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Sample records for induced grain boundary

  1. 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.

  2. Anisotropic Radiation-Induced Segregation in 316L Austenitic Stainless Steel with Grain Boundary Character

    Energy Technology Data Exchange (ETDEWEB)

    Christopher M. Barr; Gregory A. Vetterick; Kinga A. Unocic; Khalid Hattar; Xian-Ming Bai; Mitra L. Taheri

    2014-04-01

    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 __3 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.

  3. 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.

  4. 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.

  5. Impact of Surface Chemistry on Grain Boundary Induced Intrinsic Stress Evolution during Polycrystalline Thin Film Growth

    Science.gov (United States)

    Qi, Y.; Sheldon, B. W.; Guo, H.; Xiao, X.; Kothari, A. K.

    2009-02-01

    First principles calculations were integrated with cohesive zone and growth chemistry models to demonstrate that adsorbed species can significantly alter stresses associated with grain boundary formation during polycrystalline film growth. Using diamond growth as an example, the results show that lower substrate temperatures increase the hydrogen content at the surface, which reduces tensile stress, widens the grain boundary separations, and permits additional atom insertions that can induce compressive stress. More generally, this work demonstrates that surface heteroatoms can lead to behavior which is not readily described by existing models of intrinsic stress evolution.

  6. Photoconductivity induced by nanoparticle segregated grain-boundary in spark plasma sintered BiFeO3

    Science.gov (United States)

    Nandy, Subhajit; Mocherla, Pavana S. V.; Sudakar, C.

    2017-05-01

    Photoconductivity studies on spark plasma sintered BiFeO3 samples with two contrasting morphologies, viz., nanoparticle-segregated grain boundary (BFO-AP) and clean grain boundary (BFO-AA), show that their photo-response is largely influenced by the grain boundary defects. Impedance analyses at 300 K and 573 K clearly demarcate the contributions from grain, grain-boundary, and the nanoparticle-segregated grain-boundary conductivities. I-V characteristics under 1 sun illumination show one order of higher conductivity for BFO-AP, whereas conductivity decreases for BFO-AA sample. Larger photocurrent in BFO-AP is attributed to the extra conduction path provided by oxygen vacancies on the nanoparticle surfaces residing at the grain boundaries. Creation of photo-induced traps under illumination and the absence of surface conduction channels in BFO-AA are surmised to result in a decreased conductivity on illumination.

  7. 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.

  8. Theoretical progress in non-equilibrium grain-boundary segregation (Ⅰ): Thermally induced non-equilibrium grain-boundary segregation and intergranular embrittlement

    Institute of Scientific and Technical Information of China (English)

    2009-01-01

    The research progress of non-equilibrium grain-boundary segregation theories in the last 20 years is reviewed. Based on studies by the present authors, the critical time of non-equilibrium segregation and its impact on the development of non-equilibrium segregation theories are described. Quasi- thermo- dynamics and kinetics for thermal non-equilibrium grain boundary segregation are detailed along with a non-equilibrium grain boundary cosegregation model. The experimental validation of the theories and their application to the reversible temper embrittlement of steels and the intermediate temperature brittleness in metals and alloys are also addressed.

  9. Radiation-induced segregation and corrosion behavior on Σ3 coincidence site lattice and random grain boundaries in proton-irradiated type-316L austenitic stainless steel

    Energy Technology Data Exchange (ETDEWEB)

    Sakaguchi, N., E-mail: sakaguchi@eng.hokudai.ac.jp [Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Hokkaido (Japan); Endo, M.; Watanabe, S. [Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Hokkaido (Japan); Kinoshita, H. [Fukushima National College of Technology, Iwaki 970-8034, Fukushima (Japan); Yamashita, S. [Fuels and Materials Department, O-arai Research and Development Center, Japan Atomic Energy Agency, Ibaraki 311-1393 (Japan); Kokawa, H. [Graduate School of Engineering, Tohoku University, Sendai 980-8579 (Japan)

    2013-03-15

    The behavior of radiation-induced segregation (RIS) and intergranular corrosion at random grain boundaries and Σ3 coincidence site lattice (CSL) boundaries in proton-irradiated 316L stainless steel was examined. The frequency of the CSL boundaries was enhanced up to 86.6% by grain boundary engineering treatment prior to irradiation. Significant nickel enrichment and chromium depletion were induced at the random grain boundary owing to the RIS. At faceted Σ3 CSL boundaries, chromium depletion occurred at the asymmetrical boundary facet plane whereas no RIS was observed at the coherent twin boundary. After the electrochemical etching test, an intergranular corrosion groove was found along the random grain boundaries because of the low chromium concentration (∼12%) at the boundaries. At the faceted Σ3 CSL boundaries, the discontinuous groove along the asymmetric facet plane was completely disrupted by the non-corrosive coherent twin boundary.

  10. 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.

  11. Texture, local misorientation, grain boundary and recrystallization fraction in pipeline steels related to hydrogen induced cracking

    Energy Technology Data Exchange (ETDEWEB)

    Mohtadi-Bonab, M.A., E-mail: m.mohtadi@usask.ca; Eskandari, M.; Szpunar, J.A.

    2015-01-03

    In the present study, API X60 and X60SS pipeline steels were cathodically charged by hydrogen for 8 h using 0.2 M sulfuric acid and 3 g/l ammonium thiocyanate. After charging, SEM observations showed that the hydrogen induced cracking (HIC) appeared at the center of cross section in the X60 specimen. However, HIC did not appear in the X60SS steel. Therefore, electron backscatter diffraction (EBSD) technique was used to analyze the center of cross section of as-received X60SS, X60 and HIC tested X60 specimens. The results showed that the HIC crack not only can propagate through 〈100〉||ND oriented grains but also its growth may happen in various orientations. In HIC tested X60 specimen, an accumulation of low angle grain boundaries around the crack path documented that full recrystallization was not achieved during hot rolling. Kernel Average Misorientaion (KAM) histogram illustrated that the deformation is more concentrated in as-received and HIC tested X60 specimens rather than in as-received X60SS specimen. Moreover, the concentration of coincidence site lattice (CSL) boundary in HIC tested X60 specimen was very low compared with other samples. The recrystallization area fraction in X60SS steel was very high. This high amount of recrystallization fraction with no stored energy is one of the main reasons for high HIC resistance of this steel to HIC. The orientation distribution function (ODF) of the recrystallized, substructured and deformed fractions in as-received X60SS and X60 steel showed relative close orientations in both as-received specimens.

  12. Grain Boundary Induced Bias Instability in Soluble Acene-Based Thin-Film Transistors

    Science.gov (United States)

    Nguyen, Ky V.; Payne, Marcia M.; Anthony, John E.; Lee, Jung Hun; Song, Eunjoo; Kang, Boseok; Cho, Kilwon; Lee, Wi Hyoung

    2016-09-01

    Since the grain boundaries (GBs) within the semiconductor layer of organic field-effect transistors (OFETs) have a strong influence on device performance, a substantial number of studies have been devoted to controlling the crystallization characteristics of organic semiconductors. We studied the intrinsic effects of GBs within 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT) thin films on the electrical properties of OFETs. The GB density was easily changed by controlling nulceation event in TES-ADT thin films. When the mixing time was increased, the number of aggregates in as-spun TES-ADT thin films were increased and subsequent exposure of the films to 1,2-dichloroethane vapor led to a significant increase in the number of nuleation sites, thereby increasing the GB density of TES-ADT spherulites. The density of GBs strongly influences the angular spread and crystallographic orientation of TES-ADT spherulites. Accordingly, the FETs with higher GB densities showed much poorer electrical characteristics than devices with lower GB density. Especially, GBs provide charge trapping sites which are responsible for bias-stress driven electrical instability. Dielectric surface treatment with a polystyrene brush layer clarified the GB-induced charge trapping by reducing charge trapping at the semiconductor-dielectric interface. Our study provides an understanding on GB induced bias instability for the development of high performance OFETs.

  13. Recursive Determination for Effect of Alloying Element on Impurity Induced Low Angle Grain Boundary Embrittlement

    Institute of Scientific and Technical Information of China (English)

    2001-01-01

    The effect of alloying element (Ni, Co, Mn) on P doped Fe 5.3° low angle grain boundary (GB) embrittlement was investigated by the Recursion method. The model of dislocations was used to construct the atomic structure for the P doped GB. The result indicated that the role of impurity and alloying element segregation to GB can be studied with BOI and the difference between their segregation energies at GB and at free surface (FS) (ΔE=Egbseg-Efsseg). The BOI results showed that P leads the “loosening” of the 5.3° low angle GB and decreases the cohesion strength of P doped GB when the alloying element (Ni, Co, or Mn) is added into the P doped 5.3° low angle GB. The ΔE value reveals that the alloying element Ni, Co and Mn have higher energy at P doped 5.3° low angle GB, indicating it serves as a GB embrittler. The BOI results and ΔE calculation were comparable with each other, and they are also consistent with the experimental results, which confirm the embrittling effect of alloying element (Ni, Co, Mn) on P-induced GB embrittlement.

  14. Defect induced d{sup 0} ferromagnetism in a ZnO grain boundary

    Energy Technology Data Exchange (ETDEWEB)

    Assa Aravindh, Sasikala Devi; Schwingenschloegl, Udo; Roqan, Iman S, E-mail: iman.roqan@kaust.edu.sa [Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 2955-6900 (Saudi Arabia)

    2015-12-14

    Several experimental studies have referred to the grain boundary (GB) defect as the origin of ferromagnetism in zinc oxide (ZnO). However, the mechanism of this hypothesis has never been confirmed. Present study investigates the atomic structure and the effect of point defects in a ZnO GB using the generalized gradient approximation+U approximation. The relaxed GB possesses large periodicity and channels with 8 and 10 numbered atoms having 4 and 3 fold coordination. The Zn vacancy (V{sub Zn}) shows a tendency to be attracted to the GB, relative to the bulk-like region. Although no magnetization is obtained from point defect-free GB, V{sub Zn} induces spin polarization as large as 0.68 μ{sub B}/atom to the O sites at the GB. Ferromagnetic exchange energy >150 eV is obtained by increasing the concentration of V{sub Zn} and by the injection of holes into the system. Electronic structure analysis indicates that the spin polarization without external dopants originates from the O 2p orbitals, a common feature of d{sup 0} semiconductors.

  15. Defect induced d0 ferromagnetism in a ZnO grain boundary

    KAUST Repository

    Devi, Assa Aravindh Sasikala

    2015-12-08

    Several experimental studies have referred to the grain boundary(GB) defect as the origin of ferromagnetism in zinc oxide (ZnO). However, the mechanism of this hypothesis has never been confirmed. Present study investigates the atomic structure and the effect of point defects in a ZnOGB using the generalized gradient approximation+U approximation. The relaxed GB possesses large periodicity and channels with 8 and 10 numbered atoms having 4 and 3 fold coordination. The Znvacancy (VZn) shows a tendency to be attracted to the GB, relative to the bulk-like region. Although no magnetization is obtained from point defect-free GB, VZn induces spin polarization as large as 0.68 μB/atom to the O sites at the GB.Ferromagnetic exchange energy >150 eV is obtained by increasing the concentration of VZn and by the injection of holes into the system. Electronic structure analysis indicates that the spin polarization without external dopants originates from the O 2p orbitals, a common feature of d0semiconductors.

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

    Science.gov (United States)

    Zientarski, Tomasz; Chocyk, Dariusz

    2016-10-01

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

  17. 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.

  18. Grain Boundary Induced Bias Instability in Soluble Acene-Based Thin-Film Transistors

    OpenAIRE

    Ky V. Nguyen; Payne, Marcia M.; Anthony, John E.; Jung Hun Lee; Eunjoo Song; Boseok Kang; Kilwon Cho; Wi Hyoung Lee

    2016-01-01

    Since the grain boundaries (GBs) within the semiconductor layer of organic field-effect transistors (OFETs) have a strong influence on device performance, a substantial number of studies have been devoted to controlling the crystallization characteristics of organic semiconductors. We studied the intrinsic effects of GBs within 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT) thin films on the electrical properties of OFETs. The GB density was easily changed by controlling nulceatio...

  19. Grain Boundary Energies in Copper.

    Science.gov (United States)

    Omar, Ramli

    Available from UMI in association with The British Library. Requires signed TDF. The dependence of grain boundary energy on boundary orientation was studied in copper annealed at 1000 ^circC. Grain boundary orientations and the disorientations across the boundaries were measured. A rotation matrix notation is used to interpret selected area electron channelling patterns observed in a scanning electron microscope. The Herring and Shewmon torque terms were investigated using wire specimens having a "bamboo" structure. The Herring torque terms were determined using the Hess relation. The (110) section of the Sigma 11 gamma-plot (i.e. the variation of grain boundary energy with boundary orientation) was evaluated. In this plot, minima in energies were found at the (311) and (332) mirror planes. Sigma 3 and Sigma9 boundaries were investigated in sheet specimens. The (110) and (111) sections of the Sigma3 gamma -plot were evaluated. In addition to the sharp cusps occurring at the Sigma3 {111} planes, the further shallower cusps occur at the incoherent Sigma 3 boundaries with the interfacial planes approximately parallel to {322} in one crystal and {11.44} in the other crystal. Flat and curved Sigma9 boundaries were investigated. The break up of Sigma9 boundaries into two Sigma3 boundaries and the relation between the Sigma3 and Sigma 9 gamma-plots was also examined. The (110) section of the Sigma9 gamma-plot was constructed.

  20. "Atomic Force Masking" Induced Formation of Effective Hot Spots along Grain Boundaries of Metal Thin Films.

    Science.gov (United States)

    Kim, Kwang Hyun; Chae, Soo Sang; Jang, Seunghun; Choi, Won Jin; Chang, Hyunju; Lee, Jeong-O; Lee, Tae Il

    2016-11-30

    We present an interesting phenomenon, "atomic force masking", which is the deposition of a few-nanometer-thick gold film on ultrathin low-molecular-weight (LMW) polydimethylsiloxane (PDMS) engineered on a polycrystalline gold thin film, and demonstrated the formation of hot spot based on SERS. The essential principle of this atomic force masking phenomenon is that an LMW PDMS layer on a single crystalline grain of gold thin film would repel gold atoms approaching this region during a second cycle of evaporation, whereas new nucleation and growth of gold atoms would occur on LMW PDMS deposited on grain boundary regions. The nanostructure formed by the atomic force masking, denoted here as "hot spots on grain boundaries" (HOGs), which is consistent with finite-difference time-domain (FDTD) simulation, and the mechanism of atomic force masking were investigated by carrying out systematic experiments, and density functional theory (DFT) calculations were made to carefully explain the related fundamental physics. Also, to highlight the manufacturing advantages of the proposed method, we demonstrated the simple synthesis of a flexible HOG SERS, and we used this substrate in a swabbing test to detect a common pesticide placed on the surface of an apple.

  1. Identical mechanism of isochronal and isothermal embrittlement in Ni(Bi) alloy: Thermo-induced non-equilibrium grain-boundary segregation of Bi

    Energy Technology Data Exchange (ETDEWEB)

    Zheng, Lei, E-mail: zhenglei_ustb@sina.com [School of Materials Science and Engineering, University of Science and Technology Beijing, 100083 Beijing (China); Institute of Materials Physics, University of Muenster, 48149 Muenster (Germany); Chellali, Reda; Schlesiger, Ralf [Institute of Materials Physics, University of Muenster, 48149 Muenster (Germany); Meng, Ye [School of Materials Science and Engineering, University of Science and Technology Beijing, 100083 Beijing (China); Baither, Dietmar; Schmitz, Guido [Institute of Materials Physics, University of Muenster, 48149 Muenster (Germany)

    2015-05-15

    Highlights: • Both isochronal and isothermal plasticity of Ni(Bi) alloy show minima. • Existing interpretations for isochronal and isothermal embrittlement are inadequate. • Both embrittlement is caused by thermo-induced non-equilibrium grain-boundary segregation of Bi. - Abstract: Isochronal and isothermal plasticity after thermal pre-treatments are obtained by tensile tests to characterize the embrittling behaviors of Ni(Bi) alloy. Both isochronal and isothermal plasticity show evident minima. Fractography observed by scanning electron microscopy displays intergranular fracture for samples of low plasticity. The microstructure is found to be free of precipitates within grains and at grain boundaries by focused ion beam and transmission electron microscopy. Atom probe analysis indicates a strong tendency of Bi segregation to grain boundaries. By these results, the existing interpretations are discussed to be inadequate and both embrittlement are confirmed to be identical in mechanism, i.e. thermo-induced non-equilibrium grain-boundary segregation of Bi.

  2. Applied Thermodynamics: Grain Boundary Segregation

    Directory of Open Access Journals (Sweden)

    Pavel Lejček

    2014-03-01

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

  3. Numerical simulation modeling on the effects of grain boundary misorientation on radiation-induced solute segregation in 304 austenitic stainless steels

    Energy Technology Data Exchange (ETDEWEB)

    Duh, T.S.; Kai, J.J. E-mail: jjkai@ess.nthu.edu.tw; Chen, F.R.; Wang, L.H

    2001-04-01

    The purpose of this study is to develop a model to describe the effects of the grain boundary misorientation on the radiation-induced solute segregation (RIS) in 304 stainless steels. A simple rate equation model with modified boundary conditions, which included the fluxes of defects diffusing along the grain boundaries to the grain boundary dislocations, was developed for RIS at boundaries with different {sigma} values. The results of the model calculations were compared to the experimental results previously reported by us. It was found that the model could clearly predict the same trends that the Cr depletion levels at special boundaries in irradiated 304 stainless steels were increasing with {sigma}. The model calculations also showed that the widths of the segregation cusps was decreasing with increasing {sigma}.

  4. Reaction-induced grain boundary cracking and anisotropic fluid flow during prograde devolatilization reactions within subduction zones

    Science.gov (United States)

    Okamoto, Atsushi; Shimizu, Hiroyuki; Fukuda, Jun-ichi; Muto, Jun; Okudaira, Takamoto

    2017-09-01

    Devolatilization reactions during prograde metamorphism are a key control on the fluid distribution within subduction zones. Garnets in Mn-rich quartz schist within the Sanbagawa metamorphic belt of Japan are characterized by skeletal structures containing abundant quartz inclusions. Each quartz inclusion was angular-shaped, and showed random crystallographic orientations, suggesting that these quartz inclusions were trapped via grain boundary cracking during garnet growth. Such skeletal garnet within the quartz schist formed related to decarbonation reactions with a positive total volume change (Δ V t > 0), whereas the euhedral garnet within the pelitic schists formed as a result of dehydration reaction with negative Δ V t values. Coupled hydrological-chemical-mechanical processes during metamorphic devolatilization reactions were investigated by a distinct element method (DEM) numerical simulation on a foliated rock that contained reactive minerals and non-reactive matrix minerals. Negative Δ V t reactions cause a decrease in fluid pressure and do not produce fractures within the matrix. In contrast, a fluid pressure increase by positive Δ V t reactions results in hydrofracturing of the matrix. This fracturing preferentially occurs along grain boundaries and causes episodic fluid pulses associated with the development of the fracture network. The precipitation of garnet within grain boundary fractures could explain the formation of the skeletal garnet. Our DEM model also suggests a strong influence of reaction-induced fracturing on anisotropic fluid flow, meaning that dominant fluid flow directions could easily change in response to changes in stress configuration and the magnitude of differential stress during prograde metamorphism within a subduction zone.

  5. Determination of grain boundary geometry using TEM

    NARCIS (Netherlands)

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

    1992-01-01

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

  6. Determination of grain boundary geometry using TEM

    NARCIS (Netherlands)

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

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

  7. Theoretical prosress in non-equilibrium 8rain-boundary segregation (I): Thermally induced non-equilibrium grain-boundary segregation and intersranular embrittlement

    Institute of Scientific and Technical Information of China (English)

    XU TingDong; WANG Kai; SONG ShenHua

    2009-01-01

    The research progress of non-equilibrium grain-boundary segregaUon theories in the last 20 years is reviewed. Based on studies by the present authors, the critical time of non-equilibrium segregation and its impact on the development of non-equilibrium segregation theories are described. Quasi-thermodynamics and kinetics for thermal non-equilibrium grain boundary segregation are detailed along with a non-equilibrium grain boundary cosegregation model. The experimental validation of the theories and their application to the reversible temper embrittlement of steels and the intermediate temperature brittleness in metals and alloys are also addressed.

  8. Grain Boundary Engineering of Electrodeposited Thin Films

    DEFF Research Database (Denmark)

    Alimadadi, Hossein

    of the favorable boundaries that break the network of general grain boundaries. Successful dedicated synthesis of a textured nickel film fulfilling the requirements of grain boundary engineered materials, suggests improved boundary specific properties. However, the textured nickel film shows fairly low......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 not yet well-understood. This, at least partly, owes to the lack of robust characterization methods for analyzing the nature of grain boundaries including the grain boundary plane characteristics, until recently. In the past decade, significant improvements in the 2-dimensional and 3-dimensional analysis...

  9. A new approach to grain boundary engineering for nanocrystalline materials

    Directory of Open Access Journals (Sweden)

    Shigeaki Kobayashi

    2016-11-01

    Full Text Available A new approach to grain boundary engineering (GBE for high performance nanocrystalline materials, especially those produced by electrodeposition and sputtering, is discussed on the basis of some important findings from recently available results on GBE for nanocrystalline materials. In order to optimize their utility, the beneficial effects of grain boundary microstructures have been seriously considered according to the almost established approach to GBE. This approach has been increasingly recognized for the development of high performance nanocrystalline materials with an extremely high density of grain boundaries and triple junctions. The effectiveness of precisely controlled grain boundary microstructures (quantitatively characterized by the grain boundary character distribution (GBCD and grain boundary connectivity associated with triple junctions has been revealed for recent achievements in the enhancement of grain boundary strengthening, hardness, and the control of segregation-induced intergranular brittleness and intergranular fatigue fracture in electrodeposited nickel and nickel alloys with initial submicrometer-grained structure. A new approach to GBE based on fractal analysis of grain boundary connectivity is proposed to produce high performance nanocrystalline or submicrometer-grained materials with desirable mechanical properties such as enhanced fracture resistance. Finally, the potential power of GBE is demonstrated for high performance functional materials like gold thin films through precise control of electrical resistance based on the fractal analysis of the grain boundary microstructure.

  10. A new approach to grain boundary engineering for nanocrystalline materials.

    Science.gov (United States)

    Kobayashi, Shigeaki; Tsurekawa, Sadahiro; Watanabe, Tadao

    2016-01-01

    A new approach to grain boundary engineering (GBE) for high performance nanocrystalline materials, especially those produced by electrodeposition and sputtering, is discussed on the basis of some important findings from recently available results on GBE for nanocrystalline materials. In order to optimize their utility, the beneficial effects of grain boundary microstructures have been seriously considered according to the almost established approach to GBE. This approach has been increasingly recognized for the development of high performance nanocrystalline materials with an extremely high density of grain boundaries and triple junctions. The effectiveness of precisely controlled grain boundary microstructures (quantitatively characterized by the grain boundary character distribution (GBCD) and grain boundary connectivity associated with triple junctions) has been revealed for recent achievements in the enhancement of grain boundary strengthening, hardness, and the control of segregation-induced intergranular brittleness and intergranular fatigue fracture in electrodeposited nickel and nickel alloys with initial submicrometer-grained structure. A new approach to GBE based on fractal analysis of grain boundary connectivity is proposed to produce high performance nanocrystalline or submicrometer-grained materials with desirable mechanical properties such as enhanced fracture resistance. Finally, the potential power of GBE is demonstrated for high performance functional materials like gold thin films through precise control of electrical resistance based on the fractal analysis of the grain boundary microstructure.

  11. Direct imaging of grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Gronsky, R.

    1979-09-01

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

  12. Determination of grain boundary geometry using TEM

    Energy Technology Data Exchange (ETDEWEB)

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

    1992-07-01

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

  13. Defect-solute interactions near irradiation grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Simonen, E.P.; Vetrano, J.S.; Heinisch, H.L.; Bruemmer, S.M.

    1993-11-01

    Defect-solute interactions control radiation-induced segregation (RIS) to interfacial sinks, such as grain boundaries, in metallic materials. The best studied system in this regard has been austenitic stainless steels. Measurements of grain boundary composition indicate that RIS of major alloying elements are in reasonable agreement with inverse-Kirkendall predictions. The steep and narrow composition profiles are shown to result from limited back diffusion near the boundary. Subsequently, defect-solute interactions that affect the near boundary defect concentrations strongly affect RIS. The variability in measured RIS may in part be caused by grain boundary characteristics.

  14. A Toolbox for Geometric Grain Boundary Characterization

    Science.gov (United States)

    Glowinski, Krzysztof; Morawiec, Adam

    Properties of polycrystalline materials are affected by grain boundary networks. The most basic aspect of boundary analysis is boundary geometry. This paper describes a package of computer programs for geometric boundary characterization based on macroscopic boundary parameters. The program allows for determination whether a boundary can be classified as near-tilt, -twist, -symmetric et cetera. Since calculations on experimental, i.e., error affected data are assumed, the program also provides distances to the nearest geometrically characteristic boundaries. The software has a number of other functions helpful in grain boundary analysis. One of them is the determination of planes of all characteristic boundaries for a given misorientation. The resulting diagrams of geometrically characteristic boundaries can be linked to experimentally determined grain boundary distributions. In computations, all symmetrically equivalent representations of boundaries are taken into account. Cubic and hexagonal holohedral crystal symmetries are allowed.

  15. 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.

  16. Grain boundaries in silicon solar cells

    Science.gov (United States)

    Kazmerski, L. L.; Russell, P. E.; Ireland, P. J.; Herrington, C. R.; Dick, J. R.; Matson, R. J.; Jones, K. M.

    The correlations between the electrical and compositional properties of grain boundaries in polycrystalline Si are examined in detail. High-resolution surface analysis techniques (AES, SIMS, XPS, EELS) and microelectrical (SAM, EBIC, minority-carrier lifetime) characterization methods are used. The direct evidence for impurity segregation to the intergrain regions is presented. Effect of illumination on the grain boundary electrical characteristics are correlated with impurity compositions. Finally, the interrelationships among heat-treatment, oxygen segregation and grain boundary electrical activity are discussed.

  17. Effect of Carbon on Grain Boundary Segregation of Phosphorus and Phosphorus-Induced Intergranular Fracture in High Purity Iron with Phosphorus

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    The segregation of carbon at grain boundaries and its effect on phosphorus segregation and phosphorus-induced intergranular fracture in the alloy of high purity iron with phosphorus were investigated by scanning Auger electron spectroscopy, impact test and SEM observation of fractured surface. The experimental results showed that the carbon segregation at grain boundaries decreases the phosphorus segregation, and a change of fracture mode from intergranular fracture to transgranular one, hence a decrease of ductile-brittle transition temperature is observed. The mechanism of these effects was discussed.

  18. Grain boundary wetness of partially molten dunite

    Science.gov (United States)

    Mu, S.; Faul, U.

    2013-12-01

    The grain scale melt distribution plays a key role for physical properties of partially molten regions in Earth's upper mantle, but our current understanding of the distribution of basaltic melt at the grain scale is still incomplete. A recent experimental study shows that wetted two-grain boundaries are a common feature of partially molten dunite at small melt fractions (Garapic et al., G3, 2013). In early ideal models which assume isotropic surface energy, the grain scale melt distribution is uniquely determined by knowing the melt fraction and the dihedral angle between two crystalline grains and the melt (von Bargen and Waff, JGR, 1986). Olivine is anisotropic in surface energy, hence the grain scale melt distribution at given melt fraction cannot be characterized by the dihedral angle alone. The grain boundary wetness, which is defined as the ratio of solid-liquid boundary area over the total interfacial area (Takei, JGR, 1998), is a more objective measure of the grain scale melt distribution. The aim of this study is to quantify the relationship between grain size, melt fraction, temperature and grain boundary wetness of partially molten dunite under dry conditions. We annealed olivine-basalt aggregates with melt fractions from 0.03% to 6% at a range of temperatures and 1 GPa in a piston cylinder for 1 to 336 hours, with resulting mean grain sizes of 10 to 60 μm. The samples were sectioned, polished and imaged at high resolution by using a field emission SEM. Each image had a size of 2048 x 1536 pixels with a resolution of 0.014 to 0.029 μm/pixel, depending on magnification. For each sample, depending on grain sizes, we made mosaics of 3 x 3 or 6 x 6 overlapping images. Measurements of melt fraction, grain boundary wetness and grain size were carried out on these high resolution mosaics by using ImageJ software. Analyses of mosaics show that grain boundary wetness increases with increasing melt fraction at constant grain size to values well above those

  19. Grain rotation mediated by grain boundary dislocations in nanocrystalline platinum

    Science.gov (United States)

    Wang, Lihua; Teng, Jiao; Liu, Pan; Hirata, Akihiko; Ma, En; Zhang, Ze; Chen, Mingwei; Han, Xiaodong

    2014-01-01

    Grain rotation is a well-known phenomenon during high (homologous) temperature deformation and recrystallization of polycrystalline materials. In recent years, grain rotation has also been proposed as a plasticity mechanism at low temperatures (for example, room temperature for metals), especially for nanocrystalline grains with diameter d less than ~15 nm. Here, in tensile-loaded Pt thin films under a high-resolution transmission electron microscope, we show that the plasticity mechanism transitions from cross-grain dislocation glide in larger grains (d>6 nm) to a mode of coordinated rotation of multiple grains for grains with d<6 nm. The mechanism underlying the grain rotation is dislocation climb at the grain boundary, rather than grain boundary sliding or diffusional creep. Our atomic-scale images demonstrate directly that the evolution of the misorientation angle between neighbouring grains can be quantitatively accounted for by the change of the Frank–Bilby dislocation content in the grain boundary. PMID:25030380

  20. Grain boundaries in high temperature superconductors

    NARCIS (Netherlands)

    Hilgenkamp, Johannes W.M.; Mannhart, J.

    2002-01-01

    Since the first days of high-Tc superconductivity, the materials science and the physics of grain boundaries in superconducting compounds have developed into fascinating fields of research. Unique electronic properties, different from those of the grain boundaries in conventional metallic

  1. Grain boundaries in high-Tc superconductors

    NARCIS (Netherlands)

    Hilgenkamp, H.; Mannhart, J.

    2002-01-01

    Since the first days of high-Tc superconductivity, the materials science and the physics of grain boundaries in superconducting compounds have developed into fascinating fields of research. Unique electronic properties, different from those of the grain boundaries in conventional metallic supercondu

  2. The Role of Grain Boundary Energy on Grain Boundary Complexion Transitions

    Energy Technology Data Exchange (ETDEWEB)

    Bojarski, Stephanie A. [Carnegie Mellon Univ., Pittsburgh, PA (United States). Dept. of Materials Science and Engineering; Rohrer, Gregory S. [Carnegie Mellon Univ., Pittsburgh, PA (United States). Dept. of Materials Science and Engineering

    2014-09-01

    Grain boundary complexions are distinct equilibrium structures and compositions of a grain boundary and complexion transformations are transition from a metastable to an equilibrium complexion at a specific thermodynamic and geometric conditions. Previous work indicates that, in the case of doped alumina, a complexion transition that increased the mobility of transformed boundaries and resulted in abnormal grain growth also caused a decrease in the mean relative grain boundary energy as well as an increase in the anisotropy of the grain boundary character distribution (GBCD). The current work will investigate the hypothesis that the rates of complexion transitions that result in abnormal grain growth (AGG) depend on grain boundary character and energy. Furthermore, the current work expands upon this understanding and tests the hypothesis that it is possible to control when and where a complexion transition occurs by controlling the local grain boundary energy distribution.

  3. Grain boundary-induced variability of charge transport in hydrogenated polycrystalline graphene

    DEFF Research Database (Denmark)

    Barrios-Vargas, Jose E.; Falkenberg, Jesper Toft; Soriano, David

    2017-01-01

    Chemical functionalization has proven to be a promising means of tailoring the unique properties of graphene. For example, hydrogenation can yield a variety of interesting effects, including a metal-insulator transition or the formation of localized magnetic moments. Meanwhile, graphene grown...... by chemical vapor deposition is the most suitable for large-scale production, but the resulting material tends to be polycrystalline. Up to now there has been relatively little focus on how chemical functionalization, and hydrogenation in particular, impacts the properties of polycrystalline graphene....... These findings suggest a tunability of electrical transport of polycrystalline graphene through selective hydrogen functionalization, and also have implications for hydrogen-induced magnetization and spin lifetime of this material....

  4. Grain Boundary (GB) Studies in Nano- and Micro- Crystalline Materials

    OpenAIRE

    Tanju, Mst Sohanazaman

    2011-01-01

    Polycrystalline materials are composed of grains and grain boundaries. The total volume of occupied grain boundaries in polycrystalline material depends on the grain size. When grain size decreases the volume fraction of grain boundaries increases. For example, when grain size is 10 nm grain boundary volume fraction is ~ 25%. In polycrystalline materials, different properties (mechanical, electrical, optical, magnetic) are affected by the size of their grains and by the atomic structure of...

  5. Grain boundary corrosion of copper canister material

    Energy Technology Data Exchange (ETDEWEB)

    Fennell, P.A.H.; Graham, A.J.; Smart, N.R.; Sofield, C.J. [AEA Technology plc, Harwell (United Kingdom)

    2001-03-01

    The proposed design for a final repository for spent fuel and other long-lived residues in Sweden is based on the multi-barrier principle. The waste will be encapsulated in sealed cylindrical canisters, which will then be placed in granite bedrock and surrounded by compacted bentonite clay. The canister design is based on a thick cast inner container fitted inside a corrosion-resistant copper canister. During fabrication of the outer copper canisters there will be some unavoidable grain growth in the welded areas. As grains grow they will tend to concentrate impurities within the copper at the new grain boundaries. The work described in this report was undertaken to determine whether there is any possibility of enhanced corrosion at grain boundaries within the copper canister. The potential for grain boundary corrosion was investigated by exposing copper specimens, which had undergone different heat treatments and hence had different grain sizes, to aerated artificial bentonite-equilibrated groundwater with two concentrations of chloride, for increasing periods of time. The degree of grain boundary corrosion was determined by atomic force microscopy (AFM) and optical microscopy. AFM showed no increase in grain boundary 'ditching' for low chloride groundwater. In high chloride groundwater the surface was covered uniformly with a fine-grained oxide. No increases in oxide thickness were observed. No significant grain boundary attack was observed using optical microscopy either. The work suggests that in aerated artificial groundwaters containing chloride ions, grain boundary corrosion of copper is unlikely to adversely affect SKB's copper canisters.

  6. YSZ thin films with minimized grain boundary resistivity

    Energy Technology Data Exchange (ETDEWEB)

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

    2016-03-31

    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 Mg2+ 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.

  7. 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.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    1994-07-01

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

  9. Structure of grain boundaries in hexagonal materials

    CERN Document Server

    Sarrazit, F

    1998-01-01

    which allows the behaviour of line-defects to be studied in complex interfacial processes. The work presented in this thesis describes experimental and theoretical aspects associated with the structure of grain boundaries in hexagonal materials. It has been found useful to classify grain boundaries as low-angle, special or general on the basis of their structure. High-angle grain boundaries were investigated in tungsten carbide (WC) using conventional electron microscopy techniques, and three examples characteristic of the interfaces observed in this material were studied extensively. Three-dimensionally periodic patterns are proposed as plausible reference configurations, and the Burgers vectors of observed interfacial dislocations were predicted using a theory developed recently. The comparison of experimental observations with theoretical predictions proved to be difficult as contrast simulation techniques require further development for analysis to be completed confidently. Another part of this work invol...

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

    Science.gov (United States)

    Hugo, Richard Charles

    1998-12-01

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

  11. 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...

  12. Wetting Effects at a Grain Boundary

    Science.gov (United States)

    Abraham, D. B.; Mustonen, Ville; Wood, A. J.

    2004-08-01

    We consider a tier of weakened bonds along the center line of a two-dimensional Ising ferromagnet strip as a model of a grain boundary. When an interface traverses such a strip at an angle, whether or not there is a continuous pinning-depinning transition at subcritical temperature depends on this angle and the degree of bond weakening. We also study the relaxation of such a system to its equilibrium state using continuous time Monte Carlo simulation with Kawasaki dynamics; this reveals a matter transport mechanism confined to the grain boundary.

  13. Geometry and crystallographic configuration of grain boundaries

    Science.gov (United States)

    Eichler, Jan; Weikusat, Ilka; Kipfstuhl, Sepp; Binder, Tobias

    2015-04-01

    Ice cores provide a unique opportunity to study fundamental mechanisms which control the internal flow of ice sheets. Different kinds of deformation processes acting on the micro-scale are responsible for the viscoplastic behavior on large scale. Careful interpretation of microstructural features such as grain size, shape, lattice orientation and the occurrence of subgrain boundaries can help us to follow these processes and to improve our understanding of ice rheology. Polarized light microscopy experienced a quick development in the last decade. A new generation of automatic fabric analyzers enables to measure c-axis orientations in µm-resolution. This high amount and quality of fabric data motivates to apply digital-image-processing routines (DIP) for the recognition and quantification of microstructural patterns. Here we present a study on grain boundaries based on the acquisition of more than 700 fabric images recorded along the NEEM ice core (Greenland). Geometrical characteristics of grain boundaries are studied as well as their cross-sectional orientations in relation to the c-axis orientations of the corresponding adjacent grains. We could follow the evolution from the initial N-type and P-type low-angle boundaries (Weikusat et al., 2011) to high angle boundaries during rotation recrystallization. In agreement with some previous studies we confirm that the established three-stage-recrystallization model may be an oversimplification. According to our results, rotation recrystallization as well as grain boundary migration are actually present in all depths with varying intensities at NEEM. I. Weikusat, A. Miyamoto, S. H. Faria, S. Kipfstuhl, N. Azuma, and T. Hondoh: Subgrain boundaries in Antarctic ice quantified by X-ray Laue diffraction. J. Glaciol., 57(201):85-94, 2011. doi: 10013/epic.36402.

  14. Precipitation Behavior of Ferrite in Grain and in Grain Boundary Induced by Deformation%普通低碳钢形变诱导铁素体晶内和晶界的析出行为

    Institute of Scientific and Technical Information of China (English)

    杨忠民; 赵燕; 王瑞珍; 车彦民; 马燕文

    2001-01-01

    利用TEM,萃取和X-ray衍射方法分析了利用形变诱导铁素体和铁素体再结晶机制获得超细晶铁素体的晶内和晶界析出碳化物的成分和析出行为。证明晶内以M3C形成弥散析出,晶界以层片状形式析出和碳为短程扩散行为。%Utilizing the methods of TEM,extraction and X-ray diffraction,the analysis have been done about the chemical component of carbide and behavior of precipitation in the ultra fine grain ferrite and grain boundary by mechanism of deformation induced ferrite and recrystallization mechanism of ferrite.The results proved that the precipitation of dissemination of carbon in grain in the form of M3C,in grain boundary in the form of layer.The behavior of carbon diffusion is short range.

  15. 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

  16. Structures and transitions in tungsten grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

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

    2017-02-07

    The objective of this study is to develop a computational methodology to predict structure, energies 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 of tungsten for magnetic fusion applications being developed by the Marian Group at UCLA.

  17. Micro- to nano-characterization of semiconductor grain boundaries

    Science.gov (United States)

    Kazmerski, Lawrence L.

    1993-12-01

    This paper examines the use of surface-analysis methods in the characterization of grain boundaries in semiconductors. The purpose is to demonstrate the utility of increasing spatial resolution in the evaluation of defects. Three distinct mechanisms for adjusting the electronic properties of defect regions of semiconductors are presented. These include the passivation of grain boundaries by chemical methods; the electrical neutralization of shallow acceptor impurities; and, the doping of the defect regions. Secondary ion mass spectrometry, Auger electron spectroscopy and specialized scanning tunneling microscopy are used to evaluate the grain-boundary structure, composition and chemistry. Complementary macro- and nano-electro-optical characterization techniques based on electron-beam-induced current and photoluminescence provide correlated information on the effects of the chemical-treatment methods.

  18. Micro- to nano-characterization of semiconductor grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Kazmerski, Lawrence L. (National Renewable Energy Laboratory, Golden, CO (United States))

    1993-12-01

    This paper examines the use of surface-analysis methods in the characterization of grain boundaries in semiconductors. The purpose is to demonstrate the utility of increasing spatial resolution in the evaluation of defects. Three distinct mechanisms for adjusting the electronic properties of defect regions of semiconductors are presented. These include the passivation of grain boundaries by chemical methods; the electrical neutralization of shallow acceptor impurities; and, the doping of the defect regions. Secondary ion mass spectrometry, Auger electron spectroscopy and specialized scanning tunneling microscopy are used to evaluate the grain-boundary structure, composition and chemistry. Complementary macro- and nano-electro-optical characterization techniques based on electron-beam-induced current and photoluminescence provide correlated information on the effects of the chemical-treatment methods

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2015-07-29

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

  20. Development of Computational Tools for Predicting Thermal- and Radiation-Induced Solute Segregation at Grain Boundaries in Fe-based Alloys

    Energy Technology Data Exchange (ETDEWEB)

    Yang, Ying [Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

    2016-09-30

    Radiation induced segregation (RIS) has been frequently reported in structural materials such as austenitic, ferritic, and ferritic-martensitic stainless steels (SS) that have been widely used in light water reactors (LWRs). RIS has been linked to secondary degradation effects in SS including irradiation induced stress corrosion cracking (IASCC). Earlier studies on thermal segregation in Fe based alloys found that metalloids elements such as P, S, Si, Ge, Sn etc. embrittle the materials when enrichment was observed at grain boundaries (GBs). RIS of Fe-Cr- Ni based austenitic steels has been modeled in the U.S. 2015 fiscal year (FY2015), which identified the pre-enrichment due to thermal segregation can have an important role on the subsequent RIS. The goal of this work is to develop thermal segregation models for alloying elements in steels for future integration with RIS modeling.

  1. Development of Computational Tools for Predicting Thermal- and Radiation-Induced Solute Segregation at Grain Boundaries in Fe-based Alloys

    Energy Technology Data Exchange (ETDEWEB)

    Yang, Ying [Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

    2016-09-30

    Radiation-induced segregation (RIS) has been frequently reported in structural materials such as austenitic, ferritic, and ferritic-martensitic stainless steels (SS) that have been widely used in light water reactors (LWRs). RIS has been linked to secondary degradation effects in SS including irradiation-induced stress corrosion cracking (IASCC). Earlier studies on thermal segregation in Fe-based alloys found that metalloids elements such as P, S, Si, Ge, Sn, etc., embrittle the materials when enrichment was observed at grain boundaries (GBs). RIS of Fe-Cr-Ni-based austenitic steels has been modeled in the U.S. 2015 fiscal year (FY2015), which identified the pre-enrichment due to thermal segregation can have an important role on the subsequent RIS. The goal of this work is to develop thermal segregation models for alloying elements in steels for future integration with RIS modeling.

  2. High resolution grain boundary analysis of neutron irradiated stainless steel using FEG-TEM

    Energy Technology Data Exchange (ETDEWEB)

    Kodama, Mitsuhiro; Ishiyama, Yoshihide; Yokota, Norikatsu [Nippon Nuclear Fuel Development Co. Ltd., Oarai, Ibaraki (Japan)

    1999-09-01

    High-resolution grain boundary analyses of irradiated SUS304 stainless steel using a field emission gun equipped transmission electron microscope were carried out in order to detect radiation-induced grain boundary segregation. The effect of probe size on the measured compositional profiles was studied. The depletion of chromium and enrichment of nickel, phosphorus and silicon were detected at a grain boundary. The measured compositional profiles were affected by the probe size which impeded their interpretation. (author)

  3. Influence of Grain Boundary on Melting

    Institute of Scientific and Technical Information of China (English)

    王暾; 周富信; 刘曰武

    2001-01-01

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

  4. Evidence for a minigap in YBCO grain boundary Josephson junctions.

    Science.gov (United States)

    Lucignano, P; Stornaiuolo, D; Tafuri, F; Altshuler, B L; Tagliacozzo, A

    2010-10-01

    Self-assembled YBaCuO diffusive grain boundary submicron Josephson junctions offer a realization of a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current-voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Their persistence at large voltages is evidence of the long lifetime of the antinodal (high energy) quasiparticles.

  5. Roles of grain boundaries in improving fracture toughness of ultrafine-grained metals

    Science.gov (United States)

    Shimokawa, T.; Tanaka, M.; Kinoshita, K.; Higashida, K.

    2011-06-01

    In order to improve the fracture toughness in ultrafine-grained metals, we investigate the interactions among crack tips, dislocations, and grain boundaries in aluminum bicrystal models containing a crack and tilt grain boundaries using molecular dynamics simulations. The results of previous computer simulations showed that grain refinement makes materials brittle if grain boundaries behave as obstacles to dislocation movement. However, it is actually well known that grain refinement increases fracture toughness of materials. Thus, the role of grain boundaries as dislocation sources should be essential to elucidate fracture phenomena in ultrafine-grained metals. A proposed mechanism to express the improved fracture toughness in ultrafine-grained metals is the disclination shielding effect on the crack tip mechanical field. Disclination shielding can be activated when two conditions are present. First, a transition of dislocation sources from crack tips to grain boundaries must occur. Second, the transformation of grain-boundary structure into a neighboring energetically stable boundary must occur as dislocations are emitted from the grain boundary. The disclination shielding effect becomes more pronounced as antishielding dislocations are continuously emitted from the grain boundary without dislocation emissions from crack tips, and then ultrafine-grained metals can sustain large plastic deformation without fracture with the drastic increase of the mobile dislocation density. Consequently, it can be expected that the disclination shielding effect can improve the fracture toughness in ultrafine-grained metals.

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

    Science.gov (United States)

    Major, Jonathan D.

    2016-09-01

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    1996-08-01

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

  8. Recrystallization at grain boundaries in deformed copper bicrystals

    NARCIS (Netherlands)

    Heller, H.W.F.; Verbraak, C.A.; Kolster, B.H.

    1984-01-01

    The role of specific grain boundaries in the nucleation of recrystallization textures is demonstrated by experiments on copper bicrystals. It is deduced that the major part of the recrystallized grains that have nucleated at the grain boundary can be traced back to having nucleated in {100} <001>, {

  9. Phenomenology of Abnormal Grain Growth in Systems with Nonuniform Grain Boundary Mobility

    Science.gov (United States)

    DeCost, Brian L.; Holm, Elizabeth A.

    2017-06-01

    We have investigated the potential for nonuniform grain boundary mobility to act as a persistence mechanism for abnormal grain growth (AGG) using Monte Carlo Potts model simulations. The model system consists of a single initially large candidate grain embedded in a matrix of equiaxed grains, corresponding to the abnormal growth regime before impingement occurs. We assign a mobility advantage to grain boundaries between the candidate grain and a randomly selected subset of the matrix grains. We observe AGG in systems with physically reasonable fractions of fast boundaries; the probability of abnormal growth increases as the density of fast boundaries increases. This abnormal growth occurs by a series of fast, localized growth events that counteract the tendency of abnormally large grains to grow more slowly than the surrounding matrix grains. Resulting abnormal grains are morphologically similar to experimentally observed abnormal grains.

  10. Percolation of open grain boundaries and change in electrical conductivity

    Science.gov (United States)

    Watanabe, T.

    2016-12-01

    Numerical experiments were conducted on the percolation of open grain boundaries to study the percolation threshold and evolution of connectivity. Open grain boundaries are a major component of pores in crustal materials. Electrical conductivity and permeability are highly sensitive to the connectivity of open brain boundaries. The length and size of the largest cluster was surveyed in a 3D array of cubic grains for various fractions of open grain boundary. For sufficiently large size of array, the percolation threshold was found to be 0.25. If more than 25% of grain boundaries are open, an interconnected network of open grain boundaries is formed. If the aggregate is saturated with brine, the electrical conduction can occur through open grain boundaries. The connectivity of open grain boundaries steeply increases to 1 around the threshold. The electrical conductivity is also expected to increase steeply. The crack density parameter for the percolation threshold is estimated to be 0.1. The large change in electrical conductivity for a small change in crack density parameter is thus expected around crack density parameter of 0.1. Simultaneous measurements on elastic wave velocity and electrical conductivity in a brine saturated granitic rock (Watanabe and Higuchi, 2015) showed a steep change in electrical conductivity around the crack density parameter of 0.1. XCT images show that open grain boundaries are the dominant pores in the sample. The steep change in conductivity must thus be related to the percolation of open grain boundaries.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    1997-12-31

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

  12. Grain boundary and triple junction diffusion in nanocrystalline copper

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-09-07

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

  13. Effect of irradiation on mechanical properties of symmetrical grain boundaries investigated by atomic simulations

    Energy Technology Data Exchange (ETDEWEB)

    Wang, X. Y.; Gao, N.; Setyawan, W.; Xu, B.; Liu, W.; Wang, Z. G.

    2017-08-01

    Tensile response of irradiated symmetric grain boundaries to externally applied strain has been studied using atomic simulation methods. The absorption of irradiation induced defects by grain boundaries has been confirmed to degrade the mechanical properties of grain boundaries through the change of its under- taken deformation mechanism. Atomic rearrangement, the formations of a stress accumulation region and vacancy-rich zone and the nucleation and movement of dislocations under stress effect have been observed after the displacement cascades in grain boundaries, which are considered as main reasons to induce above degradation. These results suggest the necessity of considering both trap- ping efficiency to defects and the mechanical property change of irradiated grain boundaries for further development of radiation resistant materials.

  14. Effect of irradiation on mechanical properties of symmetrical grain boundaries investigated by atomic simulations

    Science.gov (United States)

    Wang, X. Y.; Gao, N.; Setyawan, W.; Xu, B.; Liu, W.; Wang, Z. G.

    2017-08-01

    Tensile response of irradiated symmetric grain boundaries to the externally applied strain has been studied using atomic simulation methods. The absorption of irradiation induced defects by grain boundaries has been confirmed to degrade the mechanical properties of grain boundaries through the change of its undertaken deformation mechanism. Atomic rearrangement, the formations of a stress accumulation region and vacancy-rich zone and the nucleation and movement of dislocations under stress effect have been observed after the displacement cascades in grain boundaries, which are considered as main reasons to induce above degradation. These results suggest the necessity of considering both trapping efficiency to defects and the mechanical property change of irradiated grain boundaries for further development of radiation resistant materials.

  15. Determination of grain boundary impurity effects in polycrystalline silicon

    Science.gov (United States)

    Kazmerski, L. L.; Dick, J. R.

    1984-06-01

    An analysis is made of the relationships existing between the chemistry and composition of the intergrain regions in polycrystalline silicon, the electrooptical properties of the grain boundaries, and the performance of polycrystalline Si solar cells. The following two impurity mechanisms are emphasized: segregation of oxygen to grain boundaries during heat treatments and the passivation of grain boundaries by incorporation of hydrogen. It is shown that hydrogen is localized at the defects; the effects of hydrogen localization on the electrical characteristics of the grain boundary and of the solar cell are discussed.

  16. A NEW MODEL FOR THE DETERMINATION OF GRAIN BOUNDARY DIFFUSIVITIES

    Directory of Open Access Journals (Sweden)

    R LOUAHDI

    2001-12-01

    Full Text Available A new model, based on a surface saturation technique, is suggested to determine grain boundary diffusivity of impurities. The model is applied to the Ni-S system that is of great practical interest. The initial saturation of nickel grain boundaries with sulphur is obtained by annealing at a temperature which satisfies the thermodynamics criterion for surface saturation. In order to reduce the annealing time, dynamic (non-equilibrium segregation is induced by carrying out the anneal on cold worked nickel (e = 0.2 true strain. Both the grain boundaries and the surface were saturated after only 24 hours of annealing at a temperature as low as 450°C. The heat treatment of the cold rolled material was carried out inside the vacuum chamber of an Auger Electron Spectrometer (AES. The diffusivity, as obtained from the slope of the linear parts of the kinetics curves recorded by the AES, is found to be given by the relationship D = 2.7×10-9exp(-58.700/RT m2s-1 in the temperature range 450 to 700°C.

  17. On the relationship between grain-boundary migration and grain-boundary diffusion by molecular-dynamics simulation

    Energy Technology Data Exchange (ETDEWEB)

    Schoenfelder, B. [Argonne National Lab., IL (United States). Materials Science Div.]|[RWTH Aachen (Germany). Inst. fuer Metallkunde und Metallphysik; Keblinski, P.; Wolf, D.; Phillpot, S.R. [Argonne National Lab., IL (United States). Materials Science Div.

    1998-07-01

    A molecular-dynamics method for the simulation of the intrinsic migration behavior of individual, flat grain boundaries is presented. A constant driving force for grain-boundary migration is generated by imposing an anisotropic elastic strain on a bicrystal such that the elastic-energy densities in its two halves are different. For the model case of the large-planar-unit-cell, high-angle (001) twist boundary in Cu the authors demonstrate that the drift velocity is proportional to the applied driving force, thus enabling determination of the boundary mobility. The activation energy for grain-boundary migration is found to be distinctly lower than that for grain-boundary self-diffusion. A decrease in the related activation energies with increasing temperature is shown to arise from a crossover in the underlying mechanisms, from solid-like at low temperatures to liquid-like at high-temperatures that is accompanied by an underlying grain-boundary structural transition.

  18. Absence of grain boundary melting in solid helium

    Energy Technology Data Exchange (ETDEWEB)

    Caupin, Frederic; Sasaki, Satoshi; Balibar, Sebastien [Laboratoire de Physique Statistique de l' Ecole Normale Superieure, associe au CNRS et aux Universites Paris 6 et 7, 24 rue Lhomond, 75005 Paris (France)], E-mail: caupin@lps.ens.fr

    2008-12-10

    Crystals are often expected to start melting at their free surface or at the interface between grains. Grain boundary melting corresponds to the situation where the interface between grains is invaded by a thick liquid film at the bulk melting temperature T{sub m}. In some cases, premelting is predicted, with liquid-like layers appearing between grains at temperatures below T{sub m}. We review this topic, and describe our experiments on solid helium 4. We find that grain boundaries are not wetted by the liquid at T{sub m}: they emerge at the liquid-solid interface with a non-zero contact angle. This is consistent with a general argument which predicts that, although systems with short-range forces might show grain boundary melting and premelting, in systems with long-range forces (like helium), grain boundaries can only be wetted incompletely by the liquid at T{sub m}.

  19. Coarsening kinetics of topologically highly correlated grain boundary networks

    Science.gov (United States)

    Tang, Ming; Reed, Bryan W.; Kumar, Mukul

    2012-08-01

    We apply phase-field simulations in two dimensions to study the thermal coarsening of grain boundary (GB) networks with high fractions of twin and twin-variant boundaries, which for example are seen in grain-boundary-engineered FCC materials. Two types of grain boundary networks with similar starting special boundary fractions but different topological features were considered as initial conditions for the grain growth simulations. A lattice Monte Carlo method creates polycrystalline microstructures (Reed and Kumar (RK)), which exhibit hierarchical organization of random and special coincidence site lattice boundaries. The other type of microstructures (randomly distributed (RD)) contains random distributions of special boundaries subject only to crystallographic constraints. Under the assumption that random boundaries have larger energy and much higher mobility than special boundaries, simulations show that increasing the initial special boundary fraction in both microstructures slows down grain growth. However, the two starting microstructures exhibit very different behavior in the evolution of GB character and triple junction (TJ) distributions. The RD networks coarsened more slowly than the RK networks with comparable initial fractions of special boundaries. The observed trend in the evolution of the RK microstructures is explained by an extended von Neumann-Mullins analysis. This study demonstrates that the special boundary fraction is not a sufficient indicator of the coarsening behavior of twinned GB networks; the network topology must also be considered to correctly predict the grain growth kinetics.

  20. A numerical model of stress driven grain boundary diffusion

    Science.gov (United States)

    Sethian, J. A.; Wilkening, Jon

    2004-01-01

    The stress driven grain boundary diffusion problem is a continuum model of mass transport phenomena in microelectronic circuits due to high current densities (electromigration) and gradients in normal stress along grain boundaries. The model involves coupling many different equations and phenomena, and difficulties such as non-locality, stiffness, complex geometry, and singularities in the stress tensor near corners and junctions make the problem difficult to analyze rigorously and simulate numerically. We present a new numerical approach to this problem using techniques from semigroup theory to represent the solution. The generator of this semigroup is the composition of a type of Dirichlet to Neumann map on the grain boundary network with the Laplace operator on the network. To compute the former, we solve the equations of linear elasticity several times, once for each basis function on the grain boundary. We resolve singularities in the stress field near corners and junctions by adjoining special singular basis functions to both finite element spaces (2d for elasticity, 1d for grain boundary functions). We develop data structures to handle jump discontinuities in displacement across grain boundaries, singularities in the stress field, complicated boundary conditions at junctions and interfaces, and the lack of a natural ordering for the nodes on a branching grain boundary network. The method is used to study grain boundary diffusion for several geometries.

  1. 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.

  2. AES/STEM grain boundary analysis of stabilized zirconia ceramics

    NARCIS (Netherlands)

    Winnubst, Aloysius J.A.; Kroot, P.J.M.; Burggraaf, A.J.

    1983-01-01

    Semiquantitative Auger Electron Spectroscopy (AES) on pure monophasic (ZrO2)0.83(YO1.5)0.17 was used to determine the chemical composition of the grain boundaries. Grain boundary enrichment with Y was observed with an enrichment factor of about 1.5. The difference in activation energy of the ionic

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

    Energy Technology Data Exchange (ETDEWEB)

    Schoenfelder, B. [Institut fuer Metallkunde und Metallphysik, RWTH Aachen University, Kopernikusstrasse 14, D-52056 Aachen (Germany); Gottstein, G. [Institut fuer Metallkunde und Metallphysik, RWTH Aachen University, Kopernikusstrasse 14, D-52056 Aachen (Germany)]. E-mail: gottstein@imm.rwth-aachen.de; Shvindlerman, L.S. [Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow 142432 (Russian Federation)

    2005-04-15

    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.

  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. Grain boundaries in polycrystalline Si solar cells Impurities and passivation

    Science.gov (United States)

    Kazmerski, L. L.

    The interrelationships among the chemistry and composition of the intergrain regions, the electro-optical properties of the grain boundaries, and the resulting photovoltaic performance of polycrystalline Si solar cells are examined. Specifically, two impurity-related mechanisms are reported. The first is that of the segregation of oxygen to the grain boundaries during thermal processing of the material/devices. Direct evidence is presented that correlates this oxygen localization to the electrical activation of the grain boundaries. The second is the passivation of the grain boundaries by incorporation of hydrogen in these regions. The localization of hydrogen at the intergrain defects is shown by surface analysis methods, and the effects on the electrical characteristics of the grain boundary and the solar cells are reported.

  6. Effects of grain boundaries on electrical property of copper wires

    Institute of Scientific and Technical Information of China (English)

    严文; 陈建; 范新会

    2003-01-01

    By means of annealing at different temperatures, the copper wires with various numbers of grain boundaries were achieved. And the resistivity of copper wires was measured. The results show that with increasing the number of grain boundaries, the resistivity of copper wires increases, the relationship between the number of grain boundaries and the resistivity of cooper wires can be expressed as y=1.86×10-8e-0.90/x. Unlike dislocation and lattice vacant sites, the curve of the grain boundary vs the resistivity is not linear. Grain boundary controls the general trend of the curve, but the type and the quantity of impurity controls the details of the curve.

  7. 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.

  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.

    2016-10-01

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

  9. 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.

  10. Interactions between grain boundary faceting, migration and grain rotation: Color group and molecular dynamics simulation approaches

    Science.gov (United States)

    Huang, Yue

    Color group theory and molecular dynamics (MD) simulations were used to study the faceting and rotation of grains in nanocrystalline materials and their interactions. Color group arguments were used to determine symmetry-dictated extrema with respect to misorientation of the grains and with respect to grain boundary normal orientations. MD simulations were used to study the evolution of the system and to elucidate the interactions between grain rotation and faceting in nano-scale systems. The systems of study were fcc bicrystalline systems with two grains sharing their [110] directions. Two geometric parameters were studied: the misorientation between two grains with a common rotation axis in the [110] direction of both grains, and the grain boundary normal orientation of fcc (110) tilt grain boundaries. The symmetry-dictated extremum (SDE) with respect to misorientation around both grains' [110] direction is 90 degrees. The SDE with respect to GB normal orientations for (110) tilt GBs are located on top of the color and classical mirror planes of their dichromatic patterns. By using periodic boundary conditions and a cylindrical embedded grain structure in our simulations, grains are only free to vary the misorientation between grains around the common [110] direction, and the normal of the grain boundaries are always perpendicular to both grains [110] direction. All SDE studied in our simulation are observed to be local energy minimum states. We observed the systems reducing their excess energy through three main modes: forming facets at the boundaries, rotating between the two grains, and reduction of grain boundary area through grain shrinkage. Facets are formed in low-energy grain boundaries and oscillating rotation occurred when the initial misorientation was not a SDE. A new algorithm was developed to quantitatively measure the grain rotation. The ovsered rotations are not rigid-body rotations and have strong interaction with faceting. Systems with lower

  11. 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.

  12. Kapitza Resistance of the Grain Boundaries in Ceria

    Energy Technology Data Exchange (ETDEWEB)

    David Bai; Jian Gan; Aleksandr Chernatynskiy

    2014-06-01

    Thermal conductivity is one of the key performance metrics of the nuclear fuels. In electrical insulators, such as most ubiquitous nuclear fuel – UO2, thermal transport is due to phonons, or lattice waves. Their propagation is impeded by any lattice defect, such as impurities or vacancies, as well as larger microstructural features: grain boundaries, dislocations and pores/bubbles. Detailed description of the phonons interactions with these features is still lacking. In this work, we elucidate the dependence of the grain boundary thermal resistance, also known as a Kapitza resistance, on the type and misorientation angle of the grain boundary in model system of CeO2.

  13. Applications of surface analysis techniques to photovoltaic research: Grain and grain boundary studies

    Science.gov (United States)

    Kazmerski, L. L.

    Complementary surface analysis techniques (AES, SIMS, XPS) are applied to photovoltaic devices in order to assess the limiting factors of grain and grain boundary chemistry to the performance of polycrystalline solar cells. Results of these compositional and chemical studies are directly correlated with electrical measurements (EBIC) and with resulting device performance. Examples of grain boundary passivation in polycrystalline Si and GaAs solar cells are cited. The quality of the intragrain material used in these devices is shown to be equally important to the grain boundary activity in determining overall photovoltaic performance.

  14. Large thermoelectric power factor in Pr-doped SrTiO3-δ ceramics via grain-boundary-induced mobility enhancement

    KAUST Repository

    Mehdizadeh Dehkordi, Arash

    2014-04-08

    We report a novel synthesis strategy to prepare high-performance bulk polycrystalline Pr-doped SrTiO3 ceramics. A large thermoelectric power factor of 1.3 W m-1 K-1 at 500 °C is achieved in these samples. In-depth investigations of the electronic transport and microstructure suggest that this significant improvement results from a substantial enhancement in carrier mobility originating from the formation of Pr-rich grain boundaries. This work provides new directions to higher performance oxide thermoelectrics as well as possibly other properties and applications of this broadly functional perovskite material. © 2014 American Chemical Society.

  15. STRESS DISTRIBUTION NEAR GRAIN BOUNDARY IN ANISOTROPIC BICRYSTALS AND TRICRYSTALS

    Institute of Scientific and Technical Information of China (English)

    万建松; 岳珠峰

    2004-01-01

    The rate dependent crystallographic finite element program was implemented in ABAQUS as a UMAT for the analysis of the stress distributions near grain boundary in anisotropic bicrystals and tricrystals, taking the different crystallographic orientations into consideration. The numerical results of bicrystals model with the different crystallographic orientations shows that there is a high stress gradient near the grain boundaries. The characteristics of stress structures are dependent on the crystallographic orientations of the two grains. The existing of triple junctions in the tricrystals may result in the stress concentrations, or may not, depending on the crystallographic orientations of the three grains. The conclusion shows that grain boundary with different crystallographic orientations can have different deformation, damage, and failure behaviors. So it is only on the detail study of the stress distribution can the metal fracture be understood deeply.

  16. Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide.

    Science.gov (United States)

    van der Zande, Arend M; Huang, Pinshane Y; Chenet, Daniel A; Berkelbach, Timothy C; You, YuMeng; Lee, Gwan-Hyoung; Heinz, Tony F; Reichman, David R; Muller, David A; Hone, James C

    2013-06-01

    Recent progress in large-area synthesis of monolayer molybdenum disulphide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapour deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulphide up to 120 μm in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane electrical conductivity, whereas tilt boundaries slightly decrease the conductivity.

  17. Interactions between displacement cascades and Σ3 tilt grain boundaries in Cu

    Science.gov (United States)

    Li, Bo; Long, Xiao-Jiang; Shen, Zhao-Wu; Luo, Sheng-Nian

    2016-12-01

    With large-scale molecular dynamics simulations, we investigate systematically the interaction of displacement cascades with a set of Σ3 tilt grain boundaries (GBs) in Cu bicrystals at low ambient temperatures, as regards irradiation-induced defect production/absorption and GB migration/faceting. Except for coherent twin boundary, GBs exhibit pronounced preferential absorption of interstitials, which depends on initial primary knock-on atom distance from GB plane and inclination angle. GB migration occurs when displacement cascades overlap with a GB plane, as induced by recrystallization of thermal spike, and concurrent asymmetric grain growth. Faceting occurs via expanding coherent twin boundaries for asymmetric GBs.

  18. Grain boundary characterization in an X750 alloy

    Energy Technology Data Exchange (ETDEWEB)

    Kevin Fisher; Sebastien Teysseyre; Emmanuelle Marquis

    2012-11-01

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

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

    DEFF Research Database (Denmark)

    Alimadadi, Hossein

    by 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......Manufacturing technologies such as injection molding and micro electromechanical systems demand materials with improved mechanical properties (e.g. hardness, ductility) and high durability at elevated temperatures. Significant improvement in some of the mechanical properties is obtained......, grain boundary engineering of electrodeposited nickel to achieve high population of coherent twin boundaries and, hence, higher thermal stability is a promising method to achieve simultaneous improvement in mechanical properties and thermal stability. This is of particular scientific and practical...

  20. Atomic arrangement and charge distribution in YBCO tilt grain boundaries

    Science.gov (United States)

    Bording, J. K.; Halley, J. W.; Zhu, Y.

    2005-03-01

    It is well known that the critical current, Jc, in high-Tc superconductors is reduced at grain boundaries. Recent high resolution holography experiments show the [100] tilt grain boundaries in YBa2Cu3O7-x to have an excess negative charge localized at the boundary dislocation core. Upon doping with Ca, this charge is reduced and the critical current increased. To shed light on this behavior at an atomic scale, we carried out Tight Binding (TB) calculations of these boundaries. Our TB scheme is charge self consistent to allow charge transfer typical for ionic materials. We present the arrangement of atoms and charge in YBCO tilt grain boundaries as determined by a combination of TB calculations, recent high resolution Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy measurements.

  1. 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

  2. Migration and nucleation of helium atoms at (110) twist grain boundaries in tungsten

    Science.gov (United States)

    Feng, Ya-Xin; Shang, Jia-Xiang; Lu, Guang-Hong

    2017-04-01

    The migration and nucleation of He atoms at three typical (110) twist grain boundaries (TGBs): the low-angle grain boundary (LAGB), the ordinary high-angle grain boundary (HAGB) and the Σ 3 TGB in W are investigated using molecular dynamics simulations. The presence of TGBs can absorb He atoms from bulk and impede the growth of He bubbles. Moreover, different grain boundary (GB) structures behave differently when interacting with He atoms. The LAGB can control the He distribution on the GB plane through its screw dislocation network, suggesting a promising approach for design of radiation tolerant materials. The ordinary HAGB presents a strong trap effect due to its disordered GB structure, which may induce a large He retention at the GB and embrittlement. The Σ 3 TGB can provide a diffusion path for He atoms, although the diffusion rate is not as fast as it in bulk.

  3. Phase-Field Modeling of Grain-Boundary Grooving Under Electromigration

    Science.gov (United States)

    Mukherjee, Arnab; Ankit, Kumar; Mukherjee, Rajdip; Nestler, Britta

    2016-12-01

    In the present work, we study the phenomenon of grain-boundary grooving under electromigration using a phase-field method. The specific focus of the work is to explore the role of grain boundaries as potential electromigration pathways. We consider the evolution of grooves under the combined influence of capillary and electromigration-mediated surface diffusion and electromigration-induced grain-boundary diffusion. Mechanisms of grooving are elucidated using flux density maps that indicate various regimes depending upon the direction of net material transport. When grain-boundary atomic mobility is lower than the surface mobility, the groove depth is found to be lower than that evolving solely under surface diffusion (no electromigration). At comparable or larger values of grain-boundary atomic mobility, grooving is initially expedited but shows groove replenishment at later stages. A detailed investigation using the phase-field method reveals the influence of an incumbent healing mechanism on grain-boundary grooving which is electrically induced. The drift characteristics such as edge and root displacement and velocity are examined in light of this assuaging effect.

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

    Science.gov (United States)

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

    2014-11-01

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

  5. Electronic structure studies of nanocrystalline diamond grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Zapol, P.; Sternberg, M.; Frauenheim, T.; Gruen, D. M.; Curtiss, L. A.

    1999-11-29

    Diamond growth from hydrogen-poor plasmas results in diamond structures that are profoundly different from conventionally CVD-grown diamond. High concentration of carbon dimers in the microwave plasma results in a high rate of heterogeneous renucleation leading to formation of nanocrystalline diamond with a typical grain size of 3--10 nm. Therefore, up to 10% of carbon atoms are located in the grain boundaries. In this paper the authors report on density-functional based tight-binding molecular dynamics calculations of the structure of a {Sigma}13 twist (100) grain boundary in diamond. Beginning with a coincidence site lattice model, simulated annealing of the initial structure was performed at 1,500 K followed by relaxation toward lower temperatures. About one-half of the carbons in the grain boundary are found to be three-coordinated. Coordination numbers, bond length and bond angle distributions are analyzed and compared to those obtained in previous studies.

  6. Creep of quartz by dislocation and grain boundary processes

    Science.gov (United States)

    Fukuda, J. I.; Holyoke, C. W., III; Kronenberg, A. K.

    2015-12-01

    Wet polycrystalline quartz aggregates deformed at temperatures T of 600°-900°C and strain rates of 10-4-10-6 s-1 at a confining pressure Pc of 1.5 GPa exhibit plasticity at low T, governed by dislocation glide and limited recovery, and grain size-sensitive creep at high T, governed by diffusion and sliding at grain boundaries. Quartz aggregates were HIP-synthesized, subjecting natural milky quartz powder to T=900°C and Pc=1.5 GPa, and grain sizes (2 to 25 mm) were varied by annealing at these conditions for up to 10 days. Infrared absorption spectra exhibit a broad OH band at 3400 cm-1 due to molecular water inclusions with a calculated OH content (~4000 ppm, H/106Si) that is unchanged by deformation. Rate-stepping experiments reveal different stress-strain rate functions at different temperatures and grain sizes, which correspond to differing stress-temperature sensitivities. At 600-700°C and grain sizes of 5-10 mm, flow law parameters compare favorably with those for basal plasticity and dislocation creep of wet quartzites (effective stress exponents n of 3 to 6 and activation enthalpy H* ~150 kJ/mol). Deformed samples show undulatory extinction, limited recrystallization, and c-axis maxima parallel to the shortening direction. Similarly fine-grained samples deformed at 800°-900°C exhibit flow parameters n=1.3-2.0 and H*=135-200 kJ/mol corresponding to grain size-sensitive Newtonian creep. Deformed samples show some undulatory extinction and grain sizes change by recrystallization; however, grain boundary deformation processes are indicated by the low value of n. Our experimental results for grain size-sensitive creep can be compared with models of grain boundary diffusion and grain boundary sliding using measured rates of silicon grain boundary diffusion. While many quartz mylonites show microstructural and textural evidence for dislocation creep, results for grain size-sensitive creep may apply to very fine-grained (<10 mm) quartz mylonites.

  7. 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.

  8. Long-period helical structures and twist-grain boundary phases induced by chemical substitution in the Mn1 -x(Co,Rh ) xGe chiral magnet

    Science.gov (United States)

    Martin, N.; Deutsch, M.; Chaboussant, G.; Damay, F.; Bonville, P.; Fomicheva, L. N.; Tsvyashchenko, A. V.; Rössler, U. K.; Mirebeau, I.

    2017-07-01

    We study the evolution of helical magnetism in MnGe chiral magnet upon partial substitution of Mn for 3 d -Co and 4 d -Rh ions. At high doping levels, we observe spin helices with very long periods—more than ten times larger than in the pure compound—and sizable ordered moments. This behavior calls for a change in the energy balance of interactions leading to the stabilization of the observed magnetic structures. Strikingly, neutron scattering unambiguously shows a double periodicity in the observed spectra at x =0.5 and >0.2 for Co- and Rh-doping, respectively. In analogy with observations made in smectic liquid crystals, we suggest that it may reveal the presence of magnetic "twist grain boundary" phases, involving a dense short-range correlated network of magnetic screw dislocations. The dislocation cores are here tentatively described as smooth textures, made of nonradial double-core skyrmions.

  9. Thermally driven grain boundary migration and melting in Cu.

    Science.gov (United States)

    Li, Y H; Wang, L; Li, B; E, J C; Zhao, F P; Zhu, J; Luo, S N

    2015-02-07

    With molecular dynamics simulations, we systematically investigate melting of a set of Σ3〈110〉70.53° tilt grain boundaries (GB) in Cu bicrystals, including coherent twin boundaries (CTBs), 12 asymmetric tilt grain boundaries (ATGBs), and symmetric incoherent twin boundaries (SITBs), in the order of increasing length weight of SITB or GB energy. ATGBs decompose into CTBs and SITBs, which migrate and coalesce as a result of internal stress relaxation. GBs can be superheated or premelted, and GB melting temperature decreases exponentially with increasing SITB weight, owing to the systematics in GB microstructure. GB melting nucleates at disordered CTB-SITB junctions, and grows along SITBs and then into grain interiors, with the solid-liquid interfaces preferentially aligned with {111}.

  10. Grain boundary resistance in p- and n-type indium phosphide

    Science.gov (United States)

    Shieh, C.-L.; Wagner, S.; Kazmerski, L. L.

    1985-08-01

    The zero-bias resistance between grains in p- and n-type InP was measured as a function of temperature. Among bulk n-type, bulk p-type and thin-film p-type samples, only the first exhibited a temperature-dependent resistance. Auger electron and secondary ion mass spectrometries of grain boundaries fractured in vacuo showed pronounced segregation of In, P and of foreign impurities. An impurity-induced grain boundary defect located close to the conduction band edge appears to account best for the experimental results.

  11. 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.

  12. Grain Boundary Microstructures of Wet and Dry Recrystallizing Marble

    Science.gov (United States)

    de Bresser, H.; Urai, J.; Olgaard, D.

    2003-12-01

    We analyzed 2D grain boundary maps of samples of marble that were deformed at high temperature with and without added water. Our aim was to relate the grain boundary geometry of wet and dry marble to the observed mechanical behavior, and to obtain criteria that can help interpretation of natural calcite rocks in terms of the influence of water on their deformation. We made use of cylindrical samples of pure white, microporous Carrara marble that were axially compressed in a gas medium deformation apparatus at temperatures (T) ranging 600-1000° C, a constant confining pressure of 300 MPa and strain rates around 10-5 s-1. Samples were jacketed in sealed Pt-capsules with or without the addition of 0.4-2.1 wt% water. Microstructural analysis was carried out using Scanning Electron Microscopy (SEM) and Light Optical Microscopy. Traced grain boundary maps were made from ultra thin sections of samples, and were quantitatively analyzed using Image Analysis techniques. The strength of water-added samples was found to be slightly less than of dry samples at all temperatures investigated (weakening ~40% at T=600° C, decreasing to ˜10% at higher T), with one exception at T=800° C. Microstructurally, the samples showed grain flattening and twinning at T=600° C and development of new grains by dynamic recrystallization at higher T, dominated by grain boundary migration. Grain boundaries in wet samples showed isolated or locally continuous remnants of fluid pockets in SEM. Quantitatively, the mean grain size and grain size distribution were found to only marginally vary between dry and wet samples. Average roundness of grains in wet recrystallized samples is systematically better than in dry samples. The fractal dimension D for the relationship between grain diameter d and grain perimeter P (expressed P ˜dD) for wet samples is systematically lower than for dry samples. Thus, grain boundaries in wet-deformed samples have less irregular shapes than in dry samples. Average

  13. The Mechanisms of Grain Boundaries - Slip Transmission, Migration, and Sliding

    Energy Technology Data Exchange (ETDEWEB)

    Briant, Clyde L.

    2005-03-02

    During the last eight years, we have worked on the general problems associated with grain boundaries in metals with DOE support. This final report summarizes the work that has been performed. At the start of this work, we took a much more atomistic approach to grain boundaries. However, as we performed this research it became clear that such approaches had the drawbacks listed above, and that we were not proceeding toward the more general understanding of grain boundaries that we have hoped to achieve. We then moved toward more macroscopic based experiments that we could use to understand the structure and motion of grain boundaries. From these we were able to begin deducing some of the most important results of this work and to provide information that can be used by others to understand the role of grain boundaries in materials. We thus present this report in a topical way and provide the experimental and theoretical underpinning that is needed at each point as we go forward.

  14. Magnetic anisotropy of grain boundaries in nanocrystalline Ni

    Science.gov (United States)

    Bian, Q.; Niewczas, M.

    2017-01-01

    Temperature-dependent magnetic anisotropy due to grain boundaries in nanocrystalline Ni has been studied by simulating experimental magnetization data with the stochastic Landau-Lifshitz-Gilbert theory. In the model the grain boundary magnetic anisotropy energy is expressed as the sum of the uniaxial anisotropy and the cubic anisotropy, characterized by Kua and Kca anisotropy constants. By comparing the calculated magnetization with the experimental magnetization measurements at finite temperatures, the values of Kua and Kca can be determined. For nanocrystalline Ni it is found that with increasing temperature Kua decreases and Kca increases. At low temperatures Kua dominates the grain boundary anisotropy energy, whereas Kca is very small and it can be neglected. At room temperature Kua and Kca are of the same order with the corresponding ratio Kua /Kca ≈ 1.9 , both coefficients are much larger than the magnetocrystalline anisotropy constant.

  15. Grain boundary cavity growth under applied stress and internal pressure

    Energy Technology Data Exchange (ETDEWEB)

    Mancuso, J.F.

    1977-08-01

    The growth of grain boundary cavities under applied stress and internal gas pressure was investigated. Methane gas filled cavities were produced by the C + 4H reversible CH4 reaction in the grain boundaries of type 270 nickel by hydrogen charging in an autoclave at 500/sup 0/C with a hydrogen pressure of either 3.4 or 14.5 MPa. Intergranular fracture of nickel was achieved at a charging temperature of 300/sup 0/C and 10.3 MPa hydrogen pressure. Cavities on the grain boundaries were observed in the scanning electron microscope after fracture. Photomicrographs of the cavities were produced in stereo pairs which were analyzed so as to correct for perspective distortion and also to determine the orientational dependence of cavity growth under an applied tensile stress.

  16. Grain Boundary Assemblies in Dynamically-Recrystallized Austenitic Stainless Steel

    Directory of Open Access Journals (Sweden)

    Marina Tikhonova

    2016-11-01

    Full Text Available The grain boundary misorientation distributions associated with the development of dynamic recrystallization were studied in a high-nitrogen austenitic stainless steel subjected to hot working. Under conditions of discontinuous dynamic recrystallization, the relationships between the grain or subgrain sizes and flow stresses can be expressed by power law functions with different grain/subgrain size exponents of about −0.76 (for grain size or −1.0 (for subgrain size. Therefore, the mean grain size being much larger than the subgrain size under conditions of low flow stress gradually approaches the size of the subgrains with an increase in the flow stress. These dependencies lead to the fraction of high-angle boundaries being a function of the flow stress. Namely, the fraction of ordinary high-angle boundaries in dynamically-recrystallized structures decreases with a decrease in the flow stress. On the other hand, the fraction of special boundaries, which are associated with annealing twins, progressively increases with a decrease of the flow stress.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2009-05-15

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

  18. Complete grain boundaries from incomplete EBSD maps: the influence of segmentation on grain size determinations

    Science.gov (United States)

    Heilbronner, Renée; Kilian, Ruediger

    2017-04-01

    Grain size analyses are carried out for a number of reasons, for example, the dynamically recrystallized grain size of quartz is used to assess the flow stresses during deformation. Typically a thin section or polished surface is used. If the expected grain size is large enough (10 µm or larger), the images can be obtained on a light microscope, if the grain size is smaller, the SEM is used. The grain boundaries are traced (the process is called segmentation and can be done manually or via image processing) and the size of the cross sectional areas (segments) is determined. From the resulting size distributions, 'the grain size' or 'average grain size', usually a mean diameter or similar, is derived. When carrying out such grain size analyses, a number of aspects are critical for the reproducibility of the result: the resolution of the imaging equipment (light microscope or SEM), the type of images that are used for segmentation (cross polarized, partial or full orientation images, CIP versus EBSD), the segmentation procedure (algorithm) itself, the quality of the segmentation and the mathematical definition and calculation of 'the average grain size'. The quality of the segmentation depends very strongly on the criteria that are used for identifying grain boundaries (for example, angles of misorientation versus shape considerations), on pre- and post-processing (filtering) and on the quality of the recorded images (most notably on the indexing ratio). In this contribution, we consider experimentally deformed Black Hills quartzite with dynamically re-crystallized grain sizes in the range of 2 - 15 µm. We compare two basic methods of segmentations of EBSD maps (orientation based versus shape based) and explore how the choice of methods influences the result of the grain size analysis. We also compare different measures for grain size (mean versus mode versus RMS, and 2D versus 3D) in order to determine which of the definitions of 'average grain size yields the

  19. Surface and grain boundary scattering in nanometric Cu thin films: A quantitative analysis including twin boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Barmak, Katayun [Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027 and Department of Materials Science and Engineering and Materials Research Science and Engineering Center, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213 (United States); Darbal, Amith [Department of Materials Science and Engineering and Materials Research Science and Engineering Center, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213 (United States); Ganesh, Kameswaran J.; Ferreira, Paulo J. [Materials Science and Engineering, The University of Texas at Austin, 1 University Station, Austin, Texas 78712 (United States); Rickman, Jeffrey M. [Department of Materials Science and Engineering and Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015 (United States); Sun, Tik; Yao, Bo; Warren, Andrew P.; Coffey, Kevin R., E-mail: kb2612@columbia.edu [Department of Materials Science and Engineering, University of Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32816 (United States)

    2014-11-01

    The relative contributions of various defects to the measured resistivity in nanocrystalline Cu were investigated, including a quantitative account of twin-boundary scattering. It has been difficult to quantitatively assess the impact twin boundary scattering has on the classical size effect of electrical resistivity, due to limitations in characterizing twin boundaries in nanocrystalline Cu. In this study, crystal orientation maps of nanocrystalline Cu films were obtained via precession-assisted electron diffraction in the transmission electron microscope. These orientation images were used to characterize grain boundaries and to measure the average grain size of a microstructure, with and without considering twin boundaries. The results of these studies indicate that the contribution from grain-boundary scattering is the dominant factor (as compared to surface scattering) leading to enhanced resistivity. The resistivity data can be well-described by the combined Fuchs–Sondheimer surface scattering model and Mayadas–Shatzkes grain-boundary scattering model using Matthiessen's rule with a surface specularity coefficient of p = 0.48 and a grain-boundary reflection coefficient of R = 0.26.

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

    Science.gov (United States)

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

    1991-01-01

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

  1. Anelastic relaxations associated with local disordering in grain boundaries

    Science.gov (United States)

    Cheng, Bolin; Ge, Tingsui

    1993-04-01

    Internal friction and micro-creep measurements were performed with high-purity Al bamboo-crystal specimens. The relaxation strength was found to decrease with the decrease of the temperature of measurement and became zero at about 0.4 T sub m, (T sub m being melting temperature). This reflects the occurrence of local disordering in the bamboo boundary region at this temperature. This result conforms to the picture of grain boundary disordering constructed by atomic simulation studies.

  2. Grain boundary dynamics in ceramics superplasticity

    Directory of Open Access Journals (Sweden)

    Wakai, E.

    2001-04-01

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

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

  3. The influence of grain boundary structure on diffusional creep

    DEFF Research Database (Denmark)

    Thorsen, Peter Anker; Bilde-Sørensen, Jørgen

    1999-01-01

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

  4. Ab initio modelling of UN grain boundary interfaces

    Science.gov (United States)

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

    2012-08-01

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

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

    Science.gov (United States)

    Käshammer, Peter; Sinno, Talid

    2015-09-01

    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.

  6. Grain boundary diffusion in terms of the tempered fractional calculus

    Energy Technology Data Exchange (ETDEWEB)

    Sibatov, R.T., E-mail: ren_sib@bk.ru [Ulyanovsk State University, 432017, 42 Leo Tolstoy str., Ulyanovsk (Russian Federation); Svetukhin, V.V. [Ulyanovsk State University, 432017, 42 Leo Tolstoy str., Ulyanovsk (Russian Federation); Institute of Nanotechnology and Microelectronics of the Russian Academy of Sciences, 115487, 18 Nagatinskaya str., Moscow (Russian Federation)

    2017-06-28

    Mathematical treatment of grain-boundary diffusion based on the model first proposed by Fisher is usually formulated in terms of normal diffusion equations in a two-component nonhomogeneous medium. On the other hand, fractional equations of anomalous diffusion proved themselves to be useful in description of grain-boundary diffusion phenomena. Moreover, the most important propagation regime predicted by Fisher's model demonstrates subdiffusive behavior. However, the direct link between fractional approach and the Fisher model and its modifications has not found yet. Here, we fill this gap and show that solution of fractional subdiffusion equation offers general properties of classical solutions obtained by Whipple and Suzuoka. The tempered fractional approach is a convenient tool for studying precipitation in granular materials as the tempered subdiffusion limited process. - Highlights: • The link connected fractional diffusion approach and Fisher's model of grain-boundary diffusion is derived. • The subdiffusion exponent of grain-boundary diffusion can differ from 1/2. • Nucleation in granular materials is modeled by the process limited by tempered subdiffusion.

  7. Measurement of spatial stress gradients near grain boundaries

    NARCIS (Netherlands)

    Basu, Indranil; Ocelík, Vaclav; De Hosson, Jeff Th M.

    2017-01-01

    A correlative method based on electron back scattered diffraction and focused ion-beam–digital image correlation slit milling technique was used to quantitatively determine spatially resolved stress profiles in the vicinity of grain boundaries in pure titanium. Measured local stress gradients were

  8. 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...

  9. Chemistry of grain boundary environments in nanocrystalline Al 7075

    Energy Technology Data Exchange (ETDEWEB)

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

    2010-04-16

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

  10. Effects of grinding-induced grain boundary and interfaces on electrical transportation and structure phase transition in ZnSe under high pressure

    Science.gov (United States)

    Jie, Yang; Pei, Wang; Guo-Zhao, Zhang; Xiao-Xue, Zhou; Jing, Li; Cai-Long, Liu

    2016-06-01

    Interface and scale effects are the two most important factors which strongly affect the structure and the properties of nano-/micro-crystals under pressure. We conduct an experiment under high pressure in situ alternating current impedance to elucidate the effects of interface on the structure and electrical transport behavior of two ZnSe samples with different sizes obtained by physical grinding. The results show that (i) two different-sized ZnSe samples undergo the same phase transitions from zinc blend to cinnabar-type phase and then to rock salt phase; (ii) the structural transition pressure of the 859-nm ZnSe sample is higher than that of the sample of 478 nm, which indicates the strong scale effect. The pressure induced boundary resistance change is obtained by fitting the impedance spectrum, which shows that the boundary conduction dominates the electrical transport behavior of ZnSe in the whole experimental pressure range. By comparing the impedance spectra of two different-sized ZnSe samples at high pressure, we find that the resistance of the 478-nm ZnSe sample is lower than that of the 859-nm sample, which illustrates that the sample with smaller particle size has more defects which are due to physical grinding. Project supported by the National Natural Science Foundation of China (Grant Nos. 11404133 and 11374121) and the Program of Science and Technology Development Plan of Jilin Province, China (Grant No. 20140520105JH).

  11. Superconductivity of individual grains and inter-grain boundaries for polycrystalline FeSrYCuO

    Science.gov (United States)

    Yamaguchi, K.; Hata, Y.; Mochiku, T.; Yasuoka, H.

    2013-04-01

    Polycrystalline FeSrYCuO was synthesized and its transport and magnetic properties were studied. Diamagnetism was observed below 60 K. Zero resistivity was observed below 38 K under zero magnetic field and below 10 K under 160 kOe. A two-step transition was observed in resistivity measurement due to the superconductivity in individual grains and across inter-grain boundaries. The critical current density in individual grains, Jcintra, at 2 K under 1 kOe was deduced 3.4 × 105 A/cm2 from the Bean model. In contrast, the critical current density in inter-grain boundaries, Jcinter, at 2 K was 1.7 A/cm2 in voltage-current measurement. The two-step transition seems to result from the large difference between Jcintra and Jcinter.

  12. Incipient plasticity during nanoindentation at grain boundaries in body-centered cubic metals

    NARCIS (Netherlands)

    Soer, WA; Aifantis, KE; De Hosson, JTM

    2005-01-01

    The mechanical response to nanoinclentation near grain boundaries has been investigated in an Fe-14%Si bicrystal with a general grain boundary and two Mo bicrystals with symmetric tilt boundaries, In particular, the indentations performed on the Fe-14%Si show that as the grain boundary is approached

  13. 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...

  14. CYCLIC HARDENING BEHAVIOR OF POLYCRYSTALS WITH PENETRABLE GRAIN BOUNDARIES: TWO-DIMENSIONAL DISCRETE DISLOCATION DYNAMICS SIMULATION

    Institute of Scientific and Technical Information of China (English)

    Chuantao Hou; Zhenhuan Li; Minsheng Huang; Chaojun Ouyang

    2009-01-01

    A two-dimensional discrete dislocation dynamics (DDD) technology by Giessen and Needleman (1995), which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally analyze the micro-cyclic plastic response of polycrystals containing micron-sized grains, with special attentions to significant influence of dislocationpenetrable grain boundaries (GBs) on the micro-plastic cyclic responses of polycrystals and underlying dislocation mechanism. Toward this end, a typical polycrystalline rectangular specimen under simple tension-compression loading is considered. Results show that, with the increase of cycle accumulative strain, continual dislocation accumulation and enhanced dislocation-dislocation interactions induce the cyclic hardening behavior; however, when a dynamic balance among dislocation nucleation, penetration through GB and dislocation annihilation is approximately established, cyclic stress gradually tends to saturate. In addition, other factors, including the grain size, cyclic strain amplitude and its history, also have considerable influences on the cyclic hardening and saturation.

  15. 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.

  16. 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.)

  17. Simulation of grain growth in nanocrystalline nickel induced by ion irradiation

    CERN Document Server

    Voegeli, W; Hahn, H

    2003-01-01

    Molecular dynamics simulations of 5 keV cascades in nanocrystalline nickel with grain sizes of 5 and 10 nm are presented. If the spike volume is exceeding the grain size or overlapping the grain boundary (GB) area we observe ion-beam induced grain growth for both grain sizes. In contrast cascades located in the grain volume lead to the formation of vacancies and interstitials, where the latter are mostly accommodated by the GBs upon annealing. Finally, we show that ion-beam induced grain growth is a direct result of recrystallisation of the thermal spike and therefore inherently different to grain growth observed in long time thermal annealing simulations.

  18. 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.

  19. Direct observation of potential barrier formation at grain boundaries of SnO2 ceramics

    Science.gov (United States)

    Maestre, D.; Cremades, A.; Piqueras, J.

    2004-11-01

    Remote electron beam induced current (REBIC) and cathodoluminescence (CL) modes in the scanning electron microscope (SEM) have been used to investigate SnO2 sintered samples. The study of the electrically active boundaries present in the oxide shows a characteristic peak and trough (PAT) contrast after thermal treatments in oxygen. Temperature-dependent measurements of the REBIC contrast show the presence of a shallow defect level 60 meV below the conduction band. This level is asigned to oxygen species adsorbed on the defect-rich boundaries. Evolution of REBIC contrast of the grain boundaries with excitation density enabled us to perform local measurements of minority carrier diffusion length.

  20. CSL grain boundary distribution in alumina and zirconia ceramics

    OpenAIRE

    Vonlanthen, Pierre; Grobéty, Bernard

    2008-01-01

    The distributions of general and coincidence site lattice (CSL) grain boundaries (GBs) in texture-free alumina and zirconia ceramics sintered at two different temperatures were investigated based on electron backscatter diffraction (EBSD) measurements. Results were compared with the distributions obtained from random 2D spatial models and with calculated random distributions reported in the literature. All alumina samples independent on sintering temperature show the same characteristic devia...

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

    Institute of Scientific and Technical Information of China (English)

    郭新; 袁润章

    1996-01-01

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2001-06-27

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

  3. Ionic Segregation on Grain Boundaries in Thermally Grown Alumina Scales

    Energy Technology Data Exchange (ETDEWEB)

    Pint, Bruce A [ORNL; Unocic, Kinga A [ORNL

    2012-01-01

    This study first examined segregation behaviour in the alumina scale formed after 100 h at 1100 C on bare and MCrAlYHfSi-coated single-crystal superalloys with {approx}10 ppma La and Y. For the bare superalloy, Hf and Ti were detected on the grain boundaries of the inner columnar alumina layer. Increasing the oxidation temperature to 1200 C for 2 h did not change the segregation behavior. With the bond coating, both Y and Hf were segregated to the grain boundaries as expected. However, there was evidence of Ti-rich oxide particles near the gas interface suggesting that Ti diffused from the superalloy through the coating. To further understand these segregation observations with multiple dopants, other alumina-forming systems were examined. Alumina scale grain boundary co-segregation of Ti with Y is common for FeCrAl alloys. Co-segregation of Hf and Ti was observed in the scale formed on co-doped NiAl. No La segregation was detected in the scale formed on NiCrAl with only a 19 ppma La addition, however, the scale was adherent.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2001-06-27

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

  5. Modulus spectroscopy of grain–grain boundary binary system

    Energy Technology Data Exchange (ETDEWEB)

    Cheng, Peng-Fei, E-mail: pfcheng@xpu.edu.cn [School of Science, Xi’an Polytechnic University, Xi’an 710048 (China); Song, Jiang [School of Science, Xi’an Polytechnic University, Xi’an 710048 (China); Li, Sheng-Tao; Wang, Hui [State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049 (China)

    2015-02-15

    Understanding various polarization mechanisms in complex dielectric systems and specifying their physical origins are key issues in dielectric physics. In this paper, four different methods for representing dielectric properties were analyzed and compared. Depending on the details of the system under study, i.e., uniform or non-uniform, it was suggested that different representing approaches should be used to obtain more valuable information. Especially, for the grain–grain boundary binary non-uniform system, its dielectric response was analyzed in detail in terms of modulus spectroscopy (MS). Furthermore, it was found that through MS, the dielectric responses between uniform and non-uniform systems, grain and grain boundary, Maxwell–Wagner polarization and intrinsic polarization can be distinguished. Finally, with the proposed model, the dielectric properties of CaCu{sub 3}Ti{sub 4}O{sub 12} (CCTO) ceramics were studied. The colossal dielectric constant of CCTO at low frequency was attributed to the pseudo relaxation process of grain.

  6. Effect of the severe plastic deformation temperature on the diffusion properties of the grain boundaries in ultrafine-grained metals

    Science.gov (United States)

    Chuvil'deev, V. N.; Myshlyaev, M. M.; Nokhrin, A. V.; Kopylov, V. I.; Lopatin, Yu. G.; Pirozhnikova, O. E.; Piskunov, A. V.; Semenycheva, A. V.; Bobrov, A. A.

    2017-05-01

    A model is proposed to explain the effect of the severe plastic deformation (SPD) temperature on the diffusion properties of the grain boundaries in ultrafine-grained (UFG) metals and alloys. It is shown that an increase in the SPD temperature in UFG metals leads to an increase in the activation energy of grainboundary diffusion from (3-5) k B T m, which corresponds to the diffusion parameters of nonequilibrium grain boundaries, to (8-10) k B T m, which corresponds to the diffusion parameters of equilibrium grain boundaries ( k B is the Boltzmann constant, T m is the melting temperature). The dependence of the activation energy of grain-boundary diffusion on the SPD temperature is found to be determined by the kinetics of the competing processes of defect accumulation at grain boundaries and the diffusion accommodation of defects.

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

    Science.gov (United States)

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

    2016-10-01

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

  8. Systematic study of grain boundary atomistic structures and related properties in cubic zirconia bicrystals

    Energy Technology Data Exchange (ETDEWEB)

    Shibata, N.; Ikuhara, Y. [Inst. of Engineering Innovation, Univ. of Tokyo, Tokyo (Japan); Oba, F. [Dept. of Materials Science and Engineering, Graduate School of Engineering, Kyoto Univ., Kyoto (Japan); Yamamoto, T. [Dept. of Advanced Materials Science, Graduate School of Frontier Science, Univ. of Tokyo, Kashiwa, Chiba (Japan)

    2005-02-01

    Systematic grain boundary study of cubic zirconia has been conducted by using bicrystals. It is clearly demonstrated that grain boundary atomistic structures dramatically change according to the misorientations and plane orientations of the boundaries, resulting in a dramatic change of excess energies and solute segregation behaviors. Combining with theoretical calculations, it is found that grain boundaries possess unique coordination-deficient cation sites at the cores, and their densities have a clear correlation with these properties in high-angle grain boundaries. This result indicates that grain boundary properties in ceramics are possibly determined by the accumulation of coordination-deficient sites. Thus, systematic grain boundary study using bicrystal offers fundamental understandings of the relationship between atomistic structures and properties in ceramic grain boundaries. (orig.)

  9. UO2 Grain Growth: Developing Phase Field Models for Pore Dragging, Solute Dragging and Anisotropic Grain Boundary Energies

    Energy Technology Data Exchange (ETDEWEB)

    Ahmed, K.; Tonks, M.; Zhang, Y.; Biner, B.

    2016-09-28

    A detailed phase field model for the effect of pore drag on grain growth kinetics was implemented in MARMOT. The model takes into consideration both the curvature-driven grain boundary motion and pore migration by surface diffusion. As such, the model accounts for the interaction between pore and grain boundary kinetics, which tends to retard the grain growth process. Our 2D and 3D simulations demonstrate that the model capture all possible pore-grain boundary interactions proposed in theoretical models. For high enough surface mobility, the pores move along with the migrating boundary as a quasi-rigid-body, albeit hindering its migration rate compared to the pore-free case. For less mobile pores, the migrating boundary can separate from the pores. For the pore-controlled grain growth kinetics, the model predicts a strong dependence of the growth rate on the number of pores, pore size, and surface diffusivity in agreement with theroretical models. An evolution equation for the grain size that includes these parameters was derived and showed to agree well with numerical solution. It shows a smooth transition from boundary-controlled kinetics to pore-controlled kinetics as the surface diffusivity decreases or the number of pores or their size increases. This equation can be utilized in BISON to give accurate estimate for the grain size evolution. This will be accomplished in the near future. The effect of solute drag and anisotropy of grain boundary on grain growth will be investigated in future studies.

  10. 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.

  11. Physics of grain boundaries in polycrystalline photovoltaic semiconductors

    Energy Technology Data Exchange (ETDEWEB)

    Yan, Yanfa, E-mail: yanfa.yan@utoledo.edu; Yin, Wan-Jian; Wu, Yelong; Shi, Tingting; Paudel, Naba R. [Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Ohio 43606 (United States); Li, Chen [Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (United States); Poplawsky, Jonathan [The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (United States); Wang, Zhiwei [Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Ohio 43606 (United States); National Renewable Energy Laboratory, Golden, Colorado 80401 (United States); Moseley, John; Guthrey, Harvey; Moutinho, Helio; Al-Jassim, Mowafak M. [National Renewable Energy Laboratory, Golden, Colorado 80401 (United States); Pennycook, Stephen J. [Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996 (United States)

    2015-03-21

    Thin-film solar cells based on polycrystalline Cu(In,Ga)Se{sub 2} (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells.

  12. 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.

  13. Multiscale Modeling of Grain-Boundary Fracture: Cohesive Zone Models Parameterized From Atomistic Simulations

    Science.gov (United States)

    Glaessgen, Edward H.; Saether, Erik; Phillips, Dawn R.; Yamakov, Vesselin

    2006-01-01

    A multiscale modeling strategy is developed to study grain boundary fracture in polycrystalline aluminum. Atomistic simulation is used to model fundamental nanoscale deformation and fracture mechanisms and to develop a constitutive relationship for separation along a grain boundary interface. The nanoscale constitutive relationship is then parameterized within a cohesive zone model to represent variations in grain boundary properties. These variations arise from the presence of vacancies, intersticies, and other defects in addition to deviations in grain boundary angle from the baseline configuration considered in the molecular dynamics simulation. The parameterized cohesive zone models are then used to model grain boundaries within finite element analyses of aluminum polycrystals.

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

    Institute of Scientific and Technical Information of China (English)

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

    2012-01-01

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

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

    Science.gov (United States)

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

    2015-02-01

    scattering and that arising from electron-impurity scattering, for MS theory does not allow parameters (ℓIMP, R) to be uniquely adjusted to describe the temperature dependence of the resistivity data. The same data can be described using different sets of (R, ℓIMP); the latter parameter can be varied by two orders of magnitude in the case of small grained samples d ℓ (where ℓ is the bulk mean free path at 300 K). For samples d > ℓ, the increase of resistivity is attributed not to electrons being partially reflected by the grain boundaries, but to a decrease in the number of states at the Fermi sphere that are allowed bands of the KP potential; hence the reflectivity required by the quantum model turns out to be an order of magnitude smaller than that required by the classical MS theory. For samples d < ℓ, the resistivity increase originates mainly from Anderson localization induced by electron grain boundary scattering from disordered successive grains characterized by a localization length of the order of 110 nm and not from electrons being partially reflected by grain boundaries; the outcome is that the reflectivity required by the quantum theory turns out to be about 4 times smaller than that required by the classical MS theory.

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

    Energy Technology Data Exchange (ETDEWEB)

    Terentyev, D.; He, Xinfu

    2010-08-15

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

  17. Quantitative high resolution electron microscopy of grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

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

    1996-12-12

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

  18. Stacking fault tetrahedra formation in the neighbourhood of grain boundaries

    CERN Document Server

    Samaras, M; Van Swygenhoven, H; Victoria, M

    2003-01-01

    Large scale molecular dynamics computer simulations are performed to study the role of the grain boundary (GB) during the cascade evolution in irradiated nanocrystalline Ni. At all primary knock-on atom (PKA) energies in cascades near GBs, the damage produced after cooling down is vacancy dominated. Truncated stacking fault tetrahedra (TSFTs) are easily formed at 10 keV and higher PKA energies. At the higher energies a complex partial dislocation network forms, consisting of TSFTs. The GB acts as an interstitial sink without undergoing major structural changes.

  19. 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.

  20. 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.

  1. Distortion Regions near the Grain Boundary and Their Effects on Nanocrystalline Materials

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    The distortion regions near grain boundaries in α-Fe-C solid solution were studied by use of internal friction method. It was found that the total thickness of these regions is quite large though the thickness of real grain boundaries is usually very thin. It was also found that the smaller the grain size, the thicker the total distortion region. A model for the structure of distortion regions near grain boundaries is proposed. Their effects on nanocrystalline materials are discussed.

  2. Dynamic recrystallization and grain boundary migration in B2 FeAl

    Science.gov (United States)

    Baker, I.; Gaydosh, D. J.

    1987-01-01

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

  3. Mechanical behaviour near grain boundaries of He-implanted UO2 ceramic polycrystals

    Science.gov (United States)

    Ibrahim, M.; Castelier, É.; Palancher, H.; Bornert, M.; Caré, S.; Micha, J.-S.

    2017-01-01

    For studying the micromechanical behaviour of UO2 and characterising the intergranular interaction, polycrystals are implanted with helium ions, inducing strains in a thin surface layer. Laue X-ray micro-diffraction is used to measure the strain field in this implanted layer with a spatial resolution of about 1 μm. It allows a 2D mapping of the strain field in a dozen of grains. These measurements show that the induced strain depends mainly on the crystal orientation, and can be evaluated by a semi-analytical mechanical model. A mechanical interaction of the neighbouring grains has also been evidenced near the grain boundaries, which has been well reproduced by a finite element model. This interaction is shown to increase with the implantation energy (i.e. the implantation depth): it can be neglected at low implantation energy (60 keV), but not at higher energy (500 keV).

  4. 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.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2005-07-25

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

  6. 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.

  7. Modeling of stresses at grain boundaries with respect to occurrence of stress corrosion cracking

    Energy Technology Data Exchange (ETDEWEB)

    Kozaczek, K.J. [Oak Ridge National Lab., TN (United States); Sinharoy, A.; Ruud, C.O. [Pennsylvania State Univ., University Park, PA (United States); McIlree, A.R. [Electric Power Research Institute, Palo Alto, CA (United States)

    1995-12-31

    The distributions of elastic stresses/strains in the grain boundary regions were studied by the analytical and the finite element models. The grain boundaries represent the sites where stress concentration occurs as a result of discontinuity of elastic properties across the grain boundary and the presence of second phase particles elastically different from the surrounding matrix grains. A quantitative analysis of those stresses for steels and nickel based alloys showed that the stress concentrations in the grain boundary regions are high enough to cause a local microplastic deformation even when the material is in the macroscopic elastic regime. The stress redistribution as a result of such a plastic deformation was discussed.

  8. Long Lifetime Hole Traps at Grain Boundaries in CdTe Thin-Film Photovoltaics

    Science.gov (United States)

    Mendis, B. G.; Gachet, D.; Major, J. D.; Durose, K.

    2015-11-01

    A novel time-resolved cathodoluminescence method, where a pulsed electron beam is generated via the photoelectric effect, is used to probe individual CdTe grain boundaries. Excitons have a short lifetime (≤100 ps ) within the grains and are rapidly quenched at the grain boundary. However, a ˜47 meV shallow acceptor, believed to be due to oxygen, can act as a long lifetime hole trap, even at the grain boundaries where their concentration is higher. This provides direct evidence supporting recent observations of hopping conduction across grain boundaries in highly doped CdTe at low temperature.

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

    Science.gov (United States)

    Turi, Maria-Lynn

    1997-10-01

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

  10. Modelling of grain boundary effects in nanocrystalline/multicrystalline silicon heterojunction solar cells

    Science.gov (United States)

    Farrokh-Baroughi, Mahdi; Sivoththaman, Siva

    2006-07-01

    Heterojunction solar cells formed by nanocrystalline silicon films on fine-grained multicrystalline silicon substrates are simulated in the presence of grain boundaries. The effects of grain boundaries on the dark and illuminated current-voltage (I-V) characteristics and spectral response (SR) of heterojunction (HJ) solar cells are assessed using 1D and 2D device simulations. The grain boundary in fine-grained multicrystalline silicon is modelled in two ways: as a defective surface with continuous defect distribution throughout the bandgap, and as a hypothetical sheet with a certain recombination velocity for electrons and holes. The SR and I-V characteristics of HJs are exploited to characterize grain boundary effects on the photovoltaic properties of the solar cells and photodetectors. Simulation results show noticeable differences on the dark I-V and SR of on- and off-grain boundary HJs. Grain boundary effects become important when fine-grained multicrystalline substrates are used. Measurement results of tiny test structures fabricated on the grain boundary show consistently inferior dark I-V and SR characteristics compared to those fabricated away from the grain and allow us to quantify the recombination at the grain boundary.

  11. Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

    NARCIS (Netherlands)

    Boutz, Michel M.R.; Chen, Chu Sheng; Winnubst, Louis; Burggraaf, Anthonie J.

    1994-01-01

    The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (1150°C), the grain boundaries are covered by an ultrathi

  12. 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.

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

    Science.gov (United States)

    Kumar, Mohit; Basu, Tanmoy; Som, Tapobrata

    2016-01-01

    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/TiO2/SiO2/Si substrate. We also demonstrate an experimental evidence on local band bending at grain boundaries of AZO films using conductive atomic force microscopy and Kelvin probe force microscopy. The presence of an opposite polarization at grains and grain boundaries gives rise to a polarization-driven barrier formation at grain boundaries. With the help of conductive atomic force microscopy, we show that the polarization-driven barrier along with the defect-induced electrostatic potential barrier account for the measured local band bending at grain boundaries. The present study opens a new avenue to understand the charge transport in light of both polarization and electrostatic effects.

  14. 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.

  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. Effects of grain size and grain boundary on critical current density of high T(sub c) superconducting oxides

    Science.gov (United States)

    Zhao, Y.; Zhang, Q. R.; Zhang, H.

    1990-01-01

    By means of adding impurity elements in high T sub c oxides, the effects were studied of grain size and grain boundary on the critical current density of the following systems: YBa2Cu3O(7-y) and Bi-Pr-Sr-Ca-Cu-O. In order to only change the microstructure instead of the superconductivity of the grains in the samples, the impurity elements were added into the systems in terms of the methods like this: (1) substituting Y with the lanthanide except Pr, Ce, and Tb in YBa2Cu3O(7-y) system to finning down grains in the samples, therefore, the effect can be investigated of the grain size on the critical current density of 1:2:3 compounds; (2) mixing the high T sub c oxides with the metal elements, such as Ag, according to the composition of (high T sub c oxide)1-xAgx to metallize the grain boundaries in the samples, studying the effect of the electric conductivity of the grain boundaries on the critical current density; (3) adding SiO2, PbO2, and SnO2 into the high T sub c oxide to form impurity phases in the grain boundaries, trying to find out the effects of the impurity phases or metalloid grain boundaries on the critical current density of the high T sub c superconductors. The experimental results indicate that in the case of of the presence of the metalloid grain boundaries finning down grains fails to enhance the j sub c, but restrains it strongly, the granular high T sub c superconductors with the small size grains coupled weakly is always the low j sub c system.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2015-02-28

    (in the allowed KP bands) that transmit electrons unhindered, without reflections, while the electrons in the forbidden KP bands are localized. A distinctive feature of the quantum theory is that it provides a description of the temperature dependence of the resistivity where the contribution to the resistivity originating on electron-grain boundary scattering can be identified by a certain unique grain boundary reflectivity R, and the resistivity arising from electron-impurity scattering can be identified by a certain unique ℓ{sub IMP} mean free path attributable to impurity scattering. This is in contrast to the classical theory of Mayadas and Shatzkes (MS), that does not discriminate properly between a resistivity arising from electron-grain boundary scattering and that arising from electron-impurity scattering, for MS theory does not allow parameters (ℓ{sub IMP}, R) to be uniquely adjusted to describe the temperature dependence of the resistivity data. The same data can be described using different sets of (R, ℓ{sub IMP}); the latter parameter can be varied by two orders of magnitude in the case of small grained samples d < ℓ, and by a factor of 4 in the case of samples made out of large grains d > ℓ (where ℓ is the bulk mean free path at 300 K). For samples d > ℓ, the increase of resistivity is attributed not to electrons being partially reflected by the grain boundaries, but to a decrease in the number of states at the Fermi sphere that are allowed bands of the KP potential; hence the reflectivity required by the quantum model turns out to be an order of magnitude smaller than that required by the classical MS theory. For samples d < ℓ, the resistivity increase originates mainly from Anderson localization induced by electron grain boundary scattering from disordered successive grains characterized by a localization length of the order of 110 nm and not from electrons being partially reflected by grain boundaries; the outcome is that the

  18. Average widths of grain boundaries in nanophase alloys synthesized by mechanical attrition

    Science.gov (United States)

    Fultz, B.; Kuwano, H.; Ouyang, H.

    1995-04-01

    Many binary ferrous alloys were synthesized by mechanical attrition in a high-energy ball mill. X-ray diffractometry and transmission electron microscopy were used to measure grain sizes, which were as small as a few nanometers in several alloys. The nanocrystalline alloys showed new features in their Mössbauer spectra, which we associated with 57Fe atoms at and near grain boundaries. The experimental data on the fraction of 57Fe atoms at and near grain boundaries were correlated to the measured grain sizes to obtain an average width of the grain boundaries. The average grain-boundary widths of the fcc alloys Fe-Mn and Ni-Fe were approximately 0.5 nm, but the average widths of grain boundaries in the bcc alloys Cr-Fe, Mo-Fe, and Fe-Ti were somewhat larger than 1 nm.

  19. Intergranular brittle fracture of a low alloy steel induced by grain boundary segregation of impurities: influence of the microstructure; Rupture intergranulaire fragile d'un acier faiblement allie induite par la segregation d'impuretes aux joints de grains: influence de la microstructure

    Energy Technology Data Exchange (ETDEWEB)

    Raoul, St

    1999-07-01

    The study contributes to improve the comprehension of intergranular embrittlement induced by the phosphorus segregation along prior austenitic grain boundaries of low alloy steels used in pressurized power reactor vessel. A part of this study was performed using a A533 steel which contains chemical fluctuations (ghost lines) with two intensities. Axi-symmetrically notched specimens were tested and intergranular brittle de-cohesions were observed in the ghost lines. The fracture initiation sites observed on fracture surfaces were identified as MnS inclusions. A bimodal statistic obtained in a probabilistic model of the fracture is explained by the double population of ghost lines' intensities. A metallurgical study was performed on the same class of steel by studying the influence of the microstructure on the susceptibility to temper embrittlement. Brittle fracture properties of such microstructures obtained by dilatometric experiments were tested on sub-sized specimens to measure the V-notched fracture toughness. Fraction areas of brittle fracture modes were determined on surface fractures. A transition of the fracture mode with the microstructure is observed. It is shown that tempered microstructures of martensite and lower bainite are more susceptible to intergranular embrittlement than tempered upper bainitic microstructure. The intergranular fracture is the most brittle mode. The analysis of crystalline mis-orientations shows a grain boundary structure appreciably more coherent for tempered microstructures of martensite and lower bainite. The higher density of randomgrain boundaries is susceptible to drag the phosphorus in the upper bainitic matrix and to make the quantity of free phosphorus decreasing. Microstructure observations show a difference in the size and the spatial distribution of carbides, essentially cementite, between tempered martensite and upper bainite. It can explain the bigger susceptibility of this last microstructure to cleavage mode

  20. Grain boundary and its hydrogenated effect in stanene

    Energy Technology Data Exchange (ETDEWEB)

    Zhu, Zhili, E-mail: zlzhu@zzu.edu.cn, E-mail: jiayu@zzu.edu.cn; Sun, Qiang; Jia, Yu, E-mail: zlzhu@zzu.edu.cn, E-mail: jiayu@zzu.edu.cn [International Joint Research Laboratory for Quantum Functional Materials of Henan, and School of Physics and Engineering, Zhengzhou University, Zhengzhou, 450001 (China)

    2016-03-15

    The geometric and electronic properties of grain boundary (GB) in two-dimensional (2D) stanene have been investigated by first-principles calculations. Four typical GB structures with particularly low formation energies were found. These extended defects act as quasi-one-dimensional semiconductor or metallic wires depending on their geometric structures. Moreover, they are reactive and the adsorption of H atoms at the GB region is more stable than the stanene bulk region. A single H adsorption poses a drastic effect on the electronic behavior of GB defects, and the band structures can be tuned by the coverages of H adsorption at these GB defects in stanene. The present results indicate that GBs are important defects in stanene which may be useful for nanomaterial devices.

  1. 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.)

  2. Recombination by grain-boundary type in CdTe

    Energy Technology Data Exchange (ETDEWEB)

    Moseley, John, E-mail: john.moseley@nrel.gov; Ahrenkiel, Richard K. [National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401 (United States); Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401 (United States); Metzger, Wyatt K.; Moutinho, Helio R.; Guthrey, Harvey L.; Al-Jassim, Mowafak M. [National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401 (United States); Paudel, Naba; Yan, Yanfa [Department of Physics & Astronomy, University of Toledo, Toledo, Ohio 43606 (United States)

    2015-07-14

    We conducted cathodoluminescence (CL) spectrum imaging and electron backscatter diffraction on the same microscopic areas of CdTe thin films to correlate grain-boundary (GB) recombination by GB “type.” We examined misorientation-based GB types, including coincident site lattice (CSL) Σ = 3, other-CSL (Σ = 5–49), and general GBs (Σ > 49), which make up ∼47%–48%, ∼6%–8%, and ∼44%–47%, respectively, of the GB length at the film back surfaces. Statistically averaged CL total intensities were calculated for each GB type from sample sizes of ≥97 GBs per type and were compared to the average grain-interior CL intensity. We find that only ∼16%–18% of Σ = 3 GBs are active non-radiative recombination centers. In contrast, all other-CSL and general GBs are observed to be strong non-radiative centers and, interestingly, these GB types have about the same CL intensity. Both as-deposited and CdCl{sub 2}-treated films were studied. The CdCl{sub 2} treatment reduces non-radiative recombination at both other-CSL and general GBs, but GBs are still recombination centers after the CdCl{sub 2} treatment.

  3. 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.

  4. Tunneling Magneto-resistance in grain boundary tailored Fe3O4 nano structured thin films

    Science.gov (United States)

    Kumar, Ankit; Behera, Nilamani; Husain, Sajid; Chaudhary, Sujeet; Pandya, Dinesh K.

    2017-05-01

    The pulse DC sputtered nanostructured Fe3O4 thin films have been subjected to hydrocarbon treatment and vacuum annealing to investigate their effect on grain boundaries. Remarkably, this treatment causes a change in the crystallographic structure of the film from polycrystalline to (440) oriented one and enhancement in film conductivity. Hopping conduction mechanism changes to nearest-neighbor hoping above Verwey transition temperature of 110 K on treatment. Treatment also leads to grain boundary modification by incorporation of antiferro defects that inhibit an increase in MR despite enhanced electron conduction across grain boundaries. Our results follow the non-homogeneous grain boundary model.

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

    Data.gov (United States)

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

  6. 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.

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

    Science.gov (United States)

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

    2015-12-21

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

  8. Reducing Grain-Boundary Resistivity of Copper Nanowires by Doping

    Science.gov (United States)

    César, Mathieu; Gall, Daniel; Guo, Hong

    2016-05-01

    The resistance of doped single grain boundaries (GBs) in copper is calculated from first principles and systematically compared to its pure single GB equivalent. As a first step, a state-of-the-art ab initio method is used to calculate the resistivity of doped bulk copper for 16 doping elements at concentration 1 at. %. Results are in qualitatively excellent and quantitatively reasonable agreement with the corresponding experimental data, and allow us to determine Ag, Zn, Mg, Pd, Al, and In as best candidates for GB doping. These atoms have a minimal impact on the bulk resistivity, while they also conform to a set of established criteria for alloying with copper. Then, the specific resistivity of six twin GBs is determined for these elements over a wide spectrum of doping concentrations for the submonolayer and the monolayer GB complexions. Reduced resistivity is observed for Zn, Mg, Al, In, and other elements in two high-Σ GBs, and is qualitatively related to the segregation enthalpy as well as to a low number of empty states around the Fermi energy in the boundary plane region of the GB. The results indicate the possibility for a reduced net resistivity in copper interconnects by GB doping.

  9. Multi-phase-field study of the effects of anisotropic grain-boundary properties on polycrystalline grain growth

    Science.gov (United States)

    Miyoshi, Eisuke; Takaki, Tomohiro

    2017-09-01

    Numerical studies of the effects of anisotropic (misorientation-dependent) grain-boundary energy and mobility on polycrystalline grain growth have been carried out for decades. However, conclusive knowledge has yet to be obtained even for the simplest two-dimensional case, which is mainly due to limitations in the computational accuracy of the grain-growth models and computer resources that have been employed to date. Our study attempts to address these problems by utilizing a higher-order multi-phase-field (MPF) model, which was developed to accurately simulate grain growth with anisotropic grain-boundary properties. In addition, we also employ general-purpose computing on graphics processing units to accelerate MPF grain-growth simulations. Through a series of simulations of anisotropic grain growth, we succeeded in confirming that both the anisotropies in grain-boundary energy and mobility affect the morphology formed during grain growth. On the other hand, we found the grain growth kinetics in anisotropic systems to follow parabolic law similar to isotropic growth, but only after an initial transient period.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2010-04-01

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

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

    Institute of Scientific and Technical Information of China (English)

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

    2006-01-01

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

  12. 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.

  13. Decoherence induced by fluctuating boundaries

    CERN Document Server

    De Lorenci, V A

    2012-01-01

    The effects of fluctuating boundaries on a superposition state of a quantum particle in a box is studied. We consider a model in one space dimension in which the initial state is a coherent superposition of two energy eigenstates. The locations of the walls of the box are assumed to undergo small fluctuation with a Gaussian probability distribution. The spatial probability density of the particle contains an interference term, which is found to decay in time due to the boundary fluctuations. At late times, this term vanishes and the quantum coherence is lost. The system is now described by a density matrix rather than a pure quantum state.This model gives a simple illustration of how environment-induced decoherence can take place in quantum systems. It can also serve as an analog model for the effects of spacetime geometry fluctuations on quantum systems.

  14. Grain boundary traction signatures: Quantifying the asymmetrical dislocation emission processes under tension and compression

    Science.gov (United States)

    Li, Ruizhi; Chew, Huck Beng

    2017-06-01

    The disruption in crystallographic arrangement of atoms across a grain boundary interface generates local stress fields in the vicinity. Here, we reconstruct the continuum-equivalent grain boundary tractions from local atomic stresses near symmetrical-tilt Ni grain boundaries. We show that the resolved shear stress contribution from the grain boundary tractions, τGB, along active slip-systems either assists or prevents the emission of dislocations, depending on its direction with respect to the resolved shear stress contribution from external loading, τext. When τGB acts in the same direction as τext, Shockley partial dislocations are readily emitted from the boundary once | τGB +τext | exceeds the critical barrier stress for shear-slip. When τGB opposes τext, the higher sustainable stresses in the grain boundary structure instead triggers: (a) emission of dislocations from the bulk, or (b) reconfiguration of the grain boundary atomic structure and subsequent emission of non-Schmid dislocations or formation of extrinsic stacking faults. Our results quantitatively explain the asymmetrical grain boundary dislocation emission processes observed in molecular dynamics (MD) simulations under applied tensile and compressive loads. The relationship between the traction signatures and periodic structural units along the grain boundary is discussed.

  15. 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

  16. 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)

  17. 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.

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

    Science.gov (United States)

    Reda Chellali, Mohammed; Balogh, Zoltan; Schmitz, Guido

    2013-09-01

    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.

  19. The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation

    NARCIS (Netherlands)

    Jonas, Laura; John, Timm; King, Helen E.; Geisler, Thorsten; Putnis, Andrew

    2014-01-01

    The pseudomorphic replacement of Carrara marble by calcium phosphates was used as a model system in order to study the influence of different fluid pathways for reaction front propagation induced by fluid-rock interaction. In this model, grain boundaries present in the rock as well as the transient

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

    NARCIS (Netherlands)

    Ayas, C; Van der Giessen, E

    2009-01-01

    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 i

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

    NARCIS (Netherlands)

    Ayas, C; Van der Giessen, E

    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

  2. Modeling of grain boundary stresses in Alloy 600

    Energy Technology Data Exchange (ETDEWEB)

    Kozaczek, K.J. [Oak Ridge National Lab., TN (United States); Sinharoy, A.; Ruud, C.O. [Pennsylvania State Univ., University Park, PA (United States); Mcllree, A.R. [Electric Power Research Inst., Palo Alto, CA (United States)

    1995-04-01

    Corrosive environments combined with high stress levels and susceptible microstructures can cause intergranular stress corrosion cracking (IGSCC) of Alloy 600 components on both primary and secondary sides of pressurized water reactors. One factor affecting the IGSCC is intergranular carbide precipitation controlled by heat treatment of Alloy 600. This study is concerned with analysis of elastic stress fields in vicinity of M{sub 7}C{sub 3} and M{sub 23}C{sub 6} carbides precipitated in the matrix and at a grain boundary triple point. The local stress concentration which can lead to IGSCC initiation was studied using a two-dimensional finite element model. The intergranular precipitates are more effective stress raisers than the intragranular precipitates. The combination of the elastic property mismatch and the precipitate shape can result in a local stress field substantially different than the macroscopic stress. The maximum local stresses in the vicinity of the intergranular precipitate were almost twice as high as the applied stress.

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

    Energy Technology Data Exchange (ETDEWEB)

    Zhang, Lei, E-mail: zhang_lei@iapcm.ac.cn [CEA, DEN, Service de Recherches de Métallurgie Physique, F-91191 Gif-sur-Yvette (France); Department of Physics, Beihang University, Beijing 100191 (China); Fu, Chu-Chun, E-mail: chuchun.fu@cea.fr [CEA, DEN, Service de Recherches de Métallurgie Physique, F-91191 Gif-sur-Yvette (France); Hayward, Erin [CEA, DEN, Service de Recherches de Métallurgie Physique, F-91191 Gif-sur-Yvette (France); Lu, Guang-Hong [Department of Physics, Beihang University, Beijing 100191 (China)

    2015-04-15

    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.

  4. Disjoining potential and grain boundary premelting in binary alloys

    Science.gov (United States)

    Hickman, J.; Mishin, Y.

    2016-06-01

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

  5. 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.

  6. 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

  7. Evidence for grain boundary passivation by oxidation in polycrystalline GaAs solar cells

    Science.gov (United States)

    Kazmerski, L. L.; Ireland, P. J.

    1980-02-01

    The chemistry and composition of grain boundaries in polycrystalline GaAs grown by liquid phase epitaxy (LPE) and molecular beam epitaxy (MBE) are examined using complementary Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). The effects of an unintentional residual-oxygen partial pressure during LPE growth are investigated in terms of grain boundary passivation. Depth-compositional data verify the grain boundary localization of oxides using an in situ, UHV fracturing technique. Indications of distributions of these oxides over the grain boundary are presented. The performances of Au Schottky barrier solar cells fabricated from the polycrystalline LPE and MBE GaAs are compared and differences are explained in terms of grain boundary activity.

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

    Institute of Scientific and Technical Information of China (English)

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

    2005-01-01

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

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

    Institute of Scientific and Technical Information of China (English)

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

    2011-01-01

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

  10. Microstructural change around grain boundary in polycrystalline IN100 during creep deformation

    Energy Technology Data Exchange (ETDEWEB)

    Takahashi, Satoshi; Kubushiro, Keiji; Morishima, Keiko [Research Laboratory IHI Co. Ltd., Yokohama (Japan); Miura, Nobuhiro; Kondo, Yoshihiro [National Defense Academy, Yokosuka (Japan)

    2010-07-01

    The microstructural change of polycrystalline IN100 has been characterized. Tensile creep tests were conducted in the temperature range 950 to 1000 C at 74MPa and some tests were interrupted at 6, 16, 20, 30, 60 and 80% in creep life. Raft structure formed on (001) at the early stage of creep life in interior grains and grain boundaries were covered by {gamma}' phase. Creep fracture occurred at grain boundaries. In order to quantify the creep damage, average grain orientation spreads (GOS) of ruptured and interruptured specimens were measured. Average GOS of interrupted creep specimen after 6% of creep life consumption was about 0.6 degree and this value was almost constant until 80%, which did not show a correlation between average GOS and creep life consumption rate. Then we focused to the grain boundaries. The thickness of grain boundary {gamma}' phase increased as creep life consumption increased and the range of thickness was about 5-10{mu}m. The orientation of gamma prime phase at the grain boundary was same as that of interior of the grain and both of them were joined. Rotation angle of grain boundary {gamma}' was measured and there was a good correlation with creep life consumption. (orig.)

  11. Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy

    Science.gov (United States)

    Karvonen, Lasse; Säynätjoki, Antti; Huttunen, Mikko J.; Autere, Anton; Amirsolaimani, Babak; Li, Shisheng; Norwood, Robert A.; Peyghambarian, Nasser; Lipsanen, Harri; Eda, Goki; Kieu, Khanh; Sun, Zhipei

    2017-06-01

    Grain boundaries have a major effect on the physical properties of two-dimensional layered materials. Therefore, it is important to develop simple, fast and sensitive characterization methods to visualize grain boundaries. Conventional Raman and photoluminescence methods have been used for detecting grain boundaries; however, these techniques are better suited for detection of grain boundaries with a large crystal axis rotation between neighbouring grains. Here we show rapid visualization of grain boundaries in chemical vapour deposited monolayer MoS2 samples with multiphoton microscopy. In contrast to Raman and photoluminescence imaging, third-harmonic generation microscopy provides excellent sensitivity and high speed for grain boundary visualization regardless of the degree of crystal axis rotation. We find that the contrast associated with grain boundaries in the third-harmonic imaging is considerably enhanced by the solvents commonly used in the transfer process of two-dimensional materials. Our results demonstrate that multiphoton imaging can be used for fast and sensitive characterization of two-dimensional materials.

  12. APPLICATION OF THE ANALYTICAL ELECTRON MICROSCOPE TO THE STUDY OF GRAIN BOUNDARY CHEMISTRY

    OpenAIRE

    Hall, E

    1982-01-01

    High spatial resolution X-ray spectroscopy in the analytical electron microscope (AEM) is a powerful tool for the study of changes in chemistry which occur at grain boundaries in metals and ceramics. Two major advantages are realized through the use of the AEM in these studies : the ability to obtain accurate quantitative microchemical analysis of grain boundary regions, and the capability for determining the structural and crystallographic characteristics of the boundaries on which the chemi...

  13. Monte Carlo study on abnormal growth of Goss grains in Fe3%Si steel induced by second-phase particles

    Institute of Scientific and Technical Information of China (English)

    Dong-qun Xin; Cheng-xu He; Xue-hai Gong; Hao Wang; Li Meng; Guang Ma; Peng-fei Hou; Wen-kang Zhang

    2016-01-01

    The selective abnormal growth of Goss grains in magnetic sheets of Fe-3%Si (grade Hi-B) induced by second-phase particles (AlN and MnS) was studied using a modified Monte Carlo Potts model. The starting microstructures for the simulations were generated from electron backscatter diffraction (EBSD) orientation imaging maps of recrystallized samples. In the simulation, second-phase particles were assumed to be randomly distributed in the initial microstructures and the Zener drag effect of particles on Goss grain boundaries was as-sumed to be selectively invalid because of the unique properties of Goss grain boundaries. The simulation results suggest that normal growth of the matrix grains stagnates because of the pinning effect of particles on their boundaries. During the onset of abnormal grain growth, some Goss grains with concave boundaries in the initial microstructure grow fast abnormally and other Goss grains with convex boundaries shrink and eventually disappear.

  14. Monte Carlo study on abnormal growth of Goss grains in Fe-3%Si steel induced by second-phase particles

    Science.gov (United States)

    Xin, Dong-qun; He, Cheng-xu; Gong, Xue-hai; Wang, Hao; Meng, Li; Ma, Guang; Hou, Peng-fei; Zhang, Wen-kang

    2016-12-01

    The selective abnormal growth of Goss grains in magnetic sheets of Fe-3%Si (grade Hi-B) induced by second-phase particles (AlN and MnS) was studied using a modified Monte Carlo Potts model. The starting microstructures for the simulations were generated from electron backscatter diffraction (EBSD) orientation imaging maps of recrystallized samples. In the simulation, second-phase particles were assumed to be randomly distributed in the initial microstructures and the Zener drag effect of particles on Goss grain boundaries was assumed to be selectively invalid because of the unique properties of Goss grain boundaries. The simulation results suggest that normal growth of the matrix grains stagnates because of the pinning effect of particles on their boundaries. During the onset of abnormal grain growth, some Goss grains with concave boundaries in the initial microstructure grow fast abnormally and other Goss grains with convex boundaries shrink and eventually disappear.

  15. 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.

  16. Ferromagnetic behaviour of ZnO: the role of grain boundaries

    Directory of Open Access Journals (Sweden)

    Boris B. Straumal

    2016-12-01

    Full Text Available The possibility to attain ferromagnetic properties in transparent semiconductor oxides such as ZnO is very promising for future spintronic applications. We demonstrate in this review that ferromagnetism is not an intrinsic property of the ZnO crystalline lattice but is that of ZnO/ZnO grain boundaries. If a ZnO polycrystal contains enough grain boundaries, it can transform into the ferromagnetic state even without doping with “magnetic atoms” such as Mn, Co, Fe or Ni. However, such doping facilitates the appearance of ferromagnetism in ZnO. It increases the saturation magnetisation and decreases the critical amount of grain boundaries needed for FM. A drastic increase of the total solubility of dopants in ZnO with decreasing grain size has been also observed. It is explained by the multilayer grain boundary segregation.

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

    Science.gov (United States)

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

    2016-04-13

    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.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2012-10-05

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

  19. Incoherent twin boundary migration induced by ion irradiation in Cu

    Energy Technology Data Exchange (ETDEWEB)

    Li, N.; Misra, A. [Center for Integrated Nanotechnologies, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Wang, J.; Wang, Y. Q. [Materials Science and Technology Division, MST-8, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Serruys, Y. [CEA, DEN, Service de Recherches de Metallurgie Physique, Laboratoire JANNUS, F-91191 Gif-sur-Yvette (France); Nastasi, M. [Nebraska Center for Energy Sciences Research, University of Nebraska, Lincoln, Nebraska 68588 (United States)

    2013-01-14

    Grain boundaries can act as sinks for radiation-induced point defects. The sink capability is dependent on the atomic structures and varies with the type of point defects. Using high-resolution transmission electron microscopy, we observed that {Sigma}3{l_brace}112{r_brace} incoherent twin boundary (ITB) in Cu films migrates under Cu{sup 3+} ion irradiation. Using atomistic modeling, we found that {Sigma}3{l_brace}112{r_brace} ITB has the preferred sites for adsorbing interstitials and the preferential diffusion channels along the Shockley partial dislocations. Coupling with the high mobility of grain boundary Shockley dislocations within {Sigma}3{l_brace}112{r_brace} ITB, we infer that {Sigma}3{l_brace}112{r_brace} ITB migrates through the collective glide of grain boundary Shockley dislocations, driven by a concurrent reduction in the density of radiation-induced defects, which is demonstrated by the distribution of nearby radiation-induced defects.

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

    Science.gov (United States)

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

    2015-11-01

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

  1. Imaging the Structure of Grains, Grain Boundaries, and Stacking Sequences in Single and Multi-Layer Graphene

    Science.gov (United States)

    Muller, David

    2012-02-01

    Graphene can be produced by chemical vapor deposition (CVD) on copper substrates on up to meter scales [1, 2], making their polycrystallinity [3,4] almost unavoidable. By combining aberration-corrected scanning transmission electron microscopy and dark-field transmission electron microscopy, we image graphene grains and grain boundaries across six orders of magnitude. Atomic-resolution images of graphene grain boundaries reveal that different grains can stitch together via pentagon-heptagon pairs. We use diffraction-filtered electron imaging to map the shape and orientation of several hundred grains and boundaries over fields of view of a hundred microns. Single, double and multilayer graphene can be differentiated, and the stacking sequence and relative abundance of sequences can be directly imaged. These images reveal an intricate patchwork of grains with structural details depending strongly on growth conditions. The imaging techniques enabled studies of the structure, properties, and control of graphene grains and grain boundaries [5]. [4pt] [1] X. Li et al., Science 324, 1312 (2009).[0pt] [2] S. Bae et al., Nature Nanotechnol. 5, 574 (2010).[0pt] [3] J. M. Wofford, et al., Nano Lett., (2010).[0pt] [4] P. Y. Huang, et al., Nature 469, 389--392 (2011); arXiv:1009.4714, (2010)[0pt] [5] In collaboration with Pinshane Y. Huang, C. S. Ruiz-Vargas, A. M. van der Zande, A. W. Tsen, L. Brown, R. Hovden, F. Ghahari, W. S. Whitney, M.P. Levendorf, J. W. Kevek, S. Garg, J. S. Alden, C. J. Hustedt, Y. Zhu, N. Petrone, J. Hone, J. Park, P. L. McEuen

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2015-09-15

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

  3. Quantifying the strain due to grain boundary sliding of forsterite using atomic force microscopy

    Science.gov (United States)

    Dillman, A. M.; Kohlstedt, D. L.

    2016-12-01

    Grain boundary sliding (GBS) is a deformation process that requires either diffusion or dislocation motion in order to maintain fully dense samples and deform at a steady-state strain rate. When accommodated by diffusion (diffusion creep), this deformation is characterized by a Newtonian viscosity with a strong dependence on grain size. To better understand the combination of diffusion and sliding, explicit measurements of the degree of grain boundary sliding, taken at a very fine scale, are necessary. High purity, synthetic forsterite was chosen to study the contribution of GBS, as it can be created with a grain size of 1 μm and its relatively sluggish grain growth kinetics limit the grain size to generally right prisms, with at least one face polished flat. This face, oriented parallel to the applied stress, was imaged using atomic force microscopy after deformation. The amount of strain due to grain boundary sliding was determined using high resolution measurements of topography. A methodology was developed to account for thermal grooving of the polished face and to objectively quantify the ratio of grain boundary sliding strain to total strain, ξ. For the differential stresses applied, ξ = 63%. This ratio was independent of strain and decreased slightly with increasing grain size. This value of 63% is very similar to experimentally determined values of ξ for pure, fine grained alumina. With this new methodology for determining the strain due to grain boundary sliding, we will be able to determine the point of transition between different deformation regimes that require different contributions of grain boundary sliding.

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

    DEFF Research Database (Denmark)

    Nørbygaard, Thomas

    of twin boundary structures. Ledges on low deviation Σ3 and Σ9 were observed repeatedly and the presence of these is proposed to relate to the inactivity of such boundaries. A handful of examples showed ledged boundaries emitting dislocations into the lattice, presumably because of stress accumulation due...

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2015-11-14

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2009-07-15

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

  7. GRAIN BOUNDARY CHEMISTRY IN Ni3Al AND Ni3Si

    OpenAIRE

    Baker, I; Schulson, E.; Michael, J.; Padgett, R.

    1990-01-01

    Measurements of the Ni/Al ratio at grain boundary regions in Ni3Al have been performed, using x-ray microanalysis of thin foils in a scanning transmission electron microscope, as a function of both alloy stoichiometry and the addition of boron. It is shown that whilst grain boundaries of Al-rich and stoichiometric Ni3Al have a composition similar to the matrix composition, boundaries in Ni-rich Ni3Al can be highly Ni-enriched. The addition of B to Ni3Al produces Ni enrichment in grain boundar...

  8. 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.

  9. Boundary induced nonlinearities at small Reynolds numbers

    NARCIS (Netherlands)

    Sbragaglia, M.; Sugiyama, K.

    2007-01-01

    We investigate the importance of boundary slip at finite Reynolds numbers for mixed boundary conditions. Nonlinear effects are induced by the non-homogeneity of the boundary condition and change the symmetry properties of the flow with an overall mean flow reduction. To explain the observed drag

  10. SILICON CARBIDE GRAIN BOUNDARY DISTRIBUTIONS, IRRADIATION CONDITIONS, AND SILVER RETENTION IN IRRADIATED AGR-1 TRISO FUEL PARTICLES

    Energy Technology Data Exchange (ETDEWEB)

    Lillo, T. M.; Rooyen, I. J.; Aguiar, J. A.

    2016-11-01

    Precession electron diffraction in the transmission electron microscope was used to map grain orientation and ultimately determine grain boundary misorientation angle distributions, relative fractions of grain boundary types (random high angle, low angle or coincident site lattice (CSL)-related boundaries) and the distributions of CSL-related grain boundaries in the SiC layer of irradiated TRISO-coated fuel particles. Two particles from the AGR-1 experiment exhibiting high Ag-110m retention (>80%) were compared to a particle exhibiting low Ag-110m retention (<19%). Irradiated particles with high Ag-110m retention exhibited a lower fraction of random, high angle grain boundaries compared to the low Ag-110m retention particle. An inverse relationship between the random, high angle grain boundary fraction and Ag-110m retention is found and is consistent with grain boundary percolation theory. Also, comparison of the grain boundary distributions with previously reported unirradiated grain boundary distributions, based on SEM-based EBSD for similarly fabricated particles, showed only small differences, i.e. a greater low angle grain boundary fraction in unirradiated SiC. It was, thus, concluded that SiC layers with grain boundary distributions susceptible to Ag-110m release were present prior to irradiation. Finally, irradiation parameters were found to have little effect on the association of fission product precipitates with specific grain boundary types.

  11. How resistive must grain boundaries in polycrystalline superconductors be, to limit J c?

    Science.gov (United States)

    Wang, Guanmei; Raine, Mark J.; Hampshire, Damian P.

    2017-10-01

    Although we can use misorientation angle to distinguish the grain boundaries that can carry high critical current density ({J}{{c}}) in high temperature superconductors (HTS) from those that cannot, there is no established normal state property equivalent. In this paper, we explore the superconducting and normal state properties of the grains and grain boundaries of polycrystalline YBa2Cu3O7-x (YBCO) using complementary magnetisation and transport measurements, and calculate how resistive grain boundaries must be to limit {J}{{c}} in polycrystalline superconductors. The average resistivity of the grain boundaries, {ρ }{{GB}}, in our micro- and nanocrystalline YBCO are 0.12 Ωm and 8.2 Ωm, values which are much higher than that of the grains ({ρ }{{G}}) and leads to huge {ρ }{{GB}}/{ρ }{{G}} values of 2 × 103 and 1.6 × 105 respectively. We find that the grain boundaries in our polycrystalline YBCO are sufficiently resistive that we can expect the transport {J}{{c}} to be several tens of orders of magnitude below the potential current density of the grains in our YBCO samples, as is found experimentally. Calculations presented show that increasing {J}{{c}} values by ˜2 orders of magnitude in high fields is still possible in all state-of-the-art technological high-field superconductors. We conclude: grain boundary engineering is unlikely to improve {J}{{c}} sufficiently in randomly aligned polycrystalline YBCO, to make it technologically useful for high-field applications; in low temperature superconducting intermetallics, such as Nb3Sn, large increases in {J}{{c}} may be achieved by completely removing the grain boundaries from these materials and, as is the case for thin films of Nb, Ba(FeCo)2As2 and HTS materials, by incorporating additional artificial pinning.

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

    Institute of Scientific and Technical Information of China (English)

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

    2005-01-01

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

  13. Investigation of olivine and orthopyroxene grain boundaries by atom probe tomography

    Science.gov (United States)

    Krawczynski, M.; Skemer, P. A.; Bachhav, M.; Dong, Y.; Marquis, E. A.

    2016-12-01

    Accurate chemical analysis at grain boundaries is challenging by traditional microscopic techniques, especially for poor conducting geological samples. Atom probe tomography (APT) is a unique technique that can elucidate chemistry and 3-D distribution of elements within a sample volume at the sub-nanometer length scale. With advances in laser and sample preparation techniques in the last decade, APT is now successfully applied to a wide range of poor conducting materials like metal oxides, ceramics, and biological minerals. In this study, we apply the APT technique to investigate the grain boundary chemistry of orthopyroxene (opx) and olivine. These minerals are the most abundant in the upper mantle and their grain boundaries may be important geochemical reservoirs in Earth. Moreover, physical properties such as grain boundary diffusivity, conductivity, and mobility, are likely influenced by the presence or absence of impurities. Single crystals of opx and olivine grains, separated from a San Carlos xenolith, were deformed at 1 GPa and 1500 K. Plastic deformation promoted dynamic recrystallization, creating new grain boundaries within a chemically homogeneous medium. Needle shaped specimens of opx-opx and olivine-olivine grain boundaries were prepared using standard lift out techniques and a dual beam focused ion beam (FIB). APT analyses were performed in laser mode with laser energy of 50 pJ/pulse, repetition rate of 200 kHz, and detection rate of 1%. A 3-D distribution of elements was reconstructed and 1-D profiles across the grain boundary have been calculated. Fe, Al, and Ca show enrichments at the grain boundaries for both phases, consistent with previous studies that used STEM/EDX or EPMA techniques. Although qualitatively similar, the spatial resolution of the APT method is significantly better than other methods, and our data show that the grain-boundary enrichment of minor elements in both olivine and pyroxene compositions is limited to a region no greater

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2011-07-01

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

  15. Grain boundary sliding associated with low strain rate at 1000°C in recrystallized ODS ferritic steel

    Directory of Open Access Journals (Sweden)

    R. Kamikawa

    2016-12-01

    Full Text Available The high-temperature deformation process of the recrystallized 16CrODS ferritic steel was investigated at 1000°C for the stress loading perpendicular to the elongated grain structure. The strain rate was varied in the range from 1.0×10−2 to 1.0×10−5s−1. At the strain rate over 1.0×10−4s−1, deformation is dominated by the conventional dislocation creep. Decreasing strain rate from 1.0×10−4s−1, grain boundary sliding becomes prominent. Accommodation process for the localized stress induced by grain boundary sliding could be dislocation creep at 1.0×10−4s−1, and by diffusional creep at 1.0×10−5s−1 or less. These were verified through the observation of void formation and localized strain accumulation by KAM map.

  16. Quantitative TEM analysis of precipitation and grain boundary segregation in neutron irradiated EUROFER 97

    Science.gov (United States)

    Dethloff, Christian; Gaganidze, Ermile; Aktaa, Jarir

    2014-11-01

    Characterization of irradiation induced microstructural defects is essential for assessing the applicability of structural steels like the Reduced Activation Ferritic/Martensitic steel EUROFER 97 in upcoming fusion reactors. In this work Transmission Electron Microscopy (TEM) is used to analyze the types and structure of precipitates, and the evolution of their size distributions and densities caused by neutron irradiation to a dose of 32 displacements per atom (dpa) at 330-340 °C in the irradiation experiment ARBOR 1. A significant growth of MX and M23C6 type precipitates is observed after neutron irradiation, while the precipitate density remains unchanged. Hardening caused by MX and M23C6 precipitate growth is assessed by applying the Dispersed Barrier Hardening (DBH) model, and shown to be of minor importance when compared to other irradiation effects like dislocation loop formation. Additionally, grain boundary segregation of chromium induced by neutron irradiation was investigated and detected in irradiated specimens.

  17. 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.

  18. Investigations of the electrical neutralization and bonding mechanisms of shallow impurities in silicon grain boundaries

    Science.gov (United States)

    Kazmerski, L. L.; Nelson, A. J.; Dhere, R. G.; Abou-Elfotouh, F.

    Interactions between shallow acceptors (B, Al, Ga and In) and hydrogen in polycrystalline Si are investigated. The bonding mechanisms involved in the acceptor neutralization process at grain boundaries are examined using microanalytical techniques. Differences in the incorporation of molecular and atomic hydrogen, and corresponding variations in electrical passivation at grain boundaries, are observed. Low-temperature Auger difference spectroscopy confirms Si-H bonding to dominate B, Ga and In-doped cases, with no direct acceptor-hydrogen bonding. Al-rich grain boundaries show H-complex and hydroxyl bonding. The data confirm chemical bond strength trends with B less than Ga less than In. Volume-indexed Auger electron spectroscopy is utilized to compare bonding and H-distributions in B- and Al-rich grain boundary regions.

  19. Neutralization and bonding mechanisms of shallow acceptors at grain boundaries in polycrystalline silicon

    Energy Technology Data Exchange (ETDEWEB)

    Kazmerski, L.L.; Nelson, A.J.; Dhere, R.G.; Yahia, A.; Abou-Elfotouh, F.

    1988-05-01

    Interactions between shallow acceptors (B, Al, Ga, and In) and hydrogen in polycrystalline Si are investigated. The bonding mechanisms involved in the acceptor neutralization process at grain boundaries are examined using microanalytical techniques. Differences in the incorporation of molecular and atomic hydrogen, and corresponding variations in electrical passivation at grain boundaries, are observed. Low-temperature Auger difference spectroscopy confirms Si--H bonding to dominate B, Ga, and In-doped cases, with no direct acceptor--hydrogen bonding. Al-rich grain boundaries show H-complex and hydroxyl bonding. The data confirm chemical bond strength trends with Bgrain boundary regions.

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

    Science.gov (United States)

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

    2016-02-19

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

  1. Studies on the grain boundary effect in polycrystalline CdTe films using optical reflectance measurements

    Energy Technology Data Exchange (ETDEWEB)

    Dutta, J. (Dept. of Materials Science, Indian Association for the Cultivation of Science, Calcutta (India)); Pal, R. (Dept. of Materials Science, Indian Association for the Cultivation of Science, Calcutta (India)); Bhattacharyya, S.K. (Central Glass and Ceramic Research Inst., Calcutta (India)); Chaudhuri, S. (Dept. of Materials Science, Indian Association for the Cultivation of Science, Calcutta (India)); Pal, A.K. (Dept. of Materials Science, Indian Association for the Cultivation of Science, Calcutta (India))

    1993-11-15

    The grain boundary effect in polycrystalline CdTe films deposited at various substrate temperatures has been studied critically. The grain boundary potential, the density of trap states at the boundary region and the carrier concentration in the films were obtained by an alternative technique that utilizes the reflectance measurements of the highly resistive films deposited on a nonabsorbing substrate. The barrier height in the CdTe films decreased from 0.34 to 0.2 eV as the grain size increased from 60 to 133 nm, owing to the increase in the deposition temperature from 373 to 523 K. Correspondingly, the density of trap states in the grain boundary region decreased from 1.63x10[sup 13] to 6.15x10[sup 12] cm[sup -2]. (orig.)

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2007-04-15

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

  3. Structure and bonding properties of Y doped ∑37grain boundary in alumina

    Institute of Scientific and Technical Information of China (English)

    Wang Ya-Bin; Zhang Gang; Liu Ming-Jie; Chen Xiang-Long; Chen Jun

    2009-01-01

    The microscopic structures and the bonding properties of Y-doped and undoped (0118)/[0441]/180° (∑37) grain boundaries in alumina are investigated by using ab initio method. The formation energy of grain boundary and the segregation energy of Yto grain boundary are acquired. Electronic structures, potential distributions, bond orders and effective charges of Y-doped and undoped ∑37 GB systems are calculated. Our results reveal that the higher strength Y-O bond than Al-O bond is ascribed to the hybridization of Y(4p, 3d) with O(2s). Meanwhile, dopant Y also causes a change in potential distribution in the grain boundary region, thereby further affecting the transport property of ceramic alumina.

  4. 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.

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

    Science.gov (United States)

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

    2015-11-01

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

  6. 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.

  7. Grain boundary defect chemistry of acceptor-doped titanates: Space charge layer width

    Energy Technology Data Exchange (ETDEWEB)

    Vollman, M.; Waser, R. [Inst. fuer Werkstoffe der Elektrotechnik, Aachen (Germany)

    1994-01-01

    The grain boundary space charge depletion layers in acceptor-doped SrTiO{sub 3} ceramics were investigated by impedance spectroscopy in the time and frequency domain. Based on the layer and its dependence on the acceptor concentration, the temperature, and the oxygen partial pressure during annealing, a suggestion for a refined Schottky model is proposed. The local distribution of the donor type grain boundary states causing the depletion layer and the resulting band bending are discussed.

  8. Effects of grain boundary characteristics of steel on magnetoacoustic emission spectra

    Science.gov (United States)

    Namkung, M.; Yost, W. T.; Utrata, D.; Grainger, J. L.; Kushnick, P. W.

    1989-01-01

    Consideration is given to the effects of grain boundary characteristics on the properties of a magnetoacoustic emission spectra obtained by external ac magnetic field-driven domain wall motions. In studies with HY80 steel samples, it is found that the domain wall-defect interaction enhances as more grain boundary is introduced. The enhancement of the domain wall-defect interaction generates high amplitude magnetoacoustic emission pulses and reduces the rate of magnetoacoustic emission events by limiting domain wall motions.

  9. Meso-microstructural computational simulation of the hydrogen permeation test to calculate intergranular, grain boundary and effective diffusivities

    Energy Technology Data Exchange (ETDEWEB)

    Jothi, S., E-mail: s.jothi@swansea.ac.uk [College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP (United Kingdom); Winzer, N. [Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstraße 11, 79108 Freiburg (Germany); Croft, T.N.; Brown, S.G.R. [College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP (United Kingdom)

    2015-10-05

    Highlights: • Characterized polycrystalline nickel microstructure using EBSD analysis. • Development meso-microstructural model based on real microstructure. • Calculated effective diffusivity using experimental electrochemical permeation test. • Calculated intergranular diffusivity of hydrogen using computational FE simulation. • Validated the calculated computation simulation results with experimental results. - Abstract: Hydrogen induced intergranular embrittlement has been identified as a cause of failure of aerospace components such as combustion chambers made from electrodeposited polycrystalline nickel. Accurate computational analysis of this process requires knowledge of the differential in hydrogen transport in the intergranular and intragranular regions. The effective diffusion coefficient of hydrogen may be measured experimentally, though experimental measurement of the intergranular grain boundary diffusion coefficient of hydrogen requires significant effort. Therefore an approach to calculate the intergranular GB hydrogen diffusivity using finite element analysis was developed. The effective diffusivity of hydrogen in polycrystalline nickel was measured using electrochemical permeation tests. Data from electron backscatter diffraction measurements were used to construct microstructural representative volume elements including details of grain size and shape and volume fraction of grains and grain boundaries. A Python optimization code has been developed for the ABAQUS environment to calculate the unknown grain boundary diffusivity.

  10. Combined measurement of surface, grain boundary and lattice diffusion coefficients on olivine bi-crystals

    Science.gov (United States)

    Marquardt, Katharina; Dohmen, Ralf; Wagner, Johannes

    2014-05-01

    Diffusion along interface and grain boundaries provides an efficient pathway and may control chemical transport in rocks as well as their mechanical strength. Besides the significant relevance of these diffusion processes for various geologic processes, experimental data are still very limited (e.g., Dohmen & Milke, 2010). Most of these data were measured using polycrystalline materials and the formalism of LeClaire (1951) to fit integrated concentration depth profiles. To correctly apply this formalism, certain boundary conditions of the diffusion problem need to be fulfilled, e.g., surface diffusion is ignored, and furthermore the lattice diffusion coefficient has to be known from other studies or is an additional fitting parameter, which produces some ambiguity in the derived grain boundary diffusion coefficients. We developed an experimental setup where we can measure the lattice and grain boundary diffusion coefficients simultaneously but independent and demonstrate the relevance of surface diffusion for typical grain boundary diffusion experiments. We performed Mg2SiO4 bicrystal diffusion experiments, where a single grain boundary is covered by a thin-film of pure Ni2SiO4 acting as diffusant source, produced by pulsed laser deposition. The investigated grain boundary is a 60° (011)/[100]. This specific grain boundary configuration was modeled using molecular dynamics for comparison with the experimental observations in the transmission electron microscope (TEM). Both, experiment and model are in good agreement regarding the misorientation, whereas there are still some disagreements regarding the strain fields along the grain boundary that are of outmost importance for the strengths of the material. The subsequent diffusion experiments were carried out in the temperature range between 800° and 1450° C. The inter diffusion profiles were measured using the TEMs energy dispersive x-ray spectrometer standardized using the Cliff-Lorimer equation and EMPA

  11. Deformation of nanocrystalline binary aluminum alloys with segregation of Mg, Co and Ti at grain boundaries

    Science.gov (United States)

    Zinovev, A. V.; Bapanina, M. G.; Babicheva, R. I.; Enikeev, N. A.; Dmitriev, S. V.; Zhou, K.

    2017-01-01

    The influence of the temperature and sort of alloying element on the deformation of the nanocrystalline (NC) binary Al alloys with segregation of 10.2 at % Ti, Co, or Mg over grain boundaries has been studied using the molecular dynamics. The deformation behavior of the materials has been studied in detail by the simulation of the shear deformation of various Al bicrystals with the grain-boundary segregation of impurity atoms, namely, Ti, Co, or Mg. The deformation of bicrystals with different grain orientation has been studied. It has been found that Co introduction into grain boundaries of NC Al has a strengthening effect due to the deceleration of the grain-boundary migration (GBM) and difficulty in the grain-boundary sliding (GBS). The Mg segregation at the boundaries greatly impedes the GBM, but stimulates the development of the GBS. In the NC alloy of Al-Ti, the GBM occurs actively, and the flow-stress values are close to the values characteristic of pure Al.

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

    Institute of Scientific and Technical Information of China (English)

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

    2005-01-01

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

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

    Science.gov (United States)

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

    2016-04-01

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2013-10-15

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

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

    NARCIS (Netherlands)

    Ayas, Can; van der Giessen, Erik

    2010-01-01

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

  16. Molecular Dynamic Simulation of Lattice Distortion Region Produced by Rounded Grain Boundary in Nanocrystalline Materials

    Institute of Scientific and Technical Information of China (English)

    2002-01-01

    The distortion structure in nanocrystalline NiAl is studied using molecular dynamics simulation. The rounded grain boundaries in these nanograins are a direct source for the observed lattice distortion. The change of grain size affects directly the volume fraction of the distorted lattice in the nanograin.

  17. Grain Boundary Relaxation in Bi-Crystals: Mechanical Spectroscopy and Molecular Dynamics Simulations

    Directory of Open Access Journals (Sweden)

    Maier A.-K.

    2015-04-01

    Full Text Available Different Au-Ag-Cu samples have been studied by mechanical spectroscopy. Both polycrystals and bi-crystals show a relaxation peak at 800 K, accompanied by an elastic modulus change. Since this peak is absent in single crystals it is related to the presence of grain boundaries. Molecular dynamics simulations reveal two microscopic mechanisms, when a shear stress is applied onto a Σ5 grain boundary: at 700 K, the boundary migrates perpendicularly to the boundary plane under an external stress. At 1000 K, only sliding at the boundary is observed. These two mechanisms acting in different temperature intervals are used to model the mechanic response of a polycrystal under an applied stress. The models yield expressions for the relaxation strength Δ and for the relaxation time τ as a function of the grain size. A comparison with the mechanical spectroscopy measurements of polycrystals and the bi-crystals show that the grain boundary sliding model reproduces correctly the characteristics of the grain boundary peak.

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

    Science.gov (United States)

    Borjan, Z.

    2016-09-01

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

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

    DEFF Research Database (Denmark)

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

    1984-01-01

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

  20. Simulation of twin boundary effect on characteristics of single grain-silicon thin film transistors

    NARCIS (Netherlands)

    Yan, F.; Migliorato, P.; Ishihara, R.

    2007-01-01

    The influence of twin boundaries on the characteristics of single grain-silicon thin film transistors has been analyzed by three-dimensional simulation. The simulations show that the orientation and the location of a twin boundary could affect the field-effect mobility and the leakage current of a d

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

    DEFF Research Database (Denmark)

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

    2011-01-01

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

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

  3. The effect of electronic energy loss on irradiation-induced grain growth in nanocrystalline oxides.

    Science.gov (United States)

    Zhang, Yanwen; Aidhy, Dilpuneet S; Varga, Tamas; Moll, Sandra; Edmondson, Philip D; Namavar, Fereydoon; Jin, Ke; Ostrouchov, Christopher N; Weber, William J

    2014-05-07

    Grain growth of nanocrystalline materials is generally thermally activated, but can also be driven by irradiation at much lower temperature. In nanocrystalline ceria and zirconia, energetic ions deposit their energy to both atomic nuclei and electrons. Our experimental results have shown that irradiation-induced grain growth is dependent on the total energy deposited, where electronic energy loss and elastic collisions between atomic nuclei both contribute to the production of disorder and grain growth. Our atomistic simulations reveal that a high density of disorder near grain boundaries leads to locally rapid grain movement. The additive effect from both electronic excitation and atomic collision cascades on grain growth demonstrated in this work opens up new possibilities for controlling grain sizes to improve functionality of nanocrystalline materials.

  4. Grain boundary and lattice diffusion in nanocrystal α-iron: An atomistic simulation study

    Science.gov (United States)

    Mohammadzadeh, Roghayeh; Mohammadzadeh, Mina

    2017-09-01

    To obtain fundamental understanding on the effect of grain boundaries on the diffusion kinetics, molecular dynamics simulations (MD) were carried out on single crystal and nanocrystal (with a mean grain size of 2.5 nm) bcc iron using the second nearest-neighbor modified embedded atom method (2NN-MEAM) interatomic potential. Self-diffusion coefficient in single crystal and nanocrystal samples were calculated in the temperature range from 350 K to 1000 K. A temperature-dependence of the diffusion coefficient according to the Arrhenius law was obtained for both lattice and grain boundary diffusion. By doing so, activation energies as well as pre-exponential factors were derived from the diffusion coefficients and compared to experimental data. MD simulation results show that diffusion rate of iron atoms in nanocrystal sample is 6 to 28 orders of magnitude greater than single crystal. The trajectory of iron atoms during diffusion process verified that diffusion occurs mostly in the grain boundaries of nanocrystal iron; suggesting that grain boundary diffusion is dominant in nanocrystal iron. Based on the obtained results pure grain boundary diffusion coefficient was calculated.

  5. Grain Boundary Penetration of Various Types of Ni Layer by Molten Metals

    Science.gov (United States)

    Yang, S.; Chang, C. Y.; Zhu, Z. X.; Lin, Y. F.; Kao, C. R.

    2017-02-01

    The grain boundary penetration of three types of Ni layer, Ni foil, electroplated Ni, and electroless Ni, by molten Pb and 95Pb5Sn (wt.%) is investigated. The average grain sizes of Ni foil and electroplated Ni are 10 μm and 1 μm, respectively, while the electroless Ni is amorphous. The purpose of using two molten metals is to study the effect of intermetallic formation on grain boundary penetration. Molten Pb was able to penetrate or disintegrate all three types of Ni, including amorphous Ni, which does not contain any grain boundaries. On the other hand, the addition of merely 5 wt.% Sn into molten Pb was able to slow the penetration down substantially for all three types of Ni layer, with the greatest suppression found in electroless Ni where a grain boundary penetration event did not take place. The mechanism for the Sn effect is due to the formation of a protective Ni3Sn4 intermetallic compound at the interface acting as a barrier against grain boundary penetration.

  6. 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.

  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. Effect of Mg Addition on the Ferrite Grain Boundaries Misorientation in HAZ of Low Carbon Steels

    Institute of Scientific and Technical Information of China (English)

    Kai Zhu; Zhenguo Yang

    2011-01-01

    The relation between the Mg treatment and ferrite grain boundaries misorientation was investigated. The orientation imaging; microscopy technique based on electron backscattered diffraction technique (EBSD) was used in this work. It was found that the addition of 0.005 wt% Mg to the steel could evidently increase the ratio of acicular ferrite crystals appearing at large angles boundaries to each other, which was attributed to the nucleation of the second-phase particles by the Mg treatment. The EBSD techniques provide a powerful method to characterize and quantify the ferrite grain boundaries misorientation, in order to relate it to toughness.

  9. Molecular dynamics investigation of the grain boundary migration hysteresis of nanocrystalline Ni under cyclic shear loading

    Science.gov (United States)

    Wang, Peng; Yang, Xinhua; Peng, Di

    2017-02-01

    The deformation behavior and grain boundary (GB) response of nanocrystalline Ni under cyclic shear loading are investigated by molecular dynamics simulations. The GB migration hysteresis phenomenon, in which the GB migration displacement lags behind the change in nominal shear strain, is observed in the symmetric tilt GBs for the first time. The elementary structure transformation occurring at the two end segments of the observed GB during GB migration produces a disordered and irreversible state, while the transformation in the middle segment is reversible. Both dislocation retraction and nucleation occur during unloading. Relatively large cyclic strain amplitudes lead to disordered GB segments of greater length, such that the residual GB migration displacement increases with increasing cyclic amplitude. GB migration hysteresis vanishes after the GB becomes immobile owing to a cyclic shear induced transition to a disordered state along its entire length.

  10. Grain boundary segregation in neutron-irradiated 304 stainless steel studied by atom probe tomography

    Energy Technology Data Exchange (ETDEWEB)

    Toyama, T., E-mail: ttoyama@imr.tohoku.ac.jp [International Research Center for Nuclear Materials Science, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313 (Japan); Nozawa, Y. [International Research Center for Nuclear Materials Science, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313 (Japan); Van Renterghem, W. [SCK Bullet CEN, Nuclear Materials Science Institute, Boeretang 200, 2400 Mol (Belgium); Matsukawa, Y.; Hatakeyama, M.; Nagai, Y. [International Research Center for Nuclear Materials Science, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313 (Japan); Al Mazouzi, A. [EDF R and D, Avenue des Renardieres Ecuelles, 77818 Moret sur Loing Cedex (France); Van Dyck, S. [SCK Bullet CEN, Nuclear Materials Science Institute, Boeretang 200, 2400 Mol (Belgium)

    2012-06-15

    Radiation-induced segregation (RIS) of solute atoms at a grain boundary (GB) in 304 stainless steel (SS), neutron-irradiated to a dose of 24 dpa at 300 Degree-Sign C in the fuel wrapper plates of a commercial pressurized water reactor, was investigated using laser-assisted atom probe tomography (APT). Ni, Si, and P enrichment and Cr and Fe depletion at the GB were evident. The full-width at half-maximum of the RIS region was {approx}3 nm for the concentration profile peaks of Ni and Si. The atomic percentages of Ni, Si, and Cr at the GB were {approx}19%, {approx}7%, and {approx}14%, respectively, in agreement with previously-reported values for neutron-irradiated SS. A high number density of intra-granular Ni-Si rich precipitates formed in the matrix. A precipitate-denuded zone with a width of {approx}10 nm appeared on both sides of the GB.

  11. Strong trapping and slow diffusion of helium in a tungsten grain boundary

    Science.gov (United States)

    Wang, Xin-Xin; Niu, Liang-Liang; Wang, Shaoqing

    2017-04-01

    We have investigated the segregation, trapping and diffusion of He in a ∑3{111} W grain boundary (GB) using combined techniques of ab initio and classical atomistic simulations. We show that, with an average segregation energy of -3.20 eV, the strong He trapping can be attributed to a GB interstitial trapping or a vacancy trapping mechanism, while an average energy barrier of 1.97 eV leads to a slow diffusion of He in the GB plane. We further reveal by molecular dynamics simulations that the He diffusion will be dictated by GB migration through the motion of GB disconnections. Interestingly, we also observe a He-induced GB structural transition in classical simulations. The present work suggests that the GB does not provide fast transport channel for He, providing useful reference for the possible application of polycrystalline W under He irradiation in advanced nuclear fusion reactors.

  12. 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).

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-06-21

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2013-01-01

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

  15. Grain Boundary Resistivity of Yttria-Stabilized Zirconia at 1400°C

    Directory of Open Access Journals (Sweden)

    J. Wang

    2013-01-01

    Full Text Available The grain size dependence of the bulk resistivity of 3 mol% yttria-stabilized zirconia at 1400°C was determined from the effect of a dc electric field Ea=18.1 V/cm on grain growth and the corresponding electric current during isothermal annealing tests. Employing the brick layer model, the present annealing test results were in accordance with extrapolations of the values obtained at lower temperature employing impedance spectroscopy and 4-point-probe dc. The combined values give that the magnitude of the grain boundary resistivity ρb=133 ohm-cm. The electric field across the grain boundary width was 28–43 times the applied field for the grain size and current ranges in the present annealing test.

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

    Science.gov (United States)

    Fensin, Saryu Jindal

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

  17. 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.

  18. Grain boundary wetting phase transitions in peritectic copper—cobalt alloys

    Science.gov (United States)

    Kogtenkova, O. A.; Straumal, A. B.; Afonikova, N. S.; Mazilkin, A. A.; Kolesnikova, K. I.; Straumal, B. B.

    2016-04-01

    The transition from incomplete to complete grain boundary wetting in copper alloys with 2.2 and 4.9 wt % Co has been studied. These alloys with peritectic phase diagrams differ from previously studied systems with eutectic transformation by the fact that the melt layer separating grains from each other is not enriched, but is depleted by the second component (cobalt in this case). The fraction of completely wetted grain boundaries increases with temperature, as in eutectic systems, from zero at a temperature of 1098°C to ~80% at 1096°C. For symmetric twin boundaries, the temperature dependence of the contact angle with melt drops is constructed. As in the eutectic systems, the contact angle decreases with increasing temperature (although not to zero due to the extremely low energy of symmetric twin boundaries).

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

    Energy Technology Data Exchange (ETDEWEB)

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

    1996-08-01

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2010-01-15

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

  1. Damage Susceptibility of Grain Boundaries in HT9 Steel Subjected to High-Temperature Creep

    Science.gov (United States)

    Leng, Zhe; Field, David P.

    2012-10-01

    HT9 steel is an attractive ferritic/martensitic steel that is used in components of nuclear and fossil power plants because of its high strength and good swelling resistance. Specific phenomena (such as segregation, voiding, cracking, etc.) are prevalent along grain boundaries since these interfaces act as efficient sources for vacancies. The accumulation of vacancies in grain boundaries may result in intergranular fracture. In this study, HT9 steel was subjected to creep tests at elevated temperature (about 0.5 T m) and two different creep conditions (where creep lifetimes were about 100 and about 1000 hours, respectively). The grain boundaries in HT9 steel after creep tests were studied by the use of scanning electron microscopy in order to establish the relationship between the grain boundary structure and creep damage. Images and data obtained using electron backscatter diffraction reveal a high susceptibility of high-angle boundaries to creep cavitation, as expected. In addition, the Σ3 boundaries are also susceptible to damage under these conditions at a similar or even higher rate as compared with random high-angle boundaries.

  2. [Grain boundary and interface kinetics during ion irradiation

    Energy Technology Data Exchange (ETDEWEB)

    Atwater, H.A.

    1991-12-31

    Proposed here is renewed support of a research program focused on interface motion and phase transformation during ion irradiation, with emphasis on elemental semiconductors. Broadly speaking, the aims of this program are to explore defect kinetics in amorphous and crystalline semiconductors, and to relate defect dynamics to interface motion and phase transformations. Over the last three years, we initiated a program under DOE support to explore crystallization and amorphization of elemental semiconductors under irradiation. This research has enabled new insights about the nature of defects in amorphous semiconductors and about microstructural evolution in the early stages of crystallization. In addition, we have demonstrated almost arbitrary control over the relative rates of crystal nucleation and crystal growth in silicon. As a result, the impinged grain microstructure of thin (100 nm) polycrystalline films crystallized under irradiation can be controlled with grain sizes ranging from a few nanometers to several micrometers, which may have interesting technological implications.

  3. Roughness of grain boundaries in partly recrystallized aluminum

    DEFF Research Database (Denmark)

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

    2017-01-01

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

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

    DEFF Research Database (Denmark)

    Jones, A R.

    1981-01-01

    boundaries during recovery might stimulate nucleation of recrystallization in low stacking fault energy materials. The experimental observations also lead to the implication that the density of recrystallization nuclei formed in such materials may be directly related to the strength of the deformation...

  5. 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).

  6. Grain boundaries analysis in polycrystalline silicon by TEM

    Energy Technology Data Exchange (ETDEWEB)

    Komninou, F.; Karakostas, T.; Bleris, G.L.; Economou, N.A. (Aristoteles University, Thessaloniki (Greece))

    1982-01-01

    Polycrystalline Si interfaces were examined within the CSL's approach. The rotation relationship of every bicrystal has been analyzed with the technique of the instrumental system and the small angle description has been used for the CSL characterization. The most frequently occuring descriptions are CSL' ..sigma..=3 coherent and incoherent twins, the later being microscopically coherent. Cases of multiple boundaries were also examined and interelations were found between low or high angle boundaries for CSL's with ..sigma..>3. A special case of interest is a ..sigma..=39 CSL which is formed from a combination of ..sigma..=13b and ..sigma..=3 and is a triclinic CSL lacking 180/sup 0/ rotational operations. The results presented indicate that for polycrystalline Si the CSL model could be used in describing the interfaces occuring.

  7. An FEGSTEM Study of Grain Boundary Segregation of Phosphorus during Quenching in a 2.25Cr-1Mo Steel

    Institute of Scientific and Technical Information of China (English)

    Shenhua SONG; Luqian WENG

    2005-01-01

    Quenching-induced phosphorus segregation to prior austenite grain boundaries in a 0.077 wt pct P-doped 2.25Cr1Mo steel is examined using field emission gun scanning transmission electron microscopy (FEGSTEM). A phosphorus level of around 1.56 at. pct is observed for the water-quenched sample. In recognition of insufficiently high spatial resolution of the technique for grain boundary composition analysis, the measured results are corrected by an analytical convolution method. The corrected phosphorus segregation level may be up to about 4.7 at. pct. The quenchinginduced phosphorus segregation is nonequilibrium segregation and the migration of vacancy-phosphorus complexes plays an important role in the kinetic process. For such a reason, the mechanism for migration of the complexes is discussed in some detail.

  8. The different effects of twin boundary and grain boundary on reducing tension-compression yield asymmetry of Mg alloys

    Science.gov (United States)

    Yu, Huihui; Xin, Yunchang; Chapuis, Adrien; Huang, Xiaoxu; Xin, Renlong; Liu, Qing

    2016-07-01

    In the present study, a coarse grained AZ31 plate was refined by twin boundaries (TBs) and grain boundaries (GBs), respectively. A comparative study about the different effects of grain refinements by GBs and by TBs on tension-compression yield asymmetry was performed. Our results show that both the refinements by GBs and by TBs increase the tensile and compressive yield strengths, but to a different degree. TBs are more effective to harden twinning, but yield a lower strengthening against prismatic slip, and a much lower tension-compression yield asymmetry is thus obtained. Both the differences in boundary coherence and misorientation between GBs and TBs affect the hardening. The misorientation of TBs provides a lower geometric compatibility factor (a higher hardening) for both prismatic slip and twinning than that of GBs, which in detail is the result of the much higher angle between c-axes of the two sides of TBs (about 86°) than GBs (0–50°). It is found that, for hardening of prismatic slip, boundary coherence plays a more important role than misorientation. With regard to twinning, the different misorientation of TBs from GBs mainly accounts for their different hardening effects.

  9. Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO

    Energy Technology Data Exchange (ETDEWEB)

    Kim, M.; Browning, N.D.; Pennyscook, S.J.; Sohlberg, K.; Pantelides, S.T.

    1999-11-29

    The understanding of electrical properties of grain boundaries in perovskites is essential for their application to capacitors, varistors and positive-temperature coefficient resistors. The origin of the electrical activity is generally attributed to the existence of charged defects in grain boundaries, usually assumed to be impurities, which set up a double Schottky barrier as they are screened by dopants in the adjacent bulk crystal. Microscopic understanding of the origin of the grain boundary charge, however, has not been achieved. It is not known yet if the charged grain boundary states are an intrinsic property of a stoichiometric grain boundary, arise from nonstoichiometry, or are caused by impurities. Here, the relation between atomic structure and electronic properties is studied by combining experiment with ab-initio calculations. The starting structures for theoretical calculations were obtained from Z-contrast images combined with electron energy loss spectroscopy to res olve the dislocation Core structures comprising the boundary. Dislocation core reconstructions are typical of all grain boundaries so far observed in this material. They avoid like-ion repulsion, and provide alternative sites for cation occupation in the grain boundaries. Optimized atomic positions are found by total energy calculations. Calculated differences in vacancy formation energies between the grain boundaries and the bulk suggest that vacancy segregation can account for the postulated grain boundary charge.

  10. 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

  11. 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...

  12. Atomic structure and properties of grain boundaries in ceramics through Z-contrast electron microscopy

    Energy Technology Data Exchange (ETDEWEB)

    Pennycook, S.J.; Nellist, P.D. [Oak Ridge National Lab., TN (United States); Browning, N.D. [Illinois Univ., Chicago, IL (United States). Dept. of Physics

    1996-08-01

    The bulk properties of a large range of materials are controlled by the atomic structure and chemistry of grain boundaries, but how this occurs, at the fundamental atomic level, remains poorly understood. This is due largely to the many degrees of freedom associated with grain boundaries - five geometrical degrees of freedom along with a myriad of possibilities involving impurity segregation. Based on Z- contrast electron microscopy, a method have been developed for determining grain boundary atomic structure and chemistry directly from experimental data. The method utilizes the incoherent nature of the Z-contrast image; as there is no phase problem associated with an incoherent image, it represents a compositionally sensitive structure image which may be directly inverted to give atomic column positions. This method extracts the column locations to an accuracy of {+-}0.2 {Angstrom}, while preserving the intensity information. The procedure has been applied to SrTiO{sub 3} and YBCO.

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

    Science.gov (United States)

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

    2016-10-01

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

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

    DEFF Research Database (Denmark)

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

    2008-01-01

    In situ transmission electron microscopy is used to study dissolution of liquid single-phase Pb/Bi inclusions attached to a grain boundary in an alloy of Al99.29Pb0.65Bi0.06 at temperatures of 343, 370, and 389 degrees C, respectively. The initial size of the inclusions was smaller than 60 nm....... Dissolution of the inclusions was observed until their complete disappearance. Digitized video recordings of the process of dissolution were used to obtain the dependence of the inclusion size with time. The kinetics of the dissolution of the grain boundary inclusions can be described with a model where...... it is assumed that grain-boundary diffusion of Pb and Bi is the controlling mechanism. The high value (2.3 eV) of the apparent activation enthalpy of dissolution indicates that the process is likely governed by the large negative enthalpies of solubility of Pb and Bi in Al....

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2012-06-20

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

  16. Evolution of microstructure and grain boundary character distribution of a tin bronze annealed at different temperatures

    Energy Technology Data Exchange (ETDEWEB)

    Huang, Weijiu [College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054 (China); Chongqing Municipal Key Laboratory of Institutions of Higher Education for Mould Technology, Chongqing University of Technology, Chongqing 400054 (China); Chai, Linjiang, E-mail: chailinjiang@cqut.edu.cn [College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054 (China); Chongqing Municipal Key Laboratory of Institutions of Higher Education for Mould Technology, Chongqing University of Technology, Chongqing 400054 (China); Li, Zhijun; Yang, Xusheng [College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054 (China); Guo, Ning; Song, Bo [Faculty of Materials and Energy, Southwest University, Chongqing 400715 (China)

    2016-04-15

    Specimens cut from a rolled tin bronze sheet were annealed at 400–800 °C for 1 h and evolution of their microstructures was then characterized in details by electron channeling contrast imaging and electron backscatter diffraction techniques. Particularly, statistics on special boundaries (SBs) with Σ ≤ 29 and network connectivity of random high angle boundaries (HABs) in the annealed specimens were examined to probe optimization potentials of grain boundary character distribution (GBCD) for this material. Results show that the deformed microstructure in the as-received material begins to be recrystallized when the annealing temperature increase to 500 °C and average grain sizes surge with further increasing temperatures. As a result of the recrystallization, a large number of annealing twins (with Σ3 misorientation) are produced, leading to remarkably increased fractions of SBs (f{sub SBs}). Thanks to preexisting dense low angle boundaries, the majority of SBs in the 500 °C specimen with only partial recrystallization are Σ3{sub ic} (incoherent) boundaries, which effectively disrupt connectivity of random HABs network. Although the f{sub SBs} can be further increased (up to 72.5%) in specimens with full recrystallization (at higher temperatures), the Σ3{sub ic} boundaries would be replaced to some extent by Σ3{sub c} (coherent) boundaries which do not contribute directly to optimizing the GBCD. This work should be able to provide clear suggestions on applying the concept of grain boundary engineering to tin bronze alloys. - Highlights: • The rolled tin bronze begins to be recrystallized as temperature increases to 500 °C. • A lot of SBs are produced after recrystallization and the highest f{sub SBs} is 72.5%. • Partially recrystallized specimen has the optimum GBCD due to more Σ3{sub ic} boundaries. • The Σ3{sub ic} boundaries are replaced by Σ3{sub c} boundaries after full recrystallization.

  17. Grain boundary engineering of powder-processed Ni-base superalloy RR1000

    Science.gov (United States)

    Detrois, Martin

    Grain boundary engineering (GBE) has been used to improve the properties of various polycrystalline materials by optimization of their grain boundary network. Traditional processing routes for GBE often require multiple iterations of cold work followed by short annealing cycles where each iteration imparts a modest increase in the fraction of special grain boundaries. Multiple iterations are then required to achieve sufficiently high fractions (>50%) that result in the improved properties. Thus, this GBE approach is not suitable for the fabrication of large, complex-shaped structures and leads to added manufacturing lead time and cost. In this investigation, the Ni-base superalloy RR1000 used as turbine discs in gas turbine engines manufactured by Rolls-Royce, was considered for GBE using alternative processing routes more suitable to the forging of Ni-base superalloy components. A preliminary study of the effects of hot deformation parameters closer to typical industrial processing revealed that the length fraction of Sigma3 boundaries increased from 35% to 52% following a single deformation/anneal cycle. Deformation parameters that resulted in strain accommodation via superplastic flow did not enhance the formation of Sigma3 boundaries upon annealing. Whereas deformation parameters that resulted in a dominant dislocation-based plasticity flow mechanism promoted the formation of annealing twins. Using misorientation maps and by estimating the stored strain energy from deformation, equations for the length fraction and density of Sigma3 boundaries were generated for high-temperature GBE of RR1000. The grain boundary characters obtained via high-temperature deformation, however, are less ideal than those resulting from traditional cold rolling. The underlying mechanisms responsible for the formation of Sigma3n boundaries during high-temperature GBE were further investigated. A larger starting grain size prior to deformation was found to be unfavorable to the

  18. Phase-field simulations of the interaction between a grain boundary and an evolving second-phase particle

    Science.gov (United States)

    Chang, Kunok; Moelans, Nele

    2015-04-01

    We performed phase-field simulations to analyse the interaction of a migrating grain boundary with an evolving second-phase particle. It is found that depending on the difference between the interfacial energies of the particle-matrix interface for the two grain orientations involved and the driving force for grain boundary movement, particles with a particle size well above the critical limit can dissolve due to passage of the boundary.

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

    OpenAIRE

    Van de Noort, R.

    2008-01-01

    Two of the three processes making up the deformation mechanism of intergranular pressure solution, being dissolution and diffusion, take place in the grain boundary fluid phase. Hence, the structure and physical properties of wet grain boundaries under stress can be expected to influence the kinetics of both dissolution and diffusion, as a result of effects such as molecular structuring of the fluid film, the grain boundary surface area available for dissolution, the dissipation of energy oth...

  20. 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.

  1. Deformation by grain boundary sliding and slip creep versus diffusional creep

    Energy Technology Data Exchange (ETDEWEB)

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

    1998-11-04

    A review is presented of the debates between the present authors and other investigators regarding the possible role of diffusional creep in the plastic flow of polycrystalline metals at low stresses. These debates are recorded in eleven papers over the past seventeen years. ln these papers it has been shown that the creep rates of materials in the so-called "diffusional creep region" are almost always higher than those predicted by the diffusional creep theory. Additionally, the predictions of grain size effects and stress exponents from diffusional creep theory are often not found in the experimental data. Finally, denuded zones have been universally considered to be direct evidence for diffusional creep; but, those reported in the literature are shown to be found only under conditions where a high stress exponent is observed. Also, the locations of the denuded zones do not match those predicted. Alternative mechanisms are described in which diffusion-controlled dislocation creep and/or grain boundary sliding are the dominant deformation processes in low-stress creep. It is proposed that denuded zones are formed by stress-directed grain boundary migration with the precipitates dissolving in the moving grain boundaries. The above observations have led us to the conclusion that grain boundary sliding and slip creep are in fact the principal mechanisms for observations of plastic flow in the so-called "diffusional creep regions".

  2. Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers.

    Science.gov (United States)

    Najmaei, Sina; Liu, Zheng; Zhou, Wu; Zou, Xiaolong; Shi, Gang; Lei, Sidong; Yakobson, Boris I; Idrobo, Juan-Carlos; Ajayan, Pulickel M; Lou, Jun

    2013-08-01

    Single-layered molybdenum disulphide with a direct bandgap is a promising two-dimensional material that goes beyond graphene for the next generation of nanoelectronics. Here, we report the controlled vapour phase synthesis of molybdenum disulphide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Furthermore, a nucleation-controlled strategy is established to systematically promote the formation of large-area, single- and few-layered films. Using high-resolution electron microscopy imaging, the atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulphide atomic layers are examined, and the primary mechanisms for grain boundary formation are evaluated. Grain boundaries consisting of 5- and 7- member rings are directly observed with atomic resolution, and their energy landscape is investigated via first-principles calculations. The uniformity in thickness, large grain sizes, and excellent electrical performance signify the high quality and scalable synthesis of the molybdenum disulphide atomic layers.

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

    CERN Document Server

    Xydou, A; Aicheler, M; Djurabekova, F

    2016-01-01

    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 $(T_m)$. 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 calculate...

  4. Yttria-Ceria stabilized tetragonal zirconia polycrystals: Sintering, grain growth and grain boundary segregation

    NARCIS (Netherlands)

    Boutz, M.M.R.; Boutz, M.M.R.; Winnubst, Aloysius J.A.; Burggraaf, Anthonie; Burggraaf, A.J.

    1994-01-01

    An analysis is presented of grain growth and densification of yttria-ceria stabilized tetragonal zirconia polycrystals (Y, Ce-TZPs) using both isothermal and non-isothermal techniques. The characteristics of Y, Ce-TZPs are compared to those of Y-TZP and Ce-TZP and the effect of increasing ceria

  5. Embrittlement of austempered nodular irons: Grain boundary phosphorus enrichment resulting from precipitate decomposition

    Science.gov (United States)

    Klug, R. C.; Hintz, M. B.; Rundman, K. B.

    1985-05-01

    The microstructures, mechanical properties, and fracture behavior were characterized for a series of Mg treated nodular cast iron specimens austenitized at 1170, 1255, and 1340 K and subsequently austempered at 640 K. The ductility and toughness of the alloy decreased as austenitization temperatures were increased, which is contrary to the behavior anticipated from the observed micro-structural evolution. Fractographic and surface chemical analyses demonstrated that the mechanical property degradation was associated with embrittlement of the austenite grain boundaries by phosphorus. The primary mechanism of grain boundary phosphorus enrichment does not appear to be equilibrium segregation, and an alternative mechanism based on the decomposition of P rich precipitates is proposed and discussed.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2012-07-01

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

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

    Directory of Open Access Journals (Sweden)

    Feng Xue

    2016-05-01

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

  8. Domain pinning near a single-grain boundary in tetragonal and rhombohedral lead zirconate titanate films

    Science.gov (United States)

    Marincel, D. M.; Zhang, H. R.; Britson, J.; Belianinov, A.; Jesse, S.; Kalinin, S. V.; Chen, L. Q.; Rainforth, W. M.; Reaney, I. M.; Randall, C. A.; Trolier-McKinstry, S.

    2015-04-01

    The interaction of grain boundaries with ferroelectric domain walls strongly influences the extrinsic contribution to piezoelectric activity in Pb Zr1 -x,TixO3 (PZT), ubiquitous in modern transducers and actuators. However, the fundamental understanding of these phenomena has been limited by complex mechanisms originating from the interplay of atomic-level domain wall pinning, collective domain wall dynamics, and emergent mesoscopic behavior. This contribution utilizes engineered grain boundaries created by depositing epitaxial PZT films with various Zr:Ti ratios onto 24° tilt SrTi O3 bicrystals. The nonlinear piezoelectric response and surface domain structure across the boundary are investigated using piezoresponse force microscopy while the cross-sectional domain structure is studied using transmission electron microscopy. The grain boundary reduces domain wall motion over a width of 800 ±70 nm for PZT 45:55 and 450 ±30 nm for PZT 52:48. Phase field modeling provides an understanding of the elastic and electric fields associated with the grain boundary and local domain configurations. This study demonstrates that complex mesoscopic behaviors can be explored to complement atomic-level pictures of the material system.

  9. 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.

  10. Relative importance of grain boundaries and size effects in thermal conductivity of nanocrystalline materials.

    Science.gov (United States)

    Dong, Huicong; Wen, Bin; Melnik, Roderick

    2014-11-13

    A theoretical model for describing effective thermal conductivity (ETC) of nanocrystalline materials has been proposed, so that the ETC can be easily obtained from its grain size, single crystal thermal conductivity, single crystal phonon mean free path (PMFP), and the Kaptiza thermal resistance. In addition, the relative importance between grain boundaries (GBs) and size effects on the ETC of nanocrystalline diamond at 300 K has been studied. It has been demonstrated that with increasing grain size, both GBs and size effects become weaker, while size effects become stronger on thermal conductivity than GBs effects.

  11. Characterization of Iron Grains near the P/T Boundary in the Meishan Section of China

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

    The Meishan section of China has been confirmed as the "Global Stratotype Section and Point" of the P/T boundary. In the section, the authors found several types of iron grains, including pyrite, pure iron grains and goethite. From the research of macro minerals, it is easy to find that the grains rich in iron appear from the bottom of the event layer of the section. In other words, it is probably residue of the geochemical catastrophe of that time. Therefore, it is important to trace the source of these iron grains and their relationships, which probably provides evidence for volcanic eruption or impact-volcanoes and has directive significance to the crisis during the P/T transitional period. Through the study of the characterization and relationships of these iron grains, the authors make a preliminary discussion on the P/T mass extinction.

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

    DEFF Research Database (Denmark)

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

    2015-01-01

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

  13. Annealing-induced Grain Refinement in a Nanostructured Ferritic Steel

    Institute of Scientific and Technical Information of China (English)

    Limin Wang; Zhenbo Wangt; Sheng Guo; Ke Lu

    2012-01-01

    A nanostructured surface layer with a mean ferrite grain size of -8 nm was produced on a Fe-gCr steel by means of surface mechanical attrition treatment. Upon annealing, ferrite grains coarsen with increasing temperature and their sizes increase to -40 nm at 973 K. Further increasing annealing temperature leads to an obvious reduction of ferrite grain sizes, to -14 nm at 1173 K. The annealing-induced grain refinement is analyzed in terms of phase transformations in the nanostructured steel.

  14. Electron scattering at surfaces and grain boundaries in thin Au films

    Energy Technology Data Exchange (ETDEWEB)

    Henriquez, Ricardo [Departamento de Física, Universidad Técnica Federico Santa María, Av. España 1680, Casilla 110-V, Valparaíso (Chile); Flores, Marcos; Moraga, Luis [Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2008, Casilla 487-3, Santiago 8370449 (Chile); Kremer, German [Bachillerato, Universidad de Chile, Las Palmeras 3425, Santiago 7800024 (Chile); González-Fuentes, Claudio [Departamento de Física, Universidad Técnica Federico Santa María, Av. España 1680, Casilla 110-V, Valparaíso (Chile); Munoz, Raul C., E-mail: ramunoz@ing.uchile.cl [Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2008, Casilla 487-3, Santiago 8370449 (Chile)

    2013-05-15

    The electron scattering at surfaces and grain boundaries is investigated using polycrystalline Au films deposited onto mica substrates. We vary the three length scales associated with: (i) electron scattering in the bulk, that at temperature T is characterized by the electronic mean free path in the bulk ℓ{sub 0}(T); (ii) electron-surface scattering, that is characterized by the film thickness t; (iii) electron-grain boundary scattering, that is characterized by the mean grain diameter D. We varied independently the film thickness from approximately 50 nm to about 100 nm, and the typical grain size making up the samples from 12 nm to 160 nm. We also varied the scale of length associated with electron scattering in the bulk by measuring the resistivity of each specimen at temperatures T, 4 K < T < 300 K. Cooling the samples to 4 K increases ℓ{sub 0}(T) by approximately 2 orders of magnitude. Detailed measurements of the grain size distribution as well as surface roughness of each sample were performed with a Scanning Tunnelling Microscope (STM). We compare, for the first time, theoretical predictions with resistivity data employing the two theories available that incorporate the effect of both electron-surface as well as electron-grain boundary scattering acting simultaneously: the theory of A.F. Mayadas and M. Shatzkes, Phys. Rev. 1 1382 (1970) (MS), and that of G. Palasantzas, Phys. Rev. B 58 9685 (1998). We eliminate adjustable parameters from the resistivity data analysis, by using as input the grain size distribution as well as the surface roughness measured with the STM on each sample. The outcome is that both theories provide a fair representation of both the temperature as well as the thickness dependence of the resistivity data, but yet there are marked differences between the resistivity predicted by these theories. In the case of the MS theory, when the average grain diameter D is significantly smaller than ℓ{sub 0}(300) = 37 nm, the electron mean

  15. Unexpected behavior of transient current in thin PZT films caused by grain-boundary conduction

    Science.gov (United States)

    Delimova, L. A.; Guschina, E. V.; Seregin, D. S.; Vorotilov, K. A.; Sigov, A. S.

    2017-06-01

    The behavior of the transient current at different preliminary polarizations has been studied in Pb(ZrTi)O3 (PZT) films with various grain structures. To affect the grain structure, PZT films were prepared by chemical solution deposition with a two-step crystallization process using combination of seed layers with a low Pb excess and the main layers with a 30 wt. % Pb excess. Some films were prepared with a fixed Pb excess in all the deposited layers. We found that the lead excess and the seed layer crystalline structure can affect the grain-boundary conduction which, in turn, influences the polarization dependence of the transient current and the appearance of current peaks which look like the so-called negative differential resistance region in the current-voltage curves. We show that the emergence of the current peaks in the PZT films depends on (i) whether the current flows inside the ferroelectric phase (grains) or outside, along grain boundaries and (ii) whether the applied bias direction is parallel or opposite to the polarization vector. A correlation between the grain-boundary conduction and current-polarization dependences is confirmed by the local current distribution measured by conductive atomic force microscopy. Possible mechanisms responsible for specific features of the transient current and appearance of the current peaks are discussed. The effect of grain-boundary conduction on the behavior of the current may be significant and should be taken into account in ferroelectric random access memory whose readout operation assumes registration of the magnitude of the polarization switching current under positive bias.

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

    Institute of Scientific and Technical Information of China (English)

    白利平; 杜建国; 刘巍; 郭捷; 周文戈

    2003-01-01

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

  17. Quantitative TEM analysis of precipitation and grain boundary segregation in neutron irradiated EUROFER 97

    Energy Technology Data Exchange (ETDEWEB)

    Dethloff, Christian, E-mail: christian.dethloff@kit.edu; Gaganidze, Ermile; Aktaa, Jarir

    2014-11-15

    Characterization of irradiation induced microstructural defects is essential for assessing the applicability of structural steels like the Reduced Activation Ferritic/Martensitic steel EUROFER 97 in upcoming fusion reactors. In this work Transmission Electron Microscopy (TEM) is used to analyze the types and structure of precipitates, and the evolution of their size distributions and densities caused by neutron irradiation to a dose of 32 displacements per atom (dpa) at 330–340 °C in the irradiation experiment ARBOR 1. A significant growth of MX and M{sub 23}C{sub 6} type precipitates is observed after neutron irradiation, while the precipitate density remains unchanged. Hardening caused by MX and M{sub 23}C{sub 6} precipitate growth is assessed by applying the Dispersed Barrier Hardening (DBH) model, and shown to be of minor importance when compared to other irradiation effects like dislocation loop formation. Additionally, grain boundary segregation of chromium induced by neutron irradiation was investigated and detected in irradiated specimens.

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

    Institute of Scientific and Technical Information of China (English)

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

    2004-01-01

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

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

    Science.gov (United States)

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

    2015-12-01

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

  20. Review of grain interior, grain boundary, and interface effects of K in CIGS solar cells: Mechanisms for performance enhancement

    Energy Technology Data Exchange (ETDEWEB)

    Muzzillo, Christopher P.

    2017-12-01

    Introducing K into Cu(In,Ga)(Se,S)2 (CIGS) absorbers has led to recent world record power conversion efficiencies for thin film polycrystalline solar cells. In this work, the diverse phenomena associated with K in CIGS were reviewed, and overarching mechanisms were identified. The effects of K depend on its distribution among grain interiors (GIs), grain boundaries (GBs), and interfaces. High substrate Na and low temperature favor GI K incorporation, while low Na and high temperature favor segregation of K at GBs. Depositing KInSe2 (or KIn1-yGaySe2) by co-evaporation or KF post-deposition treatment onto CIGS reduces buffer interface recombination in the final solar cells. KInSe2 decomposes in air, which makes characterization difficult and may affect performance. The mechanism for reduced interface recombination could be direct passivation, beneficial compound precursor, oxidation barrier, or favorable diffusion alteration.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2003-01-01

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

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

    Science.gov (United States)

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

    2014-08-01

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

  3. Nanocompositional Electron Microscopic Analysis and Role of Grain Boundary Phase of Isotropically Oriented Nd-Fe-B Magnets

    Directory of Open Access Journals (Sweden)

    Gregor A. Zickler

    2017-01-01

    Full Text Available Nanoanalytical TEM characterization in combination with finite element micromagnetic modelling clarifies the impact of the grain misalignment and grain boundary nanocomposition on the coercive field and gives guidelines how to improve coercivity in Nd-Fe-B based magnets. The nanoprobe electron energy loss spectroscopy measurements obtained an asymmetric composition profile of the Fe-content across the grain boundary phase in isotropically oriented melt-spun magnets and showed an enrichment of iron up to 60 at% in the Nd-containing grain boundaries close to Nd2Fe14B grain surfaces parallel to the c-axis and a reduced iron content up to 35% close to grain surfaces perpendicular to the c-axis. The numerical micromagnetic simulations on isotropically oriented magnets using realistic model structures from the TEM results reveal a complex magnetization reversal starting at the grain boundary phase and show that the coercive field increases compared to directly coupled grains with no grain boundary phase independently of the grain boundary thickness. This behaviour is contrary to the one in aligned anisotropic magnets, where the coercive field decreases compared to directly coupled grains with an increasing grain boundary thickness, if Js value is > 0.2 T, and the magnetization reversal and expansion of reversed magnetic domains primarily start as Bloch domain wall at grain boundaries at the prismatic planes parallel to the c-axis and secondly as Néel domain wall at the basal planes perpendicular to the c-axis. In summary our study shows an increase of coercive field in isotropically oriented Nd-Fe-B magnets for GB layer thickness > 5 nm and an average Js value of the GB layer < 0.8 T compared to the magnet with perfectly aligned grains.

  4. Mesoscopic current transport in two-dimensional materials with grain boundaries: Four-point probe resistance and Hall effect

    DEFF Research Database (Denmark)

    Lotz, Mikkel Rønne; Boll, Mads; Østerberg, Frederik Westergaard

    2016-01-01

    We have studied the behavior of micro four-point probe (M4PP) measurements on two-dimensional (2D) sheets composed of grains of varying size and grain boundary resistivity by Monte Carlo based finite element (FE) modelling. The 2D sheet of the FE model was constructed using Voronoi tessellation......-configurations depends on the dimensionality of the current transport (i.e., one- or two-dimensional). At low grain density or low grain boundary resistivity, two-dimensional transport is observed. In contrast, at moderate grain density and high grain resistivity, one-dimensional transport is seen. Ultimately......, this affects how measurements on defective systems should be interpreted in order to extract relevant sample parameters. The Hall effect response in all M4PP configurations was only significant for moderate grain densities and fairly large grain boundary resistivity....

  5. 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 differ...

  6. Grain Boundary Engineering of a Low Stacking Fault Energy Ni-based Superalloy

    Science.gov (United States)

    McCarley, Joshua; Helmink, Randolph; Goetz, Robert; Tin, Sammy

    2017-04-01

    The effects of thermo-mechanical processing parameters on the resulting microstructure of an experimental Nickel-based superalloy containing 24 wt pct Co were investigated. Hot compression tests were performed at temperatures ranging from 1293 K to 1373 K (1020 to 1100 °C) and strain rates ranging from 0.0005 to 0.1/s. The mechanically deformed samples were also subject to annealing treatments at sub-solvus 1388 K (1115 °C) and super-solvus 1413 K (1140 °C) temperatures. This investigation sought to quantify and subsequently understand the behavior and evolution of both the grain boundary structure and length fraction of Σ3 twin boundaries in the low stacking fault energy superalloy. Over the range of deformation parameters investigated, the corresponding deformation mechanism map revealed that dynamic recrystallization or dynamic recovery was dominant. These conditions largely promoted post-deformation grain refinement and the formation of annealing twins following annealing. Samples deformed at strain rates of 0.0005 and 0.001/s at 1333 K and 1373 K (1060 °C and 1100 °C) exhibited extensive grain boundary sliding/rotation associated with superplastic flow. Upon annealing, deformation conditions that resulted predominately in superplastic flow were found to provide negligible enhancement of twin boundaries and produced little to no post-deformation grain refinement.

  7. 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...

  8. Ferromagnetic grain boundary signature in die-upset RE-Fe-B magnets

    Energy Technology Data Exchange (ETDEWEB)

    Henderson Lewis, L.; Zhu, Y.; Welch, D.O.

    1994-07-01

    Previous nanostructural and nanocompositional studies performed on the boundaries of deformed grains in two die-upset rare earth magnets with bulk compositions Nd{sub 13.75}Fe{sub 80.25}B{sub 6}, and Pr{sub 13.75}Fe{sub 80.25}B{sub 6} indicate that the intergranular phase in many grain boundaries is enriched in iron relative to the bulk. Preliminary magnetic data are presented that provide further evidence that this grain boundary phase is indeed iron-rich, and in fact appears to be ferromagnetic. Hysteresis loops were performed at 800 K on die-upset magnets with the above compositions. Each sample showed a clear hysteresis with coercivities between 34 and 40 Oe average remanence 4{pi}M{sub R} of 6.8 G for the Nd-based sample and 10.3 G for the Pr-based sample. The ferromagnetic signals measured at high temperature in these magnets are attributed to the iron-rich grain boundary phase. The implications of this conclusion with respect to coercivity are discussed.

  9. Effect of grain boundary on electrical properties of polycrystalline lanthanum nickel oxide thin films

    Energy Technology Data Exchange (ETDEWEB)

    Zhu, M.W.; Wang, Z.J.; Chen, Y.N.; Wang, H.L.; Zhang, Z.D. [Chinese Academy of Sciences, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Shenyang (China)

    2013-09-15

    In the present work, lanthanum nickel oxide (LNO) thin films were prepared by the sol-gel method. Microstructures of the films were tailored by changing sol concentration so as to investigate the effect of grain boundary on the transport properties of electrons in the polycrystalline LNO films. Based on the temperature dependence of the resistivity and the magnetic field dependence of the magnetoresistance (MR) at various temperatures, the factors that dominate the transport behavior in the polycrystalline LNO films were explored in terms of weak localization and strong localization. The results show that the grain boundary has a significant influence on the transport behavior of the electrons in LNO films at a low-temperature region, which can be captured by a variable-range hopping (VRH) model. The increase of metal-insulator (M-I) transition temperature is ascribed to Anderson localization in grain boundary. At a high-temperature region, electron-electron scattering and electron-phonon scattering predominates in the films. In this case, the existence of more grain boundary shows a minor effect on the transport behavior of the electrons but elevates the residual resistivity of the films. (orig.)

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

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

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

  11. Grain Boundary Engineering of a Low Stacking Fault Energy Ni-based Superalloy

    Science.gov (United States)

    McCarley, Joshua; Helmink, Randolph; Goetz, Robert; Tin, Sammy

    2017-02-01

    The effects of thermo-mechanical processing parameters on the resulting microstructure of an experimental Nickel-based superalloy containing 24 wt pct Co were investigated. Hot compression tests were performed at temperatures ranging from 1293 K to 1373 K (1020 to 1100 °C) and strain rates ranging from 0.0005 to 0.1/s. The mechanically deformed samples were also subject to annealing treatments at sub-solvus 1388 K (1115 °C) and super-solvus 1413 K (1140 °C) temperatures. This investigation sought to quantify and subsequently understand the behavior and evolution of both the grain boundary structure and length fraction of Σ3 twin boundaries in the low stacking fault energy superalloy. Over the range of deformation parameters investigated, the corresponding deformation mechanism map revealed that dynamic recrystallization or dynamic recovery was dominant. These conditions largely promoted post-deformation grain refinement and the formation of annealing twins following annealing. Samples deformed at strain rates of 0.0005 and 0.001/s at 1333 K and 1373 K (1060 °C and 1100 °C) exhibited extensive grain boundary sliding/rotation associated with superplastic flow. Upon annealing, deformation conditions that resulted predominately in superplastic flow were found to provide negligible enhancement of twin boundaries and produced little to no post-deformation grain refinement.

  12. 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.

  13. Localized electronic states at grain boundaries on the surface of graphene and graphite

    DEFF Research Database (Denmark)

    Luican-Mayer, Adina; Barrios-Vargas, Jose E.; Falkenberg, Jesper Toft

    2016-01-01

    ecent advances in large-scale synthesis of graphene and other 2D materials have underscored the importance of local defects such as dislocations and grain boundaries (GBs), and especially their tendency to alter the electronic properties of the material. Understanding how the polycrystalline...

  14. Point defect absorption by grain boundaries in α -iron by atomic density function modeling

    Science.gov (United States)

    Kapikranian, O.; Zapolsky, H.; Patte, R.; Pareige, C.; Radiguet, B.; Pareige, P.

    2015-12-01

    Using the atomic density function theory (ADFT), we examine the point defect absorption at [110] symmetrical tilt grain boundaries in body-centered cubic iron. It is found that the sink strength strongly depends on misorientation angle. We also show that the ADFT is able to reproduce reasonably well the elastic properties and the point defect formation volume in α -iron.

  15. Atomic Structure and Doping Response of Grain Boundary in Transition Metal Ni

    Institute of Scientific and Technical Information of China (English)

    王崇愚; 于涛

    1994-01-01

    Based on the coincidence site lattice model and by use of the molecular dynamics method, the relaxation calculations on the atomic structures of the grain boundaries of various generating functions (∑3,∑5,…, ∑19,…,∑33) in the transition metal Ni are performed. The features of atomic structures corresponding to the grain boundaries and the effects of the pre-parameter on the interface structures are given. To study the doping response relating to the properties of materials, the 23 tilt grain boundary is selected. Based on the interstice and vacancy mechanisms, the interface responses doping boron, nitrogen and phosphorus for the grain boundary are investigated. According to the criterion of the energy in molecular dynamics simulation, the most probable positions of doping impurities and effect of doping impurities on the interface structure are given, and the dependence of the fine structure on doping type and the order of the impurity amount are obtained. The analysis of the local energy for the

  16. Influence of Grain Boundary Character on Creep Void Formation in Alloy 617

    Science.gov (United States)

    Lillo, Thomas; Cole, James; Frary, Megan; Schlegel, Scott

    2009-12-01

    Alloy 617, a high-temperature creep-resistant, nickel-based alloy, is being considered for the primary heat exchanger for the Next Generation Nuclear Plant (NGNP), which will operate at temperatures exceeding 760 °C and a helium pressure of approximately 7 MPa. Observations of the crept microstructure using optical microscopy indicate creep stress does not significantly influence the creep void fraction at a given creep strain over the relatively narrow set of creep conditions studied. Void formation was found to occur only after significant creep in the tertiary regime (>5 pct total creep strain) had occurred. Also, orientation imaging microscopy (OIM) was used to characterize the grain boundaries in the vicinity of creep voids that develop during high-temperature creep tests (900 °C to 1000 °C at creep stresses ranging from 20 to 40 MPa) terminated at creep strains ranging from 5 to 40 pct. Preliminary analysis of the OIM data indicates voids tend to form on grain boundaries parallel, perpendicular, or 45 deg to the tensile axis, while few voids are found at intermediate inclinations to the tensile axis. Random grain boundaries intersect most voids, while coincident site lattice (CSL)-related grain boundaries did not appear to be consistently associated with void development. Similar results were found in oxygen-free, high-conductivity (OFHC) copper, severely deformed using equal channel angular extrusion, and creep tested at 450 °C and 14 MPa.

  17. The Effects of Grain Boundaries on the Current Transport Properties in YBCO-Coated Conductors.

    Science.gov (United States)

    Yang, Chao; Xia, Yudong; Xue, Yan; Zhang, Fei; Tao, Bowan; Xiong, Jie

    2015-12-01

    We report a detailed study of the grain orientations and grain boundary (GB) networks in Y2O3 films grown on Ni-5 at.%W substrates. Electron back scatter diffraction (EBSD) exhibited different GB misorientation angle distributions, strongly decided by Y2O3 films with different textures. The subsequent yttria-stabilized zirconia (YSZ) barrier and CeO2 cap layer were deposited on Y2O3 layers by radio frequency sputtering, and YBa2Cu3O7-δ (YBCO) films were deposited by pulsed laser deposition. For explicating the effects of the grain boundaries on the current carry capacity of YBCO films, a percolation model was proposed to calculate the critical current density (J c) which depended on different GB misorientation angle distributions. The significantly higher J c for the sample with sharper texture is believed to be attributed to improved GB misorientation angle distributions.

  18. 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.

  19. Observation and rate theory modeling of grain boundary segregation in Σ3 twin boundaries in ion-irradiated stainless steel 316

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Gyeong-Geun, E-mail: gglee@kaeri.re.kr; Jin, Hyung-Ha; Lee, Yong-Bok; Kwon, Junhyun

    2014-06-01

    Radiation-induced segregation (RIS) is the phenomenon of compositional change at point defect sinks in alloys irradiated at a moderate temperature. Owing to the potential relevance of RIS by way of the susceptibility of structural materials to irradiation-assisted stress corrosion cracking, basic research on austenitic stainless steels used in nuclear reactors has been carried out in recent years. In this work, commercial stainless steel 316 specimens were irradiated with Fe ions, and the resulting changes in Cr and Ni compositions were characterized using transmission electron microscopy and energy-dispersive X-ray spectroscopy. The samples with various grain boundary orientations, including the special Σ3 orientation, were analyzed. The ledges of a few special Σ3 twin boundaries showed significantly higher RIS compared to the coherent regions. The RIS behavior of a parallel twin pair was observed, and two profiles of RIS were found in them. The inner twins in multi-twins showed considerably lower RIS compared to the outer twins. For the calculation of RIS, time-dependent differential equations based on the rate theory were established and numerically integrated. An additional variable, representing the sink strength of the grain boundary, was introduced in the differential equations, and the concentration profiles of the Σ3 twins were calculated. The calculated results were in good agreement with the experimental results.

  20. Simulation of dc magnetic effects due to geometrically defined grain boundaries in type-II superconductors

    Energy Technology Data Exchange (ETDEWEB)

    Bartolome, E. [Escola Universitaria Salesiana de Sarria, Passeig Sant Joan Bosco, 74, 08017 Barcelona (Spain)], E-mail: ebartolome@euss.es; Granados, X.; Bozzo, B. [Institut de Ciencia de Materials de Barcelona, CSIC, Campus UAB, 08193 Bellaterra (Spain); Navau, C. [Grup d' Electromagnetisme, Universitat Autonoma de Barcelona, 08193 Bellaterra (Spain); Puig, T.; Obradors, X. [Institut de Ciencia de Materials de Barcelona, CSIC, Campus UAB, 08193 Bellaterra (Spain)

    2008-03-30

    A Bean model-based program ('Trazacorrientes') has been used to simulate the current distribution in the saturated remanent state of type-II superconducting bicrystal-like squared samples. The grain boundary was modeled by a set of periodically spaced holes geometrically defining the current transparency. Current simulations performed as a function of the boundary transparency, width and geometry are analyzed. Current distributions agree qualitatively with previously reported imaging measurements, while quantitative results can be obtained with an accuracy of {approx}5% due to present computing resolution limits. Thanks to 'Trazacorrientes' easy way of implementing irregular defects, meandering grain boundaries formed by straight facets of different local transparency could be simulated. The advantages and disadvantages of the program for the simulation of type-II superconductors with defects, among which GB's, are discussed.

  1. The Effect of Eectronic Energy Loss on Irradiation-Induced Grain Growth in Nanocrystalline Oxides

    Energy Technology Data Exchange (ETDEWEB)

    Zhang, Yanwen; Aidhy, Dilpuneet S.; Varga, Tamas; Moll, Sandra; Edmondson, P. D.; Namavar, Fereydoon; Jin, Ke; Ostrouchov, Christopher N.; Weber, William J.

    2014-01-01

    Grain growth of nanocrystalline materials is generally thermally activated, but can also be driven by irradiation at much lower temperature. In nanocrystalline ceria and zirconia, contributions from both displacement damage and ionization to the grain growth are identified. Our atomistic simulations have revealed fast grain boundary (GB) movements due to the high density of disorder near GBs. Our experimental results have shown that irradiation-induced grain growth is a function of total energy deposited, where the excitation of target electrons and displacement of lattice atoms both contribute to the overall disorder and both play important roles in grain growth. The coupling of energy deposition to the electronic and lattice structures should both be taken into consideration when engineering nanostructural materials.

  2. 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.

    2010-01-01

    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...

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

    NARCIS (Netherlands)

    van Noort, R.

    2008-01-01

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

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

    Science.gov (United States)

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

    2012-04-01

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

  5. 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.

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

    Science.gov (United States)

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

    2016-10-01

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

  7. Effect of transition metal impurities on the strength of grain boundaries in vanadium

    Energy Technology Data Exchange (ETDEWEB)

    Wu, Xuebang; Kong, Xiang-Shan; You, Yu-Wei; Liu, Wei; Liu, C. S., E-mail: csliu@issp.ac.cn [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031 (China); Chen, Jun-Ling; Luo, G.-N. [Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031 (China)

    2016-09-07

    Effects of 3d (Ti-Ni), 4d (Zr-Pd), and 5d (Hf-Pt) transition metal impurities on strength of two representative vanadium grain boundaries (GBs), symmetric Σ3(111) and asymmetric Σ5(210), were studied by first-principles calculations within the framework of the Rice-Wang thermodynamic model and within the computational tensile test. The desirable elements to increase the GB cohesion were predicted based on their segregation and strengthening behaviors across the different GB sites. It reveals that the elements Ti, Zr, Hf, Nb, and Ta are good choices for the GB cohesion enhancers. In addition, the GB strengthening by solutes is sensitive to the GB structures. The elements Cr, Mn, Fe, Co, and Ni decrease the GB strength of the Σ3(111) GB but they can increase the cohesion of the Σ5(210) GB. Furthermore, the origin of Ti-induced change of the GB strength was uncovered by analyzing the atomic bonds and electronic structures as well as the tensile strength. This work provides a theoretical guidance to screen promising alloying elements in V-based materials with improved resistance to GB decohesion and also helps us to understand the formation mechanism of Ti-rich precipitates in the V-Cr-Ti alloys under neutron or ion irradiation environments.

  8. Shear-coupled grain-boundary migration dependence on normal strain/stress

    Science.gov (United States)

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

    2017-08-01

    In specific conditions, grain-boundary (GB) migration occurs in polycrystalline materials as an alternative vector of plasticity compared to the usual dislocation activity. The shear-coupled GB migration, the expected most efficient GB based mechanism, couples the GB motion to an applied shear stress. Stresses on GB in polycrystalline materials seldom have, however, a unique pure shear component. This work investigates the influence of a normal strain on the shear coupled migration of a Σ 13 (320 )[001 ] GB in a copper bicrystal using atomistic simulations. We show that the yield shear stress inducing the GB migration strongly depends on the applied normal stress. Beyond, the application of a normal stress on this GB qualitatively modifies the GB migration: while the Σ 13 (320 )[001 ] GB shear couples following the 〈110 〉 migration mode without normal stress, we report the observation of the 〈010 〉 mode under a sufficiently high tensile normal stress. Using the nudge elastic band method, we uncover the atomistic mechanism of this 〈010 〉 migration mode and energetically characterize it.

  9. Energetics of vacancy segregation to [100] symmetric tilt grain boundaries in bcc tungsten

    Science.gov (United States)

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

    2016-11-01

    The harsh irradiation environment poses serious threat to the structural integrity of leading candidate for plasma-facing materials, tungsten (W), in future nuclear fusion reactors. It is thus essential to understand the radiation-induced segregation of native defects and impurities to defect sinks, such as grain boundaries (GBs), by quantifying the segregation energetics. In this work, molecular statics simulations of a range of equilibrium and metastable [100] symmetric tilt GBs are carried out to explore the energetics of vacancy segregation. We show that the low-angle GBs have larger absorption length scales over their high-angle counterparts. Vacancy sites that are energetically unfavorable for segregation are found in all GBs. The magnitudes of minimum segregation energies for the equilibrium GBs vary from -2.61 eV to -0.76 eV depending on the GB character, while those for the metastable GB states tend to be much lower. The significance of vacancy delocalization in decreasing the vacancy segregation energies and facilitating GB migration has been discussed. Metrics such as GB energy and local stress are used to interpret the simulation results, and correlations between them have been established. This study contributes to the possible application of polycrystalline W under irradiation in advanced nuclear fusion reactors.

  10. Energetics of vacancy segregation to [100] symmetric tilt grain boundaries in bcc tungsten.

    Science.gov (United States)

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

    2016-11-22

    The harsh irradiation environment poses serious threat to the structural integrity of leading candidate for plasma-facing materials, tungsten (W), in future nuclear fusion reactors. It is thus essential to understand the radiation-induced segregation of native defects and impurities to defect sinks, such as grain boundaries (GBs), by quantifying the segregation energetics. In this work, molecular statics simulations of a range of equilibrium and metastable [100] symmetric tilt GBs are carried out to explore the energetics of vacancy segregation. We show that the low-angle GBs have larger absorption length scales over their high-angle counterparts. Vacancy sites that are energetically unfavorable for segregation are found in all GBs. The magnitudes of minimum segregation energies for the equilibrium GBs vary from -2.61 eV to -0.76 eV depending on the GB character, while those for the metastable GB states tend to be much lower. The significance of vacancy delocalization in decreasing the vacancy segregation energies and facilitating GB migration has been discussed. Metrics such as GB energy and local stress are used to interpret the simulation results, and correlations between them have been established. This study contributes to the possible application of polycrystalline W under irradiation in advanced nuclear fusion reactors.

  11. Effect of grain boundary microstructure on superplastic deformation of Al-Li-Cu-Mg-Zr alloy

    Energy Technology Data Exchange (ETDEWEB)

    Kobayashi, S.; Yoshimura, T.; Tsurekawa, S.; Watanabe, T. [Tohoku Univ., Sendai (Japan). Dept. of Machine Intelligence and Syst. Eng.

    1999-07-01

    It is common knowledge that grain boundary sliding (GBS) is the most important deformation mechanism for superplastic deformation. In this investigation, Al-Li-Cu-Mg-Zr alloys having two distinct microstructures were produced to examine the effect of grain boundary (GB) microstructure on superplastic deformation. The effective GB microstructure to develop the superplastic deformation is discussed. Specimens with homogeneous and {l_brace}011{r_brace} textured grains, including high frequency of low-angle GBs showed superplastic behavior. The texture was weakened and most of low-angle GBs were changed into random GBs during deformation. Mean grain size increased slightly with deformation. On the other hand, specimens with heterogeneous and randomly oriented grains, with a high frequency of random GBs resulted in nonsuperplastic behavior. This microstructure was essentially unchanged by deformation. Extensive cavitation at GB triple junctions was also observed after superplastic deformation. In particular, cavities were most likely to form at the triple junctions composed of two or more random GBs. We will discuss the development of superplasticity through the optimization of GB microstructures in polycrystalline materials. (orig.)

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

    Science.gov (United States)

    Miller, Eric Todd

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

  13. Deformation-induced microstructural evolution at grain scale

    DEFF Research Database (Denmark)

    Winther, Grethe

    aswell as dislocation patterning by formation of dislocation boundaries occur. Experimental data andassociated data analysis at the grain scale and below will be presented to illustrate our current level ofunderstanding. The basis for the analysis is the crystallographic orientation of the grain as well......During plastic deformation metals develop microstructures which may be analysed on several scales,spanning from crystallographic textures averaged over the entire sample to the scale of individualgrains. Even within individual grains, intragranular phenomena in the form of orientation gradients...

  14. Mesoscopic current transport in two-dimensional materials with grain boundaries: Four-point probe resistance and Hall effect

    Science.gov (United States)

    Lotz, Mikkel R.; Boll, Mads; Østerberg, Frederik W.; Hansen, Ole; Petersen, Dirch H.

    2016-10-01

    We have studied the behavior of micro four-point probe (M4PP) measurements on two-dimensional (2D) sheets composed of grains of varying size and grain boundary resistivity by Monte Carlo based finite element (FE) modelling. The 2D sheet of the FE model was constructed using Voronoi tessellation to emulate a polycrystalline sheet, and a square sample was cut from the tessellated surface. Four-point resistances and Hall effect signals were calculated for a probe placed in the center of the square sample as a function of grain density n and grain boundary resistivity ρ GB . We find that the dual configuration sheet resistance as well as the resistance measured between opposing edges of the square sample have a simple unique dependency on the dimension-less parameter √{ n } ρ GB G 0 , where G0 is the sheet conductance of a grain. The value of the ratio R A / R B between resistances measured in A- and B-configurations depends on the dimensionality of the current transport (i.e., one- or two-dimensional). At low grain density or low grain boundary resistivity, two-dimensional transport is observed. In contrast, at moderate grain density and high grain resistivity, one-dimensional transport is seen. Ultimately, this affects how measurements on defective systems should be interpreted in order to extract relevant sample parameters. The Hall effect response in all M4PP configurations was only significant for moderate grain densities and fairly large grain boundary resistivity.

  15. Grain-boundary character distribution in recrystallized L12 ordered intermetallic alloys

    Science.gov (United States)

    Kaneno, Y.; Takasugi, T.

    2003-11-01

    The grain-boundary character distribution (GBCD) of cold-rolled and, subsequently, recrystallized Co3Ti and Ni3(Si,Ti) ordered alloys with an L12 structure was studied by the electron backscattered diffraction (EBSD) method, in association with texture. For comparison, the GBCD of recrystallized pure copper and aluminum was also determined. The recrystallization textures of the Co3Ti alloys as well as the Ni3(Si,Ti) alloy were significantly weak and different from those of the pure copper and aluminum with a strong cube texture. The GBCD of the Co3Ti alloys was characterized by a high frequency of Σ3 boundaries. On the other hand, the GBCD of the Ni3(Si,Ti) alloy was characterized by a lower frequency of Σ3 and higher frequency of random ( e.g., Σ>29) boundaries than that of the Co3Ti alloys. However, the GBCDs of the Co3Ti and Ni3(Si,Ti) alloys were similar to each other and also quite similar to those of the pure copper and aluminum, when Σ3 boundaries are excluded from the GBCD. Based on these results, the formation mechanism responsible for the recrystallization textures and the grain-boundary structure and energy of the Co3Ti and Ni3(Si,Ti) alloys were discussed, in comparison with those of pure copper and aluminum.

  16. INCOHERENT Σ3 GRAIN BOUNDARIES IN F.C.C. METALS : THE INFLUENCE OF INCLINATION ON THE BOUNDARY STRUCTURE AND ENERGY

    OpenAIRE

    1990-01-01

    The "Embedded Atom Method" (EAM) is used to study the energy and structure of different Σ3 grain boundaries defined by the inclination angle Φ. The grain boundary energy, γb, varies with Φ, i.e. the boundary plane orientation. For copper the γb (Φ) plot shows a deep energy minimum for the coherent (111) twin boundary for inclinations around the [01(-1)] axis. A second significant energy minimum is found ∼8° away from the incoherent ((-2)11) twin plane. The location of a minimum energy configu...

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2016-05-20

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

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

    Science.gov (United States)

    Gordon, Lyle M; Joester, Derk

    2015-01-01

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

  19. ROLE OF GRAIN BOUNDARY CARBIDES IN CRACKING BEHAVIOR OF Ni BASE ALLOYS

    Directory of Open Access Journals (Sweden)

    SEONG SIK HWANG

    2013-02-01

    Full Text Available The primary water stress corrosion cracking (PWSCC of Alloy 600 in a PWR has been reported in the control rod drive mechanism (CRDM, pressurizer instrumentation, and the pressurizer heater sleeves. Recently, two cases of boric acid precipitation that indicated leaking of the primary cooling water were reported on the bottom head surface of steam generators (SG in Korea. The PWSCC resistance of Ni base alloys which have intergranular carbides is higher than those which have intragranular carbides. Conversely, in oxidized acidic solutions like sodium sulfate or sodium tetrathionate solutions, the Ni base alloys with a lot of carbides at the grain boundaries and shows less stress corrosion cracking (SCC resistance. The role of grain boundary carbides in SCC behavior of Ni base alloys was evaluated and effect of intergranular carbides on the SCC susceptibility were reviewed from the literature.

  20. 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.

    2016-10-01

    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.

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

    Science.gov (United States)

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

    2012-02-01

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

  2. 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.

  3. Grain Boundary Engineering of Lithium-Ion-Conducting Lithium Lanthanum Titanate for Lithium-Air Batteries

    Science.gov (United States)

    2016-01-01

    release; distribution is unlimited. 1 1. Introduction Lithium (Li)-ion batteries are currently one of the leading energy storage device technologies...phase) were submerged in concentrated LiCl solution. The LiCl solution was made by dissolving lithium carbonate (LiCO3) into hydrogen chloride until...Direct correlations between fracture toughness and grain boundary segregation behavior in ytterbium- doped magnesium aluminate spinel. Scripta

  4. Grain Boundary Engineering of Lithium-Ion-Conducting Lithium Lanthanum Titanate for Lithium-Air Batteries

    Science.gov (United States)

    2015-01-01

    Titanate for Lithium-Air Batteries by Victoria L Blair, Claire V Weiss Brennan, and Joseph M Marsico Approved for public...TR-7584 ● JAN 2015 US Army Research Laboratory Grain Boundary Engineering of Lithium-Ion- Conducting Lithium Lanthanum Titanate for Lithium... Titanate for Lithium-Air Batteries 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) Victoria L Blair, Claire V

  5. 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.

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

    Energy Technology Data Exchange (ETDEWEB)

    Stroes-Gascoyne, S.; Moir, D.L.; Kolar, M.; Porth, R.J.; McConnell, J.L.; Kerr, A.H. [AECL Research, Pinawa, Manitoba (Canada). Whiteshell Labs.

    1995-12-31

    Combined gap and grain-boundary inventories of {sup 129}I 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 {sup 128}I 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 < 42 kW/m. For high linear power rating (HLPR) fuels (> 44 kW/m), the {sup 129}I values were considerably smaller than expected. The combined gap and grain-boundary inventories of {sup 129}I 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 {sup 129}I. Segments of used CANDU fuels were leached for 92 d (samples taken at 5, 28 and 92 d) to determine the kinetics of {sup 129}I release. Results could be fitted tentatively to half-order reaction kinetics, implying that {sup 129}I 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 {sup 129}I from used CANDU fuel.

  7. 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.

  8. Quantification of mineral behavior in four dimensions: Grain boundary and substructure dynamics in salt

    DEFF Research Database (Denmark)

    Borthwick, V. E.; Schmidt, Søren; Piazolo, S.;

    2012-01-01

    crystal; a 2D experiment conducted inside the scanning electron microscope and a 3D X-ray diffraction experiment. This allowed us to evaluate the possible effects of the free surface on grain and subgrain processes. The extent to which surface effects cause experimental artifacts in 2D studies has long...... grooving. Our results show that while it is problematic to derive absolute mobilities from 2D experiments, derived relative mobilities between boundaries with different misorientation angles can be used....

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2011-04-15

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

  10. Anisotropy of self-diffusion in forsterite grain boundaries derived from molecular dynamics simulations

    Science.gov (United States)

    Wagner, Johannes; Adjaoud, Omar; Marquardt, Katharina; Jahn, Sandro

    2016-12-01

    Diffusion rates and associated deformation behaviour in olivine have been subjected to many studies, due to the major abundance of this mineral group in the Earth's upper mantle. However, grain boundary (GB) transport studies yield controversial results. The relation between transport rate, energy, and geometry of individual GBs is the key to understand transport in aggregates with lattice preferred orientation that favours the presence and/or alignment of specific GBs over random ones in an undeformed rock. In this contribution, we perform classical molecular dynamics simulations of a series of symmetric and one asymmetric tilt GBs of Mg_2 SiO_4 forsterite, ranging from 9.58° to 90° in misorientation and varying surface termination. Our emphasis lies on unravelling structural characteristics of high- and low-angle grain boundaries and how the atomic structure influences grain boundary excess volume and self-diffusion processes. To obtain diffusion rates for different GB geometries, we equilibrate the respective systems at ambient pressure and temperatures from 1900 to 2200 K and trace their evolution for run durations of at least 1000 ps. We then calculate the mean square displacement of the different atomic species within the GB interface to estimate self-diffusion coefficients in the individual systems. Grain boundary diffusion coefficients for Mg, Si and O range from 10^{-18} to 10^{-21} m^3/s, falling in line with extrapolations from lower temperature experimental data. Our data indicate that higher GB excess volumes enable faster diffusion within the GB. Finally, we discuss two types of transport mechanisms that may be distinguished in low- and high-angle GBs.

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

    Science.gov (United States)

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

    2016-01-20

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

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

    Science.gov (United States)

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

    2016-01-01

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

  13. Frozen spatial chaos induced by boundaries

    CERN Document Server

    Eguiluz, V M; Piro, O; Balle, S; Eguiluz, Victor M.; Hernandez-Garcia, Emilio; Piro, Oreste; Balle, Salvador

    1999-01-01

    We show that rather simple but non-trivial boundary conditions could induce the appearance of spatial chaos (that is stationary, stable, but spatially disordered configurations) in extended dynamical systems with very simple dynamics. We exemplify the phenomenon with a nonlinear reaction-diffusion equation in a two-dimensional undulated domain. Concepts from the theory of dynamical systems, and a transverse-single-mode approximation are used to describe the spatially chaotic structures.

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

    Institute of Scientific and Technical Information of China (English)

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

    2001-01-01

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

  15. Grain boundary void nucleation in astroloy produced by room temperature deformation and anneal

    Science.gov (United States)

    Saegusa, T.; Uemura, M.; Weertman, J. R.

    1980-08-01

    Dyson and co-workers have shown that the creep life of a nickel base superalloy can be greatly shortened if the material is strained at room temperature before the creep test is carried out. They found that a prestrain followed by a short annealing time produces small grain boundary cavities, and it is the presence of these prenucleated voids which so seriously degrades service life at elevated temperatures. The present work explores the relationship between microstructure and prestrain void nucleation. Samples of the nickel base superalloy astroloy were given various heat treatments which led to significantly different microstructures. It was found that voids resulting from a prestrain-anneal treatment form preferentially at the ends of carbides on grain boundaries oriented roughly parallel to the prestrain tensile axis or rolling direction. Void spacing in the various microstructures is proportional to (but larger than) carbide spacing. The growth of these cavities during annealing is attributed to the presence of tensile residual stresses arising from the difference in deformability between grain boundary regions and the relatively soft matrix.

  16. Grain boundary modification to suppress lithium penetration through garnet-type solid electrolyte

    Science.gov (United States)

    Hongahally Basappa, Rajendra; Ito, Tomoko; Morimura, Takao; Bekarevich, Raman; Mitsuishi, Kazutaka; Yamada, Hirotoshi

    2017-09-01

    Garnet-type solid electrolytes are one of key materials to enable practical usage of lithium metal anode for high-energy-density batteries. However, it suffers from lithium growth in pellets on charging, which causes short circuit. In this study, grain boundaries of Li6.5La3Zr1.5Ta0.5O12 (LLZT) pellets are modified with Li2CO3 and LiOH to investigate the influence of the microstructure of grain boundaries on lithium growth and to study the mechanism of the lithium growth. In spite of similar properties (relative density of ca. 96% and total ionic conductivity of 7 × 10-4 S cm-1 at 25 °C), the obtained pellets exhibit different tolerance on the short circuit. The LLZT pellets prepared from LiOH-modified LLZT powders exhibit rather high critical current density of 0.6 mA cm-2, at which short circuit occurs. On the other hand, the LLZT pellets without grain boundary modification short-circuited at 0.15 mA cm-2. Microstructural analyses by means of SEM, STEM and EIS suggest that lithium grows through interconnected open voids, and reveal that surface layers such as Li2CO3 and LiOH are not only plug voids but also facilitate the sintering of LLZT to suppress the lithium growth. The results indicate a strategy towards short-circuit-free lithium metal batteries.

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

    Science.gov (United States)

    Chartier, Alain; Golovchuk, Bogdan; Gossé, Stéphane; Van Brutzel, Laurent

    2013-10-07

    A novel empirical potential has been developed to evaluate the thermodynamic stability of Ni(1-x)Fe(x)Cr2O4 spinels. The simulations confirm the hypothesis that the NiCr2O4-FeCr2O4 pseudo-binary 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.

  18. 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.

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

    Science.gov (United States)

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

    2016-02-01

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

  20. 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.

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

    Science.gov (United States)

    Pineau, André

    2015-03-28

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

  2. ANTIMONY GRAIN BOUNDARY SEGREGATION AND ITS SUPPRESSION BY CERIUM IN Fe-2%Mn-Sb STRUCTURAL STEELS

    Institute of Scientific and Technical Information of China (English)

    Z.X. Yuan; A.M. Guo; J. Liu; D.D. Shen; J. Jia; S.H. Song

    2003-01-01

    Antimony grain boundary segregation in Fe-2%Mn-Sb structure steels has been studied through measurements of the ductile-brittle transition temperature in conjunction with scanning electron microscopy, Auger electron spectroscopy and secondary ion mass spectroscopy. The research result reveals that during tempering or ageing after quenching at 980C, Sb segregates to grain boundaries with both equilibrium and nonequilibrium natures and brings about temper embrittlement in the steels. Cerium can relieve temper embrittlement of the steels and its segregation to grain boundaries -05 play an important role in reducing this embrittlement.

  3. Ultra-fast grain boundary diffusion and its contribution to surface segregation on a martensitic steel. Experiments and modeling

    Science.gov (United States)

    Christien, F.; Le Gall, R.

    2011-09-01

    Phosphorus surface segregation was measured by Auger Electron Spectroscopy on a 17-4 PH martensitic stainless steel at 450, 550 and 600 °C. Surface segregation was shown to be much faster than expected which was attributed to a high contribution of phosphorus diffusion along the former austenitic grain boundaries. A model of surface segregation was developed following the Darken-du Plessis approach and taking account of both bulk and grain boundary solute diffusion. The phosphorus grain boundary diffusion coefficient in 17-4 PH was estimated: DGB17-4 PH steel than in α-iron.

  4. 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.

  5. Self-healing properties of nanocrystalline materials: a first-principles analysis of the role of grain boundaries.

    Science.gov (United States)

    Xu, Jian; Liu, Jian-Bo; Li, Shun-Ning; Liu, Bai-Xin; Jiang, Yong

    2016-07-21

    Understanding the self-healing mechanisms of defects in nanocrystalline materials is of particular importance for developing structural materials that can support the extended lifetime of components under extremely hostile conditions in nuclear reactors. Since grain boundaries are prevalent in nanocrystalline materials, they must affect, to some extent, the overall self-healing properties and the resultant mechanical responses. In the present work, first principles calculations are carried out to investigate the energetic landscape of point defects (i.e. self-interstitials, He-interstitials, and vacancies) induced by the irradiation damage and the kinetics of the self-healing process in the vicinity of grain boundaries (GBs) in copper, focusing on six symmetric tilt grain boundaries that vary in their energies. Our results indicate that the interaction of vacancies with the self-interstitial- and He-interstitial-loaded GBs is very sensitive to the GB character. Low-energy GBs are generally accompanied by a higher propensity for self-healing behavior, in which the inter-granular interstitials and intra-granular vacancies recombine with each other. The recombination process is proved to be regulated by two mechanisms: the interstitial emission mechanism and the vacancy mediated mechanism. For low-energy GBs, the former mechanism demonstrates its efficiency in describing the atomic motion, while for the high-energy ones, the latter turns out to be superior. With the aid of these mechanisms, we conclude that low-energy GBs are comparatively more radiation-resistant than the high-energy counterparts, which may shed light on the rational design of high-performance structural materials based on nanocrystalline alloys.

  6. 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.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2013-02-01

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

  8. 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.

  9. Correlation of Magnetic Properties of Co/Cr Bilayer Thin Films with Grain Boundary Diffusion

    Institute of Scientific and Technical Information of China (English)

    Gaowu Qin; Bo Yang; Wenli Pei; Yuping Ren

    2009-01-01

    The microstructure and magnetic properties of Co/Cr bilayer films were examined before and after post-deposition annealing by using transmission electron microscopy (TEM), X-ray diffraction (XRD) technique and vibrating sample magnetometer (VSM). A model of grain boundary (GB) Cr-rich phase growth involving GB diffusion derived from the Cr underlayer was proposed to elucidate the kinetics of the paramagnetic Cr-rich phase growth along Co GBs within the Co layer. The correlation of the GB Cr-rich phase formation with the magnetic Co grain isolation and accordingly, improvement of magnetic properties was experimentally investigated and discussed in detail. Our analysis results are well consistent with previous micromagnetic simulations on the improvement of magnetic properties by the magnetic grain isolation. The results provide some insights into the processing-structure-property relationships of the Co/Cr bilayer films, and thus suggest that the magnetic grain isolation be feasible not only in longitudinal recording media, but also be effective in tuning the exchange coupling of magnetic grains in perpendicular recording media via the GB diffusion from underlayer and/or overlayer.

  10. 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

    2016-08-01

    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.

  11. 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.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-11-17

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

  13. High pressure study of B{sub 12}As{sub 2}: Electrical transport behavior and the role of grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Qinglin [Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409 (United States); State Key Laboratory of Superhard Materials, Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012 (China); Center for High Pressure Science and Technology Advanced Research, Shanghai 201203 (China); Liu, Cailong; Gao, Chunxiao, E-mail: cxgao599@aliyun.com, E-mail: y.ma@ttu.edu [State Key Laboratory of Superhard Materials, Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012 (China); Ma, Boheng [Department of Chemical Engineering, University of Maryland, College Park, Maryland 20742 (United States); Gao, Yang; Fitzpatrick, Matthew; Li, Yuqiang; Liu, Bao; Ma, Yanzhang, E-mail: cxgao599@aliyun.com, E-mail: y.ma@ttu.edu [Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409 (United States)

    2015-01-28

    Using a diamond anvil cell, the evolutions of alternate-current impedance spectra and direct- current resistivity in B{sub 12}As{sub 2} have been investigated up to 51.9 GPa. The results provide evidence for the existence of grain and grain boundary effects that are separated in the frequency region. The grain boundary resistance shows a relatively smaller contribution to the total resistance above 16.8 GPa. By using the double-Schottky barrier model, the space charge potential was obtained. A pressure-induced inversion of charge defect concentration in the space charge layer was found at 20.7 GPa. The high-temperature resistivity measurements indicate that the transport activation energy is determined by defect energy levels in the band gap.

  14. Slip transfer and plastic strain accumulation across grain boundaries in Hastelloy X

    Science.gov (United States)

    Abuzaid, Wael Z.; Sangid, Michael D.; Carroll, Jay D.; Sehitoglu, Huseyin; Lambros, John

    2012-06-01

    In this study, high resolution ex situ digital image correlation (DIC) was used to measure plastic strain accumulation with sub-grain level spatial resolution in uniaxial tension of a nickel-based superalloy, Hastelloy X. In addition, the underlying microstructure was characterized with similar spatial resolution using electron backscatter diffraction (EBSD). With this combination of crystallographic orientation data and plastic strain measurements, the resolved shear strains on individual slip systems were spatially calculated across a substantial region of interest, i.e., we determined the local slip system activity in an aggregate of ˜600 grains and annealing twins. The full-field DIC measurements show a high level of heterogeneity in the plastic response with large variations in strain magnitudes within grains and across grain boundaries (GBs). We used the experimental results to study these variations in strain, focusing in particular on the role of slip transmission across GBs in the development of strain heterogeneities. For every GB in the polycrystalline aggregate, we have established the most likely dislocation reaction and used that information to calculate the residual Burgers vector and plastic strain magnitudes due to slip transmission across each interface. We have also used molecular dynamics simulations (MD) to establish the energy barriers to slip transmission for selected cases yielding different magnitudes of the residual Burgers vector. From our analysis, we show an inverse relation between the magnitudes of the residual Burgers vector and the plastic strains across GBs. Also, the MD simulations reveal a higher energy barrier for slip transmission at high magnitudes of the residual Burgers vector. We therefore emphasize the importance of considering the magnitude of the residual Burgers vector to obtain a better description of the GB resistance to slip transmission, which in turn influences the local plastic strains in the vicinity of grain

  15. Ellipsometry characterization of polycrystalline ZnO layers with the modeling of carrier concentration gradient: Effects of grain boundary, humidity, and surface texture

    Energy Technology Data Exchange (ETDEWEB)

    Sago, Keisuke; Fujiwara, Hiroyuki, E-mail: fujiwara@gifu-u.ac.jp [Center of Innovative Photovoltaic Systems (CIPS), Gifu University, 1-1 Yanagido, Gifu 501-1193 (Japan); Kuramochi, Hideto; Iigusa, Hitoshi; Utsumi, Kentaro [Tokyo Research Laboratory, TOSOH Co., Ltd., 2743-1 Hayakawa, Ayase-shi, Kanagawa 252-1123 (Japan)

    2014-04-07

    Spectroscopic ellipsometry (SE) has been applied to study the effects of grain boundary, humidity, and surface texture on the carrier transport properties of Al-doped ZnO layers fabricated by dc and rf magnetron sputtering. In the SE analysis, the variation in the free carrier absorption toward the growth direction, induced by the ZnO grain growth on foreign substrates, has been modeled explicitly by adopting a multilayer model in which the optical carrier concentration (N{sub opt}) varies continuously with a constant optical mobility (μ{sub opt}). The effect of the grain boundary has been studied by comparing μ{sub opt} with Hall mobility (μ{sub Hall}). The change in μ{sub Hall}/μ{sub opt} indicates a sharp structural transition of the ZnO polycrystalline layer at a thickness of d ∼ 500 nm, which correlates very well with the structure confirmed by transmission electron microscopy. In particular, below the transition thickness, the formation of the high density grain boundary leads to the reduction in the μ{sub Hall}/μ{sub opt} ratio as well as N{sub opt}. As a result, we find that the thickness dependence of the carrier transport properties is almost completely governed by the grain boundary formation. On the other hand, when the ZnO layer is exposed to wet air at 85 °C, μ{sub Hall} reduces drastically with a minor variation of μ{sub opt} due to the enhanced grain boundary scattering. We have also characterized textured ZnO:Al layers prepared by HCl wet etching by SE. The analysis revealed that the near-surface carrier concentration increases slightly after the etching. We demonstrate that the SE technique can be applied to distinguish various rough textured structures (size ∼ 1 μm) of the ZnO layers prepared by the HCl etching.

  16. Mesoscopic current transport in two-dimensional materials with grain boundaries: Four-point probe resistance and Hall effect

    DEFF Research Database (Denmark)

    Lotz, Mikkel Rønne; Boll, Mads; Østerberg, Frederik Westergaard;

    2016-01-01

    configuration sheet resistance as well as the resistance measured between opposing edges of the square sample have a simple unique dependency on the dimension-less parameter √nρGBG0, where G0 is the sheet conductance of a grain. The value of the ratio RA/RB between resistances measured in A- and B......-configurations depends on the dimensionality of the current transport (i.e., one- or two-dimensional). At low grain density or low grain boundary resistivity, two-dimensional transport is observed. In contrast, at moderate grain density and high grain resistivity, one-dimensional transport is seen. Ultimately...

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

    Science.gov (United States)

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

    2016-05-01

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

  18. The control of brittleness and development of desirable mechanical properties in polycrystalline systems by grain boundary engineering

    Energy Technology Data Exchange (ETDEWEB)

    Watanabe, Tadao; Tsurekawa, Sadahiro

    1999-11-12

    Grain boundaries can be effectively controlled to produce or enhance their beneficial effects and also to diminish or reduce their detrimental effects on bulk properties in polycrystalline materials. Particular attention has been paid to the control of intergranular brittleness which remains a serious problem of material processing and development. Recent studies are presented and discussed, which have been successfully performed to control intergranular brittleness of intrinsically brittle materials such as the refractory metal molybdenum and the ordered intermetallic alloy Ni{sub 3}Al and to produce superplasticity in an Al-Li alloy, by grain boundary engineering through controlling a new microstructural factor termed the grain boundary character distribution (GBCD). The optimization of GBCD and the grain boundary connectivity has been found to be a key to produce desirable bulk mechanical properties in both structural and functional polycrystalline materials.

  19. Model of grain-boundary self-diffusion in α- and β-phases of titanium and zirconium

    Science.gov (United States)

    Chuvil'deev, V. N.; Semenycheva, A. V.

    2017-01-01

    A model of the grain-boundary self-diffusion process in metals undergoing phase transitions in the solid state is proposed. The model is based on the ideas and approaches of the theory of nonequilibrium grain boundaries. It is shown that the range of application of basic relations of this theory can be extended, and they can be used to calculate the parameters of grain-boundary self-diffusion in high-temperature and low-temperature phases of metals with phase transition. Based on the constructed model, activation energies of grainboundary self-diffusion in titanium and zirconium are calculated, and their anomalously low values in the low-temperature phase are explained. The calculated activation energies of grain-boundary self-diffusion are in good agreement with experimental data.

  20. 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.

  1. The influence of high grain boundary density on helium retention in tungsten

    Energy Technology Data Exchange (ETDEWEB)

    Valles, G., E-mail: gonzalovallesalberdi@hotmail.es [Instituto de Fusión Nuclear UPM, José Gutiérrez Abascal 2, 28006 Madrid (Spain); González, C. [Departamento de Física, Universidad de Oviedo, C/ Calvo Sotelo, s/n, Oviedo (Spain); Martin-Bragado, I. [IMDEA Materials Institute, C/ Enric Kandel 2, 28906 Getafe, Madrid (Spain); Iglesias, R. [Departamento de Física, Universidad de Oviedo, C/ Calvo Sotelo, s/n, Oviedo (Spain); Perlado, J.M.; Rivera, A. [Instituto de Fusión Nuclear UPM, José Gutiérrez Abascal 2, 28006 Madrid (Spain)

    2015-02-15

    Highlights: • Comparison between monocrystalline and nanostructured irradiated tungsten. • OKMC parameterization published and new DFT data. • Important role of grain boundary density on defect evolution. • Cluster pressurization much lower in nanostructured tungsten. • Promising expectations on nanocrystalline tungsten in view of results. - Abstract: In order to study the influence of a high grain boundary density on the amount, size and distribution of defects produced by pulsed helium (625 keV) irradiation in tungsten, we have carried out Object Kinetic Monte Carlo (OKMC) simulations in both monocrystalline and nanocrystalline tungsten. The parameterization of the OKMC code (MMonCa) includes binding energies obtained with our in-house Density Functional Theory (DFT) calculations. In the interior of a grain in nanocrystalline tungsten the mixed He{sub n}V{sub m} clusters are larger and have a lower He/V ratio. Thus, they are less pressurized clusters. The total elastic strain energy remains almost constant with the increasing number of pulses, contrary to its increase in monocrystalline tungsten. A better response to helium irradiation is therefore expected in nanocrystalline tungsten, opening a new path to investigate these nanostructured materials for fusion purposes.

  2. Sensitivity analysis of Immersed Boundary Method simulations of fluid flow in dense polydisperse random grain packings

    Directory of Open Access Journals (Sweden)

    Knight Chris

    2017-01-01

    Full Text Available Polydisperse granular materials are ubiquitous in nature and industry. Despite this, knowledge of the momentum coupling between the fluid and solid phases in dense saturated grain packings comes almost exclusively from empirical correlations [2–4, 8] with monosized media. The Immersed Boundary Method (IBM is a Computational Fluid Dynamics (CFD modelling technique capable of resolving pore scale fluid flow and fluid-particle interaction forces in polydisperse media at the grain scale. Validation of the IBM in the low Reynolds number, high concentration limit was performed by comparing simulations of flow through ordered arrays of spheres with the boundary integral results of Zick and Homsy [10]. Random grain packings were studied with linearly graded particle size distributions with a range of coefficient of uniformity values (Cu = 1.01, 1.50, and 2.00 at a range of concentrations (ϕ ∈ [0.396; 0.681] in order to investigate the influence of polydispersity on drag and permeability. The sensitivity of the IBM results to the choice of radius retraction parameter [1] was investigated and a comparison was made between the predicted forces and the widely used Ergun correlation [3].

  3. Study of 44Ti grain boundary self-diffusion in thin nanocrystalline TiO2 films

    Energy Technology Data Exchange (ETDEWEB)

    Straumal, Petr [Institut fuer Materialphysik, Universitaet Muenster, D-48149 Muenster (Germany); National University of Science and Technology, MISIS, 119049 Moscow (Russian Federation); Divinski, Sergiy; Wilde, Gerhard [Institut fuer Materialphysik, Universitaet Muenster, D-48149 Muenster (Germany)

    2011-07-01

    Titanium dioxide is known for its photo-catalytic properties and enhanced corrosion resistance in aqueous environments. Due to these properties TiO2 is very attractive material for light-induced self-cleaning glass, water-cleaning and producing hydrogen from water applications. Numerous works are dedicated to the diffusion of various dopants like niobium or chromium in TiO2 but so far, none studied the self-diffusion of titanium in nanocrystalline TiO2. The grain boundary self-diffusion in thin nanocrystalline TiO2 films is investigated. The oxide films are produced using a novel deposition method from metal-organic precursors at relatively low (400-500 C) temperatures. A relaxation annealing at 800 C was performed. The diffusion was measured in temperature interval between 200 C and 600 C by means of the radiotracer technique applying the 44Ti isotope and utilizing ion beam sputtering for sectioning. The diffusion was measured at different oxygen pressures. In addition, the microstructure and its possible evaluation during diffusion annealing was investigated using TEM. The results are discussed with respect of the relationship between grain boundary self-diffusion and the synthesis pathway, the oxygen pressure and resulting microstructure of the nanoscale functional oxide films.

  4. The Effect of Twin Grain Boundary Tuned by Temperature on the Electrical Transport Properties of Monolayer MoS2

    OpenAIRE

    Luojun Du; Hua Yu; Li Xie; Shuang Wu; Shuopei Wang; Xiaobo Lu; Mengzhou Liao; Jianling Meng; Jing Zhao; Jing Zhang; Jianqi Zhu; Peng Chen; Guole Wang; Rong Yang; Dongxia Shi

    2016-01-01

    Theoretical calculation and experimental measurement have shown that twin grain boundary (GB) of molybdenum disulphide (MoS2) exhibits extraordinary effects on transport properties. Precise transport measurements need to verify the transport mechanism of twin GB in MoS2. Here, monolayer molybdenum disulphide with a twin grain boundary was grown in our developed low-pressure chemical vapor deposition (CVD) system, and we investigated how the twin GB affects the electrical transport properties ...

  5. 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.

  6. Studies of the grain boundary effect in electrodeposited cadmium telluride films from optical measurements

    Energy Technology Data Exchange (ETDEWEB)

    Dutta, J.; Bhattacharyva, D.; Maiti, A.B.; Chaudhuri, S.; Pal, A.K. (Indian Association for the Cultivation of Science, Calcutta (India). Dept. of Materials Science)

    1995-01-01

    Polycrystalline CdTe films were deposited onto SnO[sub 2] coated glass substrates using electrodeposition with different deposition potentials, ranging from -670 to -725 mV, with respect to a saturated calomel electrode (SCE). The grain boundary potential (E[sub b]), the density of trap states at the intercrystalline boundary (Q[sub t]) and the carrier concentration (p) in the films were obtained. The surface roughness ([sigma][sub o]) of the films was determined by utilising reflectance measurements while the band gap ([approx] 1.49 eV) was determined from transmittance vs wavelength traces. The barrier height was found to increase from 0.23 eV to 0.25 eV with the variation of the deposition potential from -675 to -725 mV; while the corresponding variation in the density of trap states at the grain boundary region was 1.0 x 10[sup 12]-2.1x10[sup 12]. The carrier concentration was obtained from experimental values of the Debye length, determined from the optical transmittance measurements. (Author)

  7. Preparation of high-quality HTS rings for application in the magnetic bearing of cryotanks and pinning in grain boundaries

    Science.gov (United States)

    Bringmann, B.; Walter, H.; Jooss, Ch.; Leenders, A.; Freyhardt, H. C.

    2002-08-01

    Seeded melt growth of YBCO high-temperature superconductors is one of the most promising preparation techniques to obtain high-quality HTS tiles for application, e.g. in magnetic bearings. Semi-finished HTSL products of complex shapes have to be developed by different seeding and multi-seeding techniques. To obtain large hollow cylinders designed for application in the magnetic bearing of a cryotank a modified multi-seeded melt growth (MSMG) process was employed. This cryotank will be mounted for testing in a vehicle of a major German car manufacturer. The MSMG process introduces grain boundaries into the HTS tiles. For transport current investigations of [0 0 1]-tilt grain boundaries in melt textured YBCO a series of MSMG bicrystals have been prepared. They exhibit a dependence of the critical current density on misorientation angle which is much weaker than the one observed in thin-film bicrystals. The bulk samples have dimensions larger than the magnetic penetration depth along the grain boundary. Thus, flux pinning has to be taken into account. Different contributions to the longitudinal pinning force have to be considered: vortices at grain boundaries can be pinned by magnetic interaction with Abrikosov vortices in the banks, by defects in the grain boundary itself or by defects which are located next to the grain boundary.

  8. Control of one-dimensional magnetism in graphene via spontaneous hydrogenation of the grain boundary.

    Science.gov (United States)

    Yin, Wan-Jian; Wei, Su-Huai; Yan, Yanfa

    2013-06-07

    We propose that control of one-dimensional (1D) magnetism in graphene could be made easier by spontaneous hydrogenation of chemically reactive grain boundaries (GBs) in polycrystalline graphenes. Unlike pristine graphene, where hydrogen adsorption favors the formation of zero-dimensional (0D) clusters, the defect cores (pentagon, heptagon and octagon) at the GBs in polycrystalline graphene promote hydrogenation along the GBs. The hydrogenation in polycrystalline graphene starts at the GBs, proceeds gradually towards the grain interior (GI) and results in smooth 1D graphane-graphene interfaces. Our calculations show that the type (ferro- or antiferro-magnetism) and strength of the magnetism can be controlled by controlling the orientation of GBs. Since GBs in single-layer graphenes can be fabricated in a controllable way in experiments, the hydrogenation of GBs could be a unique method to realize large-area magnetic graphenes for future spintronic applications.

  9. 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.

  10. Binding energetics of substitutional and interstitial helium and di-helium defects with grain boundary structure in α-Fe

    Energy Technology Data Exchange (ETDEWEB)

    Tschopp, M. A., E-mail: mark.tschopp@gatech.edu [Dynamic Research Corporation, (on site at) U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 (United States); Center for Advanced Vehicular Systems, Mississippi State University, Starkville, Mississippi 39762 (United States); Gao, F.; Yang, L. [Pacific Northwest National Laboratory, Richland, Washington 99352 (United States); Solanki, K. N. [Arizona State University, School for Engineering of Matter, Transport and Energy, Tempe, Arizona 85287 (United States)

    2014-01-21

    The formation/binding energetics and length scales associated with the interaction between He atoms and grain boundaries in BCC α-Fe were explored. Ten different low Σ grain boundaries from the 〈100〉 and 〈110〉 symmetric tilt grain boundary systems were used. In this work, we then calculated formation/binding energies for 1–2 He atoms in the substitutional and interstitial sites (HeV, He{sub 2}V, HeInt, He{sub 2}Int) at all potential grain boundary sites within 15 Å of the boundary (52 826 simulations total). The present results provide detailed information about the interaction energies and length scales of 1–2 He atoms with grain boundaries for the structures examined. A number of interesting new findings emerge from the present study. For instance, the Σ3(112) twin boundary in BCC Fe possesses a much smaller binding energy than other boundaries, which corresponds in long time dynamics simulations to the ability of an interstitial He defect to break away from the boundary in simulations on the order of nanoseconds. Additionally, positive correlations between the calculated formation/binding energies of the He defects (R > 0.9) asserts that the local environment surrounding each site strongly influences the He defect energies and that highly accurate quantum mechanics calculations of lower order defects may be an adequate predictor of higher order defects. Various metrics to quantify or classify the local environment were compared with the He defect binding energies. The present work shows that the binding and formation energies for He defects are important for understanding the physics of He diffusion and trapping by grain boundaries, which can be important for modeling He interactions in polycrystalline steels.

  11. Interaction of run-in edge dislocations with twist grain boundaries in Al-a molecular dynamics study

    Science.gov (United States)

    Chandra, S.; Naveen Kumar, N.; Samal, M. K.; Chavan, V. M.; Patel, R. J.

    2016-06-01

    Grain boundaries play an important role in outlining the mechanical properties of crystalline materials. They act as sites for absorption/nucleation of dislocations, which are the main carriers of plastic deformation. In view of this, the interactions between edge dislocations and twist grain boundaries-dislocation pileup, dislocation absorption and dislocation emission were explored by performing molecular dynamics simulations in face-centered cubic Al using embedded atom method. The ?1 1 0? twist grain boundaries with various misorientation angles were selected for this purpose. It was found that the misorientation angle of boundary and stress anomalies arising from repeated dislocation absorption at the grain boundaries are the important parameters in determining the ability of the boundary to emit dislocations. Complex network of dislocations results in later stages of deformation, which may have a significant effect on the mechanical properties of the material. The peculiarities of dislocation nucleation, their emission from twist grain boundaries and the ramifications of this study towards development of higher length scale material models are discussed.

  12. Effect of stress-induced grain growth during room temperature tensile deformation on ductility in nanocrystalline metals

    Indian Academy of Sciences (India)

    Weichang Xu; Pinqiang Dai; Xiaolei Wu

    2010-10-01

    In the present study defect-free nanocrystalline (nc) Ni–Co alloys with the Co content ranging from 2.4–59.3% (wt.%) were prepared by pulse electrodeposition. X-ray diffraction analysis shows that only a single face-centred cubic solid solution is formed for each alloy and that the grain size reduces monotonically with increasing Co content, which is consistent with transmission electron microscopy (TEM) observations. In the nc Ni–Co alloys, both the ultimate tensile strength and the elongation to failure increase as the Co content increases. The TEM observations reveal that stress-induced grain growth during tensile deformation is significantly suppressed for the nc Ni–Co alloys rich in Co in sharp contrast to those poor in Co. We believe that sufficient solutes could effectively pin grain boundaries making grain boundary motions (e.g. grain boundary migration and/or grain rotation) during deformation more difficult. Thus, stress-induced grain growth is greatly suppressed. At the same time, shear banding plasticity instability is correspondingly delayed leading to the enhanced ductility.

  13. 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.

  14. 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.

  15. Atomistic simulations of grain boundary migration in face centred cubic metals

    OpenAIRE

    Schönfelder, Bernd

    2004-01-01

    In this work, atomistic grain boundary (GB) migration and GB self-diffusion simulations of planar [001] twist GBs as well as various planar tilt GBs in face-centred cubic bicrystals have been performed. The utilized simulation method of choice was molecular dynamics (MD). Due to the planar geometry of the studied GBs, no driving force (DF) on the GB is exerted due to GB curvature. Therefore one necessary and important feature of this work was to derive DF concepts to drive planar GBs continuo...

  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. Amorphous grain boundary layers in the ferromagnetic nanograined ZnO films

    Energy Technology Data Exchange (ETDEWEB)

    Straumal, B.B., E-mail: straumal@mf.mpg.de [National University of Science and Technology ' Moscow Institute of Steel and Alloys, MISiS' , Leninsky prospect 4, 119991 Moscow (Russian Federation); Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow district, 142432 (Russian Federation); Karlsruher Institut fuer Technologie, Institut fuer Nanotechnologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germany); Mazilkin, A.A.; Protasova, S.G. [Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow district, 142432 (Russian Federation); Max-Planck-Institut fuer Intelligente Systeme (former Institut fuer Metallforschung), Heisenbergstrasse 3, 70569 Stuttgart (Germany); Myatiev, A.A. [National University of Science and Technology ' Moscow Institute of Steel and Alloys, MISiS' , Leninsky prospect 4, 119991 Moscow (Russian Federation); Straumal, P.B. [National University of Science and Technology ' Moscow Institute of Steel and Alloys, MISiS' , Leninsky prospect 4, 119991 Moscow (Russian Federation); Institut fuer Materialphysik, Universitaet Muenster, Wilhelm-Klemm-Str. 10, D-48149 Muenster (Germany); Goering, E. [Max-Planck-Institut fuer Intelligente Systeme (former Institut fuer Metallforschung), Heisenbergstrasse 3, 70569 Stuttgart (Germany); and others

    2011-12-01

    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.

  18. Conducting grain boundaries enhancing thermoelectric performance in doped Mg2Si

    Science.gov (United States)

    Muthiah, Saravanan; Pulikkotil, Jiji; Srivastava, A. K.; Kumar, Ashok; Pathak, B. D.; Dhar, Ajay; Budhani, R. C.

    2013-07-01

    The thermoelectric properties of Pb doped Mg2Si, synthesized using reactive sintering employing spark plasma sintering, are investigated and are compared with other dopants reported in literature. While a moderate decrease in Seebeck coefficient and thermal conductivity is observed for 2 at. % of Pb doping in Mg2Si, a substantial enhancement in the material's thermoelectric figure-of-merit is observed, which is due to an enormous increase in its electrical conductivity. A brick-layer model is proposed to explain these results, wherein the inter-granular electronic conductivity is facilitated by Pb (or Mg2Pb) phases at grain boundaries, which is supported by microstructural evidences.

  19. SQUID magnetometer with YBa sub 2 Cu sub 3 O sub 7 grain boundary step contacts

    CERN Document Server

    Krämer, A

    2002-01-01

    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.

  20. Optimizing Grain Boundary Complexions to Produce Dense Pressure-Less Sintered Boron Carbide (B4C)

    Science.gov (United States)

    2008-11-14

    Unlimited 18. NUMBER OF PAGES 15 19a. NAME OF RESPONSIBLE PERSON Martin P. Harmer 19b. TELEPHONE NUMBER (Include area code) 610-758-4227 Standard...American Ceramic Society, 1993. 76(11): p. 2801-8. 26. Dillon, S.J., Tang, M., Carter, W.C., and M.P. Harmer , Complexion: A New Concept for Kinetic...Engineering in Materials Science. Acta Materialia, 2007. 55: p. 6208-18. 27. Dillon, S.J. and M.P. Harmer , Multiple Grain boundary transitions in Ceramics

  1. Effect of Interstitial Hydrogen on Cohesive Strength of Al Grain Boundary with Mg Segregation

    Institute of Scientific and Technical Information of China (English)

    Xiaoguang LIU; Xiaowei WANG; Jingyang WANG; Hongyan ZHANG

    2006-01-01

    The effect of interstitial hydrogen on the cohesion of the Al ∑=11(113) grain boundary (GB) is investigated based on the thermodynamic model of Rice-Wang using the first-principles density function calculation. The results indicate that interstitial H behaves as an embrittler from "strengthening energy" analysis. The reduced GB cohesion due to the presence of H at the GB is attributed to the low affinity between H and Al, and the weakened bonding of Al atomic pairs perpendicular to GB plane.

  2. Auger electron spectroscopy study of the grain boundary microchemistry in structural materials for nuclear systems; Estudio por espectroscopia auger de la microquimica en borde de grano en materiales estructurales para sistemas nucleares

    Energy Technology Data Exchange (ETDEWEB)

    Garcia Mazario, M.

    2007-07-01

    Grain boundary microchemical characterization, by means of Auger electron Spectroscopy, has been performed in structural materials used or candidates to be used in several nuclear systems. The study has been divided in four parts: 1. Radiation induced segregation in austenitic stainless steels of internals of nuclear reactor. The irradiation effects have been simulated by means of cold work and heat treatments. 2. Effect of the thermal aging in simulated operation conditions in the microchemistry of the grain boundaries in low activation martensitic steels candidates to first wall and blanket in future fusion reactors. 3. Effect of the spallation elements production in the grain boundary microchemistry in martensitic steels candidates to be used in future accelerator driven systems. 4. Effect of irradiation in the grain boundary microchemistry in an austenitic steel, a martensitic steel and a nickel base alloy irradiated with 800 MeV protons in a spallation source. (Author) 142 refs.

  3. Variable-charge method applied to study coupled grain boundary migration in the presence of oxygen

    Energy Technology Data Exchange (ETDEWEB)

    Elsener, A. [Paul Scherrer Institute, Villigen, PSI-Villigen CH-5232 (Switzerland); Politano, O. [Institut Carnot de Bourgogne, UMR 5209 CNRS-Universite de Bourgogne, 9 Avenue Alain Savary, 21078 Dijon Cedex (France); Derlet, P.M. [Paul Scherrer Institute, Villigen, PSI-Villigen CH-5232 (Switzerland); Van Swygenhoven, H. [Paul Scherrer Institute, Villigen, PSI-Villigen CH-5232 (Switzerland)], E-mail: helena.vs@psi.ch

    2009-04-15

    One of the important differences between simulation and experiments in grain boundary (GB)-dominated metallic structures is the lack of impurities such as oxygen in computational samples. A modified variable-charge method [Elsener A, Politano O, Derlet PM, Van Swygenhoven H. Modell Simul Mater Sci Eng 2008;16:025006] based on the Streitz and Mintmire approach [Streitz FH, Mintmire JW. Phys Rev B 1994;50:11996] is used to study coupled GB motion in an Al bicrystal with a [1 1 2] symmetrical tilt GB in the presence of substitutional O, and compared with the stick-slip process identified by Cahn and Mishin [Cahn JW, Mishin Y, Suzuki A. Acta Mater 2006;54:4953]. It is found that the critical shear stress for migration of the GB increases linearly with the number of O atoms. These observations are then rationalized in terms of the internal stress signature of the O atoms in the vicinity of the boundary.

  4. On Techniques to Characterize and Correlate Grain Size, Grain Boundary Orientation and the Strength of the SiC Layer of TRISO Coated Particles: A Preliminary Study

    Energy Technology Data Exchange (ETDEWEB)

    I.J.van Rooyen; J.L. Dunzik Gougar; T. Trowbridge; Philip M van Rooyen

    2012-10-01

    The mechanical properties of the silicon carbide (SiC) layer of the TRi-ISOtropic (TRISO) coated particle (CP) for high temperature gas reactors (HTGR) are performance parameters that have not yet been standardized by the international HTR community. Presented in this paper are the results of characterizing coated particles to reveal the effect of annealing temperature (1000 to 2100°C) on the strength and grain size of unirradiated coated particles. This work was further expanded to include possible relationships between the grain size and strength values. The comparative results of two strength measurement techniques and grain size measured by the Lineal intercept method are included. Preliminary grain boundary characterization results determined by electron backscatter diffraction (EBSD) are included. These results are also important for future fission product transport studies, as grain boundary diffusion is identified as a possible mechanism by which 110mAg, one of the fission activation products, might be released through intact SiC layers. Temperature is a parameter known to influence the grain size of SiC and therefore it is important to investigate the effect of high temperature annealing on the SiC grain size. Recommendations and future work will also be briefly discussed.

  5. Charged grain boundaries reduce the open-circuit voltage of polycrystalline solar cells—An analytical description

    Science.gov (United States)

    Gaury, Benoit; Haney, Paul M.

    2016-12-01

    Analytical expressions are presented for the dark current-voltage relation J(V) of a pn+ junction with positively charged columnar grain boundaries with high defect density. These expressions apply to non-depleted grains with sufficiently high bulk hole mobilities. The accuracy of the formulas is verified by direct comparison to numerical simulations. Numerical simulations further show that the dark J(V) can be used to determine the open-circuit potential Voc of an illuminated junction for a given short-circuit current density Jsc . A precise relation between the grain boundary properties and Voc is provided, advancing the understanding of the influence of grain boundaries on the efficiency of thin film polycrystalline photovoltaics like CdTe and Cu(In,Ga)Se2 .

  6. 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.

  7. Non-equilibrium grain boundaries in titanium nanostructured by severe plastic deformation: Computational study of sources of material strengthening

    DEFF Research Database (Denmark)

    Liu, Hongsheng; Mishnaevsky, Leon; Pantleon, Wolfgang

    2014-01-01

    A computational model of ultrafine grained (UFG) or nanostructured titanium (Ti), based on a finite element (FE) unit cell model of the material and a dislocation density based model of plastic deformation has been developed. FE simulations of tensile deformation of UFG Ti with different fractions...... and properties of the grain boundary (GB) phase have been carried out. The effect of different degrees of deviation from the equilibrium state of the grain boundaries (GBs) on the mechanical behaviour of nanostructured Ti have been investigated using the combined composite/dislocation dynamics based model....... In particular, the effects of different diffusion coefficients in the GB phase, of a high initial dislocation density in the grain boundaries, as well as of atomic scale precipitates are investigated for affecting the deformation behaviour of UFG or nanostructured Ti. © 2013 Elsevier B.V. All rights reserved....

  8. Defects in rutile and anatase polymorphs of TiO2: kinetics and thermodynamics near grain boundaries.

    Science.gov (United States)

    Uberuaga, Blas P; Bai, Xian-Ming

    2011-11-02

    The direct consequence of irradiation on a material is the creation of point defects-typically interstitials and vacancies, and their aggregates-but it is the ultimate fate of these defects that determines the material's radiation tolerance. Thus, understanding how defects migrate and interact with sinks, such as grain boundaries, is crucial for predicting the evolution of the material. We examine defect properties in two polymorphs of TiO(2)-rutile and anatase-to determine how these materials might respond differently to irradiation. Using molecular statics and temperature accelerated dynamics, we focus on two issues: how point defects interact with a representative grain boundary and how they migrate in the bulk phase. We find that grain boundaries in both polymorphs are strong sinks for all point defects, though somewhat stronger in rutile than anatase. Further, the defect kinetics are very different in the two polymorphs, with interstitial species diffusing quickly in rutile while oxygen defects-both interstitials and vacancies-are fast diffusers in anatase. These results allow us to speculate on how grain boundaries will modify the radiation tolerance of these materials. In particular, grain boundaries in rutile will lead to a space charge layer at the boundary and a vacancy-rich damage structure, while in anatase the damage structure would likely be more stoichiometric, but with larger defects consisting primarily of Ti ions.

  9. Effect of one-step recrystallization on the grain boundary evolution of CoCrFeMnNi high entropy alloy and its subsystems.

    Science.gov (United States)

    Chen, Bo-Ru; Yeh, An-Chou; Yeh, Jien-Wei

    2016-02-29

    In this study, the grain boundary evolution of equiatomic CoCrFeMnNi, CoCrFeNi, and FeCoNi alloys after one-step recrystallization were investigated. The special boundary fraction and twin density of these alloys were evaluated by electron backscatter diffraction analysis. Among the three alloys tested, FeCoNi exhibited the highest special boundary fraction and twin density after one-step recrystallization. The special boundary increment after one-step recrystallization was mainly affected by grain boundary velocity, while twin density was mainly affected by average grain boundary energy and twin boundary energy.

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

    Energy Technology Data Exchange (ETDEWEB)

    Jin, Yanan; Lai, Wensheng, E-mail: wslai@tsinghua.edu.cn

    2016-03-15

    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.

  11. 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.

  12. Atomic investigation of alloying Cr, Ti, Y additions in a grain boundary of vanadium

    Energy Technology Data Exchange (ETDEWEB)

    Zhang, Pengbo, E-mail: zhangpb@dlmu.edu.cn [Department of Physics, Dalian Maritime University, Dalian 116026 (China); Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024 (China); Li, Xiaojie; Zhao, Jijun [Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024 (China); Zheng, Pengfei; Chen, Jiming [Southwestern Institute of Physics, Chengdu 610041 (China)

    2016-01-15

    The effect of alloying additions (Cr, Ti and Y) in a vanadium (V) ∑3 (111) grain boundary (GB) is investigated by first-principles calculations. To determine site preference and segregation properties of Cr, Ti and Y in the GB and bulk, we calculate the formation energies and segregation energies for different interstitial and substitutional sites. Cr/Ti/Y atom prefers to segregate to the substitutional sites of the GB from bulk environment, whereas Cr segregation to GB is very weak. Based on the Rice and Wang's model, Cr acts as the GB cohesion, while Ti and Y are strong embrittlers. The analysis of atomic and electronic structures provides a reasonable expansion for the embrittlement behavior. Moreover, the effect of Cr, Ti and Y in the GB on solution of interstitial impurities C, N, O, H, and He are determined. The results show that Cr restrains solution of these impurities in the GB, while Ti tends to form Ti–N complex by absorbing N impurities and Y can absorbs O and He impurities. The present calculations are helpful for understanding the behavior of alloying Cr, Ti, Y additions at the grain boundary of vanadium.

  13. 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.

  14. Creep deformation of grain boundary in a highly crystalline SiC fibre.

    Science.gov (United States)

    Shibayama, Tamaki; Yoshida, Yutaka; Yano, Yasuhide; Takahashi, Heishichiro

    2003-01-01

    Silicon carbide (SiC) matrix composites reinforced by SiC fibres (SiC/SiC composites) are currently being considered as alternative materials in high Ni alloys for high-temperature applications, such as aerospace components, gas-turbine energy-conversion systems and nuclear fusion reactors, because of their high specific strength and fracture toughness at elevated temperatures compared with monolithic SiC ceramics. It is important to evaluate the creep properties of SiC fibres under tensile loading in order to determine their usefulness as structural components. However, it would be hard to evaluate creep properties by monoaxial tensile properties when we have little knowledge on the microstructure of crept specimens, especially at the grain boundary. Recently, a simple fibre bend stress relaxation (BSR) test was introduced by Morscher and DiCarlo to address this problem. Interpretation of the fracture mechanism at the grain boundary is also essential to allow improvement of the mechanical properties. In this paper, effects of stress applied by BSR test on microstructural evolution in advanced SiC fibres, such as Tyranno-SA including small amounts of Al, are described and discussed along with the results of microstructure analysis on an atomic scale by using advanced microscopy.

  15. Cs diffusion in SiC high-energy grain boundaries

    Science.gov (United States)

    Ko, Hyunseok; Szlufarska, Izabela; Morgan, Dane

    2017-09-01

    Cesium (Cs) is a radioactive fission product whose release is of concern for Tristructural-Isotropic fuel particles. In this work, Cs diffusion through high energy grain boundaries (HEGBs) of cubic-SiC is studied using an ab-initio based kinetic Monte Carlo (kMC) model. The HEGB environment was modeled as an amorphous SiC, and Cs defect energies were calculated using the density functional theory (DFT). From defect energies, it was suggested that the fastest diffusion mechanism is the diffusion of Cs interstitial in an amorphous SiC. The diffusion of Cs interstitial was simulated using a kMC model, based on the site and transition state energies sampled from the DFT. The Cs HEGB diffusion exhibited an Arrhenius type diffusion in the range of 1200-1600 °C. The comparison between HEGB results and the other studies suggests not only that the GB diffusion dominates the bulk diffusion but also that the HEGB is one of the fastest grain boundary paths for the Cs diffusion. The diffusion coefficients in HEGB are clearly a few orders of magnitude lower than the reported diffusion coefficients from in- and out-of-pile samples, suggesting that other contributions are responsible, such as radiation enhanced diffusion.

  16. How grain boundaries affect the efficiency of poly-CdTe solar-cells: A fundamental atomic-scale study of grain boundary dislocation cores using CdTe bi-crystal thin films.

    Energy Technology Data Exchange (ETDEWEB)

    Klie, Robert [Univ. of Illinois, Chicago, IL (United States)

    2016-10-25

    It is now widely accepted that grain boundaries in poly-crystalline CdTe thin film devices have a detrimental effect on the minority carrier lifetimes, the open circuit voltage and therefore the overall solar-cell performance. The goal of this project was to develop a fundamental understanding of the role of grain boundaries in CdTe on the carrier life-time, open-circuit voltage, Voc, and the diffusion of impurities. To achieve this goal, i) CdTe bi-crystals were fabricated with various misorientation angels, ii) the atomic- and electronic structures of the grain boundaries were characterized using scanning transmission electron microscopy (STEM), and iii) first-principles density functional theory modeling was performed on the structures determined by STEM to predict the grain boundary potential. The transport properties and minority carrier lifetimes of the bi-crystal grain boundaries were measured using a variety of approaches, including TRPL, and provided feedback to the characterization and modeling effort about the effectiveness of the proposed models.

  17. Methylammonium lead iodide grain boundaries exhibit depth-dependent electrical properties

    Energy Technology Data Exchange (ETDEWEB)

    MacDonald, Gordon A.; Yang, Mengjin; Berweger, Samuel; Killgore, Jason P.; Kabos, Pavel; Berry, Joseph J.; Zhu, Kai; DelRio, Frank W.

    2016-01-01

    In this communication, the nanoscale through-film and lateral photo-response and conductivity of large-grained methylammonium lead iodide (MAPbI3) thin films are studied. In perovskite solar cells (PSC), these films result in efficiencies >17%. The grain boundaries (GBs) show high resistance at the top surface of the film, and act as an impediment to photocurrent collection. However, lower resistance pathways between grains exist below the top surface of the film, indicating that there exists a depth-dependent resistance of GBs (RGB(z)). Furthermore, lateral conductivity measurements indicate that RGB(z) exhibits GB-to-GB heterogeneity. These results indicate that increased photocurrent collection along GBs is not a prerequisite for high-efficiency PSCs. Rather, better control of depth-dependent GB electrical properties, and an improvement in the homogeneity of the GB-to-GB electrical properties, must be managed to enable further improvements in PSC efficiency. Finally, these results refute the implicit assumption seen in the literature that the electrical properties of GBs, as measured at the top surface of the perovskite film, necessarily reflect the electrical properties of GBs within the thickness of the film.

  18. 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).

  19. Central role of TiO2 anatase grain boundaries on resistivity of CaCu3Ti4O12-based materials probed by Raman spectroscopy

    Science.gov (United States)

    De Almeida-Didry, Sonia; Autret, Cécile; Honstettre, Christophe; Lucas, Anthony; Zaghrioui, Mustapha; Pacreau, François; Gervais, François

    2016-11-01

    This study focuses on characterization and control of grain boundaries to enhance the properties of CaCu3Ti4O12 (CCTO) ceramics capacitors for industrial applications. A novel factor deals with TiO2 anatase revealed by Raman scattering in grain boundaries, found as a dominant parameter of largest sample resistivity, consistent with higher grain boundary resistivity and higher breakdown voltage. Four selected samples of CCTO-based compositions showing very different properties in terms of permittivity ranging from 1000 to 684 000 measured at 1 kHz, capacitance of grain boundaries ranging from 8 10-10 to 4.5 10-7 F cm-1, grain boundary resistivity ranging from 193 to 30,000,000 Ω cm and sample resistivity extending from 450 to 1011 Ω cm. The relationship between permittivity weighted by grain size and capacitance of grain boundaries confirms the internal barrier layer capacitance model over 5 orders of magnitude.

  20. Bean grain hysteresis with induced mechanical damage

    Directory of Open Access Journals (Sweden)

    Renata C. Campos

    Full Text Available ABSTRACT This study aimed to evaluate the effect of mechanical damage on the hysteresis of beans with induced mechanical damage under different conditions of temperature and relative humidity. Beans (Phaseolus vulgaris L. harvested manually with 35% water content (w.b. were used. Part of this product was subjected to induced mechanical damage by Stein Breakage Tester and controlled drying (damaged and control sample, for sorption processes. The sorption isotherms of water were analyzed for different temperature conditions: 20, 30, 40 and 50 oC; and relative humidity: 0.3; 0.4; 0.5; 0.7 and 0.9 (decimal. Equilibrium moisture content data were correlated with six mathematical models, and the Modified Oswin model was the one that best fitted to the experimental data. According to the above mentioned isotherms, it was possible to observe the phenomenon of hysteresis of damaged and control samples, and this phenomenon was more pronounced in control ones.

  1. 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 (crystals filling the void space and decoupled from surrounding crystals by a thin brine layer. In parallel to this microstructural study, we deformed the same samples in order to simulate the simple shear flow at very low mean stress as in the salt fountain. First results suggest a complicated rheology. Samples loaded at σ 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

  2. Classical molecular dynamics investigation of microstructure evolution and grain boundary diffusion in nano-polycrystalline UO2

    Science.gov (United States)

    Govers, K.; Verwerft, M.

    2013-07-01

    The High Burnup Structure (HBS) observed at pellet periphery in conventional Light Water Reactor nuclear fuels and around spots presenting high plutonium content in mixed (U, Pu) oxide fuel - MOX fuel - consists of a restructuration of the original grains into smaller ones. The process is often postulated to occur because of the accumulation of irradiation damage and the retention of fission products in the matrix. The computing power nowadays available enables for simulating larger systems at the atomic scale up to the point that nano-polycrystalline material can now be investigated by empirical potential molecular dynamics. Simulations of nano-polycrystalline UO2 structures have been carried out at various temperatures to investigate atom mobility close to grain boundaries. The variation of Arrhénius parameters for the diffusion coefficient of oxygen, uranium and xenon as a function of the distance from a grain boundary was studied, leading to the distinction of three zones: the grain boundary layers (up to 1 nm depth) presenting enhanced diffusion, an intermediate zone (1 to roughly 2 nm depth) with intermediate diffusion values and the bulk of the grains. The following Arrhénius relations for grain boundary diffusion were derived:

  3. A molecular dynamics evaluation of the effect of dopant addition on grain boundary diffusion in tin: Implication for whisker growth

    Energy Technology Data Exchange (ETDEWEB)

    Banerjee, S., E-mail: soumik.banerjee@wsu.edu [School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920 (United States); Dutta, I. [School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920 (United States); Majumdar, B.S. [Materials and Metallurgical Engineering Department, New Mexico Tech, Socorro, NM 87801 (United States)

    2016-06-01

    Addition of lead dopant has been known to be an effective way of mitigating whisker growth in tin. However, the toxic nature of lead has necessitated a search for alternative dopant species. Recent investigations have indicated that indium can be an effective agent in reducing whisker growth. In an effort to investigate if reduction in diffusivity of tin atoms near grain boundaries in presence of dopant atoms is the underlying mechanism that causes reduced whisker growth, we employed molecular dynamics simulations to model tin grain boundaries with lead and indium dopants. We simulated pure tin grain boundary as well as grain boundaries with 4% dopants by mole fraction. Our results indicate that the dopant atoms segregate near the grain boundary and forms clusters, which in turn leads to reduced diffusivity of tin atoms by up to a factor of three. Since such reduction in diffusivity alone cannot reduce whisker growth by several orders of magnitude, we conclude that other mechanisms might play a more dominant role in mitigating tin whisker growth.

  4. Direct oxygen imaging within a ceramic interface, with some observations upon the dark contrast at the grain boundary

    Energy Technology Data Exchange (ETDEWEB)

    Findlay, S.D., E-mail: scott@sigma.t.u-tokyo.ac.jp [Institute of Engineering Innovation, The University of Tokyo, Tokyo 116-0013 (Japan); Azuma, S. [Institute of Engineering Innovation, The University of Tokyo, Tokyo 116-0013 (Japan); Shibata, N. [Institute of Engineering Innovation, The University of Tokyo, Tokyo 116-0013 (Japan); PRESTO, Japan Science and Technology Agency, Saitama 332-0012 (Japan); Okunishi, E. [JEOL Ltd., Tokyo 196-8558 (Japan); Ikuhara, Y. [Institute of Engineering Innovation, The University of Tokyo, Tokyo 116-0013 (Japan); Nanostructures Research Laboratory, Japan Fine Ceramic Center, Nagoya 456-8587 (Japan); WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577 (Japan)

    2011-03-15

    Annular bright field scanning transmission electron microscopy, which has recently been established to produce directly interpretable images with both light and heavier atomic columns visible simultaneously, is shown to allow directly interpretable imaging of the oxygen columns within the {Sigma}13[12-bar 10](101-bar 4) pyramidal twin grain boundary in {alpha}-Al{sub 2}O{sub 3}. By using information in the high-angle annular dark field image and annular bright field images simultaneously, we estimate the specimen thickness and finite source size, and use them to explore in simulation the issue of dark contrast in the vicinity of the grain boundary in the annular dark field image. -- Research highlights: {yields} Simultaneous HAADF and ABF images of a pristine {Sigma}13 grain boundary in {alpha}-Al{sub 2}O{sub 3}. {yields} Oxygen columns are clearly visible in the grain boundary in the ABF image. {yields} ABF is more robust than HAADF for imaging defect structures. {yields} Thickness and source size are estimated from quantitative analysis of the contrast. {yields} Dark contrast at the grain boundary in HAADF is likely due to strain and vacancies.

  5. Localization of ductile deformation in lithosphere and rocks: the role of grain boundary sliding

    Science.gov (United States)

    Dimanov, Alexandre; Rahanel, Jean; Bornert, Michel; Bourcier, Mathieu; Gaye, Ag; Heripre, Eva; Ludwig, Wolfgang

    2017-04-01

    Ductile strain of the lithosphere localizes in multi-scale shear zones, ranging from km to mm scales. The resulting mylonites/ultramylonites present microstructural signatures of several concomitant deformation mechanisms. Besides cataclastic features, crystal plasticity dominates in volume, but grain boundary sliding and diffusive/solution mass transport act along interfaces. Considering solely the inherited natural microstructures does not make clear the chronology of appearance and the interactions between these mechanisms. Therefore, inference of the overall mylonitic rheology seems illusory. We have therefore realized over the last decade a systematic rheological characterization of the high temperature flow of various synthetic anorthite - diopside mixtures. The data clearly suggest Newtonian type of rheology as best adapted to the materials representative of the lower crust mylonites. However, the post mortem microstructures undoubtedly evidenced the coexistence of both crystal plasticity and grain boundary sliding processes. Yet, the specific roles of each mechanism in the localization process remained unclear. In order to clarify these aspects we realized a multi-scale micromechanical in situ investigation of the ductile deformation of synthetic rock-salt. The mechanical tests were combined with in-situ optical microscopy, scanning electron microscopy and X-ray tomography (MCT). Digital image correlation (DIC) techniques allowed for measurements and characterization of the multi-scale organization of 2D and 3D full strain fields. Macroscopic and mesoscopic shear bands appear at the sample and microstructure scales, respectively. DIC evidenced the development of discrete slip bands within individual grains, and hence of dominant crystal plasticity. Combination of DIC and EBSD allowed for identification of active slip systems. Conversely, DIC allowed for the identification and the precise quantification of minor activity (< 5% contribution) of grain boundary

  6. Stress-induced changes in wheat grain composition and quality.

    Science.gov (United States)

    Ashraf, M

    2014-01-01

    Abiotic stresses such as drought, salinity, waterlogging, and high temperature cause a myriad of changes in the metabolism of plants, and there is a lot of overlap in these changes in plants in response to different stresses such as drought and salinity. These stress-induced metabolic changes cause impaired crop growth thereby resulting in poor yield. The metabolic changes taking place in several plant species due to a particular abiotic stress have been revealed from the whole plant to the molecular level by researchers, but most studies have focused on organs such as leaf, stem, and root. Information on such stress-induced changes in seed or grains is infrequent in the literature. From the information that is available, it is now evident that abiotic stress can induce considerable changes in the composition and quality of cereal grains including those of wheat, the premier staple food crop in the world. Thus, the present review discusses how far different types of stresses, mainly salinity, drought, high temperature, and waterlogging, can alter the wheat grain composition and quality. By fully uncovering the stress-induced changes in the nutritional values of wheat grains it would be possible to establish whether balanced supplies of essential nutrients are available to the human population from the wheat crop grown on stress-affected areas.

  7. Neutrino Induced Coherent ρ Production in a Fine Grained Tracker

    Science.gov (United States)

    Jiang, Libo; Kullenberg, Christpher; Tian, Xinchun; Mishra, Sanjib; LBNE Collaboration

    2015-04-01

    We present simulation of neutrino induced coherent ρ-meson production in charged and neutral current interactions. Sensitivity studies of this process is presented in a fine grain tracker, a near detector option for LBNE. Measurements of coherent ρ0 and ρ+ production in NOMAD are reported.

  8. A Fresh Plutonic Igneous Angrite Containing Grain Boundary Glass From Tamassint, Northwest Africa

    Science.gov (United States)

    Irving, A. J.; Kuehner, S. M.; Rumble, D.

    2006-12-01

    Tamassint Angrite: A small fragmented stone found in June 2006 south of Tamassint oasis in the Morocco-Algeria border region represents a new type of angrite lithology, unlike the coarse grained metamorphic or fine grained "basaltic" to quench-textured examples known previously. This extremely fresh, fusion-crusted specimen has a coarse grained (0.6-12 mm) plutonic igneous cumulate texture, and is composed of Al-Ti-rich clinopyroxene (33.4%), pure anorthite (28.6%), Ca-rich olivine (18.7%) with prominent exsolution lamellae (10-50 μm wide) of kirschsteinite, ulvöspinel (18.5%), and accessory glass, troilite and metal. Subhedral anorthite grains are partially enclosed within larger ulvöspinel grains. Mineral compositions are as follows: clinopyroxene (Fs20.8-33.3Wo53-54.9, Al2O3 = 5.7 to 9.4 wt.%, TiO2 = 0.9 to 2.9 wt.%, FeO/MnO = 85-278), olivine (Fa72.6-74.7Ln3.5-3.6, CaO = 2.1 wt.%, FeO/MnO = 70-87), kirschsteinite (Fa44.7-45.4Ln46-47.2, FeO/MnO = 73-82), ulvöspinel (TiO2 = 27.6 wt.%, Al2O3 = 5.5 wt.%). Reintegration of the kirschsteinite lamellae gives a pre-exsolution olivine composition of Fa68.1Ln12.2 with 7.3 wt.% CaO. Present along grain boundaries (notably between anorthite and ulvöspinel) are narrow (5-20 μm) curvilinear zones of glass associated with secondary kirschteinite, clinopyroxene and olivine (which show similar curvilinear morphology and truncate kirschsteinite lamellae). Glass compositions plot close to a mixing line between anorthite and ulvöspinel. Replicate oxygen isotopic analyses of acid-washed minerals by laser fluorination gave δ18O = 3.881, 3.845, δ17O = 1.967, 1.927, Δ17O = -0.0745, -0.0956 per mil (for TFL slope = 0.526). Comparison With NWA 2999: We previously showed [1] that angrite Northwest Africa 2999 is a metamorphically annealed breccia with distinctive symplectites and coronas representing forward and reverse versions of the same solid state reaction. We suggested that these disequilibrium textures required burial

  9. The influence of solution composition and grain boundaries on the replacement of calcite by dolomite

    Science.gov (United States)

    Moraila Martinez, Teresita de Jesus; Putnis, Christine V.; Putnis, Andrew

    2016-04-01

    Dolomite formation is a mineral replacement reaction that affects extensive rock volumes and comprises a large fraction of oil and gas reservoirs [1,2]. The most accepted hypothesis is the 'dolomitization' of limestone by Mg-rich fluids [3]. The objective of this research is to study the replacement mechanism of calcite by dolomite, the role of grain boundaries, highlighted by Etschmann et al. (2014), and the possible influence of solutions in dolomite formation under the presence of ions that are normally in crustal aqueous fluids. To accomplish this purpose, we performed hydrothermal experiments using Carrara marble cubes of ~1.5 mm size and 7-9 mg weight as starting material, reacted with 1M (Mg,Ca)Cl2 aqueous solutions, with Mg/Ca ratios of 3 and 5 at 200°C, for different reaction times. Additional experiments were performed adding 1mM of Na2SO4, NaCl or NaF to the previous solutions. After the reaction, the product phases were identified using Raman spectroscopy, X-Ray powder diffraction (XRD), electron microprobe analysis (EMPA), and the textural evolution was studied by scanning electron microscopy (SEM). Samples reacted with aqueous solutions resulted in the replacements of the calcite rock into magnesite and dolomite. The amount and type of reaction strongly depends on the Mg/Ca ratio. Samples reacted with a Mg/Ca ratio of 5 resulted in an almost complete replacement reaction and more favorable for magnesite formation than for dolomite. When the Mg/Ca ratio was 3 dolomite formed but the replacement was located in the core of the sample. We show that grain boundaries are very important for the infiltration of solution and the progress of a replacement reaction, acting as fluid pathways. Solution composition controls the nature of the replacement product. Acknowledgment: This work is funded within a Marie Curie EU Initial Training Network- CO2-React. 1. Etschmann B., Brugger J., Pearce M.A., Ta C., Brautigan D., Jung M., Pring A. (2014). Grain boundaries as

  10. Optimization of vortex pinning at grain boundaries on ex-situ MgB2 bulks synthesized by spark plasma sintering

    Science.gov (United States)

    Naito, Tomoyuki; Endo, Yuri; Fujishiro, Hiroyuki

    2017-09-01

    Grain boundaries are well known to be the predominant pinning centers in MgB2 superconductors. To study the effects of grain boundaries on the trapped field properties of MgB2 bulk, we prepared MgB2 bulks by a spark plasma sintering method using a ball-milled starting powder. The trapped field was maximized for the bulk made from the ball-milled powder with crystallite size, τ, of 27 nm; the highest trapped field, {B}{{T}}, of 2.3 T achieved at 19.3 K was 1.2 times larger than that of the bulk made from the non ball-milled powder (τ = 50 nm). The degradation of the trapped field for the bulk from finer powder (τ = 6 nm) originated mainly from the lowered {T}{{c}}. The critical current density, {J}{{c}}, and the pinning force density, {F}{{p}}, were also maximized for the bulk from τ = 27 nm. The competition between the increase of the numerical density of grain boundaries and the degradation of superconductivity determined the vortex pinning properties for the MgB2 bulks with mechanically refined grains. The scaling analysis for the pinning force density suggested that the change in the dimension of the dominant pinning source from 2D (surface) to 0D (point) was induced by grain refining. Although the nanometric impurity particles such as MgB4, MgO and Mg-B-O were created in the bulk during both ball-milling and spark plasma sintering processes, we considered the point-contact between the refined grains was the predominant point pinning source.

  11. Grain boundary segregation in FeCrNi model alloys; Korngrenzensegregation in FeCrNi-Modellegierungen

    Energy Technology Data Exchange (ETDEWEB)

    Schlueter, B.; Schneider, F.; Mummert, K. [Institut fuer Festkoerper- und Werkstofforschung Dresden e.V. (Germany); Muraleedharan, P. [Indira Gandhi Centre for Atomic Research, Kalpakkam (India). Div. of Metallurgy

    1998-12-31

    P and S segregate at the grain boundaries and thus increase susceptibility to intergranular corrosion at those sites. This could be proven by means of nitric acid-chromate tests and potentiostatic etching tests. There is a direct connection between loss in mass, mean depth of intergranular corrosion attacks, dissolution current density, and level of segregation-induced concentration of P and S at the grain boundaries. The segregation effect at these sites was found to be most evident in specimens of the examined Fe-Cr-Ni steel which had been heat-treated for 1000 hours at 550 C. However, segregation occurs also in materials that received a heat treatment of 400 C/5000 hours, while intergranular corrosion is observed only after heat treatment of 500 C/1000 hours. Apart from segregation of P, formation of Cr-rich phosphides is observed, which leads to depletion of Cr at the precipitates. (orig./CB) [Deutsch] P und S segregieren an die KG und erhoehen dort die IK-Anfaelligkeit. Dies konnte mit Hilfe von Salpetersaeure-Chromat- und Potentiostatischem Aetztest nachgewiesen werden. Es besteht ein direkter Zusammenhang zwischen Masseverlust, mittlerer IK-Angriffstiefe, Aufloesungsstromdichte und Hoehe der segregationsbedingten Anreicherungen von P und S an den KG. Der KG-Segregationseffekt am untersuchten Fe-Cr-Ni-Stahl ist im Waermebehandlungszustand 550 C/1000 h am deutlichsten ausgepraegt. Aber auch bereits bei 400 C/5000 h findet Segregation statt. IKSpRK tritt nur im Waermebehandlungszustand 550 C/1000 h auf. Neben der P-Segregation wird die Bildung Cr-reicher Phosphide beobachtet, die zur Abreicherung von Cr an den Ausscheidungen fuehrt. (orig.)

  12. Atomistic study of hydrogen embrittlement of grain boundaries in nickel: I. Fracture

    Science.gov (United States)

    Tehranchi, A.; Curtin, W. A.

    Hydrogen ingress into a metal is a persistent source of embrittlement. Fracture surfaces are often intergranular, suggesting favorable cleave crack growth along grain boundaries (GBs) as one driver for embrittlement. Here, atomistic simulations are used to investigate the effects of segregated hydrogen on the behavior of cracks along various symmetric tilt grain boundaries in fcc Nickel. An atomistic potential for Ni-H is first recalibrated against new quantum level computations of the energy of H in specific sites within the NiΣ5(120)⟨100⟩ GB. The binding energy of H atoms to various atomic sites in the NiΣ3(111) (twin), NiΣ5(120)⟨100⟩, NiΣ99(557)⟨110⟩, and NiΣ9(221)⟨110⟩ GBs, and to various surfaces created by separating these GBs into two possible fracture surfaces, are computed and used to determine equilibrium H concentrations at bulk H concentrations typical of embrittlement in Ni. Mode I fracture behavior is then studied, examining the influence of H in altering the competition between dislocation emission (crack blunting; "ductile" behavior) and cleavage fracture ("brittle" behavior) for intergranular cracks. Simulation results are compared with theoretical predictions (Griffith theory for cleavage; Rice theory for emission) using the computed surface energies. The deformation behavior at the GBs is, however, generally complex and not as simple as cleavage or emission at a sharp crack tip, which is not unexpected due to the complexity of the GB structures. In cases predicted to emit dislocations from the crack tip, the presence of H atoms reduces the critical load for emission of the dislocations and no cleavage is found. In the cases predicted to cleave, the presence of H atoms reduces the cleavage stress intensity and makes cleavage easier, including NiΣ9(221)⟨110⟩ which emits dislocations in the absence of H. Aside from the one unusual NiΣ9(221)⟨110⟩ case, no tendency is found for H to cause a ductile

  13. Improved ductility of Cu64Zr36 metallic glass/Cu nanocomposites via phase and grain boundaries

    Science.gov (United States)

    Jian, W. R.; Wang, L.; Li, B.; Yao, X. H.; Luo, S. N.

    2016-04-01

    We investigate tensile deformation of metallic glass/crystalline interpenetrating phase nanocomposites as regards the effects of specific area of amorphous/crystalline phase interfaces, and grain boundaries. As an illustrative case, large-scale molecular dynamics simulations are performed on Cu64Zr36 metallic glass/Cu nanocomposites with different specific interface areas and grain boundary characteristics. Plastic deformation is achieved via shear bands, shear transformation zones, and crystal plasticity. Three-dimensional amorphous/crystalline interfaces serve as effective barriers to the propagation of shear transformation zones and shear bands if formed, diffuse strain localizations, and give rise to improved ductility. Ductility increases with increasing specific interface area. In addition, introducing grain boundaries into the second phase facilitates crystal plasticity, which helps reduce or eliminate mature shear bands in the glass matrix.

  14. Grain boundary effects on the structural, microstructural and transport behavior of sol - gel grown PrMnO3 nanoparticles

    Science.gov (United States)

    Keshvani, M. J.; Katba, Savan; Jethva, Sadaf; Udeshi, Malay; Pandya, D. D.; Shah, N. A.; Ravalia, Ashish; Kataria, Bharat; Kuberkar, D. G.

    2017-05-01

    In this communication, we report the results of the studies on the grain boundary (GB) effect on the structural, microstructural and transport properties of nanostructured PrMnO3 (PMO) manganites synthesized by acetate precursor based sol - gel method. As synthesized PMO samples were sintered at various temperatures to understand the role of sintering temperature in modifying the structure - property correlations in the context of GB effects. Structural analysis using X-ray diffraction studies reveal the single phasic nature of all the PMO samples while TEM analysis reveals uniform particle size distribution with agglomeration effect observed in samples sintered at higher temperatures. SEM micrographs depict the increase in grain size with sharp grain boundaries in the samples sintered at a higher temperature. Size-dependent resistivity behavior of the PMO samples have been understood in the light of grain size modification and GB effect.

  15. First-principles investigation into the effect of Cr on the segregation of multi-H at the Fe Σ3 (1 1 1) grain boundary

    Energy Technology Data Exchange (ETDEWEB)

    He, Bingling; Xiao, Wei; Hao, Wei [School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 (China); Tian, Zhixue, E-mail: tian@cp.prec.eng.osaka-u.ac.jp [Division of Precision Science and Technology and Applied Physics, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871 (Japan)

    2013-10-15

    First-principles calculations were carried out to investigate the effect of Cr on segregation of multiple H atoms in the Σ3 (1 1 1) grain boundary in bcc Fe. In the absence of Cr, four H atoms can be trapped at the interstitial site of the Fe grain boundary (areal density: 28 nm{sup −2}), and no H{sub 2} molecules are formed. The presence of Cr, however, suppresses the segregation tendency of H and only two H atoms can be trapped at this grain boundary. Although the volume expansion associated with the segregation of Cr to the grain boundary promotes H segregation, such a booting effect is unable to remedy the repulsion of H resulted from charge density increase in the grain boundary core. As a consequence, Cr mitigates H aggregation at the Σ3 (1 1 1) grain boundary in bcc Fe.

  16. Atomic-scale segregation behavior of Sn/Ti and O atΣ3[11̄0](111) grain boundary in niobium

    Institute of Scientific and Technical Information of China (English)

    Zenghui Liu; Jiaxiang Shang

    2016-01-01

    First-principle calculation was performed to illustrate the atomic arrangement and segregation behavior of Sn/Ti and O in Σ3 [11¯ 0] (111) grain boundary, and the interaction of oxygen interstitials with Sn/Ti atoms on the grain boundary was studied. The analyses on the segregation energies and geometric positions, and the electron densities show that Sn, Ti and O atoms segregate at the Σ3 grain boundary. And the preferred segregation sites of the impurities at Σ3 [11¯ 0] (111) were determined. When Sn seg-regates at grain boundary plane, the O atom prefers to the bulk-like site and shows no segregation tendency at grain boundary, whereas the segregated Ti atom can slightly enhance oxygen segregation at the grain boundary.

  17. Superplasticity and cooperative grain boundary sliding in nanocrystalline Ni{sub 3}Al

    Energy Technology Data Exchange (ETDEWEB)

    Mara, N.A. [Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)], E-mail: namara@lanl.gov; Sergueeva, A.V.; Mara, T.D. [Materials Science Division, University of California, Davis, One Shields Avenue, Davis, CA 95616 (United States); McFadden, S.X. [Sandia Laboratories, Livermore, CA 94550 (United States); Mukherjee, A.K. [Materials Science Division, University of California, Davis, One Shields Avenue, Davis, CA 95616 (United States)

    2007-08-15

    Cooperative grain boundary sliding (CGBS) has been shown to account for the majority of macroscopic strain seen in microcrystalline metallic systems undergoing superplastic deformation. While CGBS has been observed on the surface of microcrystalline samples deforming superplastically through the shifting of diamond scribe lines, there have been few transmission electron microscopy results showing such occurrences in the bulk of the material, or the details behind the micromechanism of CGBS. In this work, nanocrystalline Ni{sub 3}Al produced via high-pressure torsion is deformed superplastically in the electron microscope. High-temperature ({approx}700 deg. C) in situ tensile testing shows the nature of CGBS at the nanoscale through direct observation of this phenomenon.

  18. Molecular dynamics simulation of hydrogen dissolution and diffusion in a tungsten grain boundary

    Energy Technology Data Exchange (ETDEWEB)

    Yu, Yi; Shu, Xiaolin; Liu, Yi-Nan; Lu, Guang-Hong, E-mail: lgh@buaa.edu.cn

    2014-12-15

    We employ a classic molecular dynamics method to investigate the dissolution and diffusion properties of hydrogen (H) in a Σ5(3 1 0) tilt grain boundary (GB). A maximum binding energy of 2.5 eV and a diffusion barrier of 1.65 eV indicate that GB plays an important role in H trapping. Dynamic simulations with temperature ranging from 600 K to 1200 K verify the diffusion and the aggregation of H in the GB are closely associated with the temperature. Pair radius distribution function analysis suggests a high local GB concentration of H such as 30% at 900 K can lead to a disordered GB structure.

  19. Inductance measurements in multilevel high Tc step-edge grain boundary SQUIDS

    Science.gov (United States)

    Forrester, M. G.; Davidson, A.; Talvacchio, J.; Gavaler, J. R.; Przybysz, J. X.

    1994-10-01

    Multilevel high Tc SQUIDs, suitable for digital circuit applications, have been fabricated and tested. The devices employ a YBa2Cu3O7-δ (YBCO) ground plane, an epitaxial SrTiO3 insulator, and a YBCO microstrip layer. Junctions are formed by the step-edge grain boundary process, with a ground plane contact for the ``low'' side of each junction, using only isotropic sputtering and milling techniques. Control current is directly injected in a microstrip segment of the SQUID loop, allowing us to measure the microstrip inductance, and thus to infer the magnetic penetration depth of the YBCO. The SQUIDs are operational above 77 K, at which temperature we infer a penetration depth of 350 nm. The temperature dependence of the penetration depth is found to be in reasonable agreement with the Gorter-Casimir form close to Tc.

  20. Low-Angle Grain Boundaries in Sublimation Grown 6H-SiC Crystals

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

    High-resolution X-ray diffractometry (HRXRD) was used to assess the quality of 6H-SiC crystals grown by sublimation method. The results show the occurrence of low-angle grain boundaries (LB) is close relative to the inclination of the crystal interface. At the central faceted region with 0° inclination the crystal is of high structural perfection. However, at the region close to the facet with less than 5° inclination LB occurs slightly and at the region close to the peripheral polytype ring with more than 5° inclination LB defect occurs heavily. The density of LB can be drastically reduced by decreasing radial temperature gradient that determines the shape of the crystal growth interface.

  1. Ab initio search for cohesion-enhancing impurity elements at grain boundaries in molybdenum and tungsten

    Science.gov (United States)

    Scheiber, D.; Pippan, R.; Puschnig, P.; Romaner, L.

    2016-12-01

    We report high throughput density functional theory (DFT) calculations to simulate segregation of s- and p-elements in Mo and W. First, the preference of solutes for interstitial or substitutional positions in the bulk is evaluated and then the segregation energies for the solutes to interstitial and different substitutional sites at a grain boundary (GB) and a free surface (FS) are computed. We show that several solutes change their site preference from substitutional to interstitial position upon segregation to the GB. With the segregation energies to GB and FS, the changes in cohesion can be calculated and GB cohesion enhancing solutes can be identified. The results show striking similarity for both W and Mo. In addition, we collected the available literature data from experimental and theoretical side, which we consequently compare to our results. From our results and the comparison to literature, we identify B, C and Be as potential alloying additions for an increased GB cohesion in Mo and W.

  2. Energetics of Hydrogen Segregation to α-Fe Grain Boundaries for Modeling Stress Corrosion Cracking

    Science.gov (United States)

    Rajagopalan, M.; Adlakha, I.; Tschopp, M. A.; Solanki, K. N.

    2017-08-01

    The physics of embrittlement is dictated by the various interactions between the impurities/defects and the local structure in polycrystalline material systems. In this study, a physically motivated model that describes the degree of interaction of hydrogen (H) defects on the segregation behavior to α-Fe grain boundaries (GBs) is developed. Molecular statics simulations were performed to quantify the segregation behavior of 1-2 H atoms at various interstitial sites around the , , , and symmetric tilt GBs. The results provide insights into the concentration profile of hydrogen defects along different GBs. Furthermore, the model accurately links the intrinsic GB character by quantifying the segregation length scale for the individual GBs based on the segregation behavior of defects. Finally, the metrics provided in this work are essential to comprehensively understanding the effect of hydrogen on the macroscopic behavior of α-Fe.

  3. Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions

    Science.gov (United States)

    2017-01-01

    Trap-assisted recombination, despite being lower as compared with traditional inorganic solar cells, is still the dominant recombination mechanism in perovskite solar cells (PSCs) and limits their efficiency. We investigate the attributes of the primary trap-assisted recombination channels (grain boundaries and interfaces) and their correlation to defect ions in PSCs. We achieve this by using a validated device model to fit the simulations to the experimental data of efficient vacuum-deposited p–i–n and n–i–p CH3NH3PbI3 solar cells, including the light intensity dependence of the open-circuit voltage and fill factor. We find that, despite the presence of traps at interfaces and grain boundaries (GBs), their neutral (when filled with photogenerated charges) disposition along with the long-lived nature of holes leads to the high performance of PSCs. The sign of the traps (when filled) is of little importance in efficient solar cells with compact morphologies (fused GBs, low trap density). On the other hand, solar cells with noncompact morphologies (open GBs, high trap density) are sensitive to the sign of the traps and hence to the cell preparation methods. Even in the presence of traps at GBs, trap-assisted recombination at interfaces (between the transport layers and the perovskite) is the dominant loss mechanism. We find a direct correlation between the density of traps, the density of mobile ionic defects, and the degree of hysteresis observed in the current–voltage (J–V) characteristics. The presence of defect states or mobile ions not only limits the device performance but also plays a role in the J–V hysteresis. PMID:28540366

  4. Origin and provenance of spherules and magnetic grains at the Younger Dryas boundary

    Science.gov (United States)

    Wu, Yingzhe; Sharma, Mukul; LeCompte, Malcolm A.; Demitroff, Mark N.; Landis, Joshua D.

    2013-09-01

    One or more bolide impacts are hypothesized to have triggered the Younger Dryas cooling at ∼12.9 ka. In support of this hypothesis, varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the Younger Dryas boundary (YDB). We show that bulk sediment and/or magnetic grains/microspherules collected from the YDB sites in Arizona, Michigan, New Mexico, New Jersey, and Ohio have 187Os/188Os ratios ≥1.0, similar to average upper continental crust (= 1.3), indicating a terrestrial origin of osmium (Os) in these samples. In contrast, bulk sediments from YDB sites in Belgium and Pennsylvania exhibit 187Os/188Os ratios meteoritic contribution. The YDB site in Pennsylvania is remarkable in yielding 2- to 5-mm diameter spherules containing minerals such as suessite (Fe-Ni silicide) that form at temperatures in excess of 2000 °C. Gross texture, mineralogy, and age of the spherules appear consistent with their formation as ejecta from an impact 12.9 ka ago. The 187Os/188Os ratios of the spherules and their leachates are often low, but Os in these objects is likely terrestrially derived. The rare earth element patterns and Sr and Nd isotopes of the spherules indicate that their source lies in 1.5-Ga Quebecia terrain in the Grenville Province of northeastern North America.

  5. Dynamic process of trace boron non-equilibrium grain boundary segregation and the effect of cooling rate

    Institute of Scientific and Technical Information of China (English)

    Ping Wu; Xinlai He; Bing Cao; Sen Chen

    2003-01-01

    The dynamic process of non-equilibrium grain boundary segregation of trace boron in Fe-40%Ni alloy during cooling andthe effect of cooling rate were investigated by boron tracking autoradiography technique. The results indicate that during coolingprocess, the amount of segregated boron on grain boundary firstly increases fast, then enters a comparatively even increasing stageand increases rapidly again at the third stage. The details of each stage varied with cooling rate are explained. When thc segregationdevelops to a certain degree, the segregated boron atoms transform fiom solute status to precipitate status.

  6. Density functional theory metadynamics of silver, caesium and palladium diffusion at β-SiC grain boundaries

    Science.gov (United States)

    Rabone, Jeremy; López-Honorato, Eddie

    2015-03-01

    The use of silicon carbide in coated nuclear fuel particles relies on this materials impermeability towards fission products under normal operating conditions. Determining the underlying factors that control the rate at which radionuclides such as Silver-110m and Caesium-137 can cross the silicon carbide barrier layers, and at which fission products such as palladium could compromise or otherwise alter the nature of this layer, are of paramount importance for the safety of this fuel. To this end, DFT-based metadynamics simulations are applied to the atomic diffusion of silver, caesium and palladium along a Σ5 grain boundary and to palladium along a carbon-rich Σ3 grain boundary in cubic silicon carbide at 1500 K. For silver, the calculated diffusion coefficients lie in a similar range (7.04 × 10-19-3.69 × 10-17 m2 s-1) as determined experimentally. For caesium, the calculated diffusion rates are very much slower (3.91 × 10-23-2.15 × 10-21 m2 s-1) than found experimentally, suggesting a different mechanism to the simulation. Conversely, the calculated atomic diffusion of palladium is very much faster (7.96 × 10-11-7.26 × 10-9 m2 s-1) than the observed penetration rate of palladium nodules. This points to the slow dissolution and rapid regrowth of palladium nodules as a possible ingress mechanism in addition to the previously suggested migration of entire nodules along grain boundaries. The diffusion rate of palladium along the Σ3 grain boundary was calculated to be slightly slower (2.38 × 10-11-8.24 × 10-10 m2 s-1) than along the Σ5 grain boundary. Rather than diffusing along the precise plane of the boundary, the palladium atom moves through the bulk layer immediately adjacent to the boundary as there is greater freedom to move.

  7. Quantum theory of the effect of grain boundaries on the electrical conductivity of thin films and wires

    Science.gov (United States)

    Moraga, Luis; Henriquez, Ricardo; Solis, Basilio

    2015-08-01

    We calculate the electrical conductivity of a metallic sample under the effects of distributed impurities and a random distribution of grain boundaries by means of a quantum mechanical procedure based on Kubo formula. Grain boundaries are represented either by a one-dimensional regular array of Dirac delta potentials (Mayadas and Shatzkes model) or by its three-dimensional extension (Szczyrbowski and Schmalzbauer model). We give formulas expressing the conductivity of bulk samples, thin films and thin wires of rectangular cross-sections in the case when the samples are bounded by perfectly flat surfaces. We find that, even in the absence of surface roughness, the conductivity in thin samples is reduced from its bulk value. If there are too many grain boundaries per unit length, or their scattering strength is high enough, there is a critical value Rc of the reflectivity R of an individual boundary such that the electrical conductivity vanishes for R >Rc. Also, the conductivity of thin wires shows a stepwise dependence on R. The effect of weak random variations in the strength or separation of the grain boundaries is computed by means of the method of correlation length. Finally, the resistivity of nanometric polycrystalline tungsten films reported in Choi et al. J. Appl. Phys. (2014) 115 104308 is tentatively analyzed by means of the present formalism.

  8. Density functional theory metadynamics of silver, caesium and palladium diffusion at β-SiC grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Rabone, Jeremy, E-mail: jeremy.rabone@ec.europa.eu [European Commission, Joint Research Centre, Institute for Transuranium Elements, D-76125 Karlsruhe (Germany); López-Honorato, Eddie [Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV), Unidad Saltillo, Industria Metalúrgica 1062, Parque Industrial, Ramos Arizpe 25900, Coahuila (Mexico)

    2015-03-15

    Highlights: • DFT metadynamics of diffusion of Pd, Ag and Cs on grain boundaries in β-SiC. • The calculated diffusion rates for Pd and Ag tally with experimental release rates. • A mechanism of release other than grain boundary diffusion seems likely for Cs. - Abstract: The use of silicon carbide in coated nuclear fuel particles relies on this materials impermeability towards fission products under normal operating conditions. Determining the underlying factors that control the rate at which radionuclides such as Silver-110m and Caesium-137 can cross the silicon carbide barrier layers, and at which fission products such as palladium could compromise or otherwise alter the nature of this layer, are of paramount importance for the safety of this fuel. To this end, DFT-based metadynamics simulations are applied to the atomic diffusion of silver, caesium and palladium along a Σ5 grain boundary and to palladium along a carbon-rich Σ3 grain boundary in cubic silicon carbide at 1500 K. For silver, the calculated diffusion coefficients lie in a similar range (7.04 × 10{sup −19}–3.69 × 10{sup −17} m{sup 2} s{sup −1}) as determined experimentally. For caesium, the calculated diffusion rates are very much slower (3.91 × 10{sup −23}–2.15 × 10{sup −21} m{sup 2} s{sup −1}) than found experimentally, suggesting a different mechanism to the simulation. Conversely, the calculated atomic diffusion of palladium is very much faster (7.96 × 10{sup −11}–7.26 × 10{sup −9} m{sup 2} s{sup −1}) than the observed penetration rate of palladium nodules. This points to the slow dissolution and rapid regrowth of palladium nodules as a possible ingress mechanism in addition to the previously suggested migration of entire nodules along grain boundaries. The diffusion rate of palladium along the Σ3 grain boundary was calculated to be slightly slower (2.38 × 10{sup −11}–8.24 × 10{sup −10} m{sup 2} s{sup −1}) than along the Σ5 grain boundary. Rather

  9. An Electron Microscopy Study of Vein-like Grain Boundary Microstructure in Nitrocarburized Low Carbon Steels

    Institute of Scientific and Technical Information of China (English)

    Wanglin Chen; Cuilan Wu; Jianghua Chen; Aigui He

    2013-01-01

    The coarsened grain boundaries (GBs) with vein-like morphology frequently appear in the nitrogen diffusion layer of nitrocarburized carbon steels.The electron probe X-ray microanalysis shows that such vein-like GBs are rich in nitrogen and carbon atoms.Transmission electron microscopy and scanning electron microscopy further reveal that the coarsened GBs consist of γ'-nitrocarbide (Fe4(C,N)) and ε-nitrocarbide lamellas that are formed during nitrocarburizing due to high nitrogen concentration at GBs.it is shown that many of {111}<112> micro twins exist in the γ'-phase lamellas and thin ε-phase slices prefer to nucleate at their twin boundaries with the orientation relationship of {O001}ε//{111}γ'.Upon growing large γ'-lamellas may become faceted and thin ε-lamellas may grow thicker and become the main portions in the vein-like GBs.The microstructure evolution occurring in the vein-like GBs can be depicted as:α + [N]/[C] → γ' + [C,N] → ε-nitrocarbides.

  10. Interaction of C and Mn in a ∑3 grain boundary of bcc iron

    Science.gov (United States)

    Wicaksono, A. T.; Militzer, M.

    2017-07-01

    The interaction of alloying elements with migrating interfaces is a key aspect that determines microstructure evolution during thermo-mechanical processing of metals and alloys. Recent advances in atomic scale resolution characterization techniques and atomistic modelling have dramatically increased the potential to generate new knowledge on interfaces thereby enabling paradigm shifts in microstructure design approaches. Computational materials science now offers exciting opportunities to formulate multi-scale process models that bridge the gap between atomistic and continuums approaches. In particular, the development of advanced high strength steels with novel alloying concepts has motivated atomistic scale simulations to predict the interaction of selected alloying elements with grain boundaries and the austeniteferrite interface in iron. Most of these simulations have so far been carried out for binary systems with one solute species. The extension of this modelling work to multi-component systems is, however, essential in order to account for potential interactions between different alloying elements in industrial steels. Here, we present ab initio simulations for the interaction of C and Mn at a ∑3 boundary in bcc iron using density functional theory (DFT). The simulation results confirm the strong co-segregation of C and Mn that has been recently observed in atom probe tomography studies of the austenite-ferrite interface in Fe-Mn-C alloys.

  11. Strain localization in ultramylonitic marbles by simultaneous activation of dislocation motion and grain boundary sliding (Syros, Greece)

    Science.gov (United States)

    Rogowitz, A.; White, J. C.; Grasemann, B.

    2016-03-01

    Extreme strain localization occurred in the centre of the cross-cutting element of a flanking structure in almost pure calcite marbles from Syros, Greece. At the maximum displacement of 120 cm along the cross-cutting element, evidence of grain size sensitive deformation mechanisms can be found in the ultramylonitic marbles, which are characterized by (1) an extremely small grain size ( ˜ 3 µm), (2) grain boundary triple junctions with nearly 120° angles, (3) a weak crystallographic preferred orientation with very low texture index (J = 1.4), (4) a random misorientation angle distribution curve and (5) the presence of small cavities. Using transmission electron microscopy, a deformation sequence is observed comprising recrystallization dominantly by bulging, resulting in the development of the fine-grained ultramylonite followed by the development of a high dislocation density ( ˜ 1013 m-2) with ongoing deformation of the fine-grained ultramylonite. The arrangement of dislocations in the extremely fine-grain-sized calcite differs from microstructures created by classical dislocation creep mediated by combined glide and thermally activated climb. Instead, it exhibits extensive glide and dislocation networks characteristic of recovery accommodated by cross-slip and network-assisted dislocation movement without formation of idealized subgrain walls. The enabling of grain boundary sliding to dislocation activity is deemed central to initiating and sustaining strain softening and is argued to be an important strain localization process in calcite rocks, even at a high strain rate ( ˜ 10-9 s-1) and low temperature (300 °C).

  12. Inhibition of sensitization in reactor pipe materials by grain boundary structure control. JAERI's nuclear research promotion program, H11-023 (Contract research)

    Energy Technology Data Exchange (ETDEWEB)

    Kokawa, Hiroyuki; Shimada, Masayuki; Wang, Zhan Jie; Sato, Yutaka S. [Tohoku Univ., School of Engineering, Sendai, Miyagi (Japan); Sato, Yoshihiro [Osaka City Univ., Graduate School of Engineering, Osaka (Japan); Kiuchi, Kiyoshi [Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan). Tokai Research Establishment

    2003-03-01

    Grain boundary structure control of type 304 austenitic stainless steel and nickel base Inconel Alloy 600 used in reactor pipe materials was studied to inhibit intergranular corrosion (IGC) due to sensitization. A transmission electron microscopic study showed that low-energy grain boundaries, such as coincidence site lattice boundaries are highly resistant to sensitization, i.e., to intergranular precipitation, chromium depletion and corrosion, compared with ordinary high-angle (random) boundaries. A thermo-mechanical treatment was tried to control grain boundary structure of type 304 austenitic stainless steel for producing a highly intergranular corrosion-resistant material. The effects of process parameters in thermo-mechanical treatment such as pre-strain, annealing temperature, time, etc., on grain boundary characteristics and IGC resistance corrosion of type 304 austenitic stainless steels were examined. The excellent IGC resistance was obtained by slight pre-strain annealing at a relatively low temperature due to the optimized grain boundary character distribution. The uniform distribution of a high frequency of coincidence site lattice boundaries and consequent discontinuity of random boundary network were observed in the material. The optimum distribution can be formed by introducing low energy segments on migrating random boundaries during twin emission and boundary-boundary reactions in the grain growth without additional generation of random boundaries. (author)

  13. Superconductivity of individual grains and inter-grain boundaries for polycrystalline FeSr{sub 2}YCu{sub 2}O{sub 6+y}

    Energy Technology Data Exchange (ETDEWEB)

    Yamaguchi, K. [Department of Applied Physics, National Defense Academy, Yokosuka, Kanagawa 239-8686 (Japan); Hata, Y., E-mail: hata@nda.ac.jp [Department of Applied Physics, National Defense Academy, Yokosuka, Kanagawa 239-8686 (Japan); Mochiku, T. [Superconducting Properties Unit, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047 (Japan); Yasuoka, H. [Department of Applied Physics, National Defense Academy, Yokosuka, Kanagawa 239-8686 (Japan)

    2013-04-15

    Polycrystalline FeSr{sub 2}YCu{sub 2}O{sub 6+y} was synthesized and its transport and magnetic properties were studied. Diamagnetism was observed below 60 K. Zero resistivity was observed below 38 K under zero magnetic field and below 10 K under 160 kOe. A two-step transition was observed in resistivity measurement due to the superconductivity in individual grains and across inter-grain boundaries. The critical current density in individual grains, J{sub c}{sup intra}, at 2 K under 1 kOe was deduced 3.4 × 10{sup 5} A/cm{sup 2} from the Bean model. In contrast, the critical current density in inter-grain boundaries, J{sub c}{sup inter}, at 2 K was 1.7 A/cm{sup 2} in voltage–current measurement. The two-step transition seems to result from the large difference between J{sub c}{sup intra} and J{sub c}{sup inter}.

  14. Fluid distribution in grain boundaries of natural fine-grained rock salt deformed at low differential stress (Qom Kuh salt fountain, central Iran): Implications for rheology and transport properties

    NARCIS (Netherlands)

    Desbois, G.; Urai, J.L.; Bresser, J.H.P. de

    2012-01-01

    We used a combination of broad ion beam cross-sectioning and cryogenic SEM to image polished surfaces and corresponding pairs of fractured grain boundaries in an investigation of grain boundary microstructures and fluid distribution in naturally deformed halite from the Qom Kuh salt glacier (central

  15. Electrical characterization of CdTe grain-boundary properties from as processed CdTe/CdS solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Woods, L.M.; Robinson, G.Y. [Colorado State Univ., Fort Collins, CO (United States); Levi, D.H.; Ahrenkiel, R.K. [National Renewable Energy Lab., Golden, CO (United States); Kaydanov, V. [Colorado School of Mines, Golden, CO (United States)

    1998-09-01

    An ability to liftoff or separate the thin-film polycrystalline CdTe from the CdS, without the use of chemical etches, has enabled direct electrical characterization of the as-processed CdTe near the CdTe/CdS heterointerface. The authors use this ability to understand how a back-contact, nitric-phosphoric (NP) etch affects the grain boundaries throughout the film. Quantitative determination of the grain-boundary barrier potentials and estimates of doping density near the grain perimeter are determined from theoretical fits to measurements of the current vs. temperature. Estimates of the bulk doping are determined from high-frequency resistivity measurements. The light and dark barrier potentials change after the NP etch, and the origin of this change is postulated. Also, a variable doping density within the grains of non-etched material has been determined. These results allow a semi-quantitative grain-boundary band diagram to be drawn that should aid in determining more accurate two-dimensional models for polycrystalline CdTe solar cells.

  16. Observation of distinct, temperature dependent flux noise near bicrystal grain boundaries in YBa2Cu3O7-x films

    DEFF Research Database (Denmark)

    Bukh, K. R.; Jacobsen, Claus Schelde; Hansen, Jørn Bindslev

    2000-01-01

    The characteristics of the magnetic flux noise in high temperature superconducting thin-films of yttrium-barium-copper-oxide (YBa2Cu3O7) in the vicinity of artificial grain boundaries have been studied by means of a low critical temperature superconducting quantum interference device (SQUID...

  17. The effect of grain boundary segregation of boron in cast alloy 718 on HAZ microfissuring -- a SIMS analysis

    Energy Technology Data Exchange (ETDEWEB)

    Huang, X.; Chaturvedi, M.C. [Univ. of Manitoba, Winnipeg, Manitoba (Canada). Dept. of Mechanical and Industrial Engineering; Richards, N.L. [Bristol Aerospace Ltd., Winnipeg, Manitoba (Canada); Jackman, J. [CANMET, Ottawa, Ontario (Canada)

    1997-08-01

    Secondary ion mass spectroscopy (SIMS) has been used to examine grain boundary segregation in cast alloy 718. The relationship between the boron segregation and the microfissuring tendency in heat affected zones (HAZ) around electron beam welds is discussed in this study. It is concluded that two types of segregation, namely equilibrium and non-equilibrium segregation, occurred during the homogenization heat treatment of the base material. Water quenching after the homogenization treatment inhibited non-equilibrium segregation of boron and other trace elements owing to insufficient time for diffusion of solute-vacancy complexes to occur. Intermediate cooling rates such as air cooling enhanced both non-equilibrium and equilibrium segregation, since equilibrium segregation occurred during holding at the heat treatment temperature. The value of net segregation produced by a combination of equilibrium segregation and non-equilibrium segregation varied with temperature in a U-shape. The nature of the grain boundary in the cast alloy was examined by using an electron backscattered diffraction (EBSD) technique and it was found that 93% of the grain boundaries were of the random type ({Sigma} > 49). The weldability of this alloy was found to be closely related to the grain boundary segregation of boron, i.e., the variation of HAZ total crack length (TCL) with pre-welding heat treatment temperatures has a trend similar to that of boron segregation with temperature after air cooling. Mechanisms for the effect of boron on HAZ microfissuring have been proposed.

  18. Stress-free states of continuum dislocation fields : Rotations, grain boundaries, and the Nye dislocation density tensor

    NARCIS (Netherlands)

    Limkumnerd, Surachate; Sethna, James P.

    2007-01-01

    We derive general relations between grain boundaries, rotational deformations, and stress-free states for the mesoscale continuum Nye dislocation density tensor. Dislocations generally are associated with long-range stress fields. We provide the general form for dislocation density fields whose stre

  19. Atomistic Monte Carlo simulations of the diffusion of P and C near grain boundaries in BCC iron

    Energy Technology Data Exchange (ETDEWEB)

    Binkele, P.; Kizler, P. [MPA, Univ. Stuttgart, Stuttgart (Germany); Schmauder, S. [IMWF, Univ. Stuttgart, Stuttgart (Germany)

    2004-07-01

    It is well known that thermal ageing of steels can be caused by the segregation of phosphorus (P) and carbon (C) to grain boundaries. Atomistic Monte Carlo simulations of the diffusion of P and C to grain boundaries in bcc iron will allow, if validated, predictions of the time-dependent segregation. Simulations of the Fe-P-C system are presented, where the diffusion of Fe and P is realized via a vacancy mechanism and the diffusion of C is realized via an interstitial mechanism. Time-dependent segregations have been simulated for different temperatures and start conditions and are found to follow Johnson-Mehl-Avrami laws. A comparison of the simulation results with available AES (Auger Electron Spectroscopy) data shows close agreement with respect to P segregation. In simulations starting with a pre-filled grain boundary in increase of P and a decrease of C in the grain boundary are found where the decrease of C proceeds significantly faster than the increase of P for any temperature. The temperature-dependent ratios of the different speeds of P- and C-segregation, due to their different diffusion mechanisms, are calculated as a result of the simulations. (orig.)

  20. Exploring bainite formation kinetics distinguishing grain-boundary and autocatalytic nucleation in high and low-Si steels

    NARCIS (Netherlands)

    Ravi, A.M.; Sietsma, J.; Santofimia, M.J.

    2016-01-01

    Bainite formation in steels begins with nucleation of bainitic ferrite at austenite grain boundaries (γ/γ interfaces). This leads to creation of bainitic ferrite/austenite interfaces (α/γ interfaces). Bainite formation continues through autocatalysis with nucleation of bainitic ferrite at these

  1. Computational Capabilities for Predictions of Interactions at the Grain Boundary of Refractory Alloys

    Energy Technology Data Exchange (ETDEWEB)

    Sengupta, Debasis; Kwak, Shaun; Vasenkov, Alex; Shin, Yun Kyung; Duin, Adri van

    2014-09-30

    New high performance refractory alloys are critically required for improving efficiency and decreasing CO2 emissions of fossil energy systems. The development of these materials remains slow because it is driven by a trial-and-error experimental approach and lacks a rational design approach. Atomistic Molecular Dynamic (MD) design has the potential to accelerate this development through the prediction of mechanical properties and corrosion resistance of new materials. The success of MD simulations depends critically on the fidelity of interatomic potentials. This project, in collaboration with Penn State, has focused on developing and validating high quality quantum mechanics based reactive potentials, ReaxFF, for Ni-Fe-Al-Cr-O-S system. A larger number of accurate density functional theory (DFT) calculations were performed to generate data for parameterizing the ReaxFF potentials. These potentials were then used in molecular dynamics (MD) and molecular dynamics-Monte Carlo (MD-MC) for much larger system to study for which DFT calculation would be prohibitively expensive, and to understand a number of chemical phenomena Ni-Fe-Al-Cr-O-S based alloy systems . These include catalytic oxidation of butane on clean Cr2O3 and pyrite/Cr2O3, interfacial reaction between Cr2O3 (refractory material) and Al2O3 (slag), cohesive strength of at the grain boundary of S-enriched Cr compared to bulk Cr and Ssegregation study in Al, Al2O3, Cr and Cr2O3 with a grain structure. The developed quantum based ReaxFF potential are available from the authors upon request. During this project, a number of papers were published in peer-reviewed journals. In addition, several conference presentations were made.

  2. Computational Capabilities for Predictions of Interactions at the Grain Boundary of Refractory Alloys

    Energy Technology Data Exchange (ETDEWEB)

    Sengupta, Debasis; Kwak, Shaun; Vasenkov, Alex; Shin, Yun Kyung; Duin, Adri van

    2014-09-30

    New high performance refractory alloys are critically required for improving efficiency and decreasing CO2 emissions of fossil energy systems. The development of these materials remains slow because it is driven by a trial-and-error experimental approach and lacks a rational design approach. Atomistic Molecular Dynamic (MD) design has the potential to accelerate this development through the prediction of mechanical properties and corrosion resistance of new materials. The success of MD simulations depends critically on the fidelity of interatomic potentials. This project, in collaboration with Penn State, has focused on developing and validating high quality quantum mechanics based reactive potentials, ReaxFF, for Ni-Fe-Al-Cr-O-S system. A larger number of accurate density functional theory (DFT) calculations were performed to generate data for parameterizing the ReaxFF potentials. These potentials were then used in molecular dynamics (MD) and molecular dynamics-Monte Carlo (MD-MC) for much larger system to study for which DFT calculation would be prohibitively expensive, and to understand a number of chemical phenomena Ni-Fe-Al-Cr-O-S based alloy systems . These include catalytic oxidation of butane on clean Cr2O3 and pyrite/Cr2O3, interfacial reaction between Cr2O3 (refractory material) and Al2O3 (slag), cohesive strength of at the grain boundary of S-enriched Cr compared to bulk Cr and Ssegregation study in Al, Al2O3, Cr and Cr2O3 with a grain structure. The developed quantum based ReaxFF potential are available from the authors upon request. During this project, a number of papers were published in peer-reviewed journals. In addition, several conference presentations were made.

  3. Computational Capabilities for Predictions of Interactions at the Grain Boundary of Refractory Alloys

    Energy Technology Data Exchange (ETDEWEB)

    Sengupta, Debasis [CFD Research Corporation, Huntsville, AL (United States); Kwak, Shaun [CFD Research Corporation, Huntsville, AL (United States); Vasenkov, Alex [CFD Research Corporation, Huntsville, AL (United States); Shin, Yun Kyung [Pennsylvania State Univ., University Park, PA (United States); Duin, Adri van [Pennsylvania State Univ., University Park, PA (United States)

    2014-12-01

    New high performance refractory alloys are critically required for improving efficiency and decreasing CO2 emissions of fossil energy systems. The development of these materials remains slow because it is driven by a trial-and-error experimental approach and lacks a rational design approach. Atomistic Molecular Dynamic (MD) design has the potential to accelerate this development through the prediction of mechanical properties and corrosion resistance of new materials. The success of MD simulations depends critically on the fidelity of interatomic potentials. This project, in collaboration with Penn State, has focused on developing and validating high quality quantum mechanics based reactive potentials, ReaxFF, for Ni-Fe-Al-Cr-O-S system. A larger number of accurate density functional theory (DFT) calculations were performed to generate data for parameterizing the ReaxFF potentials. These potentials were then used in molecular dynamics (MD) and molecular dynamics-Monte Carlo (MD-MC) for much larger system to study for which DFT calculation would be prohibitively expensive, and to understand a number of chemical phenomena Ni-Fe-Al-Cr-O-S based alloy systems . These include catalytic oxidation of butane on clean Cr2O3 and pyrite/Cr2O3, interfacial reaction between Cr2O3 (refractory material) and Al2O3 (slag), cohesive strength of at the grain boundary of S-enriched Cr compared to bulk Cr and Ssegregation study in Al, Al2O3, Cr and Cr2O3 with a grain structure. The developed quantum based ReaxFF potential are available from the authors upon request. During this project, a number of papers were published in peer-reviewed journals. In addition, several conference presentations were made.

  4. Study by atomistic theory and high-resolution electron microscopies of Cu atoms at an Al grain boundary

    Energy Technology Data Exchange (ETDEWEB)

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

    2003-02-02

    New insight into the atomic segregation of copper to an aluminum grain boundary has been obtained using atomic resolution electron microscopy techniques coupled with ab-initio electronic structure calculations. We find the copper segregation to be site specific, changing the structure of the boundary by unexpectedly occupying interstitial sites. The calculated energy for segregation was found to be sufficient for essentially all of the interstitial sites to be filled. Minor elemental constituents in materials can have profound effects on their engineering performance, often through segregation to grain boundaries in the host material. One important example is the great resistance to electromigration damage in microelectronics imparted by small additions of copper to aluminum interconnects.

  5. Grain boundary corrosion and alteration phase formation during the oxidative dissolution of UO{sub 2} pellets

    Energy Technology Data Exchange (ETDEWEB)

    Wronkiewicz, D.J.; Buck, E.C.; Bates, J.K.

    1996-12-31

    Alteration behavior of UO{sub 2} pellets following reaction under unsaturated drip-test conditions at 90 C for up to 10 years was examined by solid phase and leachate analyses. Sample reactions were characterized by preferential dissolution of grain boundaries between the original press-sintered UO{sub 2} granules comprising the samples, development of a polygonal network of open channels along the intergrain boundaries, and spallation of surface granules that had undergone severe grain boundary corrosion. The development of a dense mat of alteration phases after 2 years of reaction trapped loose granules, resulting in reduced rates of particulate U release. The paragenetic sequence of alteration phases that formed on the present samples was similar to that observed in surficial weathering zones of natural uraninite (UO{sub 2}) deposits, with alkali and alkaline earth uranyl silicates representing the long-term solubility-limiting phases for U in both systems.

  6. The Effects of Heat Treatments on the M{sub 23}C{sub 6} Carbide Evolution and Grain Boundary Serration in Alloy 690

    Energy Technology Data Exchange (ETDEWEB)

    Lim, Yun Soo; Kim, Dong Jin; Hwang, Seong Sik; Kim, Sung Woo [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)

    2014-05-15

    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 M{sub 23}C{sub 6} 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.

  7. Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

    Indian Academy of Sciences (India)

    Neeraj Khare; A K Gupta; U P Moharil; A K Raychaudhuri; S P Pai; R Pinto

    2002-05-01

    La0.67Ba0.33MnO3 (LBMO) thin film is deposited on a $36.7°C SrTiO3 bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature > 175 K. At low temperature, - characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance–voltage characteristics of the bicrystal grain boundary indicates that at low temperatures ( < 175$ K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures ( > 175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.

  8. Properties of grain boundary networks in the NEEM ice core analyzed by combined transmission and reflection optical microscopy

    Science.gov (United States)

    Binder, Tobias; Weikusat, Ilka; Garbe, Christoph; Svensson, Anders; Kipfstuhl, Sepp

    2014-05-01

    Microstructure analysis of ice cores is vital to understand the processes controlling the flow of ice on the microscale. To quantify the microstructural variability (and thus occurring processes) on centimeter, meter and kilometer scale along deep polar ice cores, a large number of sections has to be analyzed. In the last decade, two different methods have been applied: On the one hand, transmission optical microscopy of thin sections between crossed polarizers yields information on the distribution of crystal c-axes. On the other hand, reflection optical microscopy of polished and controlled sublimated section surfaces allows to characterize the high resolution properties of a single grain boundary, e.g. its length, shape or curvature (further developed by [1]). Along the entire NEEM ice core (North-West Greenland, 2537 m length) drilled in 2008-2011 we applied both methods to the same set of vertical sections. The data set comprises series of six consecutive 6 x 9 cm2 sections in steps of 20 m - in total about 800 images. A dedicated method for automatic processing and matching both image types has recently been developed [2]. The high resolution properties of the grain boundary network are analyzed. Furthermore, the automatic assignment of c-axis misorientations to visible sublimation grooves enables us to quantify the degree of similarity between the microstructure revealed by both analysis techniques. The reliability to extract grain boundaries from both image types as well as the appearance of sublimation groove patterns exhibiting low misorientations is investigated. X-ray Laue diffraction measurements (yielding full crystallographic orientation) have validated the sensitivity of the surface sublimation method for sub-grain boundaries [3]. We introduce an approach for automatic extraction of sub-grain structures from sublimation grooves. A systematic analysis of sub-grain boundary densities indicates a possible influence of high impurity contents (amongst

  9. Grain boundary engineering for intergranular corrosion resistant austenitic stainless steel%抗晶间腐蚀奥氏体不锈钢的晶界工程

    Institute of Scientific and Technical Information of China (English)

    粉川博之

    2005-01-01

    Sensitization by chromium depletion due to chromium carbide precipitation at grain boundaries in austenitic stainless steels can not be prevented perfectly only by previous conventional techniques, such as reduction of carbon content, stabilization-treatment, local solution-heat-treatment, etc. Recent studies on grain boundary structure have revealed that the sensitization depends strongly on grain boundary character and atomic structure, and that low energy grain boundaries such as coincidence-site-lattice (CSL) boundaries have strong resistance to intergranular corrosion. The concept of 'grain boundary design and control' has been developed as grain boundary engineering (GBE). GBEed materials are characterized by high frequencies of CSL boundaries which are resistant to intergranular deterioration of materials, such as intergranular corrosion. A thermomechanical treatment was tried to improve the resistance to the sensitization by GBE. A type 304 austenitic stainless steel was cold-rolled and solution-heat-treated, and then sensitization-heat-treated. The grain boundary character distribution was examined by orientation imaging microscopy (OIM). The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation (EPR) and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of CSL boundaries indicated a maximum at the small reduction. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanical-treated specimen than in the base material. A high density of annealing twins were observed in the thermomechanical-treated specimen. The results suggest that the therrmomechanical treatment can introduce low energy segments in the grain boundary network by annealing twins and can arrest the percolation of intergranular corrosion from the surface. The effects of carbon

  10. Nanocrystalline, ultra-degradation-resistant zirconia: its grain boundary nanostructure and nanochemistry.

    Science.gov (United States)

    Matsui, Koji; Yoshida, Hidehiro; Ikuhara, Yuichi

    2014-04-23

    Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) has been known to be an excellent structural material with high strength and toughness since the pioneering study by Garvie et al. in 1975. However, Y-TZP is not considered an environmental or biomedical material because it undergoes an inherent tetragonal-to-monoclinic (T → M) phase transformation in humid or aqueous environment, which leads to premature failure, so-called low-temperature degradation (LTD). In this study, we demonstrate for the first time that this fatal shortcoming of Y-TZP can be resolved by controlling the grain boundary nanostructure and chemical composition distribution in Y-TZP. Nanocrystalline Y-TZP doped with Al(3+) and Ge(4+) ions exhibits no LTD for more than 4 years in hot water at 140 °C, whereas 70% of the tetragonal phase in conventional TZP transforms to the monoclinic phase within only 15 h. This innovative Y-TZP can be fabricated by pressureless sintering at 1200 °C; far below the sintering temperature for conventional Y-TZP. The developed TZP ceramics will be useful in numerous environmental-proofing applications, particularly in the biomedical engineering field.

  11. Characterization of grain boundaries in Cu(In,Ga)Se{sub 2} by atom probe tomography

    Energy Technology Data Exchange (ETDEWEB)

    Schwarz, Torsten; Cojocaru-Miredin, Oana; Choi, Pyuck-Pa; Raabe, Dierk [Max-Planck Institute for Iron Research GmbH, Duesseldorf (Germany); Wuerz, Roland [Zentrum fuer Sonnenenergie- und Wasserstoff-Forschung Baden-Wuerttemberg (ZSW), Stuttgart (Germany)

    2012-07-01

    Solar cells based on the compound semiconductor Cu(In,Ga)Se{sub 2} (CIGS) as absorber material exhibit the highest efficiency among all thin-film solar cells. This is surprising high in view of the polycrystalline defect-rich structure of the CIGS absorber films. The high efficiency has been commonly ascribed to the diffusion of alkali metal atoms from the soda-lime glass substrate into the CIGS layer, which can render the grain boundaries (GB) electrically inactive. However, the exact mechanisms of how these impurities enhance the cell efficiency are yet to be clarified. As a step towards a better understanding of CIGS solar cells, we have analyzed the composition of solar-grade CIGS layers at the atomic-scale by using pulsed laser Atom Probe Tomography (APT). To perform APT analyses on selected GBs site-specific sample preparation was carried out using the Focused Ion Beam lift-out technique. In addition, Electron Back Scattered Diffraction was performed to characterize the structure and misorientation of selected GBs. Using APT, segregation of impurities at the GBs was directly observed. APT data of various types of GBs are presented and discussed with respect to the possible effects on the cell efficiency.

  12. Angle resolved photoemission spectroscopy reveals spin charge separation in metallic MoSe2 grain boundary

    Science.gov (United States)

    Ma, Yujing; Diaz, Horacio Coy; Avila, José; Chen, Chaoyu; Kalappattil, Vijaysankar; Das, Raja; Phan, Manh-Huong; Čadež, Tilen; Carmelo, José M. P.; Asensio, Maria C.; Batzill, Matthias

    2017-02-01

    Material line defects are one-dimensional structures but the search and proof of electron behaviour consistent with the reduced dimension of such defects has been so far unsuccessful. Here we show using angle resolved photoemission spectroscopy that twin-grain boundaries in the layered semiconductor MoSe2 exhibit parabolic metallic bands. The one-dimensional nature is evident from a charge density wave transition, whose periodicity is given by kF/π, consistent with scanning tunnelling microscopy and angle resolved photoemission measurements. Most importantly, we provide evidence for spin- and charge-separation, the hallmark of one-dimensional quantum liquids. Our studies show that the spectral line splits into distinctive spinon and holon excitations whose dispersions exactly follow the energy-momentum dependence calculated by a Hubbard model with suitable finite-range interactions. Our results also imply that quantum wires and junctions can be isolated in line defects of other transition metal dichalcogenides, which may enable quantum transport measurements and devices.

  13. Contrasting the grain boundary-affected performance of zinc and indium oxide transparent conductors.

    Science.gov (United States)

    Vai, A T; Rashidi, N; Fang, Y; Kuznetsov, V L; Edwards, P P

    2016-06-08

    Zinc oxide-based transparent conductors have long been advanced for their potential as low-cost, earth-abundant replacements for the indium oxide-based materials that currently dominate in practical applications. However, this potential has yet to be realized because of the difficulties in producing zinc oxide thin films with the necessary high levels of electrical conductivity and environmental stability that are readily achieved using indium oxide. To better understand the fundamental reasons for this, polycrystalline zinc and indium oxide thin films were prepared across a range of deposition temperatures using the technique of spray pyrolysis. Electrical transport measurements of these samples both as a function of temperature and UV irradiation were correlated with film morphology to illustrate that the different grain boundary behaviour of these two materials is one of the key reasons for their divergent performance. This is a critical challenge that must be addressed before any substantial increase in the adoption of ZnO-based transparent conductors can take place.

  14. Effects of atomic grain boundary structures on primary radiation damage in α-Fe

    Energy Technology Data Exchange (ETDEWEB)

    Esfandiarpour, A. [Department of Physics, Payame Noor University (PNU), P.O. BOX 19395-3697, Tehran (Iran, Islamic Republic of); Feghhi, S.A.H., E-mail: a.feghhi@gmail.com [Department of Radiation Application, Shahid Beheshti University G.C., Tehran (Iran, Islamic Republic of); Shokri, A.A. [Department of Physics, Payame Noor University (PNU), P.O. BOX 19395-3697, Tehran (Iran, Islamic Republic of)

    2015-11-01

    In this paper, we used five different grain boundary (GB) structures including two twists, two symmetric tilts and one asymmetric tilt for α-Fe, in order to study the influence of different GB structures on the production and time evolution of defects. Energetic behavior of point defects near GBs is investigated and analyzed using Molecular Static (MS) method to calculate and compare the “defect absorbency” of each structure. The primary radiation damage state near each GB structure is simulated using Molecular Dynamic (MD) method for 3 keV and 6 keV primary-knocked on atom (PKA) with velocity vector perpendicular to the GB plane at various distances in 300 K. We found that all five GB structures can decrease the defect number in bulk region, if cascade center locates on the GB plane (prefect overlap) and increase the vacancy number, if the overlap is imperfect. This depends on the energy of PKA and its distance from GB plane. Also, the results proved that the magnitude of the observed variations depends on the atomic structure of GB. Furthermore, the GBs that have stronger “interstitial absorbency” produce an excess concentration of vacancies in the bulk region, while the edge of the cascades overlaps with GB plane. This is the result of bigger “interstitial absorbency” of GBs in comparison with vacancies.

  15. Incorporating gold into nanocrystalline silver dressings reduces grain boundary size and maintains suitable antimicrobial properties.

    Science.gov (United States)

    Unrau, Kevin R; Cavanagh, Marion H; Cheng, On Kwan; Wang, Shiman; Burrell, Robert E

    2013-12-01

    Nanocrystalline silver dressings are widely known to be potent antimicrobial and anti-inflammatory agents and have long been used to treat topical wounds. Gold is known to be a strong anti-inflammatory agent and has been used in the treatment of rheumatoid arthritis for >70 years. The purpose of this work was to study the effect of incorporating gold into nanocrystalline silver dressings from antimicrobial and anti-inflammatory perspectives. Gold and silver dressing alloys were created by direct current magnetron sputtering and compared