonly used scanning probe techniques. - Highlights: • Thermally etched low-carbon steel samples have been characterized by FIB/TEM • Grain boundary (GB) lines below the groove have been characterized in this way • Absence of ghost traces and large θ angle suggests that GB are not stationary but mobile • Observations correlate well with previous works and Mullins' investigations [22].

  13. On the small angle twist sub-grain boundaries in Ti3AlC2

    Science.gov (United States)

    Zhang, Hui; Zhang, Chao; Hu, Tao; Zhan, Xun; Wang, Xiaohui; Zhou, Yanchun

    2016-04-01

    Tilt-dominated grain boundaries have been investigated in depth in the deformation of MAX phases. In stark contrast, another important type of grain boundaries, twist grain boundaries, have long been overlooked. Here, we report on the observation of small angle twist sub-grain boundaries in a typical MAX phase Ti3AlC2 compressed at 1200 °C, which comprise hexagonal screw dislocation networks formed by basal dislocation reactions. By first-principles investigations on atomic-scale deformation and general stacking fault energy landscapes, it is unequivocally demonstrated that the twist sub-grain boundaries are most likely located between Al and Ti4f (Ti located at the 4f Wyckoff sites of P63/mmc) layers, with breaking of the weakly bonded Al–Ti4f. The twist angle increases with the increase of deformation and is estimated to be around 0.5° for a deformation of 26%. This work may shed light on sub-grain boundaries of MAX phases, and provide fundamental information for future atomic-scale simulations.

  14. Experimental observation and computer simulation on non-equilibrium grain-boundary segregation kinetics of phosphorus

    Institute of Scientific and Technical Information of China (English)

    LI Li; LI Qing-fen; LIU Er-bao

    2005-01-01

    An experimental study and computer simulation on non-equilibrium grain-boundary segregation kinetics and the critical time for phosphorus in 12Cr1MoV steel(which is used in steam pipeline of ships)are put forward in this paper. The segregation level of phosphorus with solution temperature 1050℃ at the isothermal holding temperature of 540℃,have been measured at grain-boundaries. A non-equilibrium grain-boundary segregation kinetics curve of phosphorus is given. The critical time for phosphorus non-equilibrium grain-boundary segregation is about 500h at 540℃ for the experimental steel. When the holding time is longer than 1500h, non-equilibrium segregation disappears and the level of segregation reaches full equilibrium. The simulation using the kinetic equations of non-equilibrium grain-boundary segregation is in good accordance with the experimental observation for phosphorus in steel 12Cr1MoV. The non-equilibrium grain-boundary segregation kinetic model is therefore proved.

  15. A continuum framework for grain boundary diffusion in thin film/substrate systems

    NARCIS (Netherlands)

    Ayas, Can; van der Giessen, Erik

    2010-01-01

    A two-dimensional continuum model is developed for stress relaxation in thin films through grain boundary (GB) diffusion. When a thin film with columnar grains is subjected to thermal stress, stress gradients along the GBs are relaxed by diffusion of material from the film surface into the GBs. The

  16. Subgrain Coalescence and the Nucleation of Recrystallization of Grain Boundaries in Aluminium

    DEFF Research Database (Denmark)

    Jones, A. R.; Ralph, B.; Hansen, Niels

    1979-01-01

    An experimental study has been made of recrystallization processes which have been initiated at grain boundaries in aluminium samples. The samples had an initial grain size of between 30 and 130 $\\mu $m and were deformed 50% by rolling before annealing at 573 K. The microstructures developed in t...

  17. Relationship between lattice mismatch and ionic conduction of grain boundary in YSZ

    Directory of Open Access Journals (Sweden)

    Fei Ye

    2014-02-01

    Full Text Available The grain boundary plays an important role in the electrical behaviors of solid oxide electrolytes for solid state fuel cells. To reveal the relationship between the structure and the ionic conductivity of grain boundary, the conductive properties of {1 1 1} and {1 1 0} twist grain boundaries in 8 mol% yttria-stabilized zirconia have been examined. These boundaries have a series of Σ values defined by the coincident site lattice model. It has been found that the activation energy of {1 1 1} twist grain boundary increases and then decreases with the Σ value, while that of the {1 1 0} boundary shows an opposite trend. It is suggested that the properties can reflect the balance of the effects of lattice mismatch on the diffusion ability of oxygen vacancies and the segregation of oxygen vacancies and Y3+ ions. Therefore, the properties in polycrystalline electrolyte can be adjusted by controlling the grain boundary structures.

  18. Critical Casimir effect in the Ising strips with standard normal and ordinary boundary conditions and the grain boundary

    Science.gov (United States)

    Borjan, Z.

    2016-09-01

    We consider critical Casimir force in the Ising strips with boundary conditions defined by standard normal and ordinary surface universality classes containing also the internal grain boundary. Using exact variational approach of Mikheev and Fisher we have elaborated on behaviors of Casimir amplitudes Δ++(g) , ΔOO(g) and Δ+O(g) , corresponding to normal-normal, ordinary-ordinary and mixed normal-ordinary boundary conditions, respectively, with g as a strength of the grain boundary. Closed analytic results describe Casimir amplitudes Δ++(g) and ΔOO(g) as continuous functions of the grain boundary's strength g, changing the character of the Casimir force from repulsive to attractive and vice versa for certain domains of g. Present results reveal a new type of symmetry between Casimir amplitudes Δ++(g) and ΔOO(g) . Unexpectedly simple constant result for the Casimir amplitude Δ+O(g) = π/12 we have comprehensively interpreted in terms of equilibrium states of the present Ising strip as a complex interacting system comprising two sub-systems. Short-distance expansions of energy density profiles in the vicinity of the grain boundary reveal new distant-wall correction amplitudes that we examined in detail. Analogy of present considerations with earlier more usual short-distance expansions near one of the (N), (O) and (SB) boundaries, as well as close to surfaces with variable boundary conditions refers to the set of scaling dimensions appearing in the present calculations but also to the discovery of the de Gennes-Fisher distant wall correction amplitudes.

  19. Superplastic Constitutive Equation Including Percentage of High-Angle Grain Boundaries as a Microstructural Parameter

    Science.gov (United States)

    Wang, K.; Liu, F. C.; Xue, P.; Wang, D.; Xiao, B. L.; Ma, Z. Y.

    2016-01-01

    Fifteen Al-Mg-Sc samples with subgrain/grain sizes in the range of 1.8 to 4.9 μm were prepared through the processing methods of friction stir processing (FSP), equal-channel-angular pressing (ECAP), rolling, annealing, and combinations of the above. The percentages of high-angle grain boundaries (HAGBs) of these fine-grained alloys were distributed from 39 to 97 pct. The samples processed through FSP had a higher percentage of HAGBs compared to other samples. Superplasticity was achieved in all fifteen samples, but the FSP samples exhibited better superplasticity than other samples because their fine equiaxed grains, which were mostly surrounded by HAGBs, were conducive to the occurrence of grain boundary sliding (GBS) during superplastic deformation. The dominant deformation mechanism was the same for all fifteen samples, i.e., GBS controlled by grain boundary diffusion. However, the subgrains were the GBS units for the rolled or ECAP samples, which contained high percentages of unrecrystallized grains, whereas the fine grains were the GBS units for the FSP samples. Superplastic data analysis revealed that the dimensionless A in the classical constitutive equation for superplasticity of fine-grained Al alloys was not a constant, but increased with an increase in the percentage of HAGBs, demonstrating that the enhanced superplastic deformation kinetics can be ascribed to the high percentage of HAGBs. A modified superplastic constitutive equation with the percentage of HAGBs as a new microstructural parameter was established.

  20. The Pinning by Particles of Low and High Angle Grain Boundaries during Grain Growth

    DEFF Research Database (Denmark)

    Tweed, C.J.; Ralph, B.; Hansen, Niels

    1984-01-01

    and coworkers. These estimates of local driving pressures have shown that they are similar for both the low and the high angle boundaries encountered in the samples. The pinning effects by particles at high angle boundaries are in general accord with the model due to Zener whilst those at low angle boundaries...

  1. Dislocations, boundaries and slip systems in cube grains of rolled aluminium

    DEFF Research Database (Denmark)

    Wei, Yili; Godfrey, A.; Liu, W.;

    2011-01-01

    The relationship between the dislocation content of boundaries and the active slip systems is explored by characterisation of Burgers vectors, dislocation lines and relative densities in 11 boundaries in near-cube grains in 10% rolled aluminium. To provide a good basis for comparison, all...

  2. Load relaxation studies of grain boundary sliding in Incoloy 800H

    Energy Technology Data Exchange (ETDEWEB)

    Wilson, H.; Hannula, S.P.; Korhonen, M.A.; Suzuki, H.; Li, C.Y (Cornell Univ., Ithaca, NY (USA))

    Load relaxation tests were performed on Incoloy 800H at elevated temperatures as a function of prior plastic deformation. The log stress vs. log strain rate curves obtained exhibit the typical sigmoidal shape predicted by current theories. A stress enhancement factor with a value near 0.7 can be estimated based on limiting stress values both at the high and low strain rate ends. The results of data analysis yielded long grain boundary stress vs. log grain boundary sliding rate curves. These curves are found to show grain matrix-like characteristics. The significance of these results is discussed in terms of a state variable theory.

  3. Non-equilibrium segregation of boron on grain boundary in Fe-30%Ni alloy

    Institute of Scientific and Technical Information of China (English)

    2002-01-01

    The moving boundary non-equilibrium segregation during recrystallization and the influence of pre-deformation in Fe-30%Ni Alloy at1000℃ was investigated by the PTA (particle tracking autoradiography) technique, optical and electron microscopy. The results indicated that intensity of boron segregation on moving boundary after different deformation is concerned with the pre-deformation degree and the movingspeed of the boundary. The TEM ( transmission electron microscopy) result showed that the dislocation density nearby the moving boundary increase obviously. The phenomena are discussed by the widening grain boundary mechanism.

  4. Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films

    Science.gov (United States)

    Magnfält, D.; Fillon, A.; Boyd, R. D.; Helmersson, U.; Sarakinos, K.; Abadias, G.

    2016-02-01

    Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low.

  5. Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films

    Energy Technology Data Exchange (ETDEWEB)

    Magnfält, D., E-mail: danma@ifm.liu.se; Sarakinos, K. [Nanoscale Engineering Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping (Sweden); Fillon, A.; Abadias, G. [Institut P' , Département Physique et Mécanique des Matériaux, Université de Poitiers-CNRS-ENSMA, SP2MI, Téléport 2, Bd M. et P. Curie, F-86962 Chasseneuil-Futuroscope (France); Boyd, R. D.; Helmersson, U. [Plasma and Coatings Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping (Sweden)

    2016-02-07

    Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low.

  6. Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films

    International Nuclear Information System (INIS)

    Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low

  7. Structure of high angle grain boundaries in metals and ceramic oxides

    International Nuclear Information System (INIS)

    A critical review is given of the state of our current knowledge of the structure of high angle grain boundaries in metals and in ceramic oxides. Particular attention is given to effects due to differences in the bonding and crystal structure in these solid types. The results of recent experimental work and efforts to model grain boundary structure using computer simulation methods are described. Important characteristic features of boundaries in these materials are discussed. Difficulties which are presently being encountered in efforts to determine their structure are pointed out

  8. Grain boundary effect on the nature of cleavage fracture in copper crystallite under pulsed loading

    International Nuclear Information System (INIS)

    The computer modeling of cleavage fracture in the copper crystallite, containing grain boundary under pulsed loading, is carried out. It is shown that tendency to material destruction along the grain boundary is increasing by initiation of packages of several isolated compression pulses (ICP) in the material. Increase in the ICP number in the package leads to the cleavage fracture along the boundaries, outlying over a great distance from the free surface. The cleaved fragments decompose with time into smaller ones due to growing spread of atomic velocities

  9. Yttrium segregation and oxygen diffusion along high-symmetry grain boundaries in YSZ

    Energy Technology Data Exchange (ETDEWEB)

    González-Romero, Robert L. [Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, Caixa Postal 6165, CEP 13083-970, Campinas (Brazil); Meléndez, Juan J. [Department of Physics, University of Extremadura. Avda. de Elvas, s/n, 06006, Badajoz (Spain); Institute for Advanced Scientific Computing of Extremadura (ICCAEx). Avda. de Elvas, s/n, 06006, Badajoz (Spain)

    2015-02-15

    Highlights: • A study of yttrium segregation to three grain boundaries (GB) in YSZ is performed. • A steady concentration of yttrium is reached after a short transient regime. • Segregation does not change the coefficients for oxygen diffusion along the GB. • The main effect after segregation appears for oxygen bulk diffusion. • The effect is related to the rearrangement of oxygen vacancies after segregation. - Abstract: A study by Molecular Dynamics of yttrium segregation to high-symmetry grain boundaries of yttria-stabilized zirconia has been performed for different amounts of dopants. After an initial (and short) transient, segregation reaches a steady regime in which the concentration of the defect species at the grain-boundaries does not change in time. The maximum concentration of yttrium is reached at the grain-boundary planes, while oxygen vacancies screen the electric field created by segregation. Segregation of yttrium does not change appreciably the coefficients for oxygen diffusion along the grain boundaries, but instead modifies those for bulk diffusion. This effect is rationalized in terms of the rearrangement of the oxygen vacancies at the vicinities of the yttrium cations. The activation energies vary smoothly with the concentration of yttria for all the boundaries. Our data for diffusion coefficients and activation energies compare fairly well with experimental values when segregation is explicitly taken into account.

  10. Yttrium segregation and oxygen diffusion along high-symmetry grain boundaries in YSZ

    International Nuclear Information System (INIS)

    Highlights: • A study of yttrium segregation to three grain boundaries (GB) in YSZ is performed. • A steady concentration of yttrium is reached after a short transient regime. • Segregation does not change the coefficients for oxygen diffusion along the GB. • The main effect after segregation appears for oxygen bulk diffusion. • The effect is related to the rearrangement of oxygen vacancies after segregation. - Abstract: A study by Molecular Dynamics of yttrium segregation to high-symmetry grain boundaries of yttria-stabilized zirconia has been performed for different amounts of dopants. After an initial (and short) transient, segregation reaches a steady regime in which the concentration of the defect species at the grain-boundaries does not change in time. The maximum concentration of yttrium is reached at the grain-boundary planes, while oxygen vacancies screen the electric field created by segregation. Segregation of yttrium does not change appreciably the coefficients for oxygen diffusion along the grain boundaries, but instead modifies those for bulk diffusion. This effect is rationalized in terms of the rearrangement of the oxygen vacancies at the vicinities of the yttrium cations. The activation energies vary smoothly with the concentration of yttria for all the boundaries. Our data for diffusion coefficients and activation energies compare fairly well with experimental values when segregation is explicitly taken into account

  11. Observing grain boundaries in CVD-grown monolayer transition metal dichalcogenides.

    Science.gov (United States)

    Ly, Thuc Hue; Chiu, Ming-Hui; Li, Ming-Yang; Zhao, Jiong; Perello, David J; Cichocka, Magdalena Ola; Oh, Hye Min; Chae, Sang Hoon; Jeong, Hye Yun; Yao, Fei; Li, Lain-Jong; Lee, Young Hee

    2014-11-25

    Two-dimensional monolayer transition metal dichalcogenides (TMdCs), driven by graphene science, revisit optical and electronic properties, which are markedly different from bulk characteristics. These properties are easily modified due to accessibility of all the atoms viable to ambient gases, and therefore, there is no guarantee that impurities and defects such as vacancies, grain boundaries, and wrinkles behave as those of ideal bulk. On the other hand, this could be advantageous in engineering such defects. Here, we report a method of observing grain boundary distribution of monolayer TMdCs by a selective oxidation. This was implemented by exposing directly the TMdC layer grown on sapphire without transfer to ultraviolet light irradiation under moisture-rich conditions. The generated oxygen and hydroxyl radicals selectively functionalized defective grain boundaries in TMdCs to provoke morphological changes at the boundary, where the grain boundary distribution was observed by atomic force microscopy and scanning electron microscopy. This paves the way toward the investigation of transport properties engineered by defects and grain boundaries. PMID:25343242

  12. Critical current across grain boundaries in melt-textured YBa2Cu3O

    International Nuclear Information System (INIS)

    We determine the critical current JcB of grain boundaries fabricated by ''welding'' of melt-textured YBCO with various degrees of ab-plane misalignment. JcB is determined by monitoring the magnetic moment due to persistent shielding currents, which were induced in rings containing two sections of a single grain boundary. The voltage drop across the junctions is estimated to be below 10-12 V, much smaller than in typical transport measurements. As the temperature or magnetic field is increased, an abrupt decrease (kink) is observed in the magnetic moment of the ring when the induced shielding current in the ring exceeds the critical current across the boundaries. The kink signals that flux has begun to penetrate through the grain boundaries into the bore of the ring. This behavior is confirmed by magneto-optical imaging. We observe that grain boundaries with [001] tilt mismatch angles up to 5o have critical current densities in excess of 104 A/cm2, comparable to the bulk current density. At larger mismatch angles, the critical current rapidly decreases with increasing angle. The qualitative behavior of JcB as function of temperature, magnetic field, and misorientation angle is similar to that observed YBCO grain boundaries manufactured by other methods

  13. Spatially Resolved Mapping of Electrical Conductivity around Individual Domain (Grain) Boundaries in Graphene

    Energy Technology Data Exchange (ETDEWEB)

    Li, An-Ping [ORNL; Clark, Kendal W [ORNL; Zhang, Xiaoguang [ORNL; Vlassiouk, Ivan V [ORNL; He, Guowei [Carnegie Mellon University (CMU); Feenstra, Randall [Carnegie Mellon University (CMU)

    2013-01-01

    Graphene films can now be produced on the scale of up to meters. However, all large-scale graphene films contain topological defects that can significantly affect the characteristic transport behaviors of graphene. Here, we spatially map the structures and electronic transport near specific domain and grain boundaries in graphene, and evaluate effects of different types of defect on the electronic conductivity in epitaxial graphene grown on SiC and CVD graphene on Cu subsequently transferred to a SiO2 substrate. We use a combined approach with a multi-probe scanning tunneling potentiometry to investigate both structures and transport at individual grain boundaries and domain boundaries that are defined by coalesced grains, surface steps, and changes in layer thickness. It is found that the substrate step on SiC presents a significant potential barrier for electron transport of epitaxial graphene due to the reduced charge transport from the substrate at the step edges, monolayer-bilayer boundaries exhibit a high resistivity that can change depending on directions of the current across the boundary, and the resistivity of grain boundaries changes with the transition width of the disordered region between two adjacent grains in graphene. The detailed understanding of graphene defects will provide the feedback for controlled engineering of defects in large-scale graphene films.

  14. Observing grain boundaries in CVD-grown monolayer transition metal dichalcogenides

    KAUST Repository

    Ly, Thuchue

    2014-11-25

    Two-dimensional monolayer transition metal dichalcogenides (TMdCs), driven by graphene science, revisit optical and electronic properties, which are markedly different from bulk characteristics. These properties are easily modified due to accessibility of all the atoms viable to ambient gases, and therefore, there is no guarantee that impurities and defects such as vacancies, grain boundaries, and wrinkles behave as those of ideal bulk. On the other hand, this could be advantageous in engineering such defects. Here, we report a method of observing grain boundary distribution of monolayer TMdCs by a selective oxidation. This was implemented by exposing directly the TMdC layer grown on sapphire without transfer to ultraviolet light irradiation under moisture-rich conditions. The generated oxygen and hydroxyl radicals selectively functionalized defective grain boundaries in TMdCs to provoke morphological changes at the boundary, where the grain boundary distribution was observed by atomic force microscopy and scanning electron microscopy. This paves the way toward the investigation of transport properties engineered by defects and grain boundaries. (Figure Presented).

  15. Binding of He{sub n}V clusters to α-Fe grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Tschopp, M. A., E-mail: mark.a.tschopp.civ@mail.mil [U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 (United States); Gao, F. [Pacific Northwest National Laboratory, Richland, Washington 99352 (United States); Solanki, K. N. [Arizona State University, Tempe, Arizona 85287 (United States)

    2014-06-21

    The objective of this research is to explore the formation/binding energetics and length scales associated with the interaction between He{sub n}V clusters and grain boundaries in bcc α-Fe. In this work, we calculated formation/binding energies for 1–8 He atoms in a monovacancy at all potential grain boundary (GB) sites within 15 Å of the ten grain boundaries selected (122106 simulations total). The present results provide detailed information about the interaction energies and length scales of 1–8 He atoms with grain boundaries for the structures examined. A number of interesting new findings emerge from the present study. First, the Σ3(112) “twin” GB has significantly lower binding energies for all He{sub n}V clusters than all other boundaries in this study. For all grain boundary sites, the effect of the local environment surrounding each site on the He{sub n}V formation and binding energies decreases with an increasing number of He atoms in the He{sub n}V cluster. Based on the calculated dataset, we formulated a model to capture the evolution of the formation and binding energy of He{sub n}V clusters as a function of distance from the GB center, utilizing only constants related to the maximum binding energy and the length scale.

  16. Meso-scale anisotropic hydrogen segregation near grain-boundaries in polycrystalline nickel characterized by EBSD/SIMS

    Energy Technology Data Exchange (ETDEWEB)

    Oudriss, A.; Le Guernic, Solenne; Wang, Zhaoying; Osman Hock, B.; Bouhattate, Jamaa; Conforto, E.; Zhu, Zihua; Li, Dongsheng; Feaugas, Xavier

    2016-02-15

    To study anisotropic hydrogen segregation and diffusion in nickel polycrystalline, Secondary Ion Mass Spectrometry (SIMS) and Electron Back Scattered Diffraction (EBSD) are integrated to investigate hydrogen distribution around grain boundaries. Hydrogen distribution in pre-charged samples were correlated with grain boundary character by integrating high-resolution grain microstructure from EBSD inverse pole figure map and low-resolution hydrogen concentration profile map from SIMS. This multimodal imaging instrumentation shows that grain boundaries in nickel can be categorized into two families based on behavior of hydrogen distribution crossing grain boundary: the first one includes random grain boundaries with fast hydrogen diffusivity, showing a sharp gap for hydrogen concentration profile cross the grain boundaries. The second family are special Σ3n grain boundaries with low hydrogen diffusivity, showing a smooth gradient of hydrogen concentration cross the grain boundary. Heterogeneous hydrogen distributions due to grain boundary family revealed by SIMS/EBSD on mesoscale further validate the recent hydrogen permeation data and anisotropic ab-initio calculations in nanoscale. The results highlight the fact that grain boundaries character impacts hydrogen distribution significantly.

  17. The role of grain boundaries in the storage and transport of noble gases in the mantle

    Science.gov (United States)

    Burnard, Pete G.; Demouchy, Sylvie; Delon, Rémi; Arnaud, Nicolas O.; Marrocchi, Yves; Cordier, Patrick; Addad, Ahmed

    2015-11-01

    Mantle noble gases record important and ancient isotopic heterogeneities, which fundamentally influence our understanding of mantle geodynamics, yet these heterogeneities are difficult to fully interpret without understanding the basic mechanisms of noble gas storage and transport in mantle minerals. A series of annealing experiments that mimic mantle conditions (i.e. sub-solidus with natural, polycrystalline, texturally equilibrated olivines at low noble gas partial pressures) show that intergranular interfaces (grain boundaries) are major hosts for noble gases in the mantle, and that interfaces can dramatically fractionate noble gases from their radio-parents (U + Th and K). Therefore, noble gas isotopic heterogeneities in the mantle could result from grain size variations. Fine-grained lithologies (mylonites and ultramylonites, for example) with more grain boundaries will have lower U/3He ratios (compared to a coarse grained equivalent), which, over time, will preserve higher 3He/4He ratios. As predicted by theory of points defect diffusivity, these results show that noble gas diffusion along interfaces is different from those in the grain lattice itself at low temperatures. However, for grain size relevant of the Earth's mantle, the resulting effective correlated activation energies (Ea) and pre-exponential factors (Do /a2) produce similar diffusivities at mantle temperatures for interface- and lattice-hosted helium. Therefore, grain boundaries do not significantly affect helium transport at mantle conditions and length scales.

  18. Molecular dynamics study of grain boundary structure and properties at high temperatures

    Science.gov (United States)

    Fensin, Saryu Jindal

    This thesis reports research involving the development and application of atomistic simulation methods to study the effects of high homologous temperatures on the structural, thermodynamic, kinetic and mechanical properties of grain boundaries in metals. Our interest in these properties is due to the role they play in governing the evolution of microstructure and deformation of metals during solidification processing. The interest in developing more predictive models for the formation of solidification defects highlights a need to better understand the thermodynamic driving forces underlying grain-boundary premelting and the mobility and shear strength of these interfaces at high temperatures. In this work we study two different elemental systems, namely Ni and Cu, and consider a variety of grain boundary structures characterized by different misorientation angles, twist/tilt character and zero-temperature energies. A method to calculate the disjoining potential from molecular dynamics (MD) is developed and applied to grain boundaries in Ni. The disjoining potential characterizes the variation in grain-boundary free energy as a function of the width of a premelted interfacial layer. The MD method for the calculation of this property is applied to grain boundaries that display continuous premelting transitions, as well as a boundary characterized by a disordered atomic structure displaying a finite interfacial width at the melting temperature. The disjoining potential represents an important input property to larger scale models of solidification and grain coalescence. We further develop analysis methods to characterize the change in the atomic structure of an asymmetric tilt grain boundary in elemental Cu as a function of temperature. This boundary is characterized by a potential-energy surface with multiple minima as a function of the relative translation of the grains parallel to the interface plane. The more complex structure of this boundary, relative to the

  19. Effect of grain boundary misorientation on discontinuous precipitation in an AZ91 alloy

    Indian Academy of Sciences (India)

    H Azzeddine; S Abdessameud; B Alili; Z Boumerzoug; D Bradai

    2011-12-01

    A scanning electron microscopy (SEM)-based electron backscatter diffraction (EBSD) analysis showed that the discontinuous precipitation (DP) reaction rate was dependent on the geometry of the grain boundary in Mg–9Al–1Zn (wt.%) alloys. DP converted a supersaturated solid solution, 0 (magnesium (Mg)-rich solid solution), into a two-phase + aggregate, with being a precipitated Mg17Al12 (intermetallic phase) behind a migrating reaction front. The near-special grain boundary was rather inactive, whereas most of the random high-angle boundaries promoted the reaction. Prior deformation (hot rolling to achieve up to 80% thickness reduction) had no effect on the frequency of special-grain boundaries.

  20. Atomistic simulation of grain boundary structure in a series of B2 intermetallics

    Energy Technology Data Exchange (ETDEWEB)

    Mutasa, B. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Materials Engineering; Farkas, D. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Materials Engineering

    1996-08-01

    Using molecular statics and interatomic potentials of the embedded atom type, the relaxed atomistic grain boundary structures in B2 aluminides were investigated in order to study trends in a series of B2 compounds. The compounds studied: FeAl, NiAl and CoAl show increasing anti-phase boundary energies. The atomistic structure of the {Sigma}=5(310)[100] and {Sigma}=5(210)[100] symmetrical tilt grain boundaries in these compounds was studied considering possible variations of local chemical composition on grain boundary energetics. The structures obtained for the three alloys are very similar. A discussion of the trends in energetics across this series of compounds is entered into. (orig.)

  1. Grain-boundary-enhanced carrier collection in CdTe solar cells.

    Science.gov (United States)

    Li, Chen; Wu, Yelong; Poplawsky, Jonathan; Pennycook, Timothy J; Paudel, Naba; Yin, Wanjian; Haigh, Sarah J; Oxley, Mark P; Lupini, Andrew R; Al-Jassim, Mowafak; Pennycook, Stephen J; Yan, Yanfa

    2014-04-18

    When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to carrier collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells. PMID:24785058

  2. Identification of sub-grains and low angle boundaries beyond the angular resolution of EBSD maps

    Energy Technology Data Exchange (ETDEWEB)

    Germain, L., E-mail: Lionel.germain@univ-lorraine.fr [Laboratoire d' Etude des Microstructures et de Mécanique des Matériaux (LEM3), UMR 7239, CNRS/Université de Lorraine, F-57045 Metz (France); Laboratory of Excellence on Design of Alloy Metals for low-mAss Structures (‘LabEx DAMAS’), Université de Lorraine (France); Kratsch, D. [Laboratoire d' Informatique Théorique et Appliquée (LITA), EA3079, Université de Lorraine, 57045 Metz Cedex 1 (France); Salib, M. [Laboratoire d' Etude des Microstructures et de Mécanique des Matériaux (LEM3), UMR 7239, CNRS/Université de Lorraine, F-57045 Metz (France); Laboratory of Excellence on Design of Alloy Metals for low-mAss Structures (‘LabEx DAMAS’), Université de Lorraine (France); Institut Jean Lamour (IJL), SI2M Dept., CNRS UMR 7198, Université de Lorraine, Parc de Saurupt, CS 50840, F-54011 Nancy Cedex (France); Gey, N. [Laboratoire d' Etude des Microstructures et de Mécanique des Matériaux (LEM3), UMR 7239, CNRS/Université de Lorraine, F-57045 Metz (France); Laboratory of Excellence on Design of Alloy Metals for low-mAss Structures (‘LabEx DAMAS’), Université de Lorraine (France)

    2014-12-15

    A new method called ALGrId (Anti-Leak GRain IDentification) is proposed for the detection of sub-grains beyond the relative angular resolution of Electron Backscatter Diffraction maps. It does not use any additional information such as Kikuchi Pattern Quality map nor need data filtering. It uses a modified Dijkstra algorithm which seeks the continuous set of boundaries having the highest average disorientation angle. - Highlights: • ALGrId is a new method to identify sub-grains and low angle boundaries in EBSD maps. • Unlike classical methods, ALGrId works even beyond the relative angular resolution. • If the orientation noise peaks at 0.7°, ALGrid detects 0.4°-boundaries correctly. • In the same example, the classical algorithm identifies 1.1°-boundaries only.

  3. A broken-bond model for grain boundaries in face-centered cubic metals

    Energy Technology Data Exchange (ETDEWEB)

    Wolf, D. (Materials Science Division, Argonne National Laboratory, Argonne, IL (USA))

    1990-10-01

    The interrelation between the number of nearest-neighbor atomic bonds broken upon formation of a grain boundary in an fcc metal and the related zero-temperature boundary energy is investigated by atomistic simulation. Using both a Lennard--Jones and an embedded-atom-method potential, the structures and energies of symmetrical and asymmetrical tilt and twist boundaries are determined. As in free surfaces, a practically linear relationship between the nearest-neighbor miscoordination per unit area of the grain boundary and the related interface energy is obtained. The so-called random-boundary model, in which the interactions across the interface are assumed to be entirely randomized, is shown to provide a basis for understanding the role of broken bonds in both high-angle grain boundaries and free surfaces, thus naturally permitting the analysis of ideal cleavage-fracture energies. A detailed study of low-angle boundaries shows that only the dislocation cores---but not their strain fields---give rise to broken bonds. The complementarity between the dislocation model of Read and Shockley for low-angle boundaries and a broken-bond model for high-angle boundaries is thus elucidated.

  4. Roughness of grain boundaries in partly recrystallized aluminum

    DEFF Research Database (Denmark)

    Sun, Jun; Zhang, Yubin; Juul Jensen, Dorte

    2016-01-01

    cold rolled aluminum samples. The results show that particle pinning is not the main reason accounting for recrystallization boundary roughness in the present samples. The roughness is however shown to relate to the deformation microstructure and possible effects of migration rate are discussed...

  5. The effect of strain on grains and grain boundaries in YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} coated conductors

    Energy Technology Data Exchange (ETDEWEB)

    Van der Laan, D C [Department of Physics, University of Colorado, Boulder, CO 80309 (United States); Haugan, T J; Barnes, P N [Air Force Research Laboratory, Wright-Patterson AFB, OH 45433 (United States); Abraimov, D; Kametani, F; Larbalestier, D C [National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310 (United States); Rupich, M W, E-mail: danko@boulder.nist.go [American Superconductor Corporation, Westborough, MA 01598 (United States)

    2010-01-15

    The role of grains and grain boundaries in producing reversible strain effects on the transport current critical current density (J{sub c}) of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBCO) coated conductors that are produced with metal-organic deposition (MOD) was investigated. The strain ({epsilon}) dependence of J{sub c} for full-width coated conductors is compared with that for samples in which the current transport was limited to a few or single grain boundaries by cutting narrow tracks with a laser or focused ion beam, as well as with thin films deposited on bicrystalline SrTiO{sub 3} substrates by use of pulsed-laser deposition (PLD). Our results show that the dependences of J{sub c} on {epsilon} for the grains and for the grain boundaries from the two kinds of YBCO samples can be expressed by the same function, however with a greater effective tensile strain at the grain boundaries than in the grains. The really striking result is that the grain boundary strain is 5-10 times higher for grain boundaries of in situ PLD grown bicrystals as compared to the aperiodic, meandered, nonplanar grain boundaries that develop in ex situ grown MOD-YBCO in the coated conductor of this study.

  6. Tribological characteristics of few-layer graphene over Ni grain and interface boundaries

    Science.gov (United States)

    Tripathi, Manoj; Awaja, Firas; Paolicelli, Guido; Bartali, Ruben; Iacob, Erica; Valeri, Sergio; Ryu, Seunghwa; Signetti, Stefano; Speranza, Giorgio; Pugno, Nicola Maria

    2016-03-01

    The tribological properties of metal-supported few-layered graphene depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the frictional and wetting characteristics of the FLG system. We discretely measured friction characteristics of FLG covering grains and interfacial grain boundaries of polycrystalline Ni metal substrate via an atomic force microscopy (AFM) probe. The friction coefficient of FLG covered at interfacial grain boundaries is found to be lower than that on grains in vacuum (at 10-5 Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull-off adhesion forces were addressed in the course of friction measurements to explain the role of the out-of-plane deformation of graphene layer(s). Finite element simulations showed good agreement with experiments and led to a rationalization of the observations. Thus, with interfacial grain boundaries the FLG tribology can be effectively tuned.The tribological properties of metal-supported few-layered graphene depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the frictional and wetting characteristics of the FLG system. We discretely measured friction characteristics of FLG covering grains and interfacial grain boundaries of polycrystalline Ni metal substrate via an atomic force microscopy (AFM) probe. The friction coefficient of FLG covered at interfacial grain boundaries is found to be lower than that on grains in vacuum (at 10-5 Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull

  7. Effect of grain boundaries on the conductivity of high-purity ZrO2---Y2O3 ceramics

    NARCIS (Netherlands)

    Verkerk, M.J.; Middelhuis, B.J.; Burggraaf, A.J.

    1982-01-01

    Grain boundary conductivities are determined by complex impedance measurements (1–106 Hz) on high-purity ceramics prepared by the alkoxide synthesis and on less pure ceramics obtained from a commercial powder. The grain size was varied systematically in the region 0.36–55 μm. The grain boundary cond

  8. Structure and chemistry of grain boundaries in SiO2-doped TZP

    Directory of Open Access Journals (Sweden)

    Yuichi Ikuhara, Takahisa Yamamoto, Akihide Kuwabara, Hidehiro Yoshida and Taketo Sakuma

    2001-01-01

    Full Text Available The addition of glass phase can control the grain boundary structure and hence the mechanical properties of tetragonal zirconia polycrystals (TZP. To reveal the effect of the glass dopant on the high-temperature deformation behavior of TZP, SiO2-doped TZP, (SiO2–Al2O3-doped TZP, (SiO2–MgO-doped TZP and undoped TZP were prepared and their grain boundary structure, chemical composition and chemical bonding state were investigated by high resolution electron microscopy (HREM, energy dispersive X-ray spectroscopy (EDS and electron energy loss spectroscopy (EELS using a field-emission-type transmission electron microscope (FE-TEM. It was found that no amorphous film was formed along the grain boundaries in any of the specimens examined, but amorphous pockets formed at multiple grain boundary junctions in three kinds of glass-doped specimens. In the glass-doped specimens, the segregation of yttrium, silicon and the added metal ions (Al3+ or Mg2+ was observed over a width of several nm across the grain boundaries. The addition of pure SiO2 much enhanced the ductility in TZP, although further addition of a small amount of Al2O3 or MgO to SiO2 phase resulted in a marked reduction in the tensile ductility of SiO2-doped TZP. EELS measurements and molecular orbital (MO calculations using a cluster model revealed that the ductility of TZP was related to the bond overlap population (BOP at the grain boundaries, which was influenced by the kinds of segregated dopants. That is, the presence of Si4+ increases the BOP, strengthening the grain boundary bonding strength and thus preventing cavity formation, but Al3+ and Mg2+ decrease the BOP, enhancing the grain boundary cavitation and thus reducing the ductility. Furthermore, the dynamic behavior of SiO2 in TZP was observed using a TEM in situ heating technique, and the results supported the fact that that Si segregates along the grain boundaries.

  9. Atomistic investigation of the structure and transport properties of tilt grain boundaries of UO{sub 2}

    Energy Technology Data Exchange (ETDEWEB)

    Williams, Nicholas R. [Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY (United Kingdom); AWE, Aldermaston, Reading, Berkshire RG7 4PR (United Kingdom); Molinari, Marco [Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY (United Kingdom); Parker, Stephen C., E-mail: s.c.parker@bath.ac.uk [Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY (United Kingdom); Storr, Mark T. [AWE, Aldermaston, Reading, Berkshire RG7 4PR (United Kingdom)

    2015-03-15

    We apply atomistic simulation techniques to address whether oxygen shows higher diffusivity at the grain boundary region compared to that in bulk UO{sub 2}, and whether the relative diffusivity is affected by the choice of the grain boundary. We consider coincident site lattice grain boundaries, Σ3, Σ5, Σ9, Σ11 and Σ19, expressing the {n n 1}, {n 1 1}, and {n 1 0} surfaces, and evaluate the extent that the grain boundary structures affect the diffusion of oxygen. We found that oxygen diffusion is enhanced at all boundaries and in the adjacent regions, with strong dependence on the temperature and local structure.

  10. Formation of Highly Misoriented Fragments at Hot Band Grain Boundaries During Cold Rolling of Interstitial-Free Steel

    Science.gov (United States)

    Afrin, Nasima; Quadir, Md. Zakaria; Ferry, Michael

    2015-07-01

    The deformation heterogeneities that form in the vicinity of prior hot band grain boundaries in a 75 pct cold-rolled interstitial-free steel have been investigated by 3D electron backscatter diffraction. Grain boundary-affected regions occupy a large fraction of the overall material volume. The coexistence of several features, such as steep orientation gradients up to 5 deg/ μm, high-angle boundary networks, and thin, elongated grain boundary fragments, has confirmed the highly complex nature of these regions. Most notably, these thin boundary fragments were found to be significantly misoriented from any of the deformed grains immediately adjacent to the boundary. Overall, grain boundary regions adopt the so-called `deformation banding' mode of deformations on both the micro ( e.g., steep gradients)- and nano ( e.g., thin fragments)-length scales. Grain boundary structures comprise the essential features to act as preferred sites for recrystallization. The discovery of numerous thin grain boundary fragments in the deformation microstructure provides a plausible explanation for the origin of recrystallized grains with orientations other than those found within the adjoining deformed grains in the vicinity of grain boundaries; this phenomenon has been commonly observed in texture data for many years but remained unexplained.

  11. Fractal pattern formation in thermal grooving at grain boundaries in Ag films on Si(111) surfaces

    Energy Technology Data Exchange (ETDEWEB)

    Roy, A. [Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032 (India); Sundaravel, B. [Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102 (India); Batabyal, R. [Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032 (India); Dev, B.N., E-mail: msbnd@iacs.res.in [Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032 (India)

    2012-05-31

    Growth of Ag films on Br- and H-passivated Si(111) surfaces and the annealing behaviour have been investigated by Rutherford backscattering spectrometry, scanning electron microscopy and photoemission electron microscopy techniques. Upon annealing the phenomenon of thermal grooving was observed in the Ag films. Depending on the annealing temperature, at an intermediate annealing time Ag depletion (evaporation) from the grain boundaries produces fractal patterns of Ag-depleted regions. Continued annealing eventually produces a percolated network of Ag-depleted regions (thermal grooves) along the grain boundaries and isolated Ag grains appear as the depth of the grooves reaches the substrate. For the fractal structures produced by thermal grooving, the fractal dimension has been estimated to be 1.60 {+-} 0.04. Observation of a fractal pattern in thermal grooving was not hitherto reported. A thorough analysis of the experimental results has been carried out in the context of current theories. These theories are inadequate to describe the experimental results. - Highlights: Black-Right-Pointing-Pointer Thermal grooving in Ag films grown on Si(111) surfaces has been investigated. Black-Right-Pointing-Pointer At appropriate temperatures short annealing shows partly depleted grain boundaries. Black-Right-Pointing-Pointer Depleted grain boundaries reveal a fractal pattern with a fractal dimension of 1.60. Black-Right-Pointing-Pointer Long annealing produces percolated Ag-depleted regions with isolated Ag grains. Black-Right-Pointing-Pointer A thorough analysis shows that current theories are unable to explain the results.

  12. Effect of molybdenum on grain boundary segregation in Incoloy 901 superalloy

    International Nuclear Information System (INIS)

    Highlights: ► Grain boundary segregation in superalloys can be decreased by controlling of the chemical composition of alloys. ► One of the most effective element on decreasing of grain boundary segregation is Molybdenum. ► The Mo addition up to 6.7 % have a suitable effect on decreasing of grain boundary segregation of other elements.. ► The partitioning coefficients of all elements except Fe and Ni are less than one. - Abstract: In this paper, the effect of molybdenum on the grain boundary segregation of other elements was studied in Incoloy 901 superalloy. Initially, five alloys were prepared with different percentages of Mo by using a vacuum induction furnace. Then, these alloys were remelted by Electro-slag remelting (ESR) process and after homogenizing at 1160 °C for 2 h followed by air cooling, were rolled. The effect of Mo on segregation of elements was evaluated with Scanning Electron Microscopy, Linear Analysis, and the mechanical tests. The results showed that the grain boundary segregations of elements in Incoloy 901 superalloy were decreased by increasing of molybdenum content up to 6.7% and the mechanical properties (tensile and hardness properties) were improved. Also, the segregations of elements were increased by increasing the percentage of Mo from 6.7 to 7.5, and the mechanical properties were reduced

  13. A Computational Investigation of Random Angle Grain Boundaries for CdTe Solar Cells

    Science.gov (United States)

    Buurma, Christopher; Chan, Maria; Klie, Robert; Sivananthan, Sivalingam; DOE Bridge Collaboration

    2015-03-01

    Grain boundaries (GB) in poly-CdTe solar cells play an important role in species diffusion, segregation, defect formation, and carrier recombination. Many studies on GBs in CdTe focus on either entire grain-boundary networks found in complete poly-CdTe devices, those exhibiting high symmetry such as the coincident site lattice (CSL) or symmetric tilt or twist, or on very small scale Scanning-Tunneling Electron Microscopse (STEM) viewable interfaces and dislocations. The topic of this talk is a comprehensive survey of the grain boundary parameter space regardless of the degree of symmetry found and whether the STEM channeling condition is satisfied. Our survey encompasses both near-CSL or vicinal grain boundaries decorated with nearby dislocations, as well as mixed tilt and twist interfaces with all possible symmetrically inequivalent grain boundary planes. Atomistic calculations using a Stillinger-Weber potential will be presented on a large representative sample of random-angle GBs. Trends in interfacial energies and atomistic structures as a function of tilt/twist/displacement parameters will be investigated. First principles density functional theory (DFT) calculations will be performed on a subset of these GBs to reveal their electronic structures and their implications towards PV performance. DoE Sunshot program contract DOE DEEE005956. Use of the Center for Nanoscale Materials was supported by the USDoE, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

  14. Finite temperature effect on mechanical properties of graphene sheets with various grain boundaries

    Science.gov (United States)

    Yong, Ge; Hong-Xiang, Sun; Yi-Jun, Guan; Gan-He, Zeng

    2016-06-01

    The mechanical properties of graphene sheets with various grain boundaries are studied by molecular dynamics method at finite temperatures. The finite temperature reduces the ultimate strengths of the graphenes with different types of grain boundaries. More interestingly, at high temperatures, the ultimate strengths of the graphene with the zigzag-orientation grain boundaries at low tilt angles exhibit different behaviors from those at lower temperatures, which is determined by inner initial stress in grain boundaries. The results indicate that the finite temperature, especially the high one, has a significant effect on the ultimate strength of graphene with grain boundaries, which gives a more in-depth understanding of their mechanical properties and could be useful for potential graphene applications. Project supported by the Nation Natural Science Foundation of China (Grant Nos. 11347219 and 11404147), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20140519), the Training Project of Young Backbone Teacher of Jiangsu University, the Advanced Talents of Jiangsu University, China (Grant No. 11JDG118), the Practice Innovation Training Program Projects for Industrial Center of Jiangsu University, China, and the State Key Laboratory of Acoustics, Chinese Academy of Sciences (Grant No. SKLOA201308).

  15. Effects of grain boundary sliding on the flow properties of Incoloy 800H

    Energy Technology Data Exchange (ETDEWEB)

    Wilson, H.; Korhonen, M.A.; Li Cheyu (Dept. of Materials Science and Enginering, Cornell Univ., Ithaca, NY (United States))

    1992-08-01

    The nature of grain boundary sliding (GBS) is investigated in Incoloy 800H in terms of the effects of stress, temperature and grain size on the flow behavior observed by using the load relaxation test. Flow behaviors are obtained for average grain sizes ranging from 6 to 225 {mu}m at temperatures between 614 and 746degC. The flow behavior of large-grain-size material plotted as stress vs. strain rate in a doubly logarithmic scale, exhibits a sigmoidal shape which has been commonly associated with the effects of GBS on creep deformation. For the materials of smaller grain sizes the deformation properties tend toward those characteristic of structural superplasticity. It is shown that in Incoloy 800H there may exist, as a function of the grain size, a continuous scale of flow properties ranging from the normal creep to superplastic-like behavior. (orig.).

  16. Reconstruction of 3d grain boundaries from rock thin sections, using polarised light

    Science.gov (United States)

    Markus Hammes, Daniel; Peternell, Mark

    2016-04-01

    Grain boundaries affect the physical and chemical properties of polycrystalline materials significantly by initiating reactions and collecting impurities (Birchenall, 1959), and play an essential role in recrystallization (Doherty et al. 1997). In particular, the shape and crystallographic orientation of grain boundaries reveal the deformation and annealing history of rocks (Kruhl and Peternell 2002, Kuntcheva et al. 2006). However, there is a lack of non-destructive and easy-to-use computer supported methods to determine grain boundary geometries in 3D. The only available instrument using optical light to measure grain boundary angles is still the polarising microscope with attached universal stage; operated manually and time-consuming in use. Here we present a new approach to determine 3d grain boundary orientations from 2D rock thin sections. The data is recorded by using an automatic fabric analyser microscope (Peternell et al., 2010). Due to its unique arrangement of 9 light directions the highest birefringence colour due to each light direction and crystal orientation (retardation) can be determined at each pixel in the field of view. Retardation profiles across grain boundaries enable the calculation of grain boundary angle and direction. The data for all positions separating the grains are combined and further processed. In combination with the lateral position of the grain boundary, acquired using the FAME software (Hammes and Peternell, in review), the data is used to reconstruct a 3d grain boundary model. The processing of data is almost fully automatic by using MATLAB®. Only minor manual input is required. The applicability was demonstrated on quartzite samples, but the method is not solely restricted on quartz grains and other birefringent polycrystalline materials could be used instead. References: Birchenall, C.E., 1959: Physical Metallurgy. McGraw-Hill, New York. Doherty, R.D., Hughes, D.A., Humphreys, F.J., Jonas, J.J., Juul Jensen, D., Kassner, M

  17. MARMOT simulations of Xe segregation to grain boundaries in UO2

    Energy Technology Data Exchange (ETDEWEB)

    Andersson, Anders D. [Los Alamos National Laboratory; Tonks, Michael [Idaho National Laboratory; Casillas, Luis [Los Alamos National Laboratory; Millett, Paul [Idaho National Laboratory; Vyas, Shyam [Los Alamos National Laboratory; Uberuaga, Blas P. [Los Alamos National Laboratory; Nerikar, Pankaj [IBM

    2012-06-20

    Diffusion of Xe and U in UO{sub 2} is controlled by vacancy mechanisms and under irradiation the formation of mobile vacancy clusters is important. We derive continuum thermodynamic and diffusion models for Xe and U in UO{sub 2} based on the vacancy and cluster diffusion mechanisms established from recent density functional theory (DFT) calculations. Segregation of defects to grain boundaries in UO{sub 2} is described by combining the diffusion model with models of the interaction between Xe atoms and vacancies with grain boundaries derived from separate atomistic calculations. The diffusion and segregation models are implemented in the MOOSE/MARMOT (MBM) finite element (FEM) framework and we simulate Xe redistribution for a few simple microstructures. In this report we focus on segregation to grain boundaries. The U or vacancy diffusion model as well as the coupled diffusion of vacancies and Xe have also been implemented, but results are not included in this report.

  18. Equivalent Plastic Strain Gradient Plasticity with Grain Boundary Hardening and Comparison to Discrete Dislocation Dynamics

    CERN Document Server

    Bayerschen, E; Wulfinghoff, S; Weygand, D; Böhlke, T

    2015-01-01

    The gradient crystal plasticity framework of Wulfinghoff et al. [53] incorporating an equivalent plastic strain and grain boundary yielding, is extended with additional grain boundary hardening. By comparison to averaged results from many discrete dislocation dynamics (DDD) simulations of an aluminum type tricrystal under tensile loading, the new hardening parameter in the continuum model is calibrated. It is shown that although the grain boundaries (GBs) in the discrete simulations are impenetrable, an infinite GB yield strength corresponding to microhard GB conditions, is not applicable in the continuum model. A combination of a finite GB yield strength with an isotropic bulk Voce hardening relation alone also fails to model the plastic strain profiles obtained by DDD. Instead, a finite GB yield strength in combination with GB hardening depending on the equivalent plastic strain at the GBs is shown to give a better agreement to DDD results. The differences in the plastic strain profiles obtained in DDD simu...

  19. Phase-field study of grain boundary tracking behavior in crack-seal microstructures

    CERN Document Server

    Ankit, Kumar; Selzer, Michael; Reichardt, Mathias

    2012-01-01

    In order to address the vein-growth problem in geology, a multi-phase-field model is used to capture the dynamics of crystals precipitating from a super-saturated solution. To gain a complete understanding, we investigate the influence of various boundary conditions on crystal growth (free-growth and crack-sealing) that result in formation of vein microstructures. To begin with, we consider the anisotropy in surface energy to simulate crystals (with flat facets and sharp corners) possessing different orientations and study the resulting growth competition to deduce a consistent orientation selection rule in the free-growth regime. Next, from crack-sealing simulations, we co-relate the grain boundary tracking behavior and the relative rates of crack opening and trajectory, initial grain size and wall roughness. Further, we illustrate how these parameters induce the microstructural transition between blocky (crystals growing anisotropically) to fibrous morphology (isotropic) and formation of grain boundaries. T...

  20. Physical behaviors of impure atoms during relaxation of impure NiAl-based alloy grain boundary

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    The Monte Carlo simulation with the energetics described by the embedded atom method has been employed to mainly study physical behaviors of boron atoms during relaxation of the Ni3Al-x at.% B grain boundary. During relaxation of impure Ni3Al grain boundaries, we suggest that for different types of impure atoms(Mg, B, Cr and Zr atoms etc.), as the segregating species, they have the different behaviors, but as the inducing species, they have the same behaviors, i.e. they all induce Ni atoms to substitute Al atoms. Calculations show that at the equilibrium, when x(the B bulk concentration) increases from 0.1 to 0.9 , the peak concentration of B increases, correspondently, the peak concentration of Ni maximizes but the valley concentration of Al minimizes, at x = 0.5. The calculations also show the approximate saturation of Ni at the grain boundary at x = 0.5.

  1. Relating the grain boundary microchemistry of alloy 600 to the DL-EPR response

    International Nuclear Information System (INIS)

    The double loop electrochemical potentiokinetic reactivation (DL-EPR) response of Alloy 600 was characterized for a variety of grain boundary Cr depletion profiles. Solutions containing H2SO4 with either KSCN, Na2S4O6 or SC(NH2)2 were studied for their sensitivity to pitting and variations in the grain boundary Cr minimum and width of the depletion zone. Results show that a solution consisting of 0.01IM H2SO4 + 0.0001M KSCN has good sensitivity to the grain boundary Cr minimum and produced minimal pitting. A solution of 0.5M H2SO4 + 0.001M SC(NH2)2 was slightly more sensitive to small amounts of Cr depletion than the KSCN solution. Solutions containing H2SO4 + Na2S4O6 were not successful as they did not consistently repassivate all heat treat conditions

  2. Plastic deformation-induced phosphorus segregation to ferrite grain boundaries in an interstitial free steel

    International Nuclear Information System (INIS)

    Research highlights: → Plastic deformation causes non-equilibrium grain boundary phosphorus segregation. → Deformation induced segregation increases with increasing deformation rate. → Non-equilibrium segregation is induced by supersaturated vacancy-phosphorus complex. → Model predictions show a reasonable agreement with the observations. - Abstract: Grain boundary concentration of phosphorus in an interstitial free steel is observed by virtue of Auger electron spectroscopy after the alloy is plastically deformed to different strains under different strain rates at a high temperature in the ferrite region. The results reveal that phosphorus segregates at grain boundaries during plastic deformation. The segregation increases with increasing deformation until reaching a steady value, and at the same deformation amount it increases with increasing strain rate. Model predictions are made, which shows a reasonable agreement between the predictions and the observations.

  3. The influence of tilt grain boundaries on the mechanical properties of bicrystalline graphene nanoribbons

    Science.gov (United States)

    Xu, Na; Guo, Jian-Gang; Cui, Zhen

    2016-10-01

    The mechanical properties of bicrystalline graphene nanoribbons with various tilt grain boundaries (GBs) which typically consist of repeating pentagon-heptagon ring defects are investigated based on the method of molecular structural mechanics. The GB models are constructed via the theory of disclinations in crystals, and the elastic properties and ultimate strength of bicrystalline graphene nanoribbons are calculated under uniaxial tensile loads in perpendicular and parallel directions to grain boundaries. The dependence of mechanical properties is analyzed on the chirality and misorientation angles of graphene nanoribbons, and the experimental phenomena that Young's modulus and ultimate strength of bicrystalline graphene nanoribbons can either increase or decrease with the grain boundary angles are further verified and discussed. In addition, the influence of GB on the size effects of graphene Young's modulus is also analyzed.

  4. Effect of Second-phase Particles on Static Adjustment of Calcite Grain Boundaries in Carbonate Mylonites

    Science.gov (United States)

    Ree, J.; Lee, S.; Jung, H.

    2010-12-01

    Static adjustment of grain boundaries during the waning stage of deformation with sustained heat (e.g. at the end of an orogeny) has not been studied much, although it is important for the interpretation of microstructural status during the main stage of deformation. We report here that static adjustment of calcite grain boundaries is dependent on second-phase particles in carbonate mylonites from the Geounri Shear Zone in the Taebaeksan Basin of South Korea. The carbonate mylonites consist of relic (porphyroclastic) calcites (120-400 μm) and dynamically recrystallized calcites (30-35 μm) with second-phase particles (15-20 μm) of quartz and phyllosilicates. Both calcite grains contain mechanical twins and the twins are wider (10-20 μm thick) in the relic calcites than in the dynamically recrystallized ones (1-3 μm thick). In the layers of carbonate mylonite with less than 3% of second phases, grain boundaries of calcites are straight with triple junctions. In contrast, calcite grain boundaries are lobate to wavy in the layers with more than 3% of second phases, suggesting dynamic grain boundary migration. Calcite grains in both layers show a strong lattice preferred orientation indicating dominant slip system of basal with minor one of rhomb . We interpret that the foam texture of calcite in the mylonite layers with less than 3% of second phases was produced during the waning stage of the main deformation with a sustained heat since both syntectonic and posttectonic chloritoid porphyroblasts occur in adjacent phyllonite layers in the shear zone. 3% volume fraction of second-phase particles might be a critical value above which deformation microstructures of the main phase were ‘frozen’ without static adjustment in our carbonate mylonites.

  5. Effect of -bar 2 grain boundaries on plastic deformation of WC-Co cemented carbides

    Energy Technology Data Exchange (ETDEWEB)

    Ostberg, G. [Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden)]. E-mail: gusto@fy.chalmers.se; Farooq, M.U. [Department of Materials and Manufacturing Technology, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden); Christensen, M. [Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden); Andren, H.-O. [Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden); Klement, U. [Department of Materials and Manufacturing Technology, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden); Wahnstroem, G. [Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden)

    2006-01-25

    Cutting inserts of WC-6wt.% Co were investigated before and after plastic deformation. The deformation tests were performed with a turning operation under realistic, yet controlled, conditions. SEM studies showed that after deformation the WC skeleton structure had broken up and thin lamellae of binder phase had formed in less than 10% of the grain boundaries. Ab initio calculations showed that -bar 2 twist WC/WC boundaries have a high work of separation and the interface energy is not lowered by forming two Co/WC boundaries. Electron backscattered diffraction (EBSD) measurements showed an apparent increase in the relative occurrence of -bar 2 boundaries due to poor indexing of the deformed material. The accumulation of dislocations in general boundaries is held responsible for the difficulty in identifying them compared with -bar 2 boundaries. At the -bar 2 tilt boundary the grains have the glide planes {l_brace}101-bar 0{r_brace} in common and at the -bar 2 twist boundary there are three glide planes intersecting, which facilitate dislocation movements across the -bar 2 boundaries.

  6. Thermal stability of interface voids in Cu grain boundaries with molecular dynamic simulations

    Science.gov (United States)

    Xydou, A.; Parviainen, S.; Aicheler, M.; Djurabekova, F.

    2016-09-01

    By means of molecular dynamic simulations, the stability of cylindrical voids is examined with respect to the diffusion bonding procedure. To do this, the effect of grain boundaries between the grains of different crystallographic orientations on the void closing time was studied at high temperatures from 0.7 up to 0.94 of the bulk melting temperature ({{T}\\text{m}} ). The diameter of the voids varied from 3.5 to 6.5 nm. A thermal instability occurring at high temperatures at the surface of the void placed in a grain boundary triggered the eventual closure of the void at all examined temperatures. The closing time has an exponential dependence on the examined temperature values. A model based on the defect diffusion theory is developed to predict the closing time for voids of macroscopic size. The diffusion coefficient within the grain boundaries is found to be overall higher than the diffusion coefficient in the region around the void surface. The activation energy for the diffusion in the grain boundary is calculated based on molecular dynamic simulations. This value agrees well with the experimental given in the Ashby maps for the creep in copper via Coble GB diffusion.

  7. Estimation of grain boundary diffusivity in near-α titanium polycrystals

    International Nuclear Information System (INIS)

    The role of enhanced grain boundary diffusivity in high-temperature diffusion of interstitial elements through metals is widely recognized but poorly characterized in most materials. This paper summarizes an effort to estimate grain boundary diffusivity of oxygen in a near-α titanium alloy, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, by explicitly incorporating microstructure obtained from electron backscatter diffraction into an analytical model. Attention is focused on near-surface diffusion behavior contributing to the rapid ingress of oxygen and possible crack initiation in high-temperature environments.

  8. Correlation between electrical activity and various structures of Ge grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Palais, O [Laboratoire TECSEN, UMR 6122-CNRS, Faculte des Sciences et Techniques de St Jerome, F-13397 Marseille, Cedex 20 (France); Lamzatouar, A [Laboratoire TECSEN, UMR 6122-CNRS, Faculte des Sciences et Techniques de St Jerome, F-13397 Marseille, Cedex 20 (France); Duparc, O B M Hardouin [LSI, UMR 7642-CNRS-CEA, Ecole Polytechnique, 91128 Palaiseau Cedex (France); Thibault, J [CEA/DRFMC, 17 rue des Martyrs, 38054 Grenoble (France); Chara, A [Laboratoire TECSEN, UMR 6122-CNRS, Faculte des Sciences et Techniques de St Jerome, F-13397 Marseille, Cedex 20 (France)

    2004-01-21

    The links between the electrical activity and the atomic structure of various Ge grain boundaries (GBs) are investigated. The atomic structure is studied using high resolution electron microscopy, while the electrical activity is evaluated thanks to the measurement of minority carrier lifetime by means of the contactless microwave phase shift technique. Results show that in the {sigma} = 51 GB the electrical activity depends on the atomic structure connected to the configuration of the grain boundary, i.e. tilt, twist or mixed. Lower energy structures such as {sigma} = 3 and 9 GBs appear not to be recombinant.

  9. Depairing current density through a low-angle grain boundary in a superconducting film

    Directory of Open Access Journals (Sweden)

    Feng Xue

    2016-05-01

    Full Text Available In this paper, the effect of a grain boundary (GB on the depairing current density of a high-temperature superconducting film is investigated. The modified effective free energy is proposed by considering the interaction of the superconducting condensate with the deformation of the superconductor due to the dislocations which constitute a grain boundary. After the elastic strain field of the dislocation is obtained, we analyzed the depress effect of the GB on the depairing current density of a superconducting film. The results are qualitatively agreement with the classic exponential relationship with the misorientation angles of the critical current density of high-temperature superconductors.

  10. Supercurrent through cuprate grain boundaries in the presence of strong correlations

    Energy Technology Data Exchange (ETDEWEB)

    Wolf, Fabian Alexander; Graser, Siegfried; Loder, Florian; Kopp, Thilo [Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86135 Augsburg (Germany)

    2012-07-01

    Strong correlations are known to severely reduce the mobility of charge carriers near half-filling and thus have an important influence on the current carrying properties of grain boundaries in high-T{sub c} cuprates. We apply a Gutzwiller method to investigate the critical current through microscopically reconstructed grain boundaries for a wide range of misalignment angles. In good agreement with experimental data, we find a reduction of the current by one order of magnitude as compared to an analogous weak coupling evaluation. This reduction emerges from the interplay of charge fluctuations and strong correlations.

  11. Adsorption of metal atoms at a buckled graphene grain boundary using model potentials

    Energy Technology Data Exchange (ETDEWEB)

    Helgee, Edit E.; Isacsson, Andreas [Department of Applied Physics, Chalmers University of Technology, SE-412 96, Göteborg (Sweden)

    2016-01-15

    Two model potentials have been evaluated with regard to their ability to model adsorption of single metal atoms on a buckled graphene grain boundary. One of the potentials is a Lennard-Jones potential parametrized for gold and carbon, while the other is a bond-order potential parametrized for the interaction between carbon and platinum. Metals are expected to adsorb more strongly to grain boundaries than to pristine graphene due to their enhanced adsorption at point defects resembling those that constitute the grain boundary. Of the two potentials considered here, only the bond-order potential reproduces this behavior and predicts the energy of the adsorbate to be about 0.8 eV lower at the grain boundary than on pristine graphene. The Lennard-Jones potential predicts no significant difference in energy between adsorbates at the boundary and on pristine graphene. These results indicate that the Lennard-Jones potential is not suitable for studies of metal adsorption on defects in graphene, and that bond-order potentials are preferable.

  12. Roles of grain boundaries on the semiconductor to metal phase transition of VO2 thin films

    International Nuclear Information System (INIS)

    Vanadium dioxide (VO2) thin films with controlled grain sizes are deposited on amorphous glass substrates by pulsed laser deposition. The grain boundaries (GBs) are found as the dominating defects in the thin films. The semiconductor to metal transition (SMT) properties of VO2 thin films are characterized and correlated to the GB density. The VO2 films with lower GB density exhibit a sharper SMT with a larger transition amplitude. A high resolution TEM study at GB area reveals the disordered atomic structures along the boundaries and the distorted crystal lattices near the boundaries. The VO2 SMT amplitude and sharpness could be directly related to these defects at and near the boundaries

  13. Boundary Fractal Analysis of Two Cube-oriented Grains in Partly Recrystallized Copper

    DEFF Research Database (Denmark)

    Sun, Jun; Zhang, Yubin; Dahl, Anders Bjorholm;

    2015-01-01

    The protrusions and retrusions observed on the recrystallizing boundaries affect the migration kinetics during recrystallization. Characterization of the boundary roughness is necessary in order to evaluate their effects. This roughness has a structure that can be characterized by fractal analysi......, and in this study the so-called “Minkowski sausage” method is adopted. Hereby, two cube-oriented grains in partly recrystallized microstructures are analyzed and quantitative information regarding the dimensions of protrusions/retrusions is obtained....

  14. First-principles calculations on the grain boundary de-cohesion of iron and nickel by oxygen

    International Nuclear Information System (INIS)

    It is not known in detail whether segregation of elemental oxygen bring about grain boundary de-cohesion in iron or nickel, because its experimental confirmation is difficult. From first-principles calculations, we show that a strong de-cohesion in bcc/fcc iron and fcc nickel grain boundaries can occur if elemental oxygen segregate at the grain boundaries. Oxygen adsorption in fracture surfaces brings about much larger stabilization energy than oxygen segregation in grain boundary; this significantly reduces grain boundary cohesive energy. On the other hand, we show that the formation energy per an oxygen atom of iron and nickel oxides (FeO, Fe2O3, Fe3O4, NiO) are comparable to or larger than the grain boundary segregation energy of oxygen. In addition, we show that the cohesive energy of the (001) plane of these oxides is as small as that of oxygen-segregated grain boundaries. These results indicate that a strong grain-boundary de-cohesion occurs in iron and nickel by oxygen segregation and/or oxide formation as a result of oxygen penetration from environment to grain boundaries. (authors)

  15. The effect of electron scattering from disordered grain boundaries on the resistivity of metallic nanostructures

    Science.gov (United States)

    Arenas, Claudio; Henriquez, Ricardo; Moraga, Luis; Muñoz, Enrique; Munoz, Raul C.

    2015-02-01

    We calculate the electrical resistivity of a metallic specimen, under the combined effects of electron scattering by impurities, grain boundaries, and rough surfaces limiting the film, using a quantum theory based upon the Kubo formalism. Grain boundaries are represented by a one-dimensional periodic array of Dirac delta functions separated by a distance "d" giving rise to a Kronig-Penney (KP) potential. We use the Green's function built from the wave functions that are solutions of this KP potential; disorder is included by incorporating into the theory the probability that an electron is transmitted through several successive grain boundaries. We apply this new theory to analyze the resistivity of samples S1, S2, S7 and S8 measured between 4 and 300 K reported in Appl. Surf. Science273, 315 (2013). Although both the classical and the quantum theories predict a resistivity that agrees with experimental data to within a few percent or better, the phenomena giving rise to the increase of resistivity over the bulk are remarkably different. Classically, each grain boundary contributes to the electrical resistance by reflecting a certain fraction of the incoming electrons. In the quantum description, there are states (in the allowed KP bands) that transmit electrons unhindered, without reflections, while the electrons in the forbidden KP bands are localized. A distinctive feature of the quantum theory is that it provides a description of the temperature dependence of the resistivity where the contribution to the resistivity originating on electron-grain boundary scattering can be identified by a certain unique grain boundary reflectivity R, and the resistivity arising from electron-impurity scattering can be identified by a certain unique ℓIMP mean free path attributable to impurity scattering. This is in contrast to the classical theory of Mayadas and Shatzkes (MS), that does not discriminate properly between a resistivity arising from electron-grain boundary

  16. Evidence for Grain Boundary Transport from Impedance Spectroscopy of Gabbro at 1-2 GPa up to 890℃

    Institute of Scientific and Technical Information of China (English)

    白利平; 杜建国; 刘巍; 郭捷; 周文戈

    2003-01-01

    Impedance spectra of gabbro were measured at 1-2 GPa and up to 890℃ with applied frequency of 12 to 105 Hz.At temperatures below 680℃ ,only one impedance arc corresponding to the grain interior conduction process occurs.Owing to the grain boundary transport with increasing temperature,the impurities occur at the grain boundaries,resulting in the second arc corresponding to the grain boundary conduction process over the frequency range of 12 to 103Hz above 680℃,and the resistivities of the grain interior and the grain boundary conduction mechanisms add in series.The total conductivity of this rock is dominated by the grain interior conductivity and the impurities have no significant effect on the total electrical conductivities.

  17. Grains and grain boundaries contribution to dielectric relaxations and conduction of Bi5Ti3FeO15 ceramics

    Science.gov (United States)

    Rehman, Fida; Li, Jing-Bo; Zhang, Jia-Song; Rizwan, Muhammad; Niu, Changlei; Jin, Hai-Bo

    2015-12-01

    Dielectric relaxation behaviors of Aurivillius Bi5Ti3FeO15 ceramics were investigated in a wide range of frequency and temperature via dielectric and impedance spectroscopies. We distinguished two dielectric relaxations using the combination of impedance and modulus analysis. Resistance of the grain boundary was found to be much larger than grains, whereas capacitance was at the same level. The kinetic analysis of dielectric data was carried out to evaluate the contributions of microstructure and defects to the relaxation and conduction. The possible relaxation-conduction mechanism in the ceramics was discussed. The results enable deep understanding of microstructure-defect-relaxation behaviors in Bi5Ti3FeO15 ceramics.

  18. Copper segregation to the Sigma5 (310)/[001] symmetric tilt grain boundary in aluminum

    Energy Technology Data Exchange (ETDEWEB)

    Campbell, Geoffrey H.; Plitzko, Jurgen M.; King, Wayne E.; Foiles, Stephen M.; Kisielowski, Christian; Duscher, Gerd J.M.

    2003-01-01

    New insight into the atomic segregation of copper to an aluminum grain boundary has been obtained using multiple, complementary atomic resolution electron microscopy techniques coupled with ab-initio electronic structure calculations. The copper segregation is site specific and changes the structure of the boundary by occupying interstitial sites. Minor elemental constituents in materials can have profound effects on their engineering performance. This change in structure can be associated with these strong effects. The observed structural change will alter the mass transport behavior of the boundary and has implications for the understanding of electromigration mechanisms.

  19. Role of grain boundaries in the conduction of Eu-Ni substituted Y-type hexaferrites

    Science.gov (United States)

    Ali, Irshad; Islam, M. U.; Naeem Ashiq, Muhammad; Asif Iqbal, M.; Khan, Hasan M.; Murtaza, G.

    2014-08-01

    Single phase nanostructured (Eu-Ni) substituted Y-type hexaferrites with nominal composition of Sr2Co2-xNixEuyFe12-yO22 (x=0.0-1, y=0.0-0.1) were synthesized by the microemulsion method. Temperature dependent DC electrical conductivity and drift mobility were found in good agreement with each other, reflecting semiconducting behavior. The presence of Debye peaks in imaginary electric modulus curves confirmed the existence of relaxation phenomena in given frequency range. The AC conductivity follows power law, with exponent (n) value, ranges from 0.81-0.97, indicating that the mechanism is due to polaron hopping. In the present ferrite system, Cole-Cole plots were used to separate the grain and grain boundary effects. Eu-Ni substitution leads to a remarkable rise of grain boundary resistance as compared to the grain resistance. As both AC conductivity and Cole-Cole plots are the functions of concentration, they reveal the dominant contribution of grain boundaries in the conduction mechanism. It was also observed that the AC activation energy is lower than the DC activation energy. Appreciable improved values of quality factor suggested the possible use of these synthesized materials for power applications and high frequency multilayer chip inductors.

  20. EFFECTS OF MODIFICATION OF THE CARBIDE CHARACTERISTICS THROUGH GRAIN BOUNDARY SERRATION ON CREEP-FATIGUE LIFE IN AUSTENITIC STAINLESS STEELS

    Institute of Scientific and Technical Information of China (English)

    K.J.Kim; H.U.Hong; K.S.Min; S.W.Nam

    2004-01-01

    Modification of the carbide characteristics through the grain boundary serration is investigated, using an AISI 316 and 304 stainless steels. In both steels, triangular carbides were observed at straight grain boundaries while planar carbides vere observed at the serrated grain boundaries. The serrated grain boundary energy is observed to be much lower than that of the straight one. Therefore, the carbide morphology is found to be changed from triangular to planar along the serrated boundary to reduce the interfacial energy between the carbide and the matrix. The creep-fatigue properties of these steels at 873K have been investigated. The creep-fatigue life of the sample vith planar carbide at the serrated grain boundary was found to be much longer than that with triangular carbide at the straight one. These results imply that the planar carbides with lower interfacial energy have higher cavitation resistance, resulting in the retardation of cavity nucleation and growth to increase creep-fatigue life.

  1. Dissolution kinetics of nanoscale liquid Pb/Bi inclusions at a grain boundary in aluminum

    DEFF Research Database (Denmark)

    Prokofjev, S.I.; Johnson, Erik; Zhilin, V.M.;

    2008-01-01

    In situ transmission electron microscopy is used to study dissolution of liquid single-phase Pb/Bi inclusions attached to a grain boundary in an alloy of Al99.29Pb0.65Bi0.06 at temperatures of 343, 370, and 389 degrees C, respectively. The initial size of the inclusions was smaller than 60 nm. Di...

  2. Research progress in non-equilibrium grain-boundary segregation and intergranular embrittlement

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

    This paper is a summary of the research progress made by the author in the study of non-equilibrium grain-boundary segregation and intergranular embrittlement during the last 20 years. Some new concepts and new models in this research field are proposed, and their scientific backgrounds are introduced.

  3. Structure And Mobilities Of Tungsten Grain Boundaries Calculated From Atomistic Simulations

    Energy Technology Data Exchange (ETDEWEB)

    Frolov, T. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Rudd, R. E. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

    2016-08-09

    The objective of this study is to develop a computational methodology to predict structure, energies and mobilities of tungsten grain boundaries as a function of misorientation and inclination. The energies and the mobilities are the necessary input for thermomechanical model of recrystallization being developed by the Marian Group at UCLA.

  4. Investigation of grain boundary chemistry in Al-Li 2195 welds using Auger electron spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Sanders, J.H. [National Aeronautics and Space Administration, Huntsville, AL (United States). George C. Marshall Space Flight Center

    1996-05-01

    Al-Li alloy 2195 is a low-density material with high fracture toughness that is particularly well-suited for aerospace systems. It will replace Al-Cu alloy 2219 in the Super Light Weight Tank (SLWT), a modified version of the external tank being developed for the Space Shuttle to support Space Station deployment. Recent efforts have focused on joining 2195 with variable polarity plasma arc welding, as well as repairing 2195 welds with tungsten inert gas techniques. During this study, Auger electron spectroscopy (AES) was used to examine grain boundary chemistry in 2195 welds. Results indicated that weld integrity depends on whether (and how much) the grain boundaries are covered with thin films comprised of a mixture of discontinuous Al{sub 2}O{sub 3} in Al (Al/Al{sub 2}O{sub 3}), which form during weld solidification. O was probably introduced as a contaminant in the shielding gases, occurring at low levels considered negligible for Al alloys that do not contain Li. However, oxidation kinetics in 2195 are increased by Li enrichment of small quantities of Al{sub 2}O{sub 3}, further enhancing thin film formation at the grain boundaries. Al{sub 2}O{sub 3} can ultimately occupy sufficient grain boundary area to degrade the material`s mechanical properties, producing negative effects that are compounded by the cumulative heat input of multi-pass repair welding. (orig.)

  5. Coupled grain boundary motion in aluminium: the effect of structural multiplicity.

    Science.gov (United States)

    Cheng, Kuiyu; Zhang, Liang; Lu, Cheng; Tieu, Kiet

    2016-01-01

    The shear-induced coupled grain boundary motion plays an important role in the deformation of nanocrystalline (NC) materials. It has been known that the atomic structure of the grain boundary (GB) is not necessarily unique for a given set of misorientation and inclination of the boundary plane. However, the effect of the structural multiplicity of the GB on its coupled motion has not been reported. In the present study we investigated the structural multiplicity of the symmetric tilt Σ5(310) boundary in aluminium and its influence on the GB behaviour at a temperature range of 300 K-600 K using molecular dynamic simulations. Two starting atomic configurations were adopted in the simulations which resulted in three different GB structures at different temperatures. Under the applied shear deformation each GB structure exhibited its unique GB behaviour. A dual GB behaviour, namely the transformation of one GB behaviour to another during deformation, was observed for the second starting configuration at a temperature of 500 K. The atomistic mechanisms responsible for these behaviour were analysed in detail. The result of this study implicates a strong relationship between GB structures and their behaviour, and provides a further information of the grain boundary mediated plasticity in nanocrystalline materials. PMID:27140343

  6. Role of grain boundaries in the conduction of Eu–Ni substituted Y-type hexaferrites

    Energy Technology Data Exchange (ETDEWEB)

    Ali, Irshad, E-mail: irshadalibzu@gmail.com [Department of Physics, Bahauddin Zakariya University, Multan 60800 (Pakistan); Islam, M.U. [Department of Physics, Bahauddin Zakariya University, Multan 60800 (Pakistan); Naeem Ashiq, Muhammad, E-mail: naeemashiqqau@yahoo.com [Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800 (Pakistan); Asif Iqbal, M. [National University of Sciences and Technology, EME College, Islamabad (Pakistan); Khan, Hasan M. [Department of Physics, Bahauddin Zakariya University, Multan 60800 (Pakistan); Murtaza, G. [Centre for Advanced Studies in Physics, Government College University, Lahore 54000 (Pakistan)

    2014-08-01

    Single phase nanostructured (Eu–Ni) substituted Y-type hexaferrites with nominal composition of Sr{sub 2}Co{sub 2−x}Ni{sub x}Eu{sub y}Fe{sub 12−y}O{sub 22} (x=0.0–1, y=0.0–0.1) were synthesized by the microemulsion method. Temperature dependent DC electrical conductivity and drift mobility were found in good agreement with each other, reflecting semiconducting behavior. The presence of Debye peaks in imaginary electric modulus curves confirmed the existence of relaxation phenomena in given frequency range. The AC conductivity follows power law, with exponent (n) value, ranges from 0.81–0.97, indicating that the mechanism is due to polaron hopping. In the present ferrite system, Cole–Cole plots were used to separate the grain and grain boundary effects. Eu–Ni substitution leads to a remarkable rise of grain boundary resistance as compared to the grain resistance. As both AC conductivity and Cole–Cole plots are the functions of concentration, they reveal the dominant contribution of grain boundaries in the conduction mechanism. It was also observed that the AC activation energy is lower than the DC activation energy. Appreciable improved values of quality factor suggested the possible use of these synthesized materials for power applications and high frequency multilayer chip inductors. - Highlights: • Single phase nanostructures were synthesized by the micro-emulsion method. • Substitution leads to a remarkable rise of grain boundary resistance. • The AC activation energy is lower than the DC activation energy. • Improved values of quality factor make these materials useful for high frequency multilayer chip inductors.

  7. Grain boundary microstructure and fatigue crack growth in Allvac 718Plus superalloy

    International Nuclear Information System (INIS)

    Research highlights: → Hold-time crack growth rates for Allvac 718Plus are shown to be affected by delta-phase. → Delta-phase is shown to alter microstructure and chemistry at and about grain boundaries. → Two types of oxides were found and their presence was linked to the different fracture modes. - Abstract: The correlation between grain boundary microstructure and fatigue crack growth with hold-times was investigated for two conditions of the superalloy Allvac 718Plus; a Standard condition with the recommended distribution of grain boundary phases and a Clean condition with virtually no grain boundary phases. Fatigue testing was performed at 704 deg. C using 10 Hz cyclic load with intermittent hold-times of 100 s at maximum tensile load. Microstructural characterization and fractography were conducted using scanning- and transmission electron microscopy techniques. Auger electron- and X-ray photoelectron spectroscopy techniques were used for oxide analyses on fracture surfaces. It was found that in the Standard condition crack growth is mostly transgranular for 10 Hz loading and intergranular for hold-times, while for the Clean condition crack growth is intergranular in both load modes. The lower hold-time crack growth rates in the Standard condition are attributed to grain boundary δ-phase precipitates. No effect of δ-phase was observed for 10 Hz cyclic loading crack growth rates. Two different types of oxides and oxide colours were found on the fracture surfaces in the Standard condition and could be correlated to the different loading modes. For cyclic loading a bright thin Cr-enriched oxide was dominate and for hold-times a dark and slightly thicker Nb-enriched oxide was dominant These oxide types could be related to the oxidation of δ-phase and the matrix respectively. The influence of δ-phase precipitates on crack propagation is discussed.

  8. Study on Grain Boundary Structure and Corrosion Behavior of Copper Alloys

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    The grain boundary structure of Cu alloy was observed and the characteristic of the boundaries was studied,including 70∶30 cupronickel and 7 0∶30 brass. The results show that in the case of the 70∶30 cupronickel thin platelets with nickel and iron e nrichment in it precipitate intergranular so that the alloy was sensitive to sea water corrosion. In the case of 70∶30 brass the situation of grain boundary seg regation of different inclusions made the corrosion resistance of the alloy even worse. All of those were discovered through the corrosion behavior of the two d iff erent copper alloys served in various environments. The experimental methods use d here were, scanning electron microscopy(SEM), transmission electron microscopy(T EM) and energy dispersive of X ray(EDX). The intergranular corrosion morphologie s of those copper alloys served in engineering or exposed to seaw ater for a long term were given.

  9. The effect of electron scattering from disordered grain boundaries on the resistivity of metallic nanostructures

    Energy Technology Data Exchange (ETDEWEB)

    Arenas, Claudio [Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2008, Casilla 487-3, Santiago 8370449 (Chile); Synopsys Inc., Avenida Vitacura 5250, Oficina 708, Vitacura, Santiago (Chile); Henriquez, Ricardo [Departamento de Física, Universidad Técnica Federico Santa María, Av. España 1680, Casilla 110-V, Valparaíso (Chile); Moraga, Luis [Universidad Central de Chile, Toesca 1783, Santiago (Chile); Muñoz, Enrique [Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 7820436 (Chile); Munoz, Raul C., E-mail: ramunoz@ing.uchile.cl [Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2008, Casilla 487-3, Santiago 8370449 (Chile)

    2015-02-28

    Highlights: • Quantum theory of the resistivity arising from electron-grain boundary scattering in nanometric metallic structures. • The resistivity is controlled by the collective properties of the grain assembly, by the allowed Kronig-Penney (KP) bands and by the electron transmission probability across successive grains. • When the grain diameter d is larger than the electron mean free path l, the increase in resistivity arises mainly from a decrease of the number of states at the Fermi surface that are allowed KP bands. • When the grain diameter d is smaller than the electron mean free path l, the increase in resistivity arises primarily from Anderson localization caused by electron transmission across successive grains. - Abstract: We calculate the electrical resistivity of a metallic specimen, under the combined effects of electron scattering by impurities, grain boundaries, and rough surfaces limiting the film, using a quantum theory based upon the Kubo formalism. Grain boundaries are represented by a one-dimensional periodic array of Dirac delta functions separated by a distance “d” giving rise to a Kronig–Penney (KP) potential. We use the Green's function built from the wave functions that are solutions of this KP potential; disorder is included by incorporating into the theory the probability that an electron is transmitted through several successive grain boundaries. We apply this new theory to analyze the resistivity of samples S1, S2, S7 and S8 measured between 4 and 300 K reported in Appl. Surf. Science273, 315 (2013). Although both the classical and the quantum theories predict a resistivity that agrees with experimental data to within a few percent or better, the phenomena giving rise to the increase of resistivity over the bulk are remarkably different. Classically, each grain boundary contributes to the electrical resistance by reflecting a certain fraction of the incoming electrons. In the quantum description, there are states

  10. Mechanical properties of irradiated Gd2Zr2O7 pyrochlores as studied by nanoindentation technique - Effect of grains and grain boundaries

    Science.gov (United States)

    Kurpaska, L.; Jagielski, J.

    2016-07-01

    The influence of ion irradiation on nanomechanical properties of Gd2Zr2O7 pyrochlore have been studied. The polycrystalline samples were irradiated at room temperature with 320 keV Ar ions with fluences from 2 × 1014 to 1 × 1016 ions/cm2. Nanomechanical properties of grains and grains boundaries were measured by means of nanoindentation technique. The measurements were performed in the centers of the grains and at the grain boundaries and point to the conclusion that grain boundary region is usually characterized by higher hardness and Young's modulus than the center of the grain. The analysis performed suggests that the stress induced effect related to the transition to anion-deficient fluorite structure leads to the increase of recorded hardness values and may be considered as primary source of hardening. Studied phenomenon depends on the irradiation fluence and may serve as an indicator of the structure modification in the irradiated sample. Finally, nanomechanical properties of irradiated grain boundaries were interpreted in the frames of incorporation of foreign species near grain boundary.

  11. Grain boundary character distribution of CuNiSi and FeNi alloys processed by severe plastic deformation

    Science.gov (United States)

    Azzeddine, H.; Baudin, T.; Helbert, A. L.; Brisset, F.; Larbi, F. Hadj; Tirsatine, K.; Kawasaki, M.; Bradai, D.; Langdon, T. G.

    2015-04-01

    In this work the Grain Boundary Character Distribution (GBCD) in general and the relative proportion of low-Σ CSL (Coincidence Site Lattice) grain boundaries are determined through EBSD in Cu-2.5Ni-0.6Si (wt.%) and Fe-36Ni (wt.%) alloys after processing by high-pressure torsion, equal- channel angular pressing and accumulative roll bonding.

  12. Structure and properties of fluid-filled grain boundaries under stress in geological materials. Geologica Ultraiectina (290)

    NARCIS (Netherlands)

    van Noort, R.

    2008-01-01

    Two of the three processes making up the deformation mechanism of intergranular pressure solution, being dissolution and diffusion, take place in the grain boundary fluid phase. Hence, the structure and physical properties of wet grain boundaries under stress can be expected to influence the kinetic

  13. Properties of grain boundaries in bulk, melt processed Y–Ba–Cu–O fabricated using bridge-shaped seeds

    International Nuclear Information System (INIS)

    Single grain RE–Ba–Cu–O ((RE)BCO, where RE is a rare earth element or yttrium) bulk superconducting materials have significant potential for a variety of engineering applications due to their ability to trap high magnetic fields. However, it is well known that the presence of grain boundaries coupled with a high angle of misorientation (typically 5°) significantly reduces the critical current density, Jc, in all forms of high temperature superconducting materials. It is of considerable fundamental and technological interest, therefore, to investigate the grain boundary properties of bulk, film and tape (RE)BCO. We report a successful multi-seeding technique for the fabrication of fully aligned, artificial (0° misalignment) grain boundaries within large grain YBCO bulk superconductors using bridge-shaped seeds. The microstructure and critical current densities of the grain boundaries produced by this technique have been studied in detail. (paper)

  14. Properties of grain boundaries in bulk, melt processed Y-Ba-Cu-O fabricated using bridge-shaped seeds

    Science.gov (United States)

    Shi, Y.-H.; Durrell, J. H.; Dennis, A. R.; Babu, N. Hari; Mancini, C. E.; Cardwell, D. A.

    2012-04-01

    Single grain RE-Ba-Cu-O ((RE)BCO, where RE is a rare earth element or yttrium) bulk superconducting materials have significant potential for a variety of engineering applications due to their ability to trap high magnetic fields. However, it is well known that the presence of grain boundaries coupled with a high angle of misorientation (typically 5°) significantly reduces the critical current density, Jc, in all forms of high temperature superconducting materials. It is of considerable fundamental and technological interest, therefore, to investigate the grain boundary properties of bulk, film and tape (RE)BCO. We report a successful multi-seeding technique for the fabrication of fully aligned, artificial (0° misalignment) grain boundaries within large grain YBCO bulk superconductors using bridge-shaped seeds. The microstructure and critical current densities of the grain boundaries produced by this technique have been studied in detail.

  15. Multiscale Modeling of Grain Boundary Segregation and Embrittlement in Tungsten for Mechanistic Design of Alloys for Coal Fired Plants

    Energy Technology Data Exchange (ETDEWEB)

    Luo, Jian; Tomar, Vikas; Zhou, Naixie; Lee, Hongsuk

    2013-06-30

    Based on a recent discovery of premelting-like grain boundary segregation in refractory metals occurring at high temperatures and/or high alloying levels, this project investigated grain boundary segregation and embrittlement in tungsten (W) based alloys. Specifically, new interfacial thermodynamic models have been developed and quantified to predict high-temperature grain boundary segregation in the W-Ni binary alloy and W-Ni-Fe, W-Ni-Ti, W-Ni-Co, W-Ni-Cr, W-Ni-Zr and W-Ni-Nb ternary alloys. The thermodynamic modeling results have been experimentally validated for selected systems. Furthermore, multiscale modeling has been conducted at continuum, atomistic and quantum-mechanical levels to link grain boundary segregation with embrittlement. In summary, this 3-year project has successfully developed a theoretical framework in combination with a multiscale modeling strategy for predicting grain boundary segregation and embrittlement in W based alloys.

  16. Predoping effects of boron and phosphorous on arsenic diffusion along grain boundaries in polycrystalline silicon investigated by atom probe tomography

    Science.gov (United States)

    Takamizawa, Hisashi; Shimizu, Yasuo; Inoue, Koji; Nozawa, Yasuko; Toyama, Takeshi; Yano, Fumiko; Inoue, Masao; Nishida, Akio; Nagai, Yasuyoshi

    2016-10-01

    The effect of P or B predoping on As diffusion in polycrystalline Si was investigated by atom probe tomography. In all samples, a high concentration of As was found at grain boundaries, indicating that such boundaries are the main diffusion path. However, As grain-boundary diffusion was suppressed in the B-doped sample and enhanced in the P-doped sample. In a sample codoped with both P and B, As diffusion was somewhat enhanced, indicating competition between the effects of the two dopants. The results suggest that As grain-boundary diffusion can be controlled by varying the local concentration of P or B.

  17. Effect of grain-boundary flux pinning in MgB2 with columnar structure

    International Nuclear Information System (INIS)

    We studied the flux pinning properties by grain boundaries in MgB2 films prepared by using a hybrid physical chemical vapor deposition method on the c-axis oriented sapphire substrates. All the films we report here had the columnar grains with the growth direction perpendicular to the substrates and the grain sizes in the range of a few hundred nanometers. At very low magnetic fields, no discernable grain-boundary (GB) pinning effect was observed in all measuring temperatures, but above those fields, the effect of GB flux pinning was observed as enhanced critical current densities (Jcs) and reduced resistances when an external magnetic field (B) was aligned parallel to the c-axis. We interpret the B dependence of Jc in the terms of flux line lattice shear inside the columnar grains activated by dislocations of Frank-Read source while the flux lines pinned by GB act as anchors for dislocations. Magnetic field dependence of flux pinning force density for B parallel to the c-axis was reasonably explained by the above model.

  18. Transport properties through graphene grain boundaries: strain effects versus lattice symmetry

    Science.gov (United States)

    Hung Nguyen, V.; Hoang, Trinh X.; Dollfus, P.; Charlier, J.-C.

    2016-06-01

    As most materials available at the macroscopic scale, graphene samples usually appear in a polycrystalline form and thus contain grain boundaries. In the present work, the effect of uniaxial strain on the electronic transport properties through graphene grain boundaries is investigated using atomistic simulations. A systematic picture of transport properties with respect to the strain and lattice symmetry of graphene domains on both sides of the boundary is provided. In particular, it is shown that strain engineering can be used to open a finite transport gap in all graphene systems where the two domains are arranged in different orientations. This gap value is found to depend on the strain magnitude, on the strain direction and on the lattice symmetry of graphene domains. By choosing appropriately the strain direction, a large transport gap of a few hundred meV can be achieved when applying a small strain of only a few percents. For a specific class of graphene grain boundary systems, strain engineering can also be used to reduce the scattering on defects and thus to significantly enhance the conductance. With a large strain-induced gap, these graphene heterostructures are proposed to be promising candidates for highly sensitive strain sensors, flexible electronic devices and p-n junctions with non-linear I-V characteristics.

  19. Electrochemical and Numerical Studies of Surface, Grain-Boundary and Bulk Copper Diffusion Into Gold

    Science.gov (United States)

    Miller, Eric Todd

    Surface, grain-boundary, and bulk chemical diffusivities of copper into gold were measured by chronoamperometry -potentiometry applied to Cu|CuCl |Au solid state galvanic cells at 300-400^circC. The cells were constructed using a novel vapor deposition technique which is described. The automated data acquisition techniques utilizing unique hardware and custom designed software are also presented. Chronoamperometry and a two electrode limited potential cyclic voltammetry technique were comparatively used to determine cell capacitance and resistance. Both gave similar RC values at lower temperatures but diverged from each other at higher temperatures. Electron hole conductivity of CuCl could not be determined from intercept values in the chronoamperometry Cottrell analysis. The partial molar enthalpy and entropy of mixing copper into gold were determined from Emf vs temperature vs composition measurements of Cu|CuCl |Au-Cu alloy cells. The results support the regular solution model of mixing with interaction energy parameter {bf{cal Q}} = 10kJ. Diffusion coefficients were calculated from the chronoamperometry-potentiometry time/flux/concentration data in two ways: via the Cottrell equation, for an average diffusion coefficient; and via a simplex and finite difference program for the simultaneous determination of surface, grain-boundary, and bulk diffusion coefficients. This program was run on a MASPAR MP-2 massively parallel computer. The surface and grain-boundary diffusivities were numerically determinable in single and polycrystalline cathodes at short diffusion times. Bulk diffusivity was determinable at short and long diffusion times and agreed with previous data. Surface diffusivity was two orders of magnitude larger than the bulk with lower activation energy. Grain -boundary diffusivity was one order of magnitude larger than the bulk with similar activation energy. The Cottrell equation was only valid at very long diffusion times due to the transient interface

  20. Intergranular corrosion in AA5XXX aluminum alloys with discontinuous precipitation at the grain boundaries

    Science.gov (United States)

    Bumiller, Elissa

    The US Navy currently uses AA5xxx aluminum alloys for structures exposed to a marine environment. These alloys demonstrate excellent corrosion resistance over other aluminum alloys (e.g., AA2xxx or AA7xxx) in this environment, filling a niche in the marine structures market when requiring a light-weight alternative to steel. However, these alloys are susceptible to localized corrosion; more specifically, intergranular corrosion (IGC) is of concern. IGC of AA5xxx alloys due to the precipitation of beta phase on the grain boundaries is a well-established phenomenon referred to as sensitization. At high degrees of sensitization, the IGC path is a continuous anodic path of beta phase particles. At lower degrees of sensitization, the beta phase coverage at the grain boundaries is not continuous. The traditional ranges of susceptibility to IGC as defined by ASTM B928 are in question due to recent studies. These studies showed that even at mid range degrees of sensitization where the beta phase is no longer continuous, IGC may still occur. Previous thoughts on IGC of these alloy systems were founded on the idea that once the grain boundary precipitate became discontinuous the susceptibility to IGC was greatly reduced. Additionally, IGC susceptibility has been defined metallurgically by compositional gradients at the grain boundaries. However, AA5xxx alloys show no compositional gradients at the grain boundaries, yet are still susceptible to IGC. The goal of this work is to establish criteria necessary for IGC to occur given no continuous beta phase path and no compositional gradient at the grain boundaries. IGC performance of the bulk alloy system AA5083 has been studied along with the primary phases present in the IGC system: alpha and beta phases using electrochemistry and modeling as the primary tools. Numerical modeling supports that at steady-state the fissure tip is likely saturated with Mg in excess of the 4% dissolved in the matrix. By combining these results

  1. Stress induced grain boundaries in thin Co layer deposited on Au and Cu

    Science.gov (United States)

    Zientarski, Tomasz; Chocyk, Dariusz

    2016-10-01

    In this work, the structure and stress evolution in Co/Au and Co/Cu two-layer systems during deposition were studied. The growth of this system is evaluated by employing molecular dynamic simulations with potentials based on the embedded atom method theory. We used the kinematical scattering theory and the Ackland-Jones bond-angle method to the structural characterisation of deposited layers. In both systems, only compressive stress is observed during the deposition process and process relaxation of stress is visible. In Co/Au systems, creation of grains and grain boundaries is observed.

  2. The influence of grain boundary internal stress on permeability: temperature curve for Mn-Zn ferrites

    International Nuclear Information System (INIS)

    The oxygen partial pressure (PO2) utilized in homogenization or cooling stage of sintering process has been observed to significantly influence the initial permeability-temperature (μi-T) characteristics of the materials. Higher oxygen partial pressure (PO2) during homogenization period lowers the μi-value without shifting the secondary maximum peak (Tsmp). However, higher PO2 (0.1%) during cooling period shifts the Tsmp toward lower-temperature regime. This phenomenon is accounted for by the induction of compressive stress onto the ferrite grains, which in turn, is due to preferential oxidation along grain boundaries

  3. Misorientation effect of grain boundary on the formation of discontinuous precipitation in second and third generation single crystal superalloys

    Directory of Open Access Journals (Sweden)

    Yu Zhengrong

    2014-01-01

    Full Text Available [001] tilt artificial grain boundaries of Ni-based single crystal superalloys CMSX-4 and DD10 have been prepared by self-diffusion bonding. The microstructural stability of 0 ∼ 30∘ artificial grain boundaries have been investigated after heat treatment at 1100 ∘C for 0 ∼ 300 h. TCP phases and cellular colony developed on boundaries are related to misorientation angle of the bonded boundaries of DD10 and DD10 alloys as well as the bonded boundaries of CMSX-4 and DD10 alloys. The heterogeneous nucleation of TCP phase, enveloped by γ′ film, occurred along 15∘ and 20∘ boundaries. Discontinuous Precipitation (DP reaction occurred along high misorientation angle (20∘∼ 30∘ boundaries. However, no TCP phase formation existed along grain boundaries with different misorientation angles in CMSX-4/CMSX-4 bonded alloys as well as for a 0∘ boundary in DD10/DD10 and CMSX-4/DD10 bonded alloys. The current study clearly suggests that grain boundary precipitation and its morphology were influenced by the misorientation angle of grain boundary and the content of refractory elements in alloy.

  4. Cavitation and grain boundary sliding during creep of Mg-Y-Nd-Zn-Mn alloy

    Institute of Scientific and Technical Information of China (English)

    2008-01-01

    Creep of squeeze-cast Mg-3Y-2Nd-1Zn-1Mn alloy was investigated at the constant load in the stress range of 30-80 MPa.Tensile creep tests were performed at 300 ℃ up to the final fracture. Several tests at 50 MPa were interrupted after reaching the steady state creep; and another set of creep tests was interrupted after the onset of ternary creep. Fraction of cavitated dendritic boundaries was evaluated using optical microscopy. Measurement of grain boundary sliding by observation of the offset of marker lines was carried out on the surface of the crept specimens after the test interruption by scanning electron microscopy and by confocal laser scanning microscopy, The results show that the dominant creep mechanism in this alloy is dislocation creep with minor contribution of the grain boundary sliding. Creep failure took place by the nucleation, growth and coalescence of creep cavities on the boundaries predominantly oriented perpendicular to the applied stress. Increasing amount of cavitated boundaries with time of creep exposure supports the mechanism of continuous cavity nucleation and growth.

  5. Grain boundary migration induced segregation in V-Cr-Ti alloys

    Energy Technology Data Exchange (ETDEWEB)

    Gelles, D.S. [Pacific Northwest National Lab., Richland, WA (United States); Ohnuki, S.; Takahashi, H. [Univ. of Hokkaido (Japan)

    1996-10-01

    Analytical electron microscopy results are reported for a series of vanadium alloys irradiated in the HFIR JP23 experiment at 500{degrees}C. Alloys were V-5Cr-5Ti and pure vanadium which are expected to have transmuted to V-15Cr-5Ti and V-10Cr following irradiation. Analytical microscopy confirmed the expected transmutation occurred and showed redistribution of Cr and Ti resulting from grain boundary migration in V-5Cr-5Ti, but in pure V, segregation was reduced and no clear trends as a function of position near a boundary were identified.

  6. Solid on solid model for an interface crossing a grain boundary

    Energy Technology Data Exchange (ETDEWEB)

    Abraham, D B [Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP (United Kingdom); Mustonen, Ville [Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP (United Kingdom); Wood, A J [Laboratory of Computational Engineering, Helsinki University of Technology, PO Box 9203, FIN-02015 (Finland)

    2004-06-18

    Recent work has demonstrated a new structural transition occurring at an internal defect in a two-dimensional Ising model. The new behaviour is induced by boundary conditions that constrain the interface to lie at an angle across the defect line. This gives rise to the energy-entropy competition familiar from other examples of pinning-depinning transitions. We demonstrate how a horizontal solid-on-solid (SOS) model can be used to obtain comparable results to this exact calculation. This simpler model can then be easily extended to encompass a situation where the interface has a differing stiffness on either side of the grain boundary. (letter to the editor)

  7. Mapping residual organics and carbonate at grain boundaries and the amorphous interphase in mouse incisor enamel.

    Science.gov (United States)

    Gordon, Lyle M; Joester, Derk

    2015-01-01

    Dental enamel has evolved to resist the most grueling conditions of mechanical stress, fatigue, and wear. Adding insult to injury, it is exposed to the frequently corrosive environment of the oral cavity. While its hierarchical structure is unrivaled in its mechanical resilience, heterogeneity in the distribution of magnesium ions and the presence of Mg-substituted amorphous calcium phosphate (Mg-ACP) as an intergranular phase have recently been shown to increase the susceptibility of mouse enamel to acid attack. Herein we investigate the distribution of two important constituents of enamel, residual organic matter and inorganic carbonate. We find that organics, carbonate, and possibly water show distinct distribution patterns in the mouse enamel crystallites, at simple grain boundaries, and in the amorphous interphase at multiple grain boundaries. This has implications for the resistance to acid corrosion, mechanical properties, and the mechanism by which enamel crystals grow during amelogenesis.

  8. Surface studies of iridium-alloy grain boundaries associated with weld cracking

    International Nuclear Information System (INIS)

    Plutonium-238 oxide fuel pellets for the General Purpose Heat Source (GPHS) Radioisotopic Thermoelectric Generators to be used on the NASA Galileo Mission to Jupiter and the International Solar Polar Mission are produced and encapsulated in iridium alloy at the Savannah River Plant (SRP). Underbead cracks occasionally occur in the girth weld on the iridium-alloy-clad vent sets in the region where the gas tungsten arc is quenched. Grain-boundary structures and compositions were characterized by scanning electron microscopy/x-ray energy spectroscopy, electron microprobe analysis and scanning Auger microprobe analysis to determine the cause of weld quench area cracking. Results suggest that weld quench area cracking may be caused by gas porosity or liquation in the grain boundaries

  9. Fundamental Studies of the Role of Grain Boundaries on Uniform Corrosion of Advanced Nuclear Reactor Materials

    Energy Technology Data Exchange (ETDEWEB)

    Taheri, Mitra [Drexel Univ., Philadelphia, PA (United States); Motta, Arthur [Pennsylvania State Univ., University Park, PA (United States); Marquis, Emmanuelle [Univ. of Michigan, Ann Arbor, MI (United States)

    2016-05-20

    The main objective of this proposal is to develop fundamental understanding of the role of grain boundaries in stable oxide growth. To understand the process of oxide layer destabilization, it is necessary to observe the early stages of corrosion. During conventional studies in which a sample is exposed and examined after removal from the autoclave, the destabilization process will normally have already taken place, and is only examined post facto. To capture the instants of oxide destabilization, it is necessary to observe it in-situ; however significant question always arise as to the influence of the corrosion geometry and conditions on the corrosion process. Thus a combination of post facto examinations and in-situ studies is proposed, which also combines state-of-the-art characterization techniques to derive a complete understanding of the destabilization process and the role of grain boundaries.

  10. Nanoscale grain boundary channels in fracture cement enhance flow in mudrocks

    Science.gov (United States)

    Landry, Christopher J.; Eichhubl, Peter; Prodanović, Maša.; Wilkins, Scott

    2016-05-01

    Hydrocarbon production from mudrock or shale reservoirs typically exceeds estimates based on mudrock laboratory permeability measurements, with the difference attributed to natural fractures. However, natural fractures in these reservoirs are frequently completely cemented and thus assumed not to contribute to flow. We quantify the permeability of nanoscale grain boundary channels with mean apertures of 50-130 nm in otherwise completely cemented natural fractures of the Eagle Ford Formation and estimate their contribution to production. Using scanning electron imaging of grain boundary channel network geometry and a digital rock physics workflow of image reconstruction and direct flow modeling, we estimate cement permeability to be 38-750 nd, higher than reported permeability of Eagle Ford host rock (~2 nd) based on laboratory measurements. Our results suggest that effective fracture-parallel mudrock permeability can exceed laboratory values by upward of 1 order of magnitude in shale reservoirs of high macroscopic cemented fracture volume fraction.

  11. The SEM-EBIV signals near the grain boundary in a polycrystalline solar cell

    Science.gov (United States)

    Romanowski, A.; Buczkowski, A.

    1985-12-01

    A new technique of EBIV measurements has been proposed. The open-circuit voltage signal modified by a grain boundary (GB) assuming a simple equivalent circuit of a polycrystalline solar cell has been analyzed. The numerical calculations of the voltage contrast as a function of the distance between the electron-hole generation sphere and the GB were performed. The EBIC and EBIV characteristics have been compared on account of their usefulness for evaluating the carrier diffusion length. The recombination velocity at the grain boundary, and the shunt resistance of the cell. It was found that EBIV measurements provided more detailed information about the electrical properties of the GB and the junction parameters of a polycrystalline solar cell.

  12. Autonomous Filling of Grain-Boundary Cavities during Creep Loading in Fe-Mo Alloys

    Science.gov (United States)

    Zhang, S.; Fang, H.; Gramsma, M. E.; Kwakernaak, C.; Sloof, W. G.; Tichelaar, F. D.; Kuzmina, M.; Herbig, M.; Raabe, D.; Brück, E.; van der Zwaag, S.; van Dijk, N. H.

    2016-07-01

    We have investigated the autonomous repair of creep damage by site-selective precipitation in a binary Fe-Mo alloy (6.2 wt pct Mo) during constant-stress creep tests at temperatures of 813 K, 823 K, and 838 K (540 °C, 550 °C, and 565 °C). Scanning electron microscopy studies on the morphology of the creep-failed samples reveal irregularly formed deposits that show a close spatial correlation with the creep cavities, indicating the filling of creep cavities at grain boundaries by precipitation of the Fe2Mo Laves phase. Complementary transmission electron microscopy and atom probe tomography have been used to characterize the precipitation mechanism and the segregation at grain boundaries in detail.

  13. Segregation and clustering of solutes at grain boundaries in Mg–rare earth solid solutions

    International Nuclear Information System (INIS)

    The present study validates the previously reported investigations about segregation of rare-earth (RE) elements at grain boundaries in Mg–RE alloys and ultimately provides a direct visualization of the distribution of the solute atoms in the structure of a Mg–Gd alloy. It is demonstrated that Gd forms a solid solution within the Mg matrix in addition to substantial segregation at high-angle grain boundaries in the form of 1–2 nm clusters, with a postulated face-centered cubic Gd structure. The results suggest significant implications for the texture development during alloy processing and recrystallization, and thus for the mechanical behavior and properties of Mg–RE alloys

  14. Fundamental Studies of the Role of Grain Boundaries on Uniform Corrosion of Advanced Nuclear Reactor Materials

    Energy Technology Data Exchange (ETDEWEB)

    Taheri, Mitra [Drexel Univ., Philadelphia, PA (United States); Motta, Arthur [Pennsylvania State Univ., University Park, PA (United States); Marquis, Emmanuelle [Univ. of Michigan, Ann Arbor, MI (United States)

    2016-05-20

    The main objective of this proposal is to develop fundamental understanding of the role of grain boundaries in stable oxide growth. To understand the process of oxide layer destabilization, it is necessary to observe the early stages of corrosion. During conventional studies in which a sample is exposed and examined after removal from the autoclave, the destabilization process will normally have already taken place, and is only examined post facto. To capture the instants of oxide destabilization, it is necessary to observe it in situ; however, significant questions always arise as to the influence of the corrosion geometry and conditions on the corrosion process. Thus, a combination of post facto examinations and in situ studies is proposed, which also combines state-of-the-art characterization techniques to derive a complete understanding of the destabilization process and the role of grain boundaries.

  15. Dissolution of kinetics of nanoscale liquid Pb/Bi inclusions at a grain boundary in aluminium

    DEFF Research Database (Denmark)

    Prokofjev, Sergei I.; Johnson, Erik; Zhilin, Victor M.;

    2008-01-01

    In situ transmission selctron microscopy is used to study dissolution of liquid single-phase Pb/Bi inclusions attached to grain boundary in an alloy of Al99.29Pb0.65Bi0.06 at temperatures of 343, 370, and 389 °C, respectively.  The initial size of the inclusions was smaller than 60 nm.  Dissoluti...... that grain-boundary diffusion of Pb and Bi is the controlling mechanism. The high value (2.3 eV) of the apparent activation enthalpy of dissolution indicates that the process is likely governed by the large negative enthalpies of solubility of Pb and Bi in Al.  ...

  16. An improved method to identify grain boundary creep cavitation in 316H austenitic stainless steel

    Energy Technology Data Exchange (ETDEWEB)

    Chen, B., E-mail: b.chen@bristol.ac.uk [Department of Mechanical Engineering, University of Bristol, Bristol BS8 1TR (United Kingdom); Flewitt, P.E.J. [Interface Analysis Centre, University of Bristol, 121 St. Michael' s Hill, Bristol BS2 8BS (United Kingdom); H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL (United Kingdom); Smith, D.J. [Department of Mechanical Engineering, University of Bristol, Bristol BS8 1TR (United Kingdom); Jones, C.P. [Interface Analysis Centre, University of Bristol, 121 St. Michael' s Hill, Bristol BS2 8BS (United Kingdom)

    2011-04-15

    Inter-granular creep cavitation damage has been observed in an ex-service 316H austenitic stainless steel thick section weldment. Focused ion beam cross-section milling combined with ion channelling contrast imaging is used to identify the cavitation damage, which is usually associated with the grain boundary carbide precipitates in this material. The results demonstrate that this technique can identify, in particular, the early stage of grain boundary creep cavitation unambiguously in materials with complex phase constituents. -- Research highlights: {yields} FIB milling plus ion channelling contrast optimise the observation of cavity. {yields} Identification of the creep cavities unambiguously, using an FIB technique. {yields} The FIB technique can retain the polyhedral shape of cavity. {yields} Various stages of creep cavitation can be observed, using the FIB technique.

  17. Evaluation of grain boundary embrittlement of phosphorus added F82H steel by SSTT

    Science.gov (United States)

    Kim, Byung Jun; Kasada, Ryuta; Kimura, Akihiko; Tanigawa, Hiroyasu

    2012-02-01

    Non-hardening embrittlement (NHE) can be happened by a large amount of He on grain boundaries over 500-700 appm of bulk He without hardening at fusion reactor condition. Especially, at high irradiation temperatures (>≈420 °C), NHE accompanied by intergranular fracture affects the severe accident and the safety of fusion blanket system. Small specimen tests to evaluate fracture toughness and Charpy impact properties were carried out for F82H steels with different levels of phosphorous addition in order to simulate the effects of NHE on the shift of transition curve. It was found that the ductile to brittle transition temperature (DBTT) and reference temperature ( T0) after phosphorous addition is shifted to higher temperatures and accompanied by intergranular fracture at transition temperatures region. The master curve approach for evaluation of fracture toughness change by the degradation of grain boundary strength was carried out by referring to the ASTM E1921.

  18. Diffusion of Hydrogen along the Grain Boundaries in Ni3Al Alloys

    Institute of Scientific and Technical Information of China (English)

    2001-01-01

    The diffusivity of hydrogen in two Ni3Al alloys (No.1 and No.2)has been measured in the temperature range of 100℃ to 420℃ using an ultrahigh vacuum gaseous permeation technique.The diffusivity data fall into two segments, in which the hydrogen diffusivity adheres to the Arrhenius form, respectively. From the hydrogen diffusivity, it is conjectured that the hydrogen diffusivity reflects the hydrogen transportation along the grain boundaries at lower temperature and the hydrogen transportation in the lattice at higher temperature. The intergranular fracture of L12-type intermetallics induced by hydrogen at relative low temperature results from hydrogen transportation along the grain boundaries and not in the lattice.

  19. Analysis of grain boundary sinks and interstitial diffusion in neutron-irradiated SiC

    OpenAIRE

    Kondo, Sosuke; Katoh, Yutai; Snead, Lance

    2011-01-01

    The widths of the interstitial loop denuded zone (DZ) along grain boundaries were examined for 3C-SiC irradiated at 1010–1380 °C by transmission electron microscopy (TEM) in an effort to obtain the activation energy of interstitial migration. Denuded-zone widths as small as 17 nm were observed below 1130 °C, indicating that a substantial population of “TEM invisible” voids of diameter

  20. The effect of grain boundaries state on the thermal stability of a submicrocrystalline titanium alloy structure

    Science.gov (United States)

    Chuvil'deev, V. N.; Kopylov, V. I.; Nokhrin, A. V.; Lopatin, Yu. G.; Kozlova, N. A.; Tabachkova, N. Yu.; Semenycheva, A. V.; Smirnova, E. S.; Gryaznov, M. Yu.; Pirozhnikova, O. E.

    2015-06-01

    The thermal stability of the structure and the mechanical properties of submicrocrystalline (SMC) titanium alloy Ti-4Al-2V (industrial designation PT3V) are investigated. The alloy was produced by equal-channel angular pressing (ECAP). It is demonstrated that the enhanced thermal stability of the SMC alloy structure is associated with a change in the concentration of aluminum at the grain boundaries during ECAP.

  1. Influence of nano-inclusions' grain boundaries on crack propagation modes in materials

    Energy Technology Data Exchange (ETDEWEB)

    Karakasidis, T.E., E-mail: thkarak@uth.gr [Department of Civil Engineering, University of Thessaly, Pedion Areos, 38834 Volos (Greece); Charitidis, C.A. [National Technical University of Athens, School of Chemical Engineering, 9 Heroon Polytechniou st., Zografos, 157 80 Athens (Greece)

    2011-04-15

    The effect of nano-inclusions on materials' strength and toughness has attracted great interest in recent years. It has been shown that tuning the morphological and microstructural features of materials can tailor their fracture modes. The existence of a characteristic size of inclusions that favours the fracture mode (i.e. transgranular or intergranular) has been experimentally observed but also predicted by a 2D model based on energetic arguments which relates the crack propagation mode to the ratio of the interface area between the crystalline inclusion and the matrix with the area of the crystallite inclusion in a previous work. In the present work, a 3D model is proposed in order to extend the 2D model and take into account the influence of the size of grain boundary zone on the toughening/hardening behavior of the material as it was observed experimentally in the literature. The model relates crack propagation mode to the ratio of the volume of the grain boundary zone between the crystalline inclusion and the matrix with the volume of the nano-inclusion. For a ratio below a critical value, transgranular propagation is favoured while for larger values, intergranular propagation is favoured. We also demonstrate that the extent of the grain boundary region also can significantly affect this critical value. The results of the model are in agreement with the literature experimental observations related to the toughening/hardening behavior as a function of the size of crystalline inclusions as well as the width of the grain boundary regions.

  2. Dynamic recrystallization and grain boundary migration in B2 FeAl

    Science.gov (United States)

    Baker, I.; Gaydosh, D. J.

    1987-01-01

    Transmission electron microscopy and optical microscopy were used to examine polycrystalline specimens of the B2-structured alloy FeAl strained under tension to fracture at elevated temperature. Strain-induced grain boundary migration was observed above 900 K and dynamic recrystallization was found at 1000 K and 1100 K. Little evidence of dynamic recovery was evident but some networks were formed at 1100 K.

  3. Multiple time step molecular dynamics simulation for interaction between dislocations and grain boundaries

    Institute of Scientific and Technical Information of China (English)

    Xiaoyan Li; Wei Yang

    2005-01-01

    A multiple time step algorithm, called reversible reference system propagator algorithm, is introduced for thelong time molecular dynamics simulation. In contrast to the conventional algorithms, the multiple time method has better convergence, stability and efficiency. The method is validated by simulating free relaxation and the hypervelocity impact of nano-clusters. The time efficiency of the multiple time step method enables us to investigate the long time interaction between lattice dislocations and low-angle grain boundaries.

  4. Stress Field of Non-equilibrium Grain Boundaries in Nano-crystalline Metals

    Institute of Scientific and Technical Information of China (English)

    2001-01-01

    Introducing the stress distribution near grain boundaries to improve the dislocation pileup model for the Hall-Petch (H-P) relation, the continuous distribution of dislocations in the pileup could be solved by means of Tschebysheff polynomials for the Hilbert transformation. An analytical formula of the stress intensity factor for the dislocation pileup is obtained. The reverse H-P relation may be explained by the modified dislocation-pileup-model.

  5. Mn vacancy defects, grain boundaries, and A-phase stability of helimagnet MnSi.

    Science.gov (United States)

    Ou-Yang, T Y; Shu, G J; Lin, J-Y; Hu, C D; Chou, F C

    2016-01-20

    Mn vacancy defect and grain size are shown to modify the magnetic phase diagram of MnSi significantly, especially near the critical regime of A-phase (skyrmion lattice) formation and the helimagnetic phase transition. Crystals grown using controlled nonstoichiometric initial precursors creates both grain boundaries and intrinsic Mn vacancy defect of various levels in MnSi. The results of combined transport, specific heat, and AC spin susceptibility measurements are compared for MnSi single crystal samples of various manganese deficiency levels and grain sizes. The finite-size effect and Mn vacancy level dependent helical phase transition temperature T(c) have been identified and verified. The stability of A-phase in H-T phase space has been examined through AC spin susceptibility data analysis.

  6. Mn vacancy defects, grain boundaries, and A-phase stability of helimagnet MnSi

    Science.gov (United States)

    Ou-Yang, T. Y.; Shu, G. J.; Lin, J.-Y.; Hu, C. D.; Chou, F. C.

    2016-01-01

    Mn vacancy defect and grain size are shown to modify the magnetic phase diagram of MnSi significantly, especially near the critical regime of A-phase (skyrmion lattice) formation and the helimagnetic phase transition. Crystals grown using controlled nonstoichiometric initial precursors creates both grain boundaries and intrinsic Mn vacancy defect of various levels in MnSi. The results of combined transport, specific heat, and AC spin susceptibility measurements are compared for MnSi single crystal samples of various manganese deficiency levels and grain sizes. The finite-size effect and Mn vacancy level dependent helical phase transition temperature {{T}\\text{c}} have been identified and verified. The stability of A-phase in H-T phase space has been examined through AC spin susceptibility data analysis.

  7. Observation of Pseudopartial Grain Boundary Wetting in the NdFeB-Based Alloy

    Science.gov (United States)

    Straumal, B. B.; Mazilkin, A. A.; Protasova, S. G.; Schütz, G.; Straumal, A. B.; Baretzky, B.

    2016-08-01

    The NdFeB-based alloys were invented in 1980s and remain the best-known hard magnetic alloys. In order to reach the optimum magnetic properties, the grains of hard magnetic Nd2Fe14B phase have to be isolated from one another by the (possibly thin) layers of a non-ferromagnetic Nd-rich phase. In this work, we observe that the few-nanometer-thin layers of the Nd-rich phase appear between Nd2Fe14B grains due to the pseudopartial grain boundary (GB) wetting. Namely, some Nd2Fe14B/Nd2Fe14B GBs are not completely wetted by the Nd-rich melt and have the high contact angle with the liquid phase and, nevertheless, contain the 2-4-nm-thin uniform Nd-rich layer.

  8. Challenges of Engineering Grain Boundaries in Boron-Based Armor Ceramics

    Science.gov (United States)

    Coleman, Shawn P.; Hernandez-Rivera, Efrain; Behler, Kristopher D.; Synowczynski-Dunn, Jennifer; Tschopp, Mark A.

    2016-06-01

    Boron-based ceramics are appealing for lightweight applications in both vehicle and personnel protection, stemming from their combination of high hardness, high elastic modulus, and low density as compared to other ceramics and metal alloys. However, the performance of these ceramics and ceramic composites is lacking because of their inherent low fracture toughness and reduced strength under high-velocity threats. The objective of the present article is to briefly discuss both the challenges and the state of the art in experimental and computational approaches for engineering grain boundaries in boron-based armor ceramics, focusing mainly on boron carbide (B4C) and boron suboxide (B6O). The experimental challenges involve processing these ceramics at full density while trying to promote microstructure features such as intergranular films to improve toughness during shock. Many of the computational challenges for boron-based ceramics stem from their complex crystal structure which has hitherto complicated the exploration of grain boundaries and interfaces. However, bridging the gaps between experimental and computational studies at multiple scales to engineer grain boundaries in these boron-based ceramics may hold the key to maturing these material systems for lightweight defense applications.

  9. Grain boundary self-diffusion of 51Cr in Fe-Cr-Ni alloys

    International Nuclear Information System (INIS)

    The grain boundary self-diffusion characteristics P=α.δ.Dg (α is the segregation factor, δ is the grain boundary width and Dg is the grain boundary diffusion coefficient) of 51Cr were measured in the two alloys Fe-18 Cr-12 Ni and Fe-21 Cr-31 Ni. Experiments were performed in the temperature range from 973 to 1223 K using the sectioning method. In the frames of experimental errors there was found no significant difference between the results for both alloys. The temperature dependence of the triple product P for both materials can be thus described by the Arrhenius-type equation P(Cr)=(1.17+2.1-0.75).10-10.exp {-(234±19)/RT} m3/s. Within the errors, P values obtained in this paper are identical with those obtained in previous work for diffusion of 59Fe and 63Ni in the same materials. For the diffusion of all three basic constituents in both alloys the following equation is proposed P (Fe, Ni, Cr)=(6.7+9.9-4.0).10-12.exp {-(207±17)/RT} m3/s. (orig.)

  10. NON-EQUILIBRIUM SOLUTE SEGREGATION TO AUSTENITIC GRAIN BOUNDARY IN FERRUM-NICKLE ALLOY

    Institute of Scientific and Technical Information of China (English)

    P. Wu; D.Y. Yu; X.L. He

    2001-01-01

    The development of non-equilibrium segregation of boron at grain boundaries in Fe-40%Ni alloy during continuous cooling process was experimentally observed with boronParticle Tracking Autoradiography (PTA) and Transmission Electron Microscopy(TEM). The samples with 10ppm boron were cooled at 2℃/s to 1040, 980, 920,860, 780 and 640℃ respectively after pre-heat treatment of 1150℃ for 15min witha Gleeble-1500 heat simulating machine, then water quenched to room temperature.The width of segregation layer and boron depletion zone, rich factor and other pc-rameters were measured by a special image analysis system. The experimental resultsof PTA show that the grain boundary segregation of boron during cooling process is adynamic process and the development of the non-equilibrium segregation experiencesthree stages: first increases rapidly from 1150 to 1040℃, then gently from 1040 to860℃, and rapidly again from 860℃ to 640℃. The width of boron depletion zoneincreases from about 11μm at 1040℃ to 26μm at 640℃. TEM observation showsthat boron precipitates exist at grain boundaries when the samples are cooled to below860℃. The experimental phenomena are briefly discussed.

  11. Deviation of permeable coarse-grained boundary resistance from Nikuradse's observations

    Science.gov (United States)

    Cheng, Nian-Sheng; Liu, Xingnian; Chen, Xingwei; Qiao, Changkai

    2016-02-01

    Nikuradse's (1933) rough pipe study is enormously influential in the understanding of flow resistance over a sediment bed. However, the rough boundary employed in Nikuradse's study differs from permeable sediment beds in rivers. This implies that the results derived from the rough pipe experiments may not be applicable for flows over a permeable coarse-grained bed. The present study aimed to explore to what extent the flow resistance of a permeable coarse-grained boundary deviates from the Nikuradse's observations. Experiments were conducted with rough pipes, which were prepared by overlaying the inner wall with one to four layers of spherical beads. The single layer roughness resembles the experimental setup reported in Nikuradse's study, while the multilayer of grains allows significant flow to pass through the porous roughness layer. In addition, the ratio of grain diameter, k, to pipe diameter, d, was chosen to be one to two orders greater than the range (0.001 measurements also suggest the existence of a laminar flow regime, in which the friction factor is inversely proportional to the Reynolds number. The observed variations in the flow resistance are attributed to both wall permeability and large-scale roughness.

  12. Dislocation Content Measured Via 3D HR-EBSD Near a Grain Boundary in an AlCu Oligocrystal

    Science.gov (United States)

    Ruggles, Timothy; Hochhalter, Jacob; Homer, Eric

    2016-01-01

    Interactions between dislocations and grain boundaries are poorly understood and crucial to mesoscale plasticity modeling. Much of our understanding of dislocation-grain boundary interaction comes from atomistic simulations and TEM studies, both of which are extremely limited in scale. High angular resolution EBSD-based continuum dislocation microscopy provides a way of measuring dislocation activity at length scales and accuracies relevant to crystal plasticity, but it is limited as a two-dimensional technique, meaning the character of the grain boundary and the complete dislocation activity is difficult to recover. However, the commercialization of plasma FIB dual-beam microscopes have made 3D EBSD studies all the more feasible. The objective of this work is to apply high angular resolution cross correlation EBSD to a 3D EBSD data set collected by serial sectioning in a FIB to characterize dislocation interaction with a grain boundary. Three dimensional high angular resolution cross correlation EBSD analysis was applied to an AlCu oligocrystal to measure dislocation densities around a grain boundary. Distortion derivatives associated with the plasma FIB serial sectioning were higher than expected, possibly due to geometric uncertainty between layers. Future work will focus on mitigating the geometric uncertainty and examining more regions of interest along the grain boundary to glean information on dislocation-grain boundary interaction.

  13. Oxygen Permeability and Grain-Boundary Diffusion Applied to Alumina Scales

    Science.gov (United States)

    Smialek, James L.; Jacobson, Nathan S.; Gleeson, Brian; Hovis, David B.; Heuer, Arthur H.

    2013-01-01

    High-temperature oxygen permeability measurements had determined grain-boundary diffusivities (deltaD(sub gb)) in bulk polycrystalline alumina (Wada, Matsudaira, and Kitaoka). They predict that oxygen deltaD(sub gb,O) varies with oxygen pressure as P(O2)(sup -1/6) at low pressure whereas aluminum deltaD(sub gb),Al varies with P(O2)(sup +3/16) at high pressure. These relations were used to evaluate alumina scale growth in terms of diffusivity and grain size. A modified Wagner treatment for dominant inward oxygen growth produces the concise solution: ?(sub i) = k(sub p,i)×G(sub i) = 12 deltaD(sub gb,O,int), where ?(sub i) is a constant and k(sub p,i) and G(sub i) refer to instantaneous values of the scale parabolic growth constant and grain size, respectively. A commercial FeCrAl(Zr) alloy was oxidized at 1100 to 1400 degC to determine k(sub p,i), interfacial grain size, ?, and thus deltaD(sub gb,O,int). The deltaD(sub gb,O,int) values predicted from oxidation at (375 kJ/mole) were about 20 times less than those obtained above (at 298 kJ/mole), but closer than extrapolations from high-temperature bulk measurements. The experimental oxidation results agree with similar FeCrAl(X) studies, especially where both k(sub p,i) and G(sub i) were characterized. This complete approach accounts for temperature-sensitive oxidation effects of grain enlargement, equilibrium interface pressure variation, and grain-boundary diffusivity.

  14. Crossing grain boundaries in metals by slip bands, cleavage and fatigue cracks.

    Science.gov (United States)

    Pineau, André

    2015-03-28

    The size and the character (low and large angle, special boundaries, tilt and twist boundaries, twins) of the grain boundaries (GBs) in polycrystalline materials influence their strength and their fracture toughness. Recent studies devoted to nanocrystalline (NC) materials have shown a deviation from the Hall-Petch law. Special GBs formed by Σ3 twins in face-centred cubic metals are also known to have a strong effect on the mechanical behaviour of these metals, in particular their work-hardening rate. Grain orientation influences also crack path, the fracture toughness of body-centred cubic (BCC) metals and the fatigue crack growth rate of microstructurally short cracks. This paper deals both with slip transfer at GBs and with the interactions between propagating cracks with GBs. In the analysis of slip transfer, the emphasis is placed on twin boundaries (TBs) for which the dislocation reactions during slip transfer are analysed theoretically, experimentally and using the results of atomic molecular simulations published in the literature. It is shown that in a number of situations this transfer leads to a normal motion of the TB owing to the displacement of partial dislocations along the TB. This motion can generate a de-twinning effect observed in particular in NC metals. Crack propagation across GBs is also considered. It is shown that cleavage crack path behaviour in BCC metals is largely dependent on the twist component of the GBs. A mechanism for the propagation of these twisted cracks involving a segmentation of the crack front and the existence of intergranular parts is discussed and verified for a pressure vessel steel. A similar segmentation seems to occur for short fatigue cracks although, quite surprisingly, this crossing mechanism for fatigue cracks does not seem to have been examined in very much detail in the literature. Metallurgical methods used to improve the strength of the materials, via grain boundaries, are briefly discussed.

  15. High thermoelectric performance of niobium-doped strontium titanate bulk material affected by all-scale grain boundary and inclusions

    International Nuclear Information System (INIS)

    The large thermal conductivity of SrTiO3 bulk material limits its potential application for high-temperature thermoelectricity. The effects of all-scale grain boundaries and inclusions on the thermoelectric performance of Nb-doped bulk SrTiO3 materials are investigated in this study. Nano- to microscale grain boundaries and inclusions reduce the thermal conductivity by 30%. As a result, the ZT value is enhanced 2.6 times by a combination of all-sized crystals, energy filtering effect, multilevel scattering behaviors of nano/microscale grain boundaries and inclusions

  16. Atomistic studies of grain boundaries and heterophase interfaces in alloys and compounds. Final report, July 1987-August 1998

    Energy Technology Data Exchange (ETDEWEB)

    Vitek, Vaclav

    1998-08-01

    The overarching goal of the research supported by this grant was investigation of the structure and properties of interfaces in multicomponent systems by atomistic modeling. Initially, the research was devoted to studies of segregation to grain boundaries in binary disordered alloys. The next step was then studies of the structure and properties of grain boundaries in ordered compounds, specifically Ni3Al and NiAl, and grain boundary segregation in these compounds in the case of off-stoichiometry. Finally, the structure of Nb/sapphire interfaces, in particular the core configurations of the misfit dislocations, was studied.

  17. Grain and phase boundary segregation in WC-Co with small V, Cr or Mn additions

    Energy Technology Data Exchange (ETDEWEB)

    Weidow, Jonathan, E-mail: jonathan.weidow@chalmers.se [Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden); Andren, Hans-Olof [Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden)

    2010-06-15

    The effect of small additions of V, Cr or Mn on the microchemistry of interfaces in WC-Co was studied using energy-dispersive X-ray spectroscopy in a transmission electron microscope and using atom probe tomography. For WC/binder phase boundaries, segregation of V, Cr and Mn was observed, with V being the element with the largest tendency for segregation. Segregation to WC/WC grain boundaries was observed in all the materials, corresponding to half a monolayer of close packed Co. In the materials containing V or Cr, 1/3 of the Co atoms were replaced by V or Cr. In the material containing Mn, 7% of the Co atoms were replaced by Mn. Co segregation was also observed to a WC/(V, W)C{sub x} phase boundary in the material containing V.

  18. Grain and phase boundary segregation in WC-Co with small V, Cr or Mn additions

    International Nuclear Information System (INIS)

    The effect of small additions of V, Cr or Mn on the microchemistry of interfaces in WC-Co was studied using energy-dispersive X-ray spectroscopy in a transmission electron microscope and using atom probe tomography. For WC/binder phase boundaries, segregation of V, Cr and Mn was observed, with V being the element with the largest tendency for segregation. Segregation to WC/WC grain boundaries was observed in all the materials, corresponding to half a monolayer of close packed Co. In the materials containing V or Cr, 1/3 of the Co atoms were replaced by V or Cr. In the material containing Mn, 7% of the Co atoms were replaced by Mn. Co segregation was also observed to a WC/(V, W)Cx phase boundary in the material containing V.

  19. Grain Boundary Carbon in Synthetic Quartzite: Implications for Electrical Conduction in the Crust

    Science.gov (United States)

    Price, J. D.; Watson, E. B.; Wark, D. A.

    2001-12-01

    Despite the repeated implication that grain boundary graphite forms electrically connected networks in the earth's deep crust, little is known about the equilibrium microstructure of graphite at high pressures and temperatures. To evaluate this, we conducted several piston cylinder experiments designed to equilibrate carbon with crystalline SiO2. In one set of experiments, stacked single crystal (SC) disks of polished quartz were coated with 0 to 150 nm of carbon film in 50 nm increments. The stacks were positioned horizontally in graphite capsules and were heated at 1.4 GPa to 1150° C for 48 hours in one experiment, and to 1500° C for 0.05 and 5 hours in two others. In another set of experiments, we produced two polycrystalline (PC) quartzites in textural equilibrium with small amounts of carbon. A powder consisting of 75-150 μ m grains of natural crystals was fired for three days at atmospheric P and 1000° C and coated with a 30-50 nm carbon film. In one experiment, the powder was encased in a graphite capsule; in the other, a Pt capsule was used. Both were equilibrated for 120 hours at 1300° C, 1 GPa. Polished sections of the products revealed that the low-T SC run contained a thin, dark film on all interfaces including the uncoated face; the short duration, high-T SC run contained a dark film on all of the coated interfaces, but not on the uncoated interface; and the longer duration, high-T SC run contained isolated opaque blebs that increased in density with increasing thickness of the initial film. Additionally, these SC products contained a small number of fractures with thin, dark films, blebs, or dendrites. Both PC experiments produced similar products, largely composed of polygonal quartz grains and apparently unconnected small dark grains located along grain boundaries. Most of these dark grains exhibited a rounded or globular morphology, but a few showed rational faces. The results suggest that carbon films are not stable along quartz grain

  20. Influence of grain boundary characteristics on thermal stability in nanotwinned copper

    Science.gov (United States)

    Niu, Rongmei; Han, Ke; Su, Yi-feng; Besara, Tiglet; Siegrist, Theo M.; Zuo, Xiaowei

    2016-01-01

    High density grain boundaries provide high strength, but may introduce undesirable features, such as high Fermi levels and instability. We investigated the kinetics of recovery and recrystallization of Cu that was manufactured to include both nanotwins (NT) and high-angle columnar boundaries. We used the isothermal Johnson-Mehl-Avrami-Kolmogorov (JMAK) model to estimate activation energy values for recovery and recrystallization and compared those to values derived using the non-isothermal Kissinger equation. The JMAK model hinges on an exponent that expresses the growth mechanism of a material. The exponent for this Cu was close to 0.5, indicating low-dimensional microstructure evolution, which is associated with anisotropic twin coarsening, heterogeneous recrystallization, and high stability. Since this Cu was of high purity, there was a negligible impurity-drag-effect on boundaries. The twin coarsening and heterogeneous recrystallization resulted from migration of high-angle columnar boundaries with their triple junctions in one direction, assisted by the presence of high concentration vacancies at boundaries. Analyses performed by electron energy loss spectroscopy of atomic columns at twin boundaries (TBs) and in the interior showed similar plasma peak shapes and L3 edge positions. This implies that values for conductivity and Fermi level are equal for atoms at TBs and in the interior. PMID:27514474

  1. Porous-grain-upper-boundary model and its application to Tarim Basin carbonates

    Institute of Scientific and Technical Information of China (English)

    Guo Yu-Qian; Ma Hong-Da; Shi Kai-Bo; Cao Hong; Huang Lu-Zhong; Yao Feng-Chang; Hu Tian-Yue

    2013-01-01

    Most of the carbonates in the Tarim Basin in northwest China are low-porosity and low-permeability rocks. Owing to the complexity of porosity in carbonates, conventional rock-physics models do not describe the relation between velocity and porosity for the Tarim Basin carbonates well. We propose the porous-grain-upper-boundary (PGU) model for estimating the relation between velocity and porosity for low-porosity carbonates. In this model, the carbonate sediments are treated as packed media of porous elastic grains, and the carbonate pores are divided into isolated and connected pores The PGU model is modified from the porous-grain-stiff-sand (PGST) model by replacing the critical porosity with the more practical isolated porosity. In the implementation, the effective elastic constants of the porous grains are calculated by using the differential effective medium (DEM) model. Then, the elastic constants of connected porous grains in dry rocks are calculated by using the modified upper Hashin-Shtrikman bound. The application to the Tarim carbonates shows that relative to other conventional effective medium models the PGU model matches the well log data well.

  2. Guidance to Design Grain Boundary Mobility Experiments with Molecular Dynamics and Phase-Field Modeling

    Energy Technology Data Exchange (ETDEWEB)

    Michael R Tonks; Yongfeng Zhang; S.B. Biner; Paul C Millett; Xianming Bai

    2013-02-01

    Quantitative phase-field modeling can play an important role in designing experiments to measure the grain boundary (GB) mobility. In this work, molecular dynamics (MD) simulation is employed to determine the GB mobility using Cu bicrystals. Two grain configurations are considered: a shrinking circular grain and a half loop grain. The results obtained from the half loop configuration approaches asymptotically to that obtained from the circular configuration with increasing half loop width. We then verify the phase- field model by directly comparing to the MD simulation results, obtaining excellent agreement. Next, this phase-field model is used to predict the behavior in a common experimental setup that utilizes a half loop grain configuration in a bicrystal to measure the GB mobility. With a 3D simulation, we identify the two critical times within the experiments to reach an accurate value of the GB mobility. We use a series of 2D simulations to investigate the impact of the notch angle on these two critical times and we identify an angle of 60? as an optimal value. We also show that if the notch does not have a sharp tip, it may immobilize the GB migration indefinitely.

  3. Dislocation structures and electrical conduction properties of low angle tilt grain boundaries in LiNbO3

    Science.gov (United States)

    Furushima, Yuho; Nakamura, Atsutomo; Tochigi, Eita; Ikuhara, Yuichi; Toyoura, Kazuaki; Matsunaga, Katsuyuki

    2016-10-01

    Dislocations in crystalline materials constitute unique, atomic-scale, one-dimensional structure and have a potential to induce peculiar physical properties that are not found in the bulk. In this study, we fabricated LiNbO3 bicrystals with low angle tilt grain boundaries and investigated the relationship between the atomic structure of the boundary dislocations and their electrical conduction properties. Observations by using transmission electron microscopy revealed that dislocation structures at the (0001) low angle tilt grain boundaries depend on the tilt angle of the boundaries. Specifically, the characteristic dislocation structures with a large Burgers vector were formed in the boundary with the tilt angle of 2°. It is noteworthy that only the grain boundary of 2° exhibits distinct electrical conductivity after reduction treatment, although LiNbO3 is originally insulating. This unique electrical conductivity is suggested to be due to the characteristic dislocation structures with a large Burgers vector.

  4. Characterization and Modeling of Grain Boundary Chemistry Evolution in Ferritic Steels under Irradiation

    Energy Technology Data Exchange (ETDEWEB)

    Marquis, Emmanuelle [Univ. of Michigan, Ann Arbor, MI (United States); Wirth, Brian [Univ. of Tennessee, Knoxville, TN (United States); Was, Gary [Univ. of Michigan, Ann Arbor, MI (United States)

    2016-03-28

    Ferritic/martensitic (FM) steels such as HT-9, T-91 and NF12 with chromium concentrations in the range of 9-12 at.% Cr and high Cr ferritic steels (oxide dispersion strengthened steels with 12-18% Cr) are receiving increasing attention for advanced nuclear applications, e.g. cladding and duct materials for sodium fast reactors, pressure vessels in Generation IV reactors and first wall structures in fusion reactors, thanks to their advantages over austenitic alloys. Predicting the behavior of these alloys under radiation is an essential step towards the use of these alloys. Several radiation-induced phenomena need to be taken into account, including phase separation, solute clustering, and radiation-induced segregation or depletion (RIS) to point defect sinks. RIS at grain boundaries has raised significant interest because of its role in irradiation assisted stress corrosion cracking (IASCC) and corrosion of structural materials. Numerous observations of RIS have been reported on austenitic stainless steels where it is generally found that Cr depletes at grain boundaries, consistently with Cr atoms being oversized in the fcc Fe matrix. While FM and ferritic steels are also subject to RIS at grain boundaries, unlike austenitic steels, the behavior of Cr is less clear with significant scatter and no clear dependency on irradiation condition or alloy type. In addition to the lack of conclusive experimental evidence regarding RIS in F-M alloys, there have been relatively few efforts at modeling RIS behavior in these alloys. The need for predictability of materials behavior and mitigation routes for IASCC requires elucidating the origin of the variable Cr behavior. A systematic detailed high-resolution structural and chemical characterization approach was applied to ion-implanted and neutron-irradiated model Fe-Cr alloys containing from 3 to 18 at.% Cr. Atom probe tomography analyses of the microstructures revealed slight Cr clustering and segregation to dislocations and

  5. The role of grain boundary structure and crystal orientation on crack growth asymmetry in aluminum

    Energy Technology Data Exchange (ETDEWEB)

    Adlakha, I. [School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287 (United States); Tschopp, M.A. [U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005 (United States); Solanki, K.N., E-mail: kiran.solanki@asu.edu [School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287 (United States)

    2014-11-17

    Atomistic simulations have shown that the grain boundary (GB) structure affects a number of physical, mechanical, thermal, and chemical properties, which can have a profound effect on macroscopic properties of polycrystalline materials. The research objective herein is to use atomistic simulations to explore the role that GB structure and the adjacent crystallographic orientations have on the directional asymmetry of an intergranular crack (i.e. cleavage behavior is favored along one direction, while ductile behavior along the other direction of the interface) for aluminum grain boundaries. Simulation results from seven 〈110〉 symmetric tilt grain boundaries (STGBs) show that the GB structure and the associated free volume directly influence the stress–strain response, crack growth rate, and crack tip plasticity mechanisms for middle-tension (M(T)) crack propagation specimens. In particular, the structural units present within the GB promote whether a dislocation or twinning-based mechanism operates at the crack tip during intergranular fracture along certain GBs (e.g., the ‘E’ structural unit promotes twinning at the crack tip in Al). Furthermore, the crystallography of the adjacent grains, and therefore the available slip planes, can significantly affect the crack growth rates in both directions of the crack – this creates a strong directional asymmetry in the crack growth rate in the Σ11 (113) and the Σ27 (552) STGBs. Upon comparing these results with the theoretical Rice criterion, it was found that certain GBs in this study (Σ9 (221), Σ11 (332) and Σ33 (441)) show an absence of directional asymmetry in the observed crack growth behavior, in conflict with the Rice criterion. The significance of the present research is that it provides a physical basis for the role of GB character and crystallographic orientation on intergranular crack tip deformation behavior.

  6. Influence of MnS particles on the behaviors of grain boundary migration in Fe-3%Si alloys

    Institute of Scientific and Technical Information of China (English)

    MAO WeiMin; AN ZhiGuo; LI ShuXia

    2009-01-01

    The precipitation behaviors of MnS particles at 900℃ in a hot deformed Fe-3%Si alloy were observed statistically. The ratio of MnS particles on dislocations and in grain boundaries was calculated based on a model concerning the second phase precipitation in supersaturated solid solution. It was indicated that the precipitation of MnS particles on dislocations prevailed. The coarsening process of MnS parti-cles in grain boundaries determined the boundary mobility during the secondary recrystallization. However, the density difference of precipitated MnS particles inside the grains on both sides of a boundary will determine the migration direction of the boundary as well, besides the grain size effect. It was observed that the densities of MnS particles in two neighboring grains were commonly different, and the boundary tended to move towards the area with lower particle density. The factors, e.g. dislo-cation densities in differently oriented grains will affect the density of precipitated particles, in which the Goss grains with higher particle density could grow more easily.

  7. Phase-field crystal study for the characteristics and influence factors of grain boundary segregation in binary alloys

    International Nuclear Information System (INIS)

    Grain boundary segregation strongly modifies grain boundary behaviors and affects the physical and mechanical properties of solid polycrystalline materials. In this paper, we study the grain boundary segregation characteristics and the variation law of grain boundary segregation with temperature, crystal misorientation angle, undercooling, lattice mismatch and the difference of interspecies bond energy and self-bond energy using the binary-alloy phase-field crystal model. The simulation results show that the solute atoms segregate into individual dislocation regions for the low-angle grain boundary while the solute atoms homogeneously segregate into the entire boundary for the high-angle grain boundary with nonzero initial concentration. The degree of segregation strongly increases when the temperature, the difference of interspecies bond energy and the self-bond energy decrease, and when misorientation and undercooling increase. Small lattice mismatches did not strongly affect segregation; however, the higher mismatch has obvious effects on segregation. Our simulation results agree well with theoretical and experimental results. (paper)

  8. Grain boundary precipitation treatment for improving high temperature low cycle fatigue strength of SSS113M for VHTR

    International Nuclear Information System (INIS)

    Grain boundary precipitation treatment was studied for the purpose of improving high temperature low cycle fatigue strength of a Ni-23% Cr-18% W alloy. SSS113M which had been developed as an intermediate heat exchanger material of VHTR and recognized as the best alloy in the national research project concerning nuclear steelmaking in Japan. Conventional single solid solution treatment of 13000C x 1h W.Q. does not cause any massive grain boundary precipitation in SSS113M, but additional heat treatment of 12500C x 1h W.Q. causes discontinuous grain boundary precipitation of α.W phase. This grain boundary precipitation treatment results in two- to fivefold increase of low cycle fatigue strength at 8000C as well as slightly higher creep and stress rupture strength at 10000C

  9. Micromagnetic simulation of the orientation dependence of grain boundary properties on the coercivity of Nd-Fe-B sintered magnets

    Science.gov (United States)

    Fujisaki, Jun; Furuya, Atsushi; Uehara, Yuji; Shimizu, Koichi; Ataka, Tadashi; Tanaka, Tomohiro; Oshima, Hirotaka; Ohkubo, Tadakatsu; Hirosawa, Satoshi; Hono, Kazuhiro

    2016-05-01

    This paper is focused on the micromagnetic simulation study about the orientation dependence of grain boundary properties on the coercivity of polycrystalline Nd-Fe-B sintered magnets. A multigrain object with a large number of meshes is introduced to analyze such anisotropic grain boundaries and the simulation is performed by combining the finite element method and the parallel computing. When the grain boundary phase parallel to the c-plane is less ferromagnetic the process of the magnetization reversal changes and the coercivity of the multigrain object increases. The simulations with various magnetic properties of the grain boundary phases are executed to search for the way to enhance the coercivity of polycrystalline Nd-Fe-B sintered magnets.

  10. Ab initio study of a Y-doped ∑31 grain boundary in alumina

    Institute of Scientific and Technical Information of China (English)

    CHEN Jun; XU Yun; CHEN DongQuan; ZHANG JingLin

    2008-01-01

    The atomic structures and energetics of clean and Y-doped general grain boundary (GB) ∑31/(0001) models in α-Al2O3 are studied by a series of high precision ab initio calculations. A large supercell with 700 atoms and periodic boundary conditions is adopted for undoped and Y-doped GB with different substitution sites and con-centrations. It is shown that Y atoms preferably segregate to the central column of the 7-member Al ring. This is explained as more favorable bond formation for Y in this position and lower GB energy. The calculated GB formation energy for the clean and Y-doped cases is respectively 3.99 and 3.67 J/m2. On the average, the GB region in ∑31 has a slightly lower charge density than the bulk crystalline region. In addtition, the GB induces a long ranged asymmetric electrostatic potential distri-bution on each side of the grain boundary.

  11. Ab initio study of a Y-doped Σ31 grain boundary in alumina

    Institute of Scientific and Technical Information of China (English)

    2008-01-01

    The atomic structures and energetics of clean and Y-doped general grain boundary (GB) Σ31/(0001) models in α-Al2O3 are studied by a series of high precision ab initio calculations. A large supercell with 700 atoms and periodic boundary conditions is adopted for undoped and Y-doped GB with different substitution sites and con-centrations. It is shown that Y atoms preferably segregate to the central column of the 7-member Al ring. This is explained as more favorable bond formation for Y in this position and lower GB energy. The calculated GB formation energy for the clean and Y-doped cases is respectively 3.99 and 3.67 J/m2. On the average, the GB region in Σ31 has a slightly lower charge density than the bulk crystalline region. In addtition, the GB induces a long ranged asymmetric electrostatic potential distri-bution on each side of the grain boundary.

  12. Dynamics of Nanoscale Grain-Boundary Decohesion in Aluminum by Molecular-Dynamics Simulation

    Science.gov (United States)

    Yamakov, V.; Saether, E.; Phillips, D. R.; Glaessegen, E. H.

    2007-01-01

    The dynamics and energetics of intergranular crack growth along a flat grain boundary in aluminum is studied by a molecular-dynamics simulation model for crack propagation under steady-state conditions. Using the ability of the molecular-dynamics simulation to identify atoms involved in different atomistic mechanisms, it was possible to identify the energy contribution of different processes taking place during crack growth. The energy contributions were divided as: elastic energy, defined as the potential energy of the atoms in fcc crystallographic state; and plastically stored energy, the energy of stacking faults and twin boundaries; grain-boundary and surface energy. In addition, monitoring the amount of heat exchange with the molecular-dynamics thermostat gives the energy dissipated as heat in the system. The energetic analysis indicates that the majority of energy in a fast growing crack is dissipated as heat. This dissipation increases linearly at low speed, and faster than linear at speeds approaching 1/3 the Rayleigh wave speed when the crack tip becomes dynamically unstable producing periodic dislocation bursts until the crack is blunted.

  13. Local probe microscopic studies on Al-doped ZnO: Pseudoferroelectricity and band bending at grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Kumar, Mohit; Basu, Tanmoy; Som, Tapobrata, E-mail: tsom@iopb.res.in [SUNAG Laboratory, Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005 (India)

    2016-01-07

    In this paper, based on piezoforce measurements, we show the presence of opposite polarization at grains and grain boundaries of Al-doped ZnO (AZO). The polarization can be flipped by 180° in phase by switching the polarity of the applied electric field, revealing the existence of nanoscale pseudoferroelectricity in AZO grown on Pt/TiO{sub 2}/SiO{sub 2}/Si substrate. We also demonstrate an experimental evidence on local band bending at grain boundaries of AZO films using conductive atomic force microscopy and Kelvin probe force microscopy. The presence of an opposite polarization at grains and grain boundaries gives rise to a polarization-driven barrier formation at grain boundaries. With the help of conductive atomic force microscopy, we show that the polarization-driven barrier along with the defect-induced electrostatic potential barrier account for the measured local band bending at grain boundaries. The present study opens a new avenue to understand the charge transport in light of both polarization and electrostatic effects.

  14. Energetic driving force for preferential binding of self-interstitial atoms to Fe grain boundaries over vacancies

    Energy Technology Data Exchange (ETDEWEB)

    Tschopp, Mark A.; Horstemeyer, Mark; Gao, Fei; Sun, Xin; Khaleel, Mohammad A.

    2011-05-02

    Molecular dynamics simulations of 50 Fe grain boundaries were used to understand their interaction with vacancies and self-interstitial atoms at all atomic positions within 20 °A of the boundary, which is important for designing radiation-resistant polycrystalline materials. Site-to-site variation within the boundary of both vacancy and self-interstitial formation energies is substantial, with the majority of sites having lower formation energies than in the bulk. Comparing the vacancy and self-interstitial atom binding energies for each site shows that there is an energetic driving force for interstitials to preferentially bind to grain boundary sites over vacancies. Furthermore, these results provide a valuable dataset for quantifying uncertainty bounds for various grain boundary types at the nanoscale, which can be propagated to higher scale simulations of microstructure evolution.

  15. Large modulation of carrier transport by grain-boundary molecular packing and microstructure in organic thin films

    KAUST Repository

    Rivnay, Jonathan

    2009-11-08

    Solution-processable organic semiconductors are central to developing viable printed electronics, and performance comparable to that of amorphous silicon has been reported for films grown from soluble semiconductors. However, the seemingly desirable formation of large crystalline domains introduces grain boundaries, resulting in substantial device-to-device performance variations. Indeed, for films where the grain-boundary structure is random, a few unfavourable grain boundaries may dominate device performance. Here we isolate the effects of molecular-level structure at grain boundaries by engineering the microstructure of the high-performance n-type perylenediimide semiconductor PDI8-CN 2 and analyse their consequences for charge transport. A combination of advanced X-ray scattering, first-principles computation and transistor characterization applied to PDI8-CN 2 films reveals that grain-boundary orientation modulates carrier mobility by approximately two orders of magnitude. For PDI8-CN 2 we show that the molecular packing motif (that is, herringbone versus slip-stacked) plays a decisive part in grain-boundary-induced transport anisotropy. The results of this study provide important guidelines for designing device-optimized molecular semiconductors. © 2009 Macmillan Publishers Limited. All rights reserved.

  16. Preparation of high-quality HTS rings for application in the magnetic bearing of cryotanks and pinning in grain boundaries

    Science.gov (United States)

    Bringmann, B.; Walter, H.; Jooss, Ch.; Leenders, A.; Freyhardt, H. C.

    2002-08-01

    Seeded melt growth of YBCO high-temperature superconductors is one of the most promising preparation techniques to obtain high-quality HTS tiles for application, e.g. in magnetic bearings. Semi-finished HTSL products of complex shapes have to be developed by different seeding and multi-seeding techniques. To obtain large hollow cylinders designed for application in the magnetic bearing of a cryotank a modified multi-seeded melt growth (MSMG) process was employed. This cryotank will be mounted for testing in a vehicle of a major German car manufacturer. The MSMG process introduces grain boundaries into the HTS tiles. For transport current investigations of [0 0 1]-tilt grain boundaries in melt textured YBCO a series of MSMG bicrystals have been prepared. They exhibit a dependence of the critical current density on misorientation angle which is much weaker than the one observed in thin-film bicrystals. The bulk samples have dimensions larger than the magnetic penetration depth along the grain boundary. Thus, flux pinning has to be taken into account. Different contributions to the longitudinal pinning force have to be considered: vortices at grain boundaries can be pinned by magnetic interaction with Abrikosov vortices in the banks, by defects in the grain boundary itself or by defects which are located next to the grain boundary.

  17. Control of one-dimensional magnetism in graphene via spontaneous hydrogenation of the grain boundary.

    Science.gov (United States)

    Yin, Wan-Jian; Wei, Su-Huai; Yan, Yanfa

    2013-06-01

    We propose that control of one-dimensional (1D) magnetism in graphene could be made easier by spontaneous hydrogenation of chemically reactive grain boundaries (GBs) in polycrystalline graphenes. Unlike pristine graphene, where hydrogen adsorption favors the formation of zero-dimensional (0D) clusters, the defect cores (pentagon, heptagon and octagon) at the GBs in polycrystalline graphene promote hydrogenation along the GBs. The hydrogenation in polycrystalline graphene starts at the GBs, proceeds gradually towards the grain interior (GI) and results in smooth 1D graphane-graphene interfaces. Our calculations show that the type (ferro- or antiferro-magnetism) and strength of the magnetism can be controlled by controlling the orientation of GBs. Since GBs in single-layer graphenes can be fabricated in a controllable way in experiments, the hydrogenation of GBs could be a unique method to realize large-area magnetic graphenes for future spintronic applications.

  18. Self-organized pseudo-graphene on grain boundaries in topological band insulators

    Science.gov (United States)

    Slager, Robert-Jan; Juričić, Vladimir; Lahtinen, Ville; Zaanen, Jan

    2016-06-01

    Semimetals are characterized by nodal band structures that give rise to exotic electronic properties. The stability of Dirac semimetals, such as graphene in two spatial dimensions, requires the presence of lattice symmetries, while akin to the surface states of topological band insulators, Weyl semimetals in three spatial dimensions are protected by band topology. Here we show that in the bulk of topological band insulators, self-organized topologically protected semimetals can emerge along a grain boundary, a ubiquitous extended lattice defect in any crystalline material. In addition to experimentally accessible electronic transport measurements, these states exhibit a valley anomaly in two dimensions influencing edge spin transport, whereas in three dimensions they appear as graphenelike states that may exhibit an odd-integer quantum Hall effect. The general mechanism underlying these semimetals—the hybridization of spinon modes bound to the grain boundary—suggests that topological semimetals can emerge in any topological material where lattice dislocations bind localized topological modes.

  19. Quantitatively Analyzing Strength Contribution vs Grain Boundary Scale Relation in Pure Titanium Subjected to Severe Plastic Deformation

    Science.gov (United States)

    Luo, Peng; Hu, Qiaodan; Wu, Xiaolin

    2016-05-01

    Electron backscatter diffraction was used to reveal high- and low-angle grain boundaries (HAGBs, with misorientation ≥15 deg, and LAGBs, equal channel angular pressing. Comprehensive paradigms were developed to present relations of yield strength vs HAGB grain diameter, and LAGB contribution vs LAGB linear intercept. Incorporating grain orientations (against loading axis) into the Hall-Petch relation, we quantitatively investigated the strength contributions by HAGBs and LAGBs, respectively.

  20. Large Tc depression at low angle [100] tilt grain boundaries in bulk Bi2Sr2CaCu2O8+δ bicrystals

    International Nuclear Information System (INIS)

    Large depression of Tc at 7 degree [100] tilt grain boundaries was observed in bulk Bi2Sr2CaCu2O8+δ (Bi2212) bicrystals by measuring the zero-field electrical transport properties of the grain boundaries and the constituent single crystals over an extended range of currents and voltages. The Tc-depressed region was determined to be around 20 nm, comparable to the width of the strain field associated with the observed array of grain-boundary dislocations. Superconducting coupling of the grain boundaries increases sharply as temperature decreases below the grain-boundary Tc congruent 68 K. copyright 1997 American Institute of Physics

  1. Binding energetics of substitutional and interstitial helium and di-helium defects with grain boundary structure in α-Fe

    Energy Technology Data Exchange (ETDEWEB)

    Tschopp, M. A., E-mail: mark.tschopp@gatech.edu [Dynamic Research Corporation, (on site at) U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 (United States); Center for Advanced Vehicular Systems, Mississippi State University, Starkville, Mississippi 39762 (United States); Gao, F.; Yang, L. [Pacific Northwest National Laboratory, Richland, Washington 99352 (United States); Solanki, K. N. [Arizona State University, School for Engineering of Matter, Transport and Energy, Tempe, Arizona 85287 (United States)

    2014-01-21

    The formation/binding energetics and length scales associated with the interaction between He atoms and grain boundaries in BCC α-Fe were explored. Ten different low Σ grain boundaries from the 〈100〉 and 〈110〉 symmetric tilt grain boundary systems were used. In this work, we then calculated formation/binding energies for 1–2 He atoms in the substitutional and interstitial sites (HeV, He{sub 2}V, HeInt, He{sub 2}Int) at all potential grain boundary sites within 15 Å of the boundary (52 826 simulations total). The present results provide detailed information about the interaction energies and length scales of 1–2 He atoms with grain boundaries for the structures examined. A number of interesting new findings emerge from the present study. For instance, the Σ3(112) twin boundary in BCC Fe possesses a much smaller binding energy than other boundaries, which corresponds in long time dynamics simulations to the ability of an interstitial He defect to break away from the boundary in simulations on the order of nanoseconds. Additionally, positive correlations between the calculated formation/binding energies of the He defects (R > 0.9) asserts that the local environment surrounding each site strongly influences the He defect energies and that highly accurate quantum mechanics calculations of lower order defects may be an adequate predictor of higher order defects. Various metrics to quantify or classify the local environment were compared with the He defect binding energies. The present work shows that the binding and formation energies for He defects are important for understanding the physics of He diffusion and trapping by grain boundaries, which can be important for modeling He interactions in polycrystalline steels.

  2. Interaction of run-in edge dislocations with twist grain boundaries in Al-a molecular dynamics study

    Science.gov (United States)

    Chandra, S.; Naveen Kumar, N.; Samal, M. K.; Chavan, V. M.; Patel, R. J.

    2016-06-01

    Grain boundaries play an important role in outlining the mechanical properties of crystalline materials. They act as sites for absorption/nucleation of dislocations, which are the main carriers of plastic deformation. In view of this, the interactions between edge dislocations and twist grain boundaries-dislocation pileup, dislocation absorption and dislocation emission were explored by performing molecular dynamics simulations in face-centered cubic Al using embedded atom method. The ?1 1 0? twist grain boundaries with various misorientation angles were selected for this purpose. It was found that the misorientation angle of boundary and stress anomalies arising from repeated dislocation absorption at the grain boundaries are the important parameters in determining the ability of the boundary to emit dislocations. Complex network of dislocations results in later stages of deformation, which may have a significant effect on the mechanical properties of the material. The peculiarities of dislocation nucleation, their emission from twist grain boundaries and the ramifications of this study towards development of higher length scale material models are discussed.

  3. Shaping the lens of the atom probe: Fabrication of site specific, oriented specimens and application to grain boundary analysis

    International Nuclear Information System (INIS)

    The random sampling provided by classical atom probe sample preparation methods is one of the major factors limiting the types of problems that can be addressed using this powerful technique. A focused ion beam enables not only site-specific preparation, but can also be used to give the specimen, which acts as the lens in an atom probe experiment, a specific shape. In this paper we present a technique that uses low accelerating voltages (10 and 5 kV) in the focused ion beam (FIB) to reproducibly produce specimens with selected grain boundaries <100 nm from the tip at any desired orientation. These tips have a high rate of successfully running in the atom probe and no Ga contamination within the region of interest. This technique is applied to the analysis of grain boundaries in a high purity iron wire and a strip-cast steel. Lattice resolution is achieved around the boundary in certain areas. Reconstruction of these datasets reveals the distribution of light and heavy elements around the boundary. Issues surrounding the uneven distribution of certain solute elements as a result of field-induced diffusion are discussed. -- Research highlights: → Damage free site specific samples can be made using FIB. → Crystallographic relationships across grain boundaries can be obtained from field desorption patterns. → Lattice resolution can be achieved at grain boundaries for matrix atoms. → Field evaporation artifacts are observed at grain boundaries for solute atoms.

  4. Final report: Constructing comprehensive models of grain boundaries using high-throughput experiments

    Energy Technology Data Exchange (ETDEWEB)

    Demkowicz, Michael [Texas A & M Univ., College Station, TX (United States); Schuh, Christopher [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States); Marzouk, Youssef [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)

    2016-08-29

    This is the final report on project DE-SC0008926. The goal of this project was to create capabilities for constructing, analyzing, and modeling experimental databases of the crystallographic characters and physical properties of thousands of individual grain boundaries (GBs) in polycrystalline metals. This project focused on gallium permeation through aluminum (Al) GBs and hydrogen uptake into nickel (Ni) GBs as model problems. This report summarizes the work done within the duration of this project (including the original three-year award and the subsequent one-year renewal), i.e. from August 1, 2012 until April 30, 2016.

  5. Microstructural evolution adjacent to grain boundaries under cascade damage conditions and helium production

    DEFF Research Database (Denmark)

    Trinkaus, H.; Singh, B.N.; Victoria, M.

    1996-01-01

    be absorbed by extended sinks such as dislocations and grain boundaries (GBs). The loss of SIAs by this process causes a vacancy supersaturation representing an efficient driving force for void swelling, in particular in regions adjacent to GBs. Enhanced swelling in regions adjacent to GBs has been observed...... in several metals subject to irradiation by both fast fission neutrons and 600 MeV protons. In the latter case, however, the width of the region of enhanced swelling is smaller and the amount of swelling is significantly lower than in the former case. Recently, enhanced swelling near GBs as induced...

  6. Simulation of He embrittlement at grain boundaries in bcc transition metals

    Science.gov (United States)

    Suzudo, Tomoaki; Yamaguchi, Masatake

    2015-10-01

    To investigate what atomic properties largely determine vulnerability to He embrittlement at grain boundaries (GB) of bcc metals, we introduce a computational model composed of first principles density functional theory and a He segregation rate theory model. Predictive calculations of He embrittlement at the first wall of the future DEMO fusion concept reactor indicate that variation in the He embrittlement originated not only from He production rate related to neutron irradiation, but also from the He segregation energy at the GB that has a systematic trend in the periodic table.

  7. Materials Design of Microstructure in Grain Boundary and Second Phase Particles

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    A concept of microstructure design for materials or materials microstructure engineering is proposed. The argument was suggested based on literature review and some our new research work on second phase strengthening mechanisms and mechanical property modeling of a particulate reinforced metal matrix composite. Due to development of computer technology, it is possible now for us to establish the relationship between microstructures and properties systematically and quantitatively by analytical and numerical modeling in the research scope of computerization materials. Discussions and examples on intellectual optimization of microstructure are presented on two aspects:grain boundary engineering and optimal geometry of particulate reinforcements in two-phase materials.

  8. Physics and applications of novel structures with CVD graphene: edges, grain boundaries, twisted bilayers, and hybrids

    Science.gov (United States)

    Chen, Yong P.

    2014-03-01

    In this talk, I will discuss experimental studies (including electronic transport, optical/Raman, and STM) of physical properties of various novel synthetic graphene structures formed in CVD graphene grown on Cu, including edges of graphene single crystals, grain boundaries between such single crystals, and twisted bilayer graphene. Such synthetic graphene structures could be used as playground to explore novel physics and engineer new functionalities in graphene based electronic devices. Furthermore, I will discuss graphene based ``hybrid'' materials combining CVD graphene with semiconductor and metallic nanostructures for potential optoelectronic and plasmonics applications.

  9. Atomic structure of grain boundaries in iron modeled using the atomic density function

    OpenAIRE

    Kapikranian, O.; Zapolsky, H; Domain, Ch.; Patte, R.; Pareige, C.; Radiguet, B.; Pareige, P.

    2013-01-01

    A model based on the continuous atomic density function (ADF) approach is applied to predict the atomic structure of grain boundaries (GBs) in iron. Symmetrical [100] and [110] tilt GBs in bcc iron are modeled with the ADF method and relaxed afterwards in molecular dynamics (MD) simulations. The shape of the GB energy curve obtained in the ADF model reproduces well the peculiarities of the angles of 70.53 deg. [$\\Sigma$ 3(112)] and 129.52 deg. [$\\Sigma$ 11(332)] for [110] tilt GBs. The result...

  10. Linewidth of Josephson oscillations in YBa2Cu3O7-x grain-boundary junctions

    DEFF Research Database (Denmark)

    Divin, Yu. Ya.; Mygind, Jesper; Pedersen, Niels Falsig;

    1993-01-01

    The AC Josephson effect in YBa2Cu3O7-x grain-boundary junctions (GBJs) was studied in the temperature range from 4 K to 90 K. The temperature dependence of the linewidth of millimeter-wave Josephson oscillations was measured, and it is shown that the derived effective noise temperature of GBJ might...... Josephson oscillations observed at 77 K was equal to 380 MHz, which demonstrates the applicability of GBJ, particularly in the field of radiation spectroscopy, even at liquid nitrogen temperatures...

  11. Origin of intergranular embrittlement of Al alloys induced by Na and Ca segregation: Grain boundary weakening

    International Nuclear Information System (INIS)

    Using a first-principles computational tensile test, we show that the ideal tensile strength of an Al grain boundary (GB) is reduced with both Na and Ca GB segregation. We demonstrate that the fracture occurs in the GB interface, dominated by the break of the interfacial bonds. Experimentally, we further show that the presence of Na or Ca impurity, which causes intergranular fracture, reduces the ultimate tensile strength when embrittlement occurs. These results suggest that the Na/Ca-induced intergranular embrittlement of an Al alloy originates mainly from the GB weakening due to the Na/Ca segregation

  12. Amorphous grain boundary layers in the ferromagnetic nanograined ZnO films

    International Nuclear Information System (INIS)

    Pure ZnO thin films were obtained by the wet chemistry (“liquid ceramics”) method from the butanoate precursors. Films consist of dense equiaxial nanograins and reveal ferromagnetic behaviour. The structure of the ZnO films was studied by the high-resolution transmission electron microscopy. The intergranular regions in the nanograined ZnO films obtained by the “liquid ceramics” method are amorphous. It looks like fine areas of the second amorphous phase which wets (covers) some of the ZnO/ZnO grain boundaries. Most probably these amorphous intergranular regions contain the defects which are responsible for the ferromagnetic behaviour.

  13. Numerical study of the atomic and electronic structure of some silicon grain boundaries; Etude numerique de la structure atomique et electronique de quelques joints de grains du silicium

    Energy Technology Data Exchange (ETDEWEB)

    Torrent, M

    1996-07-01

    This work contributes to the theoretical study of extended defects in covalent materials. The study is especially devoted to the tilt grain boundaries in silicon as a model material. The theoretical model is based on the self-consistent tight-binding approximation and is applied within two numerical techniques: the fast 'order N' density-matrix method and the diagonalization technique which allows the sampling of the reciprocal space. Total energy parameters of the model have been fitted in order to reproduce the silicon band structure (with a correct gap value) and the transferability of crystalline and mechanical properties of this material. A new type of boundary conditions is proposed and tested. These conditions, named 'ante-periodic' or 'Moebius', allow only one grain boundary per box instead of two and decrease the CPU time by a factor of two. The model is then applied to the study of the {sigma}=25 [001] (710) grain boundary. The results show the possible presence in this boundary of low energy non-reconstructed atomic structures which are electrically active. This confirms what had been suggested by some experimental observations. The same study is also performed for the {sigma}=13 [001] (510) grain boundary. In order to compare the intrinsic electrical activity in the previous grain boundaries with the one induced by impurities, a total energy parametrization for the silicon-nickel bond is achieved and used in preliminary calculations. Finally the two variants of the {sigma}=11 [011] (2-33) interface are studied, especially their respective interfacial energies. The result disagrees with previous calculations using phenomenological potentials. (author)

  14. Variable-charge method applied to study coupled grain boundary migration in the presence of oxygen

    Energy Technology Data Exchange (ETDEWEB)

    Elsener, A. [Paul Scherrer Institute, Villigen, PSI-Villigen CH-5232 (Switzerland); Politano, O. [Institut Carnot de Bourgogne, UMR 5209 CNRS-Universite de Bourgogne, 9 Avenue Alain Savary, 21078 Dijon Cedex (France); Derlet, P.M. [Paul Scherrer Institute, Villigen, PSI-Villigen CH-5232 (Switzerland); Van Swygenhoven, H. [Paul Scherrer Institute, Villigen, PSI-Villigen CH-5232 (Switzerland)], E-mail: helena.vs@psi.ch

    2009-04-15

    One of the important differences between simulation and experiments in grain boundary (GB)-dominated metallic structures is the lack of impurities such as oxygen in computational samples. A modified variable-charge method [Elsener A, Politano O, Derlet PM, Van Swygenhoven H. Modell Simul Mater Sci Eng 2008;16:025006] based on the Streitz and Mintmire approach [Streitz FH, Mintmire JW. Phys Rev B 1994;50:11996] is used to study coupled GB motion in an Al bicrystal with a [1 1 2] symmetrical tilt GB in the presence of substitutional O, and compared with the stick-slip process identified by Cahn and Mishin [Cahn JW, Mishin Y, Suzuki A. Acta Mater 2006;54:4953]. It is found that the critical shear stress for migration of the GB increases linearly with the number of O atoms. These observations are then rationalized in terms of the internal stress signature of the O atoms in the vicinity of the boundary.

  15. Tunable-slip boundaries for coarse-grained simulations of fluid flow

    CERN Document Server

    Smiatek, Jens; Schmid, Friederike

    2007-01-01

    On the micro- and nanoscale, classical hydrodynamic boundary conditions such as the no-slip condition no longer apply. Instead, the flow profiles exhibit ``slip`` at the surface, which is characterized by a finite slip length (partial slip). We present a new, systematic way of implementing partial-slip boundary conditions with arbitrary slip length in coarse-grained computer simulations. The main idea is to represent the complex microscopic interface structure by a spatially varying effective viscous force. An analytical equation for the resulting slip length can be derived for planar and for curved surfaces. The comparison with computer simulations of a DPD (dissipative particle dynamics) fluid shows that this expression is valid from full-slip to no-slip.

  16. The influence of low-strain thermo-mechanical processing on grain boundary network characteristics in type 304 austenitic stainless steel.

    Science.gov (United States)

    Engelberg, D L; Humphreys, F J; Marrow, T J

    2008-06-01

    Grain boundary engineering of austenitic stainless steel, through the introduction of plastic strain and thermal annealing, can be used to develop microstructures with improved resistance to inter-granular degradation. The influence of low-strain thermo-mechanical processing on grain boundary network development, with systematic variations of annealing treatments, has been investigated. Three stages of the microstructure development during grain boundary engineering in low-strain processing conditions are identified, and correlated with changes in grain boundary character and deviation distributions. Low-energy connected length segments at triple junctions, which have been proposed to be responsible for crack bridging during inter-granular stress corrosion cracking, can be influenced by the choice of the annealing treatment parameters. The development of individual grain boundary length segments of different character showed consistent trends with increasing grain size. Crack length predictions are consistent with the beneficial effect of designing microstructures with high fractions of twin grain boundaries and smaller grain size. PMID:18503670

  17. Fluid distribution in grain boundaries of natural fine-grained rock salt deformed at low shear stress: implications for rheology and transport properties

    Science.gov (United States)

    Desbois, G.; Urai, J. L.; De Bresser, J. H. P.

    2012-04-01

    We used a combination of broad ion beam (BIB) cross-sectioning and high resolution (cryogenic) SEM to image polished surfaces and corresponding pairs of fractured grain boundaries in an investigation of grain boundary (GB) microstructures and fluid distribution in naturally deformed halite from a salt glacier (Kum Quh, central Iran). At the scale of observations, four types of fluid or gas filled grain boundaries can be distinguished: (1) straight boundaries with thick (up to 10 µm) GB tubes (2) straight boundaries with narrow (about 50 nm) GB tubes (3) wavy (tens of µm wavelength) GB with isolated inclusions of a few µm, and (4) wavy (µm wavelength) GB with small (µm) isolated inclusions. Grain boundary fluid inclusions can have three types of morphologies: the inclusion of Type 1 is intruded completely in one grain, inclusion of Type 2 has its major part included in one grain with a minor part in the second grain and the inclusion of Type 3 is located in both grains. Solid second phases in GB are mainly euhedral anhydrite crystals. The mobility of the brine is shown after cutting the inclusions by BIB in vacuum and fine-grained halite forms efflorescence and precipitates on internal walls of inclusions. At cryogenic temperature, in-situ brine is seen as continuous film in GB of type (1) and (2), and in isolated inclusions in GB of type (3) and (4). The structure of halite-halite contact between isolated fluid inclusions in GB of type (3) and (4) is below the resolution of SEM. GB of type (3) and (4) are interpreted to have formed by healing of mobile fluid films. First results of deformation experiments on the same samples under shear stress corresponding to conditions of natural salt glacier, show very low strain rates (7.43x10-10 s-1 and 1x10-9 s-1), up to one order of magnitude below of expected strain rates by solution precipitation creep. Both microstructures and deformation experiments suggest interfacial energy-driven GB healing, in agreement with the

  18. On Techniques to Characterize and Correlate Grain Size, Grain Boundary Orientation and the Strength of the SiC Layer of TRISO Coated Particles: A Preliminary Study

    Energy Technology Data Exchange (ETDEWEB)

    I.J.van Rooyen; J.L. Dunzik Gougar; T. Trowbridge; Philip M van Rooyen

    2012-10-01

    The mechanical properties of the silicon carbide (SiC) layer of the TRi-ISOtropic (TRISO) coated particle (CP) for high temperature gas reactors (HTGR) are performance parameters that have not yet been standardized by the international HTR community. Presented in this paper are the results of characterizing coated particles to reveal the effect of annealing temperature (1000 to 2100°C) on the strength and grain size of unirradiated coated particles. This work was further expanded to include possible relationships between the grain size and strength values. The comparative results of two strength measurement techniques and grain size measured by the Lineal intercept method are included. Preliminary grain boundary characterization results determined by electron backscatter diffraction (EBSD) are included. These results are also important for future fission product transport studies, as grain boundary diffusion is identified as a possible mechanism by which 110mAg, one of the fission activation products, might be released through intact SiC layers. Temperature is a parameter known to influence the grain size of SiC and therefore it is important to investigate the effect of high temperature annealing on the SiC grain size. Recommendations and future work will also be briefly discussed.

  19. Non-uniform Stress Field and Stress Concentration Induced by Grain Boundary and Triple Junction of Tricrystal

    Institute of Scientific and Technical Information of China (English)

    Jiansong WAN; Zhufeng YUE

    2003-01-01

    The stress characteristics in the anisotropic bicrystal and tricrystal specimens were analyzed using the anisotropic elastic model, orthotropic Hill's model and rate-dependent crystallographic model. The finite element analysis results show that non-uniform stresses are induced by the grain boundary. For bicrystal specimens in different crystallographic orientations, there exist stress concentrations and high stress gradients nearby the boundaries. The activation and slipping of the slip systems are dependent on the crystallographic orientations of the grains and also on the relative crystallographic orientations of the two adjoining grains. For the tricrystal specimens, there is not always any stress concentrations in the triple junction, and the concentration degree depends on the relative crystallographic orientations of the three grains. Different from the bicrystal specimens, there may be or no stress concentration in the vicinity of grain boundaries for the tricrystal specimens, which depends on the relative crystallographic orientations of the three grains. The stress concentration near to the grain boundaries and triple junction can be high enough for the local plastic deformation, damage and voiding or cracking even when the whole specimen is still under the elastic state.It can be further concluded that homogeneous assumption for polycrystalline materials is not suitable to study the detailed meso- or micro-mechanisms for damaging and fracturing.

  20. Grain boundary chemistry and heat treatment effects on the ductile-to-brittle transition behavior of vanadium alloys

    International Nuclear Information System (INIS)

    One-third scale Charpy impact specimens of V-4Cr-4Ti were given the same heat treatments applied to equivalent specimens of V-5Cr-5Ti. Auger specimens of V-4Cr-4Ti were also heat treated with the Charpy specimens to enable grain boundary chemistry measurements. The microstructural, microchemical and Charpy impact response of V-4Cr-4Ti displayed trends similar to those observed for V-5Cr-5Ti. The results show that grain size plays an important role in determining the ductile-to-brittle transition temperature (DBTT) of these materials and that a threshold level of grain boundary segregant appears to be required to cause grain boundary embrittlement and intergranular fracture

  1. Grain boundary chemistry and heat treatment effects on the ductile-to-brittle transition behavior of vanadium alloys

    Energy Technology Data Exchange (ETDEWEB)

    Kurtz, R.J.; Hamilton, M.L.; Li, H. [Pacific Northwest National Lab., Richland, WA (United States)

    1998-03-01

    One-third scale Charpy impact specimens of V-4Cr-4Ti were given the same heat treatments applied to equivalent specimens of V-5Cr-5Ti. Auger specimens of V-4Cr-4Ti were also heat treated with the Charpy specimens to enable grain boundary chemistry measurements. The microstructural, microchemical and Charpy impact response of V-4Cr-4Ti displayed trends similar to those observed for V-5Cr-5Ti. The results show that grain size plays an important role in determining the ductile-to-brittle transition temperature (DBTT) of these materials and that a threshold level of grain boundary segregant appears to be required to cause grain boundary embrittlement and intergranular fracture.

  2. Analysis of grain boundary dynamics using event detection and cumulative averaging

    International Nuclear Information System (INIS)

    To analyze extended time series of high resolution images, we have employed automated frame-by-frame comparisons that are able to detect dynamic changes in the structure of a grain boundary in Au. Using cumulative averaging of images between events allowed high resolution measurements of the atomic relaxation in the interface with sufficient accuracy for comparison with atomistic models. Cumulative averaging was also used to observe the structural rearrangement of atomic columns at a moving step in the grain boundary. The technique of analyzing changing features in high resolution images by averaging between incidents can be used to deconvolute stochastic events that occur at random intervals and on time scales well beyond that accessible to single-shot imaging. - Highlights: • We have observed dynamic structural changes in extended time series of atomic resolution images. • Application of edge detection in the time domain isolates stochastic events in dynamic observations. • Splitting time series at stochastic events highlights changes in local atomic structure. • Cumulative averaging between events generates precise atomic resolution structural images

  3. Grain boundary diffusion and wetting in the analysis of intergranular penetration

    Energy Technology Data Exchange (ETDEWEB)

    Wolski, K. [Centre SMS, Ecole des Mines de St-Etienne, CNRS UMR 5546 PECM, 158, cours Fauriel, 42 023 Saint Etienne (France)], E-mail: wolski@emse.fr; Laporte, V. [Ecole Polytechnique Federale de Lausanne (EPFL), Laboratoire de Metallurgie Mecanique, IMX STI LMM - Station 12, CH-1015 Lausanne (Switzerland)

    2008-11-15

    Intergranular penetration of liquid bismuth has been analysed in two pure metals, Cu at 500 deg. C and Ni at 700 deg. C, used either as polycrystals or as oriented bicrystals. At the liquid/solid interface, large grooves have developed in Cu-Bi, while micrometer-thick films were observed in Ni-Bi. The bismuth concentration measurements obtained by Auger electron spectroscopy indicate a zone of monolayer Bi segregation followed by a diffusion-type profile over a distance of the order of 100 {mu}m for Cu-Bi and a nanometer-thick film followed by similar diffusion-type profile for Ni-Bi. In both cases the kinetics of intergranular penetration and embrittlement has been shown to be parabolic. It is concluded that no wetting occurs in Cu-Bi system at 500 deg. C while Bi wets Ni at 700 deg. C. It is postulated that the mechanism of intergranular penetration operates at a very tip of the penetration front, as opposed to the tip of liquid Bi film observed by scanning electron microscopy, and must be based on diffusion rather than wetting phenomena. Some suggestions are formulated for the future research in the area of intergranular penetration that can be split in two phenomena: grain boundary wetting above the wetting transition temperature and grain boundary diffusion below.

  4. Measuring surface and grain boundary segregation using wavelength dispersive X-ray spectroscopy

    Science.gov (United States)

    Christien, F.; Le Gall, R.

    2008-07-01

    Electron probe microanalysis (EPMA) using wavelength dispersive X-ray spectroscopy (EPMA-WDS) is applied to the quantification of surface and grain boundary monolayer segregation. The case of sulphur segregation in nickel and nickel alloys is considered. It is evidenced that EPMA-WDS is able to detect submonolayer surface segregation. The sulphur segregation can be accurately quantified from the sulphur Kα line relative intensity (ratio of the intensity measured on the sample and the intensity measured on a standard material) using the Stratagem™ software (analytical modelling of the X-ray emission in a stratified specimen based on the PhiRoZ model). The statistical accuracy of the technique and its detection limit are estimated to be as low as a few percents of a monolayer for reasonable counting times (˜ a few minutes). The advantages and drawbacks of EPMA-WDS with respect to Auger electron spectroscopy (AES) are discussed. The main advantage of EMPA-WDS is that it is almost insensitive to surface contamination and oxidation, which makes it possible to measure surface segregation on samples that have been in contact with atmosphere. The influence of specimen tilt is also discussed. The technique is also applied to the measurement of sulphur segregation on the fracture surface of an iron-nickel alloy sample broken at high temperature. It is thus demonstrated that EPMA-WDS could be a very useful tool for failure analysis in the case of grain boundary fractures.

  5. Scanning Auger electron spectroscopy studies of grain-boundary segregation in Type 304 stainless steel

    International Nuclear Information System (INIS)

    Scanning Auger electron spectroscopy studies have been conducted on grain-boundary surfaces of Type 304 stainless steel that were fractured in situ. To enhance the probability of intergranular fracture, the specimens were first subjected to creep deformation for 1000 h at 7000C. A semiquantitative surface chemical composition was calculated from the peak heights of Auger electron spectra. The concentration of Cr at the fracture surface was not different from the bulk value. This indicates that the long-term heat treatment caused healing of the sensitization. The concentrations of S, C, and Si at the fracture surface were at least an order of magnitude higher than the bulk values. Chemical composition profiles obtained by ion-sputtering indicated that segregation of S, C, P, and Si occurred within a depth of several atomic monolayers from the grain-boundary surface. Mo, Mn and Cu were not detected. The concentrations of Ni and Fe are in good agreement with the bulk chemical analysis

  6. Ultra-fast sulphur grain boundary segregation during hot deformation of nickel

    International Nuclear Information System (INIS)

    Sulphur grain boundary segregation during hot-compression of nickel (5.4 wt. ppm S) is monitored using Auger electron spectroscopy and wavelength dispersive X-ray spectroscopy. The deformation conditions (temperature/deformation rate) investigated are: 550 °C/0, 550 °C/3 × 10−5 s−1, 550 °C/3 × 10−4 s−1 and 450 °C/3 × 10−5 s−1. It is shown that plastic deformation accelerates the kinetics of sulphur grain boundary segregation by a factor of ∼103 to a few 105, depending on the deformation conditions. Very high levels of segregation (∼0.8 monolayer of sulphur) are obtained after very low deformation (∼5%). In addition a linear dependence of the segregation level with time and deformation is demonstrated. The segregation kinetics during plastic deformation is proportional to the deformation rate and almost independent of temperature. Several metallurgical mechanisms are discussed and confronted with the experimental results: dislocations dragging, pipe diffusion, dislocation collection/diffusion and acceleration by excess vacancies. It appears that the models developed in this work on the basis of the two latter mechanisms (dislocation collection/diffusion and acceleration by excess vacancies) predict the experimental data correctly

  7. Analysis of grain boundary dynamics using event detection and cumulative averaging

    Energy Technology Data Exchange (ETDEWEB)

    Gautam, A.; Ophus, C. [National Center for Electron Microscopy, LBNL, Berkeley, CA 94720 (United States); Lançon, F. [Laboratoire de Simulation Atomistique L-Sim, SP2M, INAC, CEA, 38054 Grenoble (France); Denes, P. [National Center for Electron Microscopy, LBNL, Berkeley, CA 94720 (United States); Dahmen, U., E-mail: udahmen@lbl.gov [National Center for Electron Microscopy, LBNL, Berkeley, CA 94720 (United States)

    2015-04-15

    To analyze extended time series of high resolution images, we have employed automated frame-by-frame comparisons that are able to detect dynamic changes in the structure of a grain boundary in Au. Using cumulative averaging of images between events allowed high resolution measurements of the atomic relaxation in the interface with sufficient accuracy for comparison with atomistic models. Cumulative averaging was also used to observe the structural rearrangement of atomic columns at a moving step in the grain boundary. The technique of analyzing changing features in high resolution images by averaging between incidents can be used to deconvolute stochastic events that occur at random intervals and on time scales well beyond that accessible to single-shot imaging. - Highlights: • We have observed dynamic structural changes in extended time series of atomic resolution images. • Application of edge detection in the time domain isolates stochastic events in dynamic observations. • Splitting time series at stochastic events highlights changes in local atomic structure. • Cumulative averaging between events generates precise atomic resolution structural images.

  8. Effect of the disorder in graphene grain boundaries: A wave packet dynamics study

    International Nuclear Information System (INIS)

    Chemical vapor deposition (CVD) on Cu foil is one of the most promising methods to produce graphene samples despite of introducing numerous grain boundaries into the perfect graphene lattice. A rich variety of GB structures can be realized experimentally by controlling the parameters in the CVD method. Grain boundaries contain non-hexagonal carbon rings (4, 5, 7, 8 membered rings) and vacancies in various ratios and arrangements. Using wave packet dynamic (WPD) simulations and tight-binding electronic structure calculations, we have studied the effect of the structure of GBs on the transport properties. Three model GBs with increasing disorder were created in the computer: a periodic 5–7 GB, a “serpentine” GB, and a disordered GB containing 4, 8 membered rings and vacancies. It was found that for small energies (E = EF ± 1 eV) the transmission decreases with increasing disorder. Four membered rings and vacancies are identified as the principal scattering centers. Revealing the connection between the properties of GBs and the CVD growth method may open new opportunities in the graphene based nanoelectronics.

  9. Effect of the disorder in graphene grain boundaries: A wave packet dynamics study

    Energy Technology Data Exchange (ETDEWEB)

    Vancsó, Péter, E-mail: vancso.peter@ttk.mta.hu [Institute of Technical Physics and Materials Science, Centre for Natural Sciences, PO Box 49, H-1525 Budapest (Hungary); Korean-Hungarian Joint Laboratory for Nanosciences, PO Box 49, H-1525 Budapest (Hungary); Márk, Géza I. [Institute of Technical Physics and Materials Science, Centre for Natural Sciences, PO Box 49, H-1525 Budapest (Hungary); Korean-Hungarian Joint Laboratory for Nanosciences, PO Box 49, H-1525 Budapest (Hungary); Lambin, Philippe; Mayer, Alexandre [Department of Physics of Matter and Radiation, University of Namur, 61, Rue de Bruxelles, B-5000 Namur (Belgium); Hwang, Chanyong [Center for Nano-characterization, Division of Industrial Metrology, Korea Research Institute of Standards and Science, Yuseong, Daejeon 305-340 (Korea, Republic of); Korean-Hungarian Joint Laboratory for Nanosciences, PO Box 49, H-1525 Budapest (Hungary); Biró, László P. [Institute of Technical Physics and Materials Science, Centre for Natural Sciences, PO Box 49, H-1525 Budapest (Hungary); Korean-Hungarian Joint Laboratory for Nanosciences, PO Box 49, H-1525 Budapest (Hungary)

    2014-02-01

    Chemical vapor deposition (CVD) on Cu foil is one of the most promising methods to produce graphene samples despite of introducing numerous grain boundaries into the perfect graphene lattice. A rich variety of GB structures can be realized experimentally by controlling the parameters in the CVD method. Grain boundaries contain non-hexagonal carbon rings (4, 5, 7, 8 membered rings) and vacancies in various ratios and arrangements. Using wave packet dynamic (WPD) simulations and tight-binding electronic structure calculations, we have studied the effect of the structure of GBs on the transport properties. Three model GBs with increasing disorder were created in the computer: a periodic 5–7 GB, a “serpentine” GB, and a disordered GB containing 4, 8 membered rings and vacancies. It was found that for small energies (E = EF ± 1 eV) the transmission decreases with increasing disorder. Four membered rings and vacancies are identified as the principal scattering centers. Revealing the connection between the properties of GBs and the CVD growth method may open new opportunities in the graphene based nanoelectronics.

  10. Effect of the disorder in graphene grain boundaries: A wave packet dynamics study

    Science.gov (United States)

    Vancsó, Péter; Márk, Géza I.; Lambin, Philippe; Mayer, Alexandre; Hwang, Chanyong; Biró, László P.

    2014-02-01

    Chemical vapor deposition (CVD) on Cu foil is one of the most promising methods to produce graphene samples despite of introducing numerous grain boundaries into the perfect graphene lattice. A rich variety of GB structures can be realized experimentally by controlling the parameters in the CVD method. Grain boundaries contain non-hexagonal carbon rings (4, 5, 7, 8 membered rings) and vacancies in various ratios and arrangements. Using wave packet dynamic (WPD) simulations and tight-binding electronic structure calculations, we have studied the effect of the structure of GBs on the transport properties. Three model GBs with increasing disorder were created in the computer: a periodic 5-7 GB, a "serpentine" GB, and a disordered GB containing 4, 8 membered rings and vacancies. It was found that for small energies (E = EF ± 1 eV) the transmission decreases with increasing disorder. Four membered rings and vacancies are identified as the principal scattering centers. Revealing the connection between the properties of GBs and the CVD growth method may open new opportunities in the graphene based nanoelectronics.

  11. Creep deformation of grain boundary in a highly crystalline SiC fibre.

    Science.gov (United States)

    Shibayama, Tamaki; Yoshida, Yutaka; Yano, Yasuhide; Takahashi, Heishichiro

    2003-01-01

    Silicon carbide (SiC) matrix composites reinforced by SiC fibres (SiC/SiC composites) are currently being considered as alternative materials in high Ni alloys for high-temperature applications, such as aerospace components, gas-turbine energy-conversion systems and nuclear fusion reactors, because of their high specific strength and fracture toughness at elevated temperatures compared with monolithic SiC ceramics. It is important to evaluate the creep properties of SiC fibres under tensile loading in order to determine their usefulness as structural components. However, it would be hard to evaluate creep properties by monoaxial tensile properties when we have little knowledge on the microstructure of crept specimens, especially at the grain boundary. Recently, a simple fibre bend stress relaxation (BSR) test was introduced by Morscher and DiCarlo to address this problem. Interpretation of the fracture mechanism at the grain boundary is also essential to allow improvement of the mechanical properties. In this paper, effects of stress applied by BSR test on microstructural evolution in advanced SiC fibres, such as Tyranno-SA including small amounts of Al, are described and discussed along with the results of microstructure analysis on an atomic scale by using advanced microscopy.

  12. Quantitative Determination of Grain Boundary Recombination Velocity in CdTe by Combination of Cathodoluminescence Measurements and Numerical Simulations

    Energy Technology Data Exchange (ETDEWEB)

    Kanevce, Ana; Moseley, John; Kuciauskas, Darius; Al-Jassim, Mowafak; Metzger, Wyatt K.

    2015-06-14

    We developed a 2D numerical model simulating cathodoluminescence (CL) measurements in CdTe. Using this model we analyze how various material parameters impact the CL contrast and intensity observed in the measured signal, and determine if and when we can accurately determine the value of grain boundary recombination rate. In addition to grain boundary (GB) recombination, the grain size and its ratio to the carrier diffusion length impact the results of the measurement. Holding the grain interior and GB recombination rates constant, we find that as the grain size increases and becomes larger than the diffusion length, the observed CL contrast is larger. In a small grain size material the surface recombination lowers the overall intensity of the signal, but does not impact the observed contrast significantly. In a large grain size material, high surface recombination velocity can lower the observed contrast in a measurement. This model in combination with an experiment is used to quantify the grain boundary recombination velocity in polycrystalline CdTe before and after the CdCl2 treatment.

  13. Simulation of xenon, uranium vacancy and interstitial diffusion and grain boundary segregation in UO2

    Energy Technology Data Exchange (ETDEWEB)

    Andersson, Anders D. [Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Tonks, Michael R. [Idaho National Lab. (INL), Idaho Falls, ID (United States); Casillas, Luis [Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Nerikar, Pankaj [Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Vyas, Shyam [Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Uberuaga, Blas P. [Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Stanek, Christopher R. [Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

    2014-10-31

    In light water reactor fuel, gaseous fission products segregate to grain boundaries, resulting in the nucleation and growth of large intergranular fission gas bubbles. Based on the mechanisms established from density functional theory (DFT) and empirical potential calculations 1, continuum models for diffusion of xenon (Xe), uranium (U) vacancies and U interstitials in UO2 have been derived for both intrinsic conditions and under irradiation. Segregation of Xe to grain boundaries is described by combining the bulk diffusion model with a model for the interaction between Xe atoms and three different grain boundaries in UO2 ( Σ5 tilt, Σ5 twist and a high angle random boundary),as derived from atomistic calculations. All models are implemented in the MARMOT phase field code, which is used to calculate effective Xe and U diffusivities as well as redistribution for a few simple microstructures.

  14. Atomic-scale investigations of grain boundary segregation in astrology with a three dimensional atom-probe

    Energy Technology Data Exchange (ETDEWEB)

    Blavette, D. [Rouen Univ., 76 - Mont-Saint-Aignan (France). Lab. de Microscopie Electronique]|[Institut Universitaire de France (France); Letellier, L. [Rouen Univ., 76 - Mont-Saint-Aignan (France). Lab. de Microscopie Electronique; Duval, P. [Rouen Univ., 76 - Mont-Saint-Aignan (France). Lab. de Microscopie Electronique; Guttmann, M. [Rouen Univ., 76 - Mont-Saint-Aignan (France). Lab. de Microscopie Electronique]|[Institut de Recherches de la Siderurgie Francaise (IRSID), 57 - Maizieres-les-Metz (France)

    1996-08-01

    Both conventional and 3D atom-probes were applied to the investigation of grain-boundary (GB) segregation phenomena in two-phase nickel base superalloys Astroloy. 3D images as provided by the tomographic atom-probe reveal the presence of a strong segregation of both boron and molybdenum at grain-boundaries. Slight carbon enrichment is also detected. Considerable chromium segregation is exhibited at {gamma}`-{gamma}` grain-boundaries. All these segregants are distributed in a continuous manner along the boundary over a width close to 0.5 nm. Experiments show that segregation occurs during cooling and more probably between 1000 C and 800 C. Boron and molybdenum GB enrichments are interpreted as due to an equilibrium type-segregation while chromium segregation is thought to be induced by {gamma}` precipitation at GB`s and stabilised by the presence of boron. No segregation of zirconium is detected. (orig.)

  15. Evidence of sealing and brine distribution at grain boundaries in natural fine-grained Halite (Qum Kuh salt fountain, Central Iran): implications for rheology of salt extrusions

    Science.gov (United States)

    Desbois, Guillaume; Urai, Janos L.; de Bresser, J. H. P.

    2010-05-01

    When grain boundary movement is stopped, surface energy related forces reassert themselves driving the system to its equilibrium conditions ([2], [6], [7], [8]). This could result in growth of islands and shrinking of channels and hence in healing the boundary by internal redistribution of fluid and solid in the contact region. Such islands are proposed to grow preferentially close to the contact rim and promote the healing of the grain-grain contact, which in turn prevents transport in or out the boundary region and thus traps the fluids in isolated inclusions. This contribution is focused on observation of grain boundary microstructures in natural mylonitic rocksalt collected from the distal part of Kum-Quh salt fountain (central Iran) in order to give unprecedented insight of grain boundary microstructures using argon-beam cross-sectioning to prepare high quality polished surfaces suitable for high-resolution SEM imaging. The possibility to use our SEM under cryogenic conditions allows also imaging the in-situ distribution of fluids. Results show that brine at grain boundaries occurs as thick layers (> µm in scale) corresponding to cross-sectioned wetted triple junction tubes, as filling at triple junction and as array of isolated fluids inclusions at grain-grain contacts. Close observations at islands contacts suggest the presence of a very thin fluid film (Desbois G., Zavada P., Schleder Z. and Urai J.L. (In review). Deformation and recrystallization mechanisms in naturally deformed salt fountain: microstructural evidence for a switch in deformation mechanisms with increased availability of meteoric water and decreased grain size (Qum Kuh, central Iran). Submitted to Journal of Structural Geology. [2] Ghoussoub J., and Leroy Y.M. (2001), Solid-fluid phase transformation within grain boundaries during compaction by pressure solution, J. Mech. Phys. Solids, 49, 737 2385-2430. 738 [3] Jackson, M.P.A., (1985). Natural strain in diapiric and glacial rock salt

  16. Effect of one-step recrystallization on the grain boundary evolution of CoCrFeMnNi high entropy alloy and its subsystems.

    Science.gov (United States)

    Chen, Bo-Ru; Yeh, An-Chou; Yeh, Jien-Wei

    2016-01-01

    In this study, the grain boundary evolution of equiatomic CoCrFeMnNi, CoCrFeNi, and FeCoNi alloys after one-step recrystallization were investigated. The special boundary fraction and twin density of these alloys were evaluated by electron backscatter diffraction analysis. Among the three alloys tested, FeCoNi exhibited the highest special boundary fraction and twin density after one-step recrystallization. The special boundary increment after one-step recrystallization was mainly affected by grain boundary velocity, while twin density was mainly affected by average grain boundary energy and twin boundary energy.

  17. Atomistic investigation of Cr influence on primary radiation damage in Fe-12 at.% Cr grain boundaries

    Science.gov (United States)

    Esfandiarpour, A.; Feghhi, S. A. H.; Arjhangmehr, A.

    2016-08-01

    In this paper, we investigate the influence of Cr on the primary radiation damage in Fe-12 at.% Cr with different atomic grain boundaries (GBs). Four different GB structures, two twists and two symmetric tilt boundaries are selected as the model structures. The primary radiation damage near each GB in α-Fe and Fe-12 at.% Cr is simulated using Molecular Dynamics for 9 keV primary knock-on atoms with velocity vectors perpendicular to the GB plane. In agreement with previous works, the results indicate that the atomic GBs are biased toward interstitials and due to the reduction of ‘in-cascade’ interstitial-vacancy annihilation rates, vacancies accumulate in the bulk grains. The minimum defect production occurs when the overlap between cascade center and GB plane is maximum; in contrast, the number of residual defects in the bulk (vacancies and interstitials) increases when the overlap decreases. Moreover, we find that the presence of Cr hardly affects the number of residual defects in the grain interiors, and causes a Cr-enrichment in the surviving self-interstitial atoms in bulk during relaxation of the primary cascades—also in agreement with previous studies. Further, in order to study the effect of 12 at.% Cr on the energetic and kinetic properties of vacancies near the atomic GBs, we calculate formation energies and diffusion barriers of defects using Molecular Static and climbing-Nudged Elastic Band methods. The results reveal that the vacancies energetically and kinetically tend to form and cluster around the GB plane due to the substantial reduction of their formation energies and migration barriers in layers close to the GB center and are immobile on the simulated time frame (~ps).

  18. Micromechanics stress–strain behavior prediction of dual phase steel considering plasticity and grain boundaries debonding

    International Nuclear Information System (INIS)

    Highlights: • A simulation procedure is utilized to investigate mechanical behavior of DP steels. • Cohesive elements are used for consideration of debonding on the grain boundaries. • The finite element model was first constructed from the SEM images. • Pattern of voids formation is investigated using CZM and elastic–plastic analysis. - Abstract: Stress–strain response of multiphase materials similar to dual phase (DP) steel depends on the elastic–plastic and damage behavior of all ingredient phases. DP steels typically contains of ferrite and martensite phases, but the grain boundaries of martensite phase may act as important location with possible occurrence of damage or debonding under static loading. The focus of this paper is consideration of ferrite and martensite interface debonding in addition to the elastic–plastic behavior of ferrite and martensite to predict the stress–strain behavior of DP steel using a finite element (FE) micromechanical approach. For this purpose the micromechanics representative geometry is selected from scanning electron microscopy (SEM) images and the finite element mesh is generated based on the real shape of grains. Interface elements based on the cohesive zone modeling are also used for consideration of damage or debonding on the ferrite and martensite interfaces. Therefore, the developed micro mechanic finite element model is based on the real microstructure, uses cohesive elements between martensite islands and ferrite matrix and also considers the elastic–plastic behavior of ferrite and martensite phases. Handling of such simulation procedure with two source of material nonlinearity (plasticity and cohesive zone damage) is not an easy task. It is shown that the obtained stress–strain behaviors are in well agreement with the experimental results

  19. Local compositional analysis of magnetic crystal grain and boundary in CoCrPt-SiO2 granular perpendicular recording media

    International Nuclear Information System (INIS)

    The compositions of magnetic crystal grains and boundaries of a CoCrPt-SiO2 granular perpendicular medium were investigated for plan-view and cross-sectional samples by using a transmission electron microscope equipped with an energy-dispersive X-ray spectrometer. The grain boundary composition, which is not easy to measure because of the small width around 1 nm, is estimated from the average composition of magnetic layer and that of crystal grains by considering the grain boundary volume ratio determined by structure observation. The grain boundary is shown to include Si as the major metallic element together with not small amounts of other metallic elements.

  20. Pseudo-single-crystal electrochemistry on polycrystalline electrodes: visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction.

    Science.gov (United States)

    Aaronson, Barak D B; Chen, Chang-Hui; Li, Hongjiao; Koper, Marc T M; Lai, Stanley C S; Unwin, Patrick R

    2013-03-13

    The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing 'pseudo-single-crystal' electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe(2+/3+) couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe(2+) oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale. PMID:23405963

  1. The use of cubic Nd-Ba-Cu-O seeds to create θ[100], 900-θ[100], and θ[001] tilt Y-Ba-Cu-O grain boundaries

    International Nuclear Information System (INIS)

    Using seeding techniques to control the orientation of grains, we have been able to create a wide variety of YBa2Cu3O6+x, grain boundaries. In addition to five domain samples with 90 degree[100] twist and tilt grain boundaries, we have now developed a method to produce grain boundaries in the same sample that have the misorientations θ[001] tilt, θ[100] tilt, and 90 degree ∼ θ[100], where the disorientation angle θ is fully controllable. We will demonstrate how these boundaries can be synthesized, give experimental evidence via polarized light microscopy and electron backscatter patterns (EBSP) that the intended grain boundaries were indeed formed, and discuss the importance of these boundaries in future grain boundary studies

  2. Non-destructive analysis of micro texture and grain boundary character from X-ray diffraction contrast tomography

    International Nuclear Information System (INIS)

    Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting new possibilities for mapping 3D grain shapes and crystallographic orientations in different classes of polycrystalline materials. X-ray diffraction contrast tomography (DCT) is a monochromatic beam imaging technique combining the principles of X-ray micro-tomography and three-dimensional X-ray diffraction microscopy (3DXRD). DCT provides simultaneous access to 3D grain shape, crystallographic orientation and attenuation coefficient distribution at the micrometer length scale. The microtexture of the material can be quantified in more detail by post-processing of the volume data provided by DCT. In particular one can determine the local crystallographic habit plane of the grain boundary by analysing the surface normal of the grain boundary with respect to the crystal orientation. The resulting five parameter description of the character of individual grain boundaries could previously be produced only by destructive characterization techniques. Statistical analysis of this kind of data can be expected to provide new insight into various physico-chemical processes, driven by the grain boundary energy (corrosion, coarsening).

  3. Structural and chemical analysis of grain boundaries and tellurium precipitates in commercial Cd1-xZnxTe

    International Nuclear Information System (INIS)

    The structure and chemistry of grain boundaries in commercial Cd1-xZnxTe, prepared by the high-pressure Bridgman technique, have been analyzed using transmission electron microscopy, scanning electron microscopy, infrared-light microscopy and visible-light microscopy. These analyses show that the grain boundaries inside the Cd1-xZnxTe materials are decorated with tellurium precipitates. Analysis of a tellurium precipitate at a grain boundary by transmission electron microscopy and selected-area electron diffraction found the precipitate to consist of a single, saucer-shaped grain. Electron diffraction from the precipitate was consistent with the trigonal phase of tellurium (space group P3121), which is the equilibrium phase at room temperature and atmospheric pressure. This precipitate was found to be aligned with one of the adjacent CZT grains such that the tellurium (0 bar 111) planes were nearly parallel to the CZT (111) planes. High-resolution transmission electron microscopy of the Te/Cd1-xZnxTe interface showed no tertiary phase at the interface. The structures of the grain boundaries and the Te/Cd1-xZnxTe interface are discussed and related to their possible implications on Cd1-xZnxTe gamma-ray detector performance

  4. Classical molecular dynamics investigation of microstructure evolution and grain boundary diffusion in nano-polycrystalline UO2

    Science.gov (United States)

    Govers, K.; Verwerft, M.

    2013-07-01

    The High Burnup Structure (HBS) observed at pellet periphery in conventional Light Water Reactor nuclear fuels and around spots presenting high plutonium content in mixed (U, Pu) oxide fuel - MOX fuel - consists of a restructuration of the original grains into smaller ones. The process is often postulated to occur because of the accumulation of irradiation damage and the retention of fission products in the matrix. The computing power nowadays available enables for simulating larger systems at the atomic scale up to the point that nano-polycrystalline material can now be investigated by empirical potential molecular dynamics. Simulations of nano-polycrystalline UO2 structures have been carried out at various temperatures to investigate atom mobility close to grain boundaries. The variation of Arrhénius parameters for the diffusion coefficient of oxygen, uranium and xenon as a function of the distance from a grain boundary was studied, leading to the distinction of three zones: the grain boundary layers (up to 1 nm depth) presenting enhanced diffusion, an intermediate zone (1 to roughly 2 nm depth) with intermediate diffusion values and the bulk of the grains. The following Arrhénius relations for grain boundary diffusion were derived:

  5. The influence of solution composition and grain boundaries on the replacement of calcite by dolomite

    Science.gov (United States)

    Moraila Martinez, Teresita de Jesus; Putnis, Christine V.; Putnis, Andrew

    2016-04-01

    Dolomite formation is a mineral replacement reaction that affects extensive rock volumes and comprises a large fraction of oil and gas reservoirs [1,2]. The most accepted hypothesis is the 'dolomitization' of limestone by Mg-rich fluids [3]. The objective of this research is to study the replacement mechanism of calcite by dolomite, the role of grain boundaries, highlighted by Etschmann et al. (2014), and the possible influence of solutions in dolomite formation under the presence of ions that are normally in crustal aqueous fluids. To accomplish this purpose, we performed hydrothermal experiments using Carrara marble cubes of ~1.5 mm size and 7-9 mg weight as starting material, reacted with 1M (Mg,Ca)Cl2 aqueous solutions, with Mg/Ca ratios of 3 and 5 at 200°C, for different reaction times. Additional experiments were performed adding 1mM of Na2SO4, NaCl or NaF to the previous solutions. After the reaction, the product phases were identified using Raman spectroscopy, X-Ray powder diffraction (XRD), electron microprobe analysis (EMPA), and the textural evolution was studied by scanning electron microscopy (SEM). Samples reacted with aqueous solutions resulted in the replacements of the calcite rock into magnesite and dolomite. The amount and type of reaction strongly depends on the Mg/Ca ratio. Samples reacted with a Mg/Ca ratio of 5 resulted in an almost complete replacement reaction and more favorable for magnesite formation than for dolomite. When the Mg/Ca ratio was 3 dolomite formed but the replacement was located in the core of the sample. We show that grain boundaries are very important for the infiltration of solution and the progress of a replacement reaction, acting as fluid pathways. Solution composition controls the nature of the replacement product. Acknowledgment: This work is funded within a Marie Curie EU Initial Training Network- CO2-React. 1. Etschmann B., Brugger J., Pearce M.A., Ta C., Brautigan D., Jung M., Pring A. (2014). Grain boundaries as

  6. A Fresh Plutonic Igneous Angrite Containing Grain Boundary Glass From Tamassint, Northwest Africa

    Science.gov (United States)

    Irving, A. J.; Kuehner, S. M.; Rumble, D.

    2006-12-01

    Tamassint Angrite: A small fragmented stone found in June 2006 south of Tamassint oasis in the Morocco-Algeria border region represents a new type of angrite lithology, unlike the coarse grained metamorphic or fine grained "basaltic" to quench-textured examples known previously. This extremely fresh, fusion-crusted specimen has a coarse grained (0.6-12 mm) plutonic igneous cumulate texture, and is composed of Al-Ti-rich clinopyroxene (33.4%), pure anorthite (28.6%), Ca-rich olivine (18.7%) with prominent exsolution lamellae (10-50 μm wide) of kirschsteinite, ulvöspinel (18.5%), and accessory glass, troilite and metal. Subhedral anorthite grains are partially enclosed within larger ulvöspinel grains. Mineral compositions are as follows: clinopyroxene (Fs20.8-33.3Wo53-54.9, Al2O3 = 5.7 to 9.4 wt.%, TiO2 = 0.9 to 2.9 wt.%, FeO/MnO = 85-278), olivine (Fa72.6-74.7Ln3.5-3.6, CaO = 2.1 wt.%, FeO/MnO = 70-87), kirschsteinite (Fa44.7-45.4Ln46-47.2, FeO/MnO = 73-82), ulvöspinel (TiO2 = 27.6 wt.%, Al2O3 = 5.5 wt.%). Reintegration of the kirschsteinite lamellae gives a pre-exsolution olivine composition of Fa68.1Ln12.2 with 7.3 wt.% CaO. Present along grain boundaries (notably between anorthite and ulvöspinel) are narrow (5-20 μm) curvilinear zones of glass associated with secondary kirschteinite, clinopyroxene and olivine (which show similar curvilinear morphology and truncate kirschsteinite lamellae). Glass compositions plot close to a mixing line between anorthite and ulvöspinel. Replicate oxygen isotopic analyses of acid-washed minerals by laser fluorination gave δ18O = 3.881, 3.845, δ17O = 1.967, 1.927, Δ17O = -0.0745, -0.0956 per mil (for TFL slope = 0.526). Comparison With NWA 2999: We previously showed [1] that angrite Northwest Africa 2999 is a metamorphically annealed breccia with distinctive symplectites and coronas representing forward and reverse versions of the same solid state reaction. We suggested that these disequilibrium textures required burial

  7. An analytical model for stress-induced grain growth in the presence of both second-phase particles and solute segregation at grain boundaries

    International Nuclear Information System (INIS)

    A theoretical framework that incorporates the influence of second-phase particles and solute segregation at grain boundaries (GBs) on stress-induced GB migration and grain rotation is formulated in the present paper. In our work, we modified the well-established Cahn–Taylor model to account for the drag stresses generated by second-phase particles and by solute atoms segregated at GBs. The theoretical framework is then implemented to rationalize GB migration and grain rotation using experimental data from a previously published study on stress-induced grain growth in the presence of both second-phase particles and solute segregation at GBs. The calculated grain growth results are generally consistent with the experimental data, providing support to the proposed theoretical model, despite the various assumptions involved. Moreover, the influence of second-phase particles and solute segregation at GBs on GB migration and grain rotation was also investigated using the model, and our results suggest that both second-phase particles and solute atoms segregated at GBs reduce the velocities of GB migration and grain rotation as compared to those in the case of high-purity Al

  8. Decoupled phase transitions and grain-boundary melting in supported phospholipid bilayers

    DEFF Research Database (Denmark)

    Keller, D.; Larsen, N.B.; Møller, I.M.;

    2005-01-01

    Two separate liquid-solid phase transitions are detected in the two monolayers of a mica-supported phospholipid bilayer by atomic force microscopy. The phase transitions of the two monolayers are decoupled by the stronger interaction between the lipid headgroups of the proximal monolayer and the ......Two separate liquid-solid phase transitions are detected in the two monolayers of a mica-supported phospholipid bilayer by atomic force microscopy. The phase transitions of the two monolayers are decoupled by the stronger interaction between the lipid headgroups of the proximal monolayer...... and the mica support. The transition temperature of the proximal monolayer is increased and this transition occurs over a narrower temperature range. Both transitions occur via grain-boundary melting and the variation of the width of the interfacial zone with temperature is consistent with mean-field theory....

  9. Interstitial and substitutional solute segregation at individual grain boundaries of α-iron: data revisited

    International Nuclear Information System (INIS)

    Theoretical calculations (usually density-functional-theory methods performed at 0 K) confirm the formerly assumed substitutional phosphorus segregation in α-iron. In contrast, the enthalpy–entropy compensation effect predicts that phosphorus should segregate interstitially. To resolve this discrepancy, we recalculated the values of the segregation enthalpy and entropy for the interstitial segregation of phosphorus according to the Guttmann model of segregation in multicomponent systems. This recalculation is based on earlier measured experimental data and shows that only slight changes in the values of the standard enthalpy and entropy of phosphorus, carbon and silicon segregation are obtained. Consequently, all dependences constructed previously remain qualitatively the same. By thermodynamic considerations based on the enthalpy–entropy compensation effect we quantitatively show that there is an alteration of the position of phosphorus at grain boundaries in α-Fe with increasing temperature: while substitutional segregation is preferred at 0 K, interstitial segregation occurs at temperatures of practical interest. (paper)

  10. Atomic structures and oxygen dynamics of CeO2 grain boundaries

    Science.gov (United States)

    Feng, Bin; Sugiyama, Issei; Hojo, Hajime; Ohta, Hiromichi; Shibata, Naoya; Ikuhara, Yuichi

    2016-02-01

    Material performance is significantly governed by grain boundaries (GBs), a typical crystal defects inside, which often exhibit unique properties due to the structural and chemical inhomogeneity. Here, it is reported direct atomic scale evidence that oxygen vacancies formed in the GBs can modify the local surface oxygen dynamics in CeO2, a key material for fuel cells. The atomic structures and oxygen vacancy concentrations in individual GBs are obtained by electron microscopy and theoretical calculations at atomic scale. Meanwhile, local GB oxygen reduction reactivity is measured by electrochemical strain microscopy. By combining these techniques, it is demonstrated that the GB electrochemical activities are affected by the oxygen vacancy concentrations, which is, on the other hand, determined by the local structural distortions at the GB core region. These results provide critical understanding of GB properties down to atomic scale, and new perspectives on the development strategies of high performance electrochemical devices for solid oxide fuel cells.

  11. Automatic processing of an orientation map into a finite element mesh that conforms to grain boundaries

    Science.gov (United States)

    Dancette, S.; Browet, A.; Martin, G.; Willemet, M.; Delannay, L.

    2016-06-01

    A new procedure for microstructure-based finite element modeling of polycrystalline aggregates is presented. The proposed method relies (i) on an efficient graph-based community detection algorithm for crystallographic data segmentation and feature contour extraction and (ii) on the generation of selectively refined meshes conforming to grain boundaries. It constitutes a versatile and close to automatic environment for meshing complex microstructures. The procedure is illustrated with polycrystal microstructures characterized by orientation imaging microscopy. Hot deformation of a Duplex stainless steel is investigated based on ex-situ EBSD measurements performed on the same region of interest before and after deformation. A finite element mesh representing the initial microstructure is generated and then used in a crystal plasticity simulation of the plane strain compression. Simulation results and experiments are in relatively good agreement, confirming a large potential for such directly coupled experimental and modeling analyses, which is facilitated by the present image-based meshing procedure.

  12. Energy scales in YBaCuO grain boundary biepitaxial Josephson junctions

    International Nuclear Information System (INIS)

    Self-assembled nanoscale channels may naturally arise in the growth process of grain boundaries (GBs) in high critical temperature superconductor (HTS) systems, and deeply influence the transport properties of the GB Josephson junctions (JJs). By isolating nano-channels in YBCO biepitaxial JJs and studying their properties, we sort out specific fingerprints of the mesoscopic nature of the contacts. The size of the channels combined to the characteristic properties of HTS favors a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current-voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Thouless energy emerges as a characteristic energy of these types of Josephson junctions. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed, along with elements to take into account when designing HTS nanostructures.

  13. New method of grain boundary extraction by directional optimal filtering: applying to estimate creep in metals

    Science.gov (United States)

    Journaux, Stephane; Gouton, Pierre; Thauvin, G.

    2002-07-01

    It is economically important for manufacturers of high-temperature machines to be able to measure creep so they can predict residual service life more accurately. This paper describes and refines an image analysis method for evaluating creep in laboratory test pieces. It is a preliminary study of how to extract relevant information for creep measurement by counting cavities. Sample preparation for quantification by image analysis is an important step determining the further development of the image analysis technique. Grain-boundary extraction, which involves directional information, is the major problem to be solved before measurement can be automated. The search for a crest-line extraction filter by the Canny method has led to the development of a directional wavelet transform filter. The results of this innovative filtering method are applied here.

  14. Improved ductility of Cu64Zr36 metallic glass/Cu nanocomposites via phase and grain boundaries

    Science.gov (United States)

    Jian, W. R.; Wang, L.; Li, B.; Yao, X. H.; Luo, S. N.

    2016-04-01

    We investigate tensile deformation of metallic glass/crystalline interpenetrating phase nanocomposites as regards the effects of specific area of amorphous/crystalline phase interfaces, and grain boundaries. As an illustrative case, large-scale molecular dynamics simulations are performed on Cu64Zr36 metallic glass/Cu nanocomposites with different specific interface areas and grain boundary characteristics. Plastic deformation is achieved via shear bands, shear transformation zones, and crystal plasticity. Three-dimensional amorphous/crystalline interfaces serve as effective barriers to the propagation of shear transformation zones and shear bands if formed, diffuse strain localizations, and give rise to improved ductility. Ductility increases with increasing specific interface area. In addition, introducing grain boundaries into the second phase facilitates crystal plasticity, which helps reduce or eliminate mature shear bands in the glass matrix.

  15. Avoidance of stress corrosion susceptibility in high strength aluminum alloys by control of grain boundary and matrix microstructure

    Science.gov (United States)

    Adler, P.; Deiasi, R.

    1974-01-01

    The relation of microstructure to the mechanical strength and stress corrosion resistance of highest strength and overaged tempers of BAR and 7050 aluminum alloys was investigated. Comparison is made with previously studied 7075 aluminum alloy. Optical microscopy, transmission electron microscopy, and differential scanning calorimetry were used to characterize the grain morphology, matrix microstructure, and grain boundary microstructure of these tempers. Grain boundary interparticle spacing was significant to stress corrosion crack propagation for all three alloys; increasing interparticle spacing led to increased resistance to crack propagation. In addition, the fire grain size in Bar and 7050 appears to enhance crack propagation. The highest strength temper of 7050 has a comparatively high resistance to crack initiation. Overall stress corrosion behavior is dependent on environment pH, and evaluation over a range of pH is recommended.

  16. Texture, local misorientation, grain boundary and recrystallization fraction in pipeline steels related to hydrogen induced cracking

    International Nuclear Information System (INIS)

    In the present study, API X60 and X60SS pipeline steels were cathodically charged by hydrogen for 8 h using 0.2 M sulfuric acid and 3 g/l ammonium thiocyanate. After charging, SEM observations showed that the hydrogen induced cracking (HIC) appeared at the center of cross section in the X60 specimen. However, HIC did not appear in the X60SS steel. Therefore, electron backscatter diffraction (EBSD) technique was used to analyze the center of cross section of as-received X60SS, X60 and HIC tested X60 specimens. The results showed that the HIC crack not only can propagate through 〈100〉||ND oriented grains but also its growth may happen in various orientations. In HIC tested X60 specimen, an accumulation of low angle grain boundaries around the crack path documented that full recrystallization was not achieved during hot rolling. Kernel Average Misorientaion (KAM) histogram illustrated that the deformation is more concentrated in as-received and HIC tested X60 specimens rather than in as-received X60SS specimen. Moreover, the concentration of coincidence site lattice (CSL) boundary in HIC tested X60 specimen was very low compared with other samples. The recrystallization area fraction in X60SS steel was very high. This high amount of recrystallization fraction with no stored energy is one of the main reasons for high HIC resistance of this steel to HIC. The orientation distribution function (ODF) of the recrystallized, substructured and deformed fractions in as-received X60SS and X60 steel showed relative close orientations in both as-received specimens

  17. Texture, local misorientation, grain boundary and recrystallization fraction in pipeline steels related to hydrogen induced cracking

    Energy Technology Data Exchange (ETDEWEB)

    Mohtadi-Bonab, M.A., E-mail: m.mohtadi@usask.ca; Eskandari, M.; Szpunar, J.A.

    2015-01-03

    In the present study, API X60 and X60SS pipeline steels were cathodically charged by hydrogen for 8 h using 0.2 M sulfuric acid and 3 g/l ammonium thiocyanate. After charging, SEM observations showed that the hydrogen induced cracking (HIC) appeared at the center of cross section in the X60 specimen. However, HIC did not appear in the X60SS steel. Therefore, electron backscatter diffraction (EBSD) technique was used to analyze the center of cross section of as-received X60SS, X60 and HIC tested X60 specimens. The results showed that the HIC crack not only can propagate through 〈100〉||ND oriented grains but also its growth may happen in various orientations. In HIC tested X60 specimen, an accumulation of low angle grain boundaries around the crack path documented that full recrystallization was not achieved during hot rolling. Kernel Average Misorientaion (KAM) histogram illustrated that the deformation is more concentrated in as-received and HIC tested X60 specimens rather than in as-received X60SS specimen. Moreover, the concentration of coincidence site lattice (CSL) boundary in HIC tested X60 specimen was very low compared with other samples. The recrystallization area fraction in X60SS steel was very high. This high amount of recrystallization fraction with no stored energy is one of the main reasons for high HIC resistance of this steel to HIC. The orientation distribution function (ODF) of the recrystallized, substructured and deformed fractions in as-received X60SS and X60 steel showed relative close orientations in both as-received specimens.

  18. Grain boundary defects initiation at the outer surface of dissimilar welds: corrosion mechanism studies

    International Nuclear Information System (INIS)

    Dissimilar welds located on the primary coolant system of the French PWR I plants exhibit grain boundary defects in the true austenitic zones of the first buttering layer. If grain boundaries reach the interface, they can extend to the martensitic band. Those defects are filled with compact oxides. In addition, the ferritic base metal presents some pits along the interface. Nowadays, three mechanisms are proposed to explain the initiation of those defects: stress corrosion cracking, intergranular corrosion and high temperature intergranular oxidation. This paper is dealing with the study of the mechanisms involved in the corrosion phenomenon. Intergranular corrosion tests performed on different materials show that only the first buttering layer, even with some δ ferrite, is sensitized. The results of stress corrosion cracking tests in water solutions show that intergranular cracking is possible on a bulk material representative of the first buttering layer. It is unlikely on actual dissimilar welds where the ferritic base metal protects the first austenitic layer by galvanic coupling. Therefore, the stress corrosion cracking assumption cannot explain the initiation of the defects in aqueous environment. The results of the investigations and of the corrosion studies led to the conclusion that the atmosphere could be the only possible aggressive environment. This conclusion is based on natural atmospheric exposure and accelerated corrosion tests carried out with SO2 additions in controlled atmosphere. They both induce a severe intergranular corrosion on true sensitized austenitic materials. This corrosion studies cannot conclude definitively on the causes of the defect initiation on field, but they show that the atmospheric corrosion could produce intergranular attacks in the pure austenitic zones of the first buttering layer of the dissimilar welds and that this corrosion is stress assisted. (author). 1 ref., 6 figs., 4 tabs

  19. First-principles investigation into the effect of Cr on the segregation of multi-H at the Fe Σ3 (1 1 1) grain boundary

    International Nuclear Information System (INIS)

    First-principles calculations were carried out to investigate the effect of Cr on segregation of multiple H atoms in the Σ3 (1 1 1) grain boundary in bcc Fe. In the absence of Cr, four H atoms can be trapped at the interstitial site of the Fe grain boundary (areal density: 28 nm−2), and no H2 molecules are formed. The presence of Cr, however, suppresses the segregation tendency of H and only two H atoms can be trapped at this grain boundary. Although the volume expansion associated with the segregation of Cr to the grain boundary promotes H segregation, such a booting effect is unable to remedy the repulsion of H resulted from charge density increase in the grain boundary core. As a consequence, Cr mitigates H aggregation at the Σ3 (1 1 1) grain boundary in bcc Fe

  20. A predictive model for transferability of plastic deformation through grain boundaries

    Directory of Open Access Journals (Sweden)

    T. Tsuru

    2016-01-01

    Full Text Available The material strengths of polycrystalline metals have been widely predicted according to the grain size, where yield stress is governed by slip transfer through the grain boundary (GB. The transferability of a dislocation across a GB is enormously important in the deformation process as well as the interaction between a dislocation and GB. This paper proposes a new criterion for the transferability of dislocations through a GB that considers both the intergranular crystallographic orientation of slip systems and the applied stress condition. Atomistic simulations were carried out to investigate the slip transfer event of simple bicrystals composed of Σ 3 ( 1 ̄ 12 GB than Σ 3 ( 1 ̄ 11 GBs under uniaxial deformation and to illustrate the availability of this criterion. As a result, in contrast to the predictions of conventional criteria such as the M-value, dislocations propagated more easily across the Σ 3 ( 1 ̄ 11 and Σ 3 ( 1 ̄ 12 GB under given stress states, reflecting a larger L′-value of Σ3 bicrystal associated with higher transferability.

  1. Zener Pinning of Grain Boundaries and Structural Stability of Immiscible Alloys

    Science.gov (United States)

    Koju, R. K.; Darling, K. A.; Kecskes, L. J.; Mishin, Y.

    2016-06-01

    Immiscible Cu-Ta alloys produced by mechanical alloying are currently the subject of intensive research due to their mechanical strength combined with extraordinary structural stability at high temperatures. Previous experimental and simulation studies suggested that grain boundaries (GBs) in Cu-Ta alloys are stabilized by Ta nano-clusters coherent with the Cu matrix. To better understand the stabilization effect of Ta, we performed atomistic computer simulations of GB-cluster interactions in Cu-Ta alloys with various compositions and GB velocities. The study focuses on a single plane GB driven by an applied shear stress due to the shear-coupling effect. The results of the simulations are in close quantitative agreement with the Zener model of GB pinning. This agreement and the large magnitude of the unpinning stress confirm that the structural stability of these alloys is due to the drastically decreased GB mobility rather than a reduction in GB energy. For comparison, we simulated GB motion in a random solid solution. While the latter also reduces the GB mobility, the effect is not as strong as in the presence of Ta clusters. GB motion in the random solution itself induces precipitation of Ta clusters due to short-circuit diffusion of Ta in GBs, suggesting a possible mechanism of cluster formation inside the grains.

  2. Corrosion-erosion test of SS316L grain boundary engineering material (GBEM) in lead bismuth flowing loop

    Energy Technology Data Exchange (ETDEWEB)

    Saito, Shigeru, E-mail: saito.shigeru@jaea.go.jp [JAEA, Tokai-mura, Ibaraki-ken 319-1195 (Japan); Kikuchi, Kenji [Ibaraki Univ., Frontier Research Center, Tokai-mura, Ibaraki-ken 319-1106 (Japan); Hamaguchi, Dai [JAEA, Tokai-mura, Ibaraki-ken 319-1195 (Japan); Tezuka, Masao [Sukegawa Electic Co., Takahagi-shi, Ibaraki-ken 318-0004 (Japan); Miyagi, Masanori; Kokawa, Hiroyuki [Tohoku Univ., Sendai-shi, Miyagi-ken 980-8579 (Japan); Watanabe, Seiichi [Hokkaido Univ., Sapporo-shi, Hokkai-do 060-8628 (Japan)

    2012-12-15

    To evaluate the lifetime of structural materials utilized in a spallation neutron source, corrosion tests in lead-bismuth eutectic (LBE) have been done at JAEA. Austenitic steels are preferable as the structural material for ADS. However, previous studies have revealed that austenitic steel SS316 shows severe corrosion-erosion in LBE because of LBE penetration through grain boundaries and separation of grains. So it was considered that GBE (grain-boundary engineered) materials may be effective to improve the corrosion resistance of austenitic steels in LBE. In this study, the results of corrosion tests on austenitic steel SS316L-BM (base metal) and SS316L-GBEM (grain-boundary-engineered material) under flowing LBE conditions will be reported. The corrosion test was performed using the JAEA lead-bismuth material corrosion loop (JLBL-1). The experimental conditions were as follows: The high and low temperature parts of the loop were 450 Degree-Sign C and 350 Degree-Sign C, respectively. The flow velocity at the test specimens was about 0.7 m/s. The oxygen concentration in LBE was not controlled and was estimated to have been very low. After the 3600 h of operation, macroscopic, SEM, and SIM observations and EDX analysis were carried out. The results showed that the corrosion depth and LBE penetration through the grain boundaries of the 316SS-GBEM were smaller than those of the 316SS-BM.

  3. Corrosion-erosion test of SS316L grain boundary engineering material (GBEM) in lead bismuth flowing loop

    Science.gov (United States)

    Saito, Shigeru; Kikuchi, Kenji; Hamaguchi, Dai; Tezuka, Masao; Miyagi, Masanori; Kokawa, Hiroyuki; Watanabe, Seiichi

    2012-12-01

    To evaluate the lifetime of structural materials utilized in a spallation neutron source, corrosion tests in lead-bismuth eutectic (LBE) have been done at JAEA. Austenitic steels are preferable as the structural material for ADS. However, previous studies have revealed that austenitic steel SS316 shows severe corrosion-erosion in LBE because of LBE penetration through grain boundaries and separation of grains. So it was considered that GBE (grain-boundary engineered) materials may be effective to improve the corrosion resistance of austenitic steels in LBE. In this study, the results of corrosion tests on austenitic steel SS316L-BM (base metal) and SS316L-GBEM (grain-boundary-engineered material) under flowing LBE conditions will be reported. The corrosion test was performed using the JAEA lead-bismuth material corrosion loop (JLBL-1). The experimental conditions were as follows: The high and low temperature parts of the loop were 450 °C and 350 °C, respectively. The flow velocity at the test specimens was about 0.7 m/s. The oxygen concentration in LBE was not controlled and was estimated to have been very low. After the 3600 h of operation, macroscopic, SEM, and SIM observations and EDX analysis were carried out. The results showed that the corrosion depth and LBE penetration through the grain boundaries of the 316SS-GBEM were smaller than those of the 316SS-BM.

  4. Dynamic process of trace boron non-equilibrium grain boundary segregation and the effect of cooling rate

    Institute of Scientific and Technical Information of China (English)

    Ping Wu; Xinlai He; Bing Cao; Sen Chen

    2003-01-01

    The dynamic process of non-equilibrium grain boundary segregation of trace boron in Fe-40%Ni alloy during cooling andthe effect of cooling rate were investigated by boron tracking autoradiography technique. The results indicate that during coolingprocess, the amount of segregated boron on grain boundary firstly increases fast, then enters a comparatively even increasing stageand increases rapidly again at the third stage. The details of each stage varied with cooling rate are explained. When thc segregationdevelops to a certain degree, the segregated boron atoms transform fiom solute status to precipitate status.

  5. Orientation dependence of grain-boundary energy in metals in the view of a pseudoheterophase dislocation core model

    International Nuclear Information System (INIS)

    A new dislocation model for symmetric tilt grain boundaries was developed as a basis for deriving the quantitative dependence of grain-boundary energy upon misorientation angle in the form of an expression similar to that given by Read and Shockley [Phys. Rev. 78: 275(1950)]. The range of applicability of this equation was extended to over 20 degrees. A comparison of theory and experiment was made for Bi, Ag, Cu, and Fe--Si 3 percent in the teen-degree range of misorientation angles and for Au, α-Fe, Mo, and W in the high-angle range

  6. Effects of Grain Boundaries and Dislocation Cell Walls on Void Nucleation and Growth in Aluminium during Fast Neutron Irradiation

    DEFF Research Database (Denmark)

    Horsewell, Andy; Rahman, F. A.; Singh, Bachu Narain

    1983-01-01

    High purity aluminium irradiated to fluences between 2 multiplied by 10**2**1 and 1 multiplied by 10**2**4 n. m** minus **2 (E greater than 1 Mev) at 120 degree C has been investigated by TEM. A void denuded zone is seen both at grain boundaries and dislocation cell walls. Enhanced void formation...... and growth occurs in a zone extending up to 10 mu m from grain boundaries in annealed material. In polygonized material, the presence of dislocation cell walls leads to cell size dependent void formation and growth; the swelling rate in the large cells is substantially higher than in the annealed material....

  7. Grain Boundary Peak in a Foamed Zn-A1 Eutectoid Alloy

    Institute of Scientific and Technical Information of China (English)

    魏健宁; 程和法; 宫晨利; 周正存; 李志彬; 韩福生

    2002-01-01

    We study experimentally the temperature spectra of internal friction (IF) and relative dynamic modulus (RDM) in a foamed Zn-Al eutectoid alloy. The specimens with macroscopic pores (0.5-1.0mm) were prepared in an air pressure infiltration process. The damping behaviour of the foamed Zn-A1 eutectoid alloy is characterized by IF. The IF and RDM measurements at a maximum surface shear strain of 20 x 10-6 have been made by using a multi-function internal friction apparatus at frequencies of 0.5, 1.0 and 3.0Hz from room temperature to the equilibrium eutectoid isotherm of 277°C, while continuously changing the temperature cycle. At low frequency,an IF peak is observed in the IF temperature curve. The IF peak is of a grain boundary, which is associated with the diffusive flux on a crystalline boundary between the like-phases of Al/A1. Its activation energy has been calculated to be 1.10 ± 0.06 eV and the pre-exponential factor γ0 is 10-14 s in the IF measurements.

  8. An Electron Microscopy Study of Vein-like Grain Boundary Microstructure in Nitrocarburized Low Carbon Steels

    Institute of Scientific and Technical Information of China (English)

    Wanglin Chen; Cuilan Wu; Jianghua Chen; Aigui He

    2013-01-01

    The coarsened grain boundaries (GBs) with vein-like morphology frequently appear in the nitrogen diffusion layer of nitrocarburized carbon steels.The electron probe X-ray microanalysis shows that such vein-like GBs are rich in nitrogen and carbon atoms.Transmission electron microscopy and scanning electron microscopy further reveal that the coarsened GBs consist of γ'-nitrocarbide (Fe4(C,N)) and ε-nitrocarbide lamellas that are formed during nitrocarburizing due to high nitrogen concentration at GBs.it is shown that many of {111}<112> micro twins exist in the γ'-phase lamellas and thin ε-phase slices prefer to nucleate at their twin boundaries with the orientation relationship of {O001}ε//{111}γ'.Upon growing large γ'-lamellas may become faceted and thin ε-lamellas may grow thicker and become the main portions in the vein-like GBs.The microstructure evolution occurring in the vein-like GBs can be depicted as:α + [N]/[C] → γ' + [C,N] → ε-nitrocarbides.

  9. Solute grain boundary segregation during high temperature plastic deformation in a Cr-Mo low alloy steel

    International Nuclear Information System (INIS)

    Highlights: → The segregation of P and Mo is evidently enhanced by plastic deformation. → The boundary concentrations of P and Mo increase with increasing strain. → A model with consideration of site competition in grain boundary segregation in a ternary system is developed. → Model predictions show a reasonable agreement with the observations. - Abstract: Grain boundary segregation of Cr, Mo and P to austenite grain boundaries in a P-doped 1Cr0.5Mo steel is examined using field emission gun scanning transmission electron microscopy for the specimens undeformed and deformed by 10% with a strain rate of 2 x 10-3 s-1 at 900 deg. C, and subsequently water quenched to room temperature. Before deformation, there is some segregation for Mo and P, but the segregation is considerably increased after deformation. The segregation of Cr is very small and there is no apparent difference between the undeformed and deformed specimens. Since the thermal equilibrium segregation has been attained prior to deformation, the segregation produced during deformation has a non-equilibrium characteristic. A theoretical model with consideration of site competition in grain boundary segregation between two solutes in a ternary alloy is developed to explain the experimental results. Model predictions are made, which show a reasonable agreement with the observations.

  10. Density functional theory metadynamics of silver, caesium and palladium diffusion at β-SiC grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Rabone, Jeremy, E-mail: jeremy.rabone@ec.europa.eu [European Commission, Joint Research Centre, Institute for Transuranium Elements, D-76125 Karlsruhe (Germany); López-Honorato, Eddie [Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV), Unidad Saltillo, Industria Metalúrgica 1062, Parque Industrial, Ramos Arizpe 25900, Coahuila (Mexico)

    2015-03-15

    Highlights: • DFT metadynamics of diffusion of Pd, Ag and Cs on grain boundaries in β-SiC. • The calculated diffusion rates for Pd and Ag tally with experimental release rates. • A mechanism of release other than grain boundary diffusion seems likely for Cs. - Abstract: The use of silicon carbide in coated nuclear fuel particles relies on this materials impermeability towards fission products under normal operating conditions. Determining the underlying factors that control the rate at which radionuclides such as Silver-110m and Caesium-137 can cross the silicon carbide barrier layers, and at which fission products such as palladium could compromise or otherwise alter the nature of this layer, are of paramount importance for the safety of this fuel. To this end, DFT-based metadynamics simulations are applied to the atomic diffusion of silver, caesium and palladium along a Σ5 grain boundary and to palladium along a carbon-rich Σ3 grain boundary in cubic silicon carbide at 1500 K. For silver, the calculated diffusion coefficients lie in a similar range (7.04 × 10{sup −19}–3.69 × 10{sup −17} m{sup 2} s{sup −1}) as determined experimentally. For caesium, the calculated diffusion rates are very much slower (3.91 × 10{sup −23}–2.15 × 10{sup −21} m{sup 2} s{sup −1}) than found experimentally, suggesting a different mechanism to the simulation. Conversely, the calculated atomic diffusion of palladium is very much faster (7.96 × 10{sup −11}–7.26 × 10{sup −9} m{sup 2} s{sup −1}) than the observed penetration rate of palladium nodules. This points to the slow dissolution and rapid regrowth of palladium nodules as a possible ingress mechanism in addition to the previously suggested migration of entire nodules along grain boundaries. The diffusion rate of palladium along the Σ3 grain boundary was calculated to be slightly slower (2.38 × 10{sup −11}–8.24 × 10{sup −10} m{sup 2} s{sup −1}) than along the Σ5 grain boundary. Rather

  11. Localisation of shear in ice by recrystallisation on kinks and grain boundary networks: new EBSD data in old experiments

    Science.gov (United States)

    Prior, D. J.; Golding, N.; Durham, W. B.; Seidemann, M.; Diebold, S.; De Bresser, J. H. P.

    2015-12-01

    Relatively high stress (>1MPa), low temperature (95%) success rate and have quantified the microstructures of >50 confined medium experiments. The indium jackets of many confined media experiments have an irregular rumpled surface after deformation. The "rumples" have wavelengths and amplitudes larger than the starting grain size. EBSD maps show anastomosing traces along which fine recrystallised grains are developed. The traces follow some of the original grain boundaries - the scale of connectivity of the network of recrystallized grains is on the scale of several grains (or larger) and broadly corresponds to the wavelength of the rumples on the jacket. In some samples the original grains are kinked and recrystallized grains occur along kink traces. In samples where such recrystallisation is well-developed, these bands contribute of the anastomosing network. Recrystallised grain size describes a broad piezometer relationship that is consistent with much coarser recrystallized grain sizes from higher temperature, lower stress unconfined experiments. We suggest that the dynamic recrystallisation process is a mechanism that enables localization and weakening in ice. The high stress confined media experiments give some insight into processes that may occur in terrestrial ice sheets and cannot be accessed at natural conditions. Some preliminary modeling shows that a dynamic recrystallisation process is consistent with the magnitude of weakening on ice-stream margins in Antarctica.

  12. Inhibition of sensitization in reactor pipe materials by grain boundary structure control. JAERI's nuclear research promotion program, H11-023 (Contract research)

    Energy Technology Data Exchange (ETDEWEB)

    Kokawa, Hiroyuki; Shimada, Masayuki; Wang, Zhan Jie; Sato, Yutaka S. [Tohoku Univ., School of Engineering, Sendai, Miyagi (Japan); Sato, Yoshihiro [Osaka City Univ., Graduate School of Engineering, Osaka (Japan); Kiuchi, Kiyoshi [Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan). Tokai Research Establishment

    2003-03-01

    Grain boundary structure control of type 304 austenitic stainless steel and nickel base Inconel Alloy 600 used in reactor pipe materials was studied to inhibit intergranular corrosion (IGC) due to sensitization. A transmission electron microscopic study showed that low-energy grain boundaries, such as coincidence site lattice boundaries are highly resistant to sensitization, i.e., to intergranular precipitation, chromium depletion and corrosion, compared with ordinary high-angle (random) boundaries. A thermo-mechanical treatment was tried to control grain boundary structure of type 304 austenitic stainless steel for producing a highly intergranular corrosion-resistant material. The effects of process parameters in thermo-mechanical treatment such as pre-strain, annealing temperature, time, etc., on grain boundary characteristics and IGC resistance corrosion of type 304 austenitic stainless steels were examined. The excellent IGC resistance was obtained by slight pre-strain annealing at a relatively low temperature due to the optimized grain boundary character distribution. The uniform distribution of a high frequency of coincidence site lattice boundaries and consequent discontinuity of random boundary network were observed in the material. The optimum distribution can be formed by introducing low energy segments on migrating random boundaries during twin emission and boundary-boundary reactions in the grain growth without additional generation of random boundaries. (author)

  13. Electrical characterization of CdTe grain-boundary properties from as processed CdTe/CdS solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Woods, L.M.; Robinson, G.Y. [Colorado State Univ., Fort Collins, CO (United States); Levi, D.H.; Ahrenkiel, R.K. [National Renewable Energy Lab., Golden, CO (United States); Kaydanov, V. [Colorado School of Mines, Golden, CO (United States)

    1998-09-01

    An ability to liftoff or separate the thin-film polycrystalline CdTe from the CdS, without the use of chemical etches, has enabled direct electrical characterization of the as-processed CdTe near the CdTe/CdS heterointerface. The authors use this ability to understand how a back-contact, nitric-phosphoric (NP) etch affects the grain boundaries throughout the film. Quantitative determination of the grain-boundary barrier potentials and estimates of doping density near the grain perimeter are determined from theoretical fits to measurements of the current vs. temperature. Estimates of the bulk doping are determined from high-frequency resistivity measurements. The light and dark barrier potentials change after the NP etch, and the origin of this change is postulated. Also, a variable doping density within the grains of non-etched material has been determined. These results allow a semi-quantitative grain-boundary band diagram to be drawn that should aid in determining more accurate two-dimensional models for polycrystalline CdTe solar cells.

  14. Meso-microstructural computational simulation of the hydrogen permeation test to calculate intergranular, grain boundary and effective diffusivities

    Energy Technology Data Exchange (ETDEWEB)

    Jothi, S., E-mail: s.jothi@swansea.ac.uk [College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP (United Kingdom); Winzer, N. [Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstraße 11, 79108 Freiburg (Germany); Croft, T.N.; Brown, S.G.R. [College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP (United Kingdom)

    2015-10-05

    Highlights: • Characterized polycrystalline nickel microstructure using EBSD analysis. • Development meso-microstructural model based on real microstructure. • Calculated effective diffusivity using experimental electrochemical permeation test. • Calculated intergranular diffusivity of hydrogen using computational FE simulation. • Validated the calculated computation simulation results with experimental results. - Abstract: Hydrogen induced intergranular embrittlement has been identified as a cause of failure of aerospace components such as combustion chambers made from electrodeposited polycrystalline nickel. Accurate computational analysis of this process requires knowledge of the differential in hydrogen transport in the intergranular and intragranular regions. The effective diffusion coefficient of hydrogen may be measured experimentally, though experimental measurement of the intergranular grain boundary diffusion coefficient of hydrogen requires significant effort. Therefore an approach to calculate the intergranular GB hydrogen diffusivity using finite element analysis was developed. The effective diffusivity of hydrogen in polycrystalline nickel was measured using electrochemical permeation tests. Data from electron backscatter diffraction measurements were used to construct microstructural representative volume elements including details of grain size and shape and volume fraction of grains and grain boundaries. A Python optimization code has been developed for the ABAQUS environment to calculate the unknown grain boundary diffusivity.

  15. Observation of distinct, temperature dependent flux noise near bicrystal grain boundaries in YBa2Cu3O7-x films

    DEFF Research Database (Denmark)

    Bukh, K. R.; Jacobsen, Claus Schelde; Hansen, Jørn Bindslev;

    2000-01-01

    The characteristics of the magnetic flux noise in high temperature superconducting thin-films of yttrium-barium-copper-oxide (YBa2Cu3O7) in the vicinity of artificial grain boundaries have been studied by means of a low critical temperature superconducting quantum interference device (SQUID...

  16. The effect of grain boundary segregation of boron in cast alloy 718 on HAZ microfissuring -- a SIMS analysis

    Energy Technology Data Exchange (ETDEWEB)

    Huang, X.; Chaturvedi, M.C. [Univ. of Manitoba, Winnipeg, Manitoba (Canada). Dept. of Mechanical and Industrial Engineering; Richards, N.L. [Bristol Aerospace Ltd., Winnipeg, Manitoba (Canada); Jackman, J. [CANMET, Ottawa, Ontario (Canada)

    1997-08-01

    Secondary ion mass spectroscopy (SIMS) has been used to examine grain boundary segregation in cast alloy 718. The relationship between the boron segregation and the microfissuring tendency in heat affected zones (HAZ) around electron beam welds is discussed in this study. It is concluded that two types of segregation, namely equilibrium and non-equilibrium segregation, occurred during the homogenization heat treatment of the base material. Water quenching after the homogenization treatment inhibited non-equilibrium segregation of boron and other trace elements owing to insufficient time for diffusion of solute-vacancy complexes to occur. Intermediate cooling rates such as air cooling enhanced both non-equilibrium and equilibrium segregation, since equilibrium segregation occurred during holding at the heat treatment temperature. The value of net segregation produced by a combination of equilibrium segregation and non-equilibrium segregation varied with temperature in a U-shape. The nature of the grain boundary in the cast alloy was examined by using an electron backscattered diffraction (EBSD) technique and it was found that 93% of the grain boundaries were of the random type ({Sigma} > 49). The weldability of this alloy was found to be closely related to the grain boundary segregation of boron, i.e., the variation of HAZ total crack length (TCL) with pre-welding heat treatment temperatures has a trend similar to that of boron segregation with temperature after air cooling. Mechanisms for the effect of boron on HAZ microfissuring have been proposed.

  17. RELATION BETWEEN ELECTROCHEMICAL REACTIVITY AND STRUCTURE OF GRAIN BOUNDARIES CORRELATED TO INTERGRANULAR SULFUR SEGREGATION IN ULTRA PURE NICKEL

    OpenAIRE

    Beaunier, L.; Vignaud, C.; Bouchet, D; Colliex, C.; Trebbia, P.

    1985-01-01

    In a ultra pure nickel doped with 8 and 16 ppm of sulfur, we have determined the kineticsand equilibrium state of segregated sulfur at grain boundaries by electrochemical corrosion test. The undoped nickel and the highest segregated specimen are observed in transmission electron microscopy in order to correlate the structural histogram with electrochemical reactivity study and analytical scanning transmission electron microscope experiments.

  18. Stress-free states of continuum dislocation fields : Rotations, grain boundaries, and the Nye dislocation density tensor

    NARCIS (Netherlands)

    Limkumnerd, Surachate; Sethna, James P.

    2007-01-01

    We derive general relations between grain boundaries, rotational deformations, and stress-free states for the mesoscale continuum Nye dislocation density tensor. Dislocations generally are associated with long-range stress fields. We provide the general form for dislocation density fields whose stre

  19. Influence of point defects on grain boundary diffusion in oxides. Final technical report, July 1, 1990--June 30, 1993

    Energy Technology Data Exchange (ETDEWEB)

    Stubican, V.S.

    1993-11-01

    Grain boundary diffusion coefficients of {sup 57}Co and {sup 59}Co in polycrys. NiO, NiO bicrystal, and polycrys. Fe{sub 3}O{sub 4} were determined at various oxygen pressures at 750 C. For NiO, the low oxygen pressure region (<10{sup {minus}10} MPa) displayed constant grain boundary diffusion coefficients as the oxygen pressure decreased, indicating an extrinsic region in which the impurity-induced defects dominated the intrinsic defects. At greater oxygen pressures, the intrinsic defects (Ni vacancies) dominated the extrinsic defects, causing the diffusion to increase with pressure. For Fe{sub 3}O{sub 4}, at low oxygen pressures (<10{sup {minus}16} MPa), the grain boundary diffusion coefficient increased when the pressure decreased, owing to interstitial type diffusion; at >10{sup {minus}15} MPa, the diffusion increased with pressure, owing to vacancy type diffusion. D{sub gb} of Co ions in Fe{sub 3}O{sub 4} is proportional to pO{sub 2}{sup {minus}2/3} in the low pressure region and to pO{sub 2}{sup 2/3} in the high pressure region, indicating similar mechanisms in the grain boundary diffusion and volume diffusion. Ratio of D{sub gb}/D was about 10{sup 3}.

  20. Computational Capabilities for Predictions of Interactions at the Grain Boundary of Refractory Alloys

    Energy Technology Data Exchange (ETDEWEB)

    Sengupta, Debasis; Kwak, Shaun; Vasenkov, Alex; Shin, Yun Kyung; Duin, Adri van

    2014-09-30

    New high performance refractory alloys are critically required for improving efficiency and decreasing CO2 emissions of fossil energy systems. The development of these materials remains slow because it is driven by a trial-and-error experimental approach and lacks a rational design approach. Atomistic Molecular Dynamic (MD) design has the potential to accelerate this development through the prediction of mechanical properties and corrosion resistance of new materials. The success of MD simulations depends critically on the fidelity of interatomic potentials. This project, in collaboration with Penn State, has focused on developing and validating high quality quantum mechanics based reactive potentials, ReaxFF, for Ni-Fe-Al-Cr-O-S system. A larger number of accurate density functional theory (DFT) calculations were performed to generate data for parameterizing the ReaxFF potentials. These potentials were then used in molecular dynamics (MD) and molecular dynamics-Monte Carlo (MD-MC) for much larger system to study for which DFT calculation would be prohibitively expensive, and to understand a number of chemical phenomena Ni-Fe-Al-Cr-O-S based alloy systems . These include catalytic oxidation of butane on clean Cr2O3 and pyrite/Cr2O3, interfacial reaction between Cr2O3 (refractory material) and Al2O3 (slag), cohesive strength of at the grain boundary of S-enriched Cr compared to bulk Cr and Ssegregation study in Al, Al2O3, Cr and Cr2O3 with a grain structure. The developed quantum based ReaxFF potential are available from the authors upon request. During this project, a number of papers were published in peer-reviewed journals. In addition, several conference presentations were made.

  1. Computational Capabilities for Predictions of Interactions at the Grain Boundary of Refractory Alloys

    Energy Technology Data Exchange (ETDEWEB)

    Sengupta, Debasis; Kwak, Shaun; Vasenkov, Alex; Shin, Yun Kyung; Duin, Adri van

    2014-09-30

    New high performance refractory alloys are critically required for improving efficiency and decreasing CO2 emissions of fossil energy systems. The development of these materials remains slow because it is driven by a trial-and-error experimental approach and lacks a rational design approach. Atomistic Molecular Dynamic (MD) design has the potential to accelerate this development through the prediction of mechanical properties and corrosion resistance of new materials. The success of MD simulations depends critically on the fidelity of interatomic potentials. This project, in collaboration with Penn State, has focused on developing and validating high quality quantum mechanics based reactive potentials, ReaxFF, for Ni-Fe-Al-Cr-O-S system. A larger number of accurate density functional theory (DFT) calculations were performed to generate data for parameterizing the ReaxFF potentials. These potentials were then used in molecular dynamics (MD) and molecular dynamics-Monte Carlo (MD-MC) for much larger system to study for which DFT calculation would be prohibitively expensive, and to understand a number of chemical phenomena Ni-Fe-Al-Cr-O-S based alloy systems . These include catalytic oxidation of butane on clean Cr2O3 and pyrite/Cr2O3, interfacial reaction between Cr2O3 (refractory material) and Al2O3 (slag), cohesive strength of at the grain boundary of S-enriched Cr compared to bulk Cr and Ssegregation study in Al, Al2O3, Cr and Cr2O3 with a grain structure. The developed quantum based ReaxFF potential are available from the authors upon request. During this project, a number of papers were published in peer-reviewed journals. In addition, several conference presentations were made.

  2. Computational Capabilities for Predictions of Interactions at the Grain Boundary of Refractory Alloys

    Energy Technology Data Exchange (ETDEWEB)

    Sengupta, Debasis [CFD Research Corporation, Huntsville, AL (United States); Kwak, Shaun [CFD Research Corporation, Huntsville, AL (United States); Vasenkov, Alex [CFD Research Corporation, Huntsville, AL (United States); Shin, Yun Kyung [Pennsylvania State Univ., University Park, PA (United States); Duin, Adri van [Pennsylvania State Univ., University Park, PA (United States)

    2014-12-01

    New high performance refractory alloys are critically required for improving efficiency and decreasing CO2 emissions of fossil energy systems. The development of these materials remains slow because it is driven by a trial-and-error experimental approach and lacks a rational design approach. Atomistic Molecular Dynamic (MD) design has the potential to accelerate this development through the prediction of mechanical properties and corrosion resistance of new materials. The success of MD simulations depends critically on the fidelity of interatomic potentials. This project, in collaboration with Penn State, has focused on developing and validating high quality quantum mechanics based reactive potentials, ReaxFF, for Ni-Fe-Al-Cr-O-S system. A larger number of accurate density functional theory (DFT) calculations were performed to generate data for parameterizing the ReaxFF potentials. These potentials were then used in molecular dynamics (MD) and molecular dynamics-Monte Carlo (MD-MC) for much larger system to study for which DFT calculation would be prohibitively expensive, and to understand a number of chemical phenomena Ni-Fe-Al-Cr-O-S based alloy systems . These include catalytic oxidation of butane on clean Cr2O3 and pyrite/Cr2O3, interfacial reaction between Cr2O3 (refractory material) and Al2O3 (slag), cohesive strength of at the grain boundary of S-enriched Cr compared to bulk Cr and Ssegregation study in Al, Al2O3, Cr and Cr2O3 with a grain structure. The developed quantum based ReaxFF potential are available from the authors upon request. During this project, a number of papers were published in peer-reviewed journals. In addition, several conference presentations were made.

  3. The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation

    Science.gov (United States)

    Jonas, Laura; John, Timm; King, Helen E.; Geisler, Thorsten; Putnis, Andrew

    2014-01-01

    The pseudomorphic replacement of Carrara marble by calcium phosphates was used as a model system in order to study the influence of different fluid pathways for reaction front propagation induced by fluid-rock interaction. In this model, grain boundaries present in the rock as well as the transient porosity structures developing throughout the replacement reaction enable the reaction front to progress further into the rock as well as to the center of each single grain until transformation is complete. Hydrothermal treatment of the marble using phosphate bearing solutions led to the formation of hydroxylapatite and β-TCP; the formation of the latter phase was probably promoted by the presence of ∼0.6 wt.% Mg in the parent carbonate phase. Completely transformed single grains show a distinctive zoning, both in composition and texture. Whereas areas next to the grain boundary consist of nearly pure hydroxylapatite and show a coarse porosity, areas close to the center of the single grains show a high amount of β-TCP and a very fine porous microstructure. If fluorine was added as an additional solution component, up to 3 wt.% of F were incorporated into the product apatite and the formation of β-TCP was avoided. The use of the isotope 18O as a chronometer for the replacement reaction makes it possible to reconstruct the chronological development of the calcium phosphate reaction front. Raman analysis revealed that the incorporation of 18O in the PO4 tetrahedron of hydroxylapatite results in the development of distinct profiles in the calcium phosphate reaction front perpendicular to the grain boundaries of the marble. Through the use of the 18O chronometer, it is possible to estimate and compare the time effectiveness of the different fluid pathways in this model system. The results demonstrate that the grain boundaries are an effective pathway enabling the fluid to penetrate the rock more than one order of magnitude faster compared to the newly developing channel

  4. Gas accumulation at grain boundaries during 800 MeV proton irradiation of aluminium and aluminium-alloys

    International Nuclear Information System (INIS)

    Samples of pure aluminium (99.9999%) and commercial Al-2.7%Mg (AlMg3) and Al-1.1%Mg-0.5%Si (Al6061) alloys were irradiated with 800 MeV protons at the Los Alamos Meson Physics Facility (LAMPF) at a temperature between 40-1000C to a maximum dose of 0.2 dpa. Transmission electron microscopy (TEM) showed a complete absence of voids or bubbles in the grain interiors of the aluminium and the aluminium-alloys. Bubbles were clearly visible by TEM at grain boundaries in pure Al and the AlMg3 alloy; but bubbles were not visible in the Al6061 alloy. The bubble density in the AlMg3 alloy was considerably higher than in pure Al. The amount of gas accumulation at grain boundaries was found to depend on gas generation rate, alloying and cold-work microstructure. (orig.)

  5. Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite

    International Nuclear Information System (INIS)

    Graphical abstract: -- Abstract: In an Fe–9 at.% Mn maraging alloy annealed at 450 °C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5–15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite

  6. Study by atomistic theory and high-resolution electron microscopies of Cu atoms at an Al grain boundary

    Energy Technology Data Exchange (ETDEWEB)

    Plitzko, Jurgen M.; Campbell, Geoffrey H.; King, Wayne E.; Foiles, Stephen M.; Kisielowski, Christian; Duscher, Gerd

    2003-02-02

    New insight into the atomic segregation of copper to an aluminum grain boundary has been obtained using atomic resolution electron microscopy techniques coupled with ab-initio electronic structure calculations. We find the copper segregation to be site specific, changing the structure of the boundary by unexpectedly occupying interstitial sites. The calculated energy for segregation was found to be sufficient for essentially all of the interstitial sites to be filled. Minor elemental constituents in materials can have profound effects on their engineering performance, often through segregation to grain boundaries in the host material. One important example is the great resistance to electromigration damage in microelectronics imparted by small additions of copper to aluminum interconnects.

  7. Quantification of the Existence Ratio of Non-Adhesion Grain Boundaries and Factors Governing the Strength of Coke Containing Low-Quality Coal

    Science.gov (United States)

    Kanai, Tetsuya; Yamazaki, Yoshiaki; Zhang, Xiaoqing; Uchida, Ataru; Saito, Yasuhiro; Shoji, Masakazu; Aoki, Hideyuki; Nomura, Seiji; Kubota, Yukihiro; Hayashizaki, Hideyuki; Miyashita, Shigeto

    “Non-adhesion grain boundaries” are formed when low-quality coal grains do not adhere to other grains in the carbonization process because of the low dilation of coke. To better understand the effects of non-adhesion grain boundaries on coke strength, the relationship between the existence ratio of non-adhesion grain boundaries and coke strength was investigated quantitatively. The existence ratio of non-adhesion grain boundaries were measured quantitatively by observing the fracture cross-section of coke using scanning electron microscopy (SEM). Coke strength was measured with a diametral-compression test and an I-shape drum index test. As a result, non-adhesion grain boundaries increased with an increase in the blending ratio of low-quality coal. In particular, non-adhesion grain boundaries increased rapidly when the blending ratio of low-quality coal was over 50%. When the ratio was less than 50%, low-quality coals adhered to other caking coal. However, not many low-quality coals adhered to other caking coals when the ratio was over 50%. The tensile strength of coke was not affected by the porosity of coke. However, the tensile strength and the drum index were affected by the existence ratio of non-adhesion grain boundaries. Tensile strength decreased rapidly even for a few non-adhesion grain boundaries because significant defects caused a fracture in the diametral-compression test. However, the I-shape drum index decreased linearly with the existence ratio of the non-adhesion grain boundaries because many fractures occurred during 600 rotations in the drum. The strength of coke containing low-quality coal is governed by the existence ratio of non-adhesion grain boundaries rather than mean values such as the porosity of coke.

  8. Effect of transition metal impurities on the strength of grain boundaries in vanadium

    Science.gov (United States)

    Wu, Xuebang; Kong, Xiang-Shan; You, Yu-Wei; Liu, Wei; Liu, C. S.; Chen, Jun-Ling; Luo, G.-N.

    2016-09-01

    Effects of 3d (Ti-Ni), 4d (Zr-Pd), and 5d (Hf-Pt) transition metal impurities on strength of two representative vanadium grain boundaries (GBs), symmetric Σ3(111) and asymmetric Σ5(210), were studied by first-principles calculations within the framework of the Rice-Wang thermodynamic model and within the computational tensile test. The desirable elements to increase the GB cohesion were predicted based on their segregation and strengthening behaviors across the different GB sites. It reveals that the elements Ti, Zr, Hf, Nb, and Ta are good choices for the GB cohesion enhancers. In addition, the GB strengthening by solutes is sensitive to the GB structures. The elements Cr, Mn, Fe, Co, and Ni decrease the GB strength of the Σ3(111) GB but they can increase the cohesion of the Σ5(210) GB. Furthermore, the origin of Ti-induced change of the GB strength was uncovered by analyzing the atomic bonds and electronic structures as well as the tensile strength. This work provides a theoretical guidance to screen promising alloying elements in V-based materials with improved resistance to GB decohesion and also helps us to understand the formation mechanism of Ti-rich precipitates in the V-Cr-Ti alloys under neutron or ion irradiation environments.

  9. High coercivity in rare-earth lean nanocomposite magnets by grain boundary infiltration

    Science.gov (United States)

    Madugundo, Rajasekhar; Salazar-Jaramillo, Daniel; Manuel Barandiaran, Jose; Hadjipanayis, George C.

    2016-02-01

    A significant enhancement in coercivity was achieved by grain boundary modification through low temperature infiltration of Pr75(Cu0.25Co0.75)25 eutectic alloy in rare-earth lean (Pr/Nd)-Fe-B/α-Fe nanocomposite magnets. The infiltration procedure was carried out on ribbons and hot-deformed magnets at 600-650 °C for different time durations. In Nd2Fe14B/α-Fe ribbons, the coercivity increased from 5.3 to 23.8 kOe on infiltration for 4 h. The Pr2Fe14B/α-Fe hot-deformed magnet shows an increase in coercivity from 5.4 to 22 kOe on infiltration for 6 h. The increase in the coercivity comes at the expense of remnant magnetization. X-ray diffraction studies confirm the presence of both the hard Nd2Fe14B and soft α-Fe phases. A decrease in the soft α-Fe phase content was observed after infiltration.

  10. YBCO grain boundary Josephson junction coupled with a slot dipole antenna for terahertz wave detectors

    Science.gov (United States)

    Yamada, H.; Hayasaka, T.; Toya, G.; Saito, A.; Ohshima, S.; Nakajima, K.

    2014-05-01

    We examined terahertz wave detectors that used YBa2Cu3O7-δ (YBCO) grain boundary Josephson junctions (GBJJs) coupled with a slot dipole antenna (SDA). The detectors consisted of a 220-GHz full-wavelength SDA patterned on a Au layer and a GBJJ patterned on an YBCO/bicrystal MgO film, which were separated by an insulating benzocyclobutene layer. The microbridge of the fabricated junction was 5-μm wide and was trimmed to 2 μm using an ultraviolet laser cutter to modify the junction parameters. The critical current IC, normal resistance RN, and ICRN product after the trimming at 30 K were 0.62 mA, 1.42 Ω, and 0.88 mV, respectively. The current-voltage characteristics and radio frequency (RF) wave responses of these detectors for a millimeter wave of 180-240 GHz were measured at 30 K. The coupling efficiency between the GBJJ and the SDA and the system sensitivity were obtained as -19.0 dB and 630 V/W, respectively, at 193 GHz. For the RF wave response of 180-240 GHz, the coupling efficiency was relatively flat.

  11. Gap States at Low-Angle Grain Boundaries in Monolayer Tungsten Diselenide

    KAUST Repository

    Huang, Yu Li

    2016-05-03

    Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6°. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport and other device properties highlights the importance of defect engineering in future 2D device fabrication. © 2016 American Chemical Society.

  12. Grain Boundary Induced Bias Instability in Soluble Acene-Based Thin-Film Transistors

    Science.gov (United States)

    Nguyen, Ky V.; Payne, Marcia M.; Anthony, John E.; Lee, Jung Hun; Song, Eunjoo; Kang, Boseok; Cho, Kilwon; Lee, Wi Hyoung

    2016-09-01

    Since the grain boundaries (GBs) within the semiconductor layer of organic field-effect transistors (OFETs) have a strong influence on device performance, a substantial number of studies have been devoted to controlling the crystallization characteristics of organic semiconductors. We studied the intrinsic effects of GBs within 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT) thin films on the electrical properties of OFETs. The GB density was easily changed by controlling nulceation event in TES-ADT thin films. When the mixing time was increased, the number of aggregates in as-spun TES-ADT thin films were increased and subsequent exposure of the films to 1,2-dichloroethane vapor led to a significant increase in the number of nuleation sites, thereby increasing the GB density of TES-ADT spherulites. The density of GBs strongly influences the angular spread and crystallographic orientation of TES-ADT spherulites. Accordingly, the FETs with higher GB densities showed much poorer electrical characteristics than devices with lower GB density. Especially, GBs provide charge trapping sites which are responsible for bias-stress driven electrical instability. Dielectric surface treatment with a polystyrene brush layer clarified the GB-induced charge trapping by reducing charge trapping at the semiconductor-dielectric interface. Our study provides an understanding on GB induced bias instability for the development of high performance OFETs.

  13. Nanocrystalline, Ultra-Degradation-Resistant Zirconia: Its Grain Boundary Nanostructure and Nanochemistry

    Science.gov (United States)

    Matsui, Koji; Yoshida, Hidehiro; Ikuhara, Yuichi

    2014-04-01

    Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) has been known to be an excellent structural material with high strength and toughness since the pioneering study by Garvie et al. in 1975. However, Y-TZP is not considered an environmental or biomedical material because it undergoes an inherent tetragonal-to-monoclinic (T-->M) phase transformation in humid or aqueous environment, which leads to premature failure, so-called low-temperature degradation (LTD). In this study, we demonstrate for the first time that this fatal shortcoming of Y-TZP can be resolved by controlling the grain boundary nanostructure and chemical composition distribution in Y-TZP. Nanocrystalline Y-TZP doped with Al3+ and Ge4+ ions exhibits no LTD for more than 4 years in hot water at 140°C, whereas 70% of the tetragonal phase in conventional TZP transforms to the monoclinic phase within only 15 h. This innovative Y-TZP can be fabricated by pressureless sintering at 1200°C far below the sintering temperature for conventional Y-TZP. The developed TZP ceramics will be useful in numerous environmental-proofing applications, particularly in the biomedical engineering field.

  14. Defect induced d0 ferromagnetism in a ZnO grain boundary

    KAUST Repository

    Assa Aravindh, Sasikala Devi

    2015-12-08

    Several experimental studies have referred to the grain boundary(GB) defect as the origin of ferromagnetism in zinc oxide (ZnO). However, the mechanism of this hypothesis has never been confirmed. Present study investigates the atomic structure and the effect of point defects in a ZnOGB using the generalized gradient approximation+U approximation. The relaxed GB possesses large periodicity and channels with 8 and 10 numbered atoms having 4 and 3 fold coordination. The Znvacancy (VZn) shows a tendency to be attracted to the GB, relative to the bulk-like region. Although no magnetization is obtained from point defect-free GB, VZn induces spin polarization as large as 0.68 μB/atom to the O sites at the GB.Ferromagnetic exchange energy >150 eV is obtained by increasing the concentration of VZn and by the injection of holes into the system. Electronic structure analysis indicates that the spin polarization without external dopants originates from the O 2p orbitals, a common feature of d0semiconductors.

  15. Self-passivation rule and structure of CdTe Σ3 (112) grain boundaries

    Science.gov (United States)

    Liu, Cheng-yan; Zhang, Yue-yu; Hou, Yu-sheng; Chen, Shi-you; Xiang, Hong-jun; Gong, Xin-gao

    2016-05-01

    The theoretical study of grain boundaries (GBs) in polycrystalline semiconductors is currently stalemated by their complicated nature, which is difficult to extract from any direct experimental characterization. Usually, coincidence-site-lattice models are constructed simply by aligning two symmetric planes ignoring various possible reconstructions. Here, we propose a general self-passivation rule to determine the low-energy GB reconstruction and find new configurations for the CdTe Σ3 (112) GBs. First-principles calculations show that it has lower formation energies than the prototype GBs adopted widely in previous studies. Surprisingly, the reconstructed GBs show self-passivated electronic properties without deep-level states in the band gap. Based on the reconstructed configurations, we revisited the influence of CdC l2 post-treatment on the CdTe GBs and found that the addition of both Cd and Cl atoms in the GB improves the photovoltaic properties by promoting self-passivation and inducing n -type levels, respectively. The present study provides a new route for further studies of GBs in covalent polycrystalline semiconductors and highlights that previous studies on the GBs of multinary semiconductors, which are based on the unreconstructed prototype GB models, should be revisited.

  16. Ab initio description of segregation and cohesion of grain boundaries in W–25 at.% Re alloys

    International Nuclear Information System (INIS)

    We investigate grain boundaries (GBs) in W–25 at.% Re alloys with special focus on the segregation of Re to the GBs and the changes in their cohesive properties that arise therefrom. Our simulations rely on density functional theory and a mean-field approximation, the virtual crystal approximation, to model the highly alloyed system. Based on a gamma-surface approach, the geometry of a wide range of GBs, including tilt, twist and mixed GBs, is obtained. Segregation energies are found to vary strongly between different segregation sites, with the strongest segregation energy amounting to −0.75 eVs. We show that bond-order parameters are able to identify strong segregation sites based on purely geometric information only. With a thermodynamic model based on a Bragg–Williams approach, the concentration of Re atoms is calculated as a function of GB character at 1913 K and compared with atom probe experiments. The segregation levels are similar, with a trend towards higher segregation levels observed in the calculations for certain misorientation angles. Finally, Re alloying is found to lead to a general increase in the work of separation of the GBs, suggesting a reduction in the tendency for intergranular fracture

  17. Phase-field crystal study of segregation induced grain-boundary premelting in binary alloys

    International Nuclear Information System (INIS)

    The segregation-induced grain boundary (GB) premelting is studied by phase-field crystal model. We investigate the microstructure evolution of GBs with different misorientation angles and the variation of concentration distribution during premelting process. The liquid film thickness is calculated by an excessive mass technique. The results show that for low-angle GBs, the liquid phase first appears at individual dislocations region where the solute atoms segregate, and there exist two structural transitions during premelting process, which behave as two inflection points in the curve of w versus ΔB0. For high-angle GBs, the liquid film is rather uniform and its thickness diverges logarithmically when the melting point is approached. Moreover, the higher misorientation angle and segregation the GB has, the lower temperature the liquid film occurs. The concentration variation law indicates that GB segregation and GB premelting promote each other when the liquid phase appears. The critical wetting angle is almost equal to 12.5° and this value is much closer to the reality

  18. Quantitative TEM analysis of precipitation and grain boundary segregation in neutron irradiated EUROFER 97

    Energy Technology Data Exchange (ETDEWEB)

    Dethloff, Christian, E-mail: christian.dethloff@kit.edu; Gaganidze, Ermile; Aktaa, Jarir

    2014-11-15

    Characterization of irradiation induced microstructural defects is essential for assessing the applicability of structural steels like the Reduced Activation Ferritic/Martensitic steel EUROFER 97 in upcoming fusion reactors. In this work Transmission Electron Microscopy (TEM) is used to analyze the types and structure of precipitates, and the evolution of their size distributions and densities caused by neutron irradiation to a dose of 32 displacements per atom (dpa) at 330–340 °C in the irradiation experiment ARBOR 1. A significant growth of MX and M{sub 23}C{sub 6} type precipitates is observed after neutron irradiation, while the precipitate density remains unchanged. Hardening caused by MX and M{sub 23}C{sub 6} precipitate growth is assessed by applying the Dispersed Barrier Hardening (DBH) model, and shown to be of minor importance when compared to other irradiation effects like dislocation loop formation. Additionally, grain boundary segregation of chromium induced by neutron irradiation was investigated and detected in irradiated specimens.

  19. Electronic and magnetic properties of armchair graphene nanoribbons with 558 grain boundary.

    Science.gov (United States)

    Dai, Q Q; Zhu, Y F; Jiang, Q

    2014-06-14

    Grain boundaries (GBs) that inevitably appear in CVD-grown graphene affect the electronic properties of graphene-based nanomaterials. In this paper, we introduce 558 GB (composed of a pair of pentagons and one octagon alternately) into armchair graphene nanoribbons (AGNRs) and divide them into three groups in light of the end configurations of 558 GB at the ribbon edges. By using first-principles calculations, the variations of electronic and magnetic properties with two adjustable parameters W (ribbon widths) and NZ (the distances between 558 GBs) are systematically investigated for each group. The results show that different electronic and magnetic behaviors versus W and NZ are presented for varying end configurations of 558 GB, including nonmagnetic metals, ferromagnetic metals and nonmagnetic semiconductors. By introducing 558 GB into AGNRs, the impurity states that are completely contributed by 558 GB appear around the Fermi level. Furthermore, a ferromagnetic ordering on the two zigzag chains of 558 GB occurs for the ferromagnetic metals due to the spin splitting energy bands near the Fermi level. These unique electronic and magnetic properties of AGNRs with 558 GB would find their potential applications in electronic and spintronic devices. PMID:24752487

  20. Contrasting the grain boundary-affected performance of zinc and indium oxide transparent conductors

    International Nuclear Information System (INIS)

    Zinc oxide-based transparent conductors have long been advanced for their potential as low-cost, earth-abundant replacements for the indium oxide-based materials that currently dominate in practical applications. However, this potential has yet to be realized because of the difficulties in producing zinc oxide thin films with the necessary high levels of electrical conductivity and environmental stability that are readily achieved using indium oxide. To better understand the fundamental reasons for this, polycrystalline zinc and indium oxide thin films were prepared across a range of deposition temperatures using the technique of spray pyrolysis. Electrical transport measurements of these samples both as a function of temperature and UV irradiation were correlated with film morphology to illustrate that the different grain boundary behaviour of these two materials is one of the key reasons for their divergent performance. This is a critical challenge that must be addressed before any substantial increase in the adoption of ZnO-based transparent conductors can take place. (paper)

  1. Defect induced d{sup 0} ferromagnetism in a ZnO grain boundary

    Energy Technology Data Exchange (ETDEWEB)

    Assa Aravindh, Sasikala Devi; Schwingenschloegl, Udo; Roqan, Iman S, E-mail: iman.roqan@kaust.edu.sa [Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 2955-6900 (Saudi Arabia)

    2015-12-14

    Several experimental studies have referred to the grain boundary (GB) defect as the origin of ferromagnetism in zinc oxide (ZnO). However, the mechanism of this hypothesis has never been confirmed. Present study investigates the atomic structure and the effect of point defects in a ZnO GB using the generalized gradient approximation+U approximation. The relaxed GB possesses large periodicity and channels with 8 and 10 numbered atoms having 4 and 3 fold coordination. The Zn vacancy (V{sub Zn}) shows a tendency to be attracted to the GB, relative to the bulk-like region. Although no magnetization is obtained from point defect-free GB, V{sub Zn} induces spin polarization as large as 0.68 μ{sub B}/atom to the O sites at the GB. Ferromagnetic exchange energy >150 eV is obtained by increasing the concentration of V{sub Zn} and by the injection of holes into the system. Electronic structure analysis indicates that the spin polarization without external dopants originates from the O 2p orbitals, a common feature of d{sup 0} semiconductors.

  2. Contrasting the grain boundary-affected performance of zinc and indium oxide transparent conductors.

    Science.gov (United States)

    Vai, A T; Rashidi, N; Fang, Y; Kuznetsov, V L; Edwards, P P

    2016-06-01

    Zinc oxide-based transparent conductors have long been advanced for their potential as low-cost, earth-abundant replacements for the indium oxide-based materials that currently dominate in practical applications. However, this potential has yet to be realized because of the difficulties in producing zinc oxide thin films with the necessary high levels of electrical conductivity and environmental stability that are readily achieved using indium oxide. To better understand the fundamental reasons for this, polycrystalline zinc and indium oxide thin films were prepared across a range of deposition temperatures using the technique of spray pyrolysis. Electrical transport measurements of these samples both as a function of temperature and UV irradiation were correlated with film morphology to illustrate that the different grain boundary behaviour of these two materials is one of the key reasons for their divergent performance. This is a critical challenge that must be addressed before any substantial increase in the adoption of ZnO-based transparent conductors can take place. PMID:26952740

  3. Modeling radiation damage near grain boundary in helium-doped α-iron

    Science.gov (United States)

    Xu, C. P.; Liu, X.-Y.; Gao, F.; Li, Y. H.; Wang, Y. Q.

    2014-08-01

    Molecular dynamics (MD) simulations are performed to investigate how ∑3(1 2 1) symmetric tilt grain boundary (GB) affects point defects and defect clusters in He-doped α-iron at 300 K in picosecond time scales. Molecular statics calculations are also performed and show that the formation energy is reduced in the GB, and the GB acts as a good sink for point defects, especially for interstitial He and self-interstitial atoms (SIAs). It is observed that the average size of HenVm (m > n) clusters becomes smaller in the GB-containing Fe system, where m and n represent the number of vacancies and He atoms in the cluster, respectively. It is also found that the number of HenV (n = 2, 3) clusters in the GB region decreases, while the number of the HeV clusters increases. The GBs loaded with substitutional or interstitial helium atoms are found to facilitate the growth of helium clusters in the GB region.

  4. Microstructural sensitivity of 316H austenitic stainless steel: Residual stress relaxation and grain boundary fracture

    International Nuclear Information System (INIS)

    Research highlights: → Triaxial residual macro-stresses have been measured by neutron diffraction. → Rates of stress relaxation are shown to be a function of the microstructure. → Quantification of M23C6 precipitation was undertaken by a novel approach. → Intergranular M23C6 precipitation promotes the potential to intergranular fracture. → Phosphorous segregation further enhances the potential to intergranular fracture. - Abstract: The present work considers the role of thermo-mechanical history on the generation and relaxation of residual stresses, typical of those encountered in Type 316H austenitic stainless steel thick section weldments. A series of thermo-mechanical pre-treatments have been developed and applied to simulate the critical microstructures observed within the heat affected zone of the thick section parent material. The through thickness distributions of the residual macro-stresses in cylindrical specimens have been measured by neutron diffraction and then the rates of the relaxation are shown to be a function of microstructure. The susceptibility to intergranular brittle fracture at a temperature of -196 deg. C is shown to be a function of M23C6 carbide precipitates and phosphorous segregation at the grain boundaries. Finally, the link of the present study to the understanding of the reheat cracking is briefly discussed.

  5. Localized electronic states at grain boundaries on the surface of graphene and graphite

    Science.gov (United States)

    Luican-Mayer, Adina; Barrios-Vargas, Jose E.; Toft Falkenberg, Jesper; Autès, Gabriel; Cummings, Aron W.; Soriano, David; Li, Guohong; Brandbyge, Mads; Yazyev, Oleg V.; Roche, Stephan; Andrei, Eva Y.

    2016-09-01

    Recent advances in large-scale synthesis of graphene and other 2D materials have underscored the importance of local defects such as dislocations and grain boundaries (GBs), and especially their tendency to alter the electronic properties of the material. Understanding how the polycrystalline morphology affects the electronic properties is crucial for the development of applications such as flexible electronics, energy harvesting devices or sensors. We here report on atomic scale characterization of several GBs and on the structural-dependence of the localized electronic states in their vicinity. Using low temperature scanning tunneling microscopy and spectroscopy, together with tight binding and ab initio numerical simulations we explore GBs on the surface of graphite and elucidate the interconnection between the local density of states and their atomic structure. We show that the electronic fingerprints of these GBs consist of pronounced resonances which, depending on the relative orientation of the adjacent crystallites, appear either on the electron side of the spectrum or as an electron-hole symmetric doublet close to the charge neutrality point. These two types of spectral features will impact very differently the transport properties allowing, in the asymmetric case to introduce transport anisotropy which could be utilized to design novel growth and fabrication strategies to control device performance.

  6. Contrasting the grain boundary-affected performance of zinc and indium oxide transparent conductors

    Science.gov (United States)

    Vai, A. T.; Rashidi, N.; Fang, Y.; Kuznetsov, V. L.; Edwards, P. P.

    2016-06-01

    Zinc oxide-based transparent conductors have long been advanced for their potential as low-cost, earth-abundant replacements for the indium oxide-based materials that currently dominate in practical applications. However, this potential has yet to be realized because of the difficulties in producing zinc oxide thin films with the necessary high levels of electrical conductivity and environmental stability that are readily achieved using indium oxide. To better understand the fundamental reasons for this, polycrystalline zinc and indium oxide thin films were prepared across a range of deposition temperatures using the technique of spray pyrolysis. Electrical transport measurements of these samples both as a function of temperature and UV irradiation were correlated with film morphology to illustrate that the different grain boundary behaviour of these two materials is one of the key reasons for their divergent performance. This is a critical challenge that must be addressed before any substantial increase in the adoption of ZnO-based transparent conductors can take place.

  7. Properties of grain boundary networks in the NEEM ice core analyzed by combined transmission and reflection optical microscopy

    Science.gov (United States)

    Binder, Tobias; Weikusat, Ilka; Garbe, Christoph; Svensson, Anders; Kipfstuhl, Sepp

    2014-05-01

    Microstructure analysis of ice cores is vital to understand the processes controlling the flow of ice on the microscale. To quantify the microstructural variability (and thus occurring processes) on centimeter, meter and kilometer scale along deep polar ice cores, a large number of sections has to be analyzed. In the last decade, two different methods have been applied: On the one hand, transmission optical microscopy of thin sections between crossed polarizers yields information on the distribution of crystal c-axes. On the other hand, reflection optical microscopy of polished and controlled sublimated section surfaces allows to characterize the high resolution properties of a single grain boundary, e.g. its length, shape or curvature (further developed by [1]). Along the entire NEEM ice core (North-West Greenland, 2537 m length) drilled in 2008-2011 we applied both methods to the same set of vertical sections. The data set comprises series of six consecutive 6 x 9 cm2 sections in steps of 20 m - in total about 800 images. A dedicated method for automatic processing and matching both image types has recently been developed [2]. The high resolution properties of the grain boundary network are analyzed. Furthermore, the automatic assignment of c-axis misorientations to visible sublimation grooves enables us to quantify the degree of similarity between the microstructure revealed by both analysis techniques. The reliability to extract grain boundaries from both image types as well as the appearance of sublimation groove patterns exhibiting low misorientations is investigated. X-ray Laue diffraction measurements (yielding full crystallographic orientation) have validated the sensitivity of the surface sublimation method for sub-grain boundaries [3]. We introduce an approach for automatic extraction of sub-grain structures from sublimation grooves. A systematic analysis of sub-grain boundary densities indicates a possible influence of high impurity contents (amongst

  8. Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

    Indian Academy of Sciences (India)

    Neeraj Khare; A K Gupta; U P Moharil; A K Raychaudhuri; S P Pai; R Pinto

    2002-05-01

    La0.67Ba0.33MnO3 (LBMO) thin film is deposited on a $36.7°C SrTiO3 bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature > 175 K. At low temperature, - characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance–voltage characteristics of the bicrystal grain boundary indicates that at low temperatures ( < 175$ K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures ( > 175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.

  9. Molecular dynamics simulations of grain boundary mobility in Al, Cu and γ-Fe using a symmetrical driving force

    International Nuclear Information System (INIS)

    We present a new artificial driving force for the determination of grain boundary mobility by molecular dynamics. The new driving force is a symmetric version of the synthetic driving force formerly introduced by Janssens et al 2006 Nature Mater. 5 124–7. The new version depends on two orientation parameters instead of one. We analyze the advantages and disadvantages of these two driving force methods. Grain boundary mobilities are simulated for eight symmetric CSL tilt grain boundaries in Al, Cu and γ-Fe, and two MD potentials for each of these materials. Boundary conditions are kept as similar as possible to show the influence of the different materials and to compare to the influence of the different MD potential types on simulated GB mobilities. We find that the newly introduced artificial driving force is a slight improvement, but it cannot remove the shortcomings of the original approach. Also, it is found that the differences in calculated MD mobilities between different materials are of the same order as those between different MD potentials of any one element. Sources for such differences are identified and classified by severity. (paper)

  10. Explanation of red spectral shifts at CdTe grain boundaries

    Science.gov (United States)

    Moseley, John

    The best research-cell efficiencies for CdTe thin-film solar cells have recently increased from 17.3% to 20.4%. Despite these impressive recent gains, many improvements in device technology are necessary to reach the detailed-balance efficiency limit for CdTe-based (single-junction, non-concentrator) solar cells of ~32%. Improvements will increasingly rely on knowledge of the fundamental relationships between processing, electrical properties of defects, and device performance. In this study, scanning electron microscope (SEM)-based cathodoluminescence (CL) spectrum imaging was used to examine these fundamental relationships. In CL spectrum imaging we collect a spectrum per pixel in a 256 x 256 pixel SEM image by synchronizing a cryogenic silicon charge-coupled device with the electron-beam positioning. High spatial resolution photon energy maps obtained with this technique can reveal intricate luminescence phenomena that are not apparent in spectroscopic data. CL spectrum imaging was performed at T= 25 K on the back surface of CSS-deposited CdTe thin-films in a CdTe/CdS/SnO_2/glass configuration without back contacting. Both as-deposited and CdCl2 vapor-treated samples were analyzed. Luminescence emission is detected (bands) at ~1.32 eV and ~1.50 eV, which are consistent with Z- and Y-bands. The importance of the Z-band to CdTe solar cells is discussed. For the grains in the as-deposited films, there is a significant redshift in the transition energies near the grain boundaries. For the Z-band, this behavior is due to the effect of the high GB recombination velocity (sX~1x10 4 cm/s) in as-deposited CSS films on the donor-acceptor pair transition mechanism. The concentration of the shallow donor species participating in the Z-band transition was estimated to be ~1017 cm-3 . Based on this estimate, and the spatial correlation between the Z-band and the A-center (VCd-ClTe) complex transitions, ClTe is proposed as is the shallow donor species.

  11. Dynamics of ordering in highly degenerate models with anisotropic grain-boundary potential: Effects of temperature and vortex formation

    OpenAIRE

    Jeppesen, Claus; Flyvbjerg, Henrik; Ole G. Mouritsen

    1989-01-01

    Monte Carlo computer-simulation techniques are used to elucidate the equilibrium phase behavior as well as the late-stage ordering dynamics of some two-dimensional models with ground-state ordering of a high degeneracy, Q. The models are Q-state Potts models with anisotropic grain-boundary potential on triangular lattices—essentially clock models, except that the potential is not a cosine, but a sine function of the angle between neighboring grain orientations. For not too small Q, these mode...

  12. Strain localization in ultramylonitic marbles by simultaneous activation of dislocation motion and grain boundary sliding (Syros, Greece

    Directory of Open Access Journals (Sweden)

    A. Rogowitz

    2015-09-01

    Full Text Available Extreme strain localization occurred in the center of the cross-cutting element of a flanking structure in almost pure calcite marbles from Syros, Greece. At the maximum displacement of 120 cm along the cross-cutting element evidence of grain size sensitive deformation mechanisms can be found in the ultramylonitic marbles, which are characterized by (1 an extremely small grain size (∼3 μm, (2 grain boundary triple junctions with nearly 120° angles, (3 a weak crystallographic preferred orientation with very low texture index (J=1.4, (4 a random misorientation angle distribution curve and (5 the presence of small cavities. Using transmission electron microscopy a deformation sequence is observed comprising, first recrystallization by bulging resulting in the development of the fine-grained ultramylonite followed by the evolution of a high dislocation density (∼1013 m−2 with ongoing deformation of the fine-grained ultramylonite. The arrangement of dislocations in the extremely fine grain sized calcite differs from microstructures created by classical dislocation creep mediated by combined glide and thermally activated climb. Instead, it exhibits extensive glide and dislocation networks characteristic of recovery accommodated by cross-slip and network-assisted dislocation movement without formation of idealized subgrain walls. The enabling of grain boundary sliding to dislocation activity is deemed central to initiating and sustaining strain softening and is argued to be an important strain localization process in calcite rocks, even at high strain rate (10−9 s−1 and low temperature (300 °C.

  13. Final Technical Report: Grain Boundary Complexions and Transitions in Doped Silicon

    Energy Technology Data Exchange (ETDEWEB)

    Jian Luo

    2012-10-15

    This four-year research project has advanced the fundamental knowledge of grain boundary (GB) complexions (i.e., "two-dimensional interfacial phases") and associated GB "phase" transitions in several grounds. First, a bilayer interfacial phase, which had been directly observed by microscopy only in complex ceramic systems in prior studies, has been identified in simpler systems such as Au-doped Si and Bi-doped Ni in this study, where the interpretations of the their formation mechanisms and microscopic images are less equivocal. Second, convincing evidence for the existence of a first-order GB transition from a nominally "clean" GB to a bilayer adsorption interfacial phase has been revealed for Au-doped Si; the confirmation of the first-order nature of interfacial transitions at GBs, which was rare in prior studies, is scientifically significant and technologically important. Third, the bilayer interfacial phase discovered in Bi-doped Ni has been found to be the cause of the mysterious liquid metal embrittlement phenomenon in this system; the exact atomic level mechanism of this phenomenon has puzzled the materials and physics communities for over a century. Finally, significant advancements have been made to establish phenomenological thermodynamic models for GB complexions and transitions. Since GB complexions can control the transport, mechanical and physical properties of a broad range of metallic and ceramic materials, the fundamental knowledge generated by this project can have broad impacts on materials design in general. In this regard, understanding and controlling GB phase behaviors (complexions and transitions) can be an important component for the "Materials Genome" project.

  14. Efficiency improvement of multicrystalline silicon solar cells after surface and grain boundaries passivation using vanadium oxide

    Energy Technology Data Exchange (ETDEWEB)

    Derbali, L., E-mail: rayan.slat@yahoo.fr [Photovoltaiec Laboratory, Research and Technology Center of Energy, Technopole de Borj-Cedria, BP 95, Hammam-Lif 2050 (Tunisia); Ezzaouia, H. [Photovoltaiec Laboratory, Research and Technology Center of Energy, Technopole de Borj-Cedria, BP 95, Hammam-Lif 2050 (Tunisia)

    2012-08-01

    Highlights: Black-Right-Pointing-Pointer Evaporation of vanadium pentoxide onto the front surface leads to reduce the surface reflectivity considerably. Black-Right-Pointing-Pointer An efficient surface passivation can be obtained after thermal treatment of obtained films. Black-Right-Pointing-Pointer Efficiency of the obtained solar cells has been improved noticeably after thermal treatment of deposited thin films. - Abstract: The aim of this work is to investigate the effect of vanadium oxide deposition onto the front surface of multicrystalline silicon (mc-Si) substrat, without any additional cost in the fabrication process and leading to an efficient surface and grain boundaries (GBs) passivation that have not been reported before. The lowest reflectance of mc-Si coated with vanadium oxide film of 9% was achieved by annealing the deposited film at 600 Degree-Sign C. Vanadium pentoxide (V{sub 2}O{sub 5}) were thermally evaporated onto the surface of mc-Si substrates, followed by a short annealing duration at a temperature ranging between 600 Degree-Sign C and 800 Degree-Sign C, under O{sub 2} atmosphere. The chemical composition of the films was analyzed by means of Fourier transform infrared spectroscopy (FTIR). Surface and cross-section morphology were determined by atomic force microscope (AFM) and a scanning electron microscope (SEM), respectively. The deposited vanadium oxide thin films make the possibility of combining in one processing step an antireflection coating deposition along with efficient surface state passivation, as compared to a reference wafer. Silicon solar cells based on untreated and treated mc-Si wafers were achieved. We showed that mc-silicon solar cells, subjected to the above treatment, have better short circuit currents and open-circuit voltages than those made from untreated wafers. Thus, the efficiency of obtained solar cells has been improved.

  15. Environment sensitive embedding energies of impurities, and grain boundary stability in tantalum

    International Nuclear Information System (INIS)

    Metalloid impurities have a very low solubility in tantalum, and therefore prefer to segregate at the grain boundaries (GBs). In order to analyze the energetics of the impurities on the tantalum GB, the LMTO calculations were performed on a simple 8-atom supercell emulating a typical (capped trigonal prism) GB environment. The so-called environment-sensitive embedding energies were calculated for hydrogen, boron, carbon, nitrogen, oxygen, phosphorus, and sulphur, as a function of the electron charge density due to the host atoms at the impurity site. The calculations showed that, at the electron density typical of a GB, carbon has the lowest energy (followed by Nitrogen and Boron) and thus would compete with the other impurities for the site on the GB, tending to displace them from the GB. The above energies were then used in a modified Finnis-Sinclair embedded atom approach for calculating the cohesive energies and the equilibrium interplanar distances in the vicinity of a (111) Σ3 tilt GB plane, both for the clean GB and that with an impurity. These distances were found to oscillate, returning to the value corresponding to the equilibrium spacing between (111) planes in bulk BCC tantalum by the 10th--12th plane off the GB. Carbon, nitrogen and boron somewhat dampen the deformation wave (making the oscillations less than in the clean GB), while oxygen, phosphorus and sulphur result in an increase of the oscillations. The cohesive energies follow the same trend, the GB with carbon being the most stable. Thus, carbon, nitrogen and boron may be thought of as being cohesion enhancers, while oxygen, phosphorus and sulphur result in decohesion effects

  16. Grain boundaries at the surface of consolidated MgO nanocrystals and acid-base functionality.

    Science.gov (United States)

    Vingurt, Dima; Fuks, David; Landau, Miron V; Vidruk, Roxana; Herskowitz, Moti

    2013-09-21

    The increase of the surface basicity-acidity of MgO material by factors of 1.8-3.0 due to consolidation of its nanocrystals was demonstrated by the indicator titration. It was shown that the parallel increase of surface acidity and basicity is attributed to the formation of grain boundaries (GB) after MgO aerogel densification. A simple model predicting the increase of surface acidity-basicity of MgO that correlates with the results of direct measurements was proposed. The model is based on the study of the fine atomic structure at GB surface areas in consolidated MgO nanocrystals in the framework of Density Functional Theory. It is found that the displacements of coordinatively unsaturated surface ions near the GB are significant at the distances ~3-4 atomic layers from the geometrical contact plane between nanocrystals. The detailed analysis of atomic positions inside GB demonstrated the coordination deficiency of surface atoms at the GB areas leading to the formation of stretched bonds and to creation of low coordinated surface ions due to splitting of coordination numbers of surface atoms belonging to GB areas. Density of states for electrons shows the existence of additional states in the band gap close to the bottom of the conduction band. The adsorption energy of CO2 molecules atop oxygen atoms exposed at surface GB areas is of the same order of magnitude as that reported for oxygen atoms at crystallographic edges and corners of MgO crystals. It provides additional options for bonding of molecules at the surface of nanocrystalline MgO increasing the adsorption capacity and catalytic activity. PMID:23907522

  17. Direct observation of grain boundary migration during recrystallization within the bulk of a moderately deformed aluminium single crystal

    DEFF Research Database (Denmark)

    Van Boxel, Steven; Schmidt, Søren; Ludwig, Wolfgang;

    2014-01-01

    planar boundary segments (facets) are analyzed using a method that determines the displacements of local boundary segments along parallel lines perpendicular to the facet plane. Facets are observed to form after a certain annealing time. They migrate at a constant rate for extended periods of time and...... remain planar during their migration. A change in the migration rate for one facet has been observed which is not related to changes in the experimental conditions and is most likely to be driven by the changes in grain orientation and/or the local deformation microstructure. The crystallography of the...

  18. Coercivity enhancement of Nd-Fe-B sintered magnets by grain boundary modification via reduction-diffusion process

    Energy Technology Data Exchange (ETDEWEB)

    Li, D S; Nishimoto, M; Suzuki, S; Nishiyama, K; Itoh, M; Machida, K [Center for Advanced Science and Innovation, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)], E-mail: machida@casi.osaka-u.ac.jp

    2009-02-01

    The interface between the Nd{sub 2}Fe{sub 14}B particles and the grain boundary surrounding them in Nd-Fe-B sintered magnets was modified by diffusion Dy metal selectively through the grain boundary involving the reduction of Dy{sub 2}O{sub 3} or DyF{sub 3} with CaH{sub 2} and the coercivity of them was effectively enhanced from 0.98 MA/m to 1.42 MA/m with keeping the high value of remanence. The resultant coercivity was higher than that of the magnets modified with Dy{sub 2}O{sub 3} (H{sub cJ} = 1.10 MA/m) and DyF{sub 3} (H{sub cJ} = 1.24 MA/m) without using any reductant such as CaH{sub 2}, respectively.

  19. Magnetic Properties of Grain Boundaries of Nanocrystalline Ni and of Ni Precipitates in Nanocrystalline NiCu Alloys

    International Nuclear Information System (INIS)

    Perturbed γγ-angular correlation spectroscopy (PAC) was used to investigate nanocrystalline Ni and NiCu alloys, which are prepared by pulsed electrodeposition (PED). Using diffusion for doping nanocrystalline Ni with 111In four different ordered grain boundary structures are observed, which are characterized by unique electric field gradients. The incorporation of 111In on substitutional bulk sites of Ni is caused by moving grain boundaries below 1000 K and by volume diffusion above 1000 K. The nanocrystalline NiCu alloys prepared by PED are microscopically inhomogeneous as observed by PAC. In contrast, this inhomogeneity cannot be detected by X-ray diffraction. The influence of the temperature of the electrolyte, the current density during deposition, and the optional addition of saccharin to the electrolyte on the homogeneity of nanocrystalline NiCu alloys was investigated.

  20. Effect of Grain Boundary Character Distribution on the Impact Toughness of 410NiMo Weld Metal

    DEFF Research Database (Denmark)

    Divya, M.; Das, Chitta Ranjan; Chowdhury, Sandip Ghosh;

    2016-01-01

    Grain boundary character distributions in 410NiMo weld metal were studied in the as-welded, first-stage, and second-stage postweld heat treatment (PWHT) conditions, and these were correlated with the Charpy-V impact toughness values of the material. The high impact toughness values in the weld...... metal in the as-welded and first-stage PWHT conditions compared to that in the second-stage condition are attributed to the higher fraction of low-energy I pound boundaries. A higher volume fraction of retained austenite and coarser martensite after second-stage PWHT accompanied by the formation of the...... impact toughness. In addition to this, grain refinement during 4-hour PWHT in the second stage also increased the toughness of the weld metal....

  1. Magnetic Properties of Grain Boundaries of Nanocrystalline Ni and of Ni Precipitates in Nanocrystalline NiCu Alloys

    Science.gov (United States)

    Wolf, H.; Guan, Z.; Li, X.; Wichert, Th.

    2001-11-01

    Perturbed γγ-angular correlation spectroscopy (PAC) was used to investigate nanocrystalline Ni and NiCu alloys, which are prepared by pulsed electrodeposition (PED). Using diffusion for doping nanocrystalline Ni with 111In four different ordered grain boundary structures are observed, which are characterized by unique electric field gradients. The incorporation of 111In on substitutional bulk sites of Ni is caused by moving grain boundaries below 1000 K and by volume diffusion above 1000 K. The nanocrystalline NiCu alloys prepared by PED are microscopically inhomogeneous as observed by PAC. In contrast, this inhomogeneity cannot be detected by X-ray diffraction. The influence of the temperature of the electrolyte, the current density during deposition, and the optional addition of saccharin to the electrolyte on the homogeneity of nanocrystalline NiCu alloys was investigated.

  2. Intergranular magnetoresistance in Sr2FeMoO6 from a magnetic tunnel barrier mechanism across grain boundaries.

    Science.gov (United States)

    Sarma, D D; Ray, Sugata; Tanaka, K; Kobayashi, M; Fujimori, A; Sanyal, P; Krishnamurthy, H R; Dasgupta, C

    2007-04-13

    We present magnetization (M) and magnetoresistance (MR) data for a series of Sr2FeMoO6 samples with independent control on antisite defect and grain-boundary densities, which reveal several unexpected features, including a novel switching-like behavior of MR with M. These, in conjunction with model calculations, establish that the MR in Sr2FeMoO6 is dominantly controlled by a new mechanism, derived from the magnetic polarization of grain-boundary regions acting like spin valves, leading to behavior qualitatively different from that usually encountered in tunneling MR. We show that a simple and useful experimental signature for the presence of this spin-valve-type MR (SVMR) is a wider hysteresis in MR compared to that in M. PMID:17501379

  3. Role of Grain Boundaries in the Coercivity of Magnetic Thin Films Investigated by a Two-Dimensional Ginzburg-Landau-Type Model

    Science.gov (United States)

    Iwano, Kaoru; Mitsumata, Chiharu; Ono, Kanta

    2016-07-01

    We investigate a two-dimensional Ginzburg-Landau-type model, with focus on grain boundaries that are prevalent in magnetic thin films with perpendicular magnetic anisotropy. The model covers four basic interactions, which are exchange, anisotropic, external, and dipole-dipole interactions, and describes magnetic thin films. Through numerical simulations for square grains, we find that there is a minimum grain-grain distance beyond which the interactions effectively vanish. Furthermore, we also find that magnetic reversals occur at the corners of grains and confirm that circular grains result in higher coercivity under similar packing ratio, because of the absence of corners.

  4. Formation of Graphene Grain Boundaries on Cu(100) Surface and a Route Towards Their Elimination in Chemical Vapor Deposition Growth

    OpenAIRE

    Qinghong Yuan; Guangyao Song; Deyan Sun; Feng Ding

    2014-01-01

    Grain boundaries (GBs) in graphene prepared by chemical vapor deposition (CVD) greatly degrade the electrical and mechanical properties of graphene and thus hinder the applications of graphene in electronic devices. The seamless stitching of graphene flakes can avoid GBs, wherein the identical orientation of graphene domain is required. In this letter, the graphene orientation on one of the most used catalyst surface — Cu(100) surface, is explored by density functional theory (DFT) calculatio...

  5. Desorption of Ag from Grain Boundaries in Ag Film on Br and H-Passivated Si(111) Surfaces

    International Nuclear Information System (INIS)

    Growth of Ag film on Br- and H-passivated Si(111) surfaces was examined by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and photoemission electron microscopy (PEEM) techniques. The phenomenon of thermal grooving was observed after annealing at higher temperatures. Hierarchical desorption of Ag from the grain boundaries produce a fractal structure of Ag-depleted regions. Hierarchical desorption may be used for nanopatterning of the layer.

  6. The effect of grain boundaries on the resistivity of polycrystalline silicon. Ph.D. Thesis - Va. Univ.

    Science.gov (United States)

    Fripp, A. L., Jr.

    1974-01-01

    The electrical resistivity of polycrystalline silicon films was investigated. The films were grown by the chemical vapor decomposition of silane on oxidized silicon wafers. The resistivity was found to be independent of dopant atom concentration in the lightly doped regions but was a strong function of dopant levels in the more heavily doped regions. A model, based on high dopant atom segregation in the grain boundaries, is proposed to explain the results.

  7. Effect of grain-boundaries on electrical properties of n-ZnO:Al/p-Si heterojunction diodes

    Directory of Open Access Journals (Sweden)

    Mohit Kumar

    2013-09-01

    Full Text Available We report on room temperature diode characteristics of ZnO:Al (AZO/Si heterostructures by current-voltage measurements. In this study, with increasing AZO film thickness, systematic reduction in the turn-on potential (from 3.16 to 1.80 V and the film stress are observed. Complementary capacitance-voltage studies reveal a decreasing trend in barrier height at the junction with increasing AZO film thickness. A gradual decrease in resistivity takes place with increasing AZO film thickness. Above observations are explained in the framework of AZO thickness dependent variation in grain size and in turn trap density at the grain boundaries influencing carrier transport across the adjacent grains.

  8. Effects of processing optimisation on microstructure, texture, grain boundary and mechanical properties of Fe–17Cr ferritic stainless steel thick plates

    Energy Technology Data Exchange (ETDEWEB)

    Han, Jian, E-mail: jh595@uowmail.edu.au [School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522 (Australia); Li, Huijun; Zhu, Zhixiong [School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522 (Australia); Jiang, Laizhu; Xu, Haigang; Ma, Li [Baoshan Iron and Steel Co., Ltd., Shanghai 200431 (China)

    2014-10-20

    The relationships between microstructure, texture, grain boundary and tensile strength, Charpy impact toughness of (Nb+Ti+V) stabilised Fe–17Cr ferritic stainless steel thick plates were investigated by means of optical microscopy, X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, tensile and Charpy impact testing. The results show that for Fe–17Cr ferritic stainless steel thick plate, the addition of warm rolling procedure leads to refinement of grain size, modification of texture, and then optimisation of grain boundary, including grain boundary character distribution and grain boundary connectivity. Meanwhile, the mechanical testing results indicate that optimal transformation that warm rolling procedure brings to Fe–17Cr ferritic steel thick plate is beneficial to its mechanical properties.

  9. Identical mechanism of isochronal and isothermal embrittlement in Ni(Bi) alloy: Thermo-induced non-equilibrium grain-boundary segregation of Bi

    International Nuclear Information System (INIS)

    Highlights: • Both isochronal and isothermal plasticity of Ni(Bi) alloy show minima. • Existing interpretations for isochronal and isothermal embrittlement are inadequate. • Both embrittlement is caused by thermo-induced non-equilibrium grain-boundary segregation of Bi. - Abstract: Isochronal and isothermal plasticity after thermal pre-treatments are obtained by tensile tests to characterize the embrittling behaviors of Ni(Bi) alloy. Both isochronal and isothermal plasticity show evident minima. Fractography observed by scanning electron microscopy displays intergranular fracture for samples of low plasticity. The microstructure is found to be free of precipitates within grains and at grain boundaries by focused ion beam and transmission electron microscopy. Atom probe analysis indicates a strong tendency of Bi segregation to grain boundaries. By these results, the existing interpretations are discussed to be inadequate and both embrittlement are confirmed to be identical in mechanism, i.e. thermo-induced non-equilibrium grain-boundary segregation of Bi

  10. Investigation of the grain-boundary chemistry in used CANDU fuel by x-ray photoelectron spectroscopy (XPS)

    International Nuclear Information System (INIS)

    The grain-boundary chemistry of used CANDU fuel is being systematically investigated by X-ray photoelectron spectroscopy (XPS) using a McPherson ESCA-36 instrument that has been adapted for routine studies of highly radioactive materials. Initial stages of fuel corrosion under various storage and disposal conditions can be identified from chemical-shift effects for uranium. For example, pervasive but highly selective grain-boundary oxidation has been revealed in CANDU fuels exposed to moist air at 150 deg. C for extended periods, suggesting aggressive radiolytic processes operating in a thin film of adsorbed water. Pronounced segregation of a number of fission products to cracks and grain boundaries in used CANDU fuels has been explicitly demonstrated by XPS as well. Model calculations and composition depth profiles are indicative of near monolayer films. Some correlations between fuel power history and fission-product distributions have been established and possible evidence of migration during moist-air exposure has been obtained. The key advantages and limitations of XPS in this context are discussed and illustrated with selected results. (author). 23 refs, 8 figs, 1 tab

  11. Temperature and frequency dependent electrical properties of NiCuZn ferrite with CuO-rich grain boundary segregation

    International Nuclear Information System (INIS)

    Highlights: • Sintered NiCuZn ferrite contains CuO-rich segregation along the grain boundaries. • Segregation is allied with different activation energies based on temperature zone. • Both grain boundary and segregated layer posses short-range hopping mechanism. - Abstract: Polycrystalline Ni0.9−yCuyZn0.1Fe1.98O4−δ (y = 0, 0.1 and 0.2) powder was synthesized by conventional solid-state reaction method. X-ray diffraction analysis confirmed single phase cubic spinel phase formation in the calcined powders. An additional phase was observed predominantly after sintering of y = 0.2 compound. The extra phase in the sintered sample is due to the formation of CuO-rich segregation along the grain boundary, which is confirmed by high resolution scanning electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy analysis. In order to understand the temperature and frequency dependent electrical properties of segregated ferrite, detailed impedance spectra was investigated on Ni0.7Cu0.2Zn0.1Fe1.98O4−δ in the frequency range of 1 kHz–1 MHz and temperature range of 30–250 °C

  12. Irradiation deformation near different atomic grain boundaries in α-Zr: An investigation of thermodynamics and kinetics of point defects

    Science.gov (United States)

    Arjhangmehr, A.; Feghhi, S. A. H.

    2016-03-01

    Understanding radiation performance of nanocrystalline Zr-based alloys is essential to develop internal components and external cladding materials with self-healing capabilities for longer and safer life cycles in harsh reactor environments. However, the precise role of interfaces in modifying defect production and evolution in α-Zr is not yet determined. Using atomistic simulation methods, we investigate the influence of different atomic grain boundaries (GBs) in thermodynamic and kinetic properties of defects on short timescales. We observe that the sink efficiency and sink strength of interfaces vary significantly with the boundary structures, with a preference to absorb interstitials (vacancies) when the GBs are semi-parallel (semi-perpendicular) relative to the basal planes. Further, we identify three distinct primary cascade geometries, and find that the residual defect clustering in grain interiors depends on how the atomic GBs modify the spatial distribution of defects within the crystal structure. Finally, we explain and discuss the dynamic results in terms of energetic and kinetic behaviors of defects near the pristine and damaged boundaries. Eventually, these will provide a microscopic reference for further improving the radiation response of Zr by using fine grains or by introducing a high density of dispersoids in material metallurgy.

  13. Dynamically- and chemically-induced grain boundary migration in quartz: microstructures, crystallographic fabrics, and trace element contents

    Science.gov (United States)

    Nachlas, Will; Thomas, Jay

    2016-04-01

    Grain boundary migration (GBM) is a common mechanism by which quartz recrystallizes in the Earth. In the most basic sense, GBM occurs as atoms exchange structural positions across a planar defect. Reconstitution of grains via GBM imparts a new crystallographic orientation, but its effect on the geochemistry of recrystallized grains remains uncertain and depends on the kinetic and thermodynamic properties of the moving grain boundary. Two of the dominant driving forces for GBM are lattice strain energy, controlled by the applied stress field, and chemical potential energy, controlled by differences in mineral stability. We present observations from static and dynamic recrystallization experiments showing evidence for GBM in response to both of these driving forces. In static recrystallization experiments, quartz recrystallized in response to local variations in trace-level Ti concentrations, whereas in dynamic recrystallization experiments, quartz recrystallized during dislocation creep in response to the imposed differential stress. Each case produced recrystallized quartz exhibiting diagnostic microstructures, crystallographic fabrics, and trace element contents that can be used to infer the mechanisms of quartz recrystallization and the pressure-temperature conditions at which recrystallization occurred.

  14. The contribution of grain boundary and defects to the resistivity in the ferromagnetic state of polycrystalline manganites

    International Nuclear Information System (INIS)

    In the present study we report the precise resistivity measurements for the polycrystalline bulk sample as well as highly oriented thin-films of La0.8Ca0.2MnO3. The poly crystalline sample was prepared by standard solid-state reaction route and the oriented thin film was prepared by pulsed laser deposition (PLD). The phase purity of these samples was confirmed by X-ray diffraction and the back-scattered electron imaging using scanning electron microscopy (SEM). The oxygen stoichiometry analysis was done by iodimetry titration. The resistivities of these samples were carried out with four-probe resistivity measurement setup. The observed temperature dependence of resistivity data for both the samples was fitted using the polaron model. We have found that polaronic model fits well with the experimental data of both polycrystalline and single crystal samples. A new phenomenological model is proposed and used to estimate contribution to the resistivity due to grain boundary in the ferromagnetic state of polycrystalline manganites and it has been shown that the scattering of electrons from the grain boundary (grain surface) is a function of temperature and controlled by the effective grain resistance at that temperature

  15. Fe precipitation at the grain boundaries in Zn-reacted Nd-Fe-B magnets: Experimental and first-principles studies

    Energy Technology Data Exchange (ETDEWEB)

    Yang, YuQi; Lu, Zhao; Huang, DongFang; Wang, DongDong; Guo, KeKe; Sun, ZhiQiang; Li, TaoTao [Shool of Materials Science and Engineering, Jiangxi University of Science and Technology, Ganzhou (China); Han, BaoJun [Gannan Normal University, Ganzhou (China); Zhou, Yang [Shool of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou (China)

    2016-01-15

    Traces of non-magnetic atoms at the grain boundaries can effectively increase the coercivity of sintered Nd-Fe-B magnets. These atoms also have chance to occupy crystallographic Fe sites in Nd{sub 2}Fe{sub 14}B, which may force the unbound Fe to precipitate at the grain boundaries and thus destroy the nonmagnetic properties of the grain boundaries. In order to provide the thermodynamic evidences of Fe migration to the grain boundaries, one Zn diffusion reaction in Nd{sub 2}Fe{sub 14}B is investigated by scanning electron microscopy and energy-dispersive X-ray spectroscopy experiments, which confirm that 90 atomic percent of Fe is expelled to the grain boundaries. Magnetization measurements show that when Fe atoms precipitate at the grain boundaries, magnet's coercivity rapidly declines. The first-principle calculations using linearized augmented plane wave method show that Zn atoms in 16k1, 8j2, and 4e sites have positive substitution energies and unfavorable for the Zn substitution at these sites, while the substitution energies are negative values and favorable for the Zn substitution at the sites of 8j1, 4c, and 16k2. Our experimental and theoretical calculations indicate that the doped atoms have significant effect on the formation of the Fe-enriched layer at the grain boundary. (copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  16. Atomic structure of (111) twist grain boundaries in f.c.c. metals

    NARCIS (Netherlands)

    Hosson, J.Th.M. De; Vitek, V.

    1990-01-01

    In this paper we have studied the atomic structures of (111) twist boundaries and investigated the applicability of the structural unit model which has previously been established for tilt boundaries and (001) twist boundaries. The calculations were carried out using two different descriptions of in

  17. Direct observation of nanometer-scale amorphous layers and oxide crystallites at grain boundaries in polycrystalline Sr1−xKxFe2As2 superconductors

    KAUST Repository

    Wang, Lei

    2011-06-01

    We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystallineSr0.6K0.4Fe2As2superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale crystallites sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films.

  18. Explanation of low critical currents in flat, bulk versus meandering, thin-film [001] tilt bicrystal grain boundaries in YBa2Cu3O7

    International Nuclear Information System (INIS)

    Thin-film and bulk [001] tilt, bicrystal grain boundaries in YBa2Cu3O7 exhibit a strong dependence of critical current density on misorientation angle. What is particularly difficult to understand is its thirty times lower magnitude in bulk grain boundaries which are microscopically more perfect, i.e., flatter, minimally faceted, and free of impurity phases. A plausible explanation, based on differences of the pinning of Josephson vortices in these grain boundary types, is proposed here. copyright 1998 The American Physical Society

  19. Micromechanical Modeling of Grain Boundaries Damage in a Copper Alloy Under Creep; Mikromechanische Modellierung der Korngrenzenschaedigung in einer Kupferlegierung unter Kriechbeanspruchung

    Energy Technology Data Exchange (ETDEWEB)

    Voese, Markus

    2015-07-01

    In order to include the processes on the scale of the grain structure into the description of the creep behaviour of polycrystalline materials, the damage development of a single grain boundary has been initially investigated in the present work. For this purpose, a special simulationmethod has been used, whose resolution procedure based on holomorphic functions. The mechanisms taken into account for the simulations include nucleation, growth by grain boundary diffusion, coalescence and shrinkage until complete sintering of grain boundary cavities. These studies have then been used to develop a simplified cavitation model, which describes the grain boundary damage by two state variables and the time-dependent development by a mechanism-oriented rate formulation. To include the influence of grain boundaries within continuum mechanical considerations of polycrystals, an interface model has been developed, that incorporates both damage according to the simplified cavitation model and grain boundary sliding in dependence of a phenomenological grain boundary viscosity. Furthermore a micromechanical model of a polycrystal has been developed that allows to include a material's grain structure into the simulation of the creep behaviour by means of finite element simulations. Thereby, the deformations of individual grains are expressed by a viscoplastic single crystal model and the grain boundaries are described by the proposed interface model. The grain structure is represented by a finite element model, in which the grain boundaries are modelled by cohesive elements. From the evaluation of experimental creep data, the micromechanical model of a polycrystal has been calibrated for a copper-antimony alloy at a temperature of 823 K. Thereby, the adjustment of the single crystal model has been carried out on the basis of creep rates of pure copper single crystal specimens. The experimental determination of grain boundary sliding and grain boundary porosity for coarse-grained

  20. A study on the role of grain boundary engineering in promoting high-cycle fatigue resistance and improving reliability in nickel base superalloys for propulsion systems

    Science.gov (United States)

    Gao, Yong

    High-cycle fatigue, involving the premature initiation and/or rapid propagation of small cracks to failure due to high-frequency (vibratory) loading, remains the principal cause of failures in military gas-turbine propulsion systems. The objective of this study is to examine whether the resistance to high-cycle fatigue failures can be enhanced by grain-boundary engineering, i.e., through the modification of the spatial distribution and topology of the grain boundaries in the microstructure. While grain boundary engineering has been used to obtain significant improvements in intergranular corrosion and cracking, creep and cavitation behavior, toughness and plasticity, cold-work embrittlement, and weldability, only very limited, but positive, results exist for fatigue. Accordingly, using a commercial polycrystalline nickel base gamma/gamma' superalloy, ME3, as a typical engine disk material, sequential thermomechanical processing, involving alternate cycles of strain and annealing, is used to (i) modify the proportion of special grain boundaries, and (ii) interrupt the connectivity of the random boundaries in the grain boundary network. The processed microstructures are then subjected to fracture-mechanics based high cycle fatigue testing to evaluate how the crack initiation and small- and large-crack growth properties are affected and to examine how the altered grain boundary population and connectivity can influence growth rates and overall lifetimes. The effect of such grain-boundary engineering on the fatigue-crack-propagation behavior of large (˜8 to 20 mm), through-thickness cracks at 25, 700, and 800°C was examined. Although there was little influence of an increased special boundary fraction at ambient temperatures, the resistance to near-threshold crack growth was definitively improved at elevated temperatures, with fatigue threshold-stress intensities some 10 to 20% higher than at 25°C, concomitant with a lower proportion (˜20%) of intergranular

  1. Barrier layer and grain boundary effects in Nd/Zr doped BaTiO3 ceramics

    International Nuclear Information System (INIS)

    Gradient structures with barrier layer characteristics and core-shell morphology have been developed in BaTiO3 ceramics with Nd2O3 and ZrO2 as co-dopants. Features include reduced Curie temperatures and anisotropic stress gradients, resulting from an oxidized surface layer and reduced interior, developed during air sintering. Co-doping was typically carried out through solution milling of the BaTiO3 powders with nitrate precursors of the dopant oxides, spray drying and sintering of the pressed pellets in air ambient at 1300-1320 deg. C/60-90 min with furnace cooling. Structural characterization, as well as dielectric and d.c. resistance measurements of the pellets, as-sintered and after removing equal amounts of material from both surfaces, revealed the existence of an oxidized surface layer and barrier layer microstructures consisting of graded regions of oxidized insulating surfaces over partially oxidized or conducting grain interiors. In this complex structure, the ZrO2 segregates to the grain boundary region, forming a core-shell structure, with Nd2O3 partitioning between the BaTiO3 and ZrO2 phases. The overall system was modeled in terms of an equivalent circuit and the analysis indicates that the dielectric constant and the loss behavior are strongly impacted by both the surface and grain boundary barrier characteristics, with the surface barrier effects having the more dominant effect on the dielectric properties of the doped compositions. Indications are that fine-tuning of the system to optimize the grain boundary effect could lead to extraordinary dielectric constant effects which could potentially be utilized in high energy storage devices.

  2. Effect of interfaces of grain boundary Al2CuLi plates on fracture behavior of Al–3Cu–2Li

    International Nuclear Information System (INIS)

    Transmission electron microscopy (TEM) and density functional theory (DFT) calculations were employed to investigate the interfacial characteristics and fracture behavior of Al–3Cu–2Li containing plate-like Al2CuLi (known as the T1 phase) precipitates at grain boundaries. TEM studies showed that T1 plates form at grain boundaries, with a coherent interface on one side parallel to the {1 1 1} planes of the matrix and a non-coherent interface with no preferred orientation relative to the grain on the other side. The low energy of the coherent interface leads to a serration of the grain boundaries due to the growth of the grain boundary T1 phase. Under tensile loading, the intergranular T1 phase leads to the formation of nanopores at the non-coherent side, and fracture mostly through the non-coherent grain boundary T1/matrix interface for under-, peak- and overaged conditions. DFT simulations showed that, under tensile loading, fracture is most likely to take place at the T1/Al interface, and the non-coherent side of the grain boundary is weakest as the decohesion energy is 25% lower than that of the coherent interface

  3. Computational study of the impurity induced reduction of grain boundary energies in nano- and bi-crystalline Al-Pb alloys

    International Nuclear Information System (INIS)

    Segregation of impurities with limited solubilities to grain boundaries can slow or even eliminate grain growth in nanocrystalline materials. Due to a very limited miscibility Pb is a potential candidate for thermodynamically stabilizing nanocrystalline Al. To investigate this we have used atomic modeling to characterize the structure and energy of substitutional Pb defects in bulk Al, in Al bi-crystals and in an Al nanocrystal. Monte Carlo simulations using a modified embedded-atom method (MEAM) potential fit to the results of density functional theory (DFT) calculations predict the formation of Pb clusters, in agreement with prior experiments. In addition, the simulations show strong segregation of Pb atoms to grain boundaries, a result that supports prior suggestions that Pb is distributed along grain boundaries in nanocrystals created by ball milling. Analysis of the enthalpies for Pb defects using MEAM and DFT calculations suggests that Pb impurities can help stabilize nanocrystalline Al against grain growth.

  4. An atom probe field ion microscope investigation of the role of boron in precipitates and at grain boundaries in NiAl

    International Nuclear Information System (INIS)

    This paper reports that the high resolution analytical technique of Atom Probe Field Ion Microscopy (APFIM) has been used to characterize grain boundaries and the matrix of a stoichiometric NiAl alloy doped with 0.04 (100 wppm) and 0.12 at. % (300 wppm) boron. Field ion images revealed boron segregation to the grain boundaries. Atom probe elemental analysis of the grain boundaries measured a boron coverage of up to 30% of a monolayer. Extensive atom probe analyses also revealed a fine dispersion of nanoscale boride precipitates in the matrix. The boron segregation to the grain boundaries was found to correlate with the observed suppression of intergranular fracture. However, the decrease in ductility of boron-doped NiAl is attributed in part to the precipitation hardening effect of the boride phases

  5. Asymptotic expressions for the nearest and furthest dislocations in a pile-up against a grain boundary

    KAUST Repository

    Hall, Cameron L.

    2010-10-14

    In 1965, Armstrong and Head explored the problem of a pile-up of screw dislocations against a grain boundary. They used numerical methods to determine the positions of the dislocations in the pile-up and they were able to fit approximate formulae for the locations of the first and last dislocations. These formulae were used to gain insights into the Hall-Petch relationship. More recently, Voskoboinikov et al. used asymptotic techniques to study the equivalent problem of a pile-up of a large number of screw dislocations against a bimetallic interface. In this paper, we extend the work of Voskoboinikov et al. to construct systematic asymptotic expressions for the formulae proposed by Armstrong and Head. The further extension of these techniques to more general pile-ups is also outlined. As a result of this work, we show that a pile-up against a grain boundary can become equivalent to a pile-up against a locked dislocation in the case where the mismatch across the boundary is small. © 2010 Taylor & Francis.

  6. Crystallographic Fabrics, Grain Boundary Microstructure and Shape Preferred Orientation of Deformed Banded Iron Formations and their Significance for Deformation Interpretation

    Science.gov (United States)

    Ávila, Carlos Fernando; Graça, Leonardo; Lagoeiro, Leonardo; Ferreira, Filippe

    2016-04-01

    The characterization of grain boundaries and shapes along with crystallographic preferred orientations (CPOs) are a key aspect of investigations of rock microstructures for their correlation with deformation mechanisms. Rapid developments have occurred in the studying rock microstructures due to recent improvements in analytical techniques such as Electron Backscatter Diffraction (EBSD). EBSD technique allows quick automated microtextural characteritzation. The deformed banded iron formations (BIFs) occurring in the Quadrilátero Ferrífero (QF) province in Brazil have been studied extensively with EBSD. All studies have focused mainly in CPOs. The general agreement is that dislocation creep was the dominant process of deformation, for the strong c-axis fabric of hematite crystals. This idea is substantiated by viscoplastic self-consistent models for deformation of hematite. However there are limitations to analyzing natural CPOs alone, or those generated by deformation models. The strong c-axis fabric could be taken as equally powerful an evidence for other known deformation mechanisms. Some grain boundary types in BIFs of the QF are irregular and comprise equant grains in granoblastic texture (Figure 1a). CPOs for this kind are strong and consistent with a predominance of dislocation creep. Others are very regular and long parallel to basal planes of hematites forming large elongated crystals (lepidoblastic texture, Figure 1b). Such crystals are called specularite, and their formation has been previously attributed to dislocation creep. This is erroneous because of the high strains which would be required. Their shape must be due to anisotropic grain growth. Other types lie between the above end-textures. Both types of grain shape microstructures have the same core deformation mechanism. Describing their genetic differences is crucial, since specularite owe its shape to anisotropic grain growth. It is not possible yet to confirm that dislocation creep was the

  7. Dynamics of ordering in highly degenerate models with anisotropic grain-boundary potential: Effects of temperature and vortex formation

    Science.gov (United States)

    Jeppesen, Claus; Flyvbjerg, Henrik; Mouritsen, Ole G.

    1989-11-01

    Monte Carlo computer-simulation techniques are used to elucidate the equilibrium phase behavior as well as the late-stage ordering dynamics of some two-dimensional models with ground-state ordering of a high degeneracy Q. The models are Q-state Potts models with anisotropic grain-boundary potential on triangular lattices-essentially clock models, except that the potential is not a cosine, but a sine function of the angle between neighboring grain orientations. For not too small Q, these models display two thermally driven phase transitions, one which takes the system from a low-temperature Potts-ordered phase to an intermediate phase which lacks conventional long-range order, and another transition which takes the system to the high-temperature disordered phase. The linear nature of the sine potential used makes it a marginal case in the sense that it favors neither hard domain boundaries, like the standard Potts models do, nor a wetting of the boundaries, as the standard clock models do. Thermal fluctuations nevertheless cause wetting to occur for not too small temperatures. Specifically, we have studied models with Q=12 and 48. The models are quenched from infinity to zero as well as finite temperatures within the two low-temperature phases. The order parameter is a nonconserved quantity during these quenches. The nonequilibrium ordering process subsequent to the quench is studied as a function of time by calculating the interfacial energy, ΔE, associated with the entire grain-boundary network. The time evolution of this quantity is shown to obey the growth law, ΔE(t)~t-n, over an extended time range at late times. It is found that the zero-temperature dynamics is characterized by a special exponent value which for the Q=48 model is n~=0.25 in accordance with earlier work. However, for quenches to finite temperatures in the Potts-ordered phase there is a distinct crossover to the classical Lifshitz-Allen-Cahn exponent value, n=(1/2, for both values of Q. This

  8. Effect of helium bubbles at grain boundaries on the fracture characteristics of high-density 238PuO2

    International Nuclear Information System (INIS)

    Helium bubbles that formed at grain boundaries in high density (greater than 92 percent of theoretical) 238PuO2 shards did not affect the room temperature fracture behavior as observed by scanning electron microscopy. Fracture was predominantly by brittle transgranular cleavage with only infrequent intergranular failure observed. Pores (approximately 5 μm dia) that formed within the grains during the sintering process, rather than helium bubbles, initiated fractures. Helium bubbles were observed occasionally on the fracture surfaces of 20-month-old shards that had been heated to 16000C for 8 h and subsequently crushed at room temperature. The average radius of these bubbles was approximately 1 μm. These bubbles were not interconnected, but were sometimes aligned in stringers

  9. A model bismuth oxide intergranular thin film in a ZnO twist grain boundary

    Energy Technology Data Exchange (ETDEWEB)

    Domingos, H S [INESC, Microsistemas and Nanotecnologias, Rua Alves Redol 1-9, P-1000-029 Lisbon (Portugal)

    2010-04-14

    The electronic properties of a model bismuth oxide intergranular film in ZnO were investigated using density functional plane wave calculations. It was found that oxygen excess plays a fundamental role in the appearance of electrical activity. The introduction by oxygen interstitials or zinc vacancies results in depletion of the charge in deep gap states introduced by the bismuth impurities. This makes the boundary less metallic and promotes the formation of acceptor states localized to the boundary core, resulting in Schottky barrier enhancement. The results indicate that the origin of electrical activity in thin intergranular bismuth oxide films is probably not distinct from that in decorated ZnO boundaries.

  10. Intergranular brittle fracture of a low alloy steel induced by grain boundary segregation of impurities: influence of the microstructure

    International Nuclear Information System (INIS)

    The study contributes to improve the comprehension of intergranular embrittlement induced by the phosphorus segregation along prior austenitic grain boundaries of low alloy steels used in pressurized power reactor vessel. A part of this study was performed using a A533 steel which contains chemical fluctuations (ghost lines) with two intensities. Axi-symmetrically notched specimens were tested and intergranular brittle de-cohesions were observed in the ghost lines. The fracture initiation sites observed on fracture surfaces were identified as MnS inclusions. A bimodal statistic obtained in a probabilistic model of the fracture is explained by the double population of ghost lines' intensities. A metallurgical study was performed on the same class of steel by studying the influence of the microstructure on the susceptibility to temper embrittlement. Brittle fracture properties of such microstructures obtained by dilatometric experiments were tested on sub-sized specimens to measure the V-notched fracture toughness. Fraction areas of brittle fracture modes were determined on surface fractures. A transition of the fracture mode with the microstructure is observed. It is shown that tempered microstructures of martensite and lower bainite are more susceptible to intergranular embrittlement than tempered upper bainitic microstructure. The intergranular fracture is the most brittle mode. The analysis of crystalline mis-orientations shows a grain boundary structure appreciably more coherent for tempered microstructures of martensite and lower bainite. The higher density of random grain boundaries is susceptible to drag the phosphorus in the upper bainitic matrix and to make the quantity of free phosphorus decreasing. Microstructure observations show a difference in the size and the spatial distribution of carbides, essentially cementite, between tempered martensite and upper bainite. It can explain the bigger susceptibility of this last microstructure to cleavage mode

  11. Self-healing properties of nanocrystalline materials: a first-principles analysis of the role of grain boundaries.

    Science.gov (United States)

    Xu, Jian; Liu, Jian-Bo; Li, Shun-Ning; Liu, Bai-Xin; Jiang, Yong

    2016-07-21

    Understanding the self-healing mechanisms of defects in nanocrystalline materials is of particular importance for developing structural materials that can support the extended lifetime of components under extremely hostile conditions in nuclear reactors. Since grain boundaries are prevalent in nanocrystalline materials, they must affect, to some extent, the overall self-healing properties and the resultant mechanical responses. In the present work, first principles calculations are carried out to investigate the energetic landscape of point defects (i.e. self-interstitials, He-interstitials, and vacancies) induced by the irradiation damage and the kinetics of the self-healing process in the vicinity of grain boundaries (GBs) in copper, focusing on six symmetric tilt grain boundaries that vary in their energies. Our results indicate that the interaction of vacancies with the self-interstitial- and He-interstitial-loaded GBs is very sensitive to the GB character. Low-energy GBs are generally accompanied by a higher propensity for self-healing behavior, in which the inter-granular interstitials and intra-granular vacancies recombine with each other. The recombination process is proved to be regulated by two mechanisms: the interstitial emission mechanism and the vacancy mediated mechanism. For low-energy GBs, the former mechanism demonstrates its efficiency in describing the atomic motion, while for the high-energy ones, the latter turns out to be superior. With the aid of these mechanisms, we conclude that low-energy GBs are comparatively more radiation-resistant than the high-energy counterparts, which may shed light on the rational design of high-performance structural materials based on nanocrystalline alloys. PMID:27326789

  12. Self-healing properties of nanocrystalline materials: a first-principles analysis of the role of grain boundaries.

    Science.gov (United States)

    Xu, Jian; Liu, Jian-Bo; Li, Shun-Ning; Liu, Bai-Xin; Jiang, Yong

    2016-07-21

    Understanding the self-healing mechanisms of defects in nanocrystalline materials is of particular importance for developing structural materials that can support the extended lifetime of components under extremely hostile conditions in nuclear reactors. Since grain boundaries are prevalent in nanocrystalline materials, they must affect, to some extent, the overall self-healing properties and the resultant mechanical responses. In the present work, first principles calculations are carried out to investigate the energetic landscape of point defects (i.e. self-interstitials, He-interstitials, and vacancies) induced by the irradiation damage and the kinetics of the self-healing process in the vicinity of grain boundaries (GBs) in copper, focusing on six symmetric tilt grain boundaries that vary in their energies. Our results indicate that the interaction of vacancies with the self-interstitial- and He-interstitial-loaded GBs is very sensitive to the GB character. Low-energy GBs are generally accompanied by a higher propensity for self-healing behavior, in which the inter-granular interstitials and intra-granular vacancies recombine with each other. The recombination process is proved to be regulated by two mechanisms: the interstitial emission mechanism and the vacancy mediated mechanism. For low-energy GBs, the former mechanism demonstrates its efficiency in describing the atomic motion, while for the high-energy ones, the latter turns out to be superior. With the aid of these mechanisms, we conclude that low-energy GBs are comparatively more radiation-resistant than the high-energy counterparts, which may shed light on the rational design of high-performance structural materials based on nanocrystalline alloys.

  13. Effects of Grain Boundary Barrier in ZnO/Si Heterostructure

    Institute of Scientific and Technical Information of China (English)

    LIU Bing-Ce; LIU Ci-Hui; FV Zhu-Xi; Yi Bo

    2009-01-01

    The influence of ZnO microstructure on electrical barriers is investigated using capacitance-voltage (C-V), current-voltage (I-V) and deep level transient spectroscopy (DLTS) measurements. A deep level center located at EC- 0.24 eV obtained by DLTS in the ZnO films is an intrinsic defect related to Zni. The surface states in the ZnO grains that have acceptor behavior of capturing electrons from Zni defects result in the formation of grain barriers. In addition, we find that the current transport is dominated by grain barriers after annealing at 600°C at O2 ambient. With the increment of the annealing temperature, the current transport mechanism of ZnO/Si heterostructure is mainly dominated by thermo-emission.

  14. Numerical study on the dependence of ZnO thin-film transistor characteristics on grain boundary position

    Institute of Scientific and Technical Information of China (English)

    Zhang An; Zhao xiao-Ru; Duan Li-Bing; Liu Jin-Ming; Zhao Jian-Lin

    2011-01-01

    The dependence of transistor characteristics on grain boundary (GB) position in short-channel ZnO thin film transistors (TFTs) has been investigated using two-dimensional numerical simulations. To simulate the device accurately,both tail states and deep-level states are taken into consideration. It is shown that both the transfer and output characteristics of ZnO TFTs change dramatically with varying GB position, which is different from polycrystalline Si (poly-Si)TFTs. By analysing the mechanism of the carrier transportation in the device, it is revealed that the dependence is derived from the degrees of carrier concentration descent and mobility variation with GB position.

  15. Gas accumulation at grain boundaries during 800 MeV proton irradiation of aluminium and aluminium-alloys

    DEFF Research Database (Denmark)

    Singh, Bachu Narain; Horsewell, Andy; Sommer, W. F.;

    1986-01-01

    Samples of pure aluminium (99.9999%) and commercial Al-2.7%Mg(AlMg3) and Al-1.1%Mg-0.5%Si(Al6061) alloys were irradiated with 800 MeV protons at the Los Alamos Meson Physics Facility (LAMPF) at a temperature between 40-100°C to a maximum dose of 0.2 dpa. Transmission electron microscopy (TEM) sho...... higher than in the pure Al. The amount of gas accumulation at grain boundaries was found to depend on gas generation rate, alloying and cold-work microstructure...

  16. A first principles scanning tunneling potentiometry study of an opaque graphene grain boundary in the ballistic transport regime.

    Science.gov (United States)

    Bevan, Kirk H

    2014-10-17

    We report on a theoretical interpretation of scanning tunneling potentiometry (STP), formulated within the Keldysh non-equilibrium Green's function description of quantum transport. By treating the probe tip as an electron point source/sink, it is shown that this approach provides an intuitive bridge between existing theoretical interpretations of scanning tunneling microscopy and STP. We illustrate this through ballistic transport simulations of the potential drop across an opaque graphene grain boundary, where atomistic features are predicted that might be imaged through high resolution STP measurements. The relationship between the electrochemical potential profile measured and the electrostatic potential drop across such a nanoscale defect is also explored in this model system.

  17. Clean Grain Boundary Found in C14/Body-Center-Cubic Multi-Phase Metal Hydride Alloys

    Directory of Open Access Journals (Sweden)

    Hao-Ting Shen

    2016-06-01

    Full Text Available The grain boundaries of three Laves phase-related body-center-cubic (bcc solid-solution, metal hydride (MH alloys with different phase abundances were closely examined by scanning electron microscopy (SEM, transmission electron microscopy (TEM, and more importantly, electron backscatter diffraction (EBSD techniques. By using EBSD, we were able to identify the alignment of the crystallographic orientations of the three major phases in the alloys (C14, bcc, and B2 structures. This finding confirms the presence of crystallographically sharp interfaces between neighboring phases, which is a basic assumption for synergetic effects in a multi-phase MH system.

  18. Grain Boundary Diffusion of Oxygen in c-Textured YBa2Cu3O7-δ Films

    Institute of Scientific and Technical Information of China (English)

    2001-01-01

    The dislocation structure in magnetron sputtered c-textured YBCO films deposited on (100)SrTiO3 substrates consists of vertical (//c axis) screw dislocation forests together with layered horizontal (|caxis) edge dislocation net-works. It is found that in the oxygenation process in YBCO films, oxygen diffusion is enhanced greatly by the short circuit paths which we suggest to be the horizontally ( c axis) layered edge dislocation net-works. Diffusion equation for oxygenation was solved with the help of Fishers theory of grain boundary diffusion, and an active energy 1.16 ev.``

  19. Quantification of mineral behavior in four dimensions: Grain boundary and substructure dynamics in salt

    DEFF Research Database (Denmark)

    Borthwick, V. E.; Schmidt, Søren; Piazolo, S.;

    2012-01-01

    quantitatively. The details that can be resolved are unprecedented and therefore the presented technique promises to become influential in a wide range of geoscientific investigations. Grain and subgrain scale processes are fundamental to mineral deformation and associated Earth Dynamics, and time resolved...... crystal; a 2D experiment conducted inside the scanning electron microscope and a 3D X-ray diffraction experiment. This allowed us to evaluate the possible effects of the free surface on grain and subgrain processes. The extent to which surface effects cause experimental artifacts in 2D studies has long...

  20. The impact of carbon and oxygen in alpha-titanium: ab initio study of solution enthalpies and grain boundary segregation.

    Science.gov (United States)

    Aksyonov, D A; Hickel, T; Neugebauer, J; Lipnitskii, A G

    2016-09-28

    The solution, grain boundary (GB) segregation, and co-segregation of carbon and oxygen atoms in α-titanium are studied using density functional theory. For five titanium tilt boundaries, including T1, T2, and C1 twin systems, we determine the GB structure, as well as GB energy and excess volume. The segregation energies and volumes of carbon and oxygen are calculated for 23 inequivalent interstitial voids, while for co-segregation 75 configurations are considered. It is obtained that depending on the type of the segregation void both a positive and a negative segregation process is possible. The physical reasons of segregation are explained in terms of the analysis of the void atomic geometry, excess volume and features of the electronic structure at the Fermi level. Although carbon and oxygen show qualitatively similar properties in α-Ti, several distinctions are observed for their segregation behavior and mutual interactions. PMID:27460043