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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. On the elastic stiffness of grain boundaries

    International Nuclear Information System (INIS)

    The elastic softening of grain boundaries is evaluated from the starting point of grain boundary energy. Several examples are given to illustrate the relationship between boundary energy and the extent of softening. In general, a high grain boundary energy is associated with a large excess atomic volume in the boundary region. The consequent reduction in grain boundary stiffness can represent a significant fraction of that observed in bulk crystals. (orig.)

  5. 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 is...... analysis of the grain boundaries have happened. These improvements, for example by high-resolution imaging techniques and orientation imaging microscopy for additional crystallographic information, provide the possibilities for thorough characterization of the grain boundaries and based on that, it is...

  6. Applied thermodynamics: Grain boundary segregation

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Zheng, L.; Hofmann, S.; Šob, Mojmír

    2014-01-01

    Roč. 16, č. 3 (2014), s. 1462-1484. ISSN 1099-4300 R&D Projects: GA ČR(CZ) GAP108/12/0311; GA ČR GAP108/12/0144; GA MŠk(CZ) ED1.1.00/02.0068 Institutional support: RVO:68378271 ; RVO:68081723 Keywords : interfacial segregation * Gibbs energy of segregation * enthalpy * entropy * volume * grain boundaries * iron Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.502, year: 2014

  7. Transient Solute Drag in Migrating Grain Boundaries

    Czech Academy of Sciences Publication Activity Database

    Svoboda, Jiří; Fischer, F. D.; Liendl, M.

    2011-01-01

    Roč. 59, č. 17 (2011), s. 6556-6562. ISSN 1359-6454 R&D Projects: GA MŠk(CZ) OC10029 Institutional research plan: CEZ:AV0Z20410507 Keywords : Grain boundary diffusion * Grain boundary migration * Grain boundary segregation Subject RIV: BJ - Thermodynamics Impact factor: 3.755, year: 2011

  8. Twin boundary interactions with grain boundaries investigated in pure rhenium

    International Nuclear Information System (INIS)

    The mechanical behavior of pure rhenium was investigated using uniaxial compression tests, transmission electron microscopy and electron backscatter diffraction characterization. The plasticity was characterized by a large amount of twin formation and propagation, including twin transmission across grain boundaries. In-depth analysis of the interactions of {112¯1}〈1¯1¯26〉 twins with grain boundaries found that grain boundaries with misorientation angles below ∼25° allowed twin transmission, while grain boundaries with higher angles did not. Similar to dislocation interactions with grain boundaries, twin transmission was largely dictated by the minimization of the angle between the shear vectors of the incoming and outgoing twins

  9. Grain-boundary resistance in polycrystalline metals

    Science.gov (United States)

    Reiss, G.; Vancea, J.; Hoffmann, H.

    1986-05-01

    Grain boundaries are known to reduce significantly the electrical dc conductivity of polycrystalline metallic materials. In this paper, we give a quantum mechanical calculation of the grain-boundary resistance based on the transfer matrix approach. The results show an exponential decrease of the conductivity with respect to the number of grain boundaries per mean free path in accord with an empirical model proposed recently.

  10. Grain boundary resistance in polycrystalline metals

    OpenAIRE

    Reiss, Günter; Vancea, Johann; Hoffmann, Horst

    1986-01-01

    Grain boundaries are known to reduce significantly the electrical dc conductivity of polycrystalline metallic materials. In this paper, we give a quantum mechanical calculation of the grain-boundary resistance based on the transfer-matrix approach. The results show an exponential decrease of the conductivity with respect to the number of grain boundaries per mean free path in accord with an empirical model proposed recently.

  11. 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

  12. Diffusion mechanisms in grain boundaries in solids

    International Nuclear Information System (INIS)

    A critical review is given of our current knowledge of grain-boundary diffusion in solids. A pipe mechanism of diffusion based on the well-established dislocation model seems most appropriate for small-angle boundaries. Open channels, which have atomic configurations somewhat like dislocation cores, probably play a major role in large-angle grain-boundary diffusion. Dissociated dislocations and stacking faults are not efficient paths for grain-boundary diffusion. The diffusion and computer modeling experiments are consistent with a vacancy mechanism of diffusion by a rather well-localized vacancy. The effective width of a boundary for grain-boundary diffusion is about two atomic planes. These general features of grain-boundary diffusion, deduced primarily from experiments on metals, are thought to be equally applicable for pure ceramic solids. The ionic character of many ceramic oxides may cause some differences in grain-boundary structure from that observed in metals, resulting in changes in grain-boundary diffusion behavior. 72 references, 5 figures

  13. Boundary plane distribution for Σ13 grain boundaries in magnesium

    Czech Academy of Sciences Publication Activity Database

    Ostapovets, Andriy; Molnár, P.; Lejček, P.

    2014-01-01

    Roč. 137, DEC (2014), s. 102-105. ISSN 0167-577X R&D Projects: GA MŠk(CZ) EE2.3.30.0063 Institutional support: RVO:68081723 Keywords : magnesium * grain boundaries * CSL * Grain boundary plane Subject RIV: JG - Metallurgy; JG - Metallurgy (FZU-D) Impact factor: 2.489, year: 2014

  14. 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.)

  15. Plasticity enhancement through disordering at grain boundaries

    International Nuclear Information System (INIS)

    In seeking to explain the boron effect in Ni3Al, Frost first suggested that disordering at the grain boundary might lead to enhanced plasticity by relieving some of the geometric constraints upon dislocation interactions with these normally brittle interfaces. This idea was further investigated by King and Yoo, who performed an extensive survey on the possible reactions between lattice dislocations or superdislocations and coincidence-related grain boundaries in the L12 structure. It was found that, indeed, the number of available reactions in any case was multiplied by as much as a factor of four if the requirement for maintaining chemical order in the grain boundary was removed. This suggests that it may be considerably easier to relax plastic strain at grain boundaries in the case where order is not imposed than where it is, and thus that the plastic energy contributing to the fracture process may be partly relieved. This would contribute to ductilizing the grain boundaries. There ar, in fact, several mechanisms by which grain boundary disordering may contribute to improved interfacial ductility. These mechanisms primarily relate to the fat that smaller Burgers vectors are available for the grain boundary dislocations in the case where order is not preserved, in much the same way that superdislocations are not required in the matrix if the material is disordered. This paper discusses the contributions to ductility

  16. 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.

  17. GRAIN BOUNDARY SEGREGATIONS AND HYDROGEN EMBRITTLEMENT

    OpenAIRE

    Aucouturier, M.

    1982-01-01

    The relation between grain boundary segregation and hydrogen embrittlement of metals may be discussed from two stand points : 1°) Hydrogen has a strong tendency to segregate in structure defects, among them, in grain boundaries. Hydrogen segregation controls the properties of hydrogenated materials in many cases (hydrogen diffusion, hydrogen induced cracking, electrical properties in semiconductors, etc) and more precisely their mechanical behaviour (embrittlement itself). 2°) The occurence o...

  18. 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.

  19. 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.

  20. Grain Boundary Design and Grain Boundary Character Distribution (GBCD) in Texture Polycrystalline Materials

    OpenAIRE

    Watanabe, Tadao

    1991-01-01

    The importance of grain boundary character distribution (GBCD) to materials development by grain boundary design is briefly discussed. Particular attention has been paid to the relationship between GBOD and the nature of texture in polycrystalline materials produced by different processing methods. Several important findings on the relationship are discussed.

  1. 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...

  2. Boundary plane distribution for Σ13 grain boundaries in magnesium

    Czech Academy of Sciences Publication Activity Database

    Ostapovets, Andriy; Molnár, Peter; Lejček, Pavel

    2014-01-01

    Roč. 37, Dec (2014), s. 102-105. ISSN 0167-577X R&D Projects: GA ČR GBP108/12/G043; GA ČR GPP108/12/P054 Institutional support: RVO:68378271 Keywords : magnesium * grain boudaries * csl * grain boundary plane Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.489, year: 2014

  3. 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

  4. Grain boundary engineering with gold nanoparticles

    International Nuclear Information System (INIS)

    We investigated high-TC grain boundary Josephson junctions with and without incorporated gold nanoparticles. Pulsed laser deposition was used for the deposition of YBa2Cu3O7−δ thin films on SrTiO3 bicrystal substrates with different grain boundary angles. During the deposition process, single-crystalline nanoparticles self-assembled from a thin gold layer which was sputtered on the substrate before the YBCO deposition. The interaction between nanoparticles and thin film growth significantly influences the quality of the YBCO films [1]. The critical current density and the critical temperature of the superconducting films can be increased in a defined manner. Furthermore, the nanoparticles influence the growth conditions in the region of the grain boundary and thus the properties of the later patterned Josephson junctions. The comparison between Josephson junctions with and without nanoparticles on the same substrate shows a reduction of the critical current IC and an increase of the normal state resistance RN for all investigated types of grain boundaries in the areas with gold nanoparticles. In some cases we even found an increase of the resulting ICRN product. We present the influence of light irradiation on the properties of the Josephson junctions.

  5. Developing grain boundary diagrams for multicomponent alloys

    International Nuclear Information System (INIS)

    Impurity-based, premelting-like, intergranular films (IGFs, a common type of grain boundary complexion) can form in various materials and influence sintering, creep, and microstructure development. A thermodynamic framework is presented to forecast the formation and stability of these premelting-like grain boundary complexions (a.k.a. interfacial “phases” that are thermodynamically two dimensional) in multicomponent alloys to consider the interactions of multiple alloying elements. Key thermodynamic parameters that control the interfacial segregation and disordering behaviors have been identified and systematically examined. Subsequently, ternary and quaternary grain boundary diagrams have been computed and used to forecast the sintering behaviors of W–Ni–M (M = Fe, Co, Cr, Zr, Nb and Ti) and Mo–Si–B–M (M = Ni, Co and Fe) systems. This work supports a long-range scientific goal of extending bulk computational thermodynamics and CALPHAD methods to interfaces and developing grain boundary complexion (interfacial “phase”) diagrams as extensions to bulk phase diagrams, which can be a generally useful materials science tool

  6. Grain boundary effects in nanocrystalline diamond

    Czech Academy of Sciences Publication Activity Database

    Mareš, Jiří J.; Hubík, Pavel; Krištofik, Jozef; Nesládek, Miloš

    2008-01-01

    Roč. 205, č. 9 (2008), 2163-2168. ISSN 1862-6300 R&D Projects: GA ČR(CZ) GA202/06/0040 Institutional research plan: CEZ:AV0Z10100521 Keywords : diamond film * grain boundary * superconductivity * noise * ballistic transport Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.205, year: 2008

  7. 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

  8. Silver grain boundary diffusion in Pd

    Energy Technology Data Exchange (ETDEWEB)

    Balogh, Z., E-mail: bz0015@delfin.unideb.hu [Department of Solid State Physics, University of Debrecen, P.O. Box 2, H-4010 Debrecen (Hungary); Erdelyi, Z.; Beke, D.L. [Department of Solid State Physics, University of Debrecen, P.O. Box 2, H-4010 Debrecen (Hungary); Portavoce, A.; Girardeaux, C.; Bernardini, J.; Rolland, A. [Aix-Marseille Universite, IM2NP, Faculte des Sciences et Techniques, Campus de Saint-Jerome, Avenue Escadrille Normandie Niemen - Case 142, F-13397 Marseille Cedex (France); CNRS, IM2NP (UMR 6242), Faculte des Sciences et Techniques, Campus de Saint-Jerome, Avenue Escadrille Normandie Niemen - Case 142, F-13397 Marseille Cedex (France)

    2009-02-15

    Two to ten nanometer thick polycrystalline Pd films were prepared on the (1 1 1) surface of Ag single crystal and investigations of the Ag diffusion along Pd grain boundaries were carried out using the Hwang-Balluffi method. The samples were monitored by Auger electron spectroscopy (AES) during isothermal heat treatments in the 438-563 K temperature range. Using plausible simplifying assumptions, the activation energy of the product of the grain boundary (GB) diffusion coefficient and k' (k' = c{sub s}/c{sub gb}; c{sub s} and c{sub gb} are the surface and GB concentration, respectively) was calculated (0.99 {+-} 0.08 eV) from the evaluated saturation coefficients of the surface accumulation. This energy, for weak temperature dependence of k', is approximately equal to the activation energy of the GB diffusion.

  9. Interaction of shear-coupled grain boundary motion with crack: Crack healing, grain boundary decohesion, and sub-grain formation

    Science.gov (United States)

    Aramfard, Mohammad; Deng, Chuang

    2016-02-01

    Stress-driven grain boundary motion is one of the main mechanisms responsible for microstructural evolution in polycrystalline metals during deformation. In this research, the interaction of shear-coupled grain boundary motion (SCGBM) in face-centered cubic metals with crack, which is a common type of structural defects in engineering materials, has been studied by using molecular dynamics simulations in simple bicrystal models. The influences of different parameters such as metal type, temperature, grain boundary structure, and crack geometry have been examined systematically. Three types of microstructural evolution have been identified under different circumstances, namely, crack healing, grain boundary decohesion, and sub-grain formation. The underlying atomistic mechanisms for each type of SCGBM-crack interaction, particularly grain boundary decohesion and crack healing, have also been examined. It is found that crack healing is generally favoured during the SCGBM-crack interaction at relatively high temperature in metals with relatively low stacking fault energy and grain boundary structure with relatively low misorientation angles. The results of this work may open up new opportunities for healing severely damaged materials.

  10. Challenges of interfacial classification for grain boundary engineering

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Paidar, Václav

    2005-01-01

    Roč. 21, č. 4 (2005), s. 393-398. ISSN 0267-0836 R&D Projects: GA ČR(CZ) GA106/02/0253 Institutional research plan: CEZ:AV0Z10100520 Keywords : grain boundary classification * grain boundary engineering * grain boundary segregation * iron Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.639, year: 2005

  11. 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.

  12. 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.

  13. Liquid Nucleation at Superheated Grain Boundaries

    Science.gov (United States)

    Frolov, T.; Mishin, Y.

    2011-04-01

    Grain boundaries with relatively low energies can be superheated above the melting temperature and eventually melt by heterogeneous nucleation of liquid droplets. We propose a thermodynamic model of this process based on the sharp-interface approximation with a disjoining potential. The distinct feature of the model is its ability to predict the shape and size of the critical nucleus by using a variational approach. The model reduces to the classical nucleation theory in the limit of large nuclei but is more general and remains valid for small nuclei. Contrary to the classical nucleation theory, the model predicts the existence of a critical temperature of superheating and offers a simple formula for its calculation. The model is tested against molecular dynamic simulations in which liquid nuclei at a superheated boundary were obtained by an adiabatic trapping procedure. The simulation results demonstrate a reassuring consistency with the model.

  14. Study of some properties of point defects in grain boundaries

    International Nuclear Information System (INIS)

    With the aim of deducing simple informations on the grain boundary core structure, we investigated self diffusion under hydrostatic pressure, impurity diffusion (In and Au), electromigration (Sb) along certain types of grain boundaries in Ag bicrystals, and the Moessbauer effect of 57Co located in the grain boundaries of polycrystalline Be. Our results lead to the following conclusions: the formation of a vacancy like defects is necessary to grain boundary diffusion; solute atoms may release most of their elastic energy of dissolution as they segregate at the boundary; in an electrical field, the drift of Sb ions parallel to the boundary takes place toward the anode as in the bulk. The force on the grain boundary ions is larger than in the bulk; Moessbauer spectroscopy revealed the formation of Co-rich aggregates, which may proves important in the study of early stages of grain boundary precipitation. (author)

  15. 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...

  16. Mathematical simulation of point defect interaction with grain boundaries

    International Nuclear Information System (INIS)

    Published works, where the interaction of point defects and grain boundaries was studied by mathematical simulation methods, have been analysed. Energetics of the vacancy formation both in nuclei of large-angle special grain boundaries and in lattice regions adjoining them has been considered. The data obtained permit to explain specific features of grain-boundary diffusion processes. Results of mathematical simulation of the interaction of impurity atoms and boundaries have been considered. Specific features of the helium atom interaction with large-angle grain boundaries are analysed as well

  17. 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.

  18. 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.

  19. 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

  20. 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

    2016-03-01

    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.

  1. 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.

  2. 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

  3. Measurement of krypton grain-boundary inventories in CANDU fuel

    International Nuclear Information System (INIS)

    A technique for measuring the Kr-85 grain-boundary inventory in irradiated fuel based on the conversion of UO2 to U3O8 at low temperatures has been improved. The improvements include: 1) the use of a tracer isotope to account for release from the matrix during measurement of the grain-boundary inventory and 2) the cutting of samples from known locations. With these improvements it is possible to measure radial variations in the grain-boundary inventory. The measurements of Kr-85 grain-boundary inventory can be combined with gamma mapping and ceramography to allow investigation of the connection between microstructure and fission-product distribution. (author)

  4. Interactions of impurities with a moving grain boundary

    International Nuclear Information System (INIS)

    Most theories developed to explain interaction of impurities with a moving grain boundary involve a uniform excess impurity concentration distributed along a planar grain boundary. As boundary velocity increases, the excess impurities exert a net drag force on the boundary until a level is reached whereat the drag force no longer can balance the driving force and breakaway of the boundary from these impurities occurs. In this investigation, assumptions of a uniform lateral impurity profile and a planar grain boundary shape are relaxed by allowing both forward and lateral diffusion of impurities in the vicinity of a grain boundary. It is found that the two usual regions (drag of impurities by, and breakaway of a planar grain boundary) are separated by an extensive region wherein a uniform lateral impurity profile and a planar grain boundary shape are unstable. It is suspected that, in this unstable region, grain boundaries assume a spectrum of more complex morphologies and that elucidation of these morphologies can provide the first definitive description of the breakaway process and insight to more complex phenomena such as solid-solution strengthening, grain growth and secondary recrystallization

  5. Grain-boundary migration in nonstoichiometric solid solutions of magnesium aluminate spinel 2; Effects of grain-boundary nonstoichiometry

    Energy Technology Data Exchange (ETDEWEB)

    Chiang, Y.M. (Massachusetts Inst. of Tech., Cambridge, MA (USA). Dept. of Materials Science and Engineering); Kingery, W.D. (Arizona Univ., Tucson, AZ (USA))

    1990-05-01

    The grain-boundary chemistry of magnesium aluminate spinel solid solutions MgO {center dot} nAl{sub 2}O{sub 3} has been investigated in order to understand the mechanism of grain-boundary migration. It is found that although segregation of impurity Ca and Si is common, much larger deviations in grain-boundary stoichiometry are present. There is an excess of Al and O relative to Mg at grain boundaries in all compositions. Grain-boundary migration appears to be rate-limited by solute drag from intrinsic defects accommodating lattice nonstoichiometry, rather than by extrinsic solutes, consistent with the observed impurity tolerance of grain-boundary mobility. Different rate-limiting defects are proposed for magnesia-rich and alumina-rich spinels.

  6. 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)

  7. 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.

  8. The effect of grain boundary chemistry on the slip transmission process through grain boundaries in Ni3Al

    International Nuclear Information System (INIS)

    This paper reports on the conditions established in disordered FCC systems for predicting the slip system that will be activated by a grain boundary to relieve a local stress concentration that have been applied to the ordered FCC alloy Ni3Al. The slip transfer behavior in hypo-stoichiometric Ni3Al with (0.2 at. %B) and without boron was directly observed by performing the deformation experiments in situ in the transmission electron microscope. In the boron-free and boron-doped alloys, lattice dislocations were incorporated in the grain boundary, but did not show evidence of dissociation to grain boundary dislocations or of movement in the grain boundary plane. The stress concentration associated with the dislocation pileup at the grain boundary are relieved by the emission of dislocations from the grain boundary in the boron-doped alloy. The slip system initiated in the adjoining grain obeyed the conditions established for disordered FCC systems. In the boron-free alloy, the primary stress relief mechanism was grain-boundary cracking, although dislocation emission from the grain boundary also occurred and accompanied intergranular crack advance

  9. Grain boundary strength as point defect sink strength

    International Nuclear Information System (INIS)

    Sink strength of spherical grain boundary as an absolutely absorbing surface and as finite thickness wall consisting of the edge dislocations are considered. The values of the grain boundary sink strength are shown to be critically dependent on the point defect recombination degree

  10. Calculation of local elastic constants at a metallic grain boundary

    International Nuclear Information System (INIS)

    A new atomistic-simulation method for calculating the full local elastic-constant tensor in terms of local stress and local strain for inhomogeneous systems is described. Results of simulations of an isolated high-angle twist grain boundary are presented. A dramatic reduction in resistance to shear parallel to the grain boundary is observed, and its relation to structural disorder is discussed

  11. Critical currents across low-angle grain boundaries in YBCO

    International Nuclear Information System (INIS)

    The following topics were covered in this thesis: Currents in HTSC, Shubnikov phase, HTSC thin films, magneto-optical Faraday effect, current density distribution, flux density, grain boundaries in YBa2Cu3O7-x (YBCO), currents across grain boundaries, planar defect (WL)

  12. 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...

  13. Comparative study of grain-boundary migration and grain-boundary self-diffusion of [0 0 1] twist-grain boundaries in copper by atomistic simulations

    International Nuclear Information System (INIS)

    Molecular-dynamics simulations were used to study grain-boundary migration as well as grain-boundary self-diffusion of low-angle and high-angle [0 0 1] planar twist grain boundaries (GBs) in copper. Elastic strain was imposed to drive the planar [0 0 1] twist GBs. The temperature dependence of the GB mobility was determined over a wide misorientation range. Additionally grain-boundary self-diffusion was studied for all investigated [0 0 1] planar twist GBs. A comparison of the activation energies determined shows that grain-boundary migration and self-diffusion are distinctly different processes. The behavior of atoms during grain-boundary migration was analyzed for all studied GBs. The analysis reveals that usually in absolute pure materials high-angle planar [0 0 1] twist GBs move by a collective shuffle mechanism while low-angle GBs move by a dislocation based mechanism. The obtained activation parameters were analyzed with respect to the compensation effect

  14. O(minus 2) grain boundary diffusion and grain growth in pure dense MgO

    Science.gov (United States)

    Kapadia, C. M.; Leipold, M. H.

    1973-01-01

    Grain growth behavior in fully dense compacts of MgO of very high purity was studied, and the results compared with other similar behaving materials. The activation energy for the intrinsic self-diffusion of Mg(2minus) is discussed along with the grain boundary diffusion of O(2minus). Grain boundary diffusion of O(2minus) is proposed as the controlling mechanism for grain growth.

  15. 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.

  16. 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.

  17. 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

  18. 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.

  19. 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.

  20. 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.

  1. 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

  2. YSZ thin films with minimized grain boundary resistivity.

    Science.gov (United States)

    Mills, Edmund M; Kleine-Boymann, Matthias; Janek, Juergen; Yang, Hao; Browning, Nigel D; Takamura, Yayoi; Kim, Sangtae

    2016-04-21

    In recent years, interface engineering of solid electrolytes has been explored to increase their ionic conductivity and improve the performance of solid oxide fuel cells and other electrochemical power sources. It has been observed that the ionic conductivity of epitaxially grown thin films of some electrolytes is dramatically enhanced, which is often attributed to effects (e.g. strain-induced mobility changes) at the heterophase boundary with the substrate. Still largely unexplored is the possibility of manipulation of grain boundary resistivity in polycrystalline solid electrolyte films, clearly a limiting factor in their ionic conductivity. Here we report that the ionic conductivity of yttria stabilized zirconia thin films with nano-columnar grains grown on a MgO substrate nearly reaches that of the corresponding single crystal when the thickness of the films becomes less than roughly 8 nm (smaller by a factor of three at 500 °C). Using impedance spectroscopy, the grain boundary resistivity was probed as a function of film thickness. The resistivity of the grain boundaries near the film-substrate interface and film surface (within 4 nm of each) was almost entirely eliminated. This minimization of grain boundary resistivity is attributed to Mg(2+) diffusion from the MgO substrate into the YSZ grain boundaries, which is supported by time of flight secondary ion mass spectroscopy measurements. We suggest grain boundary "design" as an attractive method to obtain highly conductive solid electrolyte thin films. PMID:27030391

  3. Hydrogen effects on nanovoid nucleation at nickel grain boundaries

    International Nuclear Information System (INIS)

    We performed molecular dynamics (MD) simulations to study hydrogen effects on nanovoid nucleation at nickel grain boundaries using an embedded atom method (EAM) potential. Monte Carlo (MC) simulations were performed to introduce hydrogen atoms in low-angle and high-angle symmetrical [0 0 1] tilt boundaries at 300 K for analysis of plasticity and nanovoid nucleation. The simulation results show that hydrogen atoms were trapped at the grain boundaries and reduced the critical stresses and strains for nanovoid nucleation. The MD results also show that the effects of hydrogen on nanovoid nucleation depended on the grain-boundary hydrogen concentration regardless of the grain-boundary misorientations. The MD results were then inserted into a new hydrogen associated void nucleation model that operates as an internal state variable in the context of continuum thermodynamic plasticity

  4. 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)

  5. 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)

  6. 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

  7. 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.

  8. Fission gas bubble percolation on crystallographically consistent grain boundary networks

    Science.gov (United States)

    Sabogal-Suárez, Daniel; David Alzate-Cardona, Juan; Restrepo-Parra, Elisabeth

    2016-07-01

    Fission gas release in nuclear fuels can be modeled in the framework of percolation theory, where each grain boundary is classified as open or closed to the release of the fission gas. In the present work, two-dimensional grain boundary networks were assembled both at random and in a crystallographically consistent manner resembling a general textured microstructure. In the crystallographically consistent networks, grain boundaries were classified according to its misorientation. The percolation behavior of the grain boundary networks was evaluated as a function of radial cracks and radial thermal gradients in the fuel pellet. Percolation thresholds tend to shift to the left with increasing length and number of cracks, especially in the presence of thermal gradients. In general, the topology and percolation behavior of the crystallographically consistent networks differs from those of the random network.

  9. Hyperfine field at grain boundary atoms in iron nanostructures

    International Nuclear Information System (INIS)

    Iron nanocrystallites of ball-milled iron powder, partially crystallized melt quenched amorphous alloys and polycrystalline multilayers were studied. The change in the hyperfine field at iron atoms due to grain boundaries does not exceed the experimental linewidth

  10. 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)

  11. 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...

  12. GRAIN BOUNDARY CORROSION, STRUCTURE AND SEGREGATION IN NICKEL BICRYSTALS

    OpenAIRE

    Vignaud, C.; Beaunier, L.; Biscondi, M.

    1990-01-01

    Nickel bicrystals with tilt boundaries are used to study the influence of the structure of the grain boundaries on the intergranular corrosion. An electrochemical test allows to obtain the evolution of the corrosion versus the misorientation of grains and the repartition of elements segregated during the elaboration and / or heat treatment at 800°C. Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) on intergranular fracture surfaces allows a corrosion-segregation-struc...

  13. 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

  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. Molecular dynamics study on microstructure of near grain boundary distortion region in small grain size nano- NiAl alloy

    International Nuclear Information System (INIS)

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

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

    DEFF Research Database (Denmark)

    Alimadadi, Hossein

    miniaturization of the grains down to nano-meter scale. However, this augments the total grain boundary energy stored in the material, hence, making the material less thermally stable. Coherent twin boundaries are of very low energy and mobility compared to all other boundaries in a FCC material. Accordingly...... interest. The evolution of microstructure in as-deposited and annealed condition was investigated with a combination of complementary microscopic techniques, electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI), ion channelling contrast imaging (ICCI), and, for the as-deposited...... state also high resolution transmission electron microscopy (TEM). Based on the obtained results, it is demonstrated that the grain boundary character including the network of special boundaries has a pronounced influence on the thermal stability of the as-deposited microstructure....

  17. 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...

  18. 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

  19. 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.

  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. 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.

  2. 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.

  3. 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...... 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...

  4. Grain boundary migration during sintering of Mo with Ni additions

    International Nuclear Information System (INIS)

    The development of the microstructure during sintering in the system Mo-Ni was investigated using powders with model geometry. Single crystalline Mo spheres (220 to 250 μm) were mixed with fine Mo powder (5 μm) and Ni (1.5 and 10 wt.%) and then sintered at 1250, 1340 and 14000C up to 2 h. Qualitative and quantitative metallography indicate a rapid movement of the grain boundaries in the Mo. Microhardness measurements and electron microprobe analysis show a difference of No concentration across the graon boundary; this is thought to act as the driving force of enhanced graon baoundary migration. The rapid migration of the grain boundaries is considered as the reason for increased grain boundary diffusivity and thus enhanced densification of Mo in the presence of small Ni contents. (orig.)

  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. Grain boundary character dependence of oxygen grain boundary diffusion in α-Al2O3 bicrystals

    International Nuclear Information System (INIS)

    We measured oxygen diffusion coefficients (Dgb) along five grain boundaries (GBs) in alumina bicrystals by tracing 18O by secondary ion mass spectrometry. Although all boundaries are classified as coincident site lattice boundaries with relatively ordered structures, Dgb varied up to 103 times among them. On the other hand, the boundaries with identical boundary planes had relatively similar diffusivities as well as similar structures, regardless of Σ values. These results suggest that Dgb are related to GB atomic structures and hence the GB character, especially GB planes.

  8. 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

  9. 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.

  10. Present state of the controversy about the grain boundary relaxation

    International Nuclear Information System (INIS)

    An analysis of the internal friction produced by grain boundary relaxation in metals, alloys and ceramics is presented. The different interpretations given in the literature to relaxation phenomena occurring at temperatures above about half the melting point which include the influence of grain boundaries and their interaction with solutes and precipitates are discussed in detail. A complete set of the experimental data disposable in this field since 1972 until today is reviewed. Finally, some recent experiments are discussed and new ones are suggested. They might solve the actual controversy about the real origin of the relaxation phenomena observed. If this is the case, a considerable amount of information already published can be taken into account with a good degree of confidence. This information contributes to the description of the structure and behaviour of grain boundaries, both being important topics for materials science. (author). 119 refs, 21 figs, 1 tab

  11. Grain boundary engineering technology : materials to reduce recovery boiler maintenance

    Energy Technology Data Exchange (ETDEWEB)

    Rivers, K.C.; MacKenzie, C.M. [Babcock and Wilcox Canada Ltd., Cambridge, ON (Canada)

    2001-05-01

    This paper presented a new technology designed to improve the mechanical reliability of recovery boilers. Thermal fatigue and environmentally-assisted stress corrosion cracking often cause cracking in 304L composite tubes in the lower recovery furnace. In March 1999, Babcock and Wilcox Canada created Integran Technologies Inc. with co-shareholders Ontario Power Generation and Nanometals to develop and market advanced materials applications using patented Grain Boundary Engineering (GBE) and nanocrystalline technologies. The main objective was to provide their customers with a better return on equipment investments. GBE is the method by which the local grain boundary structure is characterized and the material processing variables are adjusted to create an optimized grain boundary structure that improves the performance of the material beyond that which would result from conventional processing techniques. The technology has resulted in significant improvements in intergranular corrosion cracking resistance, creep resistance and fatigue resistance. 9 refs., 4 figs.

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

    International Nuclear Information System (INIS)

    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 MIPARTM 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

  13. Intercrystalline diffusion, dynamical and magnetic properties of grain boundaries

    International Nuclear Information System (INIS)

    Using the method, developed by the authors, which, combines diffusion over grain boundaries and nuclear gamma-resonance spectroscopy, electron, dynamic, magnetic, diffusion properties of impurity states of atomic probes (57Fe) in the nucleus of grain boundary (GB) of metallic polycrystals and in the regions of neighbour crystallite adjacent to GB, have been investigated. It is shown that for a wide number of metals the atomic probes in the nucleus of wide-angle GB polycrystals occupy the only state, which differs significantly as to its properties from the states in the interstitials of the metal regular lattice

  14. Grain boundary sliding in beta phase zircaloy-2

    International Nuclear Information System (INIS)

    Grain boundary sliding in zircaloy-2 was investigated in torsion using bamboo-structured wire coiled into helical springs. The rate of sliding is shown to vary with stress raised to a power slightly greater than unity with an activation energy close to that for lattice diffusion. Results are interpreted in terms of sliding controlled by diffusion between undulations in the boundary geometry. The geometry required to satisfy the observed rates of sliding is estimated

  15. The effect of grain size, microcracking and grain boundary grooving on osteoblast attachment in hydroxyapatite

    Science.gov (United States)

    Smith, Ian Orland

    This research examined the effect of particle size, microcracking and grain-boundary grooving in hydroxyapatite (HA) ceramics on osteoblast (OB) attachment, with the overall goal of understanding the role of physical characteristics in optimized scaffolds for bone tissue engineering. Bimodally porous HA scaffolds were fabricated by foaming and sintering either micron-scale or nano-scale HA powder, yielding two sets with average grain diameters of 8.6 +/- 1.9 mum and 588 +/- 55 nm, respectively. OBs were seeded onto these scaffolds and counted at 0.5, 1, 2 and 4 hours for attachment and 1, 3 and 5 days for proliferation using a hemacytometer. Results showed that OB attachment and proliferation was not significantly affected by the change in grain size and may depend more on the bimodal porosity of the implant. However, as our attempt to reduce the error in the hemacytometer counts was not fully successful, a more accurate method of counting the OBs, such as a quantifiable dye, must be used to verify this trend. While microcracks occur as a result of thermal processing of HA, these TEA-induced cracks are not easily controlled. For our studies we used Vickers-induced microcracks to quantify the effect of microcracking on OB attachment in HA. OB attachment was not significantly affected at one hour, but increased at four hours to 61% higher than on non-microcracked control specimens. This increase indicates that microcracking does have an effect on OB attachment and should be studied further, to assess its effect on OB proliferation and differentiation. It is not surprising that microcracks have a positive effect on OB attachment, as this mimics the natural process of bone remodeling. However, they are not likely to occur in nano-grained HA as a result of processing, as its small grain size falls below the known values of critical grain size for microcracking (GCR) in HA. Grain boundary grooving in dense HA is also investigated in this dissertation. OBs were seeded

  16. Radioisotope diffusion in grain textures by boundary integral method

    International Nuclear Information System (INIS)

    Aim of this contribution is to deal with radioisotope diffusion in grain texture by Boundary Integral method (BIM). Governing partial integral equation is transformed to an equivalent boundary integral equation, which is written in a discrete form and a system of linear algebraic equations is thus obtained. Advantage of BIM is that the system of equations is solved only for unknown values on the boundary. values in the domain are calculated explicitly in a down stream procedure. A given example indicates a good agreement with analytical results. (author)

  17. Filaments in the twist-grain-boundary smectic A phase

    Czech Academy of Sciences Publication Activity Database

    Lejček, Lubor; Novotná, Vladimíra; Glogarová, Milada

    2015-01-01

    Roč. 92, č. 3 (2015), "032505-1"-"032505-10". ISSN 1539-3755 R&D Projects: GA ČR GA15-02843S Institutional support: RVO:68378271 Keywords : twist- grain -boundary smectic-A phase * filament * dislocation walls Subject RIV: BK - Fluid Dynamics Impact factor: 2.288, year: 2014

  18. Grain boundary enegineering and alterations in anisotropy of interfacial properties

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Gärtnerová, Viera; Jäger, Aleš

    654-656, - (2010), s. 2350-2353. ISSN 0255-5476 R&D Projects: GA AV ČR KAN300100801; GA ČR GA106/08/0369 Institutional research plan: CEZ:AV0Z10100520 Keywords : grain boundary segregation, * anisotropy * compensation effect * thermodynamics Subject RIV: BM - Solid Matter Physics ; Magnetism

  19. 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

  20. The influence of grain boundary structure on diffusional creep

    DEFF Research Database (Denmark)

    Thorsen, Peter Anker; Bilde-Sørensen, Jørgen

    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 the...

  1. Nanosegregation phenomena at grain boundaries of metallic materials

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Janovec, J.; Konečná, R.

    2007-01-01

    Roč. 13, č. 1 (2007), s. 117-130. ISSN 1335-1532 R&D Projects: GA ČR GA202/06/0049 Institutional research plan: CEZ:AV0Z10100520 Keywords : segregation * grain boundaries * prediction * thermodynamics * ferrite alloys Subject RIV: BM - Solid Matter Physics ; Magnetism

  2. Kinetics of interstitial segregation in Cottrell atmospheres and grain boundaries

    Czech Academy of Sciences Publication Activity Database

    Svoboda, Jiří; Zickler, G. A.; Kozeschnik, E.; Fischer, F. D.

    2015-01-01

    Roč. 95, č. 9 (2015), s. 458-465. ISSN 0950-0839 R&D Projects: GA ČR(CZ) GA15-06390S Institutional support: RVO:68081723 Keywords : segregation * grain boundaries * dislocations * simulation * thermodynamic extremal principle Subject RIV: BJ - Thermodynamics Impact factor: 1.087, year: 2014

  3. A Molecular Dynamics Approach to Grain Boundary Structure and Migration

    DEFF Research Database (Denmark)

    Cotterill, R. M. J.; Leffers, Torben; Lilholt, Hans

    1974-01-01

    It has been demonstrated that grain boundary formation from the melt can be simulated by the molecular dynamics method. The space between two mutually-misoriented crystal slabs was filled with atoms in a random manner and this liquid was then cooled until crystallization occurred. The general...

  4. The influence of vortex pinning and grain boundary structure on critical currents across grain boundaries in YBa2Cu3Ox

    International Nuclear Information System (INIS)

    We have used studies of single grain boundaries in YBCO thin films and bulk bicrystals to study the influence of vortex pinning along a grain boundary on dissipation. The critical current density for transport across grain boundaries in thin films is typically more than an order of magnitude larger than that measured for transport across grain boundaries in bulk samples. For low disorientation angles, the difference in critical current density within the grains that form the boundary can contribute to the substantial differences in current density measured across the boundary. However, substantial differences exist in the critical current density across boundaries in thin film compared to bulk bicrystals even in the higher angle regime in which grain boundary dissipation dominates. The differences in critical current density in this regime can be understood on the basis of vortex pinning along the boundary

  5. Atomistic studies of grain boundaries in alloys and compounds

    Energy Technology Data Exchange (ETDEWEB)

    Vitek, V.

    1992-02-01

    In this research project we carry out theoretical, computer modeling, studies of the atomic structure of grain boundaries in binary alloys. Both ordered and disordered alloys are investigated. The goal is to analyze those structural, chemical and electronic features that distinguish alloys from pure metals and are responsible for remarkably different intergranular fracture behavior of alloys when compared with pure metals. The most important phenomenon is, of course, segregation and related structural changes in the boundary region. When studying segregation phenomena copper-bismuth is a very suitable model system since bismuth segregation occurs readily, leads to boundary faceting and thus to remarkable changes in the boundary structure, as well as to a very strong embrittlement. Our recent research concentrated on the investigation of the structure of {Sigma} = 3 (111)/(11{bar 1}) facets formed during segregation from boundaries which were originally curved.

  6. Grain boundary composition and intergranular fracture of steels. Volume 1. Detection of grain boundary segregation in steels. Final report

    Energy Technology Data Exchange (ETDEWEB)

    Bruemmer, S.M.; Charlot, L.A.; Thomas, M.T.; Jones, R.H.

    1985-01-01

    Several alternative techniques for the measurement of grain boundary composition in iron-base alloys were evaluated, including Secondary Ion Mass Spectroscopy (SIMS), Analytical Electron Microscopy (AEM), and chemical/electrochemical etching. Potential problems in grain boundary composition measurements of CrMoV and NiCrMoV steels by AES were identified as a result of AES, high-resolution AES, and AEM characterization. Significant differences were identified in comparing peak height ratios on identical specimens using PHI 545, 560, and 595 systems. These differences were due to instrument parameters that are generally not known for any particular AES sysyem. Localized ductile tearing regions and the presence of second phase particles on intergranular faces exposed by fracture led to significant point to point composition variations. Phosphorus levels varied by more than 50% across individual grain boundary facets. Copper-tin intermetallics were observed by AEM in the CrMoV steel and nickel-tin intermetallics in the NiCrMoV steel. Much of the tin documented by AES at grain boundaries could be explained by the presence of these small intermetallic precipitates.

  7. Grain boundary composition and intergranular fracture of steels. Volume 1: Detection of grain boundary segregation in steels

    Science.gov (United States)

    Bruemmer, S. M.; Charlot, L. A.; Thomas, M. T.; Jones, R. H.

    1985-01-01

    Several alternative techniques for the measurement of grain boundary composition in iron-base alloys were evaluated, including Secondary Ion Mass Spectroscopy (SIMS), Analytical Electron Microscopy (AEM), and chemical/electrochemical etching. Potential problems in grain boundary composition measurements of CrMoV and NiCrMoV steels by AES were identified as a result of AES, high-resolution AES, and AEM characterization. Significant differences were identified in comparing peak height ratios on identical specimens using PHI 545, 560, and 595 systems. These differences were due to instrument parameters that are generally not known for any particular AES system. Localized ductile tearing regions and the presence of second phase particles on intergranular faces exposed by fracture led to significant point to point composition varitions. Phosphorus levels varied by more than 50% across individual grain boundary facets. Copper-tin intermetallics were observed by AEM in the CrMoV steel and nickel-tin intermetallics in the NiCrMoV steel. Much of the tin documented by AES at grain boundaries could be explained by the presence of these small intermetallic precipitates.

  8. Predicting failure stress for grain boundaries using average and local properties

    International Nuclear Information System (INIS)

    Several factors can affect the failure stress of a grain boundary, such as grain boundary structure, energy and excess volume, in addition to its interactions with dislocations. In this paper, we focus on the influence of grain boundary energy, excess volume and plasticity at the boundary on the failure stress of a grain boundary, in copper from molecular-dynamics 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 of 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 failure strength of a boundary. However, local boundary properties related to the ability of a grain boundary to undergo plastic deformation are better markers of its strength.

  9. 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).

  10. 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).

  11. 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.

  12. 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...

  13. 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.

  14. Detecting Grain-Boundary Chromium Depletion in Inconel 600

    Science.gov (United States)

    Airey, G. P.; Vaia, A. R.; Pessall, N.; Aspden, R. G.

    1981-11-01

    Techniques to evaluate grain-boundary chromium depletion in Inconel Alloy 600 were investigated. Procedures studied were a modified Huey test, reactivation polarization, magnetic permeability measurements, and eddy current measurements. Results from these tests were correlated with susceptibility to stress-assisted intergranular cracking in polythionic acid. Thermally treated Inconel Alloy 600 steam generator tubing was the principal source of material evaluated, but experimental heats of Ni-Cr-Fe alloys with 8-18 wt.% Cr were prepared to determine the critical chromium level below which stress-assisted intergranular cracking occurs; this critical chromium content was found to be between 9.8 and 11.7 wt.%. All four techniques were considered suitable to evaluate grain-boundary chromium depletion; the modified Huey test and reactivation polarization technique showed a greater sensitivity than the magnetic permeability and eddy current measurements.

  15. 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%.

  16. 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.

  17. 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

    2016-01-01

    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. PMID:27230299

  18. Solute interaction in grain boundary segregation and cohesion

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel

    Zurich : Trans Tech Publications, 2014 - (Šandera, P.), s. 389-392 ISBN 978-3-03785-934-6. ISSN 1013-9826. - (Key Engineering Materials. 592-593). [International Conference on Materials Structure and Micromechanics of Fracture /7./. Brno (CZ), 01.07.2013-03.07.2013] R&D Projects: GA ČR GAP108/12/0144 Institutional support: RVO:68378271 Keywords : interfacial segregation * grain boundary embrittlemenmt * solute interaction modeling Subject RIV: BM - Solid Matter Physics ; Magnetism

  19. Grain boundary resistance to amorphization of nanocrystalline silicon carbide

    OpenAIRE

    Dong Chen; Fei Gao; Bo Liu

    2015-01-01

    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 in...

  20. 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)

  1. 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.

  2. Energy conserving orientational force for determining grain boundary mobility

    International Nuclear Information System (INIS)

    Current experimental methods are not able to determine the mobility of flat grain boundaries across the large misorientation phase space. We find that the synthetic driving force method proposed to achieve this feat by simulation has a deficiency concerning numerical accuracy. We introduce a new synthetic driving force method by defining a new way to differentiate between crystal orientations. In contrast to the former method, this has the advantage that energy is correctly preserved during the simulation and is thus more reliable. This also results in a closer match of the applied energy difference to the thermodynamic free energy. This reduces the necessity of a post-simulation correction of the applied energy per atom to resulting driving pressure. We compare the newly proposed version to the old one for two grain boundaries and investigate the influence of simulation parameters on the resulting mobility values. For future simulations using a synthetic grain boundary driving force, we recommend using this newly proposed version over previous methods. (paper)

  3. 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)

  4. 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).

  5. Multiple grain boundary transitions in ceramics: A case study of alumina

    International Nuclear Information System (INIS)

    There is a growing body of the literature that suggests that there are grain boundary structures that exist in ceramic systems that are not predicted by bulk thermodynamics. Transitions between the various grain boundary structures are not well understood either experimentally or theoretically. This study identifies six different types of grain boundary structures present in the alumina system. These grain boundary structures were directly correlated with significantly different grain boundary mobilities. There is a general trend towards increasing grain boundary disorder and increasing grain boundary mobility with increasing temperature. This is the first time such a broad range of behavior has been observed in a single system. The results have many implications on understanding grain boundary transport phenomena in ceramic systems

  6. Phase-field modeling for 3D grain growth based on a grain boundary energy database

    International Nuclear Information System (INIS)

    A 3D phase-field model for grain growth combined with a grain boundary (GB) energy database is proposed. The phase-field model is applied to a grain growth simulation of polycrystalline bcc Fe to investigate the effect of anisotropic GB energy on the microstructural evolution and its kinetics. It is found that the anisotropy in the GB energy results in different microstructures and slower kinetics, especially when the portion of low-angle, low-energy GBs is large. We discuss the applicability of the proposed phase-field simulation technique, based on the GB or interfacial energy database to simulations for microstructural evolution, including abnormal grain growth, phase transformations, etc., in a wider range of polycrystalline materials. (paper)

  7. Model calculations for evaluation of bulk- and grain boundary diffusion coefficients

    International Nuclear Information System (INIS)

    Model calculations are developed for evaluation of bulk- and grain boundary diffusion coefficients in polycrystalline media in the case of a semi-infinite diffusion source. The Matrix is assumed to be either quasi-homogeneous or to be composed of grain material and ''grain boundary-material''. Using Whipple's solution of the diffusion equation the flux of material from the grain boundaries into the grain is considered additionally. Examples are given to show the possibility to classify experimentally found diffusion profiles. (author)

  8. 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.

  9. 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...

  10. An atomistic modeling survey of the shear strength of twist grain boundaries in aluminum

    International Nuclear Information System (INIS)

    A computational survey of the shear strength of 343 unique grain boundaries was performed. For each boundary, the strength was surveyed as a function of shear direction. The results suggest that: (1) the shear strength cannot be comprehensively predicted by common grain boundary descriptors, (2) the shear strength depends significantly and simply on shear direction due to the faceted geometry of boundary planes, and (3) grain boundary shear strengths in an ordinary material can be represented by a simple statistical distribution

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. Electrical behavior of grain and grain boundary regions in stabilized zirconia

    International Nuclear Information System (INIS)

    The influence of dopant concentration on the electrical conductivity of the system (ZrO sub(2)) sub(1-x)(Y sub(2)O sub(3)) sub(x)(0,06 ≤ x ≤ 0,15) has been studied. Conductivity measurements were carried out by Impedance Spectroscopy. The results show a maximum of conductivity to a 9 mole% dopant concentration either for the intragrain (IG) or grain boundary (GB) contributions in all the investigate temperature range. (author)

  17. 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

  18. Microcrystalline silicon, grain boundaries and role of oxygen

    Czech Academy of Sciences Publication Activity Database

    Kočka, Jan; Stuchlíková, The-Ha; Ledinský, Martin; Stuchlík, Jiří; Mates, Tomáš; Fejfar, Antonín

    2009-01-01

    Roč. 93, č. 8 (2009), s. 1444-1447. ISSN 0927-0248 R&D Projects: GA MŠk(CZ) LC06040; GA AV ČR KAN400100701; GA ČR(CZ) GD202/05/H003; GA MŠk LC510; GA AV ČR IAA1010413 Institutional research plan: CEZ:AV0Z10100521 Keywords : microcrystalline silicon * grain boundaries * electronic transport * hydrogen * oxygen Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.858, year: 2009

  19. Elementary Mechanisms of Shear-Coupled Grain Boundary Migration

    Science.gov (United States)

    Rajabzadeh, A.; Mompiou, F.; Legros, M.; Combe, N.

    2013-06-01

    A detailed theoretical study of the elementary mechanisms occurring during the shear-coupled grain boundary (GB) migration at low temperature is performed focusing on both the energetic and structural characteristics. The migration of a Σ13(320) GB in a copper bicrystal in response to external shear displacements is simulated using a semiempirical potential. The minimum energy path of the shear-coupled GB migration is computed using the nudge elastic band method. The GB migration occurs through the nucleation and motion of GB steps identified as disconnections. Energy barriers for the GB and disconnection migrations are evaluated.

  20. 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.

  1. Oxidation embrittlement of grain boundaries in nickel alloys

    International Nuclear Information System (INIS)

    Intergranular oxidation-embrittlement of Alloy 600 has been studied by surface analysis of fractured thin foils pre-exposed to hydrogenated steam at 400 C. The results are believed to be relevant to primary-water stress corrosion cracking in nuclear power plants. Earlier work had shown that pre-exposure to this environment caused several micrometers of intergranular failure during subsequent straining. This cracking had none of the characteristics of hydrogen embrittlement and could be due to oxidation of chromium and/or carbon at the grain boundaries, or to ingress of a monolayer of oxygen. New results show that an oxide phase of chromium does indeed penetrate but most of the cracking seems to be due to oxygen segregation. Sulfur was also found on the fractured boundaries

  2. Stabilizing nanostructures in metals using grain and twin boundary architectures

    Science.gov (United States)

    Lu, K.

    2016-05-01

    Forming alloys with impurity elements is a routine method for modifying the properties of metals. An alternative approach involves the incorporation of interfaces into the crystalline lattice to enhance the metal's properties without changing its chemical composition. The introduction of high-density interfaces in nanostructured materials results in greatly improved strength and hardness; however, interfaces at the nanoscale show low stability. In this Review, I discuss recent developments in the stabilization of nanostructured metals by modifying the architectures of their interfaces. The amount, structure and distribution of several types of interfaces, such as high- and low-angle grain boundaries and twin boundaries, are discussed. I survey several examples of materials with nanotwinned and nanolaminated structures, as well as with gradient nanostructures, describing the techniques used to produce such samples and tracing their exceptional performances back to the nanoscale architectures of their interfaces.

  3. Stress relaxation in thin film/substrate systems by grain boundary diffusion: a discrete dislocation framework

    International Nuclear Information System (INIS)

    The relaxation of stress in a thin film due to grain boundary diffusion is investigated in terms of a new discrete dislocation framework. Discrete dislocations along grain boundaries are nucleated from the free surface and are then driven to 'climb' by the Peach–Koehler force, with a mobility that is determined by the grain boundary diffusivity. Application to a planar film/substrate problem with (sub-) micrometer scale columnar grains shows that the amount of relaxation is dependent on the initial stress and on the grain aspect ratio. For thin columnar grains the relaxation is faster and more effective and the opening displacements along the grain boundary are more uniform, an effect that is not captured by current continuum models. When the initial stress is low and and the grain size is small, it is necessary to account for variations in the threshold stress for diffusion among different grain boundaries to achieve realistic results

  4. In Situ Observations of the Interaction of Liquid Lead Inclusions with Grain Boundaries in Aluminum.

    Science.gov (United States)

    Gabrisch; Dahmen; Johnson

    1998-05-01

    : The evolution of liquid lead (Pb) inclusions at grain boundaries in aluminum (Al) was investigated by direct in situ TEM observation in the temperature range from 330 degrees-643 degreesC. In agreement with earlier reports on quenched alloys, the characteristic contact angle of the lens-shaped grain boundary inclusions was found to be near 120 degrees. This angle remained approximately constant over the entire temperature range, ruling out the possibility of a wetting transition. Coarsening of grain boundary inclusions was observed to proceed mainly by Ostwald ripening, although coalescence could also be observed. Inclusions at grain boundaries, at triple junctions, and at the intersection of grain boundaries with the foil surfaces adopted characteristic shapes that were shown to be equilibrium forms. At the highest temperatures, the grain boundaries were observed to detach from the inclusions and the interaction of a migrating grain boundary with inclusions could be observed. PMID:9767666

  5. 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

  6. 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

  7. Influence of SiC grain boundary character on fission product transport in irradiated TRISO fuel

    Science.gov (United States)

    Lillo, T. M.; van Rooyen, I. J.

    2016-05-01

    In this study, the fission product precipitates at silicon carbide grain boundaries from an irradiated TRISO particle were identified and correlated with the associated grain boundary characteristics. Precession electron diffraction in the transmission electron microscope provided the crystallographic information needed to identify grain boundary misorientation and boundary type (i.e., low angle, random high angle or coincident site lattice (CSL)-related). The silicon carbide layer was found to be composed mainly of twin boundaries and small fractions of random high angle and low angle grain boundaries. Most fission products were found at random, high-angle grain boundaries, with small fractions at low-angle and CSL-related grain boundaries. Palladium (Pd) was found at all types of grain boundaries while Pd-uranium and Pd-silver precipitates were only associated with CSL-related and random, high-angle grain boundaries. Precipitates containing only Ag were found only at random, high-angle grain boundaries, but not at low angle or CSL-related grain boundaries.

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

    International Nuclear Information System (INIS)

    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)

  9. 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.

  10. 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...

  11. 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.

  12. Grain boundary mobility in anion doped MgO

    Science.gov (United States)

    Kapadia, C. M.; Leipold, M. H.

    1973-01-01

    Certain anions OH(-), F(-) and Gl(-) are shown to enhance grain growth in MgO. The magnitude of their effect decreases in the order in which the anions are listed and depends on their location (solid-solution, second phase) in the MgO lattice. As most anions exhibit relatively high vapor pressures at sintering temperatures, they retard densification and invariably promote residual porosity. The role of anions on grain growth rates was studied in relation to their effect on pore mobility and pore removal; the atomic process controlling the actual rates was determined from observed kinetics in conjunction with the microstructural features. With respect to controlling mechanisms, the effects of all anions are not the same. OH(-) and F(-) control behavior through creation of a defect structure and a grain boundary liquid phase while Cl(-) promotes matter transport within pores by evaporation-condensation. Studies on an additional anion, S to the minus 2nd power gave results which were no different from undoped MgO, possibly because of evaporative losses during hot pressing. Hence, the effect of sulphur is negligible or undetermined.

  13. 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.

  14. 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.

  15. Atomistic investigation of the structure and transport properties of tilt grain boundaries of UO2

    International Nuclear Information System (INIS)

    We apply atomistic simulation techniques to address whether oxygen shows higher diffusivity at the grain boundary region compared to that in bulk UO2, 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

  16. 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

  17. Multiscale model of metal alloy oxidation at grain boundaries.

    Science.gov (United States)

    Sushko, Maria L; Alexandrov, Vitaly; Schreiber, Daniel K; Rosso, Kevin M; Bruemmer, Stephen M

    2015-06-01

    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 oxygen

  18. 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

  19. 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.

  20. Molecular dynamics simulation of mechanical behavior of a system including both a crack and grain boundaries under cyclic loading

    International Nuclear Information System (INIS)

    The evaluation of material properties has been recently executed using a molecular dynamics simulation for the purpose of the application to the analysis of semiconductor manufacture process and structural design of a micro machine. The effect of grain boundaries on the mechanical behaviors of material has been examined. These studies, however, dealt with the initial phase of brittle fracture such as crack propagation in a polycrystalline materials. On the other hand, the effect of grain boundaries on ductile fracture is not enough to be examined. Therefore, in order to observe the initial phase of ductile fracture of a material with grain boundaries, we performed a molecular dynamics simulation for a system including both a crack and two tilt grain boundaries under uniaxial tension. Three kinds of the grain boundaries were examined. The {112} grain boundary has the lowest grain boundary energy among all tilt grain boundaries. The {111} grain boundary is one of the grain boundaries which have a local-maximum grain boundary energy. The {332} grain boundary has a grain boundary energy between the preceding two grain boundaries. In a system including both the crack and the {112} grain boundaries, not only a phase transition from bcc to hcp but also ductile deformation such as dislocations and twin deformation were observed around the crack tip in order to relax stress concentration. Then, two dislocation pile-ups near the grain boundaries were formed after the dislocations reached the grain boundaries. In a system including both the crack and the {111} grain boundaries, a new crack was generated at the grain boundaries and intergranular fracture occurred. In a system including both the crack and the {332} grain boundaries, dislocations were emitted not only from the crack tip but also from contact points of a free surface and the grain boundary, then complex dislocation structure was formed. Refs. 3 (author)

  1. Ab initio calculations of grain boundaries in bcc metals

    Science.gov (United States)

    Scheiber, Daniel; Pippan, Reinhard; Puschnig, Peter; Romaner, Lorenz

    2016-03-01

    In this study, we compute grain boundary (GB) properties for a large set of GBs in bcc transition metals with a special focus on W, Mo and Fe using ab initio density functional theory (DFT) and semi-empirical second nearest neighbour modified embedded atom method (2NN-MEAM) potentials. The GB properties include GB energies, surface energies, GB excess volume and work of separation, which we analyse and then compare to experimental data. We find that the used 2NN-MEAM potentials can predict general trends of GB properties, but do not always reproduce the GB ground state structure and energy found with DFT. In particular, our results explain the experimental finding that W and Mo prefer intergranular fracture, while other bcc metals prefer transgranular cleavage.

  2. Fiske resonances in mesoscopic '0-π' grain boundary junctions

    International Nuclear Information System (INIS)

    A theory describing Fiske resonance steps in high-temperature 'd-wave' superconductive Josephson junctions has been developed. The model is an extension of the theory proposed by Kulik in 1965, which applies in the case of conventional low-temperature junctions ('s-wave' superconductors). The theory allows to derive the magnetic field dependences of the n-th order Fiske step, also in the presence of '0-π' singularities in the junction phase difference. An analysis of Fiske steps in asymmetric 0-45o [0 0 1] 'd-wave' Josephson junctions has been presented. Finally, in order to describe the phenomenology encountered in real grain boundary junctions, the presence of facets of different orientations and lengths has also been considered

  3. 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.

  4. Experimental approaches for probing interfaces and grain boundaries

    International Nuclear Information System (INIS)

    This paper focuses its attention on several research efforts performed by the authors and colleagues whose primary aim is probing interfacial and grain boundary properties. The first investigations described pertain to impurity segregation induced embrittlement which were modeled using thermodynamic considerations and were characterized by fracture testing using Auger electron spectroscopy (AES). Localized interface chemical characterization is essential in this research. The second investigation focuses on interface/impurity interaction determination. Extended X-Ray Absorption Fine Structure (EXAFS), a structure characterization tool, was used to determine the local environment of impurities in metal-impurity-metal laminate composites, where the impurities were in monolayer to submonolayer interface concentrations. The third study pertains to the interface fracture in metal-matrix (metal/metal oxide/graphite) composites where a correlation was made between electronic states and fracture path

  5. 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)

  6. Nanohardness of molybdenum in the vicinity of grain boundaries and triple junctions

    International Nuclear Information System (INIS)

    We performed a nanoindentation study of the inherent mechanical properties of grain boundaries and their triple junctions in molybdenum tricrystals of controlled geometry grown from the melt. A region with increased hardness is revealed in the vicinity of all grain boundaries studied. The width of this region is about 2 μm and the maximum relative increase of hardness at the grain boundary is about 30%. The local nanohardness at the triple junctions was found to be higher than that at the grain boundaries. This effect is analyzed in terms of the indentation size effect model of Nix and Gao, assuming that the grain boundaries represent ideal barriers for the first nucleated dislocation loops. We calibrated the parameters of the developed model using the data for nanohardness of grain boundaries. The model predicts values of nanohardness at the triple junctions which are in good agreement with the experimental results

  7. Grain-boundary migration in nonstoichiometric solid solutions of magnesium aluminate spinel 2

    International Nuclear Information System (INIS)

    The grain-boundary chemistry of magnesium aluminate spinel solid solutions MgO · nAl2O3 has been investigated in order to understand the mechanism of grain-boundary migration. It is found that although segregation of impurity Ca and Si is common, much larger deviations in grain-boundary stoichiometry are present. There is an excess of Al and O relative to Mg at grain boundaries in all compositions. Grain-boundary migration appears to be rate-limited by solute drag from intrinsic defects accommodating lattice nonstoichiometry, rather than by extrinsic solutes, consistent with the observed impurity tolerance of grain-boundary mobility. Different rate-limiting defects are proposed for magnesia-rich and alumina-rich spinels

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

    OpenAIRE

    Taylor, Seth T.

    2002-01-01

    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 re...

  9. Grain Boundary Related Effects in Aluminium during 600 MeV Proton Irradiation of Different Temperatures

    DEFF Research Database (Denmark)

    Singh, Bachu Narain; Leffers, Torben; Green, W.V.; Victoria, M.

    Samples of high-purity aluminium were irradiated with 600 MeV protons at temperatures in the range 130 to 433°C; in these experiments 615 and 125 appm of hydrogen and helium, respectively, are produced per dpa. Bubble formation and growth at grain boundaries and in the zone adjacent to the bubble......-denuded zone are described. Precipitation at grain boundaries and migration of grain boundaries during irradiation are also reported....

  10. Observation of mesoscopic conductance fluctuations in YBaCuO grain boundary Josephson Junctions

    OpenAIRE

    Tagliacozzo, A.; Born, D.; D. Stornaiuolo; Gambale, E.; Dalena, D.; Lombardi, F.; Barone, A.; Altshuler, B. L.; Tafuri, F.

    2006-01-01

    Magneto-fluctuations of the normal resistance R_N have been reproducibly observed in high critical temp erature superconductor (HTS) grain boundary junctions, at low temperatures. We attribute them to mesoscopic transport in narrow channels across the grain boundary line. The Thouless energy appears to be the relevant energy scale. Our findings have significant implications on quasiparticle relaxation and coherent transport in HTS grain boundaries.

  11. 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.

  12. 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.

  13. 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

  14. 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...

  15. 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

  16. 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.

  17. Grain boundary chemistry effects on environment-induced crack growth of iron-based alloys

    Energy Technology Data Exchange (ETDEWEB)

    Jones, R.H.

    1992-11-01

    Relation between grain boundary chemistry and environment-induced crack growth of Fe-based alloys is reviewed. The importance of the cleanliness of steels is clearly demonstrated by direct relations between grain boundary chemistry and crack growth behavior for both H and anodic dissolution-induced crack growth. Relationships between strain to failure, work of fracture, K[sub ISCC], crack velocity and fracture mode and grain boundary chemistry are presented. Only results in which the grain boundary chemistry has been measured directly by Auger electron spectroscopy (AES) on intergranular surfaces exposed by in situ fracture have been considered in this review.

  18. Grain boundary chemistry effects on environment-induced crack growth of iron-based alloys

    Energy Technology Data Exchange (ETDEWEB)

    Jones, R.H.

    1992-11-01

    Relation between grain boundary chemistry and environment-induced crack growth of Fe-based alloys is reviewed. The importance of the cleanliness of steels is clearly demonstrated by direct relations between grain boundary chemistry and crack growth behavior for both H and anodic dissolution-induced crack growth. Relationships between strain to failure, work of fracture, K{sub ISCC}, crack velocity and fracture mode and grain boundary chemistry are presented. Only results in which the grain boundary chemistry has been measured directly by Auger electron spectroscopy (AES) on intergranular surfaces exposed by in situ fracture have been considered in this review.

  19. 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.

  20. Metallographic screening of grain boundary engineered type 304 austenitic stainless steel

    International Nuclear Information System (INIS)

    An electrochemical etching method for the identification of grain boundary engineered type 304 austenitic stainless steel microstructures is described. The method can be applied for rapid microstructure screening to complement electron backscatter diffraction analysis. A threshold parameter to identify grain boundary engineered microstructure is proposed, and the application of metallographic etching for characterising the degree of grain boundary engineering discussed. - Highlights: • As-received (annealed) and grain boundary engineered microstructures were compared. • Electro-chemical polarisation in nitric acid solutions was carried out. • A metallographic screening method has been developed. • The screening method complements EBSD analysis for microstructure identification

  1. 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.

  2. Mean-field model for the growth and coarsening of stoichiometric precipitates at grain boundaries

    International Nuclear Information System (INIS)

    In this paper, a model for growth and coarsening of precipitates at grain boundaries is developed. The concept takes into account that the evolution of grain boundary precipitates involves fast short-circuit diffusion along grain boundaries as well as slow bulk diffusion of atoms from the grain interior to the grain boundaries. The mathematical formalism is based on a mean-field approximation, utilizing the thermodynamic extremal principle. The model is applied to the precipitation of aluminum nitrides in microalloyed steel in austenite, where precipitation occurs predominately at the austenite grain boundaries. It is shown that the kinetics of precipitation predicted by the proposed model differs significantly from that calculated for randomly distributed precipitates with spherical diffusion fields. Good agreement of the numerical solution is found with experimental observations as well as theoretical treatment of precipitate coarsening

  3. 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.

  4. Variation of electric properties across the grain boundaries in BiFeO3 film

    Science.gov (United States)

    Stojadinović, Bojan; Vasić, Borislav; Stepanenko, Dimitrije; Tadić, Nenad; Gajić, Radoš; Dohčević-Mitrović, Zorana

    2016-02-01

    Stark differences in charge transport properties between the interior and the boundary regions of grains in an undoped BiFeO3 thin film have been found. The material is ferroelectric and each grain is a single domain. A spatial resolution that distinguishes between the grain interior and the boundary between the grains has been achieved by using piezoelectric force microscopy and conductive atomic force microscopy measurements. The local electric properties, as well as the local band gap show hysteresis only when probed in the grain interior, but do not show hysteresis when probed in the region around the boundary between two grains. The leakage current is more pronounced at the grain boundaries, and the region that carries significant current increases with the applied voltage.

  5. 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

  6. Nano-analysis of grain boundary and triple junction transport in nanocrystalline Ni/Cu

    Energy Technology Data Exchange (ETDEWEB)

    Reda Chellali, Mohammed, E-mail: m_chel01@uni-muenster.de [Institute of Materials Physics, Westf. Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster (Germany); Laboratoire de Chimie des Polymères, Faculté des Sciences, Université d' Oran (Algeria); Balogh, Zoltan; Schmitz, Guido [Institute of Materials Physics, Westf. Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster (Germany)

    2013-09-15

    Nanocrystalline materials are distinguished by a high density of structural defects and grain boundaries. Due to the small grain size, a particular defect of the grain boundary topology, the so-called triple junction takes a dominant role for grain growth and atomic transport. We demonstrate by atom probe tomography that triple junctions in nanocrystalline Cu have 100–300 times higher diffusivity of Ni than standard high angle grain boundaries. Also, a previously unexpected systematic variation of the grain boundary width with temperature is detected. The impurity segregation layer at the grain boundaries grows from the 0.7 nm at 563 K to 2.5 nm at 643 K. This variation is clearly not controlled by simple bulk diffusion. Taking this effect into consideration, the activation energies for Ni diffusion in triple junctions and grain boundaries in Cu can be determined to be (83±10) and (120±15) kJ/mol, respectively. Thus, triple junctions are distinguished by considerably lower activation energy with respect to grain boundaries. - Highlights: ► TJs, GBs, and individual grains are clearly localized. ► Diffusion (and segregation) in TJ can be studied. ► TJs diffusivity more than 2 orders of magnitude faster than GBs. ► The chemical width of GBs grows from 0.7 nm at 563 K to 2.5 nm at 643 K.

  7. 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

  8. Point-defect recombination efficiency at grain boundaries in irradiated SiC

    International Nuclear Information System (INIS)

    We studied the atomic-scale mechanisms of radiation damage recovery, by molecular dynamics simulations of irradiation cascades in a β-SiC model system, containing one general (001) twist grain boundary in the direction approximately perpendicular to the cascade. The (001) grain boundary has a disordered atomic structure, representative of high-angle, high-energy boundaries in cubic silicon carbide. Compared to the perfect crystal model system, we find a relevant effect of grain boundaries on the annealing of cascade defects, both in terms of localization of defects, which are preferentially concentrated around the grain boundary, and of relative defect recovery efficiency. In general, C interstitials are the prevalent type of defect over the whole range of energies explored. A slight grain boundary expansion is observed, accompanied by a broadening of the central atomic planes

  9. 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.

  10. Misorientation related microstructure at the grain boundary in a nickel-based single crystal superalloy

    Energy Technology Data Exchange (ETDEWEB)

    Huang, Ming; Zhuo, Longchao [National Center for Electron Microscopy in Beijing, School of Materials Science and Engineering, The State Key Laboratory of New Ceramics and Fine Processing, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084 (China); Liu, Zhanli [Applied Mechanics Lab, School of Aerospace, Tsinghua University, Beijing 100084 (China); Lu, Xiaogang [School of Materials Science and Engineering, Shanghai University, Shanghai (China); Shi, Zhenxue; Li, Jiarong [Science and Technology on Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095 (China); Zhu, Jing, E-mail: jzhu@mail.tsinghua.edu.cn [National Center for Electron Microscopy in Beijing, School of Materials Science and Engineering, The State Key Laboratory of New Ceramics and Fine Processing, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084 (China)

    2015-07-29

    The mechanical properties of nickel-based single crystal superalloys deteriorate with increasing misorientation, thus the finished product rate of the casting of single crystal turbine airfoils may be reduced due to the formation of grain boundaries especially when the misorientation angle exceeds to some extent. To this day, evolution of the microstructures at the grain boundaries with misorientation and the relationship between the microstructures and the mechanical properties are still unclear. In this work a detailed characterization of the misorientation related microstructure at the grain boundary in DD6 single crystal superalloy has been carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques; the elemental distribution at the grain boundaries has been analyzed by energy dispersive (EDS) X-ray mapping; and the effect of precipitation of μ phases at the grain boundary on the mechanical property has been evaluated by finite element calculation. It is shown that the proportion of γ phase at the grain boundaries decreases, while the proportion of γ′ phase at the grain boundaries increases with increasing misorientation; the μ phase is precipitated at the grain boundaries when the misorientation angle exceeds about 10° and thus it could lead to a dramatic deterioration of the mechanical properties, as well as that the enrichment of Re and W gradually disappears as the misorientation angle increases. All these factors may result in the degradation of the mechanical properties at the grain boundaries as the misorientation increases. Furthermore, the finite element calculation confirms that precipitation of μ phases at the grain boundary is responsible for the significant deterioration of the mechanical properties when the misorientation exceeds about 10°. This work provides a physical imaging of the microstructure for understanding the relationship between the mechanical properties and the misorientation

  11. Characterizing twist grain boundaries in BCC Nb by molecular simulation: Structure and shear deformation

    Science.gov (United States)

    Liu, Zeng-Hui; Feng, Ya-Xin; Shang, Jia-Xiang

    2016-05-01

    Atomic scale modeling was used to study the structure, energy and shear behaviors of (110) twist grain boundaries (TWGBs) in body-centered cubic Nb. The relation between grain boundary energy (GBE) and the twist angle θ agrees well with the Read-Shockley equation in low-angle range. At higher angles, the GBEs show no distinct trend with the variation of the twist angle or the density of coincident lattice sites. All (110) twist boundaries can be classified into two types: low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs). LAGBs contain a hexagonal dislocation network (HDN) which is composed of 1/2 [ 111 ], 1/2 [ 1 bar 1 bar 1 ] and [001] screw dislocations. HAGBs can be classified into three sub-types further: special boundaries with low Σ, boundaries in the vicinity of special boundaries with similar structures and ordinary HAGBs consisting of periodic patterns. Besides, a dependence of grain boundary shear response vs the twist angle over the entire twist angle range is obtained. Pure sliding behavior is found at all TWGBs. When θ < 12°, the flow stress of LAGBs is found to be correlated with the HDNs and decreases with the increasing twist angle. For ordinary HAGBs, the magnitude of flow stress is around 0.8-1.0 GPa and the twist angle has little effect on the anisotropy mobility. For special grain boundaries with low Σ, the boundary structures govern the GBEs and shear motion behavior significantly.

  12. Properties of He clustering in α-Fe grain boundaries

    International Nuclear Information System (INIS)

    Classical molecular dynamics and density functional theory calculations are performed to study the impact of two distinct Fe grain boundaries (GBs) on the clustering properties of helium (He) and the possible He effect on GB decohesion. Several He concentrations are considered. Common properties of He clustering are found for the both GBs, which are visibly different from the bcc bulk. In particular, He clusters in the GBs are always elongated in the directions parallel to the interface and contracted in the direction normal to the GB plane, while they are isotropic in the bcc bulk. When the He number in the clusters is sufficiently large, the strong local pressure promotes the occurrence of loop punching, which is easier to trigger in the GBs than in the bulk, resulting in a lower He-to-vacancy ratio in the GB clusters. The emitted self-interstitial atoms (SIAs) can more easily dissociate from the clusters in the GBs than in the bulk, leading to relatively lower local pressures around the clusters in the GBs, and facilitating the clusters growth. He is found to decrease GB cohesion, and the embrittling effect of He increases with its concentration. But interestingly, this effect decreases with He clustering. The present findings are fully compatible with existing experimental evidence, for instance, for a stronger GB embrittlement due to He at rather low temperatures than at higher temperatures

  13. 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.)

  14. 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.

  15. 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.

  16. 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.

  17. Direct visualization of dislocation dynamics in grain-boundary scars.

    Science.gov (United States)

    Lipowsky, Peter; Bowick, Mark J; Meinke, Jan H; Nelson, David R; Bausch, Andreas R

    2005-05-01

    Mesoscale objects with unusual structural features may serve as the analogues of atoms in the design of larger-scale materials with novel optical, electronic or mechanical behaviour. In this paper we investigate the structural features and the equilibrium dynamics of micrometre-scale spherical crystals formed by polystyrene particles adsorbed on the surface of a spherical water droplet. The ground state of sufficiently large crystals possesses finite-length grain boundaries (scars). We determine the elastic response of the crystal by measuring single-particle diffusion, and quantify the fluctuations of individual dislocations about their equilibrium positions within a scar by determining the dislocation spring constants. We observe rapid dislocation glide with fluctuations over the barriers separating one local Peierls minimum from the next and rather weak binding of dislocations to their associated scars. The long-distance (renormalized) dislocation diffusion glide constant is extracted directly from the experimental data and is found to be moderately faster than single-particle diffusion. We are also able to determine the parameters of the Peierls potential induced by the underlying crystalline lattice. PMID:15834411

  18. Atomic structures and electronic properties of phosphorene grain boundaries

    Science.gov (United States)

    Guo, Yu; Zhou, Si; Zhang, Junfeng; Bai, Yizhen; Zhao, Jijun

    2016-06-01

    Grain boundary (GB) is one main type of defects in two-dimensional (2D) crystals, and has significant impact on the physical properties of 2D materials. Phosphorene, a recently synthesized 2D semiconductor, possesses a puckered honeycomb lattice and outstanding electronic properties. It is very interesting to know the possible GBs present in this novel material, and how their properties differ from those in the other 2D materials. Based on first-principles calculations, we explore the atomic structure, thermodynamic stability, and electronic properties of phosphorene GBs. A total of 19 GBs are predicted and found to be energetically stable with formation energies much lower than those in graphene. These GBs do not severely affect the electronic properties of phosphorene: the band gap of perfect phosphorene is preserved, and the electron mobilities are only moderately reduced in these defective systems. Our theoretical results provide vital guidance for experimental tailoring the electronic properties of phosphorene as well as the device applications using phosphorene materials.

  19. Hydrogen diffusion along grain boundaries in erbium oxide coatings

    International Nuclear Information System (INIS)

    Diffusion of interstitial atomic hydrogen in erbium oxide (Er2O3) was investigated using density functional theory (DFT) and molecular dynamics (MD) methods. Hydrogen diffusivity in bulk, on (0 0 1) surface, and along Σ13 (4–3–1)/[1 1 1] symmetric tilt grain boundaries (GBs) were evaluated in a temperature range of 673–1073 K, as well as hydrogen diffusion barriers. It was found that H diffusion shows the faster on (0 0 1) surface than along GBs and in bulk. Also, energy barrier of H diffusion in bulk estimated by DFT and MD methods is somewhat higher than that along GBs evaluated in the experiments. This suggests that H diffusion in Er2O3 coatings depends on GBs rather than bulk. In addition, with a correction of GB density, the simulated diffusivity along GBs in MD simulations is in good agreement with the experimental data within one order of magnitude. The discrepancy of H diffusivity between the experiments and the simulations should be reduced by considering H concentration, H diffusion direction, deviations of the initial configuration, vacancy defects, etc

  20. 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.

  1. A phase-field model of stress effect on grain boundary migration

    International Nuclear Information System (INIS)

    We developed a phase-field model to study the stress-driven grain boundary migration in elastically inhomogeneous polycrystalline materials with arbitrary elastic inhomogeneity and anisotropy. The dependence of elastic stiffness tensor on grain orientation is taken into account, and the elastic equilibrium equation is solved using the Fourier spectral iterative-perturbation method. We studied the migration of planar and curved grain boundaries under an applied stress. The relation between grain boundary migration velocity and driving force is found to be linear in the steady-state regime. Our study shows that the stress distribution depends on the relative misorientation between the grains and the nature of the applied load. As a consequence, the mechanism of grain boundary migration is different when the load is applied parallel or perpendicular to a grain boundary. The bulk mechanical driving force for grain boundary migration is provided by the difference in the level of stress in the adjoining grains which arise due to difference in elastic moduli. We further show that under certain conditions an applied stress may act as a precursor to abnormal grain growth

  2. Theory and computerized simulation of interaction of point defects with grain boundaries

    International Nuclear Information System (INIS)

    The issued results on mathematical simulation at the atomic level of formation and migration of point defects arising under radiation (of intrinsic point defects, helium atoms) in the region of grain boundary are analyzed. Simulation data on impurity atom interaction with grain boundaries are also considered

  3. Grain boundary electrochemistry of {beta}-type Nb-Ti alloy using a scanning droplet cell

    Energy Technology Data Exchange (ETDEWEB)

    Woldemedhin, Michael Teka; Hassel, Achim Walter [Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University, Altenberger Str. 69, 4040 Linz (Austria); Max-Planck-Institut fuer Eisenforschung GmbH, Max Planck Str. 1, 40237 Duesseldorf (Germany); Raabe, Dierk [Max-Planck-Institut fuer Eisenforschung GmbH, Max Planck Str. 1, 40237 Duesseldorf (Germany)

    2011-06-15

    Localized oxide spots were grown at the grain boundaries of a technically relevant 30 at.% Nb-Ti {beta}-type titanium alloy to study the local electrochemical response. The grain boundaries selected were combinations of grains having different orientations and grain boundary angle. Crystallographic information of the grains and boundary angles were revealed by electron back scattering diffraction (EBSD) technique. Cyclic voltammetry is the electrochemical technique used to grow the oxides starting from 0 V and increasing the potential in steps of 1 V till 8 V at a scan rate of 100 mV s{sup -1} in an acetate buffer of pH 6.0. Electrochemical impedance spectroscopy was used to investigate the electrical properties of the oxide/electrolyte interface in the frequency range between 100 kHz and 100 mHz. Important oxide parameters such as formation factor and dielectric number were determined from these measurements. Significant differences were observed for different grain boundaries. The semiconducting properties of the oxides at the grain boundaries were assessed by using Mott-Schottky analysis on a potentiostatically grown oxide. All the oxides showed n-type semiconducting properties where the donor concentration varies with the grain boundaries mentioned above. A flat band potential -0.25 {+-} 0.02 V versus standard hydrogen electrode is more or less the same for all the boundaries studied. (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  4. 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.

  5. 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

  6. Critical currents across low-angle grain boundaries in YBCO; Kritische Stroeme ueber Kleinwinkelkorngrenzen in YBCO

    Energy Technology Data Exchange (ETDEWEB)

    Albrecht, J.

    2001-07-01

    The following topics were covered in this thesis: Currents in HTSC, Shubnikov phase, HTSC thin films, magneto-optical Faraday effect, current density distribution, flux density, grain boundaries in YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO), currents across grain boundaries, planar defect (WL)

  7. 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.

  8. Diffusion Growth and Morphological Stability of Two-Dimensional Grain Boundary Precipitates

    Directory of Open Access Journals (Sweden)

    A.V. Koropov

    2010-01-01

    Full Text Available Diffusion growth of two-dimensional precipitates at the grain boundary of a finite thickness was considered with due regard to the diffusion fluxes of impurity atoms from the grain volume to the grain boundary. Stability of the growing precipitates of a relatively small radius was analyzed with respect to the small random perturbations of its size and subject to the surface kinetics of impurity atoms at the grain boundary/precipitate profile. Threshold radii of the precipitates, above which the amplitude of the shape defect and the amplitude of the relative shape deformation of the precipitates rise, were found.

  9. The anatomy of grain boundaries: Their structure and atomic-level solute distribution

    International Nuclear Information System (INIS)

    The full macroscopic parameters of grain boundaries in a nanostructured ferritic alloy have been experimentally measured. A new atom-probe-tomography-based method determines the five degrees of freedom of the orientation relationship of the adjacent grains, and the local variations in the habit plane and solute excesses for tungsten and chromium across the grain boundary with a spatial resolution of up to 1 nm × 1 nm. The method also distinguishes ultrafine precipitates and ferrite–ferrite regions for a full description of the grain boundary

  10. 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.

  11. 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.

  12. Flux-line pinning by the grain boundary in niobium bicrystals

    International Nuclear Information System (INIS)

    Flux-line pinning by the grain boundary in niobium bicrystals was investigated by making four-terminal measurements of the critical current and neutron diffraction measurements of the flux-line bending. The elementary pinning interaction is estimated from the critical current data by using an approximate theory of the current distribution on the grain boundary. The data reported here are mainly for a sample in which the applied magnetic field is parallel to the [111] and the [001] crystal directions in the two grains when it is in the plane of the boundary and perpendicular to the current direction. Evidence is seen of grain boundary faceting and of a flux-flow rectification effect that peaks as a function of temperature below 30K. The scale of the grain-boundary pinning is consistent with the quasiparticle-scattering theory

  13. Synergistic effect of hydrogen and impurity segregations on the grain boundary embrittlement in Nb

    International Nuclear Information System (INIS)

    Niobium and its alloys are the candidate materials for fusion reactors and can be used at high-temperatures. This paper was intended to study embrittlement of niobium by high-concentrations of hydrogen and impurity segregation at grain boundaries. Specimens of commercial Nb were subjected to heat treatment at 1100 deg. C and 500 deg. C and subsequently charged with deuterium in an electrolytic cell. The charged specimens were placed into the high-vacuum chamber of a special self-made Auger electron spectrometer. They were then fractured under high-vacuum conditions and the chemistry of grain boundaries was analysed. Carbon and oxygen were found as the main impurities on the grain boundaries and effective energies for hydrogen-impurity-grain boundary interaction have been estimated. It was found that there is a noticeable reduction of fracture strength corresponding to the grain boundary oxygen and carbon segregation levels

  14. 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.

  15. 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

  16. Exploring grain boundary energy landscapes with the activation-relaxation technique

    OpenAIRE

    Alexander, Kathleen C.; Schuh, Christopher A

    2013-01-01

    To develop a structure–kinetic property mapping for grain boundaries requires an understanding of their energy landscapes, i.e. the energy basins and nearby saddle points separating adjacent structures. We implement the activation-relaxation technique to obtain a first view of grain boundary energy landscapes for the Σ5 (1 3 0), Σ5 (2 1 0) and Σ3 (1 1 1) grain boundaries in copper. The energy landscapes of these boundaries are very different, which supports a focus on energy landscapes, rathe...

  17. Grain orientation dependence of extended planar dislocation boundaries in rolled aluminium

    DEFF Research Database (Denmark)

    Lin, F.X.; Godfrey, A.; Winther, Grethe

    2009-01-01

    The dislocation boundary alignment in 124 grains in cold-rolled aluminium has been investigated by a combination of backscattered electron channelling contrast and electron backscatter diffraction. The asymmetric slip geometry of the Copper and S orientations together with the existence of...... different symmetry variants of these orientations is exploited to analyse whether the grain orientation (and consequently the slip systems) or the symmetric rolling geometry determines the boundary alignment. The results unambiguously point at the grain orientation as the controlling factor...

  18. 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.

  19. Grain Boundary Character Distributions in Nanocrystalline Metals Produced by Different Processing Routes

    Science.gov (United States)

    Bober, David B.; Khalajhedayati, Amirhossein; Kumar, Mukul; Rupert, Timothy J.

    2016-03-01

    Nanocrystalline materials are defined by their fine grain size, but details of the grain boundary character distribution should also be important. Grain boundary character distributions are reported for ball-milled, sputter-deposited, and electrodeposited Ni and Ni-based alloys, all with average grain sizes of ~20 nm, to study the influence of processing route. The two deposited materials had nearly identical grain boundary character distributions, both marked by a Σ3 length percentage of 23 to 25 pct. In contrast, the ball-milled material had only 3 pct Σ3-type grain boundaries and a large fraction of low-angle boundaries (16 pct), with the remainder being predominantly random high angle (73 pct). These grain boundary character measurements are connected to the physical events that control their respective processing routes. Consequences for material properties are also discussed with a focus on nanocrystalline corrosion. As a whole, the results presented here show that grain boundary character distribution, which has often been overlooked in nanocrystalline metals, can vary significantly and influence material properties in profound ways.

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

    OpenAIRE

    Nguyen, Viet Hung; Hoang, Trinh Xuan; Dollfus, Philippe; Charlier, Jean-Christophe

    2016-01-01

    As most materials available in macroscopic quantities, graphene appears in a polycrystalline form and thus contains 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 the transport properties with respect to the strain and the lattice symmetry of graphene domains on both sides of the boundary is provided. In particular, it is shown tha...

  1. 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.

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

    International Nuclear Information System (INIS)

    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)

  3. 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)

  4. 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)

  5. Study on grain boundary structure analysis and control of oxide dispersion strengthened (ODS) martensitic steel cladding

    International Nuclear Information System (INIS)

    For the purpose of improving high-temperature creep strength of Oxide Dispersion Strengthened (ODS) steel for advanced Fast Reactor (FR) cladding, grain boundary structure of ODS steel cladding was analyzed. Based on the analysis, grain boundary control procedure was discussed. The derived results can be summarized as follows: (1) By investigating the effects of several heat treatments on microstructure for enlarging grain size and reducing special coincidence grain boundary, a unique martensite microstructure composed of single variant and reduction of the special coincidence grain boundary were achieved. It was revealed that furnace-cooling heat treatment (1050degC x 1 hr, furnace-cooling (30degC/hr)) induces gamma to alpha diffusion transformation and reduction of the special coincidence grain boundary. A composite microstructure composed of ferrite phase and residual alpha-phase is effective for high-temperature strength enhancement. (2) As a result of investigation on grain boundary control using recrystallization, it was shown that the special coincidence grain boundary can be reduced by the appropriate combination of cold-rolling and recrystallization heat treatment. It would be easier to reduce grain boundary sliding in recrystallized ODS ferritic steel than in ODS martensite steel. (3) The precipitation behavior of Y2O3 particles in mechanically alloyed (MA) powder was investigated using transmission electron microscope (TEM). It was shown that few Y2O3 particles precipitate by a heat treatment up to 800degC. Combination of a heat treatment at 800degC for grain coarsening and a heat treatment at 800degC-1100degC for oxide particle precipitation would be a prospective procedure for the microstructure control to fine oxide particle dispersion in coarse grain structure, which is an ideal microstructure for creep strength enhancement. (author)

  6. 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

  7. Potential barrier height at the grain boundaries of a poly-silicon nanowire.

    Science.gov (United States)

    Shamir, Assaf; Amit, Iddo; Englander, Danny; Horvitz, Dror; Rosenwaks, Yossi

    2015-09-01

    We present measurements of the potential barrier height and its dependence on grain size in poly-silicon nanowire (P-SiNW) arrays. Measurements conducted using Kelvin probe force microscopy coupled with electrostatic simulations, enabled us also to extract the density of the grain boundary interface states and their energy distribution. In addition it was shown that the barrier height scales with the grain size as the square of the grain radius. PMID:26245190

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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.

    2016-04-01

    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.

  13. Do grain boundaries assist S diffusion in polycrystalline CdS/CdTe heterojunctions?

    International Nuclear Information System (INIS)

    We report on a transmission electron microscopy and energy-dispersive x-ray spectroscopy study of S diffusion in polycrystalline CdS/CdTe heterojunctions. We find that grain boundaries significantly assist S diffusion in the CdTe layer when the CdTe is grown without the presence of oxygen, i.e., the S diffuses more easily along the grain boundaries than in the grains. However, grain boundaries do not enhance the S diffusion in CdTe when it is grown in the presence of oxygen. The reason is likely to be the formation of Cd--O bonds at the grain boundaries, which are resistance to the S diffusion

  14. 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.

  15. Irradiation induced changes in the grain boundary chemistry of high-manganese low activation martensitic steels

    International Nuclear Information System (INIS)

    The effects of neutron irradiation (10 dpa at 638 K, FFTF/MOTA) on solute segregation to the grain boundaries in high-manganese martensitic steels were investigated using Auger electron spectroscopy (AES). The AES spectrum obtained from the grain boundaries in martensitic steels was significantly influenced by neutron irradiation. Neutron irradiation caused a marked increase in the amount of intergranular segregation of manganese in 12%Cr-6%Mn-1%W steel, while no significant increases in sulfur and/or phosphorus segregations were found in the steel. In 9%Cr-2%Mn-1%W steel, a large amount of silicon as well as a small amount of manganese segregation at grain boundaries was induced by the irradiation. The following mechanisms of irradiation induced embrittlement are proposed; (1) neutron irradiation induced intergranular segregation of manganese causes a reduction of grain boundary cohesive force, (2) intergranular segregation of silicon reduces carbon concentration at grain boundaries by a site competition mechanism resulting in the weakening of the grain boundary strength. Another possible explanation involves grain boundary precipitation. (orig.)

  16. Capabilities and limitations of analytical methods used to measure radiation-induced grain boundary segregation

    International Nuclear Information System (INIS)

    The quantitative determination of instrumental effects on the measurement of radiation-induced grain boundary segregation (RIS) in stainless steel have been irradiated with 3.4 MeV protons to 1 dpa at 400 degrees C and the resulting segregation has been measured by Auger electron spectroscopy (AES) and scanning-transmission electron microscopy using energy-dispersive x-ray spectroscopy (STEM-EDS). Depletion of chromium and enrichment of nickel and impurity elements at the grain boundaries have been observed and quantified using both techniques. Determination of true grain boundary compositions, as compared to measured compositions, has been attempted using a variety of techniques Deconvolution of the measured STEM profiles is possible, along with the effects of discrete sampling at finite step spacings render the technique incapable of a reliable determination of the true grain boundary compositions. Convolution of computer simulated segregation profiles with the beam-interaction volume to fit measured profiles provides a better method to estimate the true grain boundary concentration and segregation profile shape. Direct comparisons between the simulated STEM profiles and AES measurements show good agreement and indicate that the simulated profiles provide good estimates of the true grain boundary concentration. Use of computer codes based on the Perks model are shown to seriously overestimate the amount of RIS and the width of the segregation profiles. A calculational model for monolayer-type segregation of impurities combines the STEM and AES measurements to calculate the distribution of the impurity element at the grain boundary

  17. 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.

  18. 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

  19. 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.

  20. 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

  1. 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.

  2. 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

  3. 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.

  4. Investigation and optimization of YBa2Cu3O7-δ grain boundaries and coated conductors

    International Nuclear Information System (INIS)

    With increasing misorientation angle grain boundaries strongly reduce the critical current density of high temperature superconductors. For this reason costly techniques are used in production of modern Coated Conductors to induce sharp textures in the polycrystalline superconductor layers. In this dissertation measurements of the critical current density of different grain boundary types are presented showing that out-of-plane grain boundaries exhibit, also in applied magnetic fields, much higher critical current densities than expected. In further analysis of the grain boundaries indications for a microstructural reason of the high critical current densities were found. The high critical current densities of the out-of-plane grain boundaries should in fabrication of Coated Conductors allow for a relaxation of the out-of-plane grain alignment requirements and a concomitant cost reduction. In this work also results of a industrial cooperation with Nexans are presented demonstrating that the critical current density of metal-organic deposited grain boundaries and Coated Conductor layers can be increased by selective Calcium-doping. In the experiments selective Calcium-doping most effectively increased the critical current density of weak spots. (orig.)

  5. 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

  6. Columns formed by multiple twinning in nickel layers—An approach of grain boundary engineering by electrodeposition

    DEFF Research Database (Denmark)

    Alimadadi, Hossein; da Silva Fanta, Alice Bastos; Somers, Marcel A. J.;

    2013-01-01

    grain boundaries. A peculiar arrangement of Σ3 boundaries forming five-fold junctions is observed. The resulting microstructure meets the requirements for grain boundary engineering. Twinning induced effects on the crystallographic orientation of grains result in one major texture component being a ⟨210...

  7. 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.

  8. The equilibrium crystal shape of strontium titanate and its relationship to the grain boundary plane distribution

    International Nuclear Information System (INIS)

    In this study, the equilibrium crystal shape (ECS) of a model system, strontium titanate, is compared with the grain boundary plane distribution (GBPD) as a function of temperature. Strontium titanate has a pronounced surface energy anisotropy and a grain growth anomaly, with the grain growth rate decreasing by orders of magnitude with increasing temperature. The ECS was determined from the shape of small intragranular pores and the GBPD was determined from orientation measurements on surfaces, with the relative areas of grain boundary planes in a polycrystal correlated to the surface energy of both adjacent crystal planes. The grain boundary energy has been previously proposed to be the sum of the surface energy of the adjacent grains less a binding energy that is assumed to be constant. While much experimental evidence exists for this assumption at a fixed temperature, the influence of temperature is not known. While the anisotropy of the ECS was found to decrease with temperature, the anisotropy of the GBPD increased with temperature. These findings indicate that changes in the binding energy with temperature must be considered, as the binding energy links the surface energy to the grain boundary energy. The results are discussed with respect to the grain growth anomaly of strontium titanate, in which the grain growth decreases with increasing temperature

  9. 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.).

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. Electron irradiation induced solute segregation near grain boundaries in austenitic stainless steel

    International Nuclear Information System (INIS)

    Radiation-induced solute segregation near internal defect sinks such as high angle grain boundaries was investigated, through the interaction between point defects and solute atom in austenitic stainless steel and its model alloys. Electron irradiation was performed in a high voltage electron microscope (H V E M) at a dose rate of about 2 multiple 10-3 d p a.S-1 at a temperature range of 350-600 degree C. Solute concentration profile near grain boundaries was measured by E D X in S T E M mode. Strong enrichment and depletion of solutes were observed on grain boundaries during irradiation and segregation rate went through a maximum at 450 degree C. These facts indicate that grain boundaries act as preferential sinks for radiation-induced point defects

  15. The effect of surface contact conditions on grain boundary interdiffusion in a semi-infinite bicrystal

    Czech Academy of Sciences Publication Activity Database

    Svoboda, Jiří; Fischer, F. D.; Klinger, L.; Rabkin, E.

    2014-01-01

    Roč. 94, č. 30 (2014), s. 3398-3412. ISSN 1478-6435 Institutional support: RVO:68081723 Keywords : grain boundary diffusion * liquid metals * stress analysis * interfacial thermodynamics Subject RIV: BJ - Thermodynamics Impact factor: 1.825, year: 2014

  16. Influence of annealing on grain boundary segregation of neutron irradiated type 316L stainless steel

    International Nuclear Information System (INIS)

    Type 316L stainless steel was neutron irradiated (8x1025 n/m2, E>1MeV) and annealed at 673 K - 973 K for 1 hour. After the annealing, intergranular fracture ratio measurement by SSRT, grain boundary analysis by FE-TEM with EDS and simulation of grain boundary Cr depletion healing were performed in order to consider an effect of segregation healing on IASCC. The intergranular fracture ratio was healed to 0% by annealing above 723 K, but the healing of grain boundary Cr depletion of 723 K annealed specimen was not recognized by EDS analysis. Considering about the EDS analysis result and analysis probe diameter, concentration profiles were calculated. As a result, it was considered that grain boundary segregation which depleted to about 8 mass% by neutron irradiation, healed to about 12 mass% by annealing at 723 K, so that IASCC susceptibility was decreased. (author)

  17. 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.

  18. 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.

  19. 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

  20. Atomic simulation of grain boundary sliding in Co/Cu two-phase bicrystals

    International Nuclear Information System (INIS)

    Molecular dynamic simulations of grain boundary sliding were performed on Cu/Cu one-phase bicrystal and Co/Cu two-phase bicrystal models. The grain boundary sliding and migration behaviors of the Co/Cu bicrystals were different from those of the Cu/Cu bicrystals, and the Co/Cu sliding behavior was less related to the free volume, than that of Cu/Cu. In the Co/Cu(2 2 1) grain boundary model, the pore structure units were formed on the Cu side very near the Co/Cu interface, not just at the interface. The onset of grain boundary sliding in the Co/Cu bicrystals was due to the uncorrelated atomic shuffling induced by the pore structure units.

  1. 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

    2016-02-01

    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.

  2. Molecular dynamics simulations of grain boundary migration during recrystallization employing tilt and twist dislocation boundaries to provide the driving pressure

    International Nuclear Information System (INIS)

    Molecular dynamics simulations of grain boundary migration, where the driving pressure P is the excess stored energy due to dislocation structures, have been performed. This represents recrystallization in metals. Two types of dislocation structures have been simulated: (a) tilt dislocation boundaries, where edge dislocations are arranged as parallel arrays, (b) twist dislocation boundaries, where screw dislocations are arranged in interconnected dislocation networks. The velocity v and mobility M of the migrating grain boundaries have been calculated from the simulations. v and M are higher in twist-type simulations than in tilt-type simulations, although the activation energies are similar in the two cases. v ∼ P is observed for tilt simulations where the driving pressure is changed by varying the density of dislocation boundaries and for twist simulations where the driving pressure is changed by varying the misorientation across dislocation boundaries. When the misorientations across edge dislocation boundaries are varied, however, the simulations show v ∼ P2. It is suggested that this deviation from the usual v ∼ P-relationship is due to local interactions between the grain boundary and nearby individual dislocations. Misorientation variations across grain boundaries have also been simulated, but the mobilities show little dependence on this. The present simulations result in mobilities and activation energies that are, respectively, significantly higher and somewhat lower than experimental values. A direct mimic of experimental observations is, however not the purpose of this study. Rather the present simulations are based on idealized dislocation structures and suggest that variations in the dislocation structures may play a dominant role in recrystallization dynamics and that local effects are very important phenomena, essential for the interpretation of recrystallization mechanisms

  3. 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...

  4. A molecular dynamics study of lithium grain boundary intercalation in graphite

    International Nuclear Information System (INIS)

    Lithium-ion diffusion rates in graphitic battery anodes have been shown to vary greatly in experiments, with numerous hypotheses to explain this behavior. Here, we model several grain boundaries using molecular dynamics and quantify intercalation from the free surface. A significant variation in intercalation rates is revealed as dictated by local bond structure where the grain boundary intersects the free surface. Data presented may help explain dramatic differences in diffusion rates and permit more accurate predictions of lithium-ion battery performance

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

    OpenAIRE

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

    2015-01-01

    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, gamma-Fe and Pd. We discover the chemomechanical origin of variation of adsorption energetics for interstitial hydrogen at grain boundaries. A genera...

  6. HYDROGEN EMBRITTLEMENT IN GRAIN BOUNDARIES STUDIED BY FATIGUE CRACK PROPAGATION IN Al-Zn-Mg BICRYSTALS

    OpenAIRE

    Niegel, A.; Gudladt, H.-J.; Gerold, V.

    1988-01-01

    High cycle fatigue crack propagation in grain boundaries was studied in precipitation hardened Al-Zn-Mg specimens containing a single grain boundary (gb) perpendicular to the load axis. In peak-aged bicrystals tested in wet nitrogen atmospheres, cracks propagated in an intercrystalline manner. Under cyclic loading conditions two different mechanisms contribute to crack propagation which are termed stress corrosion cracking (SCC) and intergranular corrosion fatigue (CF). In both cases, it is w...

  7. Effect of heat treatment on grain boundary precipitate formation in Nimonic PE16

    International Nuclear Information System (INIS)

    Isothermal precipitation curves for small grain boundary precipitates in Nimonic PE16 have been determined experimentally. A theoretical model which describes the growth of grain boundary precipitates is summarised and its application specific to Nimonic PE16 is considered. The fit between experimental and theoretical isothermal precipitation curves and thus the usefulness of the model in predicting improved heat treatments for the alloy is discussed. (author)

  8. 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...

  9. An analysis of segregation-induced changes in grain boundary cohesion in bcc iron

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Šob, Mojmír

    2014-01-01

    Roč. 49, č. 6 (2014), 2477-2482. ISSN 0022-2461 R&D Projects: GA ČR GAP108/12/0144; GA AV ČR IAA100100920; GA ČR(CZ) GAP108/12/0311 Institutional support: RVO:68378271 ; RVO:68081723 Keywords : grain boundary segregation * grain boundary embrittlement * iron Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.371, year: 2014

  10. In situ observation of grain boundary migration by synchrotron radiation topography

    International Nuclear Information System (INIS)

    The application of in situ synchrotron radiation (SR) transmission topography to the study of grain boundary migration in bicrystals of an Fe-6 at.%Si alloy is described. The details on experimental arrangement of the high-temperature in situ observations of dynamic processes in material are given. The pertinence of this method is documented by comparison of the data on migration of {130} grain boundary measured this way and by optical method after heating and cooling cycles

  11. 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...

  12. Nonequilibrium grain-boundary cosegregation of nitrogen and chromium in NiCrMoV steel

    Science.gov (United States)

    Zheng, Lei; Xu, Tingdong

    2005-12-01

    It is concluded in this article that nonequilibrium grain-boundary cosegregation (NCGS) of nitrogen and chromium occurs in NiCrMoV steel. That conclusion is reached from experimental observations of the parallel segregation isotherms and the maximum coverage of Cr and N at grain boundaries during the isotherms. This means that the nonequilibrium segregation of Cr induces that of N, in NiCrMoV steel.

  13. 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.

  14. Grain boundary migration in Fe-3mass%Si alloy bicrystals under a magnetic field

    Czech Academy of Sciences Publication Activity Database

    Tsurekawa, S.; Inoue, K.; Lejček, Pavel

    2010-01-01

    Roč. 50, č. 4 (2010), 591-595. ISSN 0915-1559 R&D Projects: GA ČR GA106/08/0369 Grant ostatní: JSPS(JP) 19106013 Institutional research plan: CEZ:AV0Z10100520 Keywords : grain boundary migration * magnetic field * grain boundary character * Fe-Si alloy * bicrystals Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.742, year: 2010

  15. 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.

  16. Increase of Grain Boundary Mobility by Radiation Damage under Thermal Gradient Condition using Molecular Dynamics Simulation

    Energy Technology Data Exchange (ETDEWEB)

    Kim, Woongkee; Kaviany, Massoud Kaviany; Shim, Jihoon [Pohang University of Science and Technology, Pohang (Korea, Republic of)

    2015-05-15

    Most transformations of microstructure, such as recrystallization, grain growth are based on migration of grain boundaries. These transformations greatly influence on thermal, electrical transport and mechanical properties of materials. Material industries always set aim to design and produce decent materials by controlling microstructure evolution under various conditions. Since understanding grain boundary migration is critical for predicting the microstructural evolution in material, it has been very widely investigated for several decades. Also simple information from GB migration can give the clue to depict complicated and scale-up microstructural evolution. In this work, we investigated the grain boundary migration after the radiation damage. Without radiation damage, thermal gradient driving force was insufficient to cause the movement of grain boundary. However, with introduction of radiation damage, grain boundary showed the migration behavior while it is restored from damaged state. This is due to the fact that kinetic energy of energetic particles trigger the migration of GB by increasing temperature at GB region enormously. Rapid collision supplies the energy to exceed energy barrier to make the movement of migration As temperature of local region goes up due to radiation damage, mobility of GB should rise according to the eq.2. Therefore, radiation damage act as the trigger of local gratin boundary migration.

  17. 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.

  18. Grain size and boundary-related effects on the properties of nanocrystalline barium titanate ceramics

    Czech Academy of Sciences Publication Activity Database

    Buscaglia, V.; Buscaglia, M. T.; Viviani, M.; Mitoseriu, L.; Nanni, P.; Trefiletti, V.; Piaggio, P.; Gregora, Ivan; Ostapchuk, Tetyana; Pokorný, Jan; Petzelt, Jan

    2006-01-01

    Roč. 26, - (2006), s. 2889-2898. ISSN 0955-2219 R&D Projects: GA MŠk OC 525.20 Institutional research plan: CEZ:AV0Z10100520 Keywords : grain size * grain boundaries * spectroscopy * dielectric properties * BaTiO 3 Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.576, year: 2006

  19. 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...

  20. 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.

  1. 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.

  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. Precipitation at grain boundaries in irradiated austenitic Fe-Cr-Mn alloys

    International Nuclear Information System (INIS)

    In previous work, the phase stability of Fe-Cr-Mn alloys during irradiation was investigated in a study that included simple binaries, simple ternaries and commercially produced alloys. These low activation alloys are being considered for fusion reactor service in the first wall and in other structural applications subject to high neutron doses. In addition to phase instabilities observed within the grains, grain boundaries were susceptible to varying levels of precipitation dependent upon alloy composition, displacement dose and irradiation temperature. This paper describes the grain boundary microstructures that developed in these Fe-Cr-Mn alloys during irradiation

  4. 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.

  5. Influence of slip localization on surface relief formation and grain boundary microcrack nucleation

    International Nuclear Information System (INIS)

    Slip localization is often observed in metallic polycrystals after cyclic deformation (persistent slip bands) or pre-irradiation followed by tensile deformation (channels). To evaluate its influence on surface relief formation and grain boundary microcrack nucleation, crystalline finite element (FE) computations are carried out using microstructure inputs (slip band aspect ratio/spacing). Slip bands (low critical resolved shear stress (CRSS)) are embedded in small elastic aggregates. Slip band aspect ratio and neighboring grain orientations influence strongly the surface slips. But only a weak effect of slip band CRSS, spacing and grain boundary orientation is observed. Analytical formulae are deduced which allow an easy prediction of the surface and bulk slips. The computed slips are in agreement with experimental measures (AFM/TEM measures on pre-irradiated austenitic stainless steels and nickel, copper and precipitate-strengthened alloy subjected to cyclic loading). Grain boundary normal stresses are computed for various materials and loading conditions. A square root dependence with respect to the distance to the slip band corner is found similarly to the pile-up stress field. But the equivalent stress intensity factor is considerably lower. Analytical formulae are proposed for predicting the grain boundary normal stress field depending on the microstructure lengths. Finally, an energy balance criterion is applied using the equivalent elastic energy release rate and the surface/grain boundary energies. The predicted macroscopic stresses for microcrack nucleation are compared to the experimental ones. (authors)

  6. Effect of Grain Boundaries on Krypton Segregation Behavior in Irradiated Uranium Dioxide

    International Nuclear Information System (INIS)

    Fission products, such as krypton (Kr), are known to be insoluble within UO2, segregating towards grain boundaries, eventually leading to a lowering of the thermal conductivity and fuel swelling. Recent computational studies have identified that differences in grain boundary structure have a significant effect on the segregation behavior of fission products. However, experimental work supporting these simulations is lacking. Atom probe tomography was used to measure the Kr distribution across grain boundaries in UO2. Polycrystalline depleted-UO2 samples was irradiated with 0.7 and 1.8 MeV Kr-ions and annealed to 1000℃, 1300℃, and 1600℃ for 1 hour to produce a Kr-bubble dominated microstructure. The results of this work indicate a strong dependence of Kr concentration as a function of grain boundary structure. Temperature also influences grain boundary chemistry with greater Kr concentration evident at higher temperatures, resulting in a reduced Kr concentration in the bulk. While Kr migration is active at elevated temperatures, no changes in grain size or texture were observed in the irradiated UO2 samples.

  7. 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.

  8. 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.

  9. 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

  10. Evidence for cascade overlap and grain boundary enhanced amorphization in silicon carbide irradiated with Kr ions

    International Nuclear Information System (INIS)

    Evolution of amorphous domains in silicon carbide with 1 MeV Kr2+ irradiation is investigated using high-resolution transmission electron microscopy and simulations. An unusual morphology of highly curved crystalline/amorphous boundaries is observed in the images, which is identified as a result of cascade overlap and reproduced by a coarse-grained model informed by atomistic simulations. Comparison of local amorphization fractions near grain boundaries and within grain interiors provides experimental evidence for the interstitial starvation mechanism in SiC for the first time. As a competing effect to defect sinks, interstitial starvation increases the rate of local amorphization near grain boundaries and reduces the radiation resistance of nanocrystalline silicon carbide

  11. 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

  12. 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

  13. Influence of the grain boundary atomic structure on the intergranular precipitation

    International Nuclear Information System (INIS)

    The number of intergranular precipitates after long time annealing is calculated taking into account nucleation, growth and coarsening. With intermediate supersaturation, the great number of precipitates which is observed in some boundaries may have different causes: in low misorientation boundaries and (111) twin, the maxima come from semi-coherent nucleation with one grain; in the other boundaries, the maxima are connected with a great number of high energy atomic sites. Depending on supersaturation, some maxima may disappear whereas others are reinforced

  14. 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

  15. 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

  16. 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.

  17. 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

  18. 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).

  19. Multiscale simulation of xenon diffusion and grain boundary segregation in UO2

    International Nuclear Information System (INIS)

    In light water reactor fuel, gaseous fission products segregate to grain boundaries, resulting in the nucleation and growth of large intergranular fission gas bubbles. The segregation rate is controlled by diffusion of fission gas atoms through the grains and interaction with the boundaries. Based on the mechanisms established from earlier density functional theory (DFT) and empirical potential calculations, diffusion models for xenon (Xe), uranium (U) vacancies and U interstitials in UO2 have been derived for both intrinsic (no irradiation) and irradiation conditions. 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 (Sigma 5 tilt, Sigma 5 twist and a high angle random boundary), as derived from atomistic calculations. The present model does not attempt to capture nucleation or growth of fission gas bubbles at the grain boundaries. The point defect and Xe diffusion and segregation models are implemented in the MARMOT phase field code, which is used to calculate effective Xe and U diffusivities as well as to simulate Xe redistribution for a few simple microstructures

  20. 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.

  1. 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.

  2. 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.

  3. 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

  4. 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.

  5. 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.

  6. Atomistic simulations of the effect of helium clusters on grain boundary mobility in iron

    Science.gov (United States)

    Wicaksono, A. T.; Militzer, M.; Sinclair, C. W.

    2015-08-01

    A series of molecular dynamics simulations was performed in this work to investigate the kinetic interaction between helium clusters and grain boundaries in iron. Helium cluster formation and size distributions were found to be markedly different in the bulk compared to the region of a stationary boundary. Upon reaching a steady-state cluster distribution, the spatial fluctuation of cluster-enriched boundaries was analyzed to determine the grain boundary mobility using the random walk method. Segregated clusters reduced the boundary mobility, the drag effect of clusters increasing as the bulk solute concentration increases. The drag effect was further rationalized by employing Cahn's solute drag model using the effective binding energy of He clusters and the grain boundary diffusivity of a single He atom, their magnitudes having been determined from the segregation level and from monitoring the trajectory of a solute atom in the investigated grain boundaries, respectively. The model is found to provide a satisfactory explanation of the simulation results in the zero velocity limit.

  7. 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

  8. 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

  9. Three-dimensional digital approximations of grain boundary networks in polycrystals

    International Nuclear Information System (INIS)

    In this work, we offer a set of algorithms that convert a voxellated image to a conformal surface mesh that is targeted for polycrystalline materials containing grains with a wide range of sizes and complex shapes. More specifically, we propose a simple but effective algorithm for approximating the grain boundary networks that are implicit in three-dimensional digital images of polycrystals. The algorithm segments a three-dimensional digital image of a polycrystalline microstructure and then smoothes an interpolated conformal surface mesh of the grain boundary network while maintaining certain characteristic features of the microstructure. It is found that the proposed algorithm successfully approximates the grain boundary network based only on the digital, voxellated images of the polycrystal. Simulated microstructures are used to verify that the resulting mesh qualitatively and quantitatively approximates the true structure, in terms of the displacement of the nodes, the grain volume change and the dihedral angle distribution along triple junctions after smoothing. The effect of the use of the cubic grid for mapping digital microstructures on the grain boundary approximation is also discussed. (paper)

  10. 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.

  11. 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.

  12. 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).

  13. 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

  14. Grain boundary self-diffusion in polycrystalline nickel of different purity levels

    International Nuclear Information System (INIS)

    Grain boundary self-diffusion in Ni materials of two different purity levels (99.6 and 99.999 wt.%) was measured over wide temperature intervals using the radiotracer technique and applying the 63Ni radioisotope. The diffusion experiments were performed in both Harrison's type B and type C kinetic regimes. The diffusional grain boundary width, δ, was found to be equal to 0.54 ± 0.1 nm, that is reasonably close to the accepted value of δ=0.5nm in face-centred cubic metals. The purer the material, the higher the grain boundary diffusivity and the lower the corresponding activation enthalpy of self-diffusion along general high-angle grain boundaries. Using the semi-empirical approach of Borisov et al., the average energy of high-angle grain boundaries was estimated to be about 0.79 and 0.98 J m-2 in the low- and high-purity Ni materials at room temperature, respectively.

  15. Continuum framework for dislocation structure, energy and dynamics of dislocation arrays and low angle grain boundaries

    Science.gov (United States)

    Zhu, Xiaohong; Xiang, Yang

    2014-09-01

    We present a continuum framework for dislocation structure, energy and dynamics of dislocation arrays and low angle grain boundaries that are allowed to be nonplanar or nonequilibrium. In our continuum framework, we define a dislocation density potential function on the dislocation array surface or grain boundary to describe the orientation dependent continuous distribution of dislocations in a very simple and accurate way. The continuum formulations incorporate both the long-range dislocation interaction and the local dislocation line energy, and are derived from the discrete dislocation model. The continuum framework recovers the classical Read-Shockley energy formula when the long-range elastic fields of the low angle grain boundaries are canceled out. Applications of our continuum framework in this paper are focused on dislocation structures on static planar and nonplanar low angle grain boundaries and misfitting interfaces. We present two methods under our continuum framework for this purpose, including the method based on the Franks formula and the energy minimization method. We show that for any (planar or nonplanar) low angle grain boundary, the Franks formula holds if and only if the long-range stress field in the continuum model is canceled out, and it does not necessarily hold for a total energy minimum dislocation structure.

  16. 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.).

  17. The Structure of Grain Boundaries in Strontium Titanate: Theory, Simulation, and Electron Microscopy

    Science.gov (United States)

    von Alfthan, Sebastian; Benedek, Nicole A.; Chen, Lin; Chua, Alvin; Cockayne, David; Dudeck, Karleen J.; Elsässer, Christian; Finnis, Michael W.; Koch, Christoph T.; Rahmati, Behnaz; Rühle, Manfred; Shih, Shao-Ju; Sutton, Adrian P.

    2010-08-01

    We review a combination of theoretical and experimental techniques that have been applied to the study of grain boundaries in SrTiO3, with particular attention to Σ3 and ( 100 )-oriented grain boundaries. Electron microscopy, which includes high-resolution transmission and high-angle annular dark-field methods, is discussed, with successful applications to mapping atomic columns and testing theoretical models. Then, we compare and contrast different techniques of electron holography that may be used to map electrostatic potentials. Problems with the current methods of interpretation in holography and impedance spectroscopy are highlighted in an attempt to reconcile their respective estimates of electrostatic potentials at grain boundaries. Then, standard theoretical tools for the atomistic simulation of boundary structures are critically reviewed, which include classical potentials and density functional theory. A promising genetic algorithm for discovering low-energy grain boundary structures is described and tested. Finally, the synergy of experiment, theory, and simulation that is required to understand boundaries is demonstrated, and we identify major challenges to understanding multicomponent systems.

  18. Role of Grain Boundaries and Microstructure on the Environment Assisted Cracking of Pipeline Steels

    Science.gov (United States)

    Arafin, Muhammad

    2011-12-01

    In this research, two common types of environment assisted cracking (EAC) of pipeline steels, namely the intergranular stress corrosion cracking (IGSCC) and the hydrogen induced cracking (HIC), have been studied, and computer models have been developed to simulate the intergranular stress corrosion crack propagation behaviour in pipeline steel as well as to predict the intergranular fracture susceptibility, due to mechanical loading in non-corrosive environment, of polycrystalline materials. First, a new understanding of the IGSCC resistance of pipeline steel has been obtained by studying the grain boundary character and crystallographic orientation in both cracked and non-cracked pipeline steel samples using electron backscattered diffraction (EBSD) and X-ray texture measurements. It has been found that the low-angle and certain types of special boundaries, known as the coincident site lattice (CSL) boundaries (S5, S11, and S13b types), are crack-resistant while the random high angle boundaries are prone to cracking. However, it has been also observed that the grain boundaries associated with {110} and {111} neighbour grain orientations having and rotation axis, respectively, are crack-resistant, while the cracked boundaries are mainly linked to the {100} orientation with rotation axis. Subsequently, a novel integrated modeling approach, combining Voronoi Algorithm, Markov Chain theory, and Monte Carlo simulations, has been developed in order to predict the IGSCC behaviour of pipeline steels. The model takes both the physical microstructural features, such as the grain shape and grain size distribution, as well as the grain boundary characters and their orientations with respect to the external stress axis into account. The predicted crack propagation behaviour has been found to be in excellent agreement with the experimental crack-propagation and arrest data in API X65 pipeline steel. In addition, a texture based grain boundary character reconstruction model has

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

    DEFF Research Database (Denmark)

    Wei, Yili; Godfrey, A.; Liu, W.; Liu, Q.; Huang, Xiaoxu; Hansen, Niels; Winther, Grethe

    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 the...... boundaries investigated lie in the longitudinal plane. Practically all dislocations have screw character, with Burgers vectors corresponding to the slip systems predicted active. The dislocations arrange in a regular grid and the boundaries are most likely low-energy dislocation structures....

  20. 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

  1. Tunneling transport properties in (La,Sr)2CuO4 grain boundary Josephson junctions

    International Nuclear Information System (INIS)

    We investigate tunneling transport properties in thin film grain boundary Josephson junctions (GBJ's) of epitaxially grown (La,Sr)2CuO4 (LSCO) on bicrystal substrates. These optimally doped LSCO films were made by molecular beam epitaxy producing a very smooth film at the grain boundary. Measurements of the critical current Ic at low magnetic fields B (mT range) are used to characterize the quality of the junctions. Deviations from the ideal Ic(B) pattern enable us to indicate the homogeneity of the GBJ. Measurements of the differential conductance in high magnetic fields (T range) are used to investigate quasiparticle tunneling across the grain boundary. Results are compared to theoretical predictions

  2. 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...

  3. 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...

  4. 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

  5. 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.

  6. Low-frequency noise of dc SQUID magnetometers having slotted structure along the grain boundary junction

    International Nuclear Information System (INIS)

    In order to operate high-Tc SQUID magnetometers in an environment with no or weak magnetic shielding, we incorporated slots along the wide (3 mm) grain boundary junction in the pickup coil and the flux dam of direct-coupled SQUID magnetometers. It was confirmed that the slots formed along the wide grain boundary junction suppressed vortex motion and prevented a large increase in low-frequency noise. When we applied external fields above a threshold value of the flux dam after zero-field cooling, the flux-locked-loop output fluctuated, and the low-frequency field noise Bn increased but became stable after a time. The relaxation time after field application was longer for higher fields. A slight increase in the low-frequency noise remained, which was attributed to the fluctuation of the flux trapped in the slots having grain boundaries. (author)

  7. Grain boundary composition effects on environmentally induced cracking of engineering materials

    Energy Technology Data Exchange (ETDEWEB)

    Bruemmer, S.M. (Pacific Northwest Lab., P.O. Box 999, Richland, WA (US))

    1988-06-01

    There is now clear evidence that intergranular stress corrosion cracking (IGSCC) and hydrogen embrittlement (HE) of engineering materials depend on grain boundary composition. Two examples are used to illustrate this interrelationship: (1) stress corrosion cracking (SCC) of austenitic stainless steel (SS) in high temperature water and (2) HE of NiCrMoV rotor steels in acidic solutions. Grain boundary compositions are characterized by analytical electron microscopy and scanning Auger electron spectroscopy. Environmental cracking susceptibility is indicated by slow strain rate tests (SSRTs) in appropriate aqueous environments. Direct correlations are documented between measured grain boundary compositions and environmental cracking. The ability to establish these types of correlations enables diagnosis of embrittlement susceptibility and helps identify methods to control to eliminate embrittlement by bulk chemistry or processing modifications.

  8. 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.

  9. Grain-boundary migration in nonstoichiometric solid solutions of magnesium aluminate spinel

    Energy Technology Data Exchange (ETDEWEB)

    Chiang, Y.M.; Kingery, W.D.

    1989-02-01

    The grain-boundary mobility in magnesium aluminate spinel of magnesia-rich and alumina-rich compositions has been measured from normal grain growth in dense, hot-pressed samples. Over the temperature range 1200/sup 0/ to 1800/sup 0/C, the mobility in magnesia-rich compositions is found to be greater than that in alumina-rich compositions by a factor of 10/sup 2/ to 10/sup 3/.

  10. 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.

  11. Grain boundaries as microreactors during reactive fluid flow: experimental dolomitization of a calcite marble

    Science.gov (United States)

    Etschmann, B.; Brugger, J.; Pearce, M. A.; Ta, C.; Brautigan, D.; Jung, M.; Pring, A.

    2014-08-01

    Limestone dolomitization is an example of a fluid-induced mineralogical transformation that commonly affects extensive rock volumes. To understand the mechanisms enabling these efficient replacement reactions, we investigated experimentally the dolomitization of a fractured calcite marble under flow-through conditions at mild hydrothermal conditions. Contrary to most earlier studies of coupled dissolution reprecipitation reactions that were conducted using small, individual grains, in this study, the integrity of the rock was preserved, so that the experiment explored the links between flow in a fracture and fluid-rock interaction. In these experiments, grain boundaries acted as microreactors, in which a Mg-poor `protodolomite' formed initially, and then transformed into dolomite. The difficulty in nucleating dolomite played a key role in controlling the evolution of the porosity, by allowing for (1) initial dissolution along grain boundaries, and (2) formation of coarse porosity at the reaction interface. This porosity evolution not only enabled the reaction to progress efficiently, but also controlled the mineralogy of the system, as shown by brucite replacing calcite near the fracture once the fluid along calcite grain boundaries became sufficiently connected to the fluid flowing through the fracture. This study illustrates the role of grain boundaries, porosity evolution and nucleation in controlling reaction progress as well as the nature and textures of the products in pervasive mineralogical transformations.

  12. Numerical study of the atomic and electronic structure of some silicon grain boundaries

    International Nuclear Information System (INIS)

    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 Σ=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 Σ=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 Σ=11 [011] (2-33) interface are studied, especially their respective interfacial energies. The result disagrees with previous calculations using phenomenological potentials. (author)

  13. 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.

  14. Molecular dynamics simulation of Ga penetration along grain boundaries in Al: a dislocation climb mechanism.

    Science.gov (United States)

    Nam, Ho-Seok; Srolovitz, David J

    2007-07-13

    Many systems where a liquid metal is in contact with a polycrystalline solid exhibit deep liquid grooves where the grain boundary meets the solid-liquid interface. For example, liquid Ga quickly penetrates deep into grain boundaries in Al, leading to intergranular fracture under very small stresses. We report on a series of molecular dynamics simulations of liquid Ga in contact with an Al bicrystal. We identify the mechanism for liquid metal embrittlement, develop a new model for it, and show that is in excellent agreement with both simulation and experimental data. PMID:17678231

  15. 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.

  16. 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

    2016-03-17

    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. PMID:26948836

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

    International Nuclear Information System (INIS)

    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

  18. 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.

  19. 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

  20. Diffusion mechanism at the grain boundaries in two-dimensional metals

    International Nuclear Information System (INIS)

    Full text: The present paper is concerned with the research of diffusion mechanism at the grain boundaries in two-dimensional metals by the method of molecular dynamics. The diffusion of atoms was studied on the example of three metals: Ni, Al, Cu. The packing of two-dimensional crystals was corresponded to the plane [111] of FCC lattice. To describe interatomic interactions, pair Morse potentials were used. The metals under study were heated up to the temperatures, corresponded to 0.3-0.95 relatively to the melting temperature. Severe boundary constraints were applied on the boundaries of the calculated block. It was found, that leading diffusion mechanism at the grain boundaries of two-dimensional metals was the result of interaction of grain boundary dislocation pairs. The creep of dislocation pairs in opposite sides was observed in the experiments. The rugged atomic row of one dislocation was lengthened at the expense of shortening of the other dislocation atomic row. In this connection, the chain of the displaced atoms was observed in the directions, connecting the nuclei of two dislocations. The inverse displacements of atoms - the return of atoms in the initial positions, were observed. One of the important elements of similar mechanism is the appearance of the vacancy in the chains of the displaced atoms. The vacancy is the additional insignificant factor of diffusion. The trajectory of the vacancy migration from one nucleus to the other one can have ring reserved elements, which are the cause of the appearance of ring exchange displacements of atoms at the boundary. The research showed, that the stresses, causing the migration of the boundary, temperature and density of grain boundary dislocations influence greatly on the diffusion velocity

  1. Atomic and electronic structures of YBa2Cu3O7 [001], [010] tilt and twist grain boundaries

    International Nuclear Information System (INIS)

    We study the electronic structures of YBa2Cu3O7 (YBCO) [010]([001]) tilt and twist grain boundaries using first-principles density functional theory. The Σ3(010)/[010](90°) twist grain boundary has the lowest grain-boundary energy. Across this interface, the (001)-CuO2 layers on one side link almost perfectly to the (100)-Cu3O5 layers on the other side. To explain why the super-fluid remains almost unchanged when flowing across this kind of interface (Eom et al 1991 Nature 353 544), the (100)-Cu3O5 layers should have conductivity as good as that of the (001)-CuO2 layers. The negative grain-boundary energy between such thin YBCO [010] films means that the special one-dimensional super-lattice structure along the [010] direction is more stable than a perfect YBCO lattice. A special Σ5[001] twist grain boundary with low grain-boundary energy probably has a small barrier for super-current transport because it has the same chemical composition as that in crystal and there is minimal damage to the basic crystal structure. The Σ5[001] tilt grain boundaries and the other twist grain boundaries generally have high grain-boundary energies—more than 1.0 (J m−2). (paper)

  2. 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)

  3. Study by capacitance spectroscopy of electronic levels associated with grain boundaries in germanium

    International Nuclear Information System (INIS)

    This research thesis mainly deals with the application of capacitance spectroscopy for the measurement of interface states. Methods have been developed to determine density and cross sections of electronic capture of interface states, to detect the presence of trap levels associated with localised defects or impurities in the loading area of the grain boundary. The author presents a model of potential barrier associated with the boundary, and reports the study of boundary properties when submitted to a continuous electric voltage. Mechanisms of conduction through the barrier are discussed, and processes of capture and emissions of carriers by interface levels are addressed. A brief study of the boundary behaviour under alternative current is also reported. The next parts report the crystallographic characterization of samples, the results of electrical measurements performed on the grain boundary in conditions of permanent regime (current-voltage characteristics, capacitance-voltage curves), the study of grain boundary electrical properties in transient regime, and the application of capacitance spectroscopy to the measurement of interface states

  4. Grain boundary complexion transitions in WO3- and CuO-doped TiO2 bicrystals

    International Nuclear Information System (INIS)

    Several grain boundary complexions (grain boundary interfacial phases) have been identified in TiO2 bicrystals by high-resolution transmission electron microscopy (TEM) and aberration-corrected scanning TEM (STEM). An intrinsic grain boundary with no apparent impurity segregation was observed in an undoped TiO2 bicrystal. In a WO3-doped TiO2 bicrystal, WO3 second-phase particles formed along the boundary, with a nominally clean, intrinsic-type grain boundary in between the particles. In a CuO-doped bicrystal, a remarkable series of three distinct grain boundary complexions with abrupt structural transitions was discovered coexisting at the grain boundary, and the existence of a fourth equilibrium complexion at the annealing temperature was implied. Thus, the WO3- and CuO-doped TiO2 bicrystals exhibit dramatically different solute partitioning behavior which can be understood in terms of the relative interphase boundary energies of these two systems. STEM–electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy analysis of the nanoscale lens-shaped films of amorphous material in the CuO-doped TiO2 bicrystal demonstrated an excess of CuO, as expected, yet also revealed the unintentional presence of SiOx. The multiple grain boundary complexions in CuO-doped TiO2 offer an explanation for the CuO-enhanced grain growth and sintering of TiO2 that has been reported in the literature. Conversely, the intrinsic grain boundary complexion observed in WO3-doped TiO2 is consistent with previous work showing that WO3 has no effect on grain boundary mobility in TiO2. A phenomenological thermodynamic model is proposed to explain the physical origin of these observed grain boundary complexions and the abrupt, first-order complexion transitions that are believed to occur upon cooling of the CuO-doped TiO2 bicrystal

  5. Atomic force microscopy based approach to local impedance measurements of grain interiors and grain boundaries of sensitized AISI 304 stainless steel

    International Nuclear Information System (INIS)

    The paper presents the results of the AFM based approach to local impedance spectroscopy (LIS) measurements performed within the region of austenite grain interiors and the austenite grain boundaries for both sensitized and non-sensitized AISI 304 stainless steel (SS). The local impedance investigations in the AFM contact mode were carried out after standard tests for susceptibility to intergranular corrosion (IGC). Local impedance spectra recorded in the AFM contact mode for both sensitized and non-sensitized AISI 304 stainless steel within the region of the austenite grain interiors and the austenite grain boundaries demonstrated different shapes and characters. Performed researches revealed significant differences in the charge transfer resistance values recorded for the austenite grain interiors and the grain boundaries depending on the sensitization state. Moreover, proposed AFM based approach allows identification of the IGC process even when a double-loop electrochemical potentiokinetic reactivation (DL-EPR) method does not provide evidence of it.

  6. Simulation of grain boundary sliding based on mechanics at atomic scale

    International Nuclear Information System (INIS)

    Molecular dynamics (MD) and statics simulations are being increasingly used to model the behavior of systems at the atomic scale, where atomistic details play a critical role such as in grain boundary sliding. In the last few years atomic simulations have gone beyond the realm of understanding the effects of a few atoms to that of systems of millions of atoms. Even these simulations, which demand extensive computational resources, may not be able to adequately represent the behavior of systems of interest. This necessitates a multi-scale approach to such problems. In this paper, we present the atomistic simulation studies of grain boundary sliding of symmetric tilt grain boundaries in aluminum. The simulations are in agreement with experimental results and show a clear dependence of magnitude of sliding on grain boundary energy. Asymptotic expansion homogenization (AEH) is a mathematically rigorous approach to homogenization of periodic structures, which has been used extensively in composites and porous media. We propose a methodology to adapt AEH technique to atomic scale. Refs. 2 (author)

  7. Thermodynamic effect of elastic stress on grain boundary segregation of phosphorus in a low alloy steel

    Czech Academy of Sciences Publication Activity Database

    Zheng, L.; Lejček, Pavel; Song, S.; Schmitz, G.; Meng, Y.

    2015-01-01

    Roč. 647, Oct (2015), s. 172-178. ISSN 0925-8388 R&D Projects: GA ČR GAP108/12/0144 Institutional support: RVO:68378271 Keywords : grain boundaries * segregation * elastic stress * thermodynamics * chemical potential * molar volume Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.999, year: 2014

  8. Effect of solute interaction on interfacial and grain boundary embrittlement in binary alloys

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel

    2013-01-01

    Roč. 48, č. 6 (2013), 2574-2580. ISSN 0022-2461 R&D Projects: GA ČR GAP108/12/0144 Institutional research plan: CEZ:AV0Z10100520 Keywords : interfacial segregation * grain boundary embrittlement * binary interaction * modeling * thermodynamics Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.305, year: 2013

  9. 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.

  10. 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.; Dahmen, U.

    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...

  11. Influence of phosphorus grain boundary segregation on fracture behaviour of iron-base alloys

    Czech Academy of Sciences Publication Activity Database

    Janovec, J.; Pokluda, J.; Lejček, Pavel

    567-568, - (2007), s. 33-38. ISSN 0255-5476 Institutional research plan: CEZ:AV0Z10100520 Keywords : austenitic stainless steel * Fe-Si-P alloy * grain boundary segregation precipitation * instrumented impact test * Auger elektron spectroscopy * transmission elektron microscopy Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.399, year: 2005

  12. Analysis of grain boundary corrosion in canister material for radioactive waste using transmission electron microscope

    International Nuclear Information System (INIS)

    Canister for the processed waste is sensitive to corrosion. The grain boundary corrosion is a localized corrosion type which probably takes place on AISI 304 stainless steel canister as a result of pouring the waste glass into it. This research was aimed to study AISI 304 stainless steel as candidate material for high level waste canister. A study of Cr23C6 precipitation at the grain boundary as corrosion initiating agent had been done by observation using Transmission Electron Microscope (TEM). The experiment was carried out by heating the samples at temperature of 700oC for 2 hours followed by water quenching. It was found that the Cr23C6 precipitation occurs and the diameter of the precipitates is 0.2 μm, FCC structure with lattice parameter of 10.585 A. The precipitate was separated one another. It could be said that by the treatment mentioned above, the grain boundary corrosion was insignificant. Therefore the use of AISI 304 stainless steel as canister material candidate of high level waste will be safe from grain boundary corrosion. (author)

  13. Size and shape of grain boundary network components and their atomic structures in polycrystalline nanoscale materials

    International Nuclear Information System (INIS)

    Microstructure in polycrystalline materials is composed of grain boundary plane, triple junction line, and vertex point. They are the integral parts of the grain boundary network structure and the foundation for the structure-property relations. In polycrystalline, especially nanocrystalline, materials, it becomes increasingly difficult to probe the atomistic structure of the microstructure components directly in experiment due to the size limitation. Here, we present a numerical approach using pair correlation function from atomistic simulation to obtain the detailed information for atomic order and disorder in the grain boundary network in nanocrystalline materials. We show that the atomic structures in the different microstructural components are related closely to their geometric size and shape, leading to unique signatures for atomic structure in microstructural characterization at nanoscales. The dependence varies systematically with the characteristic dimension of the microstructural component: liquid-like disorder is found in vertex points, but a certain order persists in triple junctions and grain boundaries along the extended dimensions of these microstructure components

  14. Anisotropy of grain boundary migration observed in situ by synchrotron radiation

    Czech Academy of Sciences Publication Activity Database

    Paidar, Václav; Lejček, Pavel; Polcarová, Milena; Brádler, Jaroslav; Jacques, A.

    467-470, - (2004), s. 911-916. ISSN 0255-5476 R&D Projects: GA ČR GA202/02/0916 Institutional research plan: CEZ:AV0Z1010914 Keywords : bicrystals * grain boundary motion * synchrotron radiation * X-ray topography Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.498, year: 2004

  15. On the segregation behavior of tin and antimony at grain boundaries of polycrystalline bcc iron

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Šandera, P.; Horníková, J.; Pokluda, J.; Godec, M.

    2016-01-01

    Roč. 363, Feb (2016), 140-144. ISSN 0169-4332 R&D Projects: GA ČR GAP108/12/0144 Institutional support: RVO:68378271 Keywords : grain boundary segregation * tin * antimony * Fe based alloy * AES quantification Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.711, year: 2014

  16. 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.)

  17. 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

    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....

  18. Silicalite-1 polycrystalline layers and crystal twins: Morphology and grain boundaries

    Czech Academy of Sciences Publication Activity Database

    Brabec, Libor; Kočiřík, Milan

    2007-01-01

    Roč. 102, č. 1 (2007), s. 67-74. ISSN 0254-0584 R&D Projects: GA ČR GA203/05/0846 Institutional research plan: CEZ:AV0Z40400503 Keywords : silicalite-1 * HF acid * etching * grain boundaries Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 1.871, year: 2007

  19. Interstitial and substitutional solute segregation at individual grain boundaries of α-iron: data revisited

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Hofmann, S.

    2016-01-01

    Roč. 28, č. 6 (2016), s. 064001. ISSN 0953-8984 R&D Projects: GA ČR GAP108/12/0144 Institutional support: RVO:68378271 Keywords : anisotropy * enthalpy-entropy compensation effect * grain boundary * iron solute segregation Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.346, year: 2014

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

    International Nuclear Information System (INIS)

    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. - 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

  1. 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.

  2. Atomistic studies of grain boundaries in alloys and compounds. Progress report, July 1991--June 1992

    Energy Technology Data Exchange (ETDEWEB)

    Vitek, V.

    1992-02-01

    In this research project we carry out theoretical, computer modeling, studies of the atomic structure of grain boundaries in binary alloys. Both ordered and disordered alloys are investigated. The goal is to analyze those structural, chemical and electronic features that distinguish alloys from pure metals and are responsible for remarkably different intergranular fracture behavior of alloys when compared with pure metals. The most important phenomenon is, of course, segregation and related structural changes in the boundary region. When studying segregation phenomena copper-bismuth is a very suitable model system since bismuth segregation occurs readily, leads to boundary faceting and thus to remarkable changes in the boundary structure, as well as to a very strong embrittlement. Our recent research concentrated on the investigation of the structure of {Sigma} = 3 (111)/(11{bar 1}) facets formed during segregation from boundaries which were originally curved.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. The Effects of Heat Treatments on the M23C6 Carbide Evolution and Grain Boundary Serration in Alloy 690

    International Nuclear Information System (INIS)

    Some laboratory tests revealed that Alloy 690 is resistant to IGSCC in various environments. With a prolonged service life and improved performance being demanded by the nuclear energy industry, however, the need to improve the resistance to intergranular failure in Alloy 690 should also be considered. The present work is an attempt to elucidate the effects of various heat treatments on the evolutions of intergranular carbide precipitation and the grain boundary serration (GBS) in Alloy 690 to acquire a high resistance to intergranular degradations. By isothermal treatments at 720 .deg. C for 0.1-100 hr after solution annealing, most of the grain boundaries except for the coherent twin boundaries were decorated with well-developed M23C6 carbides. The discontinuous precipitates were initiated on the grain boundary even by a heat treatment of 0.1 hr, and covered the entire grain boundary region within a heat treatment of 10 hr. With a long aging time of 100 hr, intragranular Cr carbides were precipitated on the imperfections such as the dislocations and stacking faults GBS could be introduced by a slow cooling process in this alloy, and occurred in a limited temperature range of 990-900 .deg. C under the present heat treatment conditions. The grain boundaries had a convex shape into the incoherent grain, from which it is believed that the grain boundary shape is closely associated with the grain boundary migration during serration in this alloy

  8. Non-destructive analysis of micro texture and grain boundary character from X-ray diffraction contrast tomography

    DEFF Research Database (Denmark)

    King, A.; Herbig, M.; Ludwig, W.; Reischig, P.; Lauridsen Mejdal, Erik; Marrow, T.; Buffière, J.Y.

    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...

  9. Micro-mechanical investigation for effects of helium on grain boundary fracture of austenitic stainless steel

    International Nuclear Information System (INIS)

    Highlights: • We investigate effects of helium on grain boundary fracture of stainless steel. • We conduct micro-tensile tests on helium ion-implanted type 316 stainless steel. • Brittle fracture occur on grain boundaries on which small bubbles formed densely. • Formation of bubbles both on grain boundary and in matrix promotes brittle fracture. • Grain boundary segregated helium atoms may have a role in grain boundary fracture. - Abstract: Effects of helium (He) on grain boundary (GB) fracture of austenitic stainless steel were investigated by micro-tensile tests. Micro-bicrystal tensile specimens were fabricated for non-coincidence site lattice boundaries of He ion-irradiated 316 stainless steel by focused ion beam (FIB) micro-processing. Micro-tensile tests were conducted in a vacuum at room temperature in the FIB system. Specimens containing more than 2 at.% He fractured at GBs. The criteria for brittle fracture occurrence on GBs were: (1) He concentrations higher than 2 at.%; (2) formation of He bubbles on the GBs with less than a 5 nm spacing; and (3) matrix hardening to more than 4.6 GPa (nano-indentation hardness). The fracture stress of GB brittle fracture was lower for a specimen with higher He concentration while the size and areal density of the GB He bubbles were the same. The specimens that contained 10 at.% He and had been annealed at 923 K after irradiation fractured at the GB nominally in a brittle manner; however the inter-bubble matrix at the GB experienced ductile fracture. The annealing caused He bubbles to grow but decreased the areal density so that the spacing of the GB He bubbles widened and the hardness decreased, therefore the fracture mode changed from brittle to ductile. The findings revealed that He promotes GB fracture by weakening the GB strength and hardening the matrix due to the formation of He bubbles both on GBs and in the matrix. In addition, the findings suggested that GB segregated He atoms may have a role in GB

  10. Three-dimensional heterogenous fields in metallic multicrystals with explicit account of grain boundary effects

    International Nuclear Information System (INIS)

    Full text: A significant portion of modern material technology is concerned with materials that possess a polycrystalline structure, i.e. a collection of grains with different lattice orientations. A direct consequence of the periodic structure of the lattice is the anisotropic behavior of the single crystal and, therefore, the resulting dependence of the polycrystal properties on grain orientation, shape and distribution (texture). The resulting macroscopic properties of the material including its anisotropy are ultimately dictated by the resulting texture and other microstructural effects. A number of basic strategies have been proposed to describe the aggregate behavior of the crystallites. These theories postulate some mean-field hypothesis on the response of a collection of grain orientations associated with a continuum material point and result in some averaging procedure of the response of the single crystals. In this paper we present the evolution of the inhomogeneous features of the full 3D stress and strain fields in metallic polycrystals arising from the lattice misorientation among grains and grain boundary effects. The computation of full field solutions is especially critical for the analysis of the deformation of crystals with low symmetry. In these materials, inhomogeneity of deformation stems from an increased grain constraint effect. Effective calculations of these systems require massive parallel computation, which is accomplished by partitioning the finite element mesh so as to equidistribute the number of elements among the processors while minimizing communications. Each mesh partition is assigned to a different processor and the grains are identified with the mesh partitions. Refs. 7 (author)

  11. 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)

  12. Slip band–grain boundary interactions in commercial-purity titanium

    International Nuclear Information System (INIS)

    The interaction between slip bands and grain boundaries in commercial-purity titanium was examined using cross-correlation-based electron backscatter diffraction. At a low strain level, three types of interactions were observed: blocked slip band with stress concentration; slip transfer; and blocked slip band with no stress concentration. The stress concentration induced by the blocked slip band was fitted with Eshelby’s theoretical model, from which a Hall–Petch coefficient was deduced. It was found that the Hall–Petch coefficient varies with the individual grain boundary. We investigated the geometric alignment between the slip band and various slip systems to the neighbouring grain. Stress concentration can be induced by the blocked slip band if the slip system is poorly aligned with 〈a〉 prismatic, pyramidal or basal slip systems in the neighbouring grain. Transfer of slip across the boundary occurs when there is good alignment on 〈a〉 prismatic or 〈a〉 pyramidal slip systems. Other stress-relieving mechanisms are possible when the best alignment is not with the slip system that has the lower critical resolved shear stress

  13. 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.

  14. Diffusion of volatile organics through porous snow: impact of surface adsorption and grain boundaries

    Directory of Open Access Journals (Sweden)

    T. Bartels-Rausch

    2013-07-01

    Full Text Available Release of trace gases from surface snow on earth drives atmospheric chemistry, especially in the polar regions. The gas-phase diffusion of methanol and of acetone through the interstitial air of snow was investigated in a well-controlled laboratory study in the temperature range of 223 to 263 K. The aim of this study was to evaluate how the structure of the snowpack, the interaction of the trace gases with the snow surface, and the grain boundaries influence the diffusion on timescales up to 1 h. The diffusive loss of these two volatile organics into packed snow samples was measured using a chemical ionization mass spectrometer. The structure of the snow was analysed by means of X-ray-computed micro-tomography. The observed diffusion profiles could be well described based on gas-phase diffusion and the known structure of the snow sample at temperatures ≥ 253 K. At colder temperatures, surface interactions start to dominate the diffusive transport. Parameterizing these interactions in terms of adsorption to the solid ice surface, i.e. using temperature-dependent air–ice partitioning coefficients, better described the observed diffusion profiles than the use of air–liquid partitioning coefficients. No changes in the diffusive fluxes were observed by increasing the number of grain boundaries in the snow sample by a factor of 7, indicating that for these volatile organic trace gases, uptake into grain boundaries does not play a role on the timescale of diffusion through porous surface snow. For this, a snow sample with an artificially high amount of ice grains was produced and the grain boundary surface measured using thin sections. In conclusion, we have shown that the diffusivity can be predicted when the structure of the snowpack and the partitioning of the trace gas to solid ice is known.

  15. 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.

  16. 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.

  17. 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

  18. 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

  19. Grain boundaries and current conduction mechanism in polycrystalline silicon solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Tkachenko, N.N.

    1987-03-01

    A study is made of the electrical and photoelectrical characteristics of p-n junctions in polycrystalline silicon. A complex analysis of the volt-ampere characteristic as a function of the level of low-frequency current noise suggests that the experimentally observed tunneling of carriers through the diode potential barrier and the low-frequency noise may be related to grain boundary effects in polycrystalline silicon. A relationship is established between the open-circuit emf, short-circuit current, and grain size. An efficiency of 9 percent has been achieved with the p-n junctions studied without the use of antireflection coatings. 8 references.

  20. 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.

  1. 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.

  2. 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)

  3. 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.

  4. The real-space multiple-scattering theory and the electronic structure of grain boundaries

    International Nuclear Information System (INIS)

    This paper describes the recently developed real-space multiple-scattering theory (RSMST), which is designed for performing first-principles electronic-structure calculations of extended defects, such as surfaces and interfaces including atomic relaxations and with or without impurities, without using artificial periodic boundary conditions. The authors present the results of non-charge-self-consistent RSMST calculations of the local electronic densities of states at twist and tilt grain boundaries in fcc Cu and bcc Nb, and report on progress towards the implementation of charge self-consistency and total-energy capabilities

  5. 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.

  6. 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.

  7. 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.

  8. Properties of submicron [001] tilt symmetric and asymmetric 450 bicrystal grain boundary junctions

    International Nuclear Information System (INIS)

    We have fabricated submicron YBa2Cu3O7-x [001] tilt bicrystal grain boundary junctions by a focused ion beam process. The reduction of the junction width, leading to more homogeneous grain boundary interfaces, has been proved to be very important for the study of many fundamental properties of high critical temperature superconducting junctions. In particular, experiments on 450 symmetric junctions have shown a clear evidence of midgap states, with a 0- to π- junction transition in the case of small barrier transparency; moreover, in 450 asymmetric junctions we have observed a transition from an unconventional to a Fraunhofer-like magnetic field dependence of the critical current, sign of the reduced influence of faceting on the junction properties

  9. 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

  10. Grain boundary high-Tc dc-SQUIDs with self-organized nanocrystals

    International Nuclear Information System (INIS)

    We fabricated and investigated direct current superconducting quantum interference devices (dc-SQUIDs) based on YBa2Cu3O7-x (YBCO) grain boundary Josephson junctions. Directed embedding of gold nanoparticles different sizes can modify the crystalline structure and thus the superconducting properties of the YBCO thin films and grain boundaries. We investigated the growth conditions of these particles as well as their influence on the properties of the YBCO thin films. The variation of the size and distribution of the gold nanoparticles changes the electrical properties of the dc-SQUIDs. For this kind of device the normal resistance, critical current density, the resulting IcRN-product, the London penetration depth and transfer function are analyzed. Furthermore we show noise properties for such modified dc-SQUIDs.

  11. Effect of grain boundary trapping kinetics on diffusion in polycrystalline materials: hydrogen transport in Ni

    Science.gov (United States)

    Ilin, Dmitrii N.; Kutsenko, Anton A.; Tanguy, Dome; Olive, Jean-Marc

    2016-03-01

    Due to experimental limitations, the solute distribution in polycrystalline materials is difficult to obtain directly, especially in the vicinity of grain boundaries. Using a newly developed computational method which mixes continuum diffusion equations and atomic scale jump rates, we study the interstitial diffusion in solids containing interfaces taking into account trapping kinetics. The model is applied to hydrogen diffusion in Ni in elementary configurations: fast intergranular diffusion with no segregation (in agreement with Fisher’s model), slow intergranular diffusion with trapping, diffusion through a triple junction and solute redistribution due to stress gradients across the interface. It is shown that the classical diffusion modes can be captured and a new diffusion regime with the effect of grain boundary trapping is revealed.

  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. Atomistic features in the electrochemical potential drop across a graphene grain boundary

    International Nuclear Information System (INIS)

    A recent publication presents a new computational approach to the local electrochemical potential in the vicinity of a graphene grain boundary subject to an in-plane electric current [1]. The local electrochemical potential can be measured using scanning tunneling potentiometry, a method related to scanning tunneling microscopy. The paper predicts that atomistic features should be measurable. These features reflect the local electrochemical potential drop caused by the opaque grain boundary which is non-transparent to ballistic electrons. The paper has implications not only for scanning tunneling potentiometry, but also for Kelvin probe-force microscopy which can also measure the local electrochemical potential. In addition it could help to understand electronic transport across metallic nanocontacts. (viewpoint)

  14. 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

  15. 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.

  16. 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

  17. 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.

  18. 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.

  19. Fundamental Study of Hydrogen Segregation at Vacancy and Grain Boundary in Palladium

    OpenAIRE

    Pham, Hieu H.; Cagin, Tahir

    2015-01-01

    We have studied the fundamental process of hydrogen binding at interstitial, vacancy and grain boundary (GB) in palladium crystals using Density-Functional Theory. It showed that hydrogen prefers to occupy the octahedral interstitial site in Pd matrix, however a stable H-vacancy complex with most H occupations would contain up to eight hydrogen atoms surrounding the vacancy at tetrahedral sites. Furthermore, H presence assists the pairing or formation of nearby vacancies, which in agreement w...

  20. 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.

  1. Magnetically dead layers at sp-impurity-decorated grain boundaries and surfaces in nickel

    Czech Academy of Sciences Publication Activity Database

    Všianská, Monika; Šob, Mojmír

    2011-01-01

    Roč. 84, č. 1 (2011), Art.n. 014418. ISSN 1098-0121 R&D Projects: GA AV ČR IAA100100920; GA ČR GD106/09/H035 Institutional research plan: CEZ:AV0Z20410507 Keywords : magnetically dead layers * sp-impurity-decorated grain boundaries and surfaces * nickel Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.691, year: 2011

  2. 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...

  3. Open problems in grain boundary segregation and embrittlement: case study for nickel and bcc iron

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Šob, Mojmír; Paidar, Václav

    Boston: TMS, 2015 - (Militzer, M.; Botton, G.; Chen, L.; Howe, J.; Sinclair, C.; Zurob, H.). s. 1197-1198 ISBN 978-0-692-43736-0. [International conference on solid-solid phase transformations in inorganic materials 2015 (PTM 2015). 29.06.2015-03.07.2015, Whistler] Institutional support: RVO:68378271 ; RVO:68081723 Keywords : solute segregation * interfacial embrittlement * grain boundaries * computer modeling * experimental measurements Subject RIV: BM - Solid Matter Physics ; Magnetism

  4. Radiation-induced grain boundary segregation effect on stress corrosion cracking in steel

    International Nuclear Information System (INIS)

    Stress corrosion cracking of stainless steels 304L, 316 and 316L irradiated at 673 K up to the dose of 0.8 dpa is studied. It is shown that radiation-induced reduction of chromium content in the grain boundary results in corrosion cracking of the steels in water containing dissolved oxygen. Corrosion cracking in water with dissolved hydrogen occurs in the context of radiation-induced strengthening of the steel and change in its microstructure

  5. Radiation induced segregation at grain boundary in an austenitic stainless steel under ion irradiation

    International Nuclear Information System (INIS)

    Chromium depletion near grain boundaries of austenitic stainless steel during irradiation was investigated. Specimens were kept at 1,473 K for 30 min, and were quenched into the water. Irradiations were done using 400 keV He+ ions at 573, 673 and 773 K up to 10 dpa with a dose rate of 2.4x10-4 dpa/s. After irradiation, the Cr concentration profile near the grain boundary was measured using an analytical electron microscope with a 1 nm beam diameter. At 573 K, Cr depletion is small, and its concentration at the grain boundary decreases to 15.5 mass% at 3 dpa from the initial concentration of 18.5 mass%. At 673 and 773 K, Cr concentration at the grain boundary rapidly decreases between 0 and 0.2 dpa, and then gradually approaches a constant value, 7.0 mass% at 673 K and 5.0 mass% at 773 K. Two stages are found in radiation induced segregation (RIS) behavior, one stage in which Cr depletion and Ni enrichment balance and another in which Fe depletion and Ni enrichment balance. These experimental results were compared with the calculations based on the vacancy-induced inverse Kirkendall effect. Predicted Cr segregation at 673 and 773 K above 3 dpa agrees with the experimental results. But Cr depletions at low doses which were obtained in the experiments are much faster than calculated. At 573 K in the experiments, depletion is smaller than calculated up to 10 dpa. (author)

  6. Measurement of gap and grain-boundary inventories of 129I in used CANDU fuels

    International Nuclear Information System (INIS)

    Combined gap and grain-boundary inventories of 129I in 14 used CANDU fuel elements were measured by crushing and simultaneously leaching fuel segments for 4 h in a solution containing KI carrier. From analogy with previous work a near one-to-one correlation was anticipated between the amount of stable Xe and the amount of 128I in the combined gap and grain-boundary regions of the fuel. However, the results showed that such a correlation was only apparent for low linear power rating (LLPR) fuels with an average linear power rating of 44 kW/m), the 129I values were considerably smaller than expected. The combined gap and grain-boundary inventories of 129I in the 14 fuels tested varied from 1.8 to 11.0%, with an average value of 3.6 ± 2.4% which suggests that the average value of 8.1 ± 1% used in safety assessment calculations overestimates the instant release fraction for 129I. Segments of used CANDU fuels were leached for 92 d (samples taken at 5, 28 and 92 d) to determine the kinetics of 129I release. Results could be fitted tentatively to half-order reaction kinetics, implying that 129I release is a diffusion-controlled process for LLPR fuels, and also for HLPR fuels, once the gap inventory has been leached. However, more data are needed over longer leaching periods to gain more understanding of the processes that control grain-boundary release of 129I from used CANDU fuel

  7. Thickness of interfaces between twins, glide domains and grain boundaries in oxides from HREM studies

    International Nuclear Information System (INIS)

    High-resolution electron microscope images (lattice or atomic row images) of planar interfaces can provide a precise measurement of interface thicknesses defined as the total width of the region where the atomic arrangement is different from that in the bulk. All the interfaces that are studied (A.P.B., twin and grain boundaries) are at most a few reticular distances thick, but great differences can occur between the different cases. (author)

  8. Effect of alloying additions on the hydrogen-induced grain boundary embrittlement in iron.

    Science.gov (United States)

    Tian, Z X; Yan, J X; Hao, W; Xiao, W

    2011-01-12

    Using ab initio density functional theory calculations, we have investigated the influence of Mo, V and Pd on the H-induced grain boundary embrittlement in Fe. We find that, in the high impurity concentration systems, all of the three alloying elements facilitate H embrittlement at the Σ3 (111) [Formula: see text] grain boundary in Fe. The calculated binary effects of the H-X (X = Mo, V, Pd) couples are 0.063, 0.074 and 0.040 eV, respectively. On the other hand, in the large unit cell with low impurity concentration, both Mo and V can facilitate H embrittlement, and the binary effects of pairs are 0.152 and 0.164 eV, respectively. While Pd reduces the H embrittlement on the cohesion of the Fe grain boundary with the binary effect of - 0.1 eV. The H-X (X = Mo, V, Pd) interactions are interpreted by electronic structure analyses. PMID:21406825

  9. Effect of alloying additions on the hydrogen-induced grain boundary embrittlement in iron

    International Nuclear Information System (INIS)

    Using ab initio density functional theory calculations, we have investigated the influence of Mo, V and Pd on the H-induced grain boundary embrittlement in Fe. We find that, in the high impurity concentration systems, all of the three alloying elements facilitate H embrittlement at the Σ3 (111) [1 1-bar 0] grain boundary in Fe. The calculated binary effects of the H-X (X = Mo, V, Pd) couples are 0.063, 0.074 and 0.040 eV, respectively. On the other hand, in the large unit cell with low impurity concentration, both Mo and V can facilitate H embrittlement, and the binary effects of pairs are 0.152 and 0.164 eV, respectively. While Pd reduces the H embrittlement on the cohesion of the Fe grain boundary with the binary effect of - 0.1 eV. The H-X (X = Mo, V, Pd) interactions are interpreted by electronic structure analyses.

  10. 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.

  11. Disordering and grain boundaries of (Ni,Fe)Cr2O4 spinels from atomistic calculations

    International Nuclear Information System (INIS)

    A novel empirical potential has been developed to evaluate the thermodynamic stability of Ni1-xFexCr2O4 spinels. The simulations confirm the hypothesis that the NiCr2O4-FeCr2O4 pseudobinary has normal structure spinel up to 1000 K and stabilizes as a solid solution. However, the disordering energy (normal to inverse spinel) is found higher for FeCr2O4 than for NiCr2O4 spinel. The formation energies of tilt, twist, and random grain boundaries have been calculated in pure NiCr2O4 and FeCr2O4. The same behavior has been found for both spinels. Detail analysis of the grain boundaries structure shows that the cation coordination number is a key parameter for the stability of the grain boundaries. With this criterion, we evidenced that the structural and energetic differences are caused only by nickel and iron cations. (authors)

  12. 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.)

  13. 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.

    2016-03-01

    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.

  14. Effects of grain boundaries on superconductivity in multi-phase systems

    International Nuclear Information System (INIS)

    The experimental results presented show that there are three types of effects of grain boundaries on Tc in multi-phase systems studied. The critical current density (Jc) can raise apparently by improving the properties of the boundary between the two superconducting grains. The first type is that the grain boundary is beneficial to Tc, e.g., in the Ba-Ln-Cu-O system, Tc of the multi-phase superconductors is often found higher than that of the corresponding single phase ones. The second type shows that Tc degrades, e.g., in Ba-Sr-Y-Cu-O system. The third type is zero, as in the system with nominal composition Ba2-xCaxYCu3Oy, Tc remains 78K when x is larger than 0.25. In the system with nominal composition Ba2YCu3-xPbxOy, Tc keeps constant, but Jc increases obviously when a suitable amount of Pb is doped

  15. 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 open channel resistance in the presence of large-scale roughness. The 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.

  16. 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.

  17. 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.

  18. The structure and properties of grain boundaries in Ni3Al

    International Nuclear Information System (INIS)

    This paper reports that the mechanical behavior of polycrystalline intermetallic compounds are often strongly influenced by the properties of the interfaces present. A classic example of this is the intergranular fracture exhibited by polycrystalline Ni3Al, and the dramatic increase in ductility upon the addition of small amounts of boron. It has been proposed that boron may promote the transmission of slip across grain boundaries by inducing the formation of a local region of compositional disorder. The results of experimental efforts to characterize the effect of boron on the structure and chemistry of these grain boundaries are summarized. Recent high resolution transmission electron miroscopy studies using oriented bicrystals are described which indicate that there is no apparent change in the compositional ordering to within 0.5 nm from the boundary. These experimental results are compared with the results of atomistic calculations which have been used to examine the effect of stoichiometry and boron content on the energy and degree of ordering of these boundaries. The proposed mechanisms of intergranular fracture in Ni3Al are discussed based on the experimental and theoretical work

  19. Interstitial clusters on Σ = 11(113) grain boundary in copper: Geometric structure, stability, and ability to annihilate vacancies

    Science.gov (United States)

    Liu, Liangliang; Liu, Qin; Wang, Zhu; Tang, Zheng

    2016-02-01

    The properties of interstitial clusters on Σ = 11 (113) grain boundary (GB) in copper are studied by first-principle calculations. The results show that the interstitial atoms on GB plane energetically tend to accumulate into a cluster. The geometric structures of interstitial clusters prefer to present in a line along the direction [ 1 1 bar 0] on Σ = 11 GB plane if the grain boundary and neighbor lattices remain undamaged, and the stability of clusters increases with their elongation. The ability of interstitials in clusters to annihilate vacancies is decreased compared with isolated interstitials, however, interstitial clusters still could efficiently heal vacancies within a certain distance from grain boundary.

  20. 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

  1. 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.

  2. Multiscale Modeling of Grain Boundaries in ZrB2: Structure, Energetics, and Thermal Resistance

    Science.gov (United States)

    Lawson, John W.; Daw, Murray S.; Squire, Thomas H.; Bauschlicher, Charles W., Jr.

    2012-01-01

    A combination of ab initio, atomistic and finite element methods (FEM) were used to investigate the structures, energetics and lattice thermal conductance of grain boundaries for the ultra high temperature ceramic ZrB2. Atomic models of idealized boundaries were relaxed using density functional theory. Information about bonding across the interfaces was determined from the electron localization function. The Kapitza conductance of larger scale versions of the boundary models were computed using non-equilibrium molecular dynamics. The interfacial thermal parameters together with single crystal thermal conductivities were used as parameters in microstructural computations. FEM meshes were constructed on top of microstructural images. From these computations, the effective thermal conductivity of the polycrystalline structure was determined.

  3. 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.

  4. 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.

  5. 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

  6. 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.

  7. 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

  8. 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

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

    International Nuclear Information System (INIS)

    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

  10. 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.

  11. 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)

  12. Evolution of grain boundary conduction with increasing temperature in pure and Ti doped Co ferrite materials

    Energy Technology Data Exchange (ETDEWEB)

    Vaithyanathan, V. [Research and Development Centre, Bharathiar University, Coimbatore-641 046 (India); Patro, L. N., E-mail: laxminar@chemie.uni-marburg.de, E-mail: kkamalabharathi@gmail.com [Department of Chemistry, Philips University of Marburg, 35032, Marburg (Germany); Kodam, Ugendar [Advanced Magnetic Materials Laboratory (AMMLa), Department of Physics, Indian Institute of Technology Madras, Chennai-600 036 (India); Tan, H. [Theiss Research, La Jolla, California 92037 (United States); Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (United States); Inbanathan, S. S. R. [Post Graduate and Research Department of Physics, The American College, Madurai-625002 (India); Kamala Bharathi, K., E-mail: laxminar@chemie.uni-marburg.de, E-mail: kkamalabharathi@gmail.com [Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (United States); Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742 (United States)

    2015-09-21

    We report on the structural, temperature, and frequency dependent impedance studies of Ti doped cobalt ferrite material (CoFe{sub 1.95}Ti{sub 0.05}O{sub 4}) in comparison with the pure CoFe{sub 2}O{sub 4}. XRD and Raman spectroscopy studies confirm the inverse spinel crystallization of the materials with space group of Fd-3 m. Scanning electron microscope images shows the microcrystalline nature of the particles. Homogeneity, stoichiometry, and ionic states of the ions in the composition were confirmed by energy dispersive X-ray analysis and X-ray photoelectron spectroscopic studies. Temperature and frequency dependent real (Z′) and imaginary (Z″) part of the impedance shows the existence of relaxation processes and their distribution in CoFe{sub 2}O{sub 4} and CoFe{sub 1.95}Ti{sub 0.05}O{sub 4} materials. Complex impedance spectroscopy studies at low temperatures shows that the conductivity in these materials is predominantly due to the intrinsic bulk grains. With increasing the temperature, evolution of grain boundary conduction is clearly seen through the appearance of a second semi-circle in the complex impedance plots. Room temperature total dc conductivity of both CoFe{sub 2}O{sub 4} and CoFe{sub 1.95}Ti{sub 0.05}O{sub 4} materials is found to be 5.78 × 10{sup −8} and 1.61 × 10{sup −7} S/cm, respectively. Temperature variation of dc electrical conductivity follows the Arrhenius relationship and the activation energies for CoFe{sub 2}O{sub 4} corresponding to grain (0.55 eV for CoFe{sub 2}O{sub 4}), grain boundary (0.52 eV), and total conduction (0.54 eV) are discussed. Observation of well distinguishable grain and grain boundary conductions and the low conductivity values in CoFe{sub 2}O{sub 4} and CoFe{sub 1.95}Ti{sub 0.05}O{sub 4} materials indicates that these materials are promising candidates for the high frequency applications.

  13. Evolution of grain boundary conduction with increasing temperature in pure and Ti doped Co ferrite materials

    International Nuclear Information System (INIS)

    We report on the structural, temperature, and frequency dependent impedance studies of Ti doped cobalt ferrite material (CoFe1.95Ti0.05O4) in comparison with the pure CoFe2O4. XRD and Raman spectroscopy studies confirm the inverse spinel crystallization of the materials with space group of Fd-3 m. Scanning electron microscope images shows the microcrystalline nature of the particles. Homogeneity, stoichiometry, and ionic states of the ions in the composition were confirmed by energy dispersive X-ray analysis and X-ray photoelectron spectroscopic studies. Temperature and frequency dependent real (Z′) and imaginary (Z″) part of the impedance shows the existence of relaxation processes and their distribution in CoFe2O4 and CoFe1.95Ti0.05O4 materials. Complex impedance spectroscopy studies at low temperatures shows that the conductivity in these materials is predominantly due to the intrinsic bulk grains. With increasing the temperature, evolution of grain boundary conduction is clearly seen through the appearance of a second semi-circle in the complex impedance plots. Room temperature total dc conductivity of both CoFe2O4 and CoFe1.95Ti0.05O4 materials is found to be 5.78 × 10−8 and 1.61 × 10−7 S/cm, respectively. Temperature variation of dc electrical conductivity follows the Arrhenius relationship and the activation energies for CoFe2O4 corresponding to grain (0.55 eV for CoFe2O4), grain boundary (0.52 eV), and total conduction (0.54 eV) are discussed. Observation of well distinguishable grain and grain boundary conductions and the low conductivity values in CoFe2O4 and CoFe1.95Ti0.05O4 materials indicates that these materials are promising candidates for the high frequency applications

  14. 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

  15. The disconnection mechanism of coupled migration and shear at grain boundaries

    International Nuclear Information System (INIS)

    The mechanism of coupled migration and shear is studied in a range of [0 0 0 1] tilt boundaries in hexagonal close-packed metal using atomic-scale computer simulation. Symmetrical tilt boundaries spanning the low- and high-angle regimes and comprising regular arrays of grain boundary dislocations are simulated. For each misorientation, θ, the perfect boundary (pristine) is investigated as well as one containing a disconnection. Both types of structures are subjected to incremental applied strains to determine the stress that produces coupled migration and shear. The stress for motion in the pristine case, entailing nucleation, is higher than the Peierls stress for motion when disconnections are present. We conclude that the applied stresses in our simulations exert a Peach–Koehler force on pre-existing disconnections, thereby providing a feasible mechanism with a well-defined driving force that produces coupled migration and shear. This mechanism is feasible for the lower-angle boundaries studied, and facile for the high-angle cases.

  16. 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.

  17. 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.

  18. The grain boundary segregation of phosphorus in thermally aged and irradiated C-Mn submerged-are weld metal

    International Nuclear Information System (INIS)

    The segregation of free phosphorus atoms to grain boundaries in C-Mn steels has been identified as an embrittlement mechanism. A change in the brittle fracture mechanism from transgranular to intergranular has been observed for materials with higher phosphorus grain boundary coverage. The grain boundary segregation of phosphorus in various steels used in the nuclear power industry has been thermodynamically and kinetically modelled mostly with the Langmuir-McLean model. Recent publications have also suggested that neutron irradiation can affect segregation and various attempts at modelling this are currently under way. The present paper describes a data base assembled on phosphorus grain boundary coverage measured by Auger electron spectroscopy on thermally aged and irradiated C-Mn submerged-arc weld specimens. Software tools were developed to evaluate the changes in phosphorus grain boundary coverage associated with instantaneous temperature changes and temperature gradients. The phosphorus free energy change associated with grain boundary segregation was modelled from the thermally aged data and used with the software to determine the phosphorus segregation in submerged-arc weld metals following the post weld stress relief heat treatments received prior to plant operation. The phosphorus grain boundary coverage changes arising from the thermal history of submerged-arc weld materials during irradiation were also modelled and found to compare well with data obtained on irradiated materials. It was concluded that under the irradiation conditions sampled, phosphorus grain boundary segregation in submerged-arc weld materials can be modelled successfully using only the thermal term without appealing to an irradiation induced segregation process. (author)

  19. Mesoscopic conductance fluctuations in YBa$_2$Cu$_3$O$_{7-\\delta}$ grain boundary Junction at low temperature

    OpenAIRE

    Tagliacozzo, A.; Tafuri, F.; Gambale, E.; Jouault, B.; Born, D.; Lucignano, P.; D. Stornaiuolo; Lombardi, F.; Barone, A.; Altshuler, B. L.

    2009-01-01

    The magneto-conductance in YBCO grain boundary Josephson junctions, displays fluctuations at low temperatures of mesoscopic origin. The morphology of the junction suggests that transport occurs in narrow channels across the grain boundary line, with a large Thouless energy. Nevertheless the measured fluctuation amplitude decreases quite slowly when increasing the voltage up to values about twenty times the Thouless energy, of the order of the nominal superconducting gap. Our findings show the...

  20. 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.

  1. 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.

  2. 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.

  3. Shear-coupled grain boundary migration assisted by unusual atomic shuffling

    Science.gov (United States)

    Niu, Liang-Liang; Zhang, Ying; Shu, Xiaolin; Gao, Fei; Jin, Shuo; Zhou, Hong-Bo; Lu, Guang-Hong

    2016-03-01

    Shear-coupled grain boundary (GB) migration can be an efficacious mechanism to accommodate plastic deformation when the grain size of polycrystalline materials goes small. Nevertheless, how this kind of GB motion comes into play at the atomic level has not been fully revealed. Here, we have investigated the shear-coupled migration (SCM) of typical [100] group symmetrical tilt GBs in bcc W using atomistic simulations. Depending on GB character, the SCM is found to proceed via dislocation slipping in the or mode with striking shear strength difference between them. We demonstrate that there exists an unusual atomic shuffling along the tilt axis, which greatly assists SCM to operate in the easier mode instead of the one. The present results highlight the significant role of GB character in the atomistic SCM process and contribute to the future design and fabrication of high-performance materials in GB engineering.

  4. Direct observation of resistive heating at graphene wrinkles and grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Grosse, Kyle L. [University of Illinois Urbana-Champaign; Dorgan, Vincent E. [University of Illinois at Urbana-Champaign, Urbana-Champaign; Estrada, David [University of Illinois at Urbana-Champaign, Urbana-Champaign; Wood, Joshua D. [University of Illinois at Urbana-Champaign, Urbana-Champaign; Vlassiouk, Ivan V [ORNL; Eres, Gyula [ORNL; Lyding, Joseph W [University of Illinois at Urbana-Champaign, Urbana-Champaign; King, William P. [University of Illinois at Urbana-Champaign, Urbana-Champaign; Pop, Eric [Stanford University

    2014-01-01

    We directly measure the nanometer-scale temperature rise at wrinkles and grain boundaries (GBs) in functioning graphene devices by scanning Joule expansion microscopy with 50 nm spatial and 0.2K temperature resolution. We observe a small temperature increase at select wrinkles and a large (100 K) temperature increase at GBs between coalesced hexagonal grains. Comparisons of measurements with device simulations estimate the GB resistivity (8 150 X lm) among the lowest reported for graphene grown by chemical vapor deposition. An analytical model is developed, showing that GBs can experience highly localized resistive heating and temperature rise, most likely affecting the reliability of graphene devices. Our studies provide an unprecedented view of thermal effects surrounding nanoscale defects in nanomaterials such as graphene.

  5. Direct observation of resistive heating at graphene wrinkles and grain boundaries

    Science.gov (United States)

    Grosse, Kyle L.; Dorgan, Vincent E.; Estrada, David; Wood, Joshua D.; Vlassiouk, Ivan; Eres, Gyula; Lyding, Joseph W.; King, William P.; Pop, Eric

    2014-10-01

    We directly measure the nanometer-scale temperature rise at wrinkles and grain boundaries (GBs) in functioning graphene devices by scanning Joule expansion microscopy with ˜50 nm spatial and ˜0.2 K temperature resolution. We observe a small temperature increase at select wrinkles and a large (˜100 K) temperature increase at GBs between coalesced hexagonal grains. Comparisons of measurements with device simulations estimate the GB resistivity (8-150 Ω μm) among the lowest reported for graphene grown by chemical vapor deposition. An analytical model is developed, showing that GBs can experience highly localized resistive heating and temperature rise, most likely affecting the reliability of graphene devices. Our studies provide an unprecedented view of thermal effects surrounding nanoscale defects in nanomaterials such as graphene.

  6. 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.

  7. 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.

  8. Diffusion of volatile organics through porous snow: impact of surface adsorption and grain boundaries

    Directory of Open Access Journals (Sweden)

    T. Bartels-Rausch

    2013-03-01

    Full Text Available Release of trace gases from surface snow on Earth drives atmospheric chemistry, especially in the polar regions. The gas-phase diffusion of methanol and of acetone through the interstitial air of snow was investigated in a well-controlled laboratory study in the temperature range of 223 to 263 K. The aim of this study was to evaluate how the structure of the snowpack, the interaction of the trace gases with the snow surface, and the grain boundaries influence the diffusion on timescales up to 1 h. The diffusive loss of these two volatile organics into packed snow samples was measured using a chemical ionization mass spectrometer. The structure of the snow was analyzed by means of X-ray computed micro-tomography. The observed diffusion profiles could be well described based on gas-phase diffusion and the known structure of the snow sample at temperatures ≥ 253 K. At colder temperatures surface interactions start to dominate the diffusive transport. Parameterizing these interactions in terms of adsorption to the solid ice surface, i.e. using temperature dependent air–ice partitioning coefficients, better described the observed diffusion profiles than the use of air–liquid partitioning coefficients. No changes in the diffusive fluxes were observed by increasing the number of grain boundaries in the snow sample by a factor of 7, indicating that for these volatile organic trace gases, uptake into grain boundaries does not play a role on the timescale of diffusion through porous surface snow. In conclusion, we have shown that the diffusivity can be predicted when the structure of the snowpack and the partitioning of the trace gas to solid ice is known.

  9. 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

  10. 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.

  11. 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.

  12. 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...... 72 GHz 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...

  13. 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.

  14. 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...

  15. 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...

  16. 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.

  17. Suppressed grain-boundary scattering in atomic layer deposited Nb:TiO2 thin films

    Science.gov (United States)

    Niemelä, Janne-Petteri; Hirose, Yasushi; Shigematsu, Kei; Sano, Masahito; Hasegawa, Tetsuya; Karppinen, Maarit

    2015-11-01

    We have fabricated high-quality thin films of the transparent conducting anatase Nb:TiO2 on glass substrates through atomic layer deposition, and a subsequent reductive heat treatment of the as-deposited amorphous films. Hall-effect measurements and Drude-fitting of the Vis-NIR spectra indicate that for lightly doped films deposited at temperatures around 170 °C, grain boundary scattering becomes negligible and the mobility is predominately limited by phonon-electron scattering inherent to the anatase lattice and by impurities. Simultaneously, such lighter doping leads to reduced plasma absorption, thereby improving material's performance as a transparent conductor.

  18. Grain boundary character distribution and mechanical properties of austenitic stainless steels

    International Nuclear Information System (INIS)

    The effect of grain boundaries character distributions (GBCDs) on high temperature creep and yield stress at room and high temperatures has been studied on AISI types 316 and 304 stainless stells. Different microstructural states were obtained in these steels by means of different thermo-mechanical treatments. It was established that mechanical properties depend significantly upon the microstructural state. The analysis of influences of different microstructural factors on deformation behaviour of the studied steels confirms an important role of the GBCD in the mechanical properties. (orig.)

  19. Structure and magnetism of clean and impurity-decorated grain boundaries in nickel from first principles

    Czech Academy of Sciences Publication Activity Database

    Všianská, Monika; Šob, Mojmír

    Brno : Mendelova zemědělská a lesnická univerzita, 2011 - (Trnková, L.), s. 74-77 ISBN 978-80-7375-514-0. [Pracovní setkání fyzikálních chemiků a elektrochemiků /11./. Brno (CZ), 01.06.2011-02.06.2011] R&D Projects: GA AV ČR IAA100100920; GA ČR GD106/09/H035 Institutional research plan: CEZ:AV0Z20410507 Keywords : grain boundary * magnetism * segregation * nikel Subject RIV: BM - Solid Matter Physics ; Magnetism

  20. 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.

  1. SQUID magnetometer with YBa2Cu3O7 grain boundary step contacts

    International Nuclear Information System (INIS)

    The report describes the assembly and characterisation of SQUID magnetometers consisting of a SQUID and a coupled superconducting flux antenna. Two concepts for coupling of the antenna were implemented: First, a directly coupled magnetometer in which the flux antenna is produced parallel to the SQUID, i.e. only a single YBCO layer is required; secondly, a flip-chip magnetometer where the flux antenna is produced on a different substrate and is coupled to the SQUID inductively. The SQUIDs were produced on the basis of grain boundary step contacts

  2. Why calculated energies of grain boundary segregation are unreliable when segregant solubility is low

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Šob, Mojmír; Paidar, Václav; Vítek, V.

    2013-01-01

    Roč. 68, č. 6 (2013), s. 547-550. ISSN 1359-6462 R&D Projects: GA ČR GAP108/12/0144; GA ČR(CZ) GAP108/12/0311; GA AV ČR IAA100100920 Grant ostatní: CEITEC(XE) CZ.1.05/1.1.00/02.0068 Institutional support: RVO:68378271 ; RVO:68081723 Keywords : Interface segregation * grain boundary embrittlement * solid solubilitys * DFT Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.968, year: 2013

  3. 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

  4. 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.

  5. Production of a grain boundary phase as conducting pathway in insulating AlN ceramics

    International Nuclear Information System (INIS)

    We report that it is possible to render aluminum nitride (AlN) ceramics electrically conductive without losing their intrinsic high thermal conductivity by precipitating a yttrium oxycarbide grain boundary phase, the volume of which is about 1/20 that of the conducting phase in conventional ceramic composites. The electrical conductivity can be controlled by varying the sintering conditions and additive composition. In addition, the electrically conductive AlN can be shaped by electric discharge machining and converted back into insulating AlN by anodic oxidation

  6. Triple junction and grain boundary diffusion in the Ni/Cu system

    International Nuclear Information System (INIS)

    Atomic transport along triple junctions (TJs) and grain boundaries (GBs) has been measured by atom probe tomography in nanocrystalline Ni/Cu bilayers. Heat treatment was chosen in the kinetic C-B regime according to generalized Harrison categories for the hierarchy of volume, GB and TJ transport. Diffusion coefficients at 623 K were found to be 4.65 x 10-20 and 1.65 x 10-17 m2 s-1 for the GB and TJ, respectively. Thus, TJs represent a significantly faster diffusion route than GBs.

  7. 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.

  8. 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)

  9. 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

  10. 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

  11. 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.

  12. 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

  13. 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.

  14. Existence of nontrivial topologically protected states at grain boundaries in bilayer graphene: signatures and electrical switching.

    Science.gov (United States)

    Jaskólski, W; Pelc, M; Chico, Leonor; Ayuela, A

    2016-03-10

    Recent experiments [L. Ju, et al., Nature, 2015, 520, 650] confirm the existence of gapless states at domain walls created in gated bilayer graphene, when the sublattice stacking is changed from AB to BA. These states are significant because they are topologically protected, valley-polarized and give rise to conductance along the domain wall. Current theoretical models predict the appearance of such states only at domain walls, which preserve the sublattice order. Here we show that the appearance of the topologically protected states in stacking domain walls can be much more common in bilayer graphene, since they can also emerge in unexpected geometries, e.g., at grain boundaries with atomic-scale topological defects. We focus on a bilayer system in which one of the layers contains a line of octagon-double pentagon defects that mix graphene sublattices. We demonstrate that gap states are preserved even with pentagonal defects. Remarkably, unlike previous predictions, the number of gap states changes by inverting the gate polarization, yielding an asymmetric conductance along the grain boundary under gate reversal. This effect, linked to defect states, should be detectable in transport measurements and could be exploited in electrical switches. PMID:26931739

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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

  20. Submicron YBa2Cu3O7-x bicrystal grain boundary junctions by focused ion beam

    International Nuclear Information System (INIS)

    Submicron YBa2Cu3O7-x bicrystal grain boundary junctions have been fabricated, for the first time, by a focused ion beam process. Although such a process has always been considered detrimental to the YBa2Cu3O7-x because of gallium contamination, high quality 24 deg. [001] tilt junctions characterized by RSJ current-voltage characteristics, ICRN products of the order of 1-4 x 104 A cm-2 at 77 K and Fraunhofer-like modulation patterns have been obtained. No significant degradation has been observed over more than 3 months. The critical current density JC and the characteristic voltage ICRN show a clear maximum for widths of the order of the Josephson penetration depth. The asymptotic normal resistance shows a typical (width)-1 dependence, indicating that the FIB process does not increase the grain boundary resistivity of submicron junctions. Experimental results clearly show that FIB is a very powerful tool for the fabrication of high critical temperature superconducting circuits, requiring a small number of submicron Josephson junctions, and for fundamental physics analysis. It also allow the final tuning or repair of superconducting or more complex integrated superconducting-semiconducting devices

  1. 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

  2. Existence of nontrivial topologically protected states at grain boundaries in bilayer graphene: signatures and electrical switching

    Science.gov (United States)

    Jaskólski, W.; Pelc, M.; Chico, Leonor; Ayuela, A.

    2016-03-01

    Recent experiments [L. Ju, et al., Nature, 2015, 520, 650] confirm the existence of gapless states at domain walls created in gated bilayer graphene, when the sublattice stacking is changed from AB to BA. These states are significant because they are topologically protected, valley-polarized and give rise to conductance along the domain wall. Current theoretical models predict the appearance of such states only at domain walls, which preserve the sublattice order. Here we show that the appearance of the topologically protected states in stacking domain walls can be much more common in bilayer graphene, since they can also emerge in unexpected geometries, e.g., at grain boundaries with atomic-scale topological defects. We focus on a bilayer system in which one of the layers contains a line of octagon-double pentagon defects that mix graphene sublattices. We demonstrate that gap states are preserved even with pentagonal defects. Remarkably, unlike previous predictions, the number of gap states changes by inverting the gate polarization, yielding an asymmetric conductance along the grain boundary under gate reversal. This effect, linked to defect states, should be detectable in transport measurements and could be exploited in electrical switches.

  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. 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

  5. Faceted shell structure in grain boundary diffusion-processed sintered Nd–Fe–B magnets

    International Nuclear Information System (INIS)

    Graphical abstract: The grain boundary diffusion process (GBDP) using a heavy rare earth elements (HRE) such as Dy and Tb is known as an effective method to enhance the coercivity of Nd–Fe–B sintered magnets without reducing remanence. This process has been industrially implemented to manufacture Nd–Fe–B based sintered magnets with high coercivity and high remanence. In this process, Dy is considered to diffuse through grain boundaries (GBs) to form (Nd1−xDyx)2Fe14B shells surrounding the Nd2Fe14B grains and the higher anisotropy field of the Dy-rich shell is considered to suppress the nucleation of reverse domains at low magnetic field. Although there are several investigations on the microstructure of HRE GBDP Nd–Fe–B magnets, no paper addressed the origin of the asymmetric formation of HRE rich shells. Based on detailed analysis of facet planes of core/shell interfaces, we propose a mechanism of the faceted core/shell microstructure formation in the GBDP sintered magnets. We believe that this gives new insights on understanding the coercivity enhancement by the GBDP. - Highlights: • Faceting was observed at the interfaces of cores and shells. • The core/shell interfaces are sharp with an abrupt change in Dy concentration. • Meting occurs at the interfaces of metalic Nd-rich/Nd2Fe14B phases above 685 °C due to eutectic reaction. • Solidification of Dy-enriched liquid phase from 900 °C can result in the shell formation. - Abstract: Dysprosium enriched shell structure formed by the grain boundary diffusion process (GBDP) of a sintered Nd–Fe–B magnet was characterized by using scanning electron microscopy, electron back-scattered diffraction and transmission electron microscopy. Faceted core–shell interfaces with an abrupt change in Dy concentration suggest the Dy-rich shells are formed by the solidification of the liquid phase during cooling from the GBDP temperature. The Nd-rich phases are almost free from Dy, and their quantity near

  6. 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.

  7. Influence of thermal barrier effect of grain boundaries on bulk cascades in alpha-zirconium revealed by molecular dynamics simulation

    Science.gov (United States)

    Jin, Yanan; Lai, Wensheng

    2016-03-01

    The effect of grain boundaries (GBs) on bulk cascades in nano-structured alpha-zirconium has been studied by molecular dynamics (MD) simulations. It turns out that the existence of GBs increases the defect productivity in grains, suggesting that the GBs may act as a thermal barrier and postpone the annihilation of defects within grains. Moreover, it is found that the thermal barrier effect of GBs facilitates the shift of symmetric tilt GBs to the grain with higher temperature, and the smaller the tilt angle is, the easier the boundary shift will be. Thus, the influence of GBs on radiation damage in the nano-structured materials comes from the competition between damage increase in grains and defect annihilation at GBs.

  8. 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.

  9. 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.)

  10. Carbide precipitation, grain boundary segregation, and temper embrittlement in NiCrMoV rotor steels

    Science.gov (United States)

    Bandyopadhyay, N.; Briant, C. L.; Hall, E. L.

    1985-05-01

    This paper presents a study of carbide precipitation, grain boundary segregation, and temper embrittlement in NiCrMoV rotor steels. One of the steels was high purity, one was doped with phosphorus, one was doped with tin, and one was commercial purity. In addition, two NiCrV steels, one high purity and one doped with phosphorus, were examined. Carbide precipitation was studied with analytical electron microscopy. It was found that after one hour of tempering at 600 ‡C only M3C carbides were precipitated in the NiCrMoV steels. These were very rich in iron. As the tempering time increased, the chromium content of the M3C carbides increased significantly, but their size did not change. Chromium rich M7C3 precipitates began to form after 20 hours of tempering, and after 50 hours of tempering Mo-rich M2C carbides were precipitated. Also, after 100 hours of tempering, the matrix formed bands rich in M3C or M7C3 and M2C particles. Tempering occurred more rapidly in the NiCrV steels. Grain boundary segregation was studied with Auger electron spectroscopy. It was found that the amount of phosphorus and tin segregation that occurred during a step-cooling heat treatment after tempering was less if a short time tempering treatment had been used. It will be proposed that this result occurs because the low temperature tempering treatments leave more carbon in the matrix. Carbon then compctes with phosphorus and tin for sites at grain boundaries. This compctition appears to affect phosphorus segregation more than tin segregation. In addition to these two impurity elements, molybdenum and nickel segregated during low temperature aging. The presence of molybdenum in the steel did not appear to affect phosphorus segregation. Finally, it will be shown that all of the steels that contain phosphorus and/or tin exhibit some degree of temper embrittlement when they are aged at 520 ‡C or are given a step-cooling heat treatment. Of the NiCrMoV steels, the phosphorus-doped steel showed

  11. 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

  12. A two-dimensional study of coupled grain boundary motion using the level set method

    International Nuclear Information System (INIS)

    The coupled motion of a closed non-circular grain boundary (GB) in a bicrystal, with both isotropic and anisotropic GB energies, is studied using the level set method. The kinetic relations, obtained within the framework of linear irreversible thermodynamics, govern the overall dynamics, including normal motion (migration) of the GB, viscous sliding along the GB and tangential motion of the grains which is geometrically coupled with the migration. The shape accommodation necessary to maintain coherency of relatively rotating and non-deforming grains is accomplished by allowing for diffusion along the GB. We solve the governing equations for the coupled motion in order to determine the shape and the misorientation evolution of an isolated GB under various constitutive assumptions. First, assuming both GB energy and kinetic coefficients to be isotropic, we study the interplay between kinetic coefficients for initially circular, near-circular and non-circular GBs, as well as the impact of stress and initial conditions on the GB dynamics. Next, we study the influence of anisotropy in the GB energy, mobility and geometric coupling for various combinations of parameters and initial conditions. Allowing for geometric coupling can in fact lead to shapes distinctly different to those that are usually predicted on the basis of migration alone. Our numerical scheme provides a general framework in which to study these and other related problems of GB motion. (paper)

  13. 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

  14. 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).

  15. 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.

  16. 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

  17. 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

  18. 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

  19. 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).

  20. HREM investigation of the structure of the Σ5(310)/[001] symmetric tilt grain boundaries in Nb

    International Nuclear Information System (INIS)

    This paper reports on atomistic simulations using interatomic potentials for Nb developed employing the embedded atom method (EAM) and the model generalized pseudopotential theory (MGPT) that have indicated a possible cusp at the Σ5 (310) orientation in the energy vs tilt angle curves for left-angle 001 right-angle symmetric tilt grain boundaries. In addition, the most stable structure predicted using EAM exhibits shifts of one crystal relative to the other along the tilt axis and along the direction perpendicular to the tilt axis lying in the boundary plane. The structure predicted using the MGPT was mirror symmetric across the plane of the grain boundary. This boundary has been prepared for experimental study using the ultra high vacuum diffusion bonding method. A segment of this boundary has been studied using high resolution electron microscopy