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Sample records for boundaries grain

  1. Grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1991-01-01

    The present document is a progress report describing the work accomplished to date during the second year of our four-year grant (February 15, 1990--February 14, 1994) to study grain boundaries. The research was focused on the following three major efforts: Study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; study of short-circuit diffusion along grain boundaries; and development of a Thin-film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals.

  2. Grain boundaries

    Science.gov (United States)

    Balluffi, R. W.; Bristowe, P. D.

    The present document is a progress report describing the work accomplished to date during the second year of our four-year grant (February 15, 1990 to February 14, 1994) to study grain boundaries. The research was focused on the following three major efforts: study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; study of short-circuit diffusion along grain boundaries; and development of a Thin-film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals.

  3. Grain boundaries: Progress report

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1988-02-01

    Quantitative measurements of grain boundary structure factors using x-ray diffraction have been performed on low angle (001) twist boundaries in gold. Also, a computer atomistic simulation program is being implemented to examine the equilibrium properties of a series of boundaries in gold. Simulation of boundaries at room temperature have been performed. Electron microscopy of grain boundary melting in aluminum was also performed. Results indicated an absence of melting. (CBS)

  4. Grain Boundary Complexions

    Science.gov (United States)

    2014-05-01

    deter- mine bulk materials behavior and properties such as superplasticity, creep, fatigue, corrosion , strength and conductivity [2]. Grain boundary...interface (i.e. lattice mismatch accommodated by interface dislocations ), wetting transitions will not occur. A wetting transition is possible in the case...melting only starts around dislocations at low- angle grain boundaries; the grain boundary structure con- sists of isolated liquid pools separated by

  5. 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.

  6. Progress report on grain boundaries

    Science.gov (United States)

    Balluffi, R. W.; Bristowe, P. D.

    1989-06-01

    The research was focused on the following three major areas: (1) study of the atomic structure of grain boundaries by means of X-ray diffraction, transmission electron microscopy and computer modeling; (2) study of grain boundary phase transitions by electron microscopy and computer modeling; (3) investigation of the mechanism of high angle grain boundary migration. Results are briefly discussed.

  7. Progress report on grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1989-06-01

    The research was focused on the following three major areas: (1) study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; (2) study of grain boundary phase transitions by electron microscopy and computer modeling; (3) investigation of the mechanism of high angle grain boundary migration. Results are briefly discussed. 20 refs.

  8. Grain boundary melting in ice

    OpenAIRE

    Thomson, E. S.; Hansen-Goos, Hendrik; Wilen, L. A.; Wettlaufer, J. S.

    2012-01-01

    We describe an optical scattering study of grain boundary premelting in water ice. Ubiquitous long ranged attractive polarization forces act to suppress grain boundary melting whereas repulsive forces originating in screened Coulomb interactions and classical colligative effects enhance it. The liquid enhancing effects can be manipulated by adding dopant ions to the system. For all measured grain boundaries this leads to increasing premelted film thickness with increasing electrolyte concentr...

  9. Grain boundary melting in ice

    Science.gov (United States)

    Thomson, E. S.; Hansen-Goos, Hendrik; Wettlaufer, J. S.; Wilen, L. A.

    2013-03-01

    We describe an optical scattering study of grain boundary premelting in water ice. Ubiquitous long ranged attractive polarization forces act to suppress grain boundary melting whereas repulsive forces originating in screened Coulomb interactions and classical colligative effects enhance it. The liquid enhancing effects can be manipulated by adding dopant ions to the system. For all measured grain boundaries this leads to increasing premelted film thickness with increasing electrolyte concentration. Although we understand that the interfacial surface charge densities qs and solute concentrations can potentially dominate the film thickness, we cannot directly measure them within a given grain boundary. Therefore, as a framework for interpreting the data we consider two appropriate qs dependent limits; one is dominated by the colligative effect and other is dominated by electrostatic interactions.

  10. 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.

  11. Structure and properties of grain boundaries

    Science.gov (United States)

    Balluffi, R. W.; Bristowe, P. D.

    1984-03-01

    Results were obtained in the following areas: determination of relative grain boundary energies by the rotating crystallite method; simple structural unit model for core dependent properties of tilt boundaries; twist boundary energies for metals with long ranged pairwise interatomic potentials; structural unit/grain boundary dislocation model for grain boundary structure; detection of expansion of boundaries using diffraction; effect of secondary relaxations on diffraction from high-(SIGMA) 001 twist boundaries; and mechanism of grain boundary migration.

  12. 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

  13. Emission of dislocations from grain boundaries by grain boundary dissociation

    Science.gov (United States)

    Hoagland, Richard G.; Valone, Steven M.

    2015-01-01

    In this article, we examine the conditions that favour the emission of Shockley partial dislocations (SPDs) that standoff from a grain boundary (GB) plane by a few lattice parameters as part of the atomic structure of some GBs. To do so, we consider GBs to be formed by the operation of arrays of intrinsic grain boundary dislocations (GBDs) that create the tilt and twist misorientation, and the lattice mismatch between the two crystal grains adjoining the GB. The conditions to be considered that favour SPDs are the following: (1) Frank's rule, (2) the proper sequential arrangement of partial dislocations to bound an intrinsic stacking fault and (3) the equilibrium stand-off distance (ESD). We apply an isotropic elasticity analysis to compute the ESD, in the absence of an applied stress, for SPDs emerging from asymmetric tilt GBs in two FCC metals, Cu and Al. The ESD is shown to be dependent on the glide plane orientation relative to the GB plane and on the position of the glide planes, relative to the position of the GBDs. An applied stress increases the ESD up to a critical stress that removes the SPDs without limit from the GB. We examine the effect of the stacking fault energy on the ESD and critical stress. The critical stress is effectively linearly dependent on the stacking fault energy. Finally, we present results of atomistic simulations of asymmetric tilt Σ11[1 0 1]{4 1 4}||{2 5 2} GBs in Cu bicrystal models subject to shock loading that behave in a manner similar to the elasticity predictions. The atomistic simulations reveal additional behaviour associated with elastic incompatibility between the two grains in the bicrystal models.

  14. Special Grain Boundaries in Ultrafine-Grained Tungsten

    Science.gov (United States)

    Dudka, O. V.; Ksenofontov, V. A.; Sadanov, E. V.; Starchenko, I. V.; Mazilova, T. I.; Mikhailovskij, I. M.

    2016-07-01

    Field ion microscopy and computer simulation were used for the study of an atomic structure high-angle grain boundary in hard-drawn ultrafine-grained tungsten wire. These boundaries with special misorientations are beyond the scope of the coincident site lattice model. It was demonstrated that the special non-coincident grain boundaries are the plane-matching boundaries, and rigid-body displacements of adjacent nanograins are normal to the misorientation axis. The vectors of rigid-body translations of grains are described by broad asymmetric statistical distribution. Mathematical modeling showed that special incommensurate boundaries with one grain oriented along the {211} plane have comparatively high cohesive energies. The grain-boundary dislocations ½ were revealed and studied at the line of local mismatch of {110} atomic planes of adjacent grains.

  15. Influence of grain size and grain boundary recombination velocity on ...

    African Journals Online (AJOL)

    The plot of the diffusion capacitance allowed us to study the influence of the following parameters: grain size, grain boundary recombination velocity, junction recombination velocity and illumination wavelength on this capacitance. This study pointed out that junction and grain boundary recombination velocities play an ...

  16. 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...... to engineer new materials. In this study, one of the most widely used electrolytes for electrodeposition is chosen for the synthesis of nickel films and based on thorough characterization of the boundaries the potentials in grain boundary engineering are outlined. The internal structure of the nickel films...

  17. Ferroelectric domain continuity over grain boundaries

    DEFF Research Database (Denmark)

    Mantri, Sukriti; Oddershede, Jette; Damjanovic, Dragan

    2017-01-01

    Formation and mobility of domain walls in ferroelectric materials is responsible for many of their electrical and mechanical properties. Domain wall continuity across grain boundaries has been observed since the 1950's and is speculated to affect the grain boundary-domain interactions, thereby...... orientation. We have also incorporated the effect of grain boundary ferroelectric polarization charge created when any two domains meet at the grain boundary plane. The probability of domain wall continuity for three specific grain misorientations is studied. Use of this knowledge to optimize processing...

  18. 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

  19. Effect of grain boundary misorientation on discontinuous ...

    Indian Academy of Sciences (India)

    Administrator

    showed that the discontinuous precipitation (DP) reaction rate was dependent on the geometry of the grain boundary in ... 80% thickness reduction) had no effect on the frequency of special-grain boundaries. Keywords. AZ91 alloy .... increasing solute concentration, the influence of the ener- getics and kinetics is diminished ...

  20. Grain boundaries. Progress report, February 15, 1990--October 15, 1990

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1990-12-31

    The following are reported: structural studies (dislocation structure of Ni/Ag interphase boundary, structural complexity in grain boundaries with covalent bonding, relation between microscopic properties of two semiconducting grain boundaries and their orientations, diffraction effects due to double positioning in (111) Au bicrystals, sensitivity of diffraction profiles to grain boundary segregation, solute segregation at grain boundaries in Au, 4-body interatomic potential for Si for defect calculations); boundary migration studies (molecular dynamics study of grain boundary migration without participation of grain boundary dislocations); study of short-circuit diffusion along grain boundaries and its dependence on boundary structure; and thin-film deposition/bonding apparatus for manufacturing high-purity bicrystals.

  1. Volume dependence of computed grain boundary energy

    Energy Technology Data Exchange (ETDEWEB)

    Bristowe, P.D.; Brokman, A.

    1980-08-01

    Over the past five years there have been numerous studies of grain boundary structure using the method of computer molecular statics which assume pairwise central potentials for the interatomic interaction. Emphasis is usually placed on relative grain boundary energies but these may be inaccurate due to various, but related, approximations and constraints implicity imposed on the calculation-namely central forces, finite model size, fixed border conditions and volume dependent contributions to the energy of the system. It is the purpose of this work to clarify how these particular properties of the model can affect the computed grain boundary energy and demonstrate instances in which the quoted energy has strictly been inaccurate. The implication of these results, especially on how they affect the method of relaxation and the resulting grain boundary structure is discussed.

  2. Grain boundaries: Annual technical progress report

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1988-04-28

    The present document is a progress report describing the work accomplished on the study of grain of gold. The research that was proposed initially for this period consisted of studies of the atomistics structure of grain boundaries by means of combined x-ray diffraction and computer modeling and of grain boundary phase transitions by electron microscopy and computer modeling. Progress has been made on both of these areas which is described in more detail. A list of reports describing the research completely during the first year is presented.

  3. 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...

  4. An Optical Study of Ice Grain Boundaries

    Science.gov (United States)

    Thomson, Erik S.

    The equilibrium phase geometry and evolution of polycrystals underlies the nature of materials. In particular, grain boundaries dominate the total interfacial area within polycrystalline materials. Our experimental studies are motivated by the importance of the structure, evolution, and thermodynamic behavior of grain boundaries near bulk melting temperatures. Ice is singled out as a material of interest due to its geophysical importance and its advantageous optical properties. An experimental apparatus and light reflection technique is designed to measure grain boundary melting in ice bicrystals, in thermodynamic equilibrium The technique allows continuous monitoring of reflected light intensity from the grain boundary as the temperature and solutal composition are systematically varied. For each sample the individual crystal orientations are also measured. The type and concentration of impurity in the liquid is controlled and the temperature is continuously recorded and controlled over a range near the melting point. An optical model of the interface is developed in order to convert experimental reflection data into a physical measurement of the liquidity of the grain boundary. Solutions are found for reflection and transmission amplitude coefficients for waves propagating from an arbitrarily oriented uniaxial anisotropic material into an isotropic material. This general model is used to determine solutions for three layer, ice/water/ice, systems with crystals of arbitrary orientation, and is broadly applicable to layered materials. Experimental results show thicker grain boundary liquid layers than expected from classical colligative effects. A physically realistic model of intermolecular interactions succeeds in bounding the measurements. These measurements may have important implications for understanding a wide range of effects in polycrystalline materials. Likewise, the experimental techniques and optical theory may be applied to other systems of broad

  5. 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...

  6. 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

  7. 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.

  8. Mechanically induced grain boundary motion in Al-bicrystals

    Energy Technology Data Exchange (ETDEWEB)

    Gorkaya, Tatiana; Molodov, Dmitri A.; Gottstein, Guenter [Institut fuer Metallkunde und Metallphysik der RWTH Aachen (Germany)

    2010-07-01

    The mechanically induced migration of planar grain boundaries in Al-bicrystals was experimentally measured. The novel tensile/compression module for scanning electron microscope was utilized for in-situ measurements of grain boundary motion at elevated temperatures. From the measured temperature dependence of boundary mobility the migration activation parameters for investigated boundaries were determined. Normal boundary motion was observed to be coupled to a shear of the crystal in the region traversed by the grain boundary during its motion. The measured ratios of the normal grain boundary motion to the lateral translation of grains were compared with geometrical models of stress induced boundary migration.

  9. The relationship between grain boundary structure, defect mobility, and grain boundary sink efficiency

    Science.gov (United States)

    Uberuaga, Blas Pedro; Vernon, Louis J.; Martinez, Enrique; Voter, Arthur F.

    2015-01-01

    Nanocrystalline materials have received great attention due to their potential for improved functionality and have been proposed for extreme environments where the interfaces are expected to promote radiation tolerance. However, the precise role of the interfaces in modifying defect behavior is unclear. Using long-time simulations methods, we determine the mobility of defects and defect clusters at grain boundaries in Cu. We find that mobilities vary significantly with boundary structure and cluster size, with larger clusters exhibiting reduced mobility, and that interface sink efficiency depends on the kinetics of defects within the interface via the in-boundary annihilation rate of defects. Thus, sink efficiency is a strong function of defect mobility, which depends on boundary structure, a property that evolves with time. Further, defect mobility at boundaries can be slower than in the bulk, which has general implications for the properties of polycrystalline materials. Finally, we correlate defect energetics with the volumes of atomic sites at the boundary. PMID:25766999

  10. On the structural unit/grain boundary dislocation model for grain boundary structure

    Science.gov (United States)

    Balluffi, R. W.; Bristowe, P. D.

    1984-08-01

    The applicability of the structural unit/grain boundary dislocation (GBD) model to the atomistic structures of [001] and [011] tilt boundary series recently calculated by Sutton, Vitek and Wang is studied in detail. It is found that a hierarchy of structural unit/GBD descriptions exists which may be used to describe such boundaries. Each description corresponds to a different choice of "delimiting boundary" structural units which are mixed together to make up all boundaries in the series. In all cases the structural units exhibit systematic distortions as the tilt angle changes. As the number of delimiting boundaries in a series is increased, and the range of structures is "quantized" on a finer scale, the distortions of the structural units decreases, but the amount of information regarding the GBD structure is decreased. It is pointed out that different members of the hierarchy of descriptions can be used advantageously for different purposes.

  11. 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.

  12. 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.

  13. 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.

  14. Towards realistic molecular dynamics simulations of grain boundary mobility

    Energy Technology Data Exchange (ETDEWEB)

    Zhou, J., E-mail: zhou@imm.rwth-aachen.de [Institut fuer Metallkunde und Metallphysik, Rheinisch-Westfaelische Technische Hochschule (RWTH), Aachen (Germany); Mohles, V. [Institut fuer Metallkunde und Metallphysik, Rheinisch-Westfaelische Technische Hochschule (RWTH), Aachen (Germany)

    2011-09-15

    In order to investigate grain boundary migration by molecular dynamics (MD) simulations a new approach involving a crystal orientation-dependent driving force has been developed by imposing an appropriate driving force on grain boundary atoms and enlarging the effective range of driving force. The new approach has been validated by the work of the driving force associated with the motion of grain boundaries. With the new approach the relation between boundary migration velocity and driving force is found to be nonlinear, as was expected from rate theory for large driving forces applied in MD simulations. By evaluating grain boundary mobility nonlinearly for a set of symmetrical <1 1 1> tilt boundaries in aluminum at high temperature, high-angle grain boundaries were shown to move much faster than low-angle grain boundaries. This agrees well with experimental findings for recrystallization and grain growth. In comparison with the available data the simulated mobility of a 38.21{sup o{Sigma}}7 boundary was found to be significantly lower than other MD simulation results and comparable with the experimental values. Furthermore, the average volume involved during atomic jumps for boundary migration is determined in MD simulations for the first time. The large magnitude of the volume indicates that grain boundary migration is accomplished by the correlated motion of atom groups.

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

    Science.gov (United States)

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

    1973-01-01

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

  16. Low energy planes for tilt grain boundaries in gold

    Energy Technology Data Exchange (ETDEWEB)

    Goodhew, P. J.; Tan, T. Y.; Balluffi, R. W.

    1977-05-01

    Thin film bicrystals of gold were annealed to create a large variety of grain boundaries of controlled crystal misorientation. The faceting of these boundaries was studied, and some low-energy boundary planes were identified. Tilt boundaries with a (100) misorientation axis were found to facet more readily than those with a (100) axis. The results were considered in terms of O-lattices and coincident site lattices, but neither of these approaches is able to explain all the observations. Consequently, no simple and general criteria, based on geometrical considerations, were found for the occurrence of grain boundary planes of low energy. A further set of observations relates to the dissociation of some specific high-angle grain boundaries into a twin and another low-energy boundary. Boundaries with a ..sigma..33 coincidence site lattice relationship frequently appear to be characterized by a relatively low energy. 12 figures, 2 tables.

  17. Modelling of grain refinement driven by negative grain boundary energy

    Czech Academy of Sciences Publication Activity Database

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

    2017-01-01

    Roč. 97, č. 23 (2017), s. 1963-1977 ISSN 1478-6435 R&D Projects: GA ČR(CZ) GA15-06390S Institutional support: RVO:68081723 Keywords : grain refinement * grain nucleation * distribution concept * jump on distribution function Subject RIV: BJ - Thermodynamics Impact factor: 1.505, year: 2016

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

    KAUST Repository

    Gurses, Ercan

    2011-12-01

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

  19. Rheology of grain-boundary sliding and grain interlocking at high temperatures

    Energy Technology Data Exchange (ETDEWEB)

    Sakai, M.; Muto, H. [Toyohashi Univ. of Technology (Japan). Dept. of Materials Science

    1999-03-01

    A two-dimensional array of elastic hexagonal grains embedded in an contiguous fluid is used as a model for grain-boundary sliding and grain interlocking. The viscoelastic constitutive equation, in a phenomenological sense, is of a nonlinear Maxwell type, comprising a strain-dependent dashpot and an elastic spring connected in series. The squeezing-in/out processes and mechanisms of grain-boundary fluid essentially give rise to the rheological nonlinearity. The experimental results in stress relaxation tests of a {beta}-spodumene glass-ceramic under simple shear are characterized in a light of the nonlinear constitutive equation. It is emphasized that stress relaxation test is one of the important test techniques which enable one to study quantitatively the rheological behavior of polycrystalline ceramics with grain-boundary sliding and grain interlocking without any difficulties and ambiguities accompanied by stress-induced grain-boundary cavities which so often appear in conventional creep tests. (orig.) 9 refs.

  20. 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.

  1. Influence of subgrain boundaries on coarsening of grain structures

    Science.gov (United States)

    Zöllner, D.; Skrotzki, W.

    2017-05-01

    In the present work, the influence of subgrain boundaries on the coarsening kinetics of individual grains embedded in an average environment as well as within a grain structure is investigated. It is found that a specific introduction of subgrain boundaries not only influences the speed with which grains shrink or grow, but in contrast to the von Neumann-Mullins-law, a distinct manipulation of the location of the subgrain boundaries allows even grains with few edges to grow, while grains with many edges shrink. During these circumstances one fact stays the same: the area of the individual grains is a linear function of annealing time as long as the environment does not change.

  2. Grain boundaries: Final report, June 16, 1984-June 15, 1987

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1987-01-01

    During the three-year period coming to a close on June 15, 1987, we have conducted a relatively broad investigation of the structure and properties of grain boundaries on a basic level. The work mainly consisted of a combined experimental and computer simulation attack on the problem of determining the atomic structure and corresponding properties of high-angle grain boundaries in metals. The experimental work has relied heavily on techniques developed for producing thin film specimens containing planar grain boundaries of predetermined and controlled geometry. Studies of the atomistic and dislocation structures of these boundaries have been carried out using electron microscopy in the diffraction, diffraction contrast and lattice imaging modes. Of particular interest have been experiments aimed at deciding questions such as ''to what extent are grain boundaries ordered structures.'' and ''to what extent can grain boundary structures be described by dislocation and structural unit models.''. Efforts have been made to measure relative grain boundary energies. The computer simulation work has involved both molecular statics and molecular dynamics calculations using pair potentials and more sophisticated ab initio methods. Emphasis has been on calculating boundary structures and energies in situations where the results can be compared with experiments. Efforts have been made to determine whether the calculated results fit dislocation and structural unit models.

  3. Grain boundaries: Progress report, June 15, 1988--June 14, 1989

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1989-02-01

    The present document is a progress report describing the work accomplished during the second year (June 15, 1988-June 14, 1989) of our three-year grant to study grain boundaries which started 15 June 1987. The research that was proposed initially for this period consisted of the following: (1) study of the atomistic structure of grain boundaries by means by combined x-ray diffraction and computer modeling; (2) study of grain boundary phase transitions by electron microscopy and computer modeling. 12 refs.

  4. Grain boundaries. Progress report, February 15, 1991--October 15, 1991

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1991-12-31

    The present document is a progress report describing the work accomplished to date during the second year of our four-year grant (February 15, 1990--February 14, 1994) to study grain boundaries. The research was focused on the following three major efforts: Study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; study of short-circuit diffusion along grain boundaries; and development of a Thin-film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals.

  5. Control and Characterization of Individual Grains and Grain Boundaries in Graphene Grown by Chemical Vapour Deposition

    Science.gov (United States)

    2011-06-01

    field (B). Figure 5c presents low temperature (4.3 K) magnetoresistance (Rxx(B)) measurements across the grain boundary compared to Rxx(B) measured within...voltage leads labelled in the legend. c, Four-terminal magnetoresistance (Rxx) measured at 4.3 K within each graphene grain and across the grain boundary...graphene nanoribbons. Nature 444, 347–349 (2006). 37. Huang, M. Y. et al. Phonon softening and crystallographic orientation of strained graphene studied by

  6. Structural unit/grain boundary dislocation model for grain boundary structure

    Science.gov (United States)

    Balluffi, R. W.; Bristowe, P. D.

    1983-12-01

    The applicability of the structural unit/grain boundary dislocation (GBD) model to recently calculated core structures of a number of series of boundaries is considered. The following questions are posed: (1) Is there a best choice of structural units? (2) Can the distortions of these units be understood in a systematic way? (3) Are the distortions of the units of a given type sufficiently small to justify the model? Questions (1) and (2) are discussed in detail and answered affirmatively. The answer to Question (3) is qualified. Relaxations occur in all boundaries tested which tend to improve the uniformity of the units, localize the corresponding GBDs and, hence, increase the applicability of the model. The model therefore stands as the most physically viable model for representing the systmatics of the computed core structures. However, in some cases, these relaxations are relatively weak, the units exhibit significant systematic distortions, and the corresponding GBDs are partially delocalized. In such cases the model must be considered an approximation, particularly for many quantitative purposes.

  7. Structural unit/grain boundary dislocation model for grain boundary structure

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1983-12-01

    The applicability of the structural unit/grain boundary dislocation (GBD) model to recently calculated core structures of a number of series of boundaries is considered. The following questions are posed: (1) Is there a best choice of structural units. (2) Can the distortions of these units be understood in a systematic way. (3) Are the distortions of the units of a given type sufficiently small to justify the model. Questions (1) and (2) are discussed in detail and answered affirmatively. The answer to Question (3) is qualified. It is pointed out that relaxations occur in all boundaries tested which tend to improve the uniformity of the units, localize the corresponding GBDs and, hence, increase the applicability of the model. The model therefore stands as the most physically viable model for representing the systematics of the computed core structures. However, in some cases, these relaxations are relatively weak, the units exhibit significant systematic distortions, and the corresponding GBDs are partially delocalized. In such cases the model must be considered an approximation, particularly for many quantitative purposes.

  8. Dynamical simulation of structural multiplicity in grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Majid, I.; Bristowe, P.D.

    1987-06-01

    Work on a computer simulation study of a low-energy high-angle boundary structure which is not periodic have been recently reported. This result is of interest since grain boundary structures are usually assumed to have a periodicity corresponding to the appropriate coincidence site lattice (CSL) and many experimental observations of the structure of grain boundaries performed using conventional and high-resolution electron microscopy, electron diffraction and x-ray diffraction appear to support this work. However, this work, using empirical interatomic pair potentials and the relaxation method of molecular statics, have simulated a ..sigma.. = 5 36.87/sup 0/ (001) twist boundary and found a low energy structure having a larger repeat cell than the CSL and is composed of two different types of structural unit that are randomly distributed in the boundary plane. This result, which has been termed the multiplicity of grain boundary structures, has also been found in the simulation of tilt boundaries. The multiplicity phenomenon is of special interest in twist boundaries since it is used as a structural model to explain the x-ray scattering from a ..sigma.. = 5 boundary in gold. These scattering patterns had previously remained unexplained using stable structures that had simple CSL periodicity. Also, the effect of having a multiple number of low energy structural units coexisting in the grain boundary is of more general interest since it implies that the boundary structures may be quasi-periodic and, in some circumstances, may even result in a roughening of the boundary plane. This paper extends this work by showing, using molecular dynamics, that a multiplicity of structural units can actually nucleate spontaneously in a high-angle grain boundary at finite temperatures.

  9. Dynamical simulation of structural multiplicity in grain boundaries

    Science.gov (United States)

    Majid, I.; Bristowe, P. D.

    1987-06-01

    Work on a computer simulation study of a low-energy high-angle boundary structure which is not periodic have been recently reported. This result is of interest since grain boundary structures are usually assumed to have a periodicity corresponding to the appropriate coincidence site lattice (CSL) and many experimental observations of the structure of grain boundaries performed using conventional and high-resolution electron microscopy, electron diffraction and X-ray diffraction appear to support this work. However, this work, using empirical interatomic pair potentials and the relaxation method of molecular statics, have simulated a Sigma=5 (36.87 deg)(001) twist boundary and found a low energy structure having a larger repeat cell than the CSL and is composed of two different types of structural unit randomly distributed in the boundary plane. This result, termed the multiplicity of grain boundary structures, has also been found in the simulation of tilt boundaries. The multiplicity phenomenon is of special interest in twist boundaries since it is used as a structural model to explain the X-ray scattering from a Sigma=5 boundary in gold. These scattering patterns had previously remained unexplained using stable structures that had simple CSL periodicity. Also, the effect of having a multiple number of low energy structural units coexisting in the grain boundary is of more general interest since it implies that the boundary structures may be quasi-periodic and, in some circumstances, may even result in a roughening of the boundary plane. This work is extended by showing, using molecular dynamics, that a multiplicity of structural units can actually nucleate spontaneously in a high-angle grain boundary at finite temperatures.

  10. Grain boundaries in ceramics and ceramic-metal interfaces

    Energy Technology Data Exchange (ETDEWEB)

    Clarke, D.R.; Wolf, D.

    1986-01-01

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

  11. 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......A study has been made using transmission electron microscopy of the pinning of grain boundaries in aluminium during grain growth by fine dispersions of alumina particles. The boundary parameters have been determined with precision and the pinning effects measured using an approach due to Ashby...

  12. 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......, 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...... interest. The evolution of microstructure in as-deposited and annealed condition was investigated with a combination of complementary microscopic techniques, electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI), ion channelling contrast imaging (ICCI), and, for the as...

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

    Energy Technology Data Exchange (ETDEWEB)

    Sabogal-Suárez, Daniel; David Alzate-Cardona, Juan, E-mail: jdalzatec@unal.edu.co; Restrepo-Parra, Elisabeth

    2016-07-15

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

  14. 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

  15. Grain damage, phase mixing and plate-boundary formation

    Science.gov (United States)

    Bercovici, David; Skemer, Philip

    2017-07-01

    The generation of plate tectonics on Earth relies on complex mechanisms for shear localization, as well as for the retention and reactivation of weak zones in the cold ductile lithosphere. Pervasive mylonitization, wherein zones of high deformation coincide with extensive mineral grain size reduction, is an important clue to this process. In that regard, the grain-damage model of lithospheric weakening provides a physical framework for both mylonitization and plate generation, and accounts for the competition between grain size reduction by deformation and damage, and healing by grain growth. Zener pinning at the evolving interface between mineral components, such as olivine and pyroxene, plays a key role in helping drive grains to small mylonitic sizes during deformation, and then retards their growth once deformation ceases. The combined effects of damage and pinning, however, rely on the efficiency of inter-grain mixing between phases (e.g., olivine and pyroxene) and grain dispersal, which likely depends on grain size itself. Here we present a new model for inter-grain mixing and damage and the onset of rapid mixing. The model considers the competition between the formation of new grains behind a receding interphase triple junction (e.g., olivine growing into a boundary between two pyroxene grains) and their severance or spalling during progressive deformation and damage. The newly formed grains of one phase are then transported along the opposing phase's grain-boundaries and the two phases become dispersed at the grain-scale in a growing mixed layer. The small intermixed grains also affect the grain evolution of the surrounding host grains by Zener pinning, and hence influence the rheology and growth of the mixed layer. As the grains in the mixed layer shrink, subsequently spalled new grains are also smaller, causing a feedback that leads to more rapid mixing and shear localization in the mixed layer. The early stages of mixing can be compared to laboratory

  16. Theoretical Analysis of Non-equilibrium Grain Boundaries Diffusion Properties Recovery during Ultra-fine Grain Metals and Alloys Annealing

    OpenAIRE

    Vladimir N. Chuvil’deev; Vladimir I. Kopylov; Aleksey V. Nokhrin; Olga E. Pirozhnikova

    2013-01-01

    The article presents the results of theoretical analysis of non-equilibrium grain boundaries diffusion properties recovery during ultra-fine grain (UFG) materials annealing, produced by severe plastic deformation (SPD) method. The paper proves that activation energy and grain boundary diffusion coefficient of UFG materials depend on density of defects, cumulated by grain boundary during SPD.Annealing causes diffusion redistribution of defects in grain boundaries, which results in diffusion pr...

  17. Computed Structure of Grain Boundaries Compared with TEM Observations

    NARCIS (Netherlands)

    Hosson, J.Th.M. De; Heringa, J.R.; Schapink, F.W.; Evans, J.H.; Veen, A. van

    1984-01-01

    Employing computer simulation techniques several studies of the relaxation of atoms in coincidence type grain boundaries have been performed in recent years. Often it is difficult to obtain a clear representation of the relaxed boundary structure, especially in the case of small atomic

  18. Atomistic simulation of dislocation emission in nanosized grain boundaries

    Science.gov (United States)

    Derlet, P. M.; van Swygenhoven, H.; Hasnaoui, A.

    2003-11-01

    The present work deals with the atomic mechanism responsible for the emission of partial dislocations from grain boundaries (GB) in nanocrystalline metals. It is shown that, in a 12 nm grain-size sample, GBs containing grain-boundary dislocations (GBDs) can emit a partial dislocation during deformation by local atomic shuffling and stress-assisted free-volume migration. As in previous work, the nucleation occurs at a GBD, which, upon nucleation and propagation, is removed. In the present case, free-volume migration occurs away from the nucleation region both before and after the nucleation event.

  19. Structure and properties of grain boundaries. Progress report, 15 July 1983-14 July 1984

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1984-03-01

    Results were obtained in the following areas: determination of relative grain boundary energies by the rotating crystallite method; simple structural unit model for core-dependent properties of tilt boundaries; twist boundary energies for metals with long-ranged pairwise interatomic potentials; structural unit/grain boundary dislocation model for grain boundary structure; detection of expansion of boundaries using diffraction; effect of secondary relaxations on diffraction from high-..sigma.. (001) twist boundaries; and mechanism of grain boundary migration.

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

    DEFF Research Database (Denmark)

    Nørbygaard, Thomas

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

  1. Grain Boundary Sliding in Deforming Wehrlite: Rheology and Microstructure

    Science.gov (United States)

    Zhao, N.; Hirth, G.; Cooper, R. F.; Kruckenberg, S. C.

    2016-12-01

    Elastic anisotropy of Earth's upper mantle used to be attributed exclusively to dislocation creep. However, recent experimental results suggest that crystallographic preferred orientation (CPO) in olivine, which contributes to elastic anisotropy, could also form during grain boundary sliding [e.g., 1-3]. Nevertheless, the fundamental problem of how CPO forms during grain boundary sliding is not fully understood. Our current efforts examine the grain-size-sensitive flow of wehrlite, to characterize the influence of the second phase (clinopyroxene) both on olivine CPO formation as well as the propensity of grain boundary sliding and accumulated strain to effect solid-state phase separation (i.e., metamorphic layering). Creep tests on fine-grain-size (2-5 µm) olivine and clinopyroxene aggregates (T =1100-1200ºC; P = 1.5 GPa; γ=3-7) have been conducted. These reveal strong type-B fabric for olivine. Characterization of effects of grain size, temperature and applied strain rate reveal the grain size dependence, stress exponent and activation energy of the flow kinetics of wehrlite. The stress exponent, which is similar to stress exponent for harzburgite reported by Sundberg & Cooper [1], and grain-size dependence suggest that the dominant deformation mechanism in our experiments may be grain boundary sliding. A large stress drop in early segments of experiments suggest an evolution of microstructure. The Fourier transform of backscatter images demonstrates that there exists a direction of foliation, defined by Ol-Cpx heterophase boundaries, which may be the key to understand the development of CPO formation. [1] Sundberg, M. & Cooper, R. F., J. Geophys. Res., 2008. [2] Miyazaki, T., Sueyoshi, K., and Hiraga, T., Nature, 2013. [3] Tielke, J. A., L. N. Hansen, M. Tasaka, C. Meyers, M. E. Zimmerman, and D. L. Kohlstedt, J. Geophys. Res., 2016.

  2. Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Sheikhzada, Ahmad [Old Dominion Univ., Norfolk, VA (United States)

    2017-05-01

    As a major component of linear particle accelerators, superconducting radio-frequency (SRF) resonator cavities are required to operate with lowest energy dissipation and highest accelerating gradient. SRF cavities are made of polycrystalline materials in which grain boundaries can limit maximum RF currents and produce additional power dissipation sources due to local penetration of Josephson vortices. The essential physics of vortex penetration and mechanisms of dissipation of vortices driven by strong RF currents along networks of grain boundaries and their contribution to the residual surface resistance have not been well understood. To evaluate how GBs can limit the performance of SRF materials, particularly Nb and Nb3Sn, we performed extensive numerical simulations of nonlinear dynamics of Josephson vortices in grain boundaries under strong dc and RF fields. The RF power due to penetration of vortices both in weakly-coupled and strongly-coupled grain boundaries was calculated as functions of the RF field and frequency. The result of this calculation manifested a quadratic dependence of power to field amplitude at strong RF currents, an illustration of resistive behavior of grain boundaries. Our calculations also showed that the surface resistance is a complicated function of field controlled by penetration and annihilation of vortices and antivortices in strong RF fields which ultimately saturates to normal resistivity of grain boundary. We found that Cherenkov radiation of rapidly moving vortices in grain boundaries can produce a new instability causing generation of expanding vortex-antivortex pair which ultimately drives the entire GB in a resistive state. This effect is more pronounced in polycrystalline thin film and multilayer coating structures in which it can cause significant increase in power dissipation and results in hysteresis effects in I-V characteristics, particularly at low temperatures.

  3. Faceting of twin grain boundaries in polysilicon films

    Energy Technology Data Exchange (ETDEWEB)

    Nakhodkin, Nikolay; Rodionova, Tatyana [Department of Radiophysics, Kiev National Taras Shevchenko University (Ukraine); Kulish, Nikolay [Department of Physics, Kiev National Taras Shevchenko University (Ukraine)

    2010-02-15

    The faceting of grain boundaries (GB) under annealing in phosphorus-doped polysilicon films, produced by low-pressure chemical vapor deposition, has been investigated by transmission electron microscopy. It has been shown that the facet types and facet density depend on annealing temperature. It has been found that GB facets are generally parallel with close-packed planes in coincidence site lattice. A correlation between GB faceting and grain-growth mechanisms has been considered. It has been shown that faceting takes place both under normal and abnormal grain growth. It is revealed that twinning plays the key role for GB faceting under normal grain growth. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

  4. Structural disjoining potential for grain-boundary premelting and grain coalescence from molecular-dynamics simulations

    Science.gov (United States)

    Fensin, Saryu J.; Olmsted, David; Buta, Dorel; Asta, Mark; Karma, Alain; Hoyt, J. J.

    2010-03-01

    We describe a molecular-dynamics framework for the direct calculation of the short-ranged structural forces underlying grain-boundary premelting and grain coalescence in solidification. The method is applied in a comparative study of (i) a Σ9⟨115⟩120° twist and (ii) a Σ9⟨110⟩{411} symmetric tilt boundary in a classical embedded-atom model of elemental Ni. Although both boundaries feature highly disordered structures near the melting point, the nature of the temperature dependence of the width of the disordered regions in these boundaries is qualitatively different. The former boundary displays behavior consistent with a logarithmically diverging premelted layer thickness as the melting temperature is approached from below, while the latter displays behavior featuring a finite grain-boundary width at the melting point. It is demonstrated that both types of behavior can be quantitatively described within a sharp-interface thermodynamic formalism involving a width-dependent interfacial free energy, referred to as the disjoining potential. The disjoining potential for boundary (i) is calculated to display a monotonic exponential dependence on width, while that of boundary (ii) features a weak attractive minimum. The results of this work are discussed in relation to recent simulation and theoretical studies of the thermodynamic forces underlying grain-boundary premelting.

  5. 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.

  6. Cross-sectional measurement of grain boundary segregation using WDS

    Energy Technology Data Exchange (ETDEWEB)

    Christien, F., E-mail: frederic.christien@emse.fr [Laboratoire Georges Friedel, CNRS, Ecole des Mines de Saint-Etienne, 158 Cours Fauriel, 42023 Saint-Etienne (France); Risch, P. [Institut des Matériaux Jean Rouxel (IMN), CNRS, Université de Nantes, Rue Christian Pauc, 44306 Nantes (France)

    2016-11-15

    A new method is proposed for the quantification of grain boundary segregation using Wavelength Dispersive Spectroscopy (WDS) in a Scanning Electron Microscope (SEM). Analyses are undertaken on a simple metallographically polished section of material. The method is demonstrated for the model system of sulphur segregation to nickel grain boundaries. Quantification was carried out from sulphur concentration profiles acquired across 11 grain boundaries of a nickel specimen containing 5.4 wt ppm of sulphur in the bulk and equilibrated at 550 °C. The average sulphur grain boundary concentration determined is µ=35.2 ng cm{sup −2}=6.6×10{sup 14} atoms cm{sup −2}≈0.5 monolayer, which is in good agreement with a previous quantification obtained from SIMS (Secondary Ion Mass Spectrometry) on the same material. However this is lower by a factor of two than the quantification obtained using “surface” techniques on fractured specimens of the same material. With the conditions of analysis used in this study, the limit of detection of the method developed is found to be better than 10% of a sulphur monolayer. - Highlights: • Impurity grain boundary segregation can be measured using WDS in a SEM. • The method proposed is quantitative. • The specimen preparation is simple: metallographical section.

  7. Theoretical Analysis of Non-equilibrium Grain Boundaries Diffusion Properties Recovery during Ultra-fine Grain Metals and Alloys Annealing

    Directory of Open Access Journals (Sweden)

    Vladimir N. Chuvil’deev

    2013-01-01

    Full Text Available The article presents the results of theoretical analysis of non-equilibrium grain boundaries diffusion properties recovery during ultra-fine grain (UFG materials annealing, produced by severe plastic deformation (SPD method. The paper proves that activation energy and grain boundary diffusion coefficient of UFG materials depend on density of defects, cumulated by grain boundary during SPD.Annealing causes diffusion redistribution of defects in grain boundaries, which results in diffusion properties change. Diffusion properties recovery rate depends on grain size and it is much higher in UFG materials than in coarse-grained materials.

  8. Measurement of grain boundary triple line energy in copper

    Energy Technology Data Exchange (ETDEWEB)

    Zhao, B. [Institut fuer Metallkunde und Metallphysik, RWTH Aachen University, 52056 Aachen (Germany); Verhasselt, J.Ch. [Sparing Roehl Henseler, Rethelstr. 123, 40237 Duesseldorf (Germany); Shvindlerman, L.S. [Institut fuer Metallkunde und Metallphysik, RWTH Aachen University, 52056 Aachen (Germany)] [Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow District 142432 (Russian Federation)] [State Technological University, Moscow Institute of Steel and Alloys (Russian Federation); Gottstein, G., E-mail: Gottstein@imm.rwth-aachen.de [Institut fuer Metallkunde und Metallphysik, RWTH Aachen University, 52056 Aachen (Germany)

    2010-10-15

    Recent studies have demonstrated that grain boundary triple junctions are crystal defects with specific thermodynamic and kinetic properties. In this study we address the energy of triple lines. Previously, a geometrical model was proposed to determine the grain boundary line tension. The current study introduces a thermodynamically correct approach which allows direct and precise measurement of the triple line energy. The experimental technique utilizes the measurement of the surface topography of a crystal in the vicinity of a triple junction by atomic force microscopy. The grain boundary triple line tension {gamma}{sub TP}{sup l} of a random triple line in a copper tricrystal was measured to be 6.3 {+-} 2.8 x 10{sup -9} J m{sup -1}.

  9. The sensitivity of diffraction profiles to grain boundary segregation

    Energy Technology Data Exchange (ETDEWEB)

    Bristowe, P.D. (Massachusetts Inst. of Tech., Cambridge, MA (United States). Dept. of Materials Science and Engineering)

    1991-01-01

    In this paper, the relrod profiles of a number of weak grain boundary reflections are computed with and without the presence of substitutional impurities, The {Epsilon}13 (22.6{degrees}) (001) twist in gold is chosen since, for this system, a number of relrod profiles have been measured that exhibit fine structure. To perform the calculations, a molecular statics method is used together with embedded-atom potentials. The first objective is to compare the measured and computed relrod profiles for pure gold to establish that the fine structure is intrinsic to the grain boundary. The second objective is to determine by calculation the sensitivity of these relrods to the presence of low concentrations of impurity, in this case nickel. It will be seen that many of the relrods respond in such a way as to make them useful indicator of grain boundary segregation.

  10. Grain boundary assisted photocurrent collection in thin film solar cells

    Directory of Open Access Journals (Sweden)

    Harndt Susanna

    2015-01-01

    Full Text Available The influence of absorber grain boundaries on the photocurrent transport in chalcopyrite based thin film solar cells has been calculated using a two dimensional numerical model. Considering extreme cases, the variation in red response is more expressed than in one dimensional models. These findings may offer an explanation for the strong influence of buffer layer preparation on the spectral response of cells with small grained absorbers.

  11. (Investigations of ultrasonic wave interactions with grain boundaries and grain imperfections)

    Energy Technology Data Exchange (ETDEWEB)

    1990-01-01

    The main objective of our research is to obtain a better understanding of ultrasonic wave interaction with interfaces in polycrystalline materials. This report discusses two recently developed experimental techniques: scanning acoustic microscope and optical point sensors. As for general wave propagation problems in anisotropic media, four major topics are discussed in separate sections. First, single boundaries between large bicrystals are considered. The reflection and transmission coefficients of such interfaces are calculated for imperfect boundary conditions by using the finite interface stiffness approach. Ultrasonic transmission through multiple-grain structures are investigated by computer simulation based on the statistical evaluation of repeated acoustical wave interactions with individual grain boundaries. The number of grains interacting with the propagating acoustical wave is considered to be high enough to approximate the wave-material interaction as scattering on elastic inhomogeneities. The grain scattering induced attenuation of Rayleigh waves is investigated in polycrystalline materials. 41 refs., 43 figs.

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

    DEFF Research Database (Denmark)

    Jones, A R.

    1981-01-01

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

  13. 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.

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

    DEFF Research Database (Denmark)

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

    2009-01-01

    A general, twisted and tilted, grain boundary in copper has been simulated using the adaptive kinetic Monte Carlo method to study the atomistic structure of the non-crystalline region and the mechanism of annealing events that occur at low temperature. The simulated time interval spanned 67 mu s...... at 135 K. Similar final configurations were obtained starting from different initial structures: (i) by bringing the two grains into contact without any intermediate layer, and (ii) by inserting an amorphous region between the grains. The results obtained were analyzed with a radial distribution function...

  15. Structural complexity in grain boundaries with covalent bonding

    Energy Technology Data Exchange (ETDEWEB)

    Tarnow, E.; Dallot, P.; Bristowe, P.D.; Joannopoulos, J.D. (Massachusetts Institute of Technology, Cambridge, MA (USA)); Francis, G.P.; Payne, M.C. (Cavendish Laboratory, University of Cambridge, Cambridge (United Kingdom))

    1990-08-15

    The structural properties of two short-period twist boundaries in germanium are explored using a state-of-the-art total-energy calculation. The structures of these boundaries are found to be very complex, with boundary bonds that are distorted and weak. These systems are found to exhibit a large degeneracy in the number of local energy minima. Thus the boundaries have difficulty in arriving at a locally ordered state. The situation may be unique to the {ital semiconducting} {ital twist} grain boundaries due to the inherent frustration present between the tendency to form directional bonds and the imposed twist geometry which makes the bond formation improbable. This study focuses on the energy, coordination, volume change, and electronic states characteristic of the local minima. A trend towards dimerization is found especially in the highest-angle twist boundary.

  16. Structural complexity in grain boundaries with covalent bonding

    Science.gov (United States)

    Tarnow, E.; Dallot, P.; Bristowe, P. D.; Joannopoulos, J. D.; Francis, G. P.; Payne, M. C.

    1990-08-01

    The structural properties of two short-period twist boundaries in germanium are explored using a state-of-the-art total-energy calculation. The structures of these boundaries are found to be very complex, with boundary bonds that are distorted and weak. These systems are found to exhibit a large degeneracy in the number of local energy minima. Thus the boundaries have difficulty in arriving at a locally ordered state. The situation may be unique to the semiconducting twist grain boundaries due to the inherent frustration present between the tendency to form directional bonds and the imposed twist geometry which makes the bond formation improbable. This study focuses on the energy, coordination, volume change, and electronic states characteristic of the local minima. A trend towards dimerization is found especially in the highest-angle twist boundary.

  17. Phase-field crystal study of grain-boundary premelting

    Science.gov (United States)

    Mellenthin, Jesper; Karma, Alain; Plapp, Mathis

    2008-11-01

    We study the phenomenon of grain-boundary premelting for temperatures below the melting point in the phase-field crystal model of a pure material with hexagonal ordering in two dimensions. We investigate the structures of symmetric tilt boundaries as a function of misorientation θ for two different inclinations and compute in the grand canonical ensemble the “disjoining potential” V(w) that describes the fundamental interaction between crystal-melt interfaces as a function of the premelted layer width w , which is defined here in terms of the excess mass of the grain boundary via a Gibbs construction. The results reveal qualitatively different behaviors for high-angle grain boundaries that are uniformly wetted, with w diverging logarithmically as the melting point is approached from below, and low-angle boundaries that are punctuated by liquid pools surrounding dislocations, separated by solid bridges. The latter persist over a superheated range of temperature. This qualitative difference between high- and low-angle boundaries is reflected in the w dependence of the disjoining potential that is purely repulsive [ V'(w)<0 for all w ] for misorientations larger than a critical angle θc , but switches from repulsive at small w to attractive at large w for θ<θc . In the latter case, V(w) has a minimum that corresponds to a premelted boundary of finite width at the melting point. Furthermore, we find that the standard wetting condition γgb(θc)=2γsl gives a much too low estimate of θc when a low-temperature value of the grain-boundary energy γgb is used. In contrast, a reasonable lower-bound estimate can be obtained if γgb is extrapolated to the melting point, taking into account both the elastic softening of the material at high homologous temperature and local melting around dislocations.

  18. 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.

  19. Dislocation Nucleation on Grain Boundaries: Low Angle Twist and Asymmetric Tilt Boundaries

    Directory of Open Access Journals (Sweden)

    Erman Guleryuz

    2016-07-01

    Full Text Available We investigate the mechanisms of incipient plasticity at low angle twist and asymmetric tilt boundaries in fcc metals. To observe plasticity of grain boundaries independently of the bulk plasticity, we simulate nanoindentation of bicrystals. On the low angle twist boundaries, the intrinsic grain boundary (GB dislocation network deforms under load until a dislocation segment compatible with glide on a lattice slip plane is created. The half loops are then emitted into the bulk of the crystal. Asymmetric twist boundaries considered here did not produce bulk dislocations under load. Instead, the boundary with a low excess volume nucleated a mobile GB dislocation and additional GB defects. The GB sliding proceeded by motion of the mobile GB dislocation. The boundary with a high excess volume sheared elastically, while bulk-nucleated dislocations produced plastic relaxation.

  20. Diffusive Fractionation of Lithium Isotopes in Olivine Grain Boundaries

    Science.gov (United States)

    Homolova, V.; Watson, E. B.

    2012-12-01

    Diffusive fractionation of isotopes has been documented in silicate melts, aqueous fluids, and single crystals. In polycrystalline rocks, the meeting place of two grains, or grain boundaries, may also be a site of diffusive fractionation of isotopes. We have undertaken an experimental and modeling approach to investigate diffusive fractionation of lithium (Li) isotopes by grain boundary diffusion. The experimental procedure consists of packing a Ni metal capsule with predominantly ground San Carlos olivine and subjecting the capsule to 1100C and 1GPa for two days in a piston cylinder apparatus to create a nominally dry, 'dunite rock'. After this synthesis step, the capsule is sectioned and polished. One of the polished faces of the 'dunite rock' is then juxtaposed to a source material of spodumene and this diffusion couple is subject to the same experimental conditions as the synthesis step. Li abundances and isotopic profiles (ratios of count rates) were analyzed using LA-ICP-MS. Li concentrations linearly decrease away from the source from 550ppm to the average concentration of the starting olivine (2.5ppm). As a function of distance from the source, the 7Li/6Li ratio decreases to a minimum before increasing to the background ratio of the 'dunite rock'. The 7Li/6Li ratio minimum coincides with the lowest Li concentrations above average 'dunite rock' abundances. The initial decrease in the 7Li/6Li ratio is similar to that seen in other studies of diffusive fractionation of isotopes and is thought to be caused by the higher diffusivity (D) of the lighter isotope relative to the heavier isotope. The relationship between D and mass (m) is given by (D1/D2) =(m2/m1)^β, where β is an empirical fractionation factor; 1 and 2 denote the lighter and heavier isotope, respectively. A fit to the Li isotopic data reveals an effective DLi of ~1.2x10^-12 m/s^2 and a β of 0.1. Numerical modelling was utilized to elucidate the relationship between diffusive fractionation

  1. 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

  2. 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...

  3. Thermal activated grain boundary creep in polycrystalline copper ...

    African Journals Online (AJOL)

    Creep deformation in metals and alloys at intermediate temperatures and low stresses are attributed to power-law and diffusion mechanisms. ... By superposition of the rate equations, the net strain rate is determined as the sum of three independent creep mechanisms of Cobble diffusion creep, gra-in boundary sliding and ...

  4. 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...

  5. 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.

  6. A grain boundary sliding model for cavitation, crack growth and ...

    African Journals Online (AJOL)

    A model is presented for cavity growth, crack propagation and fracture resulting from grain boundary sliding (GBS) during high temperature creep deformation. The theory of cavity growth by GBS was based on energy balance criteria on the assumption that the matrix is sufficiently plastic to accommodate misfit strains ...

  7. Formation of intra-island grain boundaries in pentacene monolayers

    NARCIS (Netherlands)

    Zhang, Jian; Wu, Yu; Duhm, Steffen; Rabe, Juergen P.; Rudolf, Petra; Koch, Norbert; Rabe, Jürgen P.

    2011-01-01

    To assess the formation of intra-island grain boundaries during the early stages of pentacene film growth, we studied sub-monolayers of pentacene on pristine silicon oxide and silicon oxide with high pinning centre density (induced by UV/O(3) treatment). We investigated the influence of the kinetic

  8. Atomistic studies of grain boundaries in alloys and compounds

    Energy Technology Data Exchange (ETDEWEB)

    Vitek, V.

    1992-02-01

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

  9. The Potential Link Between High Angle Grain Boundary Morphology and Grain Boundary Deformation in a Nickel-Based Superalloy (Postprint)

    Science.gov (United States)

    2015-06-01

    of strain accumulation at grain boundaries of nickel-based superalloys, in: Superalloys 2012, TMS, Warendale, PA. [5] T. Turner, P. Shade, M. Groeber ...1993) 2021–2024, http://dx.doi. org/10.1116/1.586537, URL WOS:A1993MM97200011. [10] M.D. Uchic, M.A. Groeber , D.M. Dimiduk, J. Simmons, 3D

  10. 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

  11. Ab initio study of a grain boundary in gold

    Science.gov (United States)

    Needels, M.; Rappe, A. M.; Bristowe, P. D.; Joannopoulos, J. D.

    1992-10-01

    The total energy of a grain boundary in a transition metal is calculated in an ab initio manner. Such calculations are now feasible using a plane-wave basis set provided that an optimally convergent pseudopotential is used. The investigation focuses on resolving two competing atomic models for the Σ=5 [001] twist boundary in gold. It is found that the model of lower energy, which involves small atomic displacements, corresponds to a structure determined both experimentally, using quantitative x-ray-diffraction techniques, and theoretically, using the embedded-atom method.

  12. Ab initio study of a grain boundary in gold

    Energy Technology Data Exchange (ETDEWEB)

    Needels, M.; Rappe, A.M.; Bristowe, P.D.; Joannopoulos, J.D. (Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States))

    1992-10-15

    The total energy of a grain boundary in a transition metal is calculated in an {ital ab} {ital initio} manner. Such calculations are now feasible using a plane-wave basis set provided that an optimally convergent pseudopotential is used. The investigation focuses on resolving two competing atomic models for the {Sigma}=5 (001) twist boundary in gold. It is found that the model of lower energy, which involves small atomic displacements, corresponds to a structure determined both experimentally, using quantitative x-ray-diffraction techniques, and theoretically, using the embedded-atom method.

  13. Grain boundary embrittlement and cohesion enhancement in copper

    Energy Technology Data Exchange (ETDEWEB)

    Paxton, Anthony; Lozovoi, Alexander [Atomistic Simulation Centre, Queen' s University Belfast, BT7 1NN (United Kingdom); Schweinfest, Rainer [Science+Computing ag, Hagellocher Weg 71-5, 720270 T ubingen (Germany); Finnis, Michael [Imperial College London, Exhibition Road, London SW7 2AZ (United Kingdom)

    2008-07-01

    There has been a long standing debate surrounding the mechanism of grain boundary embrittlement and cohesion enhancement in metals. Embrittlement can lead to catastrophic failure such as happened in the Hinkley Point disaster, or indeed in the case of the Titanic. This kind of embrittlement is caused by segregation of low solubility impurities to grain boundaries. While the accepted wisdom is that this is a phenomenon driven by electronic or chemical factors, using language such as charge transfer and electronegativity difference; we believe that in copper, at least, both cohesion enhancement and reduction are caused by a simple size effect. We have developed a theory that allows us to separate unambiguously, if not uniquely, chemical and structural factors. We have studied a large number of solutes in copper using first principles atomistic simulation to support this argument, and the results of these calculations are presented here.

  14. Multilevel model of polycrystalline materials: grain boundary sliding description

    Science.gov (United States)

    Sharifullina, E.; Shveykin, A.; Trusov, P.

    2017-12-01

    Material behavior description in a wide range of thermomechanical effects is one of the topical areas in mathematical modeling. Inclusion of grain boundary sliding as an important mechanism of polycrystalline material deformation at elevated temperatures and predominant deformation mechanism of metals and alloys in structural superplasticity allows to simulate various deformation regimes and their transitions (including superplasticity regime with switch-on and switch-off regimes). The paper is devoted to description of grain boundary sliding in structure of two-level model, based on crystal plasticity, and relations for determination the contribution of this mechanism to inelastic deformation. Some results are presented concerning computational experiments of polycrystalline representative volume deformation using developed model.

  15. 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.

  16. Z-contrast imaging and grain boundaries in semiconductors

    Energy Technology Data Exchange (ETDEWEB)

    Chisholm, M.F.; Pennycook, S.J.

    1996-03-01

    Interest in grain boundaries in semiconductors is linked to the application of polycrystalline semiconductors as photovoltaic and interconnect materials. In real devices such as solar cells and MOS structures as well as future devices such as flat-panel displays, the intergranular regions of the polycrystalline solid have a significant effect on the flow of electronic current. These grain boundary barriers exist because the chemical potential of the boundary atoms are shifted from the bulk value by the change in local symmetry. The chemical potential is also changed by impurities, other structural defects, and other phases in the boundary. The lack of knowledge on the atomic structure of grain boundaries is, at present, the greatest barrier to advancements in the understanding of the electrical properties of these defects. The advances of the last few years have provided the tools with which to probe these interfaces at the true atomic scale. One such tool is the high-resolution scanning transmission electron microscope installed at Oak Ridge National Laboratory (VG Microscopes HB603) that can form a 1.27 {Angstrom} electron probe. Images are formed by scanning the probe across a thin sample and using an annular detector to collect electrons scattered to high angles. Because the annular detector collects electrons scattered over a wide range of angles, phase correlation and dynamical diffraction effects are averaged by this annular integration. Thus, an image with incoherent characteristics is produced and retained to relatively large specimen thickness. The key advantage of incoherent imaging is that when the microscope is focused to produce maximum image contrast, the bright image features directly correspond to the positions of the atomic columns.

  17. Segregation-induced ordered superstructures at general grain boundaries in a nickel-bismuth alloy

    Science.gov (United States)

    Yu, Zhiyang; Cantwell, Patrick R.; Gao, Qin; Yin, Denise; Zhang, Yuanyao; Zhou, Naixie; Rohrer, Gregory S.; Widom, Michael; Luo, Jian; Harmer, Martin P.

    2017-10-01

    The properties of materials change, sometimes catastrophically, as alloying elements and impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that regular atomic patterns often arise as a result of this solute segregation at high-symmetry boundaries, but it is not known whether superstructures exist at general grain boundaries in polycrystals. In bismuth-doped polycrystalline nickel, we found that ordered, segregation-induced grain boundary superstructures occur at randomly selected general grain boundaries, and that these reconstructions are driven by the orientation of the terminating grain surfaces rather than by lattice matching between grains. This discovery shows that adsorbate-induced superstructures are not limited to special grain boundaries but may exist at a variety of general grain boundaries, and hence they can affect the performance of polycrystalline engineering alloys.

  18. Physics of grain boundaries in polycrystalline photovoltaic semiconductors

    Science.gov (United States)

    Yan, Yanfa; Yin, Wan-Jian; Wu, Yelong; Shi, Tingting; Paudel, Naba R.; Li, Chen; Poplawsky, Jonathan; Wang, Zhiwei; Moseley, John; Guthrey, Harvey; Moutinho, Helio; Pennycook, Stephen J.; Al-Jassim, Mowafak M.

    2015-03-01

    Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 (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.

  19. 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.

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

    Energy Technology Data Exchange (ETDEWEB)

    Taylor, Seth Thomas [Univ. of California, Berkeley, CA (United States)

    2002-01-01

    Solute segregation to ceramic grain boundaries governs material processing and microstructure evolution, and can strongly influence material properties critical to engineering performance. Understanding the evolution and implications of grain boundary chemistry is a vital component in the greater effort to engineer ceramics with controlled microstructures. This study examines solute segregation to engineered grain boundaries in titanium-doped sapphire (Al2O3) bicrystals, and explores 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.

  1. 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.

  2. Grain Boundary Phase Transformations in Nanostructured Conducting Oxides

    Science.gov (United States)

    Straumal, B. B.; Myatiev, A. A.; Straumal, P. B.; Mazilkin, A. A.

    Nanostructured conducting oxides are very promising for various applications like varistors (doped zinc oxide), electrolytes for the solid oxide fuel cells (SOFC) (ceria, zirconia, yttria), semipermeable membranes, and sensors (perovskite-type oxides). Grain boundary (GB) phases crucially determine the properties of nanograined oxides. GB phase transformations (wetting, prewetting, pseudopartial wetting) proceed in the conducting oxides. Novel GB lines appear in the conventional bulk phase diagrams. They can be used for the tailoring of properties of nanograined conducting oxides, particularly by using the novel synthesis method of liquid ceramics.

  3. Dislocation-Pairing Transitions in Hot Grain Boundaries

    Science.gov (United States)

    Olmsted, David L.; Buta, Dorel; Adland, Ari; Foiles, Stephen M.; Asta, Mark; Karma, Alain

    2011-01-01

    We report the finding of a novel grain-boundary structural phase transition in both molecular-dynamics and phase-field-crystal simulations of classical models of bcc Fe. This transition is characterized by pairing of individual dislocations with mixed screw and edge components. We demonstrate that this type of transition is driven by a combination of factors including elastic softening, core interaction, and core disordering. At high homologous temperatures the occurrence of this transition is shown to prevent premelting at misorientation angles where it would otherwise be expected.

  4. Tailoring Electrical Transport Across Grain Boundaries in Polycrystalline Graphene

    Science.gov (United States)

    Tsen, Adam W.; Brown, Lola; Levendorf, Mark P.; Ghahari, Fereshte; Huang, Pinshane Y.; Havener, Robin W.; Ruiz-Vargas, Carlos S.; Muller, David A.; Kim, Philip; Park, Jiwoong

    2012-06-01

    Graphene produced by chemical vapor deposition (CVD) is polycrystalline, and scattering of charge carriers at grain boundaries (GBs) could degrade its performance relative to exfoliated, single-crystal graphene. However, the electrical properties of GBs have so far been addressed indirectly without simultaneous knowledge of their locations and structures. We present electrical measurements on individual GBs in CVD graphene first imaged by transmission electron microscopy. Unexpectedly, the electrical conductance improves by one order of magnitude for GBs with better interdomain connectivity. Our study suggests that polycrystalline graphene with good stitching may allow for uniformly high electrical performance rivaling that of exfoliated samples, which we demonstrate using optimized growth conditions and device geometry.

  5. 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.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    1999-08-06

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

  7. 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.

  8. 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.

  9. Relationship between Equilibrium Fluctuations and Shear-Coupled Motion of Grain Boundaries

    Science.gov (United States)

    Karma, Alain; Trautt, Zachary T.; Mishin, Yuri

    2012-08-01

    We derive general analytical expressions relating the equilibrium fluctuations of a grain boundary to key parameters governing its motion coupled to shear deformation. We validate these expressions by molecular dynamics simulations for symmetrical tilt boundaries and demonstrate how they can be used to extract the misorientation dependence of the grain-boundary mobility. The results shed light on fundamental relationships between equilibrium and nonequilibrium grain-boundary properties and provide new means to predict those properties.

  10. 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.

  11. 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.

  12. Achieving Radiation Tolerance through Non-Equilibrium Grain Boundary Structures.

    Science.gov (United States)

    Vetterick, Gregory A; Gruber, Jacob; Suri, Pranav K; Baldwin, Jon K; Kirk, Marquis A; Baldo, Pete; Wang, Yong Q; Misra, Amit; Tucker, Garritt J; Taheri, Mitra L

    2017-09-25

    Many methods used to produce nanocrystalline (NC) materials leave behind non-equilibrium grain boundaries (GBs) containing excess free volume and higher energy than their equilibrium counterparts with identical 5 degrees of freedom. Since non-equilibrium GBs have increased amounts of both strain and free volume, these boundaries may act as more efficient sinks for the excess interstitials and vacancies produced in a material under irradiation as compared to equilibrium GBs. The relative sink strengths of equilibrium and non-equilibrium GBs were explored by comparing the behavior of annealed (equilibrium) and as-deposited (non-equilibrium) NC iron films on irradiation. These results were coupled with atomistic simulations to better reveal the underlying processes occurring on timescales too short to capture using in situ TEM. After irradiation, NC iron with non-equilibrium GBs contains both a smaller number density of defect clusters and a smaller average defect cluster size. Simulations showed that excess free volume contribute to a decreased survival rate of point defects in cascades occurring adjacent to the GB and that these boundaries undergo less dramatic changes in structure upon irradiation. These results suggest that non-equilibrium GBs act as more efficient sinks for defects and could be utilized to create more radiation tolerant materials in future.

  13. Effect of grain size and grain boundary defects on electrical and magnetic properties of Cr doped ZnO nanoparticles

    Science.gov (United States)

    Aljawfi, Rezq Naji; Rahman, F.; Batoo, Khalid M.

    2014-05-01

    Nanostructure of Zn1-xCrxO (x = 0.0, 0.05 and 0.1) were synthesized successfully through sol-gel route. The effects of grain size and grain boundary defects on the electrical and magnetic properties have been investigated. X-ray diffraction (XRD) and selected area electron diffraction (SAED) results reveal the single-phase character. Crystallite sizes were obtained from the XRD patterns whose values are decreasing from ˜27 to ˜16 nm with increase in Cr content from 0% to 10% respectively. X-ray photoelectron spectroscopy (XPS) confirms the incorporation of Cr3+/Cr2+ ions in the lattice structure of ZnO, which causes decreasing in the valence electron density of Zn. Dielectric constant (ɛ) has been explained in the light of Maxwell-Wagner interfacial model, which differentiates between the structure of grain-core and grain-boundary. Complex impedance spectroscopy has been used to separate the grain and grain boundary contributions, the high resistivity values (107 Ω) can be attributed to the dominance of grain boundary resistance. The samples exhibit room temperature ferromagnetic (RT-FM) behavior, which has been discussed based on BMP model and effect of grain/grain boundary structure.

  14. Multiscale model of metal alloy oxidation at grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Sushko, Maria L., E-mail: maria.sushko@pnnl.gov; Alexandrov, Vitaly; Schreiber, Daniel K.; Rosso, Kevin M.; Bruemmer, Stephen M. [Pacific Northwest National Laboratory, Richland, Washington 99352 (United States)

    2015-06-07

    High temperature intergranular oxidation and corrosion of metal alloys is one of the primary causes of materials degradation in nuclear systems. In order to gain insights into grain boundary oxidation processes, a mesoscale metal alloy oxidation model is established by combining quantum Density Functional Theory (DFT) and mesoscopic Poisson-Nernst-Planck/classical DFT with predictions focused on Ni alloyed with either Cr or Al. Analysis of species and fluxes at steady-state conditions indicates that the oxidation process involves vacancy-mediated transport of Ni and the minor alloying element to the oxidation front and the formation of stable metal oxides. The simulations further demonstrate that the mechanism of oxidation for Ni-5Cr and Ni-4Al is qualitatively different. Intergranular oxidation of Ni-5Cr involves the selective oxidation of the minor element and not matrix Ni, due to slower diffusion of Ni relative to Cr in the alloy and due to the significantly smaller energy gain upon the formation of nickel oxide compared to that of Cr{sub 2}O{sub 3}. This essentially one-component oxidation process results in continuous oxide formation and a monotonic Cr vacancy distribution ahead of the oxidation front, peaking at alloy/oxide interface. In contrast, Ni and Al are both oxidized in Ni-4Al forming a mixed spinel NiAl{sub 2}O{sub 4}. Different diffusivities of Ni and Al give rise to a complex elemental distribution in the vicinity of the oxidation front. Slower diffusing Ni accumulates in the oxide and metal within 3 nm of the interface, while Al penetrates deeper into the oxide phase. Ni and Al are both depleted from the region 3–10 nm ahead of the oxidation front creating voids. The oxide microstructure is also different. Cr{sub 2}O{sub 3} has a plate-like structure with 1.2–1.7 nm wide pores running along the grain boundary, while NiAl{sub 2}O{sub 4} has 1.5 nm wide pores in the direction parallel to the grain boundary and 0.6 nm pores in the perpendicular

  15. Observations of dislocation interactions with recrystallized grain boundaries

    Science.gov (United States)

    Price, C. W.

    1988-08-01

    Possible evidence of the interaction of individual dislocations with recrystallized grain boundaries (GBs) was found in aluminum specimens that had been statically recrystallized after hot deformation. Matrix dislocations from the deformed material appeared to terminate on the recrystallized GB; however, GB dislocations (GBDs) that should have formed from the absorption of the matrix dislocations were not present. In addition, subboundary structure immediately in front of the moving GB appeared to have undergone accelerated dissociation. These observations suggest that rapid GB motion is not associated with lateral movement of GBDs, and short-range dislocation-GB interaction forces, probably image forces, could cause climb-induced vacancy supersaturations to form in front of the GB. This supersaturated vacancy atmosphere could constitute an important contribution to rapid GB migration.

  16. Dislocations and Grain Boundaries in Semiconducting Rubrene Single-Crystals

    Energy Technology Data Exchange (ETDEWEB)

    Chapman,B.; Checco, A.; Pindak, R.; Siegrist, T.; Kloc, C.

    2006-01-01

    Assessing the fundamental limits of the charge carrier mobilities in organic semiconductors is important for the development of organic electronics. Although devices such as organic field effect transistors (OFETs), organic thin film transistors (OTFTs) and organic light emitting diodes (OLEDs) are already used in commercial applications, a complete understanding of the ultimate limitations of performance and stability in these devices is still lacking at this time. Crucial to the determination of electronic properties in organic semiconductors is the ability to grow ultra-pure, fully ordered molecular crystals for measurements of intrinsic charge transport. Likewise, sensitive tools are needed to evaluate crystalline quality. We present a high-resolution X-ray diffraction and X-ray topography analysis of single-crystals of rubrene that are of the quality being reported to show mobilities as high as amorphous silicon. We show that dislocations and grain boundaries, which may limit charge transfer, are prominent in these crystals.

  17. Computer simulations of the structure and properties of grain boundary dislocations

    Energy Technology Data Exchange (ETDEWEB)

    Bristowe, P.D.

    1985-10-01

    The core structure of intrinsic grain boundary dislocations embedded in (001) twist grain boundaries has been modelled using conventional atomistic techniques and a variety of empirical and pseudo-potentials representing fcc metals. In particular the width of the dislocation core region in the boundary plane has been studied as a function of the periodicity of the boundary ..sigma.. and interatomic potential. Generally, the core width increases with increasing ..sigma.. and is wider than that of lattice dislocations. Also the core width of twist grain boundary dislocations can be wider than that of tilt grain boundary dislocations. The presence of resolvable cusps on the computed energy/misorientation curve is related to the degree of core localization and the formation of domains of relatively low energy structure. The results are compared to transmission electron microscope observations, previous calculations, and the structural unit/grain boundary dislocation model.

  18. Phonon thermal transport through tilt grain boundaries in strontium titanate

    Energy Technology Data Exchange (ETDEWEB)

    Zheng, Zexi; Chen, Xiang; Yang, Shengfeng; Xiong, Liming; Chen, Youping [Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611 (United States); Deng, Bowen; Chernatynskiy, Aleksandr [Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 (United States)

    2014-08-21

    In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO{sub 3}. Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO{sub 3} contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scattering at the GBs, but the absence of certain modes in the GB region appears to be responsible for phonon reflections at GBs. This work has also demonstrated phonon mode conversion and simultaneous generation of new modes. Some of the new modes have the same frequency as the initial wave packet, while some have the same wave vector but lower frequencies.

  19. Confined and chemically flexible grain boundaries in polycrystalline compound semiconductors

    Energy Technology Data Exchange (ETDEWEB)

    Abou-Ras, Daniel; Schmidt, Sebastian S.; Caballero, Raquel; Unold, Thomas [Helmholtz-Zentrum Berlin fuer Materialien und Energie GmbH, Berlin (Germany); Schock, Hans-Werner [Department of Engineering, George Holt Building, Liverpool (United Kingdom); Koch, Christoph T. [Max-Planck Institute for Intelligent Systems, Stuttgart (Germany); Schaffer, Bernhard [SuperSTEM, STFC Daresbury Laboratories, Keckwick Lane, Warrington (United Kingdom); SUPA School of Physics and Astronomy, University of Glasgow, Glasgow (United Kingdom); Schaffer, Miroslava [SuperSTEM, STFC Daresbury Laboratories, Keckwick Lane, Warrington (United Kingdom); Department of Engineering, George Holt Building, Liverpool (United Kingdom); Choi, Pyuck-Pa; Cojocaru-Miredin, Oana [Max-Planck-Institut fuer Eisenforschung GmbH, Department of Microstructure Physics and Alloy Design, Duesseldorf (Germany)

    2012-08-15

    Grain boundaries (GBs) in polycrystalline Cu(In,Ga)Se{sub 2} thin films exhibit only slightly enhanced recombination, as compared with the grain interiors, allowing for very high power-conversion efficiencies of more than 20% in the corresponding solar-cell devices. This work highlights the specific compositional and electrical properties of Cu(In,Ga)Se{sub 2} GBs by application of appropriate subnanometer characterisation techniques: inline electron holography, electron energy-loss spectroscopy, and atom-probe tomography. It is found that changes of composition at the GBs are confined to regions of only about 1 nm in width. Therefore, these compositional changes are not due to secondary phases but atomic or ionic redistribution within the atomic planes close to the GBs. For different GBs in the Cu(In,Ga)Se{sub 2} thin film investigated, different atomic or ionic redistributions are also found. This chemical flexibility makes polycrystalline Cu(In,Ga)Se{sub 2} thin films particularly suitable for photovoltaic applications. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

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

    Energy Technology Data Exchange (ETDEWEB)

    Hanning, F., E-mail: Fabian.Hanning@googlemail.com; Engelberg, D.L., E-mail: Dirk.engelberg@manchester.ac.uk

    2014-08-15

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

  1. 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...

  2. Progress report on grain boundaries, February 15, 1990--October 15, 1990

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1990-01-01

    The present document is a progress report describing the work accomplished to date during the first year of our four-year grant (February 15, 1990--February 14, 1994) to study grain boundaries. The research was focused on the following four major efforts: study of the aromatic structure of grain boundaries by means of x-ray diffraction, transmission electro microscopy and computer modeling; study of grain boundary migration; study of short-circuit diffusion along grain boundaries; and the development of Thin-Film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals.

  3. 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.

  4. Determination of grain boundary mobility during recrystallization by statistical evaluation of electron backscatter diffraction measurements

    Energy Technology Data Exchange (ETDEWEB)

    Basu, I., E-mail: basu@imm.rwth-aachen.de; Chen, M.; Loeck, M.; Al-Samman, T.; Molodov, D.A.

    2016-07-15

    One of the key aspects influencing microstructural design pathways in metallic systems is grain boundary motion. The present work introduces a method by means of which direct measurement of grain boundary mobility vs. misorientation dependence is made possible. The technique utilizes datasets acquired by means of serial electron backscatter diffraction (EBSD) measurements. The experimental EBSD measurements are collectively analyzed, whereby datasets were used to obtain grain boundary mobility and grain aspect ratio with respect to grain boundary misorientation. The proposed method is further validated using cellular automata (CA) simulations. Single crystal aluminium was cold rolled and scratched in order to nucleate random orientations. Subsequent annealing at 300 °C resulted in grains growing, in the direction normal to the scratch, into a single deformed orientation. Growth selection was observed, wherein the boundaries with misorientations close to Σ7 CSL orientation relationship (38° 〈111〉) migrated considerably faster. The obtained boundary mobility distribution exhibited a non-monotonic behavior with a maximum corresponding to misorientation of 38° ± 2° about 〈111〉 axes ± 4°, which was 10–100 times higher than the mobility values of random high angle boundaries. Correlation with the grain aspect ratio values indicated a strong growth anisotropy displayed by the fast growing grains. The observations have been discussed in terms of the influence of grain boundary character on grain boundary motion during recrystallization. - Highlights: • Statistical microstructure method to measure grain boundary mobility during recrystallization • Method implementation independent of material or crystal structure • Mobility of the Σ7 boundaries in 5N Al was calculated as 4.7 × 10{sup –8} m{sup 4}/J ⋅ s. • Pronounced growth selection in the recrystallizing nuclei in Al • Boundary mobility values during recrystallization 2–3 orders of

  5. Detection of expansion at large angle grain boundaries using electron diffraction

    Science.gov (United States)

    Balluffi, R. W.; Bristowe, P. D.

    1984-02-01

    Lamarre and Sass (LS) (Scripta Metall. 17: 1141(1983) observed a grain boundary electron diffraction effect from a large angle twist boundary which is used to obtain the volume expansion at the grain boundary in a direction normal to it. The case where the intensity from the grain boundary region, is close to lattice reflections on the same element of the boundary diffraction lattice is considered. Analysis of this complex problem show that the simplified model of LS is misleading in this case.

  6. Detection of expansion at large angle grain boundaries using electron diffraction

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1984-02-01

    Lamarre and Sass (LS) (Scripta Metall. 17: 1141(1983)) observed a grain boundary electron diffraction effect from a large angle twist boundary which they claim can be used to obtain the volume expansion at the grain boundary in a direction normal to it. This paper considers the case where the intensity from the grain boundary region, is close to lattice reflections on the same element of the boundary diffraction lattice. Analysis of this complex problem show that the simplified model of LS is misleading in this case. (DLC)

  7. 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.

  8. 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.

  9. Elastically mediated interactions between grain boundaries and precipitates in two-phase coherent solids

    Science.gov (United States)

    Xu, Ye-Chuan; Geslin, Pierre-Antoine; Karma, Alain

    2016-10-01

    We investigate analytically and numerically the interaction between grain boundaries and second phase precipitates in two-phase coherent solids in the presence of misfit strain. Our numerical study uses amplitude equations that describe the interaction of composition and stress [R. Spatschek and A. Karma, Phys. Rev. B 81, 214201 (2010), 10.1103/PhysRevB.81.214201] and free-energies corresponding to two-dimensional hexagonal and three-dimensional BCC crystal structures that exhibit isotropic and anisotropic elastic properties, respectively. We consider two experimentally motivated geometries where (i) a lamellar precipitate nucleates along a planar grain boundary that is centered inside the precipitate, and (ii) a circular precipitate nucleates inside a grain at a finite distance to an initially planar grain boundary. For the first geometry, we find that the grain boundary becomes morphologically unstable due to the combination of long-range elastic interaction between the grain boundary and compositional domain boundaries, and shear-coupled grain boundary motion. We characterize this instability analytically by extending the linear stability analysis carried out recently [P.-A. Geslin, Y.-C. Xu, and A. Karma, Phys. Rev. Lett. 114, 105501 (2015), 10.1103/PhysRevLett.114.105501] to the more general case of elastic anisotropy. The analysis predicts that elastic anisotropy hinders but does not suppress the instability. Simulations also reveal that, in a well-developed nonlinear regime, this instability can lead to the break-up of low-angle grain boundaries when the misfit strain exceeds a threshold that depends on the grain boundary misorientation. For the second geometry, simulations show that the elastic interaction between an initially planar grain boundary and an adjacent circular precipitate causes the precipitate to migrate to and anchor at the grain boundary.

  10. Structural unit/grain boundary dislocation model for twist boundaries in cubic crystals

    Science.gov (United States)

    Bristowe, P. D.; Belluffi, R. W.

    1984-10-01

    The systemics of 001 twist boundary structure is presented formally in terms of a structure unit/grain boundary dislocation (GBD) hierarchical model and the earlier model of Sutton is generalized. By comparison with experimental observation and atomistic calculation using pair potential models, the physical significance of the individual members of the hierarchy is determined. Comparison with experiment indicates a strong 110 type primary relaxation for theta is equal to or 36.9 deg. and a significant secondary relaxation near Sigma 5 which must result from oblique perturbations in the array of primary GBDs. On the other hand, comparison with available calculated results indicate a strong 110 type relaxation at low angles but a progressively weaker relaxation at higher angles. Also, no evidence is found for any significant secondary relaxations when at least one pair potential is employed.

  11. Grain and grain boundary characters in surface layer of untreated and plasma nitrocarburized 18Ni maraging steel with nanocrystalline structure

    Energy Technology Data Exchange (ETDEWEB)

    Yan, M.F., E-mail: yanmufu@hit.edu.cn [School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 (China); Wu, Y.Q. [School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 (China); Liu, R.L. [College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001 (China)

    2013-05-15

    The nanocrystallized 18Ni maraging steel was plasma nitrocarburized at 460 °C for 4 h in a mixture gas of N{sub 2}, H{sub 2} and C{sub 2}H{sub 5}OH. The surface phase compositions of the specimens were analyzed using X-ray diffractometer. The grain shape and size, and grain boundaries in the subsurface layers of the samples were characterized by electron backscattering diffraction and transmission electron microscopy. The results show that the nitrocarburized layers are composed of α-Fe, γ′-Fe{sub 4}N and FeN{sub 0.049} phases. Most α-Fe and γ′-Fe{sub 4}N grains show in columnar shape. The major and minor axes of some α-Fe grains are elongated and shortened after nitrocarburizing, respectively. In the subsurface layers of the untreated and nitrocarburized specimens, the average areas of γ′-Fe{sub 4}N and α-Fe grains both with a dimension of nanometer are 0.395 μm{sup 2} and 0.397 μm{sup 2}, respectively. The γ′-Fe{sub 4}N grain boundaries are mainly high angle boundaries with a very small fraction of low angle boundaries. Coincidence site lattice boundaries in the subsurface layer of the untreated specimen are composed mainly of Σ3, Σ11 and Σ13b, and their fraction decreases after nitrocarburizing.

  12. A Generalized Finite Element Method for polycrystals with discontinuous grain boundaries

    NARCIS (Netherlands)

    Simone, A.; Duarte, C. A.; Van der Giessen, E.

    2006-01-01

    We present a Generalized Finite Element Method for the analysis of polycrystals with explicit treatment of grain boundaries. Grain boundaries and junctions, understood as loci of possible displacement discontinuity, are inserted into finite elements by exploiting the partition of unity property of

  13. A creep rupture model accounting for cavitation at sliding grain boundaries

    NARCIS (Netherlands)

    Giessen, Erik van der; Tvergaard, Viggo

    1991-01-01

    An axisymmetric cell model analysis is used to study creep failure by grain boundary cavitation at facets normal to the maximum principal tensile stress, taking into account the influence of cavitation and sliding at adjacent inclined grain boundaries. It is found that the interaction between the

  14. 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.

  15. 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.

  16. Comparisons between computed and observed grain boundary structures and properties in metals

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.; Babcock, S.E.; Chan, S.W.; Kvam, E.P.; Liu, J.S.

    1984-10-01

    A detailed assessment is given of the extent to which agreement has been obtained between experimentally observed and calculated structures and/or properties of grain boundaries in metals. All of the calculations employed pair-wise interaction models. Published work as well as new results obtained by the authors are cited. Topics include: boundary width; primary and secondary relaxations (i.e., grain boundary dislocations); magnitude of the boundary energy; lattice translations across the boundary and atomic positions in the boundary core; and fast diffusion in boundaries. It is concluded in general that the pair-potential modeling has been a valuable tool in grain boundary research and has often produced detailed results which are in at least semi-quantitative agreement with observations. However, in some cases the agreement is poor and as might be expected, it is clear that the technique is not capable of producing realistic results in all cases. Various problems are pointed out and discussed.

  17. 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.

  18. 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.

  19. Effects of Grain Boundary Morphologies on Stress Corrosion Cracking of Alloy 600

    Directory of Open Access Journals (Sweden)

    Kim H.P.

    2017-06-01

    Full Text Available Effects of grain boundary morphologies on stress corrosion cracking (SCC of Alloy 600 have been studied in 40% NaOH at 315°C using C-ring specimens. The configuration of the grain boundary and the intergranular carbide density were controlled by heat treatment. SCC tests were performed at +150 mV above the corrosion potential. The specimen with a serrated grain boundary showed higher SCC resistance than that with a straight grain boundary. This appears to be caused by the fact that the specimen with the serrated grain boundary has longer SCC path. SCC resistance also increased with intergranular carbide density probably due to enhanced relaxation of stress at intergranular carbide.

  20. Atomic structure and electronic properties of MgO grain boundaries in tunnelling magnetoresistive devices

    Science.gov (United States)

    Bean, Jonathan J.; Saito, Mitsuhiro; Fukami, Shunsuke; Sato, Hideo; Ikeda, Shoji; Ohno, Hideo; Ikuhara, Yuichi; McKenna, Keith P.

    2017-04-01

    Polycrystalline metal oxides find diverse applications in areas such as nanoelectronics, photovoltaics and catalysis. Although grain boundary defects are ubiquitous their structure and electronic properties are very poorly understood since it is extremely challenging to probe the structure of buried interfaces directly. In this paper we combine novel plan-view high-resolution transmission electron microscopy and first principles calculations to provide atomic level understanding of the structure and properties of grain boundaries in the barrier layer of a magnetic tunnel junction. We show that the highly [001] textured MgO films contain numerous tilt grain boundaries. First principles calculations reveal how these grain boundaries are associated with locally reduced band gaps (by up to 3 eV). Using a simple model we show how shunting a proportion of the tunnelling current through grain boundaries imposes limits on the maximum magnetoresistance that can be achieved in devices.

  1. Structure and transport at grain boundaries in polycrystalline olivine: An atomic-scale perspective

    Science.gov (United States)

    Mantisi, Boris; Sator, Nicolas; Guillot, Bertrand

    2017-12-01

    Structure and transport properties at grain boundaries in polycrystalline olivine have been investigated at the atomic scale by molecular dynamics simulation (MD) using an empirical ionocovalent interaction potential. On the time scale of the simulation (a few tens of nanoseconds for a system size of ∼650,000 atoms) grain boundaries and grain interior were identified by mapping the atomic displacements along the simulation run. In the investigated temperature range (1300-1700 K) the mean thickness of the grain boundary phase is evaluated between 0.5 and 2 nm, a value which depends on temperature and grain size. The structure of the grain boundary phase is found to be disordered (amorphous-like) and is different from the one exhibited by the supercooled liquid. The self-diffusion coefficients of major elements in the intergranular region range from ∼10-13 to 10-10 m2/s between 1300 and 1700 K (with DSigb < DOgb < DFegb < DMggb) and are only one order of magnitude smaller than those evaluated in the supercooled melt. In using a newly derived expression for the bulk self-diffusion coefficient it is concluded that the latter one is driven by the grain boundary contribution as long as the grain size is smaller than a centimeter. In assuming that the electrical conduction at grain boundaries is purely ionic, the macroscopic grain boundary conductivity is found to be two orders of magnitude lower than in molten olivine, and one order of magnitude higher than the lattice conductivity. A consequence is that the conductivity of the olivine polycrystal is dominated by the grain interior contribution as soon as the grain size is larger than a micrometer or so. The grain boundary viscosity has been evaluated from the Green-Kubo relation expressing the viscosity as function of the stress tensor time correlation function. In spite of a slow convergence of the calculation by MD, the grain boundary viscosity was estimated about ∼105 Pa s at 1500 K, a value in agreement with

  2. 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.

  3. Simultaneous three-dimensional imaging and manipulation of grain boundaries in colloidal crystals

    Science.gov (United States)

    Edmond, Kazem V.; Liu, Yanyan; Curran, Arran; Aarts, Dirk G. A. L.; Sacanna, Stefano; Dullens, Roel P. A.

    Characterizing the properties of grains and grain boundaries is critical for understanding and controlling material properties. We investigate the dynamics of grain boundaries in crystalline materials using concentrated colloidal suspensions of microspheres. The micron-sized particles are suspended in a mixture of solvents whose refractive index and density nearly match those of the particles, enabling three-dimensional visualization and negating gravitational effects. Throughout the sample we disperse specially designed core-shell particles whose cores have a higher refractive index that can be optically trapped. Via optical tweezing, these core-shell particles enable us to directly interact with and probe grain boundaries in 3D within the colloidal crystal. We use a uniquely developed optical microscopy system that combines confocal imaging with holographic trapping, enabling quantitative imaging and precise manipulation simultaneously in three dimensions. Our experiments provide direct insight into the properties of grain boundaries in crystals.

  4. 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.

  5. 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.

  6. Electronic properties of a grain boundary in Sb-doped ZnO

    Energy Technology Data Exchange (ETDEWEB)

    Carlsson, J.M.; Hellsing, B. [Experimental Physics, Chalmers and Goeteborg University, Gothenburg (Sweden); Domingos, H.S.; Bristowe, P.D. [Department of Materials Science and Metallurgy, University of Cambridge, Cambridge (United Kingdom)]. E-mail: pdb1000@cus.cam.ac.uk

    2001-11-05

    The electronic properties of a {sigma}=13 32.2 deg. [0001] tilt grain boundary in ZnO have been investigated using first-principles calculations. Two atomic models for the boundary have been considered, one of which contains structural units that are consistent with those observed for this orientation using electron microscopy. Doping both the grain boundary models with antimony reveals a strong driving force for segregation. Analysis of the electronic densities of states, bond populations and Mulliken charges shows that antimony creates a localized impurity state in the grain boundary and acts as a donor dopant. The resulting charge accumulation at the grain boundary together with the presence of local bonds that are metallic in character, will influence the mechanism for charge transport across the interface and this is discussed in relation to varistor applications. (author)

  7. Electronic properties of a grain boundary in Sb-doped ZnO

    Science.gov (United States)

    Carlsson, J. M.; Hellsing, B.; Domingos, H. S.; Bristowe, P. D.

    2001-11-01

    The electronic properties of a Σ = 13 32.2° [0001] tilt grain boundary in ZnO have been investigated using first-principles calculations. Two atomic models for the boundary have been considered, one of which contains structural units that are consistent with those observed for this orientation using electron microscopy. Doping both the grain boundary models with antimony reveals a strong driving force for segregation. Analysis of the electronic densities of states, bond populations and Mulliken charges shows that antimony creates a localized impurity state in the grain boundary and acts as a donor dopant. The resulting charge accumulation at the grain boundary together with the presence of local bonds that are metallic in character, will influence the mechanism for charge transport across the interface and this is discussed in relation to varistor applications.

  8. Morphological Instability of Grain Boundaries in Two-Phase Coherent Solids

    Science.gov (United States)

    Geslin, Pierre-Antoine; Xu, Yechuan; Karma, Alain

    2015-03-01

    We show both computationally and analytically that grain boundaries that exhibit shear-coupled motion become morphologically unstable in solid alloys that phase separate into coherent domains of distinct chemical compositions. We carry out simulations of continuum models demonstrating that this instability is mediated by long-range elastic interaction between compositional domains and grain boundaries. In addition, we perform a linear stability analysis that predicts the range of unstable wavelengths in good quantitative agreement with simulations. In nonlinear stages, this pattern-forming instability leads to the breakup of low-angle grain boundaries, thereby strongly impacting microstructural evolution in a wide range of phase-separating materials.

  9. Modeling of the grain boundary segregation of helium in {alpha}-Fe

    Energy Technology Data Exchange (ETDEWEB)

    Suzudo, Tomoaki, E-mail: suzudo.tomoaki@jaea.go.jp [Japan Atomic Energy Agency, 2-4 Shirane Shirakata, Tokai-mura 319-1195 (Japan); Kaburaki, Hideo, E-mail: kaburaki.hideo@jaea.go.jp [Japan Atomic Energy Agency, 2-4 Shirane Shirakata, Tokai-mura 319-1195 (Japan); Yamaguchi, Masatake, E-mail: yamaguchi.masatake@jaea.go.jp [Japan Atomic Energy Agency, 2-4 Shirane Shirakata, Tokai-mura 319-1195 (Japan)

    2011-10-01

    We proposed a kinetic theory model of helium segregation from the bulk to the grain boundary in {alpha}-Fe. This model is constructed on the basis of the ab initio results of large segregation energy and large deduction in strength at the grain boundary due to helium. The simulation results of the model showed the typical segregation features as function of bulk concentration of helium and temperature in general agreement with experiment. In addition, the results are compared with McLean's model for monoatomic-layered grain boundary segregation in equilibrium, and the validity of this equilibrium model is discussed.

  10. 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.)

  11. Gas immersion laser diffusion for efficient cell fabrication and grain boundary research

    Energy Technology Data Exchange (ETDEWEB)

    Turner, G.B.; Aldrich, D.; Press, R.; Pressley, R.; Tarrant, D.

    1983-09-01

    A new laser method has been developed for making n+/p and p+/n solar cells using gaseous doping without a furnace. Efficiencies of 13% have been achieved for the method, called Gas Immersion Laser Diffusion (GILD). When the process, which melts the surface without heating the bulk, is applied to polycrystalline Si, many of the grain boundaries exhibit enhanced photocurrent collection, instead of the usual recombination. Comparison of the same grain boundaries with and without high temperature heat treatment indicates that recombination is activated by high temperature. The enhanced collection is apparently a grown-in feature of these grain boundaries which can only be retained with low temperature processing.

  12. Structural unit/grain boundary dislocation model for twist boundaries in cubic crystals

    Energy Technology Data Exchange (ETDEWEB)

    Bristowe, P.D.; Balluffi, R.W.

    1984-10-01

    The systematics of (001) twist boundary structure is presented formally in terms of a structural unit/grain boundary dislocation hierarchical model and the earlier model of Sutton is generalized. By comparison with experimental observation and atomistic calculation using pair-potential models the physical significance of the individual members of the hierarchy is determined. Comparison which experiment indicates a strong <110> type primary relaxation for theta less than or equal to 36.9/sup 0/ and a significant secondary relaxation near ..sigma..5 which must result from oblique perturbations in the array of primary GBD's. On the other hand, comparison with available calculated results indicates a strong <110> type primary relaxation at low angles but a progressively weaker relaxation at higher angles. Also, no evidence is found for any significant secondary relaxations when at least one pair potential is employed. However, very recent studies indicate stronger secondary relaxations with other potentials, and this, in future work, should lead to better agreement between calculations and experiment.

  13. Grain- to multiple-grain-scale deformation processes during diffusion creep of forsterite + diopside aggregate: 2. Grain boundary sliding-induced grain rotation and its role in crystallographic preferred orientation in rocks

    Science.gov (United States)

    Maruyama, G.; Hiraga, T.

    2017-08-01

    Polycrystalline samples composed of either tabular or equiaxed forsterite grains +diopside (5 and 20 vol %) were deformed with a grid etched onto the lateral surface. In Part 1 of this study, we identified grain boundary sliding (GBS) and rigid body-like grain rotation during deformation by diffusion creep where samples with tabular forsterite grains were shown to develop low-index plane grain boundaries that result in crystallographic preferred orientation (CPO). Here we examine how grain rotation depends on the sample strain, grain size, phases, grain shapes, and orientations relative to the compression axis and long axes of tabular forsterite grains. Based on these results, we model grain rotation due to GBS that occurs preferentially along low-index plane boundaries. The model reproduces all of the characteristics of grain rotation and together with the observed grain rotation rates in tabular and equiaxed grain samples, we estimate that low-index plane boundaries have a lower viscosity by a factor of 3 relative to general grain boundaries, which results in the development of CPO during diffusion creep. The observed constant rotation rate of 0.4 (radian/strain) in equiaxed-grain samples and in tabular-grain samples deformed to a strain of >0.5 is considered to be a minimum and further, a material-independent rotation rate during diffusion creep, indicating grain rotation as a primary microprocess during diffusion creep. We discuss the possible consequences of GBS-induced grain rotation and CPO development in rock microstructure and the seismic properties of the Earth's mantle.

  14. Contribution of grain boundary related strain accommodation to deformation of ultrafine-grained palladium

    Energy Technology Data Exchange (ETDEWEB)

    Ivanisenko, Yu., E-mail: julia.ivanisenko@kit.edu [Karlsruhe Institute of Technology, Institute of Nanotechnology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germany); Enikeev, N.A. [Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, K. Marx Str. 12, 450000 Ufa (Russian Federation); Laboratory for Mechanics of Bulk Nanostructured Materials, Saint Petersburg State University, Universitetsky Prospekt 28, Peterhof, 198504 Saint Petersburg (Russian Federation); Yang, K. [Robert Bosch GmbH, AE/EAI2, D71701 Schwieberdingen (Germany); Smoliakov, A.; Soloviev, V.P. [Russian Federal Nuclear Center All-Russian Research Institute of Experimental Physics (VNIIEF), Mira Ave, 37, 607188 Sarov (Russian Federation); Fecht, H. [Institut für Mikro, und Nanomaterialien, Universität Ulm, D-89081 Ulm (Germany); Hahn, H. [Karlsruhe Institute of Technology, Institute of Nanotechnology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germany); KIT-TUD Joint Research Laboratory Nanomaterials, Technische Universität Darmstadt, D-64287 Darmstadt (Germany)

    2016-06-21

    Ultrafine-grained Pd specimens with a mean grain size of 130 nm were compressed by 10% in a scanning electron microscope and the strain-induced change in orientations of grains was measured by in-situ electron-backscattering diffraction. A comparison of grain orientations before and after compression straining revealed substantial grain rotations. The analysis of the results performed using polycrystal plasticity simulation showed that the variation of orientations with strain cannot be explained only by crystallographic dislocation slip. A large portion of strain is proved to be accommodated via cooperative non-crystallographic grain rotation.

  15. 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.

  16. 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.

  17. The effects of doping a grain boundary in ZnO with various concentrations of Bi

    Science.gov (United States)

    Carlsson, Johan M.; Hellsing, Bo; Domingos, Helder S.; Bristowe, Paul D.

    2003-06-01

    We have made a systematic study of the Bi-decoration process in a Σ=13 [0 0 0 1] tilt grain boundary in ZnO by first-principles calculations. This grain boundary is taken as a model system for studying the microscopic properties of commercial Bi-doped ZnO-varistors. The calculations show that the decoration process is strongly site dependent and that there is a considerable segregation energy for the Bi-atoms at low concentration. Increasing the concentration lowers the segregation energy which sets an upper limit of approximately 32% for the Bi-concentration in this grain boundary. This implies that the Bi-atoms stay in the grain boundary region rather than diffusing into the ZnO grains during the manufacturing process, but the maximum Bi-concentration is limited which is consistent with the experimental observations. Bi-impurities in ZnO act as donors at low impurity concentration, but a localized Bi-Bi-bond is formed at higher Bi-concentration in the grain boundary. This Bi-state is located in the band gap of ZnO and it may be responsible for the varistor effect observed in Bi-decorated grain boundaries.

  18. 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

  19. Grain boundary analysis and ionic conductivity of superplastic cubic zirconia for solid oxide fuel cell electrolytes

    Science.gov (United States)

    Martin, Michael Craig

    Yttrium stabilized zirconia (YSZ) is the material most commonly used for solid oxide fuel cell (SOFC) electrolytes because it has high oxygen ion conductivity at elevated temperatures. Manufacturing and sealing of the SOFC YSZ electrolyte is relatively expensive and cost could be reduced if the ceramic could be net shape formed. Methods to net shape form YSZ by superplastic deformation have been developed by introducing SiO2 as a second phase, but the impact of this approach on ionic conductivity was not known. This dissertation focuses on understanding how SiO2 affects the ionic conductivity of YSZ. The present work necessitated the design and fabrication of an appropriate impedance spectroscopy test capability and the preparation and evaluation of a matrix of samples with various silica amounts and grain sizes. Impedance spectroscopy is the figure of merit used to measure and evaluate ionic conductivity. Impedance spectroscopy at temperatures from 350 to 700°C and analytical electron microscopy were used to characterize grain boundary conductivity and grain boundary segregation of in 8 mol% yttrium cubic stabilized zirconia (8Y-CSZ). 1 to 10 wt% of silica was added as an intergranular phase. Grain growth experiments were conducted at temperatures of 1350°C to 1600°C for times from 0.1 to 100 hours. Grain boundary widths were determined from impedance spectroscopy data using a brick layer model. Average grain boundary widths were also determined from analytical electron microscopy conducted at Oak Ridge National Laboratory and the amount of yttrium and silicon segregation at grain boundaries was determined from chemical composition line scans. Results indicate that the addition of intergranular SiO2 to 8Y-CSZ leads to smaller grain size (due to grain boundary pinning) therefore increased grain boundary area and reduced total ionic conductivity. For a constant grain size, the specific grain boundary and the total ionic conductivity is not significantly affected

  20. Evidence for Vacancy Mechanism in Grain Boundary Diffusion in bcc Iron: A Molecular-Dynamics Study

    Science.gov (United States)

    Kwok, Thomas; Ho, P. S.; Yip, Sidney; Balluffi, R. W.; Bristowe, P. D.; Brokman, A.

    1981-10-01

    The dominance of vacancy migration in grain boundary self-diffusion has been established by molecular-dynamics simulation of a bcc iron Σ=5 tilt boundary. The data yielded a reasonable value of the activation energy for migration and showed that the jump processes are highly structure dependent. The use of a temperature-dependent transition-probability matrix to describe the diffusion of the vacancies in the grain boundary is suggested. Formation of one type of boundary interstitial was observed, and the interstitial was found to be immobile during its lifetime.

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

    CERN Document Server

    Wong, H; Poon, M C

    1999-01-01

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

  2. 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.

  3. Photoluminescence characterization of the grain boundary thermal stability in chemical vapor deposition grown WS2

    Science.gov (United States)

    Cai, Shuang; Zhao, Weiwei; Zafar, Amina; Wu, Zhangting; Tao, Yi; Bi, Kedong; Wei, Zhiyong; Ni, Zhenhua; Chen, Yunfei

    2017-10-01

    Monolayer transition metal dichalcogenides (TMDs) such as MoS2 and WS2 have been considered as promising candidate materials in nanophotonic applications. However, the structure stability of TMDs based optoelectronic devices is highly sensitive to the working environment. Here we present a successive photoluminescence study of the thermal stability characterization of grain boundary in chemical vapor deposition grown monolayer WS2. Results show that PL intensity enhancement in grain boundaries can be significantly weakened during the annealing process. Transformation temperature starts around 210 °C, substantially lower than the surrounding non-grain-boundary area. First-principle calculations results show that the PL quenching of grain boundaries is caused by the increased structural defects induced by annealing process, which makes the transition of electrons more difficult. Our results provide a route for characterizing the structure stability of two dimensional (2D) semiconductors, calling for extra attention to nanophotonic device working condition.

  4. Phase-field model of strain-induced grain-boundary premelting

    Science.gov (United States)

    Wang, Nan; Spatschek, Robert; Karma, Alain

    2008-03-01

    Grain-boundary premelting depends in a complex way on the relative magnitude of the solid-liquid interfacial free-energy and grain boundary energy as well as temperature and strain. We study this dependence in a bicrystal geometry using a phenomenological three-order parameter phase-field model. This model describes the short scale attractive or repulsive interaction between crystal-melt interfaces and macroscopic linear elasticity including the important effect of the density contrast between solid and liquid. The model exhibits a rich behavior characterized by single or multiple premelting transitions between dry or wet grain boundaries with different liquid layer thicknesses as a function of applied tensile stress. The results have important implications for the phenomenon of liquid metal embrittlement associated with the stress-driven penetration of nanometric liquid films along grain boundaries.

  5. 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.

  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 stability governs hardening and softening in extremely fine nanograined metals

    Science.gov (United States)

    Hu, J.; Shi, Y. N.; Sauvage, X.; Sha, G.; Lu, K.

    2017-03-01

    Conventional metals become harder with decreasing grain sizes, following the classical Hall-Petch relationship. However, this relationship fails and softening occurs at some grain sizes in the nanometer regime for some alloys. In this study, we discovered that plastic deformation mechanism of extremely fine nanograined metals and their hardness are adjustable through tailoring grain boundary (GB) stability. The electrodeposited nanograined nickel-molybdenum (Ni-Mo) samples become softened for grain sizes below 10 nanometers because of GB-mediated processes. With GB stabilization through relaxation and Mo segregation, ultrahigh hardness is achieved in the nanograined samples with a plastic deformation mechanism dominated by generation of extended partial dislocations. Grain boundary stability provides an alternative dimension, in addition to grain size, for producing novel nanograined metals with extraordinary properties.

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

    Directory of Open Access Journals (Sweden)

    Fei Ye

    2014-02-01

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

  9. 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...

  10. Detection of grain-boundary resistance to slip transfer using nanoindentation

    NARCIS (Netherlands)

    Soer, WA; De Hosson, JTM

    2005-01-01

    Nanoindentation measurements near a high-angle grain boundary in a Fe-14%Si bicrystal showed dislocation pile-up and transmission across the boundary. The latter is observed as a characteristic displacement jump, from which the Hall-Petch slope can be calculated as a measure for the slip

  11. Ab initio determination of the structure of a grain boundary by simulated quenching

    Energy Technology Data Exchange (ETDEWEB)

    Payne, M.C.; Bristowe, P.D.; Joannopoulos, J.D.

    1987-03-30

    Results of the first completely ab initio investigation of the microscopic structure of a grain boundary in a semiconductor are presented. By use of the molecular-dynamics--simulated annealing method for performing total-energy calculations within the local-density--functional and pseudopotential approximation, the ..sigma.. = 5 (001) twist boundary in germanium is studied. A number of rotation-and-translation states are investigated leading to a prediction for the structure of this geometry. Evidence for the possible presence of novel defects and glasslike tunneling mode states at grain boundaries is presented.

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

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.; Sun, C.P.

    1979-12-01

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

  13. Ab-initio determination of the structure of a grain boundary by simulated quenching

    Energy Technology Data Exchange (ETDEWEB)

    Payne, M.C.; Bristowe, P.D.; Joannopoulos, J.D.

    1987-02-01

    Results of the first completely ab-initio investigation of the microscopic structure of a grain boundary in a semiconductor are presented. Using the molecular dynamics simulated annealing method for performing total energy calculations within the local density functional and pseudopotential approximations, the ..sigma.. = 5(001) twist boundary in germanium is studied. A number of rotation-and-translation states are investigated leading to a prediction for the structure of this geometry. Evidence for the possible presence of novel defects and glass-like tunneling mode states at grain boundaries is presented.

  14. Ab initio determination of the structure of a grain boundary by simulated quenching

    Science.gov (United States)

    Payne, M. C.; Bristowe, P. D.; Joannopoulos, J. D.

    1987-03-01

    Results of the first completely ab initio investigation of the microscopic structure of a grain boundary in a semiconductor are presented. By use of the molecular-dynamics-simulated annealing method for performing total-energy calculations within the local-density-functional and pseudopotential approximations, the Sigma = 5 (001) twist boundary is germanium is studied. A number of rotation-and-translation states are investigated leading to a prediction for the structure of this geometry. Evidence for the possible presence of novel defects and glasslike tunneling-mode states at grain boundaries is presented.

  15. Molecular dynamics simulation of grain-boundary diffusion of vacancies in bcc iron

    Science.gov (United States)

    Kwok, T.; Ho, P. S.; Yip, S.; Balluffi, R.; Brokman, A. W.; Bristowe, P. D.

    1981-06-01

    The jumping of vacancies in a bcc iron sigma = 5 tilt boundary was simulated by computer molecular dynamics. The data yielded a reasonable value of the activation energy for migration and showed that the jump processes are highly structure dependent. The use of a temperature dependent transition probability matrix to describe the diffusion of the vacancies in the grain boundary is suggested. Formation of one type of boundary interstitial was observed which was found to be immobile.

  16. Molecular dynamics simulation of grain-boundary diffusion of vacancies in bcc iron

    Energy Technology Data Exchange (ETDEWEB)

    Kwok, T.; Ho, P. S.; Yip, S.; Balluffi, R. W.; Bristowe, P. D.; Brokman, A.

    1981-06-01

    The jumping of vacancies in a bcc iron ..sigma.. = 5 tilt boundary was simulated by computer molecular dynamics. The data yielded a reasonable value of the activation energy for migration and showed that the jump processes are highly structure-dependent. The use of a temperature dependent transition probability matrix to describe the diffusion of the vacancies in the grain boundary is suggested. Formation of one type of boundary interstitial was observed which was found to be immobile.

  17. Structure of Grain Boundaries and Aspects of Deformation Behavior in Nickel-Aluminum Alloys.

    Science.gov (United States)

    Kerans, Ronald James

    The structures of two grain boundaries in Ni _3Al were determined using transmission electron microscopy (TEM). The line directions and spacings of secondary grain boundary dislocations (gbds) were calculated for coincidence site lattice (CSL) relationships near the experimentally determined misorientations. The structures of the boundaries were found to be in good agreement with Sigma 9 and Sigma 31b relationships of the ordered lattice. The grain boundary dislocations were found to be characteristic of the ordered structure. No grouping of partial gbds was observed. Lattice dislocations consisting of pairs of 1/3 (112) superpartials bounding extended superlattice intrinsic stacking faults (SISFs) were observed by TEM. They were found to independently react to APB coupled 1/2 (110) pairs in the commonly observed pure screw orientation. Implications with regard to Kear-Wilsdorf type locking mechanisms and deformation behavior are discussed. Boron was observed to have strong effects upon the population of boundary types, grain size, and grain growth. In addition, small amounts of B completely eliminate widely extended stacking faults. It is concluded that effects of B other than influencing boundary cohesion are important.

  18. 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.

  19. 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).

  20. Grain-boundary engineering markedly reduces susceptibility to intergranular hydrogen embrittlement in metallic materials

    Energy Technology Data Exchange (ETDEWEB)

    Bechtle, Sabine; Kumar, Mukul; Somerday, Brian P.; Launey, Maximilien E.; Ritchie, Robert O.

    2009-05-10

    The feasibility of using 'grain-boundary engineering' techniques to reduce the susceptibility of a metallic material to intergranular embrittlement in the presence of hydrogen is examined. Using thermomechanical processing, the fraction of 'special' grain boundaries was increased from 46% to 75% (by length) in commercially pure nickel samples. In the presence of hydrogen concentrations between 1200 and 3400 appm, the high special fraction microstructure showed almost double the tensile ductility; also, the proportion of intergranular fracture was significantly lower and the J{sub c} fracture toughness values were some 20-30% higher in comparison with the low special fraction microstructure. We attribute the reduction in the severity of hydrogen-induced intergranular embrittlement to the higher fraction of special grain boundaries, where the degree of hydrogen segregation at these boundaries is reduced.

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

    Energy Technology Data Exchange (ETDEWEB)

    Ahmed, K. [Idaho National Lab. (INL), Idaho Falls, ID (United States); Tonks, M. [Idaho National Lab. (INL), Idaho Falls, ID (United States); Zhang, Y. [Idaho National Lab. (INL), Idaho Falls, ID (United States); Biner, B. [Idaho National Lab. (INL), Idaho Falls, ID (United States)

    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.

  2. Understanding the Effect of Grain Boundary Character on Dynamic Recrystallization in Stainless Steel 316L

    Science.gov (United States)

    Beck, Megan; Morse, Michael; Corolewski, Caleb; Fritchman, Koyuki; Stifter, Chris; Poole, Callum; Hurley, Michael; Frary, Megan

    2017-08-01

    Dynamic recrystallization (DRX) occurs during high-temperature deformation in metals and alloys with low to medium stacking fault energies. Previous simulations and experimental research have shown the effect of temperature and grain size on DRX behavior, but not the effect of the grain boundary character distribution. To investigate the effects of the distribution of grain boundary types, experimental testing was performed on stainless steel 316L specimens with different initial special boundary fractions (SBF). This work was completed in conjunction with computer simulations that used a modified Monte Carlo method which allowed for the addition of anisotropic grain boundary energies using orientation data from electron backscatter diffraction (EBSD). The correlation of the experimental and simulation work allows for a better understanding of how the input parameters in the simulations correspond to what occurs experimentally. Results from both simulations and experiments showed that a higher fraction of so-called "special" boundaries ( e.g., Σ3 twin boundaries) delayed the onset of recrystallization to larger strains and that it is energetically favorable for nuclei to form on triple junctions without these so-called "special" boundaries.

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

    Science.gov (United States)

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

    2016-03-01

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

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

    Directory of Open Access Journals (Sweden)

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

    2001-01-01

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

  6. A comparison of [0 0 1] low-angle tilt grain boundaries of (1 0 0) and (1 1 0) grain boundary planes in YBa 2Cu 3O 7- δ coated conductors

    Science.gov (United States)

    Kung, H.; Hirth, J. P.; Foltyn, S. R.; Arendt, P. N.; Jia, Q. X.; Maley, M. P.

    2001-08-01

    The microstructure and grain boundary structure in YBa 2Cu 3O 7- δ (YBCO) thick film coated conductors are characterized by transmission electron microscopy. The films contain low-angle [0 0 1] tilt grain boundaries with periodic arrays of edge dislocations parallel to the c-axis. A majority of the grain boundary planes are of either the (1 0 0) or the (1 1 0) type. Grain boundary dislocations (GBDs) with a [1 0 0] Burgers vector were observed in tilt boundaries with (1 0 0) boundary planes. However, partial dislocations, separated by stacking faults, were found at boundaries with near (1 1 0) grain boundary planes. Extensive g· b and g· R analyses confirmed the partials to be of 1/2[1 1 0] type. These results suggest that the characteristics of dislocation structures, which have been proposed in various grain boundary-based flux-pinning models, depend not only on the misorientation angle θ, but also on the types of grain boundary plane. The effect of dissociated GBDs on the transport properties of low angle grain boundaries in YBCO coated conductors is discussed.

  7. Covalently Connecting Crystal Grains with Polyvinylammonium Carbochain Backbone To Suppress Grain Boundaries for Long-Term Stable Perovskite Solar Cells.

    Science.gov (United States)

    Li, Han; Liang, Chao; Liu, Yingliang; Zhang, Yiqiang; Tong, Jincheng; Zuo, Weiwei; Xu, Shengang; Shao, Guosheng; Cao, Shaokui

    2017-02-22

    Grain boundaries act as rapid pathways for nonradiative carrier recombination, anion migration, and water corrosion, leading to low efficiency and poor stability of organometal halide perovskite solar cells (PSCs). In this work, the strategy suppressing the crystal grain boundaries is applied to improve the photovoltaic performance, especially moisture-resistant stability, with polyvinylammonium carbochain backbone covalently connecting the perovskite crystal grains. This cationic polyelectrolyte additive serves as nucleation sites and template for crystal growth of MAPbI3 and afterward the immobilized adjacent crystal grains grow into the continuous compact, pinhole-free perovskite layer. As a result, the unsealed PSC devices, which are fabricated under low-temperature fabrication protocol with a proper content of polymer additive PVAm·HI, currently exhibit the maximum efficiency of 16.3%. Remarkably, these unsealed devices follow an "outside-in" corrosion mechanism and respectively retain 92% and 80% of the initial PCE value after being exposed under ambient environment for 50 days and 100 days, indicating the superiority of carbochain polymer additives in solving the long-term stability problem of PSCs.

  8. RETRACTED ARTICLE: The Effect of Solute Atoms on Grain Boundary Migration: A Solute Pinning Approach

    Science.gov (United States)

    Hersent, Emmanuel; Marthinsen, Knut; Nes, Erik

    2012-12-01

    The effect of solute atoms on grain boundary migration has been modeled on the basis of the idea that solute atoms will locally perturb the collective rearrangements of solvent atoms associated with boundary migration. The consequence of such perturbations is the cusping of the boundary and corresponding stress concentrations on the solute atoms which will promote thermal activation of these atoms out of the boundary. This thermal activation is considered to be the rate-controlling mechanism in boundary migration. It is demonstrated that the present statistical approach is capable of explaining, in phenomenological terms, the known effects of solute atoms on boundary migration. The experimental results on the effect of copper on boundary migration in aluminum, due to Gordon and Vandermeer, have been well accounted for.

  9. 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

  10. Grain Boundary Structures and Collective Dynamics of Inversion Domains in Binary Two-Dimensional Materials

    Science.gov (United States)

    Taha, Doaa; Mkhonta, S. K.; Elder, K. R.; Huang, Zhi-Feng

    2017-06-01

    Understanding and controlling the properties and dynamics of topological defects is a lasting challenge in the study of two-dimensional materials, and is crucial to achieve high-quality films required for technological applications. Here grain boundary structures, energies, and dynamics of binary two-dimensional materials are investigated through the development of a phase field crystal model that is parametrized to match the ordering, symmetry, energy, and length scales of hexagonal boron nitride. Our studies reveal some new dislocation core structures for various symmetrically and asymmetrically tilted grain boundaries, in addition to those obtained in previous experiments and first-principles calculations. We also identify a defect-mediated growth dynamics for inversion domains governed by the collective atomic migration and defect core transformation at grain boundaries and junctions, a process that is related to inversion symmetry breaking in binary lattice.

  11. 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.

  12. Changes in the diffusion properties of nonequilibrium grain boundaries upon recrystallization and superplastic deformation of submicrocrystalline metals and alloys

    Science.gov (United States)

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

    2017-08-01

    The effect of an increase in the coefficient of the grain-boundary diffusion upon recrystallization and superplastic deformation of submicrocrystalline (SMC) materials prepared by severe plastic deformation has been studied. It is shown that the coefficient of the grain-boundary diffusion of the SMC materials is dependent on the intensity of the lattice dislocation flow whose value is proportional to the rate of the grain boundary migration upon annealing of SMC metals or the rate of the intragrain deformation under conditions of superplastic deformation of SMC alloys. It is found that, at a high rate of grain boundary migrations and high rates of superplastic deformation, the intensity of the lattice dislocation flow bombarding grain boundaries of SMC materials is higher than the intensity of their diffusion accommodation, which leads to an increase in the coefficient of the grain-boundary diffusion and a decrease in the activation energy. The results of the numerical calculations agree well with the experimental data.

  13. 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.

  14. On the detection of expansion at large angle grain boundaries using electron diffraction

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1984-06-01

    A model of the type shown by Brokman and Balluffi in this article, should hold for grain boundary reflections from twist boundaries occurring at positions in reciprocal space on elements of the ''boundary diffraction lattice'' (BDL) which do not contain lattice reflections. In such cases, the intensities scattered by the boundary region and the two unperturbed lattices can be separated to a good approximation, and the scattering from the boundary region can be considered independently. However, in the present case, where the intensity from the grain boundary region, considered separately, would appear close to lattice reflections on the same element of the BDL, it is not clear that this is the case. It may be necessary to perform a more complicated analysis taking account of the total scattering from the grain boundary region and the two adjoining lattices. The purpose of this article is to present such an analysis. The results show that the more complicated analysis is required and a more realistic explanation of the observed result is discussed.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2006-01-25

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

  16. Electronic Properties of Grain Boundaries in GaAs: A Study of Oriented Bicrystals Prepared by Epitaxial Lateral Overgrowth.

    Science.gov (United States)

    1984-05-10

    adequately described by arrays of dislocations, termed grain boundary dislocations ( GBDs ), formed in the crystallographic lattice appropriate for the...In this limit the structure can no longer be assumed to be based on discrete GBDs and it becomes necessary to adopt other formalisms to describe the...unique "crystallography" associated with a grain boundary.(7 9) In contrast to the limited applicability of the GBD model, this grain boundary

  17. 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...

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

    Science.gov (United States)

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

    2011-04-01

    Inter-granular creep cavitation damage has been observed in an ex-service 316H austenitic stainless steel thick section weldment. Focused ion beam cross-section milling combined with ion channelling contrast imaging is used to identify the cavitation damage, which is usually associated with the grain boundary carbide precipitates in this material. The results demonstrate that this technique can identify, in particular, the early stage of grain boundary creep cavitation unambiguously in materials with complex phase constituents. Copyright © 2011 Elsevier B.V. All rights reserved.

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

    Directory of Open Access Journals (Sweden)

    Edit E. Helgee

    2016-01-01

    Full Text Available 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.

  20. Grain boundary-mediated nanopores in molybdenum disulfide grown by chemical vapor deposition.

    Science.gov (United States)

    Elibol, Kenan; Susi, Toma; O Brien, Maria; Bayer, Bernhard C; Pennycook, Timothy J; McEvoy, Niall; Duesberg, Georg S; Meyer, Jannik C; Kotakoski, Jani

    2017-01-26

    Molybdenum disulfide (MoS 2 ) is a particularly interesting member of the family of two-dimensional (2D) materials due to its semiconducting and tunable electronic properties. Currently, the most reliable method for obtaining high-quality industrial scale amounts of 2D materials is chemical vapor deposition (CVD), which results in polycrystalline samples. As grain boundaries (GBs) are intrinsic defect lines within CVD-grown 2D materials, their atomic structure is of paramount importance. Here, through atomic-scale analysis of micrometer-long GBs, we show that covalently bound boundaries in 2D MoS 2 tend to be decorated by nanopores. Such boundaries occur when differently oriented MoS 2 grains merge during growth, whereas the overlap of grains leads to boundaries with bilayer areas. Our results suggest that the nanopore formation is related to stress release in areas with a high concentration of dislocation cores at the grain boundaries, and that the interlayer interaction leads to intrinsic rippling at the overlap regions. This provides insights for the controlled fabrication of large-scale MoS 2 samples with desired structural properties for applications.

  1. On grain boundary facetting under stress during high temperature deformation of Al and Al-Ga alloys

    Energy Technology Data Exchange (ETDEWEB)

    Weygand, D.; Doisneau-Cottignies, B.; Brechet, Y. [Domaine Univ. de Grenoble, St. Martin d`Heres (France). LTPCM; Blandin, J.J. [GPMM, Domaine Universitaire de Grenoble, 38402 St. Martin d`Heres (France)

    1997-08-30

    Aluminuim of high purity and aluminium with 50 ppm gallium were deformed under various strain rates and temperatures in order to investigate the possible influence of liquid like grain boundaries on grain boundary sliding and macroscopic deformation behaviour. Under all conditions, serrated grain boundaries were observed to develop during the tensile test. Those serrated structures were correlated to the development of subgrains observed by optical metallography and transmission electron microscopy. In the absence of an applied stress, further annealing lead to disappearing of the serrated grain boundaries when the annealing temperature was higher than the tensile test temperature under which the serrations were developed. (orig.) 9 refs.

  2. Computer simulation study of the structure of vacancies in grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Brokman, A.; Bristowe, P.D.; Balluffi, R.W.

    1981-10-01

    The structure of vacancies in grain boundaries has been investigated by computer molecular statics employing pairwise potentials. In order to gain an impression of the vacancy structures which may occur generally, a number of variables was investigated, including metal type, boundary type, degree of lattice coincidence, and choice of boundary site. In all cases the vacancies remained as distinguishable point defects in the relatively irregular boundary structures. However, it was found that the vacancy often induced relatively large atomic displacements in the core of the boundary. These displacements often occurred only in the direct vicinity of the vacancy, but in certain cases they were widely distributed in the boundary, sometimes at surprisingly large distances. In certain cases the displacements included a large inward relaxation of one, or more, of the atoms neighboring the vacancy, and the initial vacant site became effectively ''split''.

  3. Computer simulation study of the structure of vacancies in grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Brokman, A.; Bristowe, P.D.; Balluffi, R.W.

    1981-01-01

    The structure of vacancies in grain boundaries has been investigated by computer molecular statics employing pairwise potentials. In order to gain an impression of the vacancy structures which may occur generally, a number of variables was investigated including: metal type, boundary type, degree of lattice coincidence and choice of boundary site. In all cases the vacancies remained as distinguishable point defects in the relatively irregular boundary structures. However, it was found that the vacancy often induced relatively large atomic displacements in the core of the boundary. These displacements often occurred only in the direct vicinity of the vacancy, but in certain cases they were widely distributed in the boundary, sometimes at surprisingly large distances.

  4. 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....

  5. 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

  6. Diffuse-interface polycrystal plasticity: expressing grain boundaries as geometrically necessary dislocations

    Science.gov (United States)

    Admal, Nikhil Chandra; Po, Giacomo; Marian, Jaime

    2017-12-01

    The standard way of modeling plasticity in polycrystals is by using the crystal plasticity model for single crystals in each grain, and imposing suitable traction and slip boundary conditions across grain boundaries. In this fashion, the system is modeled as a collection of boundary-value problems with matching boundary conditions. In this paper, we develop a diffuse-interface crystal plasticity model for polycrystalline materials that results in a single boundary-value problem with a single crystal as the reference configuration. Using a multiplicative decomposition of the deformation gradient into lattice and plastic parts, i.e. F( X,t)= F L( X,t) F P( X,t), an initial stress-free polycrystal is constructed by imposing F L to be a piecewise constant rotation field R 0( X), and F P= R 0( X)T, thereby having F( X,0)= I, and zero elastic strain. This model serves as a precursor to higher order crystal plasticity models with grain boundary energy and evolution.

  7. Imaging the grain boundaries in polycrystalline MoS2 monolayer by non-invasive second harmonic generation

    Science.gov (United States)

    Cheng, Jinxin; Jiang, Tao; Ji, Qingqing; Zhang, Yanfeng; Gong, Xingao; Liu, Wei-Tao; Wu, Shiwei

    2015-03-01

    Atomically thin transition metal dichalcogenide monolayers have showed intriguing physical properties for high performance quantum electronics. In order to utilize them in technological applications at industrial scale, mass production of this two dimensional materials via chemical vapor deposition (CVD) is demanded and urged. Despite the success of growing large-scale monolayer, limited grain size and emergence of grain boundary remain as the major hurdle being single crystalline sheets. To resolve this issue, it is necessary to image the grain and grain boundary, and further understand their formation with statistical significance. Here we used second harmonic generation (SHG) microscopy, a noninvasive coherent imaging technique, to image the grain and grain boundary in CVD grown monolayer molybdenum disulfide. The destructive interference between neighboring grains enabled us to pinpoint the location of grain boundary; the anisotropic polarization pattern permitted us to determine the type of grain boundary. Furthermore, this high-throughput characterization technique allows statistical analysis of hundreds of grain and grain boundary, unambiguously revealing that the CVD growth mechanism of monolayer MoS2.

  8. 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.

  9. First-principles study of a tilt grain boundary in rutile

    Science.gov (United States)

    Dawson, I.; Bristowe, P. D.; Lee, M.-H.; Payne, M. C.; Segall, M. D.; White, J. A.

    1996-11-01

    The atomic and electronic structure of a tilt grain boundary in rutile TiO2 has been calculated in an ab initio manner. The method employs a plane-wave basis set and optimized pseudopotentials and is carried out within the local-density approximation of density-functional theory. The study focuses on the structure and energy of the ∑=15 36.9° (210)[001] tilt boundary, which is relaxed to equilibrium using a conjugate gradients iterative minimization technique. The calculations confirm the stability of a proposed atomic model for the boundary and provide some insight into its electronic structure.

  10. An ab initio investigation of a grain boundary in a transition metal oxide

    Energy Technology Data Exchange (ETDEWEB)

    Dawson, I.; Bristowe, P.D. [Univ. of Cambridge (United Kingdom). Dept. of Materials Science and Metallurgy; Payne, M.C.; Lee, M.H. [Univ. of Cambridge (United Kingdom). Cavendish Lab.

    1996-12-31

    The authors have used ab initio total energy plane wave pseudopotential methods to perform the first completely ab initio investigation of the atomic and electronic structure of a grain boundary in a transition metal oxide. The {Sigma} = 15 (210)[001] tilt boundary in rutile TiO{sub 2} is studied using the conjugate gradients iterative minimization technique for performing total energy calculations within the LDA and pseudopotential approximations. The stability of the experimentally observed translation state of the boundary is confirmed, and some insight is gained into its electronic structure.

  11. 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.

  12. 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.

  13. Solid State Theory of Photovoltaic Materials: Nanoscale Grain Boundaries and Doping CIGS

    Energy Technology Data Exchange (ETDEWEB)

    Zunger, A

    2005-01-01

    We use modern first-principles electronic structure theory to investigate (1) why are grain boundaries in chalcopyrites passive; (2) can chalcopyrites be doped by transition metals, and; (3) can hot electrons and carrier multiplication be efficient in quantum-dot solar cells.

  14. Surface roughness and grain boundary scattering effects on the electrical conductivity of thin films

    NARCIS (Netherlands)

    Palasantzas, George

    1998-01-01

    In this work, we investigate surface/interface roughness and grain boundary scattering effects on the electrical conductivity of polycrystalline thin films in the Born approximation. We assume for simplicity a random Gaussian roughness convoluted with a domain size distribution ~e^-πr^2/ζ^2 to

  15. Young Investigator Program: Quasi-Liquid Grain Boundary Films in Refractory Metals

    Science.gov (United States)

    2010-01-15

    Nishimura T, Tanaka H. Pressureless Sintering and Physical Properties of Zrb2-Based Composites with Zrsi2 Additive. Scripta Materialia 2008; 58:579. [16... Nova Science Publ., 1990. p.363. [95] Zhang X-F, Sixta ME, DeJonghe LC. Grain Boundary Evolution in Hot-Pressed ABC-SiC. J. Am. Ceram. Soc. 2000; 83

  16. In situ TEM nanoindentation and dislocation-grain boundary interactions : a tribute to David Brandon

    NARCIS (Netherlands)

    de Hosson, J.T.M.; Soer, W.A.; Minor, A.M.; Shan, Z.W.; Stach, E.A.; Asif, S.A.S.; Warren, O.L.

    2006-01-01

    As a tribute to the scientific work of Professor David Brandon, this paper delineates the possibilities of utilizing in situ transmission electron microscopy to unravel dislocation-grain boundary interactions. In particular, we have focused on the deformation characteristics of Al-Mg films. To this

  17. In-situ transmission electron microscopy : on moving dislocations and mobile grain boundaries

    NARCIS (Netherlands)

    De Hosson, J. T. M.; Soer, W.

    This paper delineates the possibilities of utilizing in situ transmission electron microscopy to unravel dislocation-g rain boundary interactions. In situ nanoindentation experiments have been conducted in TEM on ultrafine-grained Al and Al-Mg films with varying Mg contents. The observed propagation

  18. 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

    . 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...

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

    Science.gov (United States)

    2016-01-01

    uniaxially pressed into pellets at 5,000 psig in a 13-mm die. The pressed pellets were then vacuum sealed into plastic bags and cold isostatically pressed at...US); 2014 Dec. Report No.: ARL-TR-7145. 10. Ban CW, Choi GM. The effect of sintering on the grain boundary conductivity of lithium lanthanum

  20. 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.

  1. Simulation of the structure of vacancies in high angle grain boundaries

    Science.gov (United States)

    Bristowe, P. D.

    1980-06-01

    A hard sphere dynamic model and a bubble raft model provided insight into the structure of a variety of defects in two dimensional crystalline and amorphous systems. The computed results form part of a wider investigation of vacancies and interstitials in various grain boundaries in which the binding energies are analyzed and related to the defect structure and form of the interatomic potential.

  2. Structure of large-angle grain boundaries in metals and ceramic oxides

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.; Brokman, A.

    1982-08-01

    Recent progress in the understanding of grain boundary structure in metals and ceramic oxides is critically reviewed with emphasis on both experimental and theoretical work. The review essentially updates an earlier review presented at the 1979 Fall Meeting: i.e., R.W. Balluffi, P.D. Bristowe and C.P. Sun, J. Amer. Ceram. Soc. 64, 25 (1981).

  3. Structure of large-angle grain boundaries in metals and ceramic oxides

    Science.gov (United States)

    Balluffi, R. W.; Bristowe, P. D.; Brokman, A.

    1982-08-01

    Recent progress in the understanding of grain boundary structure in metals and ceramic oxides is critically reviewed with emphasis on both experimental and theoretical work. The review essentially updates an earlier review presented at the 1979 Fall Meeting: i.e., R. W. Balluffi, P. D. Bristowe and C. P. Sun, J. Amer. Ceram. Soc. 64,25(1981).

  4. LBIC characterization of a grain boundary in the Si, p bicrystal

    Energy Technology Data Exchange (ETDEWEB)

    Boyeaux, J.P.; Masri, K.; Mayet, L.; Stcherbanioff, L.; Laugier, A.

    1987-01-01

    A high performance light-beam-induced current (LBIC) analyser designed for photovoltaic characterization is described. The photoresponse and its transient behaviour can be stored in a computer under a matrix form corresponding to the investigated area. Numerical image processing is performed and the local transport properties can be displayed using grey levels or arbitrarily chosen colors. In this work, the LBIC apparatus is used for a quantitative characterization of grain boundary electrical activity. For this purpose, a Schottky device has been elaborated with a Si-type p-bicrystal. An electrical activity of the ..sigma.. 13 grain boundary is revealed. The diffusion length in the grain is determined by Ioannou's method. Then the experimental photocurrent profile can be compared with the theoretical one obtained by Marek's model.

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

    Energy Technology Data Exchange (ETDEWEB)

    Vitek, V.

    1992-02-01

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2015-02-28

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

  7. 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

    parameter description of the character of individual grain boundaries could previously be produced only by destructive characterization techniques. Statistical analysis of this kind of data can be expected to provide new insight into various physico-chemical processes, driven by the grain boundary energy...

  8. Method of making quasi-grain boundary-free polycrystalline solar cell structure and solar cell structure obtained thereby

    Science.gov (United States)

    Gonzalez, Franklin N.; Neugroschel, Arnost

    1984-02-14

    A new solar cell structure is provided which will increase the efficiency of polycrystalline solar cells by suppressing or completely eliminating the recombination losses due to the presence of grain boundaries. This is achieved by avoiding the formation of the p-n junction (or other types of junctions) in the grain boundaries and by eliminating the grain boundaries from the active area of the cell. This basic concept can be applied to any polycrystalline material; however, it will be most beneficial for cost-effective materials having small grains, including thin film materials.

  9. 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.

  10. Low-temperature fracture of high purity iron and its relationship to the grain boundary character

    Energy Technology Data Exchange (ETDEWEB)

    Ofuji, T. (Tohoku Univ., Sendai (Japan). Graduate School of Engineering Daido Special Steel Co. Ltd., Nagoya (Japan)); Suzuki, S. (Tohoku Univ., Sendai (Japan). Inst. of Materials Research Nippon Steel Corp., Tokyo (Japan)); Takai, S. (Tohoku Univ., Sendai (Japan). Inst. of Materials Research); Kimura, H. (Tohoku Univ., Sendai (Japan). Inst. of Materials Research)

    1992-02-01

    Mode of fracture and ductile-brittle transition temperature (DBTT) of high purity iron( 99.999% or higher ) was investigated by using two sets of specimens of different grain boundary character. The specimens having bamboo-type grain structure with high angle boundaries have fractured in the intergranular mode and their DBTT is between 110 and 125 K. Specimens with coarse grain structure have shown fractures in transgranular mode at and below 50 K. DBTT for intergranular fracture(IGF), if any, has been below 4.2 K and this has been in contrast with the the occurance of IGF even at 77K for less pure iron specimens(99.99 % or below). It has been concluded that DBTT for IGF, which has been the common fracture mode in pure iron depends strongly on the purity and grain boundary character of iron spocimens. DBTT has ranged from 125 to 4.2 K or below. Also, specimens of 99.99 % purity have been more susceptible to IGF than the specimens with 99.999 % purity. 8 refs., 6 figs.

  11. Computer simulation study of the structure of vacancies in grain boundaries

    Science.gov (United States)

    Brokman, A.; Bristowe, P. D.; Balluffi, R. W.

    1981-10-01

    The structure of vacancies in grain boundaries has been investigated by computer molecular statics employing pairwise potentials. In order to gain an impression of the vacancy structures which may occur generally, a number of variables was investigated, including metal type, boundary type, degree of lattice coincidence, and choice of boundary site. In all cases the vacancies remained as distinguishable point defects in the relatively irregular boundary structures. However, it was found that the vacancy often induced relatively large atomic displacements in the core of the boundary. These displacements often occurred only in the direct vicinity of the vacancy, but in certain cases they were widely distributed in the boundary, sometimes at surprisingly large distances. In certain cases the displacements included a large inward relaxation of one, or more, of the atoms neighboring the vacancy, and the initial vacant site became effectively ″split″. These results were classified and discussed in relation to the variables listed above. Several binding energies to the boundary were also calculated. Finally, the relevance of the results to the mechanism of boundary self-diffusion was discussed.

  12. Atomic structures of symmetric tilt grain boundaries in hexagonal close packed (hcp) crystals

    Science.gov (United States)

    Wang, J.; Beyerlein, I. J.

    2012-03-01

    Using molecular dynamics (MD) simulations, the dislocation structures of [1 \\bar {2} 1 0] symmetric tilt grain boundaries (STGBs) in hexagonal close packed (hcp) crystal structures are studied. STGBs over the entire range of possible rotation angles θ from 0° to 90° are found to have an ordered atomic structure. Formation energy calculations reveal four local minimum-energy boundaries that correspond to coherent grain boundaries (GBs). Deviations in tilt from the basal plane (θ = 0°, P_B(1) ) , prismatic plane (θ = 90°, P_B(6) ) , or one of these four minimum-energy boundaries, P_B(2) ,P_B(3) ,P_B(4) ,P_B(5) , result in the formation of a tilt wall (edge-type grain boundary dislocations, GBDs) superimposed on the nearest GB structure P_B(i) in θ-space. As θ deviates far from the rotation angle of one P_B(i) and draws closer to that of an adjacent P_B(j) , an abrupt transition in STGB base boundary structure and GBD Burgers vector occurs. For all θ, the sign and spacing of GBDs depend on θ, and their Burgers vector is either one or two times the interplanar spacing of PB. We present a simple model that generalizes the results to other c/a ratios. Subsequent MD simulations show that (1) the model forecasts the STGB structure to first-order and (2) STGBs with two distinct atomic structures can have remarkably different responses when interacting with basal lattice dislocations originating from the adjoining crystals.

  13. 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.

  14. Determination of vacancy mechanism for grain boundary self-diffusion by computer simulation

    Science.gov (United States)

    Balluffi, R. W.; Kwok, T.; Bristowe, P. D.; Brokman, A.; Ho, P. S.; Yip, S.

    1981-06-01

    It is currently established that the fast self diffusion which occurs along grain boundaries (GBs) in metals must occur by a point defect exchange mechanism. For example, it is known that rapid GB diffusion can transport a net current of atoms along GBs during both sintering and diffusional creep, and that the two species in a binary substitutional alloy diffuse at different rates in GBs. The results of an effort to establish the GB self diffusion mechanism in a bcc iron (UC OMEGA) = 5 (36.90) 001 (310) tilt boundary using the combined methods of computer molecular statics and molecular dynamics simulation are presented.

  15. Kinetics and Grain Boundary Selectivity of Discontinuous Precipitation in Binary Ni-Cr Alloy

    Science.gov (United States)

    Keskar, N.; Pattanaik, A. K.; Mani Krishna, K. V.; Srivastava, D.; Dey, G. K.

    2017-06-01

    A supersaturated Ni-Cr alloy (42 wt pct Cr) was subjected to a series of aging heat treatments in the two-phase region in the temperature range of 923 K to 1123 K (650 °C to 850 °C) for different time periods. The resultant microstructures were seen to be composed of varying volume fractions of continuous (CP) and discontinuous precipitation (DP). The DP dominated at lower temperatures, while CP dominated at higher temperatures and the expected DP termination temperature was estimated to be 1138 K (865 °C). The kinetics of DP followed the Turnbull model at lower temperatures and the Aaronson-Liu model at higher temperatures. The nucleation and growth of DP cells, which occurred via the `precipitate driven grain boundary migration,' was seen to be a strong function of the nature of the participating grain boundaries.

  16. Formation of Cavities at and Away from Grain Boundaries during 600 MeV Proton Irradiation

    DEFF Research Database (Denmark)

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

    1982-01-01

    ) regions beyond the CCZ containing a very low density of cavities. At the dose level of 2 dpa, a dense population of very small cavities is resolved on the grain boundaries and also in their immediate vicinity (in the CDZ). Furthermore, at the dose levels of 0.6 and 2 dpa, a well defined dual size...... were carried out at 120 degree C (0,42*Tm where Tm is the melting temperature in K). Transmission electron microscopy on specimens irradiated to 0.2 and 0.6 dpa has shown the presence of (a) cavity-denuded zones (CDZ) along grain boundaries, (b) cavity-containing zones (CCZ) adjacent to the CDZ and (c...... distribution of cavities is observed in the CCZ. The results are discussed in terms of agglomeration of helium atoms which are considered, during irradiation, to diffuse mainly via vacancies....

  17. Phase-field modeling of grain-boundary premelting using obstacle potentials

    Science.gov (United States)

    Bhogireddy, V. Sai Pavan Kumar; Hüter, C.; Neugebauer, J.; Steinbach, I.; Karma, A.; Spatschek, R.

    2014-07-01

    We investigate the multiorder parameter phase field model of Steinbach and Pezzolla [Physica D 134, 385 (1999), 10.1016/S0167-2789(99)00129-3] concerning its ability to describe grain boundary premelting. For a single order parameter situation solid-melt interfaces are always attractive, which allows us to have (unstable) equilibrium solid-melt-solid coexistence above the bulk melting point. The temperature-dependent melt layer thickness and the disjoining potential, which describe the interface interaction, are affected by the choice of the thermal coupling function and the measure to define the amount of the liquid phase. Due to the strictly finite interface thickness the interaction range also is finite. For a multiorder parameter model we find either purely attractive or purely repulsive finite-ranged interactions. The premelting transition is then directly linked to the ratio of the grain boundary and solid-melt interfacial energy.

  18. Rate dependence of grain boundary sliding via time-scaling atomistic simulations

    Science.gov (United States)

    Hammami, Farah; Kulkarni, Yashashree

    2017-02-01

    Approaching experimentally relevant strain rates has been a long-standing challenge for molecular dynamics method which captures phenomena typically on the scale of nanoseconds or at strain rates of 107 s-1 and higher. Here, we use grain boundary sliding in nanostructures as a paradigmatic problem to investigate rate dependence using atomistic simulations. We employ a combination of time-scaling computational approaches, including the autonomous basin climbing method, the nudged elastic band method, and kinetic Monte Carlo, to access strain rates ranging from 0.5 s-1 to 107 s-1. Combined with a standard linear solid model for viscoelastic behavior, our simulations reveal that grain boundary sliding exhibits noticeable rate dependence only below strain rates on the order of 10 s-1 but is rate independent and consistent with molecular dynamics at higher strain rates.

  19. 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.

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

    DEFF Research Database (Denmark)

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

    2008-01-01

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

  1. 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.

  2. Grain boundary high-T{sub c} dc-SQUIDs with self-organized nanocrystals

    Energy Technology Data Exchange (ETDEWEB)

    Koch, Stefanie; Michalowski, Peter; Katzer, Christian; Westerhausen, Markus; Schmidl, Frank; Seidel, Paul [Friedrich-Schiller-Universitaet Jena, Institut fuer Festkoerperphysik, Helmholtzweg 5, 07743 Jena (Germany)

    2012-07-01

    We fabricated and investigated direct current superconducting quantum interference devices (dc-SQUIDs) based on YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) grain boundary Josephson junctions. Directed embedding of gold nanoparticles different sizes can modify the crystalline structure and thus the superconducting properties of the YBCO thin films and grain boundaries. We investigated the growth conditions of these particles as well as their influence on the properties of the YBCO thin films. The variation of the size and distribution of the gold nanoparticles changes the electrical properties of the dc-SQUIDs. For this kind of device the normal resistance, critical current density, the resulting I{sub c}R{sub N}-product, the London penetration depth and transfer function are analyzed. Furthermore we show noise properties for such modified dc-SQUIDs.

  3. Grain boundary sliding mechanism during high temperature deformation of AZ31 Magnesium alloy

    Energy Technology Data Exchange (ETDEWEB)

    Roodposhti, Peiman Shahbeigi, E-mail: pshahbe@ncsu.edu [North Carolina State University (United States); University of Connecticut (United States); Sarkar, Apu; Murty, Korukonda Linga [North Carolina State University (United States); Brody, Harold [University of Connecticut (United States); Scattergood, Ronald [North Carolina State University (United States)

    2016-07-04

    High temperature tensile creep tests were conducted on AZ31 Magnesium alloy at low stress range of 1–13 MPa to clarify the existence of grain boundary sliding (GBS) mechanism during creep deformation. Experimental data within the GBS regime shows the stress exponent is ~2 and the activation energy value is close to that for grain boundary diffusion. Analyses of the fracture surface of the sample revealed that the GBS provides many stress concentrated sites for diffusional cavities formation and leads to premature failure. Scanning electron microscopy images show the appearances of both ductile and brittle type fracture mechanism. X-ray diffraction line profile analysis (based on Williamson-Hall technique) shows a reduction in dislocation density due to dynamic recovery (DRV). A correlation between experimental data and Langdon's model for GBS was also demonstrated.

  4. 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.

  5. Concurrent atomistic and continuum simulation of bi-crystal strontium titanate with tilt grain boundary

    OpenAIRE

    Yang, Shengfeng; Chen, Youping

    2015-01-01

    In this paper, we present the development of a concurrent atomistic–continuum (CAC) methodology for simulation of the grain boundary (GB) structures and their interaction with other defects in ionic materials. Simulation results show that the CAC simulation allows a smooth passage of cracks through the atomistic–continuum interface without the need for additional constitutive rules or special numerical treatment; both the atomic-scale structures and the energies of the four different [001] ti...

  6. Planar defects and grain-boundary disclinations in the superconducting compund YBaCuO

    Energy Technology Data Exchange (ETDEWEB)

    Sadanov, E.V.; Ksenofontov, V.A. ( Khar' kovskii Fiziko-Tekhnicheskii Institut, Kharkov (Ukrainian SSR))

    1989-08-01

    Planar defects separating variously oriented regions of the crystal were observed in the structure of the YBa2Cu3O(7-x) high-temperature superconductor. Also observed were rotation-type linear grain-boundary defects, which were centers of the elastic perturbation of the lattice. Both types of defects can have a substantial effect on the energy properties of superconducting materials. 5 refs.

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

    Science.gov (United States)

    Ruggles, Timothy; Hochhalter, Jacob; Homer, Eric

    2016-01-01

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

  8. 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.

  9. 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.

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

    Energy Technology Data Exchange (ETDEWEB)

    Cermak, J. (Ceskoslovenska Akademie Ved, Brno. Ustav Fyzikalni Metalurgie (Czechoslovakia))

    1990-03-01

    The grain boundary self-diffusion characteristics P={alpha}.{delta}.D{sub g} ({alpha} is the segregation factor, {delta} is the grain boundary width and D{sub g} is the grain boundary diffusion coefficient) of {sup 51}Cr were measured in the two alloys Fe-18 Cr-12 Ni and Fe-21 Cr-31 Ni. Experiments were performed in the temperature range from 973 to 1223 K using the sectioning method. In the frames of experimental errors there was found no significant difference between the results for both alloys. The temperature dependence of the triple product P for both materials can be thus described by the Arrhenius-type equation P(Cr)=(1.17{sup +2.1}{sub -0.75}).10{sup -10}.exp {l brace}-(234{plus minus}19)/RT{r brace} m{sup 3}/s. Within the errors, P values obtained in this paper are identical with those obtained in previous work for diffusion of {sup 59}Fe and {sup 63}Ni in the same materials. For the diffusion of all three basic constituents in both alloys the following equation is proposed P (Fe, Ni, Cr)=(6.7{sup +9.9}{sub -4.0}).10{sup -12}.exp {l brace}-(207{plus minus}17)/RT{r brace} m{sup 3}/s. (orig.).

  11. Enhanced rate performance of Li4Ti5O12 anodes with bridged grain boundaries

    Science.gov (United States)

    Feng, Xu-Yong; Li, Xiang; Tang, Mingxue; Gan, Alberic; Hu, Yan-Yan

    2017-06-01

    Excellent rate performance of Li4+xTi5O12 (0 exchange at 8a and 16c sites. In this paper, we reveal that inter-particle connectivity within LTO electrodes affects 8a and 16c site occupancies upon discharge and impacts Li ion diffusion. LTO electrodes of the same primary crystal structure but of different grain boundary structures were prepared and they showed significantly different electrochemical performance. LTO electrodes with a percolated 3D structural network and bridged grain boundaries offered balanced 8a-16c occupancy, Li ion exchange at 8a and 16c sites upon discharge, high ionic conductivities, and good rate performance. While LTO electrodes with isolated clusters of particles showed strong rate dependence of 8a-16c occupancy, a lack of Li ion exchange at 8a and 16c sites, large over-potential, and substantial capacity decay upon fast charging. Bridged grain boundaries in LTO secondary particles facilitate apparent solid-solution process during electrochemical cycling by maintaining Li site exchange and thus enhance the rate performance of LTO electrodes.

  12. 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.

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

    Science.gov (United States)

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

    2013-01-01

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

  14. 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.

  15. 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

    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 ρGB. We find that the dual......, 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....

  16. Intrinsic Compressive Stress in Polycrystalline Films is Localized at Edges of the Grain Boundaries

    Science.gov (United States)

    Vasco, Enrique; Polop, Celia

    2017-12-01

    The intrinsic compression that arises in polycrystalline thin films under high atomic mobility conditions has been attributed to the insertion or trapping of adatoms inside grain boundaries. This compression is a consequence of the stress field resulting from imperfections in the solid and causes the thermomechanical fatigue that is estimated to be responsible for 90% of mechanical failures in current devices. We directly measure the local distribution of residual intrinsic stress in polycrystalline thin films on nanometer scales, using a pioneering method based on atomic force microscopy. Our results demonstrate that, at odds with expectations, compression is not generated inside grain boundaries but at the edges of gaps where the boundaries intercept the surface. We describe a model wherein this compressive stress is caused by Mullins-type surface diffusion towards the boundaries, generating a kinetic surface profile different from the mechanical equilibrium profile by the Laplace-Young equation. Where the curvatures of both profiles differ, an intrinsic stress is generated in the form of Laplace pressure. The Srolovitz-type surface diffusion that results from the stress counters the Mullins-type diffusion and stabilizes the kinetic surface profile, giving rise to a steady compression regime. The proposed mechanism of competition between surface diffusions would explain the flux and time dependency of compressive stress in polycrystalline thin films.

  17. Charge accumulation and barrier formation at grain boundaries in ZnO decorated with bismuth

    Science.gov (United States)

    Domingos, H. S.; Carlsson, J. M.; Bristowe, P. D.; Hellsing, B.

    2002-12-01

    Density functional plane-wave pseudopotential calculations have been performed on two high-angle grain boundaries in ZnO which have been decorated with various quantities of Bi. The results show that both grain boundaries, which have significantly different structures, can accommodate up to about 30% of substitutional Bi in qualitative agreement with experimental observations. The segregation of Bi to the boundaries results in local charge accumulation which is localized within Bi-Bi bonds or on Bi atoms. The charge accumulation in both boundaries results in fluctuations in potential across the interface and the formation of a barrier to electron transport. However, there is no evidence for a deep acceptor level usually associated with the Schottky barrier model. The present results suggest an alternative mechanism in which electrons are trapped in Bi-Bi bonds and depleted in an external field. However, defect states have not been ruled out and it is suggested that if they exist they are caused by more complex defects than those considered here.

  18. Origin of the low grain boundary conductivity in lithium ion conducting perovskites: Li3xLa0.67-xTiO3.

    Science.gov (United States)

    Wu, Jian-Fang; Guo, Xin

    2017-02-22

    Although the bulk conductivity of lithium ion conducting Li3xLa0.67-xTiO3 electrolytes reaches the level of 10-3 S cm-1, the grain boundary conductivity is orders of magnitude lower; the origin of the low grain boundary conductivity should be thoroughly understood as a prerequisite to improve the overall conductivity. Samples with grain sizes ranging from 25 nm to 3.11 μm were prepared. According to SEM and TEM investigations, the grain boundaries are free of any second phase; however, the grain boundary conductivity is still ∼4 orders of magnitude lower than the bulk conductivity. The grain boundary conductivity decreases with decreasing grain size, indicating that the low grain boundary conductivity is not dominated only by the crystallographic grain boundary. Since electrons are attracted to the grain boundaries, as reflected by the dramatically enhanced grain boundary conductivity when electrons are introduced, the grain boundary core in Li3xLa0.67-xTiO3 should be positively charged, causing the depletion of lithium ions in the adjacent space-charge layers. The very low grain boundary conductivity can be accounted for by the lithium ion depletion in the space-charge layer.

  19. Impact of Wide-Ranging Nanoscale Chemistry on Band Structure at Cu(In, Ga)Se2 Grain Boundaries.

    Science.gov (United States)

    Stokes, Adam; Al-Jassim, Mowafak; Diercks, David; Clarke, Amy; Gorman, Brian

    2017-10-26

    The relative chemistry from grain interiors to grain boundaries help explain why grain boundaries may be beneficial, detrimental or benign towards device performance. 3D Nanoscale chemical analysis extracted from atom probe tomography (APT) (10's of parts-per-million chemical sensitivity and sub-nanometer spatial resolution) of twenty grain boundaries in a high-efficiency Cu(In, Ga)Se2 solar cell shows the matrix and alkali concentrations are wide-ranging. The concentration profiles are then related to band structure which provide a unique insight into grain boundary electrical performance. Fluctuating Cu, In and Ga concentrations result in a wide distribution of potential barriers at the valence band maximum (VBM) (-10 to -160 meV) and the conduction band minimum (CBM) (-20 to -70 meV). Furthermore, Na and K segregation is not correlated to hampering donors, (In, Ga)Cu and VSe, contrary to what has been previously reported. In addition, Na and K are predicted to be n-type dopants at grain boundaries. An overall band structure at grain boundaries is presented.

  20. 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

  1. 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.

  2. Morphological study on pentacene thin-film transistors: the influence of grain boundary on the electrical properties

    Energy Technology Data Exchange (ETDEWEB)

    Chen, Fang-Chung; Chen, Ying-Pin; Huang, Yu-Jen [Department of Photonics and Display Institute, National Chiao Tung University, Hsinchu 30010, Taiwan (China); Chien, Shang-Chieh, E-mail: fcchen@mail.nctu.edu.t [Department of Photonics and Institute of Electro-optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (China)

    2010-10-13

    We have prepared organic thin-film transistors (OTFTs) featuring pentacene molecules deposited at various substrate temperatures onto either hexamethyldisilazane (HMDS)- or poly({alpha}-methylsyrene) (P{alpha}MS)-treated SiO{sub 2} surfaces. As a result, we obtained different grain boundary densities in the conducting channel. Since the surface-modified devices featured similar grain boundary densities in their active layers, but displayed different electrical performances, we suspected that different trap states probably existed at the grain boundaries for the two different kinds of OTFTs. In addition, the surface morphologies of the initial layers featured grain boundaries that were rather blurred for the thin films prepared on the P{alpha}MS-treated substrates, whereas shallow boundaries appeared for the pentacene layers on the HMDS-treated surfaces. Therefore, we deduced that the different surface treatment processes resulted in different Schwoebel (step-edge) barriers, and hence, different morphologies. These results suggested that different trap states existed at the grain boundaries of the two types of surface-treated devices, leading to variations in the electrical performance, even though the grain boundary densities were similar.

  3. 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 (?/? interfaces). This leads to creation of bainitic ferrite/austenite interfaces (?/? interfaces). Bainite formation continues through autocatalysis with nucleation of bainitic ferrite at these

  4. 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].

  5. 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.

  6. Sensitivity analysis of Immersed Boundary Method simulations of fluid flow in dense polydisperse random grain packings

    Science.gov (United States)

    Knight, Chris; Abdol Azis, Mohd Hazmil; O'Sullivan, Catherine; van Wachem, Berend; Dini, Daniele

    2017-06-01

    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].

  7. Stress-free states of continuum dislocation fields: Rotations, grain boundaries, and the Nye dislocation density tensor

    OpenAIRE

    Limkumnerd, Surachate; Sethna, James P.

    2006-01-01

    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 stress fields vanish. We explain that a grain boundary (a dislocation wall satisfying Frank's formula) has vanishing stress in the continuum limit. We show that the general stress-free state can be wri...

  8. 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.

  9. Grain Boundary Analysis of the Garnet-like Oxides Li7+X-YLa3-XAXZr2-YNbYO12 (A = Sr or Ca

    Directory of Open Access Journals (Sweden)

    Shingo Ohta

    2016-07-01

    Full Text Available Garnet-like oxides having the formula Li7+X-YLa3-XAXZr2-YNbYO12 (A = Sr or Ca were synthesized using a solid-state reaction and their bulk and grain boundary resistivities were assessed by AC impedance measurements. A difference in grain boundary resistivity was identified between the Sr and Ca materials and so the grain boundaries were examined using electron probe microanalysis (EPMA. The difference in the grain boundary resistivities was attributed to the core-shell structure of the Sr-substituted samples. In contrast, the Ca-substituted materials exhibited accumulations of impurities at the grain boundaries.

  10. 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.

  11. Internal microstructure observation of enhanced grain-boundary sliding at room temperature in AZ31 magnesium alloy

    Energy Technology Data Exchange (ETDEWEB)

    Ando, D., E-mail: daisuke.ando.c4@tohoku.ac.jp; Sutou, Y.; Koike, J.

    2016-06-01

    The origin of grain boundary sliding (GBS) is known to be slip-induced due to plastic incompatibility near the grain boundary at room temperature. In this study, the relationship between GBS and crystal orientation was investigated in AZ31 Mg alloy rolled sheets at room temperature. The GBS tendency was determined as related to basal dislocation slip where the GBS boundaries were generally located between the grains with respectively high and low or high and high Schmid factors for basal slip. The results indicate that GBS is attributed to the plastic incompatibility caused by anisotropic basal and prismatic slip. Furthermore, GBS was located in regions with localized deformation near grain boundaries. Cross-sectional focused ion beam/transmission electron microscopy (FIB/TEM) observations of these regions revealed seriately arranged subgrains adjacent to a grain boundary. Therefore, we propose that RT-GBS in AZ31 can be caused by localized crystal rotation due to dynamic recover and recrystallization by stress concentration near the grain boundary but not ordinary GBS.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-01-01

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

  13. 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.

  14. Multiscale modelling of dislocation/grain-boundary interactions: I. Edge dislocations impinging on Σ11 (1 1 3) tilt boundary in Al

    Science.gov (United States)

    Dewald, M. P.; Curtin, W. A.

    2007-01-01

    Dislocation and grain-boundary processes contribute significantly to plastic behaviour in polycrystalline metals, but a full understanding of the interaction between these processes and their influence on plastic response has yet to be achieved. The coupled atomistic discrete-dislocation method is used to study edge dislocation pile-ups interacting with a Σ11-lang1 1 3rang symmetric tilt boundary in Al at zero temperature under various loading conditions. Nucleation of grain-boundary dislocations (GBDs) at the dislocation/grain-boundary intersection is the dominant mechanism of deformation. Dislocation pile-ups modify both the stress state and the residual defects at the intersection, the latter due to multiple dislocation absorption into the boundary, and so change the local grain-boundary/dislocation interaction phenomena as compared with cases with a single dislocation. The deformation is irreversible upon unloading and reverse loading if multiple lattice dislocations absorb into the boundary and damage in the form of microvoids and loss of crystalline structure accumulates around the intersection. Based on these results, the criteria for dislocation transmission formulated by Lee, Robertson and Birnbaum are extended to include the influences of grain-boundary normal stress, shear stress on the leading pile-up dislocation and minimization of step height at the intersection. Two possible yield loci for the onset of GBD nucleation versus compressive stress and relevant shear stresses are derived from the simulations. These results, and similar studies on other boundaries and dislocation characters, guide the formulation of continuum constitutive behaviours for use in discrete-dislocation or strain-gradient plasticity modelling.

  15. Intergranular fracture stress and phosphorus grain boundary segregation of a Mn-Ni-Mo steel

    Energy Technology Data Exchange (ETDEWEB)

    Naudin, C.; Frund, J.M. [EDF, Moret sur Loing (France). Direction des Etudes et Recherches; Pineau, A. [Ecole des Mines de Paris, Evry (France). Centre des Materiaux

    1999-04-09

    Nuclear Reactor Pressure Vessel (RPV) steel A508 class 3 which is a low alloyed steel is not usually sensitive to reversible temper embrittlement when properly heat treated. However heterogeneous zones may be present in particular near the inner side of the vessel. These zones result from the segregation of the alloying elements (C, Mn, Ni, Mo) and impurities (S, P) taking place during solidification of the material. They are called segregated zones (or ghost lines). They can reach 2 mm thick along the radius and 30 mm long through the circumferential direction. Their susceptibility to reversible temper embrittlement is mainly due to grain boundary phosphorus segregation triggering brittle intergranular fracture when the material is tested at low temperature. In this material like in other steels the influence of some other alloying elements (Mo, Mn...) is clearly significant and should also be taken into account. But phosphorus effect has proved to be predominant. The aim of the present study is therefore to find out a quantitative relationship between grain boundary phosphorus segregation and critical intergranular fracture stress. A synthetic steel with a chemical composition representative of an average segregated zone was prepared for the present study. A number of heat treatments were applied to reach different embrittlement conditions. Then brittle fracture properties were obtained by performing cryogenic fracture tests on notched tensile specimens while the corresponding grain boundary phosphorus levels were measured by Auger electron spectroscopy. Systematic fractographic observations were carried out. Moreover an attempt to determine the influence of temperature on the critical intergranular fracture stress was made.

  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. 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...... be as low as the physical temperature in the temperature range investigated. This makes it possible to use the resistively shunted junction (RSJ) model with thermal fluctuations to get a limiting performance of high-T c devices utilizing the AC Josephson effect. The lowest value of the linewidth of 72 GHz...

  18. 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.

  19. 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

    morphology affects the electronic properties is crucial for the development of applications such as flexible electronics, energy harvesting devices or sensors. We here report on atomic scale characterization of several GBs and on the structural-dependence of the localized electronic states in their vicinity......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...

  20. Simulation of the structure of vacancies in high angle grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Bristowe, P.D.; Brokman, A.; Spaepen, F.; Balluffi, R.W.

    1980-06-01

    Since the modeling approach used in an earlier work is used at an atomic level, this is the most appropriate and reliable technique available. To complement this study, however, we have also employed a hard sphere dynamic model and a bubble raft model because in the past they have provided useful qualitative insight into the structure of a variety of defects in two-dimensional crystalline and amorphous systems. The computed results form part of a wider investigation of vacancies and interstitials in various grain boundaries in which the binding energies are analyzed and related to the defect structure and form of the interatomic potential.

  1. Atomistic simulations on intergranular fracture toughness of copper bicrystals with symmetric tilt grain boundaries

    Science.gov (United States)

    Cui, Cheng Bin; Beom, Hyeon Gyu

    2018-01-01

    The intergranular fracture toughness of Cu bicrystals with symmetric tilt grain boundaries was investigated using atomistic simulations. Mode I fracture of Cu bicrystals with an intergranular crack was considered. The boundary conditions were specified by the near-tip displacement fields obtained based on linear elastic fracture mechanics (LEFM). Based on the energy interpretation of the energy release rate, a two-specimen method was adopted to determine the fracture toughness. The simulation results of the fracture toughness matched well with those determined using LEFM. In contrast to the toughness obtained using the Griffith energy criterion, the atomistic simulation results for the same bicrystal were not constants, but dependent on the crack-tip circumstances. This behavior was mainly associated with the different local stress conditions and fracture patterns observed for the different models.

  2. Method for computing short-range forces between solid-liquid interfaces driving grain boundary premelting

    Science.gov (United States)

    Hoyt, J. J.; Olmsted, David; Jindal, Saryu; Asta, Mark; Karma, Alain

    2009-02-01

    We present a molecular dynamics based method for accurately computing short-range structural forces resulting from the overlap of spatially diffuse solid-liquid interfaces at wetted grain boundaries close to the melting point. The method is based on monitoring the fluctuations of the liquid layer width at different temperatures to extract the excess interfacial free energy as a function of this width. The method is illustrated for a high-energy Σ9 twist boundary in pure Ni. The short-range repulsion driving premelting is found to be dominant in comparison to long-range dispersion and entropic forces and consistent with previous experimental findings that nanometer-scale layer widths may be observed only very close to the melting point.

  3. Density functional theory study of stoichiometric and nonstoichiometric ZnO grain boundaries

    Science.gov (United States)

    Körner, Wolfgang; Bristowe, Paul D.; Elsässer, Christian

    2011-07-01

    We present a density functional theory analysis of stoichiometric and nonstoichiometric ZnO tilt grain boundaries (GBs) that reveals under which conditions such extrinsically undoped GBs may become electrically active. In the case of ZnO the self-interaction correction (SIC) scheme used allows a more accurate description of the formation energies as well as the electronic levels than the local density approximation (LDA). The results obtained with the SIC scheme deviate in some crucial ways from the LDA results of recent years. First, stoichiometric and nonstoichiometric ZnO GBs can show occupied deep levels whenever oxygen atoms are undercoordinated. Second, ZnO GBs with an oxygen excess at the boundary plane can exhibit unoccupied deep levels which may account for an experimentally observed weak varistor effect found in undoped polycrystalline ZnO.

  4. Explicit calculation for grain boundary barrier height in polysilicon TFTs based on quasi-two-dimensional approach

    Science.gov (United States)

    Deng, Wanling; Huang, Junkai

    2013-09-01

    A physical-based explicit calculation to the height of grain boundary barrier has been derived based on the quasi-two-dimensional approach at discrete grain boundaries. The analytical solution is obtained by using the Lambert W function, combining both the uniform distributed deep states and the exponential tail states. The proposed scheme is demonstrated as an accurate and computationally efficient solution in a closed form, which can serve as a basis for the discrete-grain-based models of mobility and drain current in polysilicon thin film transistors. It is verified successfully by comparisons with both numerical simulation and experimental data.

  5. 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...

  6. 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.

  7. Special grain boundaries in the nugget zone of friction stir welded AA6061-T6 under various welding parameters

    Energy Technology Data Exchange (ETDEWEB)

    Tao, Wang [Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061 (China); Yong, Zou, E-mail: yzou@sdu.edu.cn [Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061 (China); Xuemei, Liu [Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061 (China); Matsuda, Kenji [Department of Materials Science and Technology, Faculty of Engineering, University of Toyama, Toyama 930-8555 (Japan)

    2016-08-01

    The age hardenable AA6061-T6 plate was butt welded by friction stir welding. The total heat input, generated by friction between the tool and work piece and plastic deformation, results in a consumption of meta-stable phases in the nugget zone. Precipitation phenomena were closely related to the diffusion of the solute atoms. The existence of special grain boundaries like Σ1a and Σ3 will increase the difficulty in diffusion, which will improve the hardness in the nugget zone. Furthermore, the formation of Σ3 grain boundaries can result from an impingement of re-crystallized grains coming from texture components in twin relationship already. An appropriate strain level may benefit the development of the twin components with a similar intensity. The welding parameters have an effect on heat source mode and the strain level. Then, the type of dynamic re-crystallization and distribution of the special grain boundaries was altered by changing the parameters.

  8. 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.

  9. Dissociation of grain boundary dislocations in SrBi2Ta2O9 ferroelectric thin films

    Science.gov (United States)

    Zhu, Xinhua; Zhu, Jianmin; Zhou, Shunhua; Li, Qi; Liu, Zhiguo; Ming, Naiben

    2001-08-01

    In this work, the dissociation of grain boundary dislocations (GBDs) is reported in SrBi2Ta2O9 (SBT) ferroelectric thin films with c-axis orientation grown by pulsed-laser deposition on Pt/TiO2/SiO2/Si(100) substrates. Small-angle (8.2°) [001] tilt grain boundaries with a boundary plane close to the (110) plane exhibit partial GBDs separated by stacking faults. The dissociated grain-boundary structures have twice the number of GBDs and interdislocation core channel width smaller than that Frank's geometrical rule predicts. At the equilibrium, the repulsive elastic force between partial dislocations is balanced by an attractive force produced by the formation of a stacking fault between the partials. Based on this, the stacking fault energy is evaluated to be 0.27-0.29 J/m2. The relationship between the leakage current of SBT films and dissociation of GBDs is also discussed.

  10. 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.

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

    DEFF Research Database (Denmark)

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

    2013-01-01

    Complementary microscopic and diffraction based methods revealed a peculiar microstructure of electrodeposited nickel. For the as-deposited layer, thus, without any additional treatment, multiple twinning yields a high population of Σ3n boundaries, which interrupts the network of normal high angle...... grain boundaries. A peculiar arrangement of Σ3 boundaries forming five-fold junctions is observed. The resulting microstructure meets the requirements for grain boundary engineering. Twinning induced effects on the crystallographic orientation of grains result in one major texture component being a ⟨210......⟩ fiber axis and additional minor orientations originating from first and second generation twins of ⟨210⟩, i.e., ⟨542⟩ and ⟨20 2 1⟩....

  12. 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.)

  13. Diffusion to dislocation creep transition in upper-mantle from Si grain-boundary diffusion

    Science.gov (United States)

    Fei, Hongzhan; Koizumi, Sanae; Sakamoto, Naoya; Hashiguchi, Minako; Yurimoto, Hisayoshi; Yamazaki, Daisuke; Katsura, Tomoo

    2014-05-01

    Dislocation creep causes non-Newtonian viscosity and seismic anisotropy, whereas diffusion creep causes Newtonian viscosity and no or weak seismic anisotropy. Determination of deformation mechanism in the Earth's interior is thus essential to understand mantle dynamics. Although previous deformation studies on olivine suggested the dislocation to diffusion creep transition with depth in the upper mantle, recent studies suggested possible misinterpretation of those results due to experimental difficulties. Since the olivine creep is considered to be controlled by silicon diffusion, we measured silicon grain-boundary diffusion coefficient in Mg-olivine as a function of temperature, pressure, and water content to estimate the diffusion creep rate. The experimental results show a small activation enthalpy (240-260 kJ/mol), small activation volume (1.8±0.2 cm3/mol), and small water-content exponent (0.22±0.05). The smaller activation energy of grain-boundary diffusion than that of dislocation creep predicts a diffusion to dislocation creep transition in the upper mantle, which is in contrast with the previous model. The Gutenberg discontinuity could be caused by this creep-mechanism transition. The weak seismic anisotropy in lithosphere is interpreted as fossil anisotropy formed at spreading ridges. The dominance of diffusion creep in upper lithosphere accounts for the Newtonian rheology suggested by postglacial rebound.

  14. 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.

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

    Science.gov (United States)

    Barrios Vargas, Jose E.; Toft Falkenberg, Jesper; Soriano, David; Cummings, Aron W.; Brandbyge, Mads; Roche, Stephan

    2017-06-01

    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. In this work, we use numerical simulations to study the electrical properties of hydrogenated polycrystalline graphene. We find a strong correlation between the spatial distribution of the hydrogen adsorbates and the charge transport properties. Charge transport is weakly sensitive to hydrogenation when adsorbates are confined to the grain boundaries, while a uniform distribution of hydrogen degrades the electronic mobility. This difference stems from the formation of the hydrogen-induced resonant impurity states, which are inhibited when the honeycomb symmetry is locally broken by the grain boundaries. 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.

  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. Separating grain-boundary and bulk recombination with time-resolved photoluminescence microscopy

    Energy Technology Data Exchange (ETDEWEB)

    Kuciauskas, Darius [National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA; Lu, Dingyuan [First Solar, 1035 Walsh Ave., Santa Clara, California 95050, USA; Grover, Sachit [First Solar, 1035 Walsh Ave., Santa Clara, California 95050, USA; Xiong, Gang [First Solar, 1035 Walsh Ave., Santa Clara, California 95050, USA; Gloeckler, Markus [First Solar, 1035 Walsh Ave., Santa Clara, California 95050, USA

    2017-12-04

    Two-photon excitation (2PE) microscopy allows contactless and non-destructive cross-sectional analysis of grain-boundary (GB) and grain-interior (GI) properties in polycrystalline solar cells, with measurements of doping uniformity, space-charge field distribution, and carrier dynamics in different regions of the device. Using 2PE time-resolved microscopy, we analyzed charge-carrier lifetimes near the GBs and in the GI of polycrystalline thin-film CdTe solar cells doped with As. When the grain radius is larger than the minority-carrier diffusion length, GI lifetimes are interpreted as the bulk lifetimes ..tau..B, and GB recombination velocity SGB is extracted by comparing recombination rates in the GI and near GBs. In As-doped CdTe solar cells, we find ..tau..B = 1.0-2.4 ns and SGB = (1-4) x 105 cm/s. The results imply the potential to improve solar cell voltage via GB passivation and reduced recombination center concentration in the GI.

  18. Atomistic study of hydrogen embrittlement of grain boundaries in nickel: II. Decohesion

    Science.gov (United States)

    Tehranchi, A.; Curtin, W. A.

    2017-10-01

    Atomistic simulations of bicrystal samples containing a grain boundary are used to examine the effect of hydrogen atoms on the nucleation of intergranular cracks in Ni. Specifically, the theoretical strength is obtained by rigid separation of the two crystals above and below the GB and the yield strength (point of dislocation emission) is obtained by standard tension testing normal to the GB. These strengths are computed in pure Ni and Ni with H segregated to the grain boundaries under conditions typical of H embrittlement in Ni, and in artificially highly-H-saturated states. In all GBs studied here, the theoretical strength \\hat{σ } is not significantly reduced by the presence of the hydrogen atoms. Similarly, with the exception of the Ni {{Σ }}27(115) boundary, the yield strength {σ }{{y}} is not significantly altered by the presence of segregated H atoms. In all cases, the theoretical strengths are ˜25 GPa and the yield strengths are ˜10 GPa, so that (i) the theoretical strength is always well above the yield strength, with or without H, and (ii) both strengths are far above the bulk plastic flow stress, {σ }{{y}}{{B}} of Ni and Ni alloys. Significant reductions in fracture energy (25%-45%) are only achieved for some of the artificially high-H-segregation cases and then only when all the H around the GB is allow to diffuse locally to the fracture surface, which corresponds to unlikely out-of-equilibrium segregation plus local kinetics. Complementing recent work showing that H does not change the ability of GB cracks to emit dislocations and blunt, the present work indicates that equilibrium segregation of hydrogen atoms to GBs has little effect on lowering the GB strength and energy, and so does not significantly facilitate nucleation of intergranular cracks.

  19. Effect of Trace Levels of Si on Microstructure and Grain Boundary Segregation in DOP-26 Iridium Alloy

    Science.gov (United States)

    Pierce, Dean; Muralidharan, Govindarajan; Heatherly, Lee; Fox, Ethan

    2017-10-01

    The thermodynamics and kinetics of Silicon (Si) segregation to grain boundaries in Iridium alloy DOP-26 with added trace levels of Si of 6, 11, 29, and 36 wppm was studied by Auger Electron Spectroscopy. The four alloys were annealed at 1500 or 1535 °C for 19 or 76 hours followed by cooling at three different rates. Si enrichment at the grain boundaries (GB) increased with increasing bulk Si content, with the grain boundary Si enrichment factors ranging from 62 to 344, depending on the bulk Si content and the cooling rate. Grain boundary Si contents increased with decreasing cooling rate in all alloys, indicating that Si GB segregation is influenced by both thermodynamic and kinetic factors in the alloys and temperature ranges of the study. A Langmuir-McLean isotherm-based model was successfully used to predict the temperature dependence of GB Si segregation in DOP-26 alloys with Si additions and estimate the temperature independent free energy of Si segregation to grain boundaries in DOP-26.

  20. TEM analysis of grain boundary phase in Mn-Zn ferrites; Mn-Zn ferrite ryukaiso no nano kozo kaiseki

    Energy Technology Data Exchange (ETDEWEB)

    Sato, T.; Otsuki, E. [Tokin Corp., Sendai (Japan)

    1996-12-15

    A control of nano-structure in sintering process is necessary to decrease the power loss of Mn-Zn ferrites. The purpose of this paper is to investigate the forming process of nano-structure during sintering as well as the structural change of spinel matrix and grain boundary which are affected by the oxygen partial pressure of sintering atmosphere by using High-Resolution Transmission Electron Microscopy (HRTEM). As a result, it was revealed by the TEN images (bright, dark-field and lattice-fringe images) and the electron diffraction analyses that the thickness of the grain boundary layers was found to increase with increasing oxygen partial pressure of sintering atmosphere. And the reduction of power loss by adding SiO2 and CaO is thought of due to the formation of highly resistive grain boundary layers. 7 refs., 5 figs.

  1. Magnetic Skyrmion Formation at Lattice Defects and Grain Boundaries Studied by Quantitative Off-Axis Electron Holography.

    Science.gov (United States)

    Li, Zi-An; Zheng, Fengshan; Tavabi, Amir Hossein; Caron, Jan; Jin, Chiming; Du, Haifeng; Kovács, András; Tian, Mingliang; Farle, Michael; Dunin-Borkowski, Rafal E

    2017-03-08

    We use in situ Lorentz microscopy and off-axis electron holography to investigate the formation and characteristics of skyrmion lattice defects and their relationship to the underlying crystallographic structure of a B20 FeGe thin film. We obtain experimental measurements of spin configurations at grain boundaries, which reveal inversions of crystallographic and magnetic chirality across adjacent grains, resulting in the formation of interface spin stripes at the grain boundaries. In the absence of material defects, we observe that skyrmions lattices possess dislocations and domain boundaries, in analogy to atomic crystals. Moreover, the distorted skyrmions can flexibly change their size and shape to accommodate local geometry, especially at sites of dislocations in the skyrmion lattice. Our findings provide a detailed understanding of the elasticity of topologically protected skyrmions and their correlation with underlying material defects.

  2. Simultaneous visualization of graphene grain boundaries and wrinkles with structural information by gold deposition.

    Science.gov (United States)

    Yu, Seong Uk; Park, Beomjin; Cho, Yeonchoo; Hyun, Seung; Kim, Jin Kon; Kim, Kwang S

    2014-08-26

    Although line defects such as grain boundaries (GBs) and wrinkles are unavoidable in graphene, difficulties in identification preclude studying their impact on electronic and mechanical properties. As previous methods focus on a single type of line defect, simultaneous measurements of both GBs and wrinkles with detailed structural information have not been reported. Here, we introduce effective visualization of both line defects by controlled gold deposition. Upon depositing gold on graphene, single lines and double lines of gold nanoparticles (NPs) are formed along GBs and wrinkles, respectively. Moreover, it is possible to analyze whether a GB is stitched or overlapped, whether a wrinkle is standing or folded, and the width of the standing collapsed wrinkle. Theoretical calculations show that the characteristic morphology of gold NPs is due to distinct binding energies of line defects, which are correlated to disrupting diffusion of NPs. Our approach could be further exploited to investigate the defect structures of other two-dimensional materials.

  3. 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.

  4. Further investigations on dc-SQUID gradiometers based on growth modified bi-crystal grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Michalowski, Peter; Katzer, Christian; Kuhwald, Daniel; Koch, Stefanie; Schmidl, Frank; Seidel, Paul [Friedrich-Schiller-Universitaet Jena, Institut fuer Festkoerperphysik, Helmholtzweg 5, Jena (Germany)

    2012-07-01

    The properties of grain boundaries of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBCO) films grown on bi-crystal substrates can be modified by gold nano crystallites self-assembling from an intermediate gold layer during pulsed laser deposition. These gold particles act as additional pinning centers in the YBCO layer. Using a new layout, which enables us to restrict the gold nano crystallites only to the Superconducting QUantum Interference Device (SQUID) or the antenna structures, we fabricated dc-SQUID gradiometers. We present results of the temperature dependence of the London penetration depth as well as of noise measurements carried out with ac- and dc-bias. In addition we investigated the dependence of the superconducting properties on the thickness of the initial gold layer.

  5. Energy scales in YBaCuO grain boundary biepitaxial Josephson junctions

    Energy Technology Data Exchange (ETDEWEB)

    Tafuri, F., E-mail: tafuri@na.infn.it [Dip. Ingegneria dell' Informazione, Seconda Universita di Napoli, 81031 Aversa (CE) (Italy); CNR-SPIN, UOS Napoli, Monte S. Angelo via Cinthia, 80126 Napoli (Italy); Dip. Scienze Fisiche, Universita di Napoli Federico II, Monte S. Angelo via Cinthia, 80126 Napoli (Italy); Stornaiuolo, D. [DPMC, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4 (Switzerland); CNR-SPIN, UOS Napoli, Monte S. Angelo via Cinthia, 80126 Napoli (Italy); Lucignano, P. [CNR-ISC, sede di Tor Vergata, Via del Fosso del Cavaliere 100, 00133 Roma (Italy); Dip. Scienze Fisiche, Universita di Napoli Federico II, Monte S. Angelo via Cinthia, 80126 Napoli (Italy); Galletti, L. [Dip. Scienze Fisiche, Universita di Napoli Federico II, Monte S. Angelo via Cinthia, 80126 Napoli (Italy); Longobardi, L. [Dip. Ingegneria dell' Informazione, Seconda Universita di Napoli, 81031 Aversa (CE) (Italy); Massarotti, D. [Dip. Scienze Fisiche, Universita di Napoli Federico II, Monte S. Angelo via Cinthia, 80126 Napoli (Italy); CNR-SPIN, UOS Napoli, Monte S. Angelo via Cinthia, 80126 Napoli (Italy); Montemurro, D. [NEST and Scuola Normale Superiore, Piazza San Silvestro 12, I-56127 Pisa (Italy); Papari, G. [INPAC - Institute for Nanoscale Physics and Chemistry, Nanoscale Superconductivity and Magnetism Pulsed Fields Group, K.U. Leuven, Celestijnenlaan 200 D, B-3001 Leuven (Belgium); Dip. Scienze Fisiche, Universita di Napoli Federico II, Monte S. Angelo via Cinthia, 80126 Napoli (Italy); Barone, A.; Tagliacozzo, A. [Dip. Scienze Fisiche, Universita di Napoli Federico II, Monte S. Angelo via Cinthia, 80126 Napoli (Italy); CNR-SPIN, UOS Napoli, Monte S. Angelo via Cinthia, 80126 Napoli (Italy)

    2012-09-15

    Self-assembled nanoscale channels may naturally arise in the growth process of grain boundaries (GBs) in high critical temperature superconductor (HTS) systems, and deeply influence the transport properties of the GB Josephson junctions (JJs). By isolating nano-channels in YBCO biepitaxial JJs and studying their properties, we sort out specific fingerprints of the mesoscopic nature of the contacts. The size of the channels combined to the characteristic properties of HTS favors a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current-voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Thouless energy emerges as a characteristic energy of these types of Josephson junctions. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed, along with elements to take into account when designing HTS nanostructures.

  6. Atomic simulations of twist grain boundary structures and deformation behaviors in aluminum

    Directory of Open Access Journals (Sweden)

    Qing Yin

    2017-01-01

    Full Text Available The structures and behaviors of grain boundaries (GBs have profound effects on the mechanical properties of polycrystalline materials. In this paper, twist GBs in aluminum were investigated with molecular dynamic simulations to reveal their atomic structures, energy and interactions with dislocations. One hundred twenty-six twist GBs were studied, and the energy of all these twist GBs were calculated. The result indicates that and twist GBs have lower energy than twist GBs because of their higher interplanar spacing. In addition, 12 types of twist GBs in aluminum were chosen to explore the deformation behaviors. Low angle twist GBs with high density of network structures can resist greater tension because mutually hindering behaviors between partial dislocations increase the twist GB strength.

  7. The role of grain boundaries in the mechanism of plasma immersion hydrogenation of nanocrystalline magnesium films

    Energy Technology Data Exchange (ETDEWEB)

    Pranevicius, L. [Vytautas Magnus University, Physics Department, 58 K. Donelaicio St., 44248 Kaunas (Lithuania)]. E-mail: l.pranevicius@gmf.vdu.lt; Milcius, D. [Lithuanian Energy Institute, Surface Treatment Laboratory, 3 Breslaujos St., 44403 Kaunas (Lithuania); Pranevicius, L.L. [Vytautas Magnus University, Physics Department, 58 K. Donelaicio St., 44248 Kaunas (Lithuania); Lithuanian Energy Institute, Surface Treatment Laboratory, 3 Breslaujos St., 44403 Kaunas (Lithuania); Templier, C. [Laboratoire de Metallurgie Physique, Universite de Poitiers, Bd. 3, Teleport 2, B.P. 179, 86960 Futuroscope (France); Bobrovaite, B. [Vytautas Magnus University, Physics Department, 58 K. Donelaicio St., 44248 Kaunas (Lithuania); Barnackas, I. [Vytautas Magnus University, Physics Department, 58 K. Donelaicio St., 44248 Kaunas (Lithuania)

    2006-04-15

    In this paper, attention in focused on the nanostructured magnesium films for hydrogen storage. It is shown that 2 {mu}m thick Mg film is transformed into MgH{sub 2} film under high-flux and fluence hydrogen plasma immersion ion implantation at 450 K for 15 min. All hydrogen desorbs at temperature about 530 K, which corresponds to the decomposition of MgH{sub 2} {sup {yields}} Mg + H{sub 2}{up_arrow}. The macroscopic and microscopic observations show that magnesium film undergoes a high deformation and restructuring during hydrogenation-dehydrogenation reaction. The suggested hydrogenation model is based upon the incorporation of excess of hydrogen atoms in grain boundaries of nanocrystalline Mg film driven by the increase in surface chemical potential associated with the implantation flux. The results provide new aspects of hydriding of thin nanocrystalline film materials under highly non-equalibrium conditions on the surface.

  8. High coercivity in rare-earth lean nanocomposite magnets by grain boundary infiltration

    Energy Technology Data Exchange (ETDEWEB)

    Madugundo, Rajasekhar, E-mail: mraja@udel.edu [Department of Physics and Astronomy, University of Delaware, Newark, DE 19716 (United States); Salazar-Jaramillo, Daniel [BCMaterials, Bizkaia Science and Technology Park, E-48160 Derio (Spain); Manuel Barandiaran, Jose [BCMaterials, Bizkaia Science and Technology Park, E-48160 Derio (Spain); Department of Electricity & Electronics, University of the Basque Country (UPV/EHU), E-48080 Bilbao (Spain); Hadjipanayis, George C., E-mail: hadji@udel.edu [Department of Physics and Astronomy, University of Delaware, Newark, DE 19716 (United States)

    2016-02-15

    A significant enhancement in coercivity was achieved by grain boundary modification through low temperature infiltration of Pr{sub 75}(Cu{sub 0.25}Co{sub 0.75}){sub 25} eutectic alloy in rare-earth lean (Pr/Nd)–Fe–B/α-Fe nanocomposite magnets. The infiltration procedure was carried out on ribbons and hot-deformed magnets at 600–650 °C for different time durations. In Nd{sub 2}Fe{sub 14}B/α-Fe ribbons, the coercivity increased from 5.3 to 23.8 kOe on infiltration for 4 h. The Pr{sub 2}Fe{sub 14}B/α-Fe hot-deformed magnet shows an increase in coercivity from 5.4 to 22 kOe on infiltration for 6 h. The increase in the coercivity comes at the expense of remnant magnetization. X-ray diffraction studies confirm the presence of both the hard Nd{sub 2}Fe{sub 14}B and soft α-Fe phases. A decrease in the soft α-Fe phase content was observed after infiltration. - Highlights: • Enhancement in coercivity was achieved by grain boundary modification. • Coercivity increased from 5.3 to 23.8 kOe in Nd{sub 2}Fe{sub 14}B/α-Fe on infiltration. • Pr{sub 2}Fe{sub 14}B/α-Fe deformed magnet shows an increase in coercivity from 5.4 to 22 kOe. • The increase in the coercivity comes at the expense of remnant magnetization. • A decrease in the soft α-Fe phase content was observed after infiltration.

  9. 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

    2013-09-17

    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 (187)Os/(188)Os 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 (187)Os/(188)Os ratios <1.0 and at face value suggest mixing with extraterrestrial Os with (187)Os/(188)Os of ∼0.13. However, the Os concentration in bulk sample and magnetic grains from Belgium is 2.8 pg/g and 15 pg/g, respectively, much lower than that in average upper continental crust (=31 pg/g), indicating no 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 (187)Os/(188)Os 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.

  10. A new mechanism for fluid migration in midcrustal shear zones based on viscous grain boundary sliding and creep cavitation

    Science.gov (United States)

    Fusseis, F.; Regenauer-Lieb, K.; Liu, J.

    2009-04-01

    The common perception of fluid migration pathways in midcrustal shear zones relies mostly on hydraulic fractures forming by tensile failure where pore fluid pressure equalizes the combined lithostatic pressure and the tensile strength of the rock. Where the associated pervasive fracturing on multiple scales does not occur but the presence of a fluid phase is indicated, different models are needed to explain fluid transfer. Here we present the ‘granular fluid pump', a model that is capable of explaining fluid migration in fine-grained ultramylonites typical for midcrustal shear zones. We investigated the porosity evolution across the margin of a shear zone in central Australia that formed in granitic gneiss at upper-greenschist facies conditions (Fliervoet et al., 1997). The strain gradient in the sample, which is macroscopically expressed by the reorientation of a preexisting foliation and a substantial grain size reduction, is interpreted as a proxy for time. Our investigation showed that the microstructural evolution of the sample is characterized by two major processes: a) The progressive reduction in abundance and size of potassium feldspar and plagioclase clasts, which decay, with increasing strain, in hydration reactions to form a fine-grained (4% and that the relative frequency of the smallest detectable pores (1.3-3.9 microns) increases from tens of microns) tend to cluster in plagioclase grains, a form of porosity that is associated with feldspar decay and a removal of chemical components. Towards the shear zone center, pores are smaller (micron-sized and below) and generally occur along grain boundaries. Most pores occur in ‘pore sheets' that characterize the multiphase ultramylonitic layers. These grain boundary pores often occur at triple junctions. Lobes extending into neighboring grains and pockmarked surfaces evidence dissolution; little crystallites in grain boundary pores indicate precipitation and mineral growth. Both observations indicate the

  11. The Role of Pore-Formers on Grain Interior and Grain Boundary Conductivity in Tape-Cast Porous Sheets for Electrochemical Flue Gas Purification

    DEFF Research Database (Denmark)

    Schmidt, Cristine Grings; Andersen, Kjeld Bøhm; Stamate, Eugen

    2018-01-01

    Ce0.9Gd0.1O1.95 (CGO) electrolytes for electrochemical flue gas purification were fabricated by means of tape casting with different types, shapes and sizes of pore-formers. The sintered bodies were characterized with electrochemical impedance spectroscopy, to investigate the role of the different...... pore-formers on the electrochemical properties of the cast tapes. A strong effect of the different pore-formers on the conductivity (both grain interior and grain boundary conductivities) was observed. In addition, the conductivity data were also correlated with previously obtained gas permeability...

  12. 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

  13. Determination of vacancy mechanism for grain boundary self-diffusion by computer simulation

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R. W.; Kwok, T.; Bristowe, P. D.; Brokman, A.; Ho, P. S.; Yip, S.

    1981-06-01

    It is currently well established that the fast self-diffusion which occurs along grain boundaries (GBs) in metals must occur by a point defect exchange mechanism. For example, it is known that rapid GB diffusion can transport a net current of atoms along GBs during both sintering and diffusional creep, and that the two species in a binary substitutional alloy diffuse at different rates in GBs. However, it has not been possible to establish firmly whether the defect mechanism involves the exchange of atoms with vacancy or interstitial point defects. It has been suspected that the vacancy exchange mechanism must apply but it has been difficult to prove this hypothesis because of a lack of detailed information at the atomistic level. The results are presented of an effort to establish the GB self-diffusion mechanism in a bcc iron ..sigma.. = 5 (36.9/sup 0/) (001) (310) tilt boundary using the combined methods of computer molecular statics and molecular dynamics simulation.

  14. 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 (operating along with crystal slip

  15. 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

  16. 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.

  17. Stress-free states of continuum dislocation fields : Rotations, grain boundaries, and the Nye dislocation density tensor

    NARCIS (Netherlands)

    Limkumnerd, Surachate; Sethna, James P.

    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

  18. Effects of Grain Boundaries and Dislocation Cell Walls on Void Nucleation and Growth in Aluminium during Fast Neutron Irradiation

    DEFF Research Database (Denmark)

    Horsewell, Andy; Rahman, F. A.; Singh, Bachu Narain

    1983-01-01

    High purity aluminium irradiated to fluences between 2 multiplied by 10**2**1 and 1 multiplied by 10**2**4 n. m** minus **2 (E greater than 1 Mev) at 120 degree C has been investigated by TEM. A void denuded zone is seen both at grain boundaries and dislocation cell walls. Enhanced void formation...

  19. Influence of point defects on grain boundary diffusion in oxides. Final technical report, July 1, 1990--June 30, 1993

    Energy Technology Data Exchange (ETDEWEB)

    Stubican, V.S.

    1993-11-01

    Grain boundary diffusion coefficients of {sup 57}Co and {sup 59}Co in polycrys. NiO, NiO bicrystal, and polycrys. Fe{sub 3}O{sub 4} were determined at various oxygen pressures at 750 C. For NiO, the low oxygen pressure region (<10{sup {minus}10} MPa) displayed constant grain boundary diffusion coefficients as the oxygen pressure decreased, indicating an extrinsic region in which the impurity-induced defects dominated the intrinsic defects. At greater oxygen pressures, the intrinsic defects (Ni vacancies) dominated the extrinsic defects, causing the diffusion to increase with pressure. For Fe{sub 3}O{sub 4}, at low oxygen pressures (<10{sup {minus}16} MPa), the grain boundary diffusion coefficient increased when the pressure decreased, owing to interstitial type diffusion; at >10{sup {minus}15} MPa, the diffusion increased with pressure, owing to vacancy type diffusion. D{sub gb} of Co ions in Fe{sub 3}O{sub 4} is proportional to pO{sub 2}{sup {minus}2/3} in the low pressure region and to pO{sub 2}{sup 2/3} in the high pressure region, indicating similar mechanisms in the grain boundary diffusion and volume diffusion. Ratio of D{sub gb}/D was about 10{sup 3}.

  20. Effect of Grain Boundary Character Distribution on the Impact Toughness of 410NiMo Weld Metal

    DEFF Research Database (Denmark)

    Divya, M.; Das, Chitta Ranjan; Chowdhury, Sandip Ghosh

    2016-01-01

    Grain boundary character distributions in 410NiMo weld metal were studied in the as-welded, first-stage, and second-stage postweld heat treatment (PWHT) conditions, and these were correlated with the Charpy-V impact toughness values of the material. The high impact toughness values in the weld me...

  1. 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

  2. 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...

  3. Effect of Grain Orientation and Boundary Distributions on Hydrogen-Induced Cracking in Low-Carbon-Content Steels

    Science.gov (United States)

    Masoumi, Mohammad; Coelho, Hana Livia Frota; Tavares, Sérgio Souto Maior; Silva, Cleiton Carvalho; de Abreu, Hamilton Ferreira Gomes

    2017-08-01

    Hydrogen-induced cracking (HIC) causes considerable economic losses in a wide range of steels exposed to corrosive environments. The effect of crystallographic texture and grain boundary distributions tailored by rolling at 850 °C in three different steels with a body-centered cube structure was investigated on HIC resistance. The x-ray and electron backscattered diffraction techniques were used to characterize texture evolutions during the rolling process. The findings revealed a significant improvement against HIC based on texture engineering. In addition, increasing the number of {111} and {110} grains, associated with minimizing the number of {001} grains in warm-rolled samples, reduced HIC susceptibility. Moreover, the results showed that boundaries associated with low {hkl} indexing and denser packing planes had more resistance against crack propagation.

  4. Molecular dynamic study of grain boundary embrittlement for [101] tilt copper bicrystals induced by bismuth segregation

    Science.gov (United States)

    Peng, Ba-yi; Wu, Xi-jun; Zhou, Fu-xin; Tang, Qi-heng

    1992-02-01

    The selective bismuth segregation and the microprocess of fracture for the three [101] tilt copper bicrystals Σ9(2¯1¯2) 38.94°, Σ11(3¯2¯3) 50.48°, and Σ33(5¯4¯5) 58.99° have been studied by a molecular dynamics technique. The results show that the Bi segregation and the fracture behavior of the Cu-Bi bicrystals are strongly dependent on the grain boundary (GB) structure. The Bi segregation is strongly related to the polyhedra constructing the GB cores and the stress fields of the GB dislocations (GBDs), and the GB embrittlement of copper induced by the Bi segregation is determined by the segregated concentration and the distribution of Bi atoms. With the increase of the relative number of pentagonal bipyramids and the localization of the stress fields of the GBDs in the GBs, the bicrystals Σ9, Σ11, and Σ33 show a decreasing propensity for the Bi segregation and subsequent different fracture behaviors. The severe intergranular brittle fracture that happens in the Σ9 bicrystal is mainly caused by the breaking of weakened Cu-Cu bonds, which is related to the highly concentrated Bi segregation at the GB core. In the case of the Σ11 bicrystal, the segregation of Bi atoms at the GB shows an inhomogeneous distribution characteristic, so that the fracture is intergranular but with a large amount of shear deformation. The transgranular fraction that appears in the Σ33 bicrystal is related to the low concentration of the Bi segregation and the dispersive distribution of the Bi atoms along the GB and in the grains.

  5. 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.

  6. Quantification of the Existence Ratio of Non-Adhesion Grain Boundaries and Factors Governing the Strength of Coke Containing Low-Quality Coal

    Science.gov (United States)

    Kanai, Tetsuya; Yamazaki, Yoshiaki; Zhang, Xiaoqing; Uchida, Ataru; Saito, Yasuhiro; Shoji, Masakazu; Aoki, Hideyuki; Nomura, Seiji; Kubota, Yukihiro; Hayashizaki, Hideyuki; Miyashita, Shigeto

    “Non-adhesion grain boundaries” are formed when low-quality coal grains do not adhere to other grains in the carbonization process because of the low dilation of coke. To better understand the effects of non-adhesion grain boundaries on coke strength, the relationship between the existence ratio of non-adhesion grain boundaries and coke strength was investigated quantitatively. The existence ratio of non-adhesion grain boundaries were measured quantitatively by observing the fracture cross-section of coke using scanning electron microscopy (SEM). Coke strength was measured with a diametral-compression test and an I-shape drum index test. As a result, non-adhesion grain boundaries increased with an increase in the blending ratio of low-quality coal. In particular, non-adhesion grain boundaries increased rapidly when the blending ratio of low-quality coal was over 50%. When the ratio was less than 50%, low-quality coals adhered to other caking coal. However, not many low-quality coals adhered to other caking coals when the ratio was over 50%. The tensile strength of coke was not affected by the porosity of coke. However, the tensile strength and the drum index were affected by the existence ratio of non-adhesion grain boundaries. Tensile strength decreased rapidly even for a few non-adhesion grain boundaries because significant defects caused a fracture in the diametral-compression test. However, the I-shape drum index decreased linearly with the existence ratio of the non-adhesion grain boundaries because many fractures occurred during 600 rotations in the drum. The strength of coke containing low-quality coal is governed by the existence ratio of non-adhesion grain boundaries rather than mean values such as the porosity of coke.

  7. Properties of grain boundary networks in the NEEM ice core analyzed by combined transmission and reflection optical microscopy

    Science.gov (United States)

    Binder, Tobias; Weikusat, Ilka; Garbe, Christoph; Svensson, Anders; Kipfstuhl, Sepp

    2014-05-01

    Microstructure analysis of ice cores is vital to understand the processes controlling the flow of ice on the microscale. To quantify the microstructural variability (and thus occurring processes) on centimeter, meter and kilometer scale along deep polar ice cores, a large number of sections has to be analyzed. In the last decade, two different methods have been applied: On the one hand, transmission optical microscopy of thin sections between crossed polarizers yields information on the distribution of crystal c-axes. On the other hand, reflection optical microscopy of polished and controlled sublimated section surfaces allows to characterize the high resolution properties of a single grain boundary, e.g. its length, shape or curvature (further developed by [1]). Along the entire NEEM ice core (North-West Greenland, 2537 m length) drilled in 2008-2011 we applied both methods to the same set of vertical sections. The data set comprises series of six consecutive 6 x 9 cm2 sections in steps of 20 m - in total about 800 images. A dedicated method for automatic processing and matching both image types has recently been developed [2]. The high resolution properties of the grain boundary network are analyzed. Furthermore, the automatic assignment of c-axis misorientations to visible sublimation grooves enables us to quantify the degree of similarity between the microstructure revealed by both analysis techniques. The reliability to extract grain boundaries from both image types as well as the appearance of sublimation groove patterns exhibiting low misorientations is investigated. X-ray Laue diffraction measurements (yielding full crystallographic orientation) have validated the sensitivity of the surface sublimation method for sub-grain boundaries [3]. We introduce an approach for automatic extraction of sub-grain structures from sublimation grooves. A systematic analysis of sub-grain boundary densities indicates a possible influence of high impurity contents (amongst

  8. A study of photovoltaic loss-mechanisms due to defects and grain boundaries in polycrystalline silicon

    Science.gov (United States)

    Sopori, B. L.

    1980-01-01

    Various photovoltaic loss mechanisms associated with defects and grain boundaries (gbs) in polycrystalline silicon have been experimentally studied. Analysis was carried out on two types of substrates/cells viz. Wacker Silso and laser-crystallized RTR ribbons. Solar cells were fabricated on selected regions of the substrates and their characteristics related to the substrate structure. Mechanisms related to photovoltaic losses are divided into two categories: electronic and physical. Parameters describing electronic loss mechanisms, such as changes in minority carrier diffusion length, dark current and local photo-current losses were measured, and their dependence on density and type of defects was determined. A variety of analytical techniques were used for this study. These include I-V characterization of solar cells, I-V characterization of gbs, and light intensity dependences of some material parameters. Loss mechanisms associated with physical effects are defect-defect and impurity-defect interactions. It is shown that physical effects such as impurity segregation and defect annihilation can lead to significant loss/gain in photovoltaic characteristics.

  9. Nanolines of transition metals ruled by grain boundaries in graphene: An ab initio study

    Energy Technology Data Exchange (ETDEWEB)

    Lima, F.D.C. de, E-mail: felipe.lima@ufu.br; Miwa, R.H., E-mail: hiroki@infis.ufu.br

    2017-06-15

    We have performed an ab initio investigation of the energetic stability, and the electronic properties of transition metals (TMs = Mn, Fe, Co, and Ru) adsorbed on graphene, upon the presence of grain boundaries (GBs); where we found an energetic preference for the TMs lying on the GB sites (TM/GB). Further energy barrier calculations, of the transition metals in TM/GB, reveal that the GBs promote the formation of energetically favorable diffusion channels on graphene. By increasing the concentration of the TM adatoms, the energetic stability of the TM/GB systems has been strengthened; giving rise to TM nanolines (TM-NLs). The electronic properties of those TM-NLs were characterized through extensive electronic band structure calculations, where the energy bands of the TM/GB systems indicate the appearance of an anisotropic spin-polarized electronic current along the TM-NLs on graphene. - Highlights: • Formation of transition metal (TM) nanolines on graphene ruled by extended defects. • Those extended defects give rise to diffusion pipes of TMs on graphene. • The electronic band structure calculations indicate the formation of spin-polarized current upon the presence of TM nanolines. • The formation of those TM nanolines support the recent experimental findings.

  10. New constraints on upper mantle creep mechanism inferred from silicon grain-boundary diffusion rates

    Science.gov (United States)

    Fei, Hongzhan; Koizumi, Sanae; Sakamoto, Naoya; Hashiguchi, Minako; Yurimoto, Hisayoshi; Marquardt, Katharina; Miyajima, Nobuyoshi; Yamazaki, Daisuke; Katsura, Tomoo

    2016-01-01

    The creep in the Earth's interior is dominated either by diffusion creep which causes Newtonian mantle flow, or by dislocation creep which results in non-Newtonian mantle flow. Although previous deformation studies on olivine claimed a transition from dislocation creep to diffusion creep with depth in the upper mantle, they might misunderstand the creep rates due to experimental difficulties. Since creep in olivine is controlled by silicon diffusion, we measured the silicon grain-boundary diffusion coefficient in well-sintered iron-free olivine aggregates as a function of temperature, pressure, and water content, showing activation energy, activation volume, and water content exponent of 220 ± 30 kJ /mol, 4.0 ± 0.7 cm3 /mol, and 0.26 ± 0.07, respectively. Our results based on Si diffusion in forsterite predict that diffusion creep dominates at low pressures and low temperatures, whereas dislocation creep dominates under high pressure and high temperature conditions. Water has negligible effects on both diffusion and dislocation creep. There is a transition from diffusion creep in the shallow upper mantle to dislocation creep in deeper regions. This explains the seismic anisotropy increases at the Gutenberg discontinuity beneath oceans and at the mid-lithosphere discontinuity beneath continents.

  11. Gap States at Low-Angle Grain Boundaries in Monolayer Tungsten Diselenide

    KAUST Repository

    Huang, Yu Li

    2016-05-03

    Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6°. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport and other device properties highlights the importance of defect engineering in future 2D device fabrication. © 2016 American Chemical Society.

  12. Influence of point defects on grain boundary mobility in bcc tungsten.

    Science.gov (United States)

    Borovikov, Valery; Tang, Xian-Zhu; Perez, Danny; Bai, Xian-Ming; Uberuaga, Blas P; Voter, Arthur F

    2013-01-23

    Atomistic computer simulations were performed to study the influence of radiation-induced damage on grain boundary (GB) sliding processes in bcc tungsten (W), the divertor material in the ITER tokamak and the leading candidate for the first wall material in future fusion reactors. In particular, we calculated the average sliding-friction force as a function of the number of point defects introduced into the GB for a number of symmetric tilt GBs. In all cases the average sliding-friction force at fixed shear strain rate depends on the number of point defects introduced into the GB, and in many cases introduction of these defects reduces the average sliding-friction force by roughly an order of magnitude. We have also observed that as the number of interstitials in the GB is varied, the direction of the coupled GB motion sometimes reverses, causing the GB to migrate in the opposite direction under the same applied shear stress. This could be important in the microstructural evolution of polycrystalline W under the harsh radiation environment in a fusion reactor, in which high internal stresses are present and frequent collision cascades generate interstitials and vacancies.

  13. Concurrent atomistic and continuum simulation of bi-crystal strontium titanate with tilt grain boundary.

    Science.gov (United States)

    Yang, Shengfeng; Chen, Youping

    2015-03-08

    In this paper, we present the development of a concurrent atomistic-continuum (CAC) methodology for simulation of the grain boundary (GB) structures and their interaction with other defects in ionic materials. Simulation results show that the CAC simulation allows a smooth passage of cracks through the atomistic-continuum interface without the need for additional constitutive rules or special numerical treatment; both the atomic-scale structures and the energies of the four different [001] tilt GBs in bi-crystal strontium titanate obtained by CAC compare well with those obtained by existing experiments and density function theory calculations. Although 98.4% of the degrees of freedom of the simulated atomistic system have been eliminated in a coarsely meshed finite-element region, the CAC results, including the stress-strain responses, the GB-crack interaction mechanisms and the effect of the interaction on the fracture strength, are comparable with that of all-atom molecular dynamics simulation results. In addition, CAC simulation results show that the GB-crack interaction has a significant effect on the fracture behaviour of bi-crystal strontium titanate; not only the misorientation angle but also the atomic-level details of the GB structure influence the effect of the GB on impeding crack propagation.

  14. 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.

  15. First-principles investigation of the energetics of point defects at a grain boundary in tungsten

    Energy Technology Data Exchange (ETDEWEB)

    Chai, Jun; Li, Yu-Hao; Niu, Liang-Liang; Qin, Shi-Yao; Zhou, Hong-Bo, E-mail: hbzhou@buaa.edu.cn; Jin, Shuo; Zhang, Ying; Lu, Guang-Hong

    2017-02-15

    Tungsten (W) and W alloys are considered as the most promising candidates for plasma facing materials in future fusion reactor. Grain boundaries (GBs) play an important role in the self-healing of irradiation defects in W. Here, we investigate the stability of point defects [vacancy and self-interstitial atoms (SIA’s)] in a Σ5(3 1 0) [0 0 1] tilt W GB by calculating the energetics using a first-principles method. It is found that both the vacancy and SIA are energetically favorable to locate at neighboring sites of the GB, suggesting the vacancy and SIA can easily segregate to the GB region with the segregation energy of 1.53 eV and 7.5 eV, respectively. This can be attributed to the special atomic configuration and large available space of the GB. The effective interaction distance between the GB and the SIA is ∼6.19 Å, which is ∼2 Å larger than that of the vacancy-GB, indicating the SIA are more preferable to locate at the GB in comparison with the vacancy. Further, the binding energy of di-vacancies in the W GB are much larger than that in bulk W, suggesting that the vacancy energetically prefers to congregate in the GB.

  16. 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.

  17. 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.

  18. 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

    2017-02-06

    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.

  19. Atomistic simulation of the trapping capability of He-vacancy defects at Ni {\\sum}^{}3\\left(1\\bar{1}2\\right)[110] grain boundary

    Science.gov (United States)

    Gong, Hengfeng; Wang, Chengbin; Zhang, Wei; Huai, Ping; Lu, Wei; Zhu, Zhiyuan

    2016-12-01

    He atoms tend to cluster and precipitate into bubbles that prefer to grow in the grain boundaries, resulting in high temperature He embrittlement with significantly degraded material properties. This is a major bottleneck in employing Ni-based alloys for applications such as molten salt reactors (MSRs). This paper focuses on understanding how the local grain boundary structure interacts with He atoms and how the local atomistic environment in the grain boundary influences the binding energy of He defects. Using molecular dynamics simulations, we have investigated the trapping capability of the Ni {\\sum}3≤ft(1 \\bar{1} 2\\right)≤ft[1 1 0\\right] grain boundary to He defects (He N ) and to He-vacancy defects (He N V M ). The two defects in the Ni grain boundary exhibit geometries with high symmetry. The binding energy of an interstitial He atom to He N V M defects is found to be generally larger in pure Ni than that in the grain boundary. We compared the binding energy of He N defects to the Ni vacancy and to the Ni grain boundary, finding that the Ni vacancy possesses a higher trapping strength to He N . We also found that the binding strength of He N to the grain boundary is stronger than that of He N V M to the grain boundary. The He-vacancy ratio in He N V M defects does not significantly affect the binding energy in the grain boundary plane. The current work will provide insight in understanding the experimentally observed He bubble formation in Ni-based alloys and bridge atomic scale events and damage with macroscopic failure.

  20. Grain boundary engineering in sintered Nd-Fe-B permanent magnets for efficient utilization of heavy rare earth elements

    Energy Technology Data Exchange (ETDEWEB)

    Loewe, Konrad

    2016-10-18

    The first part of the thesis investigates the diffusion of rare-earth (RE) elements in commercial sintered Nd-Fe-B based permanent magnets. A strong temperature dependence of the diffusion distance and resulting change in magnetic properties were found. A maximum increase in coercivity of ∼+350 kA/m using a Dy diffusion source occurred at the optimum annealing temperature of 900 C. After annealing for 6 h at this temperature, a Dy diffusion distance of about 4 mm has been observed with a scanning Hall probe. Consequently, the maximum thickness of grain boundary diffusion processed magnets with homogeneous properties is also only a few mm. The microstructural changes in the magnets after diffusion were investigated by electron microscopy coupled with electron probe microanalysis. It was found that the diffusion of Dy into sintered Nd-Fe-B permanent magnets occurs along the grain boundary phases, which is in accordance with previous studies. A partial melting of the Nd-Fe-B grains during the annealing process lead to the formation of so - called (Nd,Dy)-Fe-B shells at the outer part of the grains. These shells are μm thick at the immediate surface of the magnet and become thinner with increasing diffusion distance towards the center of the bulk. With scanning transmission electron microscopy coupled with electron probe analysis a Dy content of about 1 at.% was found in a shell located about 1.5 mm away from the surface of the magnet. The evaluation of diffusion speeds of Dy and other RE (Tb, Ce, Gd) in Nd-Fe-B magnets showed that Tb diffuses significantly faster than Dy, and Ce slightly slower than Dy, which is attributed to differences in the respective phase diagrams. The addition of Gd to the grain boundaries has an adverse effect on coercivity. Exemplary of the heavy rare earth element Tb, the nano - scale elemental distribution around the grain boundaries after the diffusion process was visualized with high resolution scanning transmission electron microscopy

  1. Characterization of boundary roughness of two cube grains in partly recrystallized copper

    DEFF Research Database (Denmark)

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

    2015-01-01

    Protrusions and retrusions typically form on recrystallizing boundaries and thus the boundaries often appear rough. Characterization of the boundary roughness is necessary in order to evaluate the effects of protrusions and retrusions on boundary migration. In the current work, a variable termed ...

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

    Science.gov (United States)

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

    2013-01-01

    We use the phase-field-crystal (PFC) method to investigate the equilibrium premelting and nonequilibrium shearing behaviors of [001] symmetric tilt grain boundaries (GBs) at high homologous temperature over the complete range of misorientation 0<θ<90∘ in classical models of bcc Fe. We characterize the dependence of the premelted layer width W as a function of temperature and misorientation. In addition, we compute the thermodynamic disjoining potential whose derivative with respect to W represents the structural force between crystal-melt interfaces due to the spatial overlap of density waves. The disjoining potential is also computed by molecular dynamics (MD) simulations, for quantitative comparison with PFC simulations, and coarse-grained amplitude equations (AE) derived from PFC that provide additional analytical insights. We find that, for GBs over an intermediate range of misorientation (θmin<θ<θmax), W diverges as the melting temperature is approached from below, corresponding to a purely repulsive disjoining potential, while for GBs outside this range (θ<θmin or θmax<θ<90∘) W remains finite at the melting point. In the latter case, W corresponds to a shallow attractive minimum of the disjoining potential. The misorientation range where W diverges predicted by PFC simulations is much smaller than the range predicted by MD simulations when the small dimensionless parameter ɛ of the PFC model is matched to liquid structure factor properties. However, it agrees well with MD simulations with a lower ɛ value chosen to match the ratio of bulk modulus and solid-liquid interfacial free energy, consistent with the amplitude-equation prediction that θmin and 90∘-θmax scale as ˜ɛ1/2. The incorporation of thermal fluctuations in PFC is found to have a negligible effect on this range. In response to a shear stress parallel to the GB plane, GBs in PFC simulations exhibit coupled motion normal to this plane or sliding. Furthermore, the coupling factor

  3. First-principles calculations of an oxygen deficient {sigma}=3(111)[101-bar] grain boundary in strontium titanate

    Energy Technology Data Exchange (ETDEWEB)

    Astala, R.; Bristowe, P.D. [Department of Materials Science and Metallurgy, University of Cambridge, Cambridge (United Kingdom)

    2002-07-01

    An oxygen vacancy in a {sigma}=3(111)[101-bar] grain boundary (GB) in SrTiO{sub 3} is modelled using DFT plane-wave pseudopotential methods. The formation energy of the vacancy in the boundary is found to depend on its charge state and on the electron chemical potential. There is a strong driving force for segregation when the vacancy is in the charge neutral state and this results in an electrostatic potential barrier at the GB. The vacancy is found to act as a donor and the GB becomes n-type. The defect induces small atomic relaxations in an otherwise highly ordered GB structure. (author)

  4. Development of Grain Boundary Precipitate-Free Zones in a Ni-Mo-Cr-W Alloy

    Science.gov (United States)

    Song, Jie; Field, Robert; Konitzer, Doug; Kaufman, Michael

    2017-05-01

    In this study, the morphology and development of precipitate-free zones (PFZs) near grain boundaries (GBs) in low coefficient of thermal expansion (CTE) Ni-Mo-Cr-W alloys (based on Haynes 244) have been investigated as a function of thermal history and composition using electron microscopy techniques. It is shown that the formation of wide, continuous PFZs adjacent to GBs can be largely attributed to a vacancy depletion mechanism. It is proposed that variations in the vacancy distributions that develop after solution heat treatment (SHT) and subsequent quenching and aging greatly influence the development of the γ'-Ni2(Mo,Cr) precipitates during the aging process and result in the development of PFZs of varying sizes. The relatively large (5 to 10 μm) PFZs are distinct from the smaller, more common PFZs that result from solute depletion due to GB precipitation that are typically observed after prolonged aging. During the course of this investigation, heat treatment parameters, such as aging time, SHT temperature, cooling rate after SHT, and heating rate to the aging temperature—all of which change vacancy concentration and distribution adjacent to GBs—were investigated and observed to have significant influence on both the size and morphology of the observed PFZs. In contrast to results from other Ni-based alloys studied previously, PFZ development in the current alloys was observed across a broad range of aging temperatures. This appears to be due to the high misfit strain energy of the γ' precipitates, resulting in a nucleation process that is sensitive to vacancy concentration. It is also shown that a slightly modified alloy with higher Mo concentrations develops smaller, more typical PFZs; this is presumably due to an increased driving force for γ' precipitation which overshadows the influence of misfit strain energy, thereby decreasing the sensitivity of precipitation on vacancy concentration.

  5. Segregation of sp-impurities at grain boundaries and surfaces: comparison of fcc cobalt and nickel

    Science.gov (United States)

    Všianská, M.; Vémolová, H.; Šob, M.

    2017-12-01

    We perform systematic ab initio investigations of the segregation of 12 non-magnetic sp-impurities (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Σ5(210) grain boundary (GB) and (210) free surface (FS) in fcc ferromagnetic cobalt and analyse their effect on structural, magnetic and mechanical properties; the results are compared with those obtained previously for nickel. It turns out that there is a slight enhancement of magnetization at the clean GB and FS with respect to bulk cobalt (4.7% and 17%, respectively). However, segregated sp-impurities sharply reduce this magnetization. As shown previously, in nickel most of the above impurities nearly destroy or substantially reduce the magnetic moments at the FS and, when segregated interstitially (i.e. Si, P, S, Ge, As, and Se), also at the GB, so that they provide atomically thin, magnetically dead layers, which may be very desirable in spintronics. The reduction of magnetic moments at the Σ5(210) GB in fcc ferromagnetic cobalt is, in absolute values, very similar to that in nickel. However, as the magnetic moment in bulk cobalt is higher, we do not observe magnetically dead layers here. Further, we find the preferred segregation sites at the Σ5(210) GB for the sp-impurities studied, and their segregation enthalpies and strengthening/embrittling energies with their decomposition into their chemical and mechanical components. It turns out that interstitially segregated Si is a GB cohesion enhancer, and interstitially segregated P, S, Ge, As, and Se and substitutionally segregated Al, Ga, In, Sn, Sb and Te are GB embrittlers in fcc cobalt. As there is essentially no experimental information on GB segregation in cobalt, most of the present results are theoretical predictions which may motivate future experimental work.

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

    Directory of Open Access Journals (Sweden)

    Lyle M Gordon

    2015-03-01

    Full Text Available 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.

  7. Clean Grain Boundary Found in C14/Body-Center-Cubic Multi-Phase Metal Hydride Alloys

    OpenAIRE

    Hao-Ting Shen; Kwo-Hsiung Young; Tiejun Meng; Bendersky, Leonid A

    2016-01-01

    The grain boundaries of three Laves phase-related body-center-cubic (bcc) solid-solution, metal hydride (MH) alloys with different phase abundances were closely examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and more importantly, electron backscatter diffraction (EBSD) techniques. By using EBSD, we were able to identify the alignment of the crystallographic orientations of the three major phases in the alloys (C14, bcc, and B2 structures). This finding...

  8. Fabrication of 45 degrees template grain boundary junctions using a CaO lift-off technique

    NARCIS (Netherlands)

    IJsselsteijn, R.P.J.; Terpstra, D.; Flokstra, Jakob; Rogalla, Horst

    1994-01-01

    45 degrees grain boundary junctions have been made using (100) MgO substrates, a CeO2 template layer and an YBa2Cu3O7 top layer. To minimize the damage to the MgO surface, which will occur if the CeO2 is structured using ion milling, the CeO2 layer has been structured using the CaO lift-off

  9. Molecular dynamics study on the diffusion behavior of Li in the grain boundaries of α-Fe

    Energy Technology Data Exchange (ETDEWEB)

    Yu, Xingang, E-mail: xgyu@ucas.ac.cn [School of Engineering Science, University of Chinese Academy of Sciences, Beijing 100049 (China); Liu, Chengrui [Beijing Institute of Control Engineering, Beijing 100190 (China); Han, Tiansi [Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Beijing 100190 (China); Gan, Xianglai [College of Materials Science and Engineering, Hunan University, Changsha 410082 (China)

    2016-11-01

    Highlights: • Confirmed the strong binding effect between the grain boundaries and lithium interstitials. • Determined the critical temperatures for a lithium atom to diffuse in the grain boundaries. • Revealed that the diffusion mechanism of a lithium atom depends on the grain boundary structure. - Abstract: Liquid lithium has been considered as a candidate material for several components of future fusion devices. Since the containment materials are usually ferrous alloys, molecular dynamics simulations were performed to study the diffusion behavior of lithium atoms along <110> tilt grain boundaries (GB) including Σ9{114}, Σ11{113}, Σ3{112} and Σ11{332} in α-Fe. The binding energies of a Li interstitial to the GBs were calculated. The results suggest that all the GBs have strong binding effect on the Li atom. The critical temperatures for the Li atom to diffuse were determined. The diffusion process of a Li interstitial in the GBs was systematically analyzed. It turns out that the diffusion mechanism depends on the GB structures. For Σ11{113} and GB Σ9{114}, the Li atom was trapped by the Frenkel defect around the GBs at 300 K and 400 K respectively and therefore the diffusion was slowed down rapidly. For Σ3{112}, no defects were formed around GB and the Li atom diffused into Fe bulk at 700 K and above. For Σ3{112}, the diffusion process is driven by the movement of the GB. Finally, the diffusion coefficient, as well as the activation energy, was evaluated.

  10. Bi-epitaxial template grain boundaries with different in-plane angles on (100) MgO substrates

    NARCIS (Netherlands)

    IJsselsteijn, R.P.J.; IJsselsteijn, R.P.J.; Hilgenkamp, Johannes W.M.; Eisenberg, M.; Eisenberg, M.; Vittoz, C.; Flokstra, Jakob; Rogalla, Horst

    1993-01-01

    Using laser ablation CeO2 and YBa2Cu3O7−d layers have been deposited on (100) oriented MgO substrates for the creation of bi-epitaxial grain boundary weak links. X-Ray Diffraction measurements were used to measure the (in-plane) orientations of the layers. The CeO2 layer has been structured using Ar

  11. Atomic configuration and electronic structure of extended defects from the point of view of positron annihilation: a grain boundary example

    Energy Technology Data Exchange (ETDEWEB)

    Sob, M.; Turek, I. [Akademie Ved Ceske Republiky, Brno (Czech Republic). Ustav Fyzikalni Metalurgie; Vitek, V. [Pennsylvania Univ., Philadelphia, PA (United States). Dept. of Materials Science and Engineering

    1997-10-01

    A quantum-mechanical approach for determining the electronic structure and atomic configuration of extended defects in metals is described and importance of theoretical methods for interpretation of positron annihilation spectroscopy data is stressed. As an example, atomic configuration of the {Sigma} = 5(210)/[001] tilt grain boundary in tungsten is determined and local densities of states at atoms in defect region are discussed. (author). 19 refs, 3 figs.

  12. Interfacial segregation and grain boundary embrittlement: an overview and critical assessment of experimental data and calculated results

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Šob, Mojmír; Paidar, Václav

    2017-01-01

    Roč. 87, Jun (2017), s. 83-139 ISSN 0079-6425 R&D Projects: GA ČR GBP108/12/G043; GA ČR(CZ) GA16-24711S; GA MŠk(CZ) LQ1601 Institutional support: RVO:68378271 ; RVO:68081723 Keywords : solute segregation * interfacial embrittlement * grain boundary * free surface * computer modeling * measurements of local composition Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 31.140, year: 2016

  13. Grain boundary segregation of elements of groups 14 and 15 and its consequences for intergranular cohesion of ferritic iron

    Czech Academy of Sciences Publication Activity Database

    Lejček, Pavel; Šandera, P.; Horníková, J.; Řehák, Petr; Pokluda, J.

    2017-01-01

    Roč. 52, č. 10 (2017), s. 5822-5834 ISSN 0022-2461 R&D Projects: GA ČR GAP108/12/0144; GA MŠk(CZ) LQ1601 Institutional support: RVO:68378271 ; RVO:68081723 Keywords : grain boundary segregation * segregation enthalpy * intergranular fracture * strengthening/embrittling energy Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.599, year: 2016

  14. 3D Microstructural Characterization of Uranium Oxide as a Surrogate Nuclear Fuel: Effect of Oxygen Stoichiometry on Grain Boundary Distributions

    Energy Technology Data Exchange (ETDEWEB)

    Rudman, K. [Arizona State Univ., Tempe, AZ (United States); Dickerson, P. [Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Byler, Darrin David [Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Peralta, P. [Arizona State Univ., Tempe, AZ (United States); Lim, H. [Arizona State Univ., Tempe, AZ (United States); McDonald, R. [Arizona State Univ., Tempe, AZ (United States); Dickerson, R. [Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Mcclellan, Kenneth James [Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

    2017-09-06

    The initial microstructure of an oxide fuel can play a key role in its performance. At low burn-ups, the diffusion of fission products can depend strongly on grain size and grain boundary (GB) characteristics, which in turn depend on processing conditions and oxygen stoichiometry. Serial sectioning techniques using Focused Ion Beam were developed to obtain Electron Backscatter Diffraction (EBSD) data for depleted UO2 pellets that were processed to obtain 3 different oxygen stoichiometries. The EBSD data were used to create 3D microstructure reconstructions and to gather statistical information on the grain and GB crystallography, with emphasis on identifying the character (twist, tilt, mixed) for GBs that meet the Coincident Site Lattice (CSL) criterion as well as GBs with the most common misorientation angles. Data on dihedral angles at triple points were also collected. The results were compared across different samples to understand effects of oxygen content on microstructure evolution.

  15. Effect of grain-boundaries on electrical properties of n-ZnO:Al/p-Si heterojunction diodes

    Directory of Open Access Journals (Sweden)

    Mohit Kumar

    2013-09-01

    Full Text Available We report on room temperature diode characteristics of ZnO:Al (AZO/Si heterostructures by current-voltage measurements. In this study, with increasing AZO film thickness, systematic reduction in the turn-on potential (from 3.16 to 1.80 V and the film stress are observed. Complementary capacitance-voltage studies reveal a decreasing trend in barrier height at the junction with increasing AZO film thickness. A gradual decrease in resistivity takes place with increasing AZO film thickness. Above observations are explained in the framework of AZO thickness dependent variation in grain size and in turn trap density at the grain boundaries influencing carrier transport across the adjacent grains.

  16. Interactions between edge lattice dislocations and Σ11 symmetrical tilt grain boundary: comparisons among several FCC metals and interatomic potentials

    Science.gov (United States)

    Yu, Wenshan; Wang, Zhiqiang

    2014-07-01

    Interactions between edge dislocations and a ? symmetrical tilt grain boundary (GB) in face-centred cubic metals of Ni and Al are studied via a quasicontinuum method (QCM). A variety of embedding atom method potentials are used, and the results are compared to previous studies of Cu [W.S. Yu, Z.Q. Wang, Acta Mater., 60 (2012) 5010]. Different potentials do not significantly affect the edge dislocation-GB interactions in these metals. Edge dislocations can easily transmit across grain boundaries in Ni and Cu, even for a single incoming dislocation. However, slip-transmission in Al occurs only after the GB absorbs many incoming dislocations. Stable nucleation of grain boundary dislocations (GBD) in Cu and Ni plays an important role in the slip-transmissions. The slip transmission in Al is found to be difficult due to the metastable nucleation of GBD. The incoming leading and trailing partials in Al are absorbed together by the GB because of the larger values of ? (?, ? and ? are the shear modulus, magnitude of Burgers vector of a partial dislocation and the stable stacking fault (SF) energy, respectively). The parameter ? (? as the unstable SF energy) [Z.H. Jin et al., Acta. Mater. 56 (2008) 1126] incorporates ? and ?, and can be used to measure the slip transmission ability of an edge dislocation in these metals. It is also shown that certain loading conditions can help enhance the nucleation of GBDs and GBD dipoles in Al, such that the incoming, leading and trailing partial dislocations can be absorbed separately.

  17. Slip, twinning, and fracture at a grain boundary in the L1/sub 2/ ordered structure: A. sigma. = 9 tilt boundary

    Energy Technology Data Exchange (ETDEWEB)

    Yoo, M.H.; King, A.H.

    1988-09-01

    The role of interaction between slip dislocations and a ..sigma.. = 9 tilt boundary in localized microplastic deformation, cleavage, or intergranular fracture in the L1/sub 2/ ordered structure has been analyzed by using the anisotropic elasticity theory of dislocations and fracture. Screw superpartials cross slip easily at the boundary onto the (11-bar1) and the (001) planes at low and high temperatures, respectively. Transmission of primary slip dislocations onto the conjugate slip system occurs with a certain degree of difficulty, which is eased by localized disordering. When the transmission is impeded, cleavage fracture on the (1-bar11) plane is predicted to occur, not intergranular fracture, unless a symmetric double pileup occurs simultaneously. Absorption (or emission) of superpartials occurs only when the boundary region is disordered. Slip initiation from pre-existing sources near the boundary can occur under the local stress concentration. Implications of the present result on the inherent brittleness of grain boundaries in Ni/sub 3/ Al and its improvement by boron segregation are discussed.

  18. Direct observation of nanometer-scale amorphous layers and oxide crystallites at grain boundaries in polycrystalline Sr1−xKxFe2As2 superconductors

    KAUST Repository

    Wang, Lei

    2011-06-01

    We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystallineSr0.6K0.4Fe2As2superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale crystallites sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films.

  19. Micromechanical Modeling of Grain Boundaries Damage in a Copper Alloy Under Creep; Mikromechanische Modellierung der Korngrenzenschaedigung in einer Kupferlegierung unter Kriechbeanspruchung

    Energy Technology Data Exchange (ETDEWEB)

    Voese, Markus

    2015-07-01

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

  20. A study on the role of grain boundary engineering in promoting high-cycle fatigue resistance and improving reliability in nickel base superalloys for propulsion systems

    Science.gov (United States)

    Gao, Yong

    High-cycle fatigue, involving the premature initiation and/or rapid propagation of small cracks to failure due to high-frequency (vibratory) loading, remains the principal cause of failures in military gas-turbine propulsion systems. The objective of this study is to examine whether the resistance to high-cycle fatigue failures can be enhanced by grain-boundary engineering, i.e., through the modification of the spatial distribution and topology of the grain boundaries in the microstructure. While grain boundary engineering has been used to obtain significant improvements in intergranular corrosion and cracking, creep and cavitation behavior, toughness and plasticity, cold-work embrittlement, and weldability, only very limited, but positive, results exist for fatigue. Accordingly, using a commercial polycrystalline nickel base gamma/gamma' superalloy, ME3, as a typical engine disk material, sequential thermomechanical processing, involving alternate cycles of strain and annealing, is used to (i) modify the proportion of special grain boundaries, and (ii) interrupt the connectivity of the random boundaries in the grain boundary network. The processed microstructures are then subjected to fracture-mechanics based high cycle fatigue testing to evaluate how the crack initiation and small- and large-crack growth properties are affected and to examine how the altered grain boundary population and connectivity can influence growth rates and overall lifetimes. The effect of such grain-boundary engineering on the fatigue-crack-propagation behavior of large (˜8 to 20 mm), through-thickness cracks at 25, 700, and 800°C was examined. Although there was little influence of an increased special boundary fraction at ambient temperatures, the resistance to near-threshold crack growth was definitively improved at elevated temperatures, with fatigue threshold-stress intensities some 10 to 20% higher than at 25°C, concomitant with a lower proportion (˜20%) of intergranular

  1. The effect of inclination angle on the plastic deformation behavior of bicrystalline silver nanowires with Σ3 asymmetric tilt grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Yuan, Lin, E-mail: yuanlin@hit.edu.cn [School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 (China); National Key Laboratory for Precision Hot Processing of Metals, Harbin 150001 (China); Jing, Peng; Shan, Debin; Guo, Bin [School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 (China); National Key Laboratory for Precision Hot Processing of Metals, Harbin 150001 (China)

    2017-01-15

    Atomistic simulations were used to investigate the plastic deformation behavior of bicrystalline silver nanowires with Σ3 asymmetric tilt grain boundaries at 0.1 K. The calculated grain boundary energies of Σ3 asymmetric tilt grain boundaries corresponded well with the energies measured in experiments and predicted by the theoretical description. The Σ3 asymmetric tilt grain boundaries with low inclination angles were composed of a replication of twin boundary segments separated by small ledges. The results demonstrated that the combination effect of Schmid factor and non-Schmid factors could explain dislocations emission into grain 1 only in models with low inclination angles (< 64.76°). At the latter stage of plastic deformation, free surfaces served as additional dislocation sources. Parallelly arranged operative slip systems were the fundamental features of plastic deformation. In addition, a number of stacking faults and multiple stacking faults were formed during plastic deformation. The hindrance of stacking faults to dislocation motion and the interactions between dislocations leaded to the observed strain hardening in nanowires with inclination angles at and above 29.50°. The low stacking fault energy of silver was responsible for the appearance of strain hardening. Dislocations emitted from grain 2 interacted with each other contributing to the observed strain hardening. Grain boundaries were completely eliminated by successive emission of dislocations from grain boundaries in nanowires with an inclination angle of 35.26° and 54.74°. A detailed understanding of the relationship between strength and grain boundary structures as well as specific plastic deformation would push forward the application of nanocrystalline materials and provide insights into the synthesis of nanocrystalline materials with superior strength and ductility.

  2. Analysis of noise generation and electric conduction at grain boundaries in CVD-grown MoS2 field effect transistors

    Science.gov (United States)

    Kim, Jae-Keun; Song, Younggul; Kim, Tae-Young; Cho, Kyungjune; Pak, Jinsu; Choi, Barbara Yuri; Shin, Jiwon; Chung, Seungjun; Lee, Takhee

    2017-11-01

    Grain boundaries in a chemical vapour deposition (CVD)-grown monolayer of MoS2 induce significant effects on the electrical and low frequency noise characteristics of the MoS2. Here, we investigated the electrical properties and noise characteristics of MoS2 field effect transistors (FETs) made with CVD-grown monolayer MoS2. The electrical and noise characteristics of MoS2 FETs were analysed and compared for the MoS2 channel layers with and without grain boundaries. The grain boundary in the CVD-grown MoS2 FETs can be the dominant noise source, and dependence of the extracted Hooge parameters on the gate voltage indicated the domination of the correlated number-mobility fluctuation at the grain boundaries. The percolative noise characteristics of the single grain regions of MoS2 were concealed by the noise generated at the grain boundary. This study can enhance understanding of the electrical transport hindrance and significant noise generation by trapped charges at grain boundaries of the CVD-grown MoS2 devices.

  3. Visualization of Grain Structure and Boundaries of Polycrystalline Graphene and Two-Dimensional Materials by Epitaxial Growth of Transition Metal Dichalcogenides.

    Science.gov (United States)

    Ago, Hiroki; Fukamachi, Satoru; Endo, Hiroko; Solís-Fernández, Pablo; Yunus, Rozan Mohamad; Uchida, Yuki; Panchal, Vishal; Kazakova, Olga; Tsuji, Masaharu

    2016-03-22

    The presence of grain boundaries in two-dimensional (2D) materials is known to greatly affect their physical, electrical, and chemical properties. Given the difficulty in growing perfect large single-crystals of 2D materials, revealing the presence and characteristics of grain boundaries becomes an important issue for practical applications. Here, we present a method to visualize the grain structure and boundaries of 2D materials by epitaxially growing transition metal dichalcogenides (TMDCs) over them. Triangular single-crystals of molybdenum disulfide (MoS2) epitaxially grown on the surface of graphene allowed us to determine the orientation and size of the graphene grains. Grain boundaries in the polycrystalline graphene were also visualized reflecting their higher chemical reactivity than the basal plane. The method was successfully applied to graphene field-effect transistors, revealing the actual grain structures of the graphene channels. Moreover, we demonstrate that this method can be extended to determine the grain structure of other 2D materials, such as tungsten disulfide (WS2). Our visualization method based on van der Waals epitaxy can offer a facile and large-scale labeling technique to investigate the grain structures of various 2D materials, and it will also contribute to understand the relationship between their grain structure and physical properties.

  4. Influence of microstructure on grain boundary sliding of alloys 600 and 690; Influence de la microstructure sur le glissement intergranulaire des alliages 600 et 690

    Energy Technology Data Exchange (ETDEWEB)

    Kergaravat, J.F.; Guetaz, L.; Baillin, X.; Robert, G.

    1995-12-31

    The influence of deformation and damage mechanisms, and more especially of the grain boundary sliding effect, on the stress corrosion of nickel base alloys used in nuclear industry (exchanger tubes), has been experimentally examined. The grain boundary sliding effect has been measured at 500 C and 320 C on several samples of alloy 690 and 600 (in the mill annealed and mill annealed heat treated conditions). (author). 4 figs., 1 tab.

  5. Delineation of Prior Austenite Grain Boundaries in a Low Alloy High Performance Steel (Preprint)

    Science.gov (United States)

    2017-07-31

    property relationships in steels, since prior austenite grain size plays an important role in defining the microstructural scale of low-temperature...the microstructure-24 property relationships in steels, since prior austenite grain size plays an important role in 25 defining the microstructural...steels [8]. A solution 73 containing 10g CrO3, 50g NaOH, 1.5g picric acid, and 100ml distilled water was also reported to 74 provide significantly

  6. Atomic Structures of [0bar{1}10] Symmetric Tilt Grain Boundaries in Hexagonal Close-Packed (hcp) Crystals

    Science.gov (United States)

    Wang, Jian; Beyerlein, Irene J.

    2012-10-01

    Molecular dynamics simulation and interface defect theory are used to determine the relaxed equilibrium atomic structures of symmetric tilt grain boundaries (STGBs) in hexagonal close-packed (hcp) crystals with a [0bar{1}10] tilt axis. STGBs of all possible rotation angles θ from 0 deg to 90 deg are found to have an ordered atomic structure. They correspond either to a coherent, defect-free boundary or to a tilt wall containing an array of distinct and discrete intrinsic grain boundary dislocations (GBDs). The STGBs adopt one of six base structures, PB^{(i)} , i = 1, …, 6, and the Burgers vector of the GBDs is related to the interplanar spacing of the base structure on which it lies. The base structures correspond to the basal plane ( θ = 0 deg, PB^{(1)} ); one of four minimum-energy, coherent boundaries, (bar{2}111),(bar{2}112),(bar{2}114) , and (bar{2}116)( {PB^{(2)} - PB^{(5)} } ) ; and the ( {11bar{2}0} ) plane ( θ = 90 deg, PB^{(6)} ). Based on these features, STGBs can be classified into one of six possible structural sets, wherein STGBs belonging to the same set i contain the same base boundary structure PB^{(i)} and an array of GBDs with the same Burgers vector b_{{GB}}^{(i)} , which vary only in spacing and sign with θ. This classification is shown to apply to both Mg and Ti, two metals with different c/ a ratios and employing different interatomic potentials in simulation. We use a simple model to forecast the misorientation range of each set for hcp crystals of general c/ a ratio, the predictions of which are shown to agree well with the molecular dynamics (MD) simulations for Mg and Ti.

  7. The effect of cold rolling on the grain boundary character and creep rupture properties of INCONEL alloy 718

    Energy Technology Data Exchange (ETDEWEB)

    Boehlert, C.J. [Alfred Univ., School of Ceramic Engineering and Materials Science, Alfred, NY (United States); Civelekoglu, S. [Alfred Univ., School of Ceramic Engineering and Materials Science, Alfred, NY (United States); Eisinger, N.; Smith, G.; Crum, J. [Special Metals Corp., Huntington (United States)

    2003-07-01

    In order to evaluate the effects of sheet processing on the grain boundary character distribution (GBCD) of INCONEL trademark alloy 718 (IN 718), electron backscattered diffraction (EBSD) mapping was performed on samples cold rolled between 0-40%. Increased cold rolling increased the fraction of low-angle boundaries at the expense of the coincident site lattice boundaries. The tensile-creep rupture life (T{sub r}) and elongation-to-failure ({epsilon}{sub f}) were evaluated at 649 C and 758 MPa, and the data indicated that increased cold rolling significantly increased both the T{sub r} and {epsilon}{sub f} values. In addition the GBCD and room-temperature (RT) tensile properties were evaluated for superplastically formed INCONEL trademark alloy 718 (IN 718SPF). The tensile results indicated the exceptional strength of the fine-grained IN 718SPF material, however the GBCD parameters were intermediate to those of the 10% and 20% cold rolled IN 718 materials. (orig.)

  8. Asymptotic expressions for the nearest and furthest dislocations in a pile-up against a grain boundary

    KAUST Repository

    Hall, Cameron L.

    2010-10-14

    In 1965, Armstrong and Head explored the problem of a pile-up of screw dislocations against a grain boundary. They used numerical methods to determine the positions of the dislocations in the pile-up and they were able to fit approximate formulae for the locations of the first and last dislocations. These formulae were used to gain insights into the Hall-Petch relationship. More recently, Voskoboinikov et al. used asymptotic techniques to study the equivalent problem of a pile-up of a large number of screw dislocations against a bimetallic interface. In this paper, we extend the work of Voskoboinikov et al. to construct systematic asymptotic expressions for the formulae proposed by Armstrong and Head. The further extension of these techniques to more general pile-ups is also outlined. As a result of this work, we show that a pile-up against a grain boundary can become equivalent to a pile-up against a locked dislocation in the case where the mismatch across the boundary is small. © 2010 Taylor & Francis.

  9. A phase-field simulation study of irregular grain boundary migration during recrystallization

    DEFF Research Database (Denmark)

    Moelans, N.; Zhang, Yubin; Godfrey, A.

    2015-01-01

    We present simulation results based on a phase-field model that describes the migration of recrystallization boundaries into spatially varying deformation energy fields. Energy fields with 2-dimensional variations representing 2 sets of dislocation boundaries lying at equal, but opposite, angles...

  10. Grain boundary transport properties in YBa{sub 2}Cu{sub 3}O{sub x} coated conductors.

    Energy Technology Data Exchange (ETDEWEB)

    Berghuis, P.; Miller, D. J.; Kim, D. H.; Gray, K. E.; Feenstra, R.; Christen, D. K.

    2000-11-02

    Critical current data obtained as a function of magnetic field on an isolated grain boundary (GB) of a coated conductor and two other types of bicrystal GBs of YBa{sub 2}Cu{sub 3}O{sub x} show a peak in the critical current and an unusual hysteresis. These results provide support for a new mechanism for enhanced GB critical currents, arising from interactions of GB vortices with pinned Abrikosov vortices in the banks of a GB, as suggested by Gurevich and Cooley. A substantial fraction of this enhancement, which can exceed a factor of ten, also occurs upon surpassing the critical current of the grains after zero field cooling. A bulk GB and thin film GBs show qualitatively identical results.

  11. 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.

  12. Clean Grain Boundary Found in C14/Body-Center-Cubic Multi-Phase Metal Hydride Alloys

    Directory of Open Access Journals (Sweden)

    Hao-Ting Shen

    2016-06-01

    Full Text Available The grain boundaries of three Laves phase-related body-center-cubic (bcc solid-solution, metal hydride (MH alloys with different phase abundances were closely examined by scanning electron microscopy (SEM, transmission electron microscopy (TEM, and more importantly, electron backscatter diffraction (EBSD techniques. By using EBSD, we were able to identify the alignment of the crystallographic orientations of the three major phases in the alloys (C14, bcc, and B2 structures. This finding confirms the presence of crystallographically sharp interfaces between neighboring phases, which is a basic assumption for synergetic effects in a multi-phase MH system.

  13. Role of grain boundary diffusion on ion-induced composition change in alloys at elevated temperatures. [A/sup +/ ions

    Energy Technology Data Exchange (ETDEWEB)

    Morita, K.; Hayashibara, M.; Ohno, H.; Itoh, N. (Nagoya Univ. (Japan). Dept. of Crystalline Materials Science)

    1984-05-01

    We prepared nickel specimens which contain gold impurity only near the grain boundaries and measured thermal segregation of gold onto the surface and the change in the composition induced by bombardment with Ar/sup +/ ions. It is found that irradiation causes composition change over a depth much larger than the thickness of the altered layer for Ni-Au alloys. It is also found that when a two-layered Ni-Au film is bombarded with gold atoms from the nickel side at elevated temperatures, the nickel is protected by a thin gold film segregated on the nickel surface.

  14. Contribution of precipitate on migrated grain boundaries to ductility-dip cracking in Alloy 625 weld joints

    Science.gov (United States)

    Lee, Dong Jin; Kim, Youn Soo; Shin, Yong Taek; Jeon, Eon Chan; Lee, Sang Hwa; Lee, Hyo-Jong; Lee, Sung Keun; Lee, Jun Hee; Lee, Hae Woo

    2010-10-01

    We investigated the crack properties in Alloy 625 weld metals and their characteristics using experimentally designed filler wires fabricated by varying the niobium and manganese contents in the flux with the shield metal arc welding (SMAW) process. The fast diffusivity of niobium on the migrated grain boundary (MGB) under strong restraint tensile stress, which was induced by the hardened matrix in weld metal containing high niobium and manganese, accelerated the growth of niobium carbide (NbC) in multipass deposits. Coalescence of microvoids along with incoherent NbC and further propagation induced ductility-dip cracking (DDC) on MGB.

  15. A first principles scanning tunneling potentiometry study of an opaque graphene grain boundary in the ballistic transport regime.

    Science.gov (United States)

    Bevan, Kirk H

    2014-10-17

    We report on a theoretical interpretation of scanning tunneling potentiometry (STP), formulated within the Keldysh non-equilibrium Green's function description of quantum transport. By treating the probe tip as an electron point source/sink, it is shown that this approach provides an intuitive bridge between existing theoretical interpretations of scanning tunneling microscopy and STP. We illustrate this through ballistic transport simulations of the potential drop across an opaque graphene grain boundary, where atomistic features are predicted that might be imaged through high resolution STP measurements. The relationship between the electrochemical potential profile measured and the electrostatic potential drop across such a nanoscale defect is also explored in this model system.

  16. Molecular dynamics simulations of self-diffusion near a symmetrical tilt grain boundary in UO{sub 2}

    Energy Technology Data Exchange (ETDEWEB)

    Vincent-Aublant, E.; Delaye, J.M. [CEA-Marcoule, DEN/DTCD/SECM, B.P. 17171, 30207 Bagnols sur Ceze cedex (France); Van Brutzel, L. [CEA-Saclay, DEN-DANS/DPC/SCP/LM2T, 91191 Gif-sur-Yvette (France)

    2008-07-01

    Molecular dynamics (MD) simulations have been used to study the influence of symmetrical tilt grain boundaries (GBs) in stoichiometric UO{sub 2} on uranium and oxygen self-diffusions. The study was performed on a large range of temperature varying from 300 K to 2100 K. First, the effect of the temperature on the structure and the formation energies of 6 relaxed tilt GBs was investigated. The {sigma}5 and {sigma}41 GBs geometries were chosen to study the diffusion. O and U diffusion coefficients have been calculated and compared to those obtained in a perfect stoichiometric UO{sub 2} as well as in over and under-stoichiometric matrices. (authors)

  17. Josephson oscillations and noise temperatures in YBa2Cu3O7-x grain-boundary junctions

    DEFF Research Database (Denmark)

    Yu, Ya. Divin; Mygind, Jesper; Pedersen, Niels Falsig

    1992-01-01

    The ac Josephson effect was studied in YBa2Cu3O7−x grain-boundary junctions (GBJ) in the temperature range from 4 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 temperatures may be as low...... as the physical temperature in the temperature range investigated. In the millimeter-wave range, linewidths as low as 380 MHz were found at liquid-nitrogen temperatures. Applied Physics Letters is copyrighted by The American Institute of Physics....

  18. Gas accumulation at grain boundaries during 800 MeV proton irradiation of aluminium and aluminium-alloys

    DEFF Research Database (Denmark)

    Singh, Bachu Narain; Horsewell, Andy; Sommer, W. F.

    1986-01-01

    Samples of pure aluminium (99.9999%) and commercial Al-2.7%Mg(AlMg3) and Al-1.1%Mg-0.5%Si(Al6061) alloys were irradiated with 800 MeV protons at the Los Alamos Meson Physics Facility (LAMPF) at a temperature between 40-100°C to a maximum dose of 0.2 dpa. Transmission electron microscopy (TEM) sho...... higher than in the pure Al. The amount of gas accumulation at grain boundaries was found to depend on gas generation rate, alloying and cold-work microstructure...

  19. Recombination in Perovskite Solar Cells : Significance of Grain Boundaries, Interface Traps, and Defect Ions

    NARCIS (Netherlands)

    Sherkar, Tejas; Momblona, Cristina; Gil-Escrig, Lidon; Avila, Jorge; Sessolo, Michele; Bolink, Henk J.; Koster, Lambert

    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

  20. Ground boundaries

    Science.gov (United States)

    Balluffi, R. W.; Bristowe, P. D.

    The present document is a progress report describing the work accomplished on the study of grain boundaries in Ag, Au, Ni, Si, and Ge. Research was focused on the following four major efforts: study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; grain boundary migration; short-circuit diffusion along grain boundaries; and development of Thin-Film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals.

  1. In-situ quantification of the effect of solute on the mobility, character and driving pressure of grain boundaries during recrystallization in aluminum alloys

    Science.gov (United States)

    Taheri, Mitra L.

    Aluminum alloys exhibit recrystallization kinetics that vary strongly with composition. The conventional understanding is that certain alloying elements, e.g. chromium, retard grain boundary motion due to the formation of tine dispersions of second phase particles, giving rise to particle drag of boundaries. There is countervailing evidence, however, that suggests that solute drag provides the stronger influence on the mobility of grain boundaries. This thesis presents new evidence of this pronounced effect of solute drag based on in-situ annealing (both SEM and TEM) and EBSD experiments involving recrystallization in aluminum alloys with varying composition in which individual boundaries move under the driving pressure of stored energy from prior plastic strain. This driving pressure is calculated both macroscopically (via Calorimetry, Microhardness) and microscopically (via Orientation Imaging Microscopy, Transmission Electron Microscopy). In all alloy compositions studied, a compensation effect is noted with respect to grain boundary mobility maxima for certain boundary types. A shift occurs in the misorientation associated with maximum mobility at 38--39° observed at low temperatures, to misorientation axes towards , and is seen in alloys when annealed at higher temperatures. A faceting/defaceting transition is also observed which is consistent with the changes in maximum mobility boundary type with increased temperature. These observations are supported by analysis of activation energy for boundary migration for each alloy. Evidence for irregular motion of boundaries from in-situ observations is discussed in reference to new theoretical results that suggest that boundaries migrating in the presence of solutes should move sporadically, provided that the length scale at which observations are made is small enough. Z-Contrast Imaging using a Scanning Transmission Electron Microscope coupled with TEM EDX analysis suggested variable Zr segregation to grain

  2. Evaluation of Mechanical Properties of Σ5(210/[001] Tilt Grain Boundary with Self-Interstitial Atoms by Molecular Dynamics Simulation

    Directory of Open Access Journals (Sweden)

    Liang Zhang

    2017-01-01

    Full Text Available Grain boundary (GB can serve as an efficient sink for radiation-induced defects, and therefore nanocrystalline materials containing a large fraction of grain boundaries have been shown to have improved radiation resistance compared with their polycrystalline counterparts. However, the mechanical properties of grain boundaries containing radiation-induced defects such as interstitials and vacancies are not well understood. In this study, we carried out molecular dynamics simulations with embedded-atom method (EAM potential to investigate the interaction of Σ5(210/[001] symmetric tilt GB in Cu with various amounts of self-interstitial atoms. The mechanical properties of the grain boundary were evaluated using a bicrystal model by applying shear deformation and uniaxial tension. Simulation results showed that GB migration and GB sliding were observed under shear deformation depending on the number of interstitial atoms that segregated on the boundary plane. Under uniaxial tension, the grain boundary became a weak place after absorbing self-interstitial atoms where dislocations and cracks were prone to nucleate.

  3. Excitonic localization in AlN-rich AlxGa1−xN/AlyGa1−yN multi-quantum-well grain boundaries

    KAUST Repository

    Ajia, Idris A.

    2014-09-22

    AlGaN/AlGaN multi-quantum-wells (MQW) with AlN-rich grains have been grown by metal organic chemical vapor deposition. The grains are observed to have strong excitonic localization characteristics that are affected by their sizes. The tendency to confine excitons progressively intensifies with increasing grain boundary area. Photoluminescence results indicate that the MQW have a dominant effect on the peak energy of the near-bandedge emission at temperatures below 150 K, with the localization properties of the grains becoming evident beyond 150 K. Cathodoluminescence maps reveal that the grain boundary has no effect on the peak intensities of the AlGaN/AlGaN samples.

  4. Solution-mass transfer and grain boundary sliding in mafic shear zones - comparison between experiments and nature

    Science.gov (United States)

    Marti, Sina; Heilbronner, Renée; Stünitz, Holger; Plümper, Oliver; Drury, Martyn

    2017-04-01

    Grain size sensitive creep (GSSC) mechanisms are widely recognized to be the most efficient deformation mechanisms in shear zones. With or without initial fracturing and fluid infiltration, the onset of heterogeneous nucleation leading to strong grain size reduction is a frequently described process for the initiation of GSSC. Phase mixing due to reaction and heterogeneous nucleation during GSSC impedes grain growth, sustaining small grain sizes as a prerequisite for GSSC. Here we present rock deformation experiments on 'wet' plagioclase - pyroxene mixtures at T=800°C, P=1.0 and 1.5GPa and strain rates of 2e-5 - 2e-6 1/s, performed with a Griggs-type solid medium deformation apparatus. Microstructural criteria are used to show that both, grain boundary sliding (GBS) and solution-mass transfer processes are active and are interpreted to be the dominant strain accommodating processes. Displacement is localized within shear bands formed by fine-grained ( 300 - 500nm) plagioclase (Pl) and the syn-kinematic reaction products amphibole (Amph), quartz (Qz) and zoisite (Zo). We compare our experiments with a natural case - a sheared mafic pegmatite (P-T during deformation 0.7 - 0.9 GPa, 610 - 710 °C; Getsinger et al., 2013) from Northern Norway. Except for the difference in grain size of the experimental and natural samples, microstructures are strikingly alike. The experimental and natural P- and especially T-conditions are very similar. Consequently, extrapolation from experiments to nature must be made without a significant 'temperature-time' trade-off, which is normally taken advantage of when relating experimental to natural strain rates. We will discuss under which assumptions extrapolation to nature in our case is likely feasible. Syn-kinematic reactions during GBS and solution-mass transport are commonly interpreted to result in an ordered (anticlustered) phase mixture. However, phase mixing in our case is restricted: Mixing is extensive between Pl + Zo + Qz and

  5. EBSD Study on Grain Boundary and Microtexture Evolutions During Friction Stir Processing of A413 Cast Aluminum Alloy

    Science.gov (United States)

    Shamanian, Morteza; Mostaan, Hossein; Safari, Mehdi; Szpunar, Jerzy A.

    2016-07-01

    The as-cast Al alloys contain heterogeneous distributions of non-deforming particles due to non-equilibrium solidification effects. Therefore, these alloys have poor tribological and mechanical behaviors. It is well known that using friction stir processing (FSP), very fine microstructure is created in the as-cast Al alloys, while their wear resistance can be improved. In this research work, FSP is used to locally refine a surface layer of the coarse as-cast microstructure of cast A413 Al alloy. The main objective of this study is to investigate the effect of FSP on microstructure and microtexture evolutions in A413 cast Al alloy. The grain boundary character distribution, grain structure, and microtexture evolutions in as-cast and friction stir processed A413 Al alloy are analyzed by electron back scatter diffraction technique. It is found that with the FSP, the fraction of low ∑boundary such as ∑3, 7, and 9 are increased. The obtained results show that there are no deformation texture components in the structure of friction stir processed samples. However, some of the main recrystallization texture components such as BR and cubeND are formed during FSP which indicate the occurrence of dynamic recrystallization phenomenon due to the severe plastic deformation induced by the rotation of tool.

  6. Carrier mobility enhancement of nano-crystalline semiconductor films: Incorporation of redox -relay species into the grain boundary interface

    Science.gov (United States)

    Desilva, L. A.; Bandara, T. M. W. J.; Hettiarachchi, B. H.; Kumara, G. R. A.; Perera, A. G. U.; Rajapaksa, R. M. G.; Tennakone, K.

    Dye-sensitized and perovskite solar cells and other nanostructured heterojunction electronic devices require securing intimate electronic contact between nanostructured surfaces. Generally, the strategy is solution phase coating of a hole -collector over a nano-crystalline high-band gap n-type oxide semiconductor film painted with a thin layer of the light harvesting material. The nano-crystallites of the hole - collector fills the pores of the painted oxide surface. Most ills of these devices are associated with imperfect contact and high resistance of the hole conducting layer constituted of nano-crystallites. Denaturing of the delicate light harvesting material forbid sintering at elevated temperatures to reduce the grain boundary resistance. It is found that the interfacial and grain boundary resistance can be significantly reduced via incorporation of redox species into the interfaces to form ultra-thin layers. Suitable redox moieties, preferably bonded to the surface, act as electron transfer relays greatly reducing the film resistance offerring a promising method of enhancing the effective hole mobility of nano-crystalline hole-collectors and developing hole conductor paints for application in nanostructured devices.

  7. A model bismuth oxide intergranular thin film in a ZnO twist grain boundary.

    Science.gov (United States)

    Domingos, H S

    2010-04-14

    The electronic properties of a model bismuth oxide intergranular film in ZnO were investigated using density functional plane wave calculations. It was found that oxygen excess plays a fundamental role in the appearance of electrical activity. The introduction by oxygen interstitials or zinc vacancies results in depletion of the charge in deep gap states introduced by the bismuth impurities. This makes the boundary less metallic and promotes the formation of acceptor states localized to the boundary core, resulting in Schottky barrier enhancement. The results indicate that the origin of electrical activity in thin intergranular bismuth oxide films is probably not distinct from that in decorated ZnO boundaries. © 2010 IOP Publishing Ltd

  8. Role of Hf4+ Doping on Oxygen Grain Boundary Diffusion in Alumina

    Science.gov (United States)

    2014-09-01

    aluminate spinel can be observed as indicated by the arrow point to the left Figure 5.11: Cs-corrected HAADF-STEM image of 500ppm Hf02-doped GB...RHS) Enlarged image of dotted box in Figure 1 showing Hf rich particles, nickel aluminate spinel and La hexaaluminates within the oxidation zone...second phase particles and nickel aluminate spinel were readily identified using EDS. Moreover, brighter confrast existed in the grain boimdary region

  9. Determination of vacancy mechanism for grain boundary self-diffusion by computer simulation

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Kwok, T.; Bristowe, P.D.; Brokman, A.; Ho, P.S.; Yip, S.

    1981-08-01

    This note reports efforts to establish the GB self-diffusion mechanism in a bcc iron /SIGMA/.5 (36.9/degree/) (001) (310) tilt boundary using the combined methods of computer molecular statics and molecular dynamics simulation to provide quantitative microscopic evidence in favor of the vacancy mechanism. 16 refs.

  10. Research on the Grain Boundary Liquation Mechanism in Heat Affected Zones of Laser Forming Repaired K465 Nickel-Based Superalloy

    Directory of Open Access Journals (Sweden)

    Qiuge Li

    2016-03-01

    Full Text Available The damaged K465 nickel-based superalloy parts were repaired by laser forming repair technology. The cracking characteristics and grain boundary liquation in heat affected zones were investigated by optical microscopy (OM, scanning electron microscopy (SEM, and transmission electron microscopy (TEM. It was found that the cracks originated from the heat-affected zone and extended to the repaired zone. The calculation by Thermol-Cale software showed that the larger γ′ particles at grain boundaries partly dissolved in the γ phase, which made the solutes’ concentration at the γ′/γ interface meet the eutectic-type liquation reaction condition of γ + γ′ → L. Then, grain boundaries liquation occurred and liquid films appeared with the temperature increasing in a rapid heating process. However, the intragranular γ′ phase completely dissolved into the γ phase with no devotion to the liquid film. The dissolution of M5B3 borides at grain boundaries could promote grain boundary liquation.

  11. The Role of the Grain Boundary on the Room Temperature Resistivity of Pb(Fe1/2Nb1/2)O3

    Science.gov (United States)

    Lee, Sang-Bop; Lee, Kwang-Ho; Kim, Hwan

    2002-08-01

    The role of the grain boundary on the room temperature resistivity of Pb(Fe1/2Nb1/2)O3 (PFN) was investigated using parameters such as temperature dependence of resistivity, complex impedance spectroscopy and X-ray photoelectron spectroscopy. The low resistivity of PFN has been reported to be due to the electron hopping between Fe2+ and Fe3+ driven by the reduction of PFN. However a reconsideration of the reduction equilibrium constant (KRe) revealed that this theory could not fully explain the effect of the sintering temperature on the room temperature resistivity. The role of the grain boundary on the total resistivity was introduced in order to account for this behavior, which was confirmed by complex impedence spectroscopy. Furthermore, the annealing data and X-ray photoelectron spectroscopy (XPS) results showed that the grain boundary properties were irreversibly changed at 1423 K, which appeared to be due to Pb volatilization.

  12. Characteristic dielectric behaviour of the wide temperature range twist grain boundary phases of unsymmetrical liquid crystal dimers

    Energy Technology Data Exchange (ETDEWEB)

    Pandey, M B [Physics Department, University of Allahabad, Allahabad-211 002 (India); Dhar, R [Physics Department, University of Allahabad, Allahabad-211 002 (India); Achalkumar, A S [Centre for Liquid Crystal Research, Jalahalli, Bangalore-560 103 (India); Yelamaggad, C V [Centre for Liquid Crystal Research, Jalahalli, Bangalore-560 103 (India)

    2007-10-31

    The investigated optically active dimeric compound, 4-n-undecyloxy-4{sup '}-(cholesteryloxycarbonyl-1-butyloxy)chalcone, shows wide temperature ranges of two twist grain boundary (TGB) phases, TGBA and TGBC*. Comprehensive dielectric studies have been carried out for this compound in the frequency range 1 Hz-10 MHz for different conditions of molecular anchoring. This compound shows negative dielectric anisotropy ({delta}{epsilon}'={epsilon}{sub parallel}'-{epsilon}{sub perpendicul=} a{sub r}'<0). Various electrical parameters, namely the dielectric permittivity, dielectric anisotropy, DC conductivity and activation energy, have been determined for these TGB phases. Weak relaxation processes have been detected in the TGBA and TGBC* phases, presumably due to amplitude (soft mode) and phase (Goldstone mode) fluctuations.

  13. Deformation behavior around grain boundaries for SCC propagation in hardened low-carbon austenitic stainless steel by micro hardness test

    Energy Technology Data Exchange (ETDEWEB)

    Nagashima, N.; Hayakawa, M. [National Inst. for Materials Science (NIMS), Ibaraki (Japan); Tsukada, T; Kaji, Y.; Miwa, Y. [Japan Atomic Energy Agency (JAEA), Ibaraki (Japan); Ando, M.; Nakata, K. [Japan Nuclear Energy Safety Organization (JNES), Tokyo (Japan)

    2007-07-01

    Stress corrosion cracking (SCC) was found in shroud and PLR piping made of low-carbon austenitic stainless steels in Japanese BWR plants. The intergranular type (IG) SCC propagated in hardened heat affected zones (HAZ) around welds. Strength behavior and local plastic deformation for a low-carbon austenitic stainless steel 316L, rolled at the reductions in area of 10, 30% at room temperature to simulate the hardened HAZ, were measured by a micro-hardness test machine and observed by atomic force microscopy (AFM), respectively. The tensile deformation at yield point (0.2% plastic strain) had given to the work-hardened 316L to simulate the plastic zone at the crack tip. It is suggested that one of the IGSCC propagation mechanism for 316L was related with the intergranular strength behavior and local plastic deformation around grain boundaries. (author)

  14. 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.)

  15. Microstructure and crystallographic preferred orientation of polycrystalline microgarnet aggregates developed during progressive creep, recovery, and grain boundary sliding

    Science.gov (United States)

    Massey, M.A.; Prior, D.J.; Moecher, D.P.

    2011-01-01

    Optical microscopy, electron probe microanalysis, and electron backscatter diffraction methods have been used to examine a broad range of garnet microstructures within a high strain zone that marks the western margin of a major transpression zone in the southern New England Appalachians. Garnet accommodated variable states of finite strain, expressed as low strain porphyroclasts (Type 1), high strain polycrystalline aggregates (Type 2), and transitional morphologies (Type 3) that range between these end members. Type 1 behaved as rigid porphyroclasts and is characterized by four concentric Ca growth zones. Type 2 help define foliation and lineation, are characterized by three Ca zones, and possess a consistent bulk crystallographic preferred orientation of (100) symmetrical to the tectonic fabric. Type 3 show variable degrees of porphyroclast associated with aggregate, where porphyroclasts display complex compositional zoning that corresponds to lattice distortion, low-angle boundaries, and subgrains, and aggregate CPO mimics porphyroclast orientation. All aggregates accommodated a significant proportion of greenschist facies deformation through grain boundary sliding, grain rotation and impingement, and pressure solution, which lead to a cohesive behavior and overall strain hardening of the aggregates. The characteristic CPO could not have been developed in this manner, and was the result of an older phase of partitioned amphibolite facies dislocation creep, recovery including chemical segregation, and recrystallization of porphyroclasts. This study demonstrates the significance of strain accommodation within garnet and its affect on composition under a range of PT conditions, and emphasizes the importance of utilizing EBSD methods with studies that rely upon a sound understanding of garnet. ?? 2010 Elsevier Ltd.

  16. Effects of grain boundaries at the electrolyte/cathode interfaces on oxygen reduction reaction kinetics of solid oxide fuel cells

    Energy Technology Data Exchange (ETDEWEB)

    Choi, Min Gi; Koo, Ja Yang; Ahn, Min Woo; Lee, Won Young [Dept. of Mechanical Engineering, Sungkyunkwan University, Suwon (Korea, Republic of)

    2017-04-15

    We systematically investigated the effects of grain boundaries (GBs) at the electrolyte/cathode interface of two conventional electrolyte materials, i.e., yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC). We deposited additional layers by pulsed laser deposition to control the GB density on top of the polycrystalline substrates, obtaining significant improvements in peak power density (two-fold for YSZ and three-fold for GDC). The enhanced performance at high GB density in the additional layer could be ascribed to the accumulation of oxygen vacancies, which are known to be more active sites for oxygen reduction reactions (ORR) than grain cores. GDC exhibited a higher enhancement than YSZ, due to the easier formation, and thus higher concentration, of oxygen vacancies for ORR. The strong relation between the concentration of oxygen vacancies and the surface exchange characteristics substantiated the role of GBs at electrolyte/cathode interfaces on ORR kinetics, providing new design parameters for highly performing solid oxide fuel cells.

  17. Effects of Ca doping and O deficiency on the charge distribution in the vicinity of a 45° [001] grain boundary in YBa2Cu3O7

    KAUST Repository

    Schwingenschlögl, Udo

    2012-02-01

    The charge redistribution at grain boundaries is critical for the applicability of high-T c superconductors in electronic devices, because it determines the transport across the material. We investigate the charge transfer and the alterations of the electronic states due to local doping of a normal-state 45°-tilted [001] grain boundary in YBa 2Cu 3O 7 by means of first-principles calculations. Considering Ca doping and O deficiency as prototypical modifications we demonstrate that the redistribution of the charge carriers in the CuO 2 planes displays a very complex spatial pattern, which deviates even qualitatively from the naive expectation. Copyright © EPLA, 2012.

  18. Grain boundary character distribution in a hot rolled 316 L stainless steel; Distribuicao de tipos de contornos de grao em um aco inoxidavel 316L laminado a quente

    Energy Technology Data Exchange (ETDEWEB)

    Lopes, L.C.R. [Rio Grande do Sul Univ., Porto Alegre, RS (Brazil). Dept. de Metalurgia; Thomson, C.B. [Wales Univ., Swansea (United Kingdom)

    1995-12-31

    The texture and the grain boundary character distribution of a 316 L stainless steel under hot rolled condition have been studied. Electron back-scatter diffraction technique associated to the scanning electron microscopy is used to determine the crystallographic orientation of grains individually. The material presented a random texture. However, the misorientation axis distribution represented by an inverse pole figure showed a non-random distribution with a high proportion <111> misorientation axis. The disorientation angle distribution indicates a proportion of about 60% of boundaries with 60 deg C misorientation angle. A 25% proportion of CSL interfaces of {Sigma} = 3 type was found 17 refs., 9 figs., 2 tabs.

  19. Grain boundary selective oxidation and intergranular stress corrosion crack growth of high-purity nickel binary alloys in high-temperature hydrogenated water

    Energy Technology Data Exchange (ETDEWEB)

    Bruemmer, S. M.; Olszta, M. J.; Toloczko, M. B.; Schreiber, D. K.

    2018-02-01

    The effects of alloying elements in Ni-5at%X binary alloys on intergranular (IG) corrosion and stress corrosion cracking (SCC) have been assessed in 300-360°C hydrogenated water at the Ni/NiO stability line. Alloys with Cr or Al additions exhibited grain boundary oxidation and IGSCC, while localized degradation was not observed for pure Ni, Ni-Cu or Ni-Fe alloys. Environment-enhanced crack growth was determined by comparing the response in water and N2 gas. Results demonstrate that selective grain boundary oxidation of Cr and Al promoted IGSCC of these Ni alloys in hydrogenated water.

  20. Martensitic transformation of pure iron at a grain boundary: Atomistic evidence for a two-step Kurdjumov-Sachs–Pitsch pathway

    Energy Technology Data Exchange (ETDEWEB)

    Meiser, Jerome; Urbassek, Herbert M., E-mail: urbassek@rhrk.uni-kl.de [Physics Department and Research Center OPTIMAS, University Kaiserslautern, Erwin-Schrödinger-Straße, D-67663 Kaiserslautern (Germany)

    2016-08-15

    Using classical molecular dynamics simulations and the Meyer-Entel interaction potential, we study the martensitic transformation pathway in a pure iron bi-crystal containing a symmetric tilt grain boundary. Upon cooling the system from the austenitic phase, the transformation starts with the nucleation of the martensitic phase near the grain boundary in a plate-like arrangement. The Kurdjumov-Sachs orientation relations are fulfilled at the plates. During further cooling, the plates expand and merge. In contrast to the orientation relation in the plate structure, the complete transformation proceeds via the Pitsch pathway.

  1. Evidences of grain boundary capacitance effect on the colossal dielectric permittivity in (Nb + In) co-doped TiO2 ceramics.

    Science.gov (United States)

    Li, Jinglei; Li, Fei; Li, Chao; Yang, Guang; Xu, Zhuo; Zhang, Shujun

    2015-02-06

    The (Nb + In) co-doped TiO2 ceramics were synthesized by conventional solid-state sintering (CSSS) and spark plasma sintering (SPS) methods. The phases and microstructures were studied by X-ray diffraction, Raman spectra, field-emission scanning electron microscopy and transmission electron microscopy, indicating that both samples were in pure rutile phase while showing significant difference in grain size. The dielectric and I-V behaviors of SPS and CSSS samples were investigated. Though both possess colossal permittivity (CP), the SPS samples exhibited much higher dielectric permittivity/loss factor and lower breakdown electric field when compared to their CSSS counterparts. To further explore the origin of CP in co-doped TiO2 ceramics, the I-V behavior was studied on single grain and grain boundary in CSSS sample. The nearly ohmic I-V behavior was observed in single grain, while GBs showed nonlinear behavior and much higher resistance. The higher dielectric permittivity and lower breakdown electric field in SPS samples, thus, were thought to be associated with the feature of SPS, by which reduced space charges and/or impurity segregation can be achieved at grain boundaries. The present results support that the grain boundary capacitance effect plays an important role in the CP and nonlinear I-V behavior of (Nb + In) co-doped TiO2 ceramics.

  2. A molecular dynamics study of tilt grain boundary resistance to slip and heat transfer in nanocrystalline silicon

    Energy Technology Data Exchange (ETDEWEB)

    Chen, Xiang; Chen, Youping [Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611 (United States); Xiong, Liming [Department of Aerospace Engineering, Iowa State University, Ames, Iowa 50011 (United States); Chernatynskiy, Aleksandr [Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 (United States)

    2014-12-28

    We present a molecular dynamics study of grain boundary (GB) resistance to dislocation-mediated slip transfer and phonon-mediated heat transfer in nanocrystalline silicon bicrystal. Three most stable 〈110〉 tilt GBs in silicon are investigated. Under mechanical loading, the nucleation and growth of hexagonal-shaped shuffle dislocation loops are reproduced. The resistances of different GBs to slip transfer are quantified through their constitutive responses. Results show that the Σ3 coherent twin boundary (CTB) in silicon exhibits significantly higher resistance to dislocation motion than the Σ9 GB in glide symmetry and the Σ19 GB in mirror symmetry. The distinct GB strengths are explained by the atomistic details of the dislocation-GB interaction. Under thermal loading, based on a thermostat-induced heat pulse model, the resistances of the GBs to transient heat conduction in ballistic-diffusive regime are characterized. In contrast to the trend found in the dislocation-GB interaction in bicrystal models with different GBs, the resistances of the same three GBs to heat transfer are strikingly different. The strongest dislocation barrier Σ3 CTB is almost transparent to heat conduction, while the dislocation-permeable Σ9 and Σ19 GBs exhibit larger resistance to heat transfer. In addition, simulation results suggest that the GB thermal resistance not only depends on the GB energy but also on the detailed atomic structure along the GBs.

  3. Jointed magnetic skyrmion lattices at a small-angle grain boundary directly visualized by advanced electron microscopy.

    Science.gov (United States)

    Matsumoto, Takao; So, Yeong-Gi; Kohno, Yuji; Sawada, Hidetaka; Ishikawa, Ryo; Ikuhara, Yuichi; Shibata, Naoya

    2016-10-24

    The interactions between magnetic skyrmions and structural defects, such as edges, dislocations, and grain boundaries (GBs), which are all considered as topological defects, will be important issues when magnetic skyrmions are utilized for future memory device applications. To investigate such interactions, simultaneous visualization of magnetic skyrmions and structural defects at high spatial resolution, which is not feasible by conventional techniques, is essential. Here, taking advantages of aberration-corrected differential phase-contrast scanning transmission electron microscopy, we investigate the interaction of magnetic skyrmions with a small-angle GB in a thin film of FeGe 1-x Si x . We found that the magnetic skyrmions and the small-angle GB can coexist each other, but a domain boundary (DB) was formed in the skyrmion lattice along the small-angle GB. At the core of the DB, unexpectedly deformed magnetic skrymions, which appear to be created by joining two portions of magnetic skyrmions in the adjacent lattices, were formed to effectively compensate misorientations between the two adjacent magnetic skyrmion lattices. These observations strongly suggest the flexible nature of individual magnetic skyrmions, and also the significance of defect engineering for future device applications.

  4. The chromium doping of Ni{sub 3}Fe alloy and restructuring of grain boundary ensemble at the phase transition A1→L1{sub 2}

    Energy Technology Data Exchange (ETDEWEB)

    Perevalova, Olga [Institute of Strength Physics and Material Science, Siberian Division of the Russian Academy of Sciences, Akademicheskii Av., 2/4, Tomsk, 634021 (Russian Federation); Konovalova, Elena, E-mail: knv123@yandex.ru [Surgut State University, Lenina Av., 1, Surgut, 628400 (Russian Federation); Koneva, Nina; Kozlov, Eduard [Tomsk State University of Architecture and Building, Solyanaya Sq., 2, Tomsk, 634003 (Russian Federation)

    2016-01-15

    The grain boundary structure of the Ni{sub 3}(Fe,Cr) alloy is studied in states with a short and long-range order formed at the phase transition A1→L1{sub 2}. It is found that the new boundaries of general and special types are formed during an ordering annealing, wherein the special boundaries share increases. The spectrum of special boundaries is changed due to decreasing of ∑3 boundary share. It leads to weakening of the texture in the alloy with atomic long-range order. The features of change of the special boundaries spectrum at the phase transition A1→L1{sub 2} in the Ni{sub 3}(Fe,Cr) alloy are determined by decreasing of the stacking fault energy and the atomic mean square displacement at the chromium doping.

  5. SEM observation of grain boundary structures in quartz-iron oxide rocks deformed at intermediate metamorphic conditions

    Directory of Open Access Journals (Sweden)

    Leonardo Lagoeiro

    2011-09-01

    Full Text Available Several studies have demonstrated the effect of a second phase on the distribution of fluid phase and dissolution of quartz grains. However, as most observations came from aggregates deformed under hydrostatic stress conditions and mica-bearing quartz rocks, 3-D distribution of pores on quartz-quartz (QQB and quartz-hematite boundaries (QHB has been studied. Several fracture surfaces oriented according to finite strain ellipsoid were analyzed. The pore distribution characterizes the porosity and grain shape as highly anisotropic, which results from the nature and orientation of boundaries. QHB have physical/chemical properties very different from QQB, once the hematite plates have strong effect on wetting behavior of fluid, likewise micas in quartzites. They are pore-free flat surfaces, normal to compression direction, suggesting that they were once wetted with a continuous fluid film acting as faster diffusion pathway. At QQB, the pores are faceted, isolated, close to its edges reflecting the crystallographic control and an interconnected network of fluid along grain junctions. The QQB facing the extension direction are sites of fluid concentration. As consequence, the anisotropic dissolution and grain growth were responsible for the formation of hematite plates and tabular quartz grains significantly contributing for the generation of the foliation observed in the studied rocks.Muitos estudos têm demonstrado o efeito de uma segunda fase sobre a distribuição de fase fluida e dissolução de grãos de quartzo. Entretanto, como a maioria das observações vêm de agregados deformados sob condições de tensão hidrostática e em rochas quartzosas ricas em mica, a distribuição 3D de poros e bordas quartzo-quartzo (BQQ e quartzo-hematita (BQH tem sido estudada. Várias superfícies de fraturas orientadas segundo o elipsóide de deformação finita foram analisadas. A distribuição dos poros caracteriza a porosidade e a forma dos grãos como

  6. 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.

  7. Characterisation of multicrystalline silicon solar cells. Development of characterisation method for the combined effect of dislocations and grain boundaries on the minority carrier lifetime

    Energy Technology Data Exchange (ETDEWEB)

    Stokkan, Gaute

    2004-07-01

    The thesis has sections on theoretical background, mathematical models, experimental work such as lifetime measurements, dislocation density and grain boundary mapping, simulation of electrical activity mapping and conclusions and future work. Various mathematical models and nafion surface passivation are studied as well.

  8. Production of NiSi phase by grain boundary diffusion induced solid state reaction between Ni{sub 2}Si and Si(1 0 0) substrate

    Energy Technology Data Exchange (ETDEWEB)

    Shenouda, S.S., E-mail: shenouda_physics@yahoo.com [University of Debrecen, Department of Solid State Physics, H-4010, Debrecen, P.O. Box 2 (Hungary); Ain Shams University, Department of Physics, Faculty of Education, Roxy, Cairo (Egypt); Langer, G.A.; Katona, G.L.; Daróczi, L. [University of Debrecen, Department of Solid State Physics, H-4010, Debrecen, P.O. Box 2 (Hungary); Csik, A. [Institute Nuclear Research, Hungarian Academy of Sciences (ATOMKI), H-4001, Debrecen, P.O. Box 51 (Hungary); Beke, D.L. [University of Debrecen, Department of Solid State Physics, H-4010, Debrecen, P.O. Box 2 (Hungary)

    2014-11-30

    Graphical abstract: Sketch of grain boundary diffusion of Si (blue) into Ni{sub 2}Si film (yellow) forming NiSi (red). - Highlights: • New processing method for creation of homogeneous thin NiSi contacts from nanocrystalline-Ni{sub 2}Si/Si(1 0 0) substrate. • The NiSi forms by grain-boundary diffusion and reaction in a certain low temperature-time and thickness-time window. • The nucleated NiSi growths perpendicular to the grain boundaries and its velocity can also be determined. - Abstract: We report a process to obtain thin (5–20 nm thick) NiSi layers on Si(1 0 0) substrate from magnetron deposited Ni{sub 2}Si thin films at low temperatures (180–200°C). The time evolution of transformation was followed by means of Secondary Neutral Mass Spectrometry, transmission electron microscopy and resistance measurements. It is shown that there exist certain temperature-time and thickness-time windows inside of which the formation of NiSi takes place. The NiSi phase, formed along the grain boundaries of Ni{sub 2}Si and grew by the motion of these interfaces, gradually consumes the Ni{sub 2}Si phase. From the depth profiles of the first stage of the process, using the linear dependence of the average composition inside the film on the annealing time, the velocity of the grain boundary diffusion induced interface motion was also estimated. The normalized value of the resistance, proportional to the amount of the new phase, showed similar time evolution and yielded similar value for the interface velocity.

  9. The cause of ‘weak-link’ grain boundary behaviour in polycrystalline Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 superconductors

    Science.gov (United States)

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

    2018-02-01

    The detrimental effects of grain boundaries have long been considered responsible for the low critical current densities ({J}{{c}}) in high temperature superconductors. In this paper, we apply the quantitative approach used to identify the cause of the ‘weak-link’ grain boundary behaviour in YBa2Cu3O7 (Wang et al 2017 Supercond. Sci Technol. 30 104001), to the Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 materials that we have fabricated. Magnetic and transport measurements are used to characterise the grain and grain boundary properties of micro- and nanocrystalline materials. Magnetisation measurements on all nanocrystalline materials show non-Bean-like behaviour and are consistent with surface pinning. Bi2Sr2CaCu2O8: our microcrystalline material has very low grain boundary resistivity ({ρ }{{GB}}), which is similar to that of the grains ({ρ }{{G}}) such that {ρ }{{GB}}≈ {ρ }{{G}}=2× {10}-5 {{Ω }}{{m}} (assuming a grain boundary thickness (d) of 1 nm) equivalent to an areal resistivity of {ρ }{{G}}=2× {10}-14 {{{Ω }}{{m}}}2. The transport {J}{{c}} values are consistent with well-connected grains and very weak grain boundary pinning. However, unlike low temperature superconductors (LTS) in which decreasing grain size increases the pinning along the grain boundary channels, any increase in pinning produced by making the grains in our Bi2Sr2CaCu2O8 materials nanocrystalline was completely offset by a decrease in the depairing current density of the grain boundaries caused by their high resistivity. We suggest a different approach to increasing {J}{{c}} from that used in LTS materials, namely incorporating additional strong grain and grain boundary pinning sites in microcrystalline materials to produce high {J}{{c}} values. Bi2Sr2Ca2Cu3O10: both our micro- and nanocrystalline samples have {ρ }{{GB}}/{ρ }{{G}} of at least 103. This causes strong suppression of {J}{{c}} across the grain boundaries, which explains the low transport {J}{{c}} values we find

  10. The origin of room temperature ferromagnetism mediated by Co–VZn complexes in the ZnO grain boundary

    KAUST Repository

    Devi, Assa Aravindh Sasikala

    2016-05-20

    Ferromagnetism in polycrystalline ZnO doped with Co has been observed to be sustainable in recent experiments. We use first-principle calculations to show that Co impurities favorably substitute at the grain boundary (GB) rather than in the bulk. We reveal that room-temperature ferromagnetism (RTFM) at the Co-doped ZnO GB in the presence of Zn vacancies is due to ferromagnetic exchange coupling of a pair of closely associated Co atoms in the GB, with a ferromagnetic exchange coupling energy of ∼300 meV, which is in contrast to a previous study that suggested the O vacancy-Co complex induced ferromagnetism. Electronic structure analysis was used to predict the exchange coupling mechanism, showing that the hybridization of O p states with Co and Zn d states enhances the magnetic polarization originating from the GB. Our results indicate that RTFM originates from Co clusters at interfaces or in GBs. © 2016 The Royal Society of Chemistry.

  11. Highly-translucent, strong and aging-resistant 3Y-TZP ceramics for dental restoration by grain boundary segregation.

    Science.gov (United States)

    Zhang, Fei; Vanmeensel, Kim; Batuk, Maria; Hadermann, Joke; Inokoshi, Masanao; Van Meerbeek, Bart; Naert, Ignace; Vleugels, Jef

    2015-04-01

    Latest trends in dental restorative ceramics involve the development of full-contour 3Y-TZP ceramics which can avoid chipping of veneering porcelains. Among the challenges are the low translucency and the hydrothermal stability of 3Y-TZP ceramics. In this work, different trivalent oxides (Al2O3, Sc2O3, Nd2O3 and La2O3) were selected to dope 3Y-TZP ceramics. Results show that dopant segregation was a key factor to design hydrothermally stable and high-translucent 3Y-TZP ceramics and the cation dopant radius could be used as a controlling parameter. A large trivalent dopant, oversized as compared to Zr(4+), exhibiting strong segregation at the ZrO2 grain boundary was preferred. The introduction of 0.2 mol% La2O3 in conventional 0.1-0.25 wt.% Al2O3-doped 3Y-TZP resulted in an excellent combination of high translucency and superior hydrothermal stability, while retaining excellent mechanical properties. Copyright © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  12. Fast and scalable prediction of local energy at grain boundaries: machine-learning based modeling of first-principles calculations

    Science.gov (United States)

    Tamura, Tomoyuki; Karasuyama, Masayuki; Kobayashi, Ryo; Arakawa, Ryuichi; Shiihara, Yoshinori; Takeuchi, Ichiro

    2017-10-01

    We propose a new scheme based on machine learning for the efficient screening in grain-boundary (GB) engineering. A set of results obtained from first-principles calculations based on density functional theory (DFT) for a small number of GB systems is used as a training data set. In our scheme, by partitioning the total energy into atomic energies using a local-energy analysis scheme, we can increase the training data set significantly. We use atomic radial distribution functions and additional structural features as atom descriptors to predict atomic energies and GB energies simultaneously using the least absolute shrinkage and selection operator, which is a recent standard regression technique in statistical machine learning. In the test study with fcc-Al [110] symmetric tilt GBs, we could achieve enough predictive accuracy to understand energy changes at and near GBs at a glance, even if we collected training data from only 10 GB systems. The present scheme can emulate time-consuming DFT calculations for large GB systems with negligible computational costs, and thus enable the fast screening of possible alternative GB systems.

  13. Numerical simulation of grain boundary effects in Cu(In,Ga)Se{sub 2} thin-film solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Taretto, K. [Departamento de Electrotecnia, Facultad de Ingenieria, Universidad Nacional del Comahue, Buenos Aires 1400, 8300 Neuquen (Argentina); Rau, U. [Institute of Physical Electronics, University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart (Germany)]. E-mail: uwe.rau@ipe.uni-stuttgart.de; Werner, J.H. [Institute of Physical Electronics, University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart (Germany)

    2005-06-01

    Two-dimensional numerical simulations of polycrystalline Cu(In,Ga)Se{sub 2} thin-film solar cells show that grain boundary (GB) recombination can deteriorate the photovoltaic power conversion efficiency of these devices by about 9% absolute with respect to a starting value of 21.7% that would hold for a material without GBs. The achieved record efficiencies of 19% are only possible if the recombination velocity S at GBs is kept below S=10{sup 3} cm s{sup -1}. Comparing devices that have all defects homogeneously distributed in the bulk to devices where the same number of defects is concentrated at GBs only unveils that the latter situation is more favorable because of kinetic restrictions. The efficiency difference between the homogeneous and the concentrated cases is, however, only 1% (absolute). We further model the possible effect of an additional hole barrier at the GB by assuming asymmetric capture cross-sections for electrons and holes. We find that the positive consequence of this feature is rather limited and much dependent on the specific properties of the GB defects. For example, the efficiency improves by 2% when introducing a hole barrier of 120 meV at a GB with midgap defects. The same improvement would result from a reduction of the GB defects by a factor of 2.5.

  14. Impedance spectroscopy of nanocrystalline MgFe2O4 and MnFe2O4 ferrite ceramics: Effect of grain boundaries on the electrical properties

    Directory of Open Access Journals (Sweden)

    Sekulić Dalibor L.

    2016-01-01

    Full Text Available Two ferrite ceramic materials, MgFe2O4 and MnFe2O4, were successfully fabricated by a conventional sintering of nanosized powders (at 1373 K for 2 h synthesized by soft mechanochemical route. The particle size and morphology of powders were studied using X-ray diffraction (XRD and transmission electron microscopy (TEM. XRD analysis was carried out for the determination of phase purity, crystal structure and average crystallite size of sintered ferrites. Both mechanosynthesized ferrite samples show mean crystallite sizes in the nm-range. Over the frequency range of 100 Hz to 1 MHz, impedance spectra of prepared ferrite ceramics are investigated at and above room temperature. Changes in the impedance plane plots with temperature have been discussed and correlated to the microstructure of materials. An equivalent circuit model is applied to explore the electrical parameters (resistance and capacitance associated with grains and grain boundaries. Complex impedance analysis indicates the dominance of grain boundary effects which control the overall electrical behaviour of studied ferrites. The decrease in grain boundary resistance with temperature suggests a thermally activated conduction mechanism. [Projekat Ministarstva nauke Republike Srbije, br. III43008 i br. III45003

  15. 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.

  16. Ground boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.

    1990-01-01

    The present document is a progress report describing the work accomplished on the study of grain boundaries in Ag, Au, Ni, Si, and Ge. Research was focused on the following four major efforts: study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; grain boundary migration; short-circuit diffusion along grain boundaries; and development of Thin-Film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals. 10 refs., 1 fig.

  17. Quantification of elemental segregation to lath and grain boundaries in low-alloy steel by STEM X-ray mapping combined with the zeta-factor method

    CERN Document Server

    Watanabe, M

    2003-01-01

    Elemental segregation to two types of boundaries in a low-alloy steel were studied by X-ray mapping using scanning transmission electron microscopy (STEM). To quantify the acquired X-ray maps, the zeta-factor method was applied, and then the compositional maps and the thickness map were obtained. Based on these quantified maps, further information about the analytical sensitivity of solute-element detection and the spatial resolution of segregation analysis were extracted. Furthermore, maps of the number of excess atoms on the boundary were also calculated from the compositional and thickness maps. It was concluded that Cr, Ni and Mo are co-segregated on the prior-austenite grain boundary and only Ni was segregated on the lath boundary. (orig.)

  18. Effect of microwave-enhanced superconductivity in YBa{sub 2}Cu{sub 3}O{sub 7} bi-crystalline grain boundary weak-links

    Energy Technology Data Exchange (ETDEWEB)

    Fu, C.M.; Chen, C.M.; Lin, H.C. [National Chiao-Tung Univ., Taiwan (China)] [and others

    1994-12-31

    We have studied systematically the effect of microwave irradiation on the temperature dependent resistivity R(T) and the current-voltage (I-V) characteristics of YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) bicrystalline grain boundary weak-links (GBWLs), with grain boundary of three different tilt angles. The superconducting transition temperature, T{sub c}, has significant enhancement upon microwave irradiation. The microwave enhanced T{sub c} is increased as a function of incidence microwave power, but limited to an optimum power level. The GBWLs of 45{degrees} tilt boundary has shown to be most sensitive to the microwave irradiation power, and the GBWLs of 36.8{degrees} tilt boundary has displayed a moderate response. In contrast, no enhancement of T{sub c} was observed in the GBWLs of 24{degrees} tilt boundary, as well as in the uniform films. Under the microwave irradiation, the R(T) dependence is hysteretic as the transition taken from superconducting state to normal state and vice versa. Mechanisms associated with the redistribution of nonequilibrium quasiparticles under microwave irradiation are discussed.

  19. Mitigation of Corrosion in 5 Series Al-Mg Alloys in Marine Environments: Grain Boundary Engineering and Cold Spray Coating Approaches

    Science.gov (United States)

    2014-03-26

    of AA5083", Electrochimica Acta , 52(2007) pp.7651- 7659 J.S. Vetrano, S.M. Bruemmer, L.M. Pawlowski, and I.M. Robertson, "Influence of the particle...boundaries in an Al-Cu alloy, Acta Metall., Vol. 18 (1970) pp. 183-187. 2] E. P. Butler and P. R. Swann, In situ observations of the nucleation and...initial growth of grain boundary precipitates in an Al-Zn-Mg alloy, Acta Metall., Vol. 24 (1976) pp. 343-352. 3] J. K. Park and A. J. Ardell

  20. Direct measurement of anisotropy of interfacial free energy from grain boundary groove morphology in transparent organic metal analong systems

    Energy Technology Data Exchange (ETDEWEB)

    Rustwick, Bryce A. [Iowa State Univ., Ames, IA (United States)

    2005-01-01

    Both academia and industry alike have paid close attention to the mechanisms of microstructural selection during the solidification process. The forces that give rise to and the principles which rule the natural selection of particular morphologies are important to understanding and controlling new microstructures. Interfacial properties play a very crucial role to the selection of such microstructure formation. In the solidification of a metallic alloy, the solid-liquid interface is highly mobile and responds to very minute changes in the local conditions. At this interface, the driving force must be large enough to drive solute diffusion, maintain local curvature, and overcome the kinetic barrier to move the interface. Therefore, the anisotropy of interfacial free energy with respect to crystallographic orientation is has a significant influence on the solidification of metallic systems. Although it is generally accepted that the solid-liquid interfacial free energy and its associated anisotropy are highly important to the overall selection of morphology, the confident measurement of these particular quantities remains a challenge, and reported values are scarce. Methods for measurement of the interfacial free energy include nucleation experiments and grain boundary groove experiments. The predominant method used to determine anisotropy of interfacial energy has been equilibrium shape measurement. There have been numerous investigations involving grain boundaries at a solid-liquid interface. These studies indicated the GBG could be used to describe various interfacial energy values, which affect solidification. Early studies allowed for an estimate of interfacial energy with respect to the GBG energy, and finally absolute interfacial energy in a constant thermal gradient. These studies however, did not account for the anisotropic nature of the material at the GBG. Since interfacial energy is normally dependent on orientation of the crystallographic plane of the

  1. The roles of carrier concentration and interface, bulk, and grain-boundary recombination for 25% efficient CdTe solar cells

    Science.gov (United States)

    Kanevce, A.; Reese, M. O.; Barnes, T. M.; Jensen, S. A.; Metzger, W. K.

    2017-06-01

    CdTe devices have reached efficiencies of 22% due to continuing improvements in bulk material properties, including minority carrier lifetime. Device modeling has helped to guide these device improvements by quantifying the impacts of material properties and different device designs on device performance. One of the barriers to truly predictive device modeling is the interdependence of these material properties. For example, interfaces become more critical as bulk properties, particularly, hole density and carrier lifetime, increase. We present device-modeling analyses that describe the effects of recombination at the interfaces and grain boundaries as lifetime and doping of the CdTe layer change. The doping and lifetime should be priorities for maximizing open-circuit voltage (Voc) and efficiency improvements. However, interface and grain boundary recombination become bottlenecks for device performance at increased lifetime and doping levels. This work quantifies and discusses these emerging challenges for next-generation CdTe device efficiency.

  2. Improvement of the Thermoelectric Figure-of-Merit of a Doped Telluride Nanocomposite by Combining Phonon Scattering with Grain Boundary-Modifying Zn-Containing Nanostructures

    Science.gov (United States)

    Rowe, Michael P.; Zhou, Li Qin; Banerjee, Debasish; Zhang, Minjuan

    2015-01-01

    Recovery of waste heat from internal combustion engines is one strategy for meeting the ever increasing demand for more fuel efficient-automobiles. Thermoelectric materials are capable of this, by solid-state conversion of thermal to electrical energy, but the efficiency of this energy conversion requires improvement. In this work the thermoelectric figure of merit ( ZT) was improved by combining phonon scattering with grain boundary modification in a bismuth antimony telluride nanocomposite material with zinc antimony grain boundaries and zinc oxide nanoparticle inclusions. The advantage of including these zinc nanostructures is discussed. By reducing thermal conductivity while increasing the power factor, ZT was been increased from 0.6 to 1.1.

  3. Engineering of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} grain boundary Josephson junctions by Au nanocrystals

    Energy Technology Data Exchange (ETDEWEB)

    Michalowski, P.; Schmidt, M.; Schmidl, F.; Grosse, V.; Kuhwald, D.; Katzer, C.; Seidel, P. [Institut fuer Festkoerperphysik, Universitaet Jena, Helmholtzweg 5, 07743 Jena (Germany); Huebner, U. [Institute of Photonic Technology, Albert-Einstein-Strasse 9, 07745 Jena (Germany)

    2011-08-15

    We prepared and investigated grain boundary Josephson junctions based on SrTiO{sub 3} bicrystal substrates. During the deposition of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBCO) gold nanocrystals forming from an intermediate gold layer can modify the crystalline structure and thus the properties of the YBCO grain boundaries. The variation of the film thickness of the Au seed layer changes the growth conditions of the YBCO film and the Au nanocrystals. The values of the characteristic I{sub C}R{sub N} product do not change whereas the values of the critical current I{sub C} decrease. (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  4. Enrichment of boron at grain boundaries of platinum-based alloys determined by electron energy loss spectroscopy in a transmission electron microscope

    Energy Technology Data Exchange (ETDEWEB)

    Preussner, Johannes [Fraunhofer Institute for Mechanics of Materials (IWM), Freiburg (Germany); Karlsruhe Institue of Technology, Karlsruhe (DE). Inst. for Reliability of Components and Systems (izbs); Fleischmann, Ernst; Voelkl, Rainer; Glatzel, Uwe [Bayreuth Univ. (Germany). Metals and Alloys

    2010-05-15

    Polycrystalline platinum-based alloys show very good creep properties at high temperatures. Small amounts of boron (less than 1 at.%) considerably increase the creep strength. Transmission electron microscopy measurements were conducted to localize the element boron in the samples. With image electron energy loss spectroscopy spectra were extracted revealing an enrichment of B at grain boundaries. The boron distribution was plotted with the three window method. (orig.)

  5. First-Principles Calculations of Electronic States and Self-Doping Effects at a 45° Grain Boundary in the High Temperature YBa2Cu3O7 Superconductor

    KAUST Repository

    Schwingenschlögl, Udo

    2009-06-03

    The charge redistribution at grain boundaries determines the applicability of high-Tc superconductors in electronic devices because the transport across the grains can be hindered considerably. We investigate the local charge transfer and the modification of the electronic states in the vicinity of the grain-grain interface by ab initio calculations for a (normal-state) 45°-tilted [001] grain boundary in YBa2Cu3O7. Our results explain the suppressed interface transport and the influence of grain boundary doping in a quantitative manner, in accordance with the experimental situation. The charge redistribution is found to be strongly inhomogeneous, which has a substantial effect on transport properties since it gives rise to a self-doping of 0.10±0.02 holes per Cu atom.

  6. Effects of Heat Treatment on Grain-Boundary β-Mg17Al12 and Fracture Properties of Resistance Spot-Welded AZ80 Mg Alloy

    Science.gov (United States)

    Niknejad, Seyed Tirdad; Liu, Lei; Nguyen, Tam; Lee, Mok-Young; Esmaeili, Shahrzad; Zhou, Norman Y.

    2013-08-01

    The distribution and morphology of β-Mg17Al12 intermetallic phase in resistance spot-welded AZ80 Mg alloy were investigated by means of optical microscopy, scanning electron microscopy, and X-ray diffraction. The influence of intermetallic phase on mechanical strength was studied by tensile shear testing and fractography. The results showed that continuous networks of β-Mg17Al12 formed along grain boundaries in both the nugget and heat-affected zone of the spot-welded AZ80 Mg alloy. Those continuous grain-boundary β-Mg17Al12 networks acted as effective crack propagation paths, which had negative effects on the weld strength. Post-weld solution heat treatment effectively reduced the amount of β-Mg17Al12 and broke the grain-boundary intermetallic networks in both the nugget and heat-affected zone. This significantly increased the weld strength of AZ80 Mg alloy and changed the fracture mode from nugget pull-out in the as-welded condition to through-thickness after heat treatment.

  7. Hexamethylenetetramine-mediated growth of grain-boundary-passivation CH3NH3PbI3 for highly reproducible and stable perovskite solar cells

    Science.gov (United States)

    Zheng, Yan-Zhen; Li, Xi-Tao; Zhao, Er-Fei; Lv, Xin-Ding; Meng, Fan-Li; Peng, Chao; Lai, Xue-Sen; Huang, Meilan; Cao, Guozhong; Tao, Xia; Chen, Jian-Feng

    2018-02-01

    Simultaneously achieving the long-term device stability and reproducibility has proven challenging in perovskite solar cells because solution-processing produced perovskite film with grain boundary is sensitive to moisture. Herein, we develop a hexamethylenetetramine (HMTA)-mediated one-step solution-processing deposition strategy that leads to the formation of high-purity and grain-boundary-passivation CH3NH3PbI3 film and thereby advances cell optoelectronic performance. Through morphological and structural characterizations and theoretical calculations, we demonstrate that HMTA fully occupies the moisture-exposed surface to build a bridge across grain boundary and coordinates with Pb ions to inhibit the formation of detrimental PbI2. Such HMTA-mediated grown CH3NH3PbI3 films achieves a decent augmentation of power conversion efficiency (PCE) from 12.70% to 17.87%. A full coverage of PbI2-free CH3NH3PbI3 surface on ZnO also boosts the device's stability and reproducibility.

  8. EBSD analysis of heterogeneous microstructures in experimentally deformed calcite: development of core and mantle subgrains, grain boundary bulges and recrystallised grains. Geologica Ultraiectina (289)

    NARCIS (Netherlands)

    Valcke, S.L.A.|info:eu-repo/dai/nl/304833452

    2008-01-01

    Geodynamic processes such as subduction and rifting are mainly controlled by high temperature plastic deformation of rocks. It is known that elements of the microstructure in plastically deformed rocks, such as subgrains and recrystallised grains, are potentially useful as indicators of past

  9. Grain boundary engineering with nano-scale InSb producing high performance InxCeyCo4Sb12+z skutterudite thermoelectrics

    Directory of Open Access Journals (Sweden)

    Han Li

    2017-12-01

    Full Text Available Thermoelectric semiconductors based on CoSb3 hold the best promise for recovering industrial or automotive waste heat because of their high efficiency and relatively abundant, lead-free constituent elements. However, higher efficiency is needed before thermoelectrics reach economic viability for widespread use. In this study, n-type InxCeyCo4Sb12+z skutterudites with high thermoelectric performance are produced by combining several phonon scattering mechanisms in a panoscopic synthesis. Using melt spinning followed by spark plasma sintering (MS-SPS, bulk InxCeyCo4Sb12+z alloys are formed with grain boundaries decorated with nano-phase of InSb. The skutterudite matrix has grains on a scale of 100–200 nm and the InSb nano-phase with a typical size of 5–15 nm is evenly dispersed at the grain boundaries of the skutterudite matrix. Coupled with the presence of defects on the Sb sublattice, this multi-scale nanometer structure is exceptionally effective in scattering phonons and, therefore, InxCeyCo4Sb12/InSb nano-composites have very low lattice thermal conductivity and high zT values reaching in excess of 1.5 at 800 K.

  10. Angular dependence of tunneling effects in Ba{sub 1{minus}x}K{sub x}BiO{sub 3} grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Kussmaul, A.; Hellman, E.S.; Hartford, E.H. Jr. [AT and T Bell Labs., Murray Hill, NJ (United States); Gupta, A. [IBM T.J. Watson Research Center, Yorktown Heights, NY (United States); Tedrow, P.M. [Francis Bitter National Magnet Lab., Cambridge, MA (United States)

    1994-12-31

    The authors report on the tunneling properties of Ba{sub 1{minus}x}K{sub x}BiO{sub 3} (BKBO) grain boundaries prepared on bicrystal substrates. They studied symmetric tilt boundaries with 5{degree}, 24{degree}, 36.8{degree} and 45{degree} misorientation on SrTiO{sub 3}, and 24{degree} on MgO substrates. The three high angle misorientations used yield clear superconductor-insulator-superconductor behavior, with very little conductance below the bias corresponding to twice the BKBO energy gap, and a strong peak at that value. The two samples prepared on MgO yielded somewhat broader tunneling characteristics, but otherwise similar features compared to the samples prepared on SrTiO{sub 3}; they showed the lowest junction resistivity and the weakest conductance increase at high bias. The linear conductance background consistently observed in superconductor-insulator-normal metal junctions is absent in the grain boundary junctions. For the 5{degree} samples, they observe weak link behavior below {Tc}, with a critical current density of 65A/cm{sup 2}. A similar angular dependence is seen in Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4{minus}{delta}}.

  11. Intrinsic stress in ZrN thin films: Evaluation of grain boundary contribution from in situ wafer curvature and ex situ x-ray diffraction techniques

    Energy Technology Data Exchange (ETDEWEB)

    Koutsokeras, L. E. [Departement Physique et Mecanique des Materiaux, Institut Pprime, CNRS-Universite de Poitiers-ENSMA, UPR 3346, SP2MI, Teleport 2, Bd M et P Curie, F 86962 Chasseneuil-Futuroscope (France); Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110 (Greece); Abadias, G. [Departement Physique et Mecanique des Materiaux, Institut Pprime, CNRS-Universite de Poitiers-ENSMA, UPR 3346, SP2MI, Teleport 2, Bd M et P Curie, F 86962 Chasseneuil-Futuroscope (France)

    2012-05-01

    Low-mobility materials, like transition metal nitrides, usually undergo large residual stress when sputter-deposited as thin films. While the origin of stress development has been an active area of research for high-mobility materials, atomistic processes are less understood for low-mobility systems. In the present work, the contribution of grain boundary to intrinsic stress in reactively magnetron-sputtered ZrN films is evaluated by combining in situ wafer curvature measurements, providing information on the overall biaxial stress, and ex situ x-ray diffraction, giving information on elastic strain (and related stress) inside crystallites. The thermal stress contribution was also determined from the in situ stress evolution during cooling down, after deposition was stopped. The stress data are correlated with variations in film microstructure and growth energetics, in the 0.13-0.42 Pa working pressure range investigated, and discussed based on existing stress models. At low pressure (high energetic bombardment conditions), a large compressive stress is observed due to atomic peening, which induces defects inside crystallites but also promotes incorporation of excess atoms in the grain boundary. Above 0.3-0.4 Pa, the adatom surface mobility is reduced, leading to the build-up of tensile stress resulting from attractive forces between under-dense neighbouring column boundary and possible void formation, while crystallites can still remain under compressive stress.

  12. Spatially resolved analytical electron microscopy at grain boundaries of {alpha}-Al{sub 2}O{sub 3}; Ortsaufgeloeste analytische Elektronenmikroskopie an Korngrenzen in {alpha}Al{sub 2}O{sub 3}

    Energy Technology Data Exchange (ETDEWEB)

    Nufer, S.

    2001-10-01

    Aluminum oxide, {alpha}-Al{sub 2}O{sub 3}, is a common structural ceramic material. The most technologically important properties are either determined or strongly influenced by the polycrystalline microstructure. For instance, the grain boundaries control the mechanical behavior (e.g. plasticity, creep, and fracture) or various transport phenomena (e.g. ion diffusion, segregation, and electrical resistivity). In order to understand the structure-properties relationships, it is therefore important to characterize the structure and chemistry of grain boundaries, both experimentally and theoretically. In this work the electronic structure of the basal and rhombohedral twin grain boundaries and the impurity excess at different tilt grain boundaries in bicrystals were investigated, using electron energy-loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS). The electronic structure of the rhombohedral twin grain boundary was determined by comparing spatially resolved EELS measurements of the O-K ionisation edge with the theoretical density of states (DOS), obtained from local density functional theory (LDFT) calculations. The interface excess of impurities was quantitatively analysed at grain boundaries with and without Y-doping. (orig.)

  13. Size effects and charge transport in metals: Quantum theory of the resistivity of nanometric metallic structures arising from electron scattering by grain boundaries and by rough surfaces

    Science.gov (United States)

    Munoz, Raul C.; Arenas, Claudio

    2017-03-01

    We discuss recent progress regarding size effects and their incidence upon the coefficients describing charge transport (resistivity, magnetoresistance, and Hall effect) induced by electron scattering from disordered grain boundaries and from rough surfaces on metallic nanostructures; we review recent measurements of the magneto transport coefficients that elucidate the electron scattering mechanisms at work. We review as well theoretical developments regarding quantum transport theories that allow calculating the increase in resistivity induced by electron-rough surface scattering (in the absence of grain boundaries) from first principles—from the parameters that describe the surface roughness that can be measured with a Scanning Tunnelling Microscope (STM). We evaluate the predicting power of the quantum version of the Fuchs-Sondheimer theory and of the model proposed by Calecki, abandoning the method of parameter fitting used for decades, but comparing instead theoretical predictions with resistivity measured in thin films where surface roughness has also been measured with a STM, and where electron-grain boundary scattering can be neglected. We also review the theory of Mayadas and Shatzkes (MS) [Phys. Rev. B 1, 1382 (1970)] used for decades, and discuss its severe conceptual difficulties that arise out of the fact that: (i) MS employed plane waves to describe the electronic states within the metal sample having periodic grain boundaries, rather than the Bloch states known since the thirties to be the solutions of the Schrödinger equation describing electrons propagating through a Krönig-Penney [Proc. R. Soc. London Ser. A 130, 499 (1931)] periodic potential; (ii) MS ignored the fact that the wave functions describing electrons propagating through a 1-D disordered potential are expected to decay exponentially with increasing distance, a fact known since the work of Anderson [Phys. Rev. 109, 1492 (1958)] in 1958 for which he was awarded the Nobel Prize in

  14. Trapping charges at grain boundaries and degradation of CH3NH3Pb(I1-x Br x )3 perovskite solar cells

    Science.gov (United States)

    Phuong Nguyen, Bich; Kim, Gee Yeong; Jo, William; Kim, Byeong Jo; Jung, Hyun Suk

    2017-08-01

    The electrical properties of CH3NH3Pb(I1-x Br x )3 (x = 0.13) perovskite materials were investigated under ambient conditions. The local work function and the local current were measured using Kelvin probe force microscopy and conductive atomic force microscopy, respectively. The degradation of the perovskite layers depends on their grain size. As the material degrades, an additional peak in the surface potential appears simultaneously with a sudden increase and subsequent relaxation of the local current. The potential bending at the grain boundaries and the intragrains is the most likely reason for the change of the local current surface of the perovskite layers. The improved understanding of the degradation mechanism garnered from this study helps pave the way toward an improved photo-conversion efficiency in perovskite solar cells.

  15. Modelling Cr depletion under a growing Cr2O3 layer on austenitic stainless steel: the influence of grain boundary diffusion

    DEFF Research Database (Denmark)

    Hansson, Anette Nørgaard; Hattel, Jesper Henri; Dahl, Kristian Vinter

    2009-01-01

    The oxidation behaviour of austenitic stainless steels in the temperature range 723–1173K is strongly influenced by the grain size of the oxidizing alloy. In this work the evolution of the concentration profiles of Cr, Ni and Fe in the substrate below a growing Cr2O3 layer is simulated...... according to a parabolic rate law as a consequence of rate limiting diffusion of Cr cations through the oxide layer; the retraction of the oxide/alloy interface associated with the removal of Cr atoms from the substrate is included in the calculations. Numerically, the movement of the oxide/alloy interface...... is formulated such that the initial mesh can be used throughout the calculation. The calculated concentration profiles of the alloying elements emphasize the importance of grain boundaries in supplying Cr from the alloy to the growing oxide layer. For temperatures of 823 and 923K the simulations predict...

  16. Electronic grain boundary properties in polycrystalline Cu(In,Ga)Se{sub 2} semiconductors for thin film solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Baier, Robert

    2012-06-25

    Solar cells based on polycrystalline Cu(In,Ga)Se{sub 2} (CIGSe) thin film absorbers reach the highest energy conversion efficiency among all thin film solar cells. The record efficiency is at least partly attributed to benign electronic properties of grain boundaries (GBs) in the CIGSe layers. However, despite a high amount of research on this phenomenon the underlying physics is not sufficiently understood. This thesis presents an elaborate study on the electronic properties of GBs in CIGSe thin films. Kelvin probe force microscopy (KPFM) was employed to investigate the electronic properties of GBs in dependence of the Ga-content. Five CIGSe thin lms with various Ga-contents were grown by means of similar three stage co-evaporation processes. Both as grown as well as chemically treated (KCN etched) thin films were analyzed. The chemical treatment was employed to remove surface oxides. No difference in electronic GB properties was found with or without the chemical treatment. Therefore, we conclude that a moderate surface oxidation does not alter the electronic properties of GBs. In general, one can observe significant variations of electronic potential barriers at GBs. Under consideration of the averaging effect of the work function signal of nanoscale potential distributions in KPFM measurements which was quantified in the course of this thesis both positive and negative potential barriers in a range between ∼-350 mV and ∼+450 mV were detected. Additionally, variations in the defect densities at GBs between ∼3.1 x 10{sup 11} cm{sup -2} and ∼2.1 x 10{sup 12} cm{sup -2} were found. However, no correlation between the electronic properties of GBs and the Ga-content of CIGSe thin films was discovered. Consequently, one cannot explain the drop in device efficiency observed for CIGSe thin film solar cells with a high Ga-content by a change of the electronic properties of GBs. Combined KPFM and electron backscatter diffraction measurements were employed for the

  17. 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.

  18. Lithium-doping inverts the nanoscale electric field at the grain boundaries in Cu2ZnSn(S,Se)4 and increases photovoltaic efficiency

    OpenAIRE

    Xin H.; Vorpahl S.M.; Collord A.D.; Braly I.L.; Uhl A.R.; Krueger B.W.; Ginger D.S.; Hillhouse H.W.

    2015-01-01

    Passive grain boundaries (GBs) are essential for polycrystalline solar cells to reach high efficiency. However the GBs in Cu2ZnSn(SSe)4 have less favorable defect chemistry compared to Cu(InGa)Se2. Here using scanning probe microscopy we show that lithium doping of Cu2ZnSn(SSe)4 changes the polarity of the electric field at the GB such that minority carrier electrons are repelled from the GB. Solar cells with lithium doping show improved performance and yield a new efficiency record of 11.8 f...

  19. Direct observation of grain boundary migration during recrystallization within the bulk of a moderately deformed aluminium single crystal

    DEFF Research Database (Denmark)

    Van Boxel, Steven; Schmidt, Søren; Ludwig, Wolfgang

    2014-01-01

    A single grain growing in the bulk of a mildly deformed (30% thickness reduction through cold rolling) aluminium single crystal with an {001}<100> orientation (Cube orientation), is monitored during recrystallization with synchrotron radiation using topo-tomography. The formation and migration of...... of the analyzed facets is not closely related to any crystallographic {111} plane of neither the growing grain nor the disappearing deformed matrix. © 2013 The Japan Institute of Metals and Materials.......A single grain growing in the bulk of a mildly deformed (30% thickness reduction through cold rolling) aluminium single crystal with an {001} orientation (Cube orientation), is monitored during recrystallization with synchrotron radiation using topo-tomography. The formation and migration...

  20. First-principles calculations of an oxygen deficient Σ = 3(111)[100953- 8984/14/25/313/bar1] grain boundary in strontium titanateFirst-principles calculations of an oxygen deficient Σ = 3(111)[10& bar 1] grain boundary in strontium titanate

    Science.gov (United States)

    Astala, R.; Bristowe, P. D.

    2002-07-01

    An oxygen vacancy in a Σ = 3(111)[10 bar 1] grain boundary (GB) in SrTiO3 is modelled using DFT plane-wave pseudopotential methods. The formation energy of the vacancy in the boundary is found to depend on its charge state and on the electron chemical potential. There is a strong driving force for segregation when the vacancy is in the charge neutral state and this results in an electrostatic potential barrier at the GB. The vacancy is found to act as a donor and the GB becomes n-type. The defect induces small atomic relaxations in an otherwise highly ordered GB structure.

  1. Grain boundary defect compensation in Ti-doped BaFe0.5Nb0.5O3 ceramics

    Science.gov (United States)

    Sun, Xiaojun; Deng, Jianming; Liu, Saisai; Yan, Tianxiang; Peng, Biaolin; Jia, Wenhao; Mei, Zaoming; Su, Hongbo; Fang, Liang; Liu, Laijun

    2016-09-01

    Giant dielectric ceramics Ba(Nb0.5Fe0.5- x Ti x )O3 (BNFT) have been fabricated by a conventional solid-state reaction. According to X-ray diffraction analysis, the crystal structure of these ceramics can be described by the cubic centrosymmetric with Pm- 3m space group. The real part ( ɛ') of dielectric permittivity and dielectric loss (tan δ) of the BNFT ceramics was measured in a frequency range from 40 Hz to 100 MHz at room temperature. The ( ɛ') of all these samples displays a high value (~6500) and a small frequency-dependence from 1 kHz to 1 MHz. We have established a link between conductivity activation energy and defect compensation at grain boundaries. The Ti4+-doped Ba(Nb0.5Fe0.5)O3 as a donor makes a great influence on the grain boundary behavior, which restricts the migration of oxygen vacancy and depresses dielectric loss factor for Ba(Nb0.5Fe0.5)O3 ceramics.

  2. Bond-Energy-Driven, Low- or High-Angle-Grain-Boundary-Movement-Mediated Synthesis of Porous Se-Te for Use in Water-Splitting Reactions.

    Science.gov (United States)

    Sasmal, Anup Kumar; Nayak, Arpan Kumar; Kartikeya, Prashant; Pradhan, Debabrata; Pal, Tarasankar

    2017-12-06

    Herein, for the first time, we applied the metal-metal-bond-energy factor to the evolution of a porous Se-Te alloy. The porous Se-Te material has been prepared from the constituents' elemental states, through only a heating-cooling process in silicone oil without the use of any reagent, surfactant, or capping agent. Surprisingly, the reaction occurred at a much lower temperature (240 °C) than the mp (450 °C) of Te0. The reaction's nucleation and growth by means of varied bond energy have been clarified for the first time. A difference in the bond energies of a hetero metal-metal bond (Se-Te) and a homo metal-metal bond (Se-Se) directs nucleation and growth toward the fabrication of a porous structure, even from the constituents' elemental states, in which low-angle-grain-boundary (LAGB) and high-angle-grain-boundary (HAGB) movements play governing roles. Proper band-gap alignment of Se and Te makes the alloy composite applicable to water-splitting reactions under Xe-arc-lamp illumination. PEC efficiency of Se-Te was found to be higher than those reported for Se and other composite materials.

  3. Magnetic cluster glass behavior and grain boundary effect in Nd0.7Ba0.3MnO3 nanoparticles

    Science.gov (United States)

    Roy, B.; Das, S.

    2008-11-01

    The magnetic and transport properties of Nd0.7Ba0.3MnO3 nanoparticles were explored by transmission electron microscopy, x-ray powder diffraction, resistivity, magnetoresistance, thermopower (S), and magnetic measurements. The metal-insulator transition behavior of the temperature dependence of resistivity for the sample with the largest particle size changes to insulating nature with the decrease in the particle size due to the enhancement of the grain boundary effect. The magnetoresistance of the nanoparticles is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. The thermopower of the samples shows a crossover from negative to positive values and at high temperatures S follows adiabatic small polaron hopping theory. The zero field cooled and field cooled (FC) magnetizations display broad ferromagnetic transition. The Curie temperature (TC) and the irreversibility temperature (Tirr) decrease considerably with the decrease in the particle size. During cooling the ac susceptibility of the nanoparticles exhibits two magnetic phase transitions with paramagnetic, ferromagnetic, and glassy phases. The frequency dependent peak in the out of phase part (χ″) of the ac susceptibility is the signature of cluster glass behavior. Large thermomagnetic irreversibility, monotonic increase in the FC magnetization, nonsaturation of the magnetization, and the observation of two distinct magnetic transitions in ac susceptibility give evidence for the cluster glass nature of the nanoparticles.

  4. Dynamics of ordering in highly degenerate models with anisotropic grain-boundary potential: Effects of temperature and vortex formation

    DEFF Research Database (Denmark)

    Jeppesen, Claus; Flyvbjerg, Henrik; Mouritsen, Ole G.

    1989-01-01

    universality class, and that all models with nonconserved order parameter, independent of ordering degeneracy and softness and origin of domain boundaries, obey the classical growth law at finite temperatures. In quenches to the Potts-ordered phase vortices and antivortices occur and annihilate mutually......-boundary network. The time evolution of this quantity is shown to obey the growth law, ΔE(t)∼t-n, over an extended time range at late times. It is found that the zero-temperature dynamics is characterized by a special exponent value which for the Q=48 model is n≃0.25 in accordance with earlier work. However......, for quenches to finite temperatures in the Potts-ordered phase there is a distinct crossover to the classical Lifshitz-Allen-Cahn exponent value, n=1 / 2, for both values of Q. This supports the conjecture that the zero-temperature dynamics for models with soft domain boundaries belong to a special...

  5. Toward a better understanding of strain incompatibilities at grain boundaries in the analysis of fatigue crack initiation at low temperature in the UdimetTM 720 Li superalloy

    Directory of Open Access Journals (Sweden)

    Larrouy Baptiste

    2014-01-01

    Full Text Available Low cycle fatigue properties of polycrystalline γ-γ′ Ni-based superalloys are dependent on many factors such as temperature, environment, grain size and distribution of the strengthening phases. Under LCF conditions at intermediate temperatures, an intergranular crack initiation could be observed. In this paper we propose to analyze the local conditions favouring such an intergranular cracking mode considering the high strength C&W UdimetTM720 Li alloy, widely used for manufacturing high pressure turbine disk for aeroengine applications. Tensile and fatigue tests were performed in air in the 20–465 ∘C range of temperature on micro-samples in order to focus on plasticity and damage processes developed near grain boundaries. A special attention was paid on the slip transfer between neighbouring grains taking into account their local crystallographic orientations. In some specific crystallographic configurations, small zones were detected at the tip of slip bands presenting an intense elastic/plastic activity. Although they are limited in size, they are associated to local crystalline rotations. High levels of local strain/stress were also evaluated in these volumes using an EBSD pattern cross correlation technique. The development of such specific zones was investigated at different stages of the tensile and LCF behaviour and was identified as leading to micro-cracks initiation for both solicitation modes.

  6. Interaction between lattice dislocations and grain boundaries in f.c.c. and ordered compounds : a computer simulation

    NARCIS (Netherlands)

    Pestman, B.J.; Hosson, J.Th.M. De; Vitek, V.; Schapink, F.W.

    1991-01-01

    The interaction of 1/2<110> screw- and 60° dislocations with symmetric [110] tilt boundaries was investigated by atomistic simulations using many-body potentials representing a pure f.c.c. metal and ordered intermetallic compounds. The calculations were performed with and without an applied shear

  7. INTERACTION BETWEEN LATTICE DISLOCATIONS AND GRAIN-BOUNDARIES IN FCC AND ORDERED COMPOUNDS - A COMPUTER-SIMULATION

    NARCIS (Netherlands)

    PESTMAN, BJ; DEHOSSON, JTM; VITEK, [No Value; SCHAPINK, FW

    1991-01-01

    The interaction of 1/2 screw- and 60-degrees dislocations with symmetric [110] tilt boundaries was investigated by atomistic simulations using many-body potentials representing a pure f.c.c. metal and ordered intermetallic compounds. The calculations were performed with and without an applied shear

  8. Interaction between short fatigue cracks and grain boundaries. Systematic experiments with focussed ion beam microscope and microstructural tomography; Ueber die Wechselwirkung kurzer Ermuedungsrisse mit Korngrenzen. Systematische Experimente mit Focussed Ion Beam Microscope und mikrostruktureller Tomographie

    Energy Technology Data Exchange (ETDEWEB)

    Schaef, Wolfgang

    2011-04-15

    Increasing the microstructural resistance of metallic materials to short fatigue crack growth is a major task of today's materials science. In this regard, grain boundaries and precipitates are well known to decelerate short cracks, but a quantitative understanding of the blocking effect is still missing. This is due to the fact that crack deceleration is influenced by many parameters: cyclic load, crack length, distance to obstacles, orientations of grains and obstacles. Even the examination of a huge number of short cracks would not be sufficient to identify the effect of these parameters independently, especially since fatigue crack growth is a 3D problem and investigations of the sub surface orientation of cracks and grain boundaries are scarce. The Focused Ion Beam Microscope (FIB) offers new methods for systematic experiments and 3D-investigation of short fatigue cracks that will help to quantify the microstructural impact on short fatigue crack growth. The ion beam is used to cut micro notches in selected grains on the surface of samples characterised by Electron Backscatter Diffraction (EBSD). Plane fatigue cracks initiate under cyclic loading in defined distances to the grain boundaries. By this technique it is possible for the first time to present quantitative data to describe the effect of grain boundaries on short fatigue cracks in nickel based superalloys (CMSX-4) as well as in mild steels.

  9. Effects of oxygen stoichiometry on the scaling behaviors of YBa{sub 2}Cu{sub 3}O{sub x} grain boundary weak-links

    Energy Technology Data Exchange (ETDEWEB)

    Wu, K.H.; Fu, C.M.; Jeng, W.J. [National Chiao-Tung Univ., Taiwan (China)] [and others

    1994-12-31

    The effects of oxygen stoichiometry on the transport properties of the pulsed laser deposited YBa{sub 2}Cu{sub 3}O{sub x} bicrystalline grain boundary weak-link junctions were studied. It is found that not only the cross boundary resistive transition foot structure can be manipulated repeatedly with oxygen annealling processes but the junction behaviors are also altered in accordance. In the fully oxygenated state i.e. with x=7.0 in YBa{sub 2}Cu{sub 3}O{sub x} stoichiometry, the junction critical current exhibits a power of 2 scaling behavior with temperature. In contrast, when annealed in the conditions of oxygen-deficient state (e.g. with x=6.9 in YBa{sub 2}Cu{sub 3}O{sub x} stoichiometry) the junction critical current switches to a linear temperature dependence behavior. The results are tentatively attributed to the modification of the structure in the boundary area upon oxygen annealing, which, in turn, will affect the effective dimension of the geometrically constrained weak-link bridges. The detailed discussion on the responsible physical mechanisms as well as the implications of the present results on device applications will be given.

  10. Grain and grain boundary transport in BaCe0.5Zr0.3Ln0.2O3-δ (Ln - Y or lanthanide) electrolytes attractive for protonic ceramic fuel cells application

    Science.gov (United States)

    Danilov, N.; Pikalova, E.; Lyagaeva, J.; Antonov, B.; Medvedev, D.; Demin, A.; Tsiakaras, P.

    2017-10-01

    Protonic ceramic fuel cells (PCFCs) belong to very attractive energy conversion systems, which are able to operate effectively at low- and intermediate temperature ranges. The improvement of their electrochemical characteristics is feasible through the optimization of their functional materials. In the present work, emphasis is given to the highly conductive and stable cerate-zirconate electrolytes of BaCe0.5Zr0.3Ln0.2O3-δ (where Ln = Y, Dy, Sm and Nd). Studying the transport properties of these materials in individual form and in PCFC assembly, it is shown that the Dy-doped sample exhibits higher grain (bulk) and grain boundary conductivity in comparison with the most studied Y-doped ones. The single PCFC based on a rather thick 30 μm electrolyte displays about 160 and 290 mW cm-2 at 600 and 700 °C, respectively. These values are comparative with those obtained for PCFCs fabricated with similar electrolytes and Co-free cathode materials, confirming the perspective of cerate-zirconates doped by other lanthanides.

  11. Influence of S, P, C on grain boundary diffusion and creep properties of Alloy 800; Einfluss von S, P, C auf die Korngrenzendiffusion und Kriecheigenschaften von Alloy 800

    Energy Technology Data Exchange (ETDEWEB)

    Lindemann, J.; Hannesen, K.; Mast, R.; Viefhaus, H. [Max-Planck-Institut fuer Eisenforschung GmbH, Duesseldorf (Germany); Grabke, H.J.

    1998-12-31

    The paper reports examinations of Alloy 800 and specific commercially available variants known under the names of 800H, 800HT, and 800LC, differing in their concentrations of C, Al, and Ti. In addition, melts also containing phosphorus (0.09 wt-%) or sulfur (0.04 wt-%) as additional alloying materials have been prepared for the experiments. The volume diffusion and the grain boundary diffusion of {sup 59}Fe in those alloys was measured at temperatures between 800 and 1000 C by means of a radioactive tracer method combined with residual activity measurements. It was found that accompanying elements like phosphorus and sulfur increase the activation energy of the grain boundary diffusion of the iron and thus delay the grain boundary self-diffusion in Alloy 800. Creep curves were measured of the same materials after age-hardening treatment for 100 hours at 800 C, measurements performed at constant temperature but at three different, constant creep stress loads. The results showed that addition of phosphorus markedly increases the lifetime of Alloy 800, and reduces the creep rupture strain. The minimum strain rate in Alloy 800 containing 0.09 wt-% of phosphorus was found to be lower by a factor of 100, as compared to the other Alloy 800 materials used. (orig./CB) [Deutsch] Alloy 800 ist ein austenitischer Fe-Ni-Cr Stahl, der relativ geringe, aber wichtige Konzentrationen von Kohlenstoff, Aluminium und Titan enthaelt. Besondere Varianten von Alloy 800, bekannt als 800H, 800HT und 800LC, unterscheiden sich in den Konzentrationen dieser Elemente. Diese kommerziellen Legierungen wurden untersucht, und zusaetzlich wurden Schmelzen mit zulegiertem Phosphor (0,09 Gew-%) bzw. Schwefel (0,04 Gew-%) hergestellt. Mittels einer radioaktiven Tracermethode in Verbindung mit Restaktivitaetsmessungen wurde die Volumen- und Korngrenzendiffusion von {sup 59}Fe in diesen Legierungen im Temperaturbereich 800-1000 C gemessen. Es wurde gefunden, dass Begleitelemente wie Phosphor und

  12. Electric characterization of grain boundaries in ionic conductors by impedance spectroscopy measurements in a bicrystal; Caracterizacion electrica de fronteras de grano en conductores ionicos mediante medidas de espectroscopia de impedancias en un bicristal

    Energy Technology Data Exchange (ETDEWEB)

    Frechero, M. A.; Rocci Riner Schmidt, M.; Diaz-Guillen, M.; Doaz-Guillen, M. R.; Dura, O.; Rivera-Calzada, A.; Santamaria, J.; Leon, C.

    2012-07-01

    Here we show impedance spectroscopy measurements on a bicrystal of the ionically conducting Yttria stabilized zirconia (YSZ). By using micrometer sized electrodes it is possible to measure ionic transport perpendicular to a single grain boundary, and characterize its electrical properties. We are thus able to obtain the microscopic parameters that determine the charge distribution at the grain boundary and the ionic transport through it, as the potential energy barrier {delta}{phi} = 0.35{+-}0.01 V at 275 degree centigrade, and the space charge layer thickness {lambda} = 5{+-}1 A. These values are significantly different from those previously obtained in polycrystalline ceramic samples of the same material, and show much better agreement with the values predicted by the Mott-Schottky model for the charge distribution and ionic transport through the grain boundary. (Author) 31 refs.

  13. Effect of VN precipitates on formation of grain boundary and intragranular ferrite in a high N-V bearing steel; V-N tenkako no ryukai oyobi ryunai ferrite hentai ni oyobosu austenite chu no VN sekishutsu no eikyo

    Energy Technology Data Exchange (ETDEWEB)

    Omori, A.; Oi, K.; Kawabata, F.; Amano, K. [Kawasaki Steel Corp., Tokyo (Japan)

    1998-11-01

    The enhancement of ferrite nucleation owing to vanadium nitride (VN) precipitated in the austenite phase was studied for a 0.14%C- 1.45%Mn-0.06%V-0.009%N steel and the isothermal ferrite transformation behavior associated with VN precipitation was also quantitatively discussed. Vanadium nitrides precipitate on the austenite grain boundary in preference to the grain interior and increase the density of grain boundary ferrites. On the other hand, VN precipitates in austenite grain interior are less effective to intragranular ferrite nucleation. The calculation based on the classical nucleation theory shows that the activation energy of VN precipitates for a critical ferrite nucleus formation is one-fifth lower than that in case of no precipitate. The ferrite nucleation potency of VN precipitates is kept high even in higher temperature range above 700degreeC. (author)

  14. Effect of solute grain boundary segregation and hardness on the ductile-to-brittle transition for a Cr-Mo low-alloy steel

    Energy Technology Data Exchange (ETDEWEB)

    Shen, D.-D. [School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081 (China); Song, S.-H. [Division of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen University Town, Xili, Shenzhen 518055 (China)]. E-mail: shsonguk@yahoo.co.uk; Yuan, Z.-X. [School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081 (China); Weng, L.-Q. [Division of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen University Town, Xili, Shenzhen 518055 (China)

    2005-03-15

    Combined solute grain boundary segregation and hardness effect on the ductile-to-brittle transition is examined for a P-doped 2.25Cr-1Mo steel by means of Auger electron spectroscopy (AES) in conjunction with hardness measurements, Charpy impact tests and scanning electron microscopy (SEM). During ageing at 540 deg. C after water quenching from 980 deg. C, the segregation of phosphorus, molybdenum and chromium increases and the hardness decreases with increasing ageing time. The ductile-to-brittle transition temperature (DBTT) increases with increasing phosphorus segregation and decreases with decreasing hardness. The phosphorus segregation effect is dominant until 100 h ageing and after that the hardness effect becomes dominant, making the DBTT decrease with further increasing ageing time although the segregation of phosphorus still increases strongly. The segregation of molybdenum has some effect on the DBTT decrease.

  15. 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.

  16. 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.

  17. Study of grain boundary segregation using the Auger electron emission technique. Annual technical progress report VI, January 1, 1976--December 31, 1976. [Summary of research activities at Michigan Tech

    Energy Technology Data Exchange (ETDEWEB)

    Stein, D.F.; Heldt, L.A.; Lee, J.K.; Pinchback, T.R.

    1976-01-01

    Studies of admirality brass stress corrosion in copper sulfate and copper nitrate have provided information on environmental contributions to SCC in acid systems. SCC susceptibility is a function of bulk corrosion rate, and is maximized when conditions favor localized attack. At a given pH and stress, solution composition determines crack initiation rates, but the crack crevice environment is not characteristic of the bulk salt solution. Crack propagation appears to be strongly related to anodic dissolution; however, contributions from thin film rupture and hydrogen embrittlement must be considered. The brittle tarnish rupture mechanism is not operative during the stress corrosion of copper alloys in any of a wide variety of environments. Theoretical models have been developed that are providing a basic understanding of segregation to grain boundaries. The statistical thermodynamic approach using a distribution of energy sites at the grain boundary has extended the McLean model and the results are consistent with sulfur segregation in Ni/sub 3/Al and Ni/sub 3/(Al,Ti). A model based on the interatomic potentials of Cu-Cu, Cu-Bi, and Bi-Bi shows the segregation of Bi should occur at grain boundaries and that the segregation should be more extensive at asymmetrical grain boundaries. This is in agreement with earlier measurements made in this program. Grain boundary diffusion experiments continue. The Mo-S-Cr system is still the most desirable one for this purpose but difficulty has been encountered in controlling the sulfur additions and keeping the Cr plate on the surface. A new closed system has been designed that appears to have solved the problem. Experiments using the Cu-Bi-Ni system have been successful so it is clear that the general approach to studying grain boundary diffusion and the effect of impurity segregation on this diffusion will be successful.

  18. Multichannel Porous TiO2 Hollow Nanofibers with Rich Oxygen Vacancies and High Grain Boundary Density Enabling Superior Sodium Storage Performance.

    Science.gov (United States)

    Wu, Ying; Jiang, Yu; Shi, Jinan; Gu, Lin; Yu, Yan

    2017-06-01

    TiO2 as an anode for sodium-ion batteries (NIBs) has attracted much recent attention, but poor cyclability and rate performance remain problematic owing to the intrinsic electronic conductivity and the sluggish diffusivity of Na ions in the TiO2 matrix. Herein, a simple process is demonstrated to improve the sodium storage performance of TiO2 by fabricating a 1D, multichannel, porous binary-phase anatase-TiO2 -rutile-TiO2 composite with oxygen-deficient and high grain-boundary density (denoted as a-TiO2-x /r-TiO2-x ) via electrospinning and subsequent vacuum treatment. The introduction of oxygen vacancies in the TiO2 matrix enables enhanced intrinsic electronic conductivity and fast sodium-ion diffusion kinetics. The porous structure offers easy access of the liquid electrolyte and a short transport path of Na+ through the pores toward the TiO2 nanoparticle. Furthermore, the high density of grain boundaries between the anatase TiO2 and rutile TiO2 offer more interfaces for a novel interfacial storage. The a-TiO2-x /r-TiO2-x shows excellent long cycling stability (134 mAh g-1 at 10 C after 4500 cycles) and superior rate performance (93 mAh g-1 after 4500 cycles at 20 C) for sodium-ion batteries. This simple and effective process could serve as a model for the modification of other materials applied in energy storage systems and other fields. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  19. Fluid-controlled grain boundary migration and switch in slip systems in a high strain, high temperature contact aureole, California, USA

    Science.gov (United States)

    Morgan, Sven S.; Nabelek, Peter I.; Student, James; Sadorski, Joseph F.

    2016-04-01

    Within the highly strained aureole surrounding the Eureka Valley-Joshua Flat-Beer Creek (EJB) composite pluton of eastern California, an inversion in microstructures and crystallographic preferred orientations (CPOs) exists with distance from the contact. An inner aureole ( slip in quartz. Within the outer aureole (250 m to 1500 m from the contact), quartzites are interbedded with pelitic schist and are completely recrystallized and microstructures are indicative of extensive GBM. CPOs are indicative of prism [c] slip. Oxygen isotope ratios in the inner aureole are only slightly shifted from their original values. Oxygen isotopes from the outer aureole are shifted more, which is consistent with equilibration with locally derived fluids. We suggest that recrystallization in the outer aureole was aided by pore water, water derived from fluid inclusions, and water generated by prograde reactions in the schists. The pore fluids in the inner aureole were also probably initially water-rich. However, during prograde reactions in the intervening calc-silicate rocks, and perhaps more importantly, between calcite cement and quartz in the quartzites, the pore fluid composition in the inner aureole changed to become dominated by CO2, which acted as a non-wetting phase and decreased the fugacity of water slowing grain boundary mobility. Low water fugacity also suppressed the activity of prism [c] slip. Therefore, we propose that dry conditions or a grain boundary fluid with a significant non-wetting component (CO2) can result in apparent temperatures of deformation that are more than 100 °C lower than the real temperatures of deformation.

  20. Engineering Mixed Ionic Electronic Conduction in La 0.8 Sr 0.2 MnO 3+ δ Nanostructures through Fast Grain Boundary Oxygen Diffusivity

    KAUST Repository

    Saranya, Aruppukottai M.

    2015-04-09

    © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Nanoionics has become an increasingly promising field for the future development of advanced energy conversion and storage devices, such as batteries, fuel cells, and supercapacitors. Particularly, nanostructured materials offer unique properties or combinations of properties as electrodes and electrolytes in a range of energy devices. However, the enhancement of the mass transport properties at the nanoscale has often been found to be difficult to implement in nanostructures. Here, an artificial mixed ionic electronic conducting oxide is fabricated by grain boundary (GB) engineering thin films of La0.8Sr0.2MnO3+δ. This electronic conductor is converted into a good mixed ionic electronic conductor by synthesizing a nanostructure with high density of vertically aligned GBs with high concentration of strain-induced defects. Since this type of GBs present a remarkable enhancement of their oxide-ion mass transport properties (of up to six orders of magnitude at 773 K), it is possible to tailor the electrical nature of the whole material by nanoengineering, especially at low temperatures. The presented results lead to fundamental insights into oxygen diffusion along GBs and to the application of these engineered nanomaterials in new advanced solid state ionics devices such are micro-solid oxide fuel cells or resistive switching memories. An electronic conductor such as La0.8Sr0.2MnO3+δ is converted into a good mixed ionic electronic conductor by synthesizing a nanostructure with excellent electronic and oxygen mass transport properties. Oxygen diffusion highways are created by promoting a high concentration of strain-induced defects in the grain boundary region. This novel strategy opens the way for synthesizing new families of artificial mixed ionic-electronic conductors by design.

  1. Change of conduction mechanism in the impedance of grain boundaries in Pr{sub 0.4}Ca{sub 0.6}MnO{sub 3}

    Energy Technology Data Exchange (ETDEWEB)

    Shah, Matiullah [EMMG, Physics Division, PINSTECH, PO Nilore, Islamabad (Pakistan); Department of Chemical and Material Engineering, PIEAS, Islamabad 45650 (Pakistan); Nadeem, M., E-mail: mnadeem@pinstech.org.pk [EMMG, Physics Division, PINSTECH, PO Nilore, Islamabad (Pakistan); Idrees, M. [EMMG, Physics Division, PINSTECH, PO Nilore, Islamabad (Pakistan); Physics Department, COMSATS Institue of Information Technology, Lahore (Pakistan); Atif, M. [Department of Physics, Air University, PAF Complex E-9, Islamabad (Pakistan); Akhtar, M.J. [EMMG, Physics Division, PINSTECH, PO Nilore, Islamabad (Pakistan)

    2013-04-15

    Freshly synthesized polycrystalline Pr{sub 0.4}Ca{sub 0.6}MnO{sub 3} sample has been analyzed by employing X-ray diffraction, magnetization and impedance spectroscopic techniques. Magnetization M(T) showed charge ordering (T{sub CO}) around 268 K and below this temperature variation of magnetic and electric field effects is discussed. With the decrease of temperature, the change in the size and shape of the impedance plane plot is explored in terms of modulation of different relaxation processes. An equivalent circuit model (RQ) is used to determine the electrical parameters of Pr{sub 0.4}Ca{sub 0.6}MnO{sub 3} sample at different temperatures. Impedance of grain boundaries derived from the fitting of experimental data using an equivalent circuit model shows a change in conduction mechanism from small polaron hopping (SPH) to the variable range hopping (VRH) model around 148 K. Activation energies calculated from the SPH model and relaxation frequencies of imaginary part of impedance are compared and discussed. Average normalized change (ANC) deduced from the impedance data illustrates a change in conduction mechanism around 148 K due to the superexchange networks of Mn{sup 3+} and Mn{sup 4+}. - Highlights: ► Different field effects originated below charge ordering temperature. ► A transition around 148 K is established across the grain boundaries. ► Conduction mechanism changes from SPH to the VRH model. ► The Relaxation frequency shows dependent as well as independent regions.

  2. Quantum-mechanical study of tensorial elastic and high-temperature thermodynamic properties of grain boundary states in superalloy-phase Ni3Al

    Science.gov (United States)

    Friák, Martin; Všianská, Monika; Holec, David; Šob, Mojmír

    2017-07-01

    Grain boundaries (GBs), the most important defects in solids and their properties are crucial for many materials properties including (in-)stability. Quantum-mechanical methods can reliably compute properties of GBs and we use them to analyze (tensorial) anisotropic elastic properties of interface states associated with GBs in one of the most important intermetallic compounds for industrial applications, Ni3Al. Selecting the Σ5(210) GBs as a case study because of its significant extra volume, we address the mechanical stability of the GB interface states by checking elasticity-based Born stability criteria. One critically important elastic constant, C 55, is found nearly three times smaller at the GB compared with the bulk, contributing thus to the reduction of the mechanical stability of Ni3Al polycrystals. Next, comparing properties of Σ5(210) GB state which is fully relaxed with those of a Σ5(210) GB state when the supercell dimensions are kept equal to those in the bulk we conclude that lateral relaxations have only marginal impact on the studied properties. Having the complete elastic tensor of Σ5(210) GB states we combine Green’s-function based homogenization techniques and an approximative approach to the Debye model to compare thermodynamic properties of a perfect Ni3Al bulk and the Σ5(210) GB states. In particular, significant reduction of the melting temperature (to 79-81% of the bulk value) is predicted for nanometer-size grains.

  3. First-principles study of twin grain boundaries in epitaxial BaSi{sub 2} on Si(111)

    Energy Technology Data Exchange (ETDEWEB)

    Baba, Masakazu; Suemasu, Takashi, E-mail: suemasu@bk.tsukuba.ac.jp [Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan); Kohyama, Masanori [Research Institute of Electrochemical Energy, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka 563-8577 (Japan)

    2016-08-28

    Epitaxial films of BaSi{sub 2} on Si(111) for solar cell applications possess three epitaxial variants and exhibit a minority carrier diffusion length (ca. 9.4 μm) much larger than the domain size (ca. 0.2 μm); thus, the domain boundaries (DBs) between the variants do not act as carrier recombination centers. In this work, transmission electron microscopy (TEM) was used to observe the atomic arrangements around the DBs in BaSi{sub 2} epitaxial films on Si(111), and the most stable atomic configuration was determined by first-principles calculations based on density functional theory to provide possible interface models. Bright-field TEM along the a-axis of BaSi{sub 2} revealed that each DB was a twin boundary between two different epitaxial variants, and that Ba{sup (II)} atoms form hexagons containing central Ba{sup (I)} atoms in both the bulk and DB regions. Four possible interface models containing Ba{sup (I)}-atom interface layers were constructed, each consistent with TEM observations and distinguished by the relationship between the Si tetrahedron arrays in the two domains adjacent across the interface. This study assessed the structural relaxation of initial interface models constructed from surface slabs terminated by Ba{sup (I)} atoms or from zigzag surface slabs terminated by Si tetrahedra and Ba{sup (II)} atoms. In these models, the interactions or relative positions between Si tetrahedra appear to dominate the relaxation behavior and DB energies. One of the four interface models whose relationship between first-neighboring Si tetrahedra across the interface was the same as that in the bulk was particularly stable, with a DB energy of 95 mJ/m{sup 2}. There were no significant differences in the partial densities of states and band gaps between the bulk and DB regions, and it was therefore concluded that such DBs do not affect the minority carrier properties of BaSi{sub 2}.

  4. Analysis of crystallographic slip and grain boundary sliding in a Ti–45Al–2Nb–2Mn (at%)–0.8 vol%TiB{sub 2} alloy by high temperature in situ mechanical testing

    Energy Technology Data Exchange (ETDEWEB)

    Muñoz-Moreno, R. [IMDEA Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid (Spain); Department of Materials Science and Engineering, Universidad Carlos III de Madrid, Avda. Universidad 30, 28911 Leganés (Spain); Ruiz-Navas, E.M. [Department of Materials Science and Engineering, Universidad Carlos III de Madrid, Avda. Universidad 30, 28911 Leganés (Spain); Boehlert, C.J. [IMDEA Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid (Spain); Department of Chemical Engineering and Materials Science, Michigan State University, 2527 Engineering Building, East Lansing, MI 48824 (United States); Llorca, J. [IMDEA Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid (Spain); Department of Materials Science, Polytechnic University of Madrid, 28040 Madrid (Spain); Torralba, J.M. [IMDEA Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid (Spain); Department of Materials Science and Engineering, Universidad Carlos III de Madrid, Avda. Universidad 30, 28911 Leganés (Spain); Pérez-Prado, M.T., E-mail: teresa.perez.prado@imdea.org [IMDEA Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid (Spain)

    2014-06-01

    This work aims to contribute to a further understanding of the fundamentals of crystallographic slip and grain boundary sliding in the γ-TiAl Ti–45Al–2Nb–2Mn (at%)–0.8 vol%TiB{sub 2} intermetallic alloy, by means of in situ high-temperature tensile testing combined with electron backscatter diffraction (EBSD). Several microstructures, containing different fractions and sizes of lamellar colonies and equiaxed γ-grains, were fabricated by either centrifugal casting or powder metallurgy, followed by heat treatment at 1300 °C and furnace cooling. in situ tensile and tensile-creep experiments were performed in a scanning electron microscope (SEM) at temperatures ranging from 580 °C to 700 °C. EBSD was carried out in selected regions before and after straining. Our results suggest that, during constant strain rate tests, true twin γ/γ interfaces are the weakest barriers to dislocations and, thus, that the relevant length scale might be influenced by the distance between non-true twin boundaries. Under creep conditions both grain/colony boundary sliding (G/CBS) and crystallographic slip are observed to contribute to deformation. The incidence of boundary sliding is particularly high in γ grains of duplex microstructures. The slip activity during creep deformation in different microstructures was evaluated by trace analysis. Special emphasis was placed in distinguishing the compliance of different slip events with the Schmid law with respect to the applied stress.

  5. High density of (pseudo) periodic twin-grain boundaries in molecular beam epitaxy-grown van der Waals heterostructure: MoTe{sub 2}/MoS{sub 2}

    Energy Technology Data Exchange (ETDEWEB)

    Diaz, Horacio Coy; Ma, Yujing; Chaghi, Redhouane; Batzill, Matthias [Department of Physics, University of South Florida, Tampa, Florida 33620 (United States)

    2016-05-09

    Growth of transition metal dichalcogenide heterostructures by molecular beam epitaxy (MBE) promises synthesis of artificial van der Waals materials with controllable layer compositions and separations. Here, we show that MBE growth of 2H-MoTe{sub 2} monolayers on MoS{sub 2} substrates results in a high density of mirror-twins within the films. The grain boundaries are tellurium deficient, suggesting that Te-deficiency during growth causes their formation. Scanning tunneling microscopy and spectroscopy reveal that the grain boundaries arrange in a pseudo periodic “wagon wheel” pattern with only ∼2.6 nm repetition length. Defect states from these domain boundaries fill the band gap and thus give the monolayer an almost metallic property. The band gap states pin the Fermi-level in MoTe{sub 2} and thus determine the band-alignment in the MoTe{sub 2}/MoS{sub 2} interface.

  6. Highly transparent conductive electrode with ultra-low HAZE by grain boundary modification of aqueous solution fabricated alumina-doped zinc oxide nanocrystals

    Directory of Open Access Journals (Sweden)

    Qiong Nian

    2015-06-01

    Full Text Available Commercial production of transparent conducting oxide (TCO polycrystalline films requires high electrical conductivity with minimal degradation in optical transparency. Aqueous solution deposited TCO films would reduce production costs of TCO films but suffer from low electrical mobility, which severely degrades both electrical conductivity and optical transparency in the visible spectrum. Here, we demonstrated that grain boundary modification by ultra-violet laser crystallization (UVLC of solution deposited aluminium-doped zinc oxide (AZO nanocrystals results in high Hall mobility, with a corresponding dramatic improvement in AZO electrical conductance. The AZO films after laser irradiation exhibit electrical mobility up to 18.1 cm2 V−1 s−1 with corresponding electrical resistivity and sheet resistances as low as 1 × 10−3 Ω cm and 75 Ω/sq, respectively. The high mobility also enabled a high transmittance (T of 88%-96% at 550 nm for the UVLC films. In addition, HAZE measurement shows AZO film scattering transmittance as low as 1.8%, which is superior over most other solution deposited transparent electrode alternatives such as silver nanowires. Thus, AZO films produced by the UVLC technique have a combined figure of merit for electrical conductivity, optical transparency, and optical HAZE higher than other solution based deposition techniques and comparable to vacuumed based deposition methods.

  7. Critical current density behaviors across a grain boundary inclined to current with different angles in YBa2Cu3O7-δ bicrystal junctions

    Science.gov (United States)

    Tao, Hua; Wei-Wei, Xu; Zheng-Ming, Ji; Da-Yuan, Guo; Qing-Yun, Wang; Xiang-Rong, Ma; Rui-Yu, Liang

    2016-06-01

    The critical current density behaviors across a bicrystal grain boundary (GB) inclined to the current direction with different angles in YBa2Cu3O7-δ bicrystal junctions in magnetic fields are investigated. There are two main reasons for the difference in critical current density in junctions at different GB inclined angles in the same magnetic field: (i) the GB plane area determines the current carrying cross section; (ii) the vortex motion dynamics at the GB affects the critical current value when the vortex starts to move along the GB by Lorentz force. Furthermore, the vortex motion in a bicrystal GB is studied by investigating transverse (Hall) and longitudinal current-voltage characteristics (I-V xx and I-V xy ). It is found that the I-V xx curve diverges from linearity at a high driving current, while the I-V xy curve keeps nearly linear, which indicates the vortices inside the GB break out of the GB by Lorentz force. Project supported by the National Natural Science Foundation of China (Grant Nos. 61501222, 61371036, and 61571219) and the School Scientific Research Fund of Nanjing Institute of Technology, China (Grant Nos. YKJ201418).

  8. Effect of the KF post-deposition treatment on grain boundary properties in Cu(In, Ga)Se2 thin films

    Science.gov (United States)

    Nicoara, N.; Lepetit, Th.; Arzel, L.; Harel, S.; Barreau, N.; Sadewasser, S.

    2017-01-01

    Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS). The passivation of grain boundaries (GBs) through (post-deposition) treatments is a crucial step for this success. For the case of CIGS, the introduction of a potassium fluoride post-deposition treatment (KF-PDT) has boosted their power conversion efficiency to the best performance of all polycrystalline solar cells. Direct and indirect effects of potassium at the interface and interface-near region in the CIGS layer are thought to be responsible for this improvement. Here, we show that also the electronic properties of the GBs are beneficially modified by the KF-PDT. We used Kelvin probe force microscopy to study the effect of the KF-PDT on the CIGS surface by spatially resolved imaging of the surface potential. We find a clear difference for the GB electronic properties: the KF-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of work function values at the GBs. This effect of the KF-PDT on the GB electronic properties is expected to contribute to the improved efficiency values observed for CIGS thin-film solar cells with KF-PDT.

  9. Commercial-scale recycling of NdFeB-type magnets with grain boundary modification yields products with 'designer properties' that exceed those of starting materials.

    Science.gov (United States)

    Zakotnik, M; Tudor, C O

    2015-10-01

    NdFeB-type magnets dominate the market for high performance magnetic materials, yet production of 'virgin' magnets via mining is environmentally, financially and energetically costly. Hence, interest is growing in 'magnet to magnet' recycling schemes that offer the potential for cheaper, more environmentally-friendly solutions to the world's growing appetite for rare-earth based magnetic materials. Unfortunately, previously described recycling processes only partially capitalise on this potential, because the methods described to date are limited to 'laboratory scale' or operate only under ideal conditions and result in products that fail to recapture the coercivity of the starting, scrap materials. Herein, we report a commercial scale process (120 kg batches) that completely recovers the properties of the starting scrap magnets. Indeed, 'grain boundary modification', via careful addition of a proprietary mix of blended elements, produces magnets with 'designer properties' that can exceed those of the starting materials and can be closely tailored to meet a wide variety of end-user applications, including high-coercivity (>2000 kA/m), sintered magnets suitable for motor applications. Copyright © 2015 Elsevier Ltd. All rights reserved.

  10. Onset of Grain Boundary Migration and Drastic Weakening of Quartzite during increasing grade of Metamorphism in the Contact Aureole surrounding the Eureka Valley-Joshua Flat-Beer Creek pluton, California, USA

    Science.gov (United States)

    Morgan, S. S.; Student, J. J.; Jakeway, J.

    2015-12-01

    The Eureka Valley-Joshua Flat-Beer Creek (EJB) pluton in eastern California is surrounded by a ~1.3 km wide intensely deformed concordant aureole of metasedimentary rocks. South of the pluton, the Harkless Quartzite can be mapped from where it is located outside the aureole, with its regional strike through the transition into the aureole and concordancy with the pluton. The transition into concordancy, which is fairly abrupt, occurs over a distance of less than 100 m. Across this transition the bedding rotates close to 90° to become subvertical. Here the metasedimentary formations in the aureole have undergone 65% shortening. A suite of Harkless Quartzite samples was collected starting at 2.3 km south of the pluton, across the transition into concordancy at 1.3 km, and to within 450m from the pluton contact. Microstructurally, the transition is defined by changes in the dominant recrystallization mechanisms. At 2.3 km from the pluton, subgrain rotation recrystallization (SGR) plus grain boundary migration (GBM) operate together and many sedimentary grains (rounded grain boundaries) exist. As the pluton is approached, SGR decreases, GBM increases, and rounded grain boundaries slowly disappear. The abrupt transition into concordancy is marked by the final disappearance of SGR and rounded grain boundaries and extensive GBM. The transition is not completely smooth, and other variables such as pinning and amount of fluid inclusions seem to have a strong local affect on the dominant recrystallization mechanism. We suspect that the onset of extensive GBM allows for the diffusion of water into the crystal lattice which results in the drastic weakening and rotation of metasedimentary formations into concordancy.

  11. Role of pH value during material synthesis and grain-grain boundary contribution on the observed semiconductor to metal like conductivity transition in Ni{sub 1.5}Fe{sub 1.5}O{sub 4} spinel ferrite

    Energy Technology Data Exchange (ETDEWEB)

    Bhowmik, R.N., E-mail: rnbhowmik.phy@pondiuni.edu.in; Aneesh Kumar, K.S.

    2016-07-01

    Ni{sub 1.5}Fe{sub 1.5}O{sub 4} ferrite samples were synthesized by maintaining different pH values (1–12) during chemical reaction at 80 {sup °}C. The as-prepared samples were annealed at 1000 {sup °}C to form a cubic spinel structure. The heat treated samples were used for the study of electrical conductivity and dielectric properties. In this work, we understand the mechanism of unusual metal-like state in ferrite samples, characterized by negative temperature coefficient of conductivity. We have discussed various aspects, e.g., hopping mechanism through superexchange paths (Fe{sup 3+}-O{sup 2−}-Fe{sup 3+} and Ni{sup 2+}-O{sup 2−}-Ni{sup 2+}), charge delocalized conduction mechanism affected by the magnetic spins order in t{sub 2g} and e{sub g} electronic energy levels of B sites cations, grain size variation, relaxation of charge carriers at grains and grain boundaries of the particles, for outlining the mechanism of thermal activated charge localization (semiconductor state) and delocalization (metal-like state) effect in our samples. We have carried out a detailed analysis of conductivity spectra (Jonscher's power law fit, scaling of conductivity), impedance spectra (Cole-Cole plot by incorporating constant phase element), modulus spectra (Bergman proposed KWW function), and dielectric loss and dielectric constant spectra to extract the conductivity and relaxation contributions from grains, grain boundaries and space charge polarization in the samples. - Highlights: • Dielectric properties of Ni{sub 1.5}Fe{sub 1.5}O{sub 4} ferrite are studied. • Mechanism of metal-like conductivity state in ferrite is understood. • Electrical contributions from grains and grain boundaries are extracted. • Temperature dependence of charge relaxation process is understood.

  12. Spinodal decomposition in fine grained materials

    Indian Academy of Sciences (India)

    Unknown

    A-rich grain boundary layer followed by a B-rich layer; the grain interior exhibits a spinodally decomposed microstructure, evolving slowly. Further, grain growth is suppressed completely during the decomposition process. Keywords. Spinodal decomposition; grain boundary effects; phase field models. 1. Introduction.

  13. Separating bulk from grain boundary Li ion conductivity in the sol–gel prepared solid electrolyte Li1.5Al0.5Ti1.5(PO4)3

    OpenAIRE

    Breuer, Stefan; Prutsch, Denise; Ma, Qianli; Epp, Viktor; Preishuber-Pflügl, Florian; Tietz, Frank; Wilkening, Martin

    2015-01-01

    Lithium aluminium titanium phosphate (LATP) belongs to one of the most promising solid electrolytes. Besides sufficiently high electrochemical stability, its use in lithium-based all-solid-state batteries crucially depends on the ionic transport properties. While many impedance studies can be found in literature that report on overall ion conductivities, a discrimination of bulk and grain boundary electrical responses via conductivity spectroscopy has rarely been reported so far. Here, we too...

  14. Investigation and optimization of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} grain boundaries and coated conductors; Untersuchung und Optimierung von YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}-Korngrenzen und Bandsupraleitern

    Energy Technology Data Exchange (ETDEWEB)

    Held, Rainer Robert Martin

    2010-01-29

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

  15. Development of Computational Tools for Modeling Thermal and Radiation Effects on Grain Boundary Segregation and Precipitation in Fe-Cr-Ni-based Alloys

    Energy Technology Data Exchange (ETDEWEB)

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

    2017-08-01

    This work aims at developing computational tools for modeling thermal and radiation effects on solute segregation at grain boundaries (GBs) and precipitation. This report described two major efforts. One is the development of computational tools on integrated modeling of thermal equilibrium segregation (TES) and radiation-induced segregation (RIS), from which synergistic effects of thermal and radiation, pre-existing GB segregation have been taken into consideration. This integrated modeling was used in describing the Cr and Ni segregation in the Fe-Cr-Ni alloys. The other effort is thermodynamic modeling on the Fe-Cr-Ni-Mo system which includes the major alloying elements in the investigated alloys in the Advanced Radiation Resistant Materials (ARRM) program. Through thermodynamic calculation, we provide baseline thermodynamic stability of the hardening phase Ni2(Cr,Mo) in selected Ni-based super alloys, and contribute knowledge on mechanistic understanding on the formation of Ni2(Cr,Mo) in the irradiated materials. The major outcomes from this work are listed in the following: 1) Under the simultaneous thermal and irradiation conditions, radiation-induced segregation played a dominant role in the GB segregation. The pre-existing GB segregation only affects the subsequent radiation-induced segregation in the short time. For the same element, the segregation tendency of Cr and Ni due to TES is opposite to it from RIS. The opposite tendency can lead to the formation of W-shape profile. These findings are consistent with literature observation of the transitory W-shape profile. 2) While TES only affects the distance of one or two atomic layers from GBs, the RIS can affect a broader distance from GB. Therefore, the W-shape due to pre-existing GB segregation is much narrower than that due to composition gradient formed during the transient state. Considering the measurement resolution of Auger or STEM analysis, the segregation tendency due to RIS should play a dominant

  16. Resonant behavior of the barrier of YBa{sub 2}Cu{sub 3}O{sub 7} grain boundary Josephson junctions fabricated on bicrystalline substrates with different geometries

    Energy Technology Data Exchange (ETDEWEB)

    Navacerrada, M.A., E-mail: mdelosangeles.navacerrada@upm.es [Grupo de Acustica Arquitectonica, Escuela Tecnica Superior de Arquitectura, Universidad Politecnica de Madrid, Avenida Juan de Herrera 4, 28040 Madrid (Spain); Lucia, M.L.; Sanchez-Quesada, F. [Departamento Fisica Aplicada III (Electricidad y Electronica), Facultad de Cc. Fisicas, Universidad Complutense, Avenida Complutense s/n, 28040 Madrid (Spain)

    2012-12-14

    We have analyzed a resonant behavior in the dielectric constant associated to the barrier of YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO) grain boundary Josephson junctions (GBJJs) fabricated on a wide variety of bicrystalline substrates: 12 Degree-Sign [0 0 1] tilt asymmetric, 24 Degree-Sign [0 0 1] tilt asymmetric, 24 Degree-Sign [0 0 1] tilt symmetric, 24 Degree-Sign [1 0 0] tilt asymmetric, 45 Degree-Sign [1 0 0] tilt asymmetric and 24 Degree-Sign [0 0 1] tilt symmetric +45 Degree-Sign [1 0 0] tilt asymmetric bicrystals. The resonance analysis allows us to estimate a more appropriate value of the relative dielectric constant, and so a more adequate value for the length L of the normal N region assuming a SNINS model for the barrier. In this work, the L dependence on the critical current density Jc has been investigated. This analysis makes possible a single representation for all the substrate geometries independently on around which axes the rotation is produced to generate the grain boundary. On the other hand, no clear evidences exist on the origin of the resonance. The resonance frequency is in the order of 10{sup 11} Hz, pointing to a phonon dynamic influence on the resonance mechanism. Besides, its position is affected by the oxygen content of the barrier: a shift at low frequencies is observed when the misorientation angle increases.

  17. Stochastic rocket dynamics under random nozzle side loads: Ornstein-Uhlenbeck boundary layer separation and its coarse grained connection to side loading and rocket response

    CERN Document Server

    Keanini, R G; Tkacik, Peter T; Weggel, David C; Knight, P Douglas

    2011-01-01

    A long-standing, though ill-understood problem in rocket dynamics, rocket response to random, altitude-dependent nozzle side-loads, is investigated. Side loads arise during low altitude flight due to random, asymmetric, shock-induced separation of in-nozzle boundary layers. In this paper, stochastic evolution of the in-nozzle boundary layer separation line, an essential feature underlying side load generation, is connected to random, altitude-dependent rotational and translational rocket response via a set of simple analytical models. Separation line motion, extant on a fast boundary layer time scale, is modeled as an Ornstein-Uhlenbeck process. Pitch and yaw responses, taking place on a long, rocket dynamics time scale, are shown to likewise evolve as OU processes. Stochastic, altitude-dependent rocket translational motion follows from linear, asymptotic versions of the full nonlinear equations of motion; the model is valid in the practical limit where random pitch, yaw, and roll rates all remain small. Comp...

  18. 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

  19. Stochastic rocket dynamics under random nozzle side loads: Ornstein-Uhlenbeck boundary layer separation and its coarse grained connection to side loading and rocket response

    Energy Technology Data Exchange (ETDEWEB)

    Keanini, R.G.; Srivastava, N.; Tkacik, P.T. [Department of Mechanical Engineering, University of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28078 (United States); Weggel, D.C. [Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28078 (United States); Knight, P.D. [Mitchell Aerospace and Engineering, Statesville, North Carolina 28677 (United States)

    2011-06-15

    A long-standing, though ill-understood problem in rocket dynamics, rocket response to random, altitude-dependent nozzle side-loads, is investigated. Side loads arise during low altitude flight due to random, asymmetric, shock-induced separation of in-nozzle boundary layers. In this paper, stochastic evolution of the in-nozzle boundary layer separation line, an essential feature underlying side load generation, is connected to random, altitude-dependent rotational and translational rocket response via a set of simple analytical models. Separation line motion, extant on a fast boundary layer time scale, is modeled as an Ornstein-Uhlenbeck process. Pitch and yaw responses, taking place on a long, rocket dynamics time scale, are shown to likewise evolve as OU processes. Stochastic, altitude-dependent rocket translational motion follows from linear, asymptotic versions of the full nonlinear equations of motion; the model is valid in the practical limit where random pitch, yaw, and roll rates all remain small. Computed altitude-dependent rotational and translational velocity and displacement statistics are compared against those obtained using recently reported high fidelity simulations [Srivastava, Tkacik, and Keanini, J. Appl. Phys. 108, 044911 (2010)]; in every case, reasonable agreement is observed. As an important prelude, evidence indicating the physical consistency of the model introduced in the above article is first presented: it is shown that the study's separation line model allows direct derivation of experimentally observed side load amplitude and direction densities. Finally, it is found that the analytical models proposed in this paper allow straightforward identification of practical approaches for: (i) reducing pitch/yaw response to side loads, and (ii) enhancing pitch/yaw damping once side loads cease. (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  20. Retention and release of hydrogen isotopes in tungsten plasma-facing components: the role of grain boundaries and the native oxide layer from a joint experiment-simulation integrated approach

    Science.gov (United States)

    Hodille, E. A.; Ghiorghiu, F.; Addab, Y.; Založnik, A.; Minissale, M.; Piazza, Z.; Martin, C.; Angot, T.; Gallais, L.; Barthe, M.-F.; Becquart, C. S.; Markelj, S.; Mougenot, J.; Grisolia, C.; Bisson, R.

    2017-07-01

    Fusion fuel retention (trapping) and release (desorption) from plasma-facing components are critical issues for ITER and for any future industrial demonstration reactors such as DEMO. Therefore, understanding the fundamental mechanisms behind the retention of hydrogen isotopes in first wall and divertor materials is necessary. We developed an approach that couples dedicated experimental studies with modelling at all relevant scales, from microscopic elementary steps to macroscopic observables, in order to build a reliable and predictive fusion reactor wall model. This integrated approach is applied to the ITER divertor material (tungsten), and advances in the development of the wall model are presented. An experimental dataset, including focused ion beam scanning electron microscopy, isothermal desorption, temperature programmed desorption, nuclear reaction analysis and Auger electron spectroscopy, is exploited to initialize a macroscopic rate equation wall model. This model includes all elementary steps of modelled experiments: implantation of fusion fuel, fuel diffusion in the bulk or towards the surface, fuel trapping on defects and release of trapped fuel during a thermal excursion of materials. We were able to show that a single-trap-type single-detrapping-energy model is not able to reproduce an extended parameter space study of a polycrystalline sample exhibiting a single desorption peak. It is therefore justified to use density functional theory to guide the initialization of a more complex model. This new model still contains a single type of trap, but includes the density functional theory findings that the detrapping energy varies as a function of the number of hydrogen isotopes bound to the trap. A better agreement of the model with experimental results is obtained when grain boundary defects are included, as is consistent with the polycrystalline nature of the studied sample. Refinement of this grain boundary model is discussed as well as the inclusion

  1. Evidence for composition variations and impurity segregation at grain boundaries in high current-density polycrystalline K- and Co-doped BaFe{sub 2}As{sub 2} superconductors

    Energy Technology Data Exchange (ETDEWEB)

    Kim, Yoon-Jun [Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (United States); Weiss, Jeremy D.; Hellstrom, Eric E.; Larbalestier, David C. [Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310 (United States); Seidman, David N., E-mail: d-seidman@northwestern.edu [Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (United States); Northwestern University Center for Atom Probe Tomography (NUCAPT), Evanston, Illinois 60208 (United States)

    2014-10-20

    Some polycrystalline forms of the K- and Co-doped BaFe{sub 2}As{sub 2} and SrFe{sub 2}As{sub 2} superconductors now have a critical current density (J{sub c}) within a factor of ∼5 of that required for real applications, even though it is known that some grain boundaries (GBs) block current, thus, raising the question of whether this blocking is intrinsic or extrinsically limited by artefacts amenable to improvement by better processing. Herein, we utilize atom-probe tomography (APT) to study the grain and GB composition in high J{sub c} K- and Co-doped BaFe{sub 2}As{sub 2} polycrystals. We find that all GBs studied show significant compositional variations on the scale of a few coherence lengths (ξ), as well as strong segregation of oxygen impurities, which we believe are largely introduced in the starting materials. Importantly, these findings demonstrate that APT enables quantitative analysis of the highest J{sub c} K-doped BaFe{sub 2}As{sub 2} samples, where analytical transmission electron microscopy (TEM) fails because of the great reactivity of thin TEM samples. The observations of major chemical perturbations at GBs make us cautiously optimistic that there is a large extrinsic component to the GB current blocking, which will be ameliorated by better processing, for which APT will likely be a crucial instrument.

  2. Concepts on Low Temperature Mechanical Grain Growth

    Energy Technology Data Exchange (ETDEWEB)

    Sharon, John Anthony [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States). Metallurgy and Materials Joining Dept.; Boyce, Brad Lee [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States). Metallurgy and Materials Joining Dept.

    2013-11-01

    In metals, as grain size is reduced below 100nm, conventional dislocation plasticity is suppressed resulting in improvements in strength, hardness, and wears resistance. Existing and emerging components use fine grained metals for these beneficial attributes. However, these benefits can be lost in service if the grains undergo growth during the component’s lifespan. While grain growth is traditionally viewed as a purely thermal process that requires elevated temperature exposure, recent evidence shows that some metals, especially those with nanocrystalline grain structure, can undergo grain growth even at room temperature or below due to mechanical loading. This report has been assembled to survey the key concepts regarding how mechanical loads can drive grain coarsening at room temperature and below. Topics outlined include the atomic level mechanisms that facilitate grain growth, grain boundary mobility, and the impact of boundary structure, loading scheme, and temperature.

  3. The electrical properties of indium oxide thin films. In-situ Hall effect measurements to investigate the influence of point defects and grain boundaries; Die elektrischen Eigenschaften von Indiumoxid-Duennschichten. In-situ Hall-Effekt-Messungen zur Aufklaerung des Einflusses von Punktdefekten und Korngrenzen

    Energy Technology Data Exchange (ETDEWEB)

    Frischbier, Mareike

    2015-08-15

    Despite the wide application of indium oxide as transparent conducting material, basic mechanisms of its high conductivity are not understood yet. However, indium is scarce and the development of alternative materials for indium oxide is necessary. This requires a detailed understanding of the conductivity mechanisms. The electrical conductivity of undoped and doped indium oxide is given by defects. Thus, the influence of point defects and grain boundaries in sputtered indium oxide thin films is investigated here. This dissertation contributes to a more profound understanding of the conductivity mechanisms of indium oxide: grain boundary scattering is identified as the most important limiting scattering mechanism and the influence of doping elements on this property is shown. Dominant point defects in oxide materials are commonly investigated in literature by conductivity relaxation measurements depending on oxygen partial pressure. Usually, carrier mobility is assumed to be constant when analysing the results. However, this assumption is incorrect and can lead to a misinterpretation of data. Therefore, relaxation measurements are conducted as part of this dissertation to directly measure charge carrier concentration and mobility for the first time. For this purpose, a new experimental setup has been developed which enables measuring the Hall effect in-situ depending on oxygen partial pressure, temperature and total pressure. Relaxation measurements are conducted as part of this dissertation to show that charge carrier mobility is not constant, but strongly depends on carrier concentration. Further measurements verify that the scattering of charge carriers at grain boundaries is one main reason. In addition, the influence of deposition parameters of the sputter deposition process and doping (dopant element and concentration) on carrier concentration and mobility at room temperature is studied. The experimental results show that the doping elements influence both

  4. Efficient Bulk Heterojunction CH3NH3PbI3-TiO2 Solar Cells with TiO2 Nanoparticles at Grain Boundaries of Perovskite by Multi-Cycle-Coating Strategy.

    Science.gov (United States)

    Shao, Jun; Yang, Songwang; Liu, Yan

    2017-05-17

    A novel bulk heterojunction (BHJ) perovskite solar cell (PSC), where the perovskite grains act as donor and the TiO2 nanoparticles act as acceptor, is reported. This efficient BHJ PSC was simply solution processed from a mixed precursor of CH3NH3PbI3 (MAPbI3) and TiO2 nanoparticles. With dissolution and recrystallization by multi-cycle-coating, a unique composite structure ranging from a MAPbI3-TiO2-dominated layer on the substrate side to a pure perovskite layer on the top side is formed, which is beneficial for the blocking of possible contact between TiO2 and the hole transport material at the interface. Scanning electron microscopy clearly shows that TiO2 nanoparticles accumulate along the grain boundaries (GBs) of perovskite. The TiO2 nanoparticles at the GBs quickly extract and reserve photogenerated electrons before they transport into the perovskite phase, as described in the multitrapping model, retarding the electron-hole recombination and reducing the energy loss, resulting in increased VOC and fill factor. Moreover, the pinning effect of the TiO2 nanoparticles at the GBs from the strong bindings between TiO2 and MAPbI3 suppresses massive ion migration along the GBs, leading to improved operational stability and diminished hysteresis. Photoluminescence (PL) quenching and PL decay confirm the efficient exciton dissociation on the heterointerface. Electrochemical impedance spectroscopy and open-circuit photovoltage decay measurements show the reduced recombination loss and improved carrier lifetime of the BHJ PSCs. This novel strategy of device design effectively combines the benefits of both planar and mesostructured architectures whilst avoiding their shortcomings, eventually leading to a high PCE of 17.42% under 1 Sun illumination. The newly proposed approach also provides a new way to fabricate a TiO2-containing perovskite active layer at a low temperature.

  5. Roles of grain boundary and oxygen vacancies in Ba{sub 0.6}Sr{sub 0.4}TiO{sub 3} films for resistive switching device application

    Energy Technology Data Exchange (ETDEWEB)

    Yan, Xiaobing, E-mail: xiaobing-yan@126.com, E-mail: mseyanx@nus.edu.sg [Key Laboratory of Digital Medical Engineering of Hebei Province, College of Electron and Information Engineering, Hebei University, Baoding 071002 (China); Department of Materials Science and Engineering, National University of Singapore, Singapore, Singapore 117576 (Singapore); Li, Yucheng; Zhao, Jianhui; Li, Yan [Key Laboratory of Digital Medical Engineering of Hebei Province, College of Electron and Information Engineering, Hebei University, Baoding 071002 (China); Bai, Gang [College of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023 (China); Zhu, Siqi [Key Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong Higher Education Institutes, Jinan University, Guangzhou 510632 (China)

    2016-01-18

    Oxygen vacancies are widely thought to be responsible for resistive switching (RS) effects based on polycrystalline oxides films. It is also well known that grain boundaries (GB) serve as reservoirs for accumulating oxygen vacancies. Here, Ar gas was introduced to enlarge the size of GB and increase the quantity of oxygen vacancies when the Ba{sub 0.6}Sr{sub 0.4}TiO{sub 3} (BST) films were deposited by pulse laser deposition technique. The experimental results indicate that the RS properties of the device exhibits better in the Ar-introduced BST films than in the O{sub 2}-grown BST films. High resolution transmission electron microscopy images show that an amorphous region GB with large size appears between two lattice planes corresponding to oxygen vacancies defects in the Ar-introduced BST. Fourier-transform infrared reflectivity spectroscopy results also reveal highly accumulated oxygen vacancies in the Ar-introduced BST films. And we propose that the conduction transport of the cell was dominantly contributed from not ions migration of oxygen vacancies but the electrons in our case according to the value of activation energies of two kinds of films.

  6. Magnetic fluctuations in nanosized goethite (alpha-FeOOH) grains

    DEFF Research Database (Denmark)

    Madsen, Daniel Esmarch; Gontard, Lionel Cervera; Kasama, Takeshi

    2009-01-01

    at the grain boundaries between nanometer-sized grains, leading to a weakened magnetic coupling between the grains. We show that the Mossbauer data of goethite can be explained by fluctuations of the sublattice magnetization directions in such weakly coupled grains. It is likely that the influence of defects...

  7. Effect of grain boundary misorientation on discontinuous ...

    Indian Academy of Sciences (India)

    Author Affiliations. H Azzeddine1 S Abdessameud1 B Alili1 Z Boumerzoug2 D Bradai1. Faculty of Physics, University of Sciences and Technology Houari Boumediene, BP 32 El-Alia, Bab-Ezzouar, Algiers, Algeria; Department of Metallurgy, University of Biskra, B.P. 145, 07000, Algeria ...

  8. Micromechanical modelling of nanocrystalline and ultrafine grained metals: A short overview

    OpenAIRE

    Mishnaevsky, Leon; Levashov, Evgeny

    2015-01-01

    An overview of micromechanical models of strength and deformation behaviour of nanostructured and ultrafine grained metallic materials is presented. Composite models of nanomaterials, polycrystal plasticity based models, grain boundary sliding, the effect of non-equilibrium grain boundaries and nanoscale properties are discussed and compared. The examples of incorporation of peculiar nanocrystalline effects (like large content of amorphous or semi-amorphous grain boundary phase, partial dislo...

  9. Cavitation at migrating boundaries during high temperature fatigue

    Energy Technology Data Exchange (ETDEWEB)

    Raman, V.

    1987-06-01

    There is growing interest in the role of migrating boundaries during high temperature deformation. One area of current interest is the manner in which grain boundary migration can influence deformation and fracture at elevated temperatures. Much of the detailed treatments of intergranular cracking and cavitation during creep deformation have centered on effects occurring at stationary grain boundaries. The conventional idea represented in numerous publications is that grain boundary sliding plays an important role in intergranular fracture at elevated temperatures. The large stress concentrations developed at irregularities on grain boundaries are frequently cited as the principal reason for the easy generation of cracks and cavities. This article concludes that high temperature fatigue can cause significant migration and sliding in Al-3% Mg and Pb-2% Sn solid solution alloys, and that microcavitation and cracking takes place at the migrating boundaries in specimens tested at large strain amplitudes and low test frequencies. Cavities may be isolated within grains due to boundary migration.

  10. Grain structure and hardness of titanium alloy VT20 after electron-beam welding

    Science.gov (United States)

    Murav'ev, V. I.; Kim, V. A.; Shpileva, A. A.

    2008-03-01

    Quantitative parameters of the microstructure (the density of grain boundaries and the fractal size of grain boundaries) that characterize the grain composition of the material are computed. The microhardness of a weld in determined. Analysis of the structural inhomogeneity of a welded joint of titanium alloy produced by electron-beam welding is performed.

  11. Grain dissection as a grain size reducing mechanism during ice microdynamics

    Science.gov (United States)

    Steinbach, Florian; Kuiper, Ernst N.; Eichler, Jan; Bons, Paul D.; Drury, Martin R.; Griera, Albert; Pennock, Gill M.; Weikusat, Ilka

    2017-04-01

    Ice sheets are valuable paleo-climate archives, but can lose their integrity by ice flow. An understanding of the microdynamic mechanisms controlling the flow of ice is essential when assessing climatic and environmental developments related to ice sheets and glaciers. For instance, the development of a consistent mechanistic grain size law would support larger scale ice flow models. Recent research made significant progress in numerically modelling deformation and recrystallisation mechanisms in the polycrystalline ice and ice-air aggregate (Llorens et al., 2016a,b; Steinbach et al., 2016). The numerical setup assumed grain size reduction is achieved by the progressive transformation of subgrain boundaries into new high angle grain boundaries splitting an existing grain. This mechanism is usually termed polygonisation. Analogue experiments suggested, that strain induced grain boundary migration can cause bulges to migrate through the whole of a grain separating one region of the grain from another (Jessell, 1986; Urai, 1987). This mechanism of grain dissection could provide an alternative grain size reducing mechanism, but has not yet been observed during ice microdynamics. In this contribution, we present results using an updated numerical approach allowing for grain dissection. The approach is based on coupling the full field theory crystal visco-plasticity code (VPFFT) of Lebensohn (2001) to the multi-process modelling platform Elle (Bons et al., 2008). VPFFT predicts the mechanical fields resulting from short strain increments, dynamic recrystallisation process are implemented in Elle. The novel approach includes improvements to allow for grain dissection, which was topologically impossible during earlier simulations. The simulations are supported by microstructural observations from NEEM (North Greenland Eemian Ice Drilling) ice core. Mappings of c-axis orientations using the automatic fabric analyser and full crystallographic orientations using electron

  12. 3D Characterization of Recrystallization Boundaries

    DEFF Research Database (Denmark)

    Zhang, Yubin; Godfrey, Andrew William; MacDonald, A. Nicole

    2016-01-01

    A three-dimensional (3D) volume containing a recrystallizing grain and a deformed matrix in a partially recrystallized pure aluminum was characterized using the 3D electron backscattering diffraction technique. The 3D shape of a recrystallizing boundary, separating the recrystallizing grain and d...

  13. Abnormal grain growth in AISI 304L stainless steel

    Energy Technology Data Exchange (ETDEWEB)

    Shirdel, M., E-mail: mshirdel1989@ut.ac.ir [School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran (Iran, Islamic Republic of); Mirzadeh, H., E-mail: hmirzadeh@ut.ac.ir [School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran (Iran, Islamic Republic of); Advanced Metalforming and Thermomechanical Processing Laboratory, School of Metallurgy and Materials Engineering, University of Tehran, Tehran (Iran, Islamic Republic of); Parsa, M.H., E-mail: mhparsa@ut.ac.ir [School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran (Iran, Islamic Republic of); Center of Excellence for High Performance Materials, School of Metallurgy and Materials Engineering, University of Tehran, Tehran (Iran, Islamic Republic of); Advanced Metalforming and Thermomechanical Processing Laboratory, School of Metallurgy and Materials Engineering, University of Tehran, Tehran (Iran, Islamic Republic of)

    2014-11-15

    The microstructural evolution during abnormal grain growth (secondary recrystallization) in 304L stainless steel was studied in a wide range of annealing temperatures and times. At relatively low temperatures, the grain growth mode was identified as normal. However, at homologous temperatures between 0.65 (850 °C) and 0.7 (900 °C), the observed transition in grain growth mode from normal to abnormal, which was also evident from the bimodality in grain size distribution histograms, was detected to be caused by the dissolution/coarsening of carbides. The microstructural features such as dispersed carbides were characterized by optical metallography, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and microhardness. Continued annealing to a long time led to the completion of secondary recrystallization and the subsequent reappearance of normal growth mode. Another instance of abnormal grain growth was observed at homologous temperatures higher than 0.8, which may be attributed to the grain boundary faceting/defaceting phenomenon. It was also found that when the size of abnormal grains reached a critical value, their size will not change too much and the grain growth behavior becomes practically stagnant. - Highlights: • Abnormal grain growth (secondary recrystallization) in AISI 304L stainless steel • Exaggerated grain growth due to dissolution/coarsening of carbides • The enrichment of carbide particles by titanium • Abnormal grain growth due to grain boundary faceting at very high temperatures • The stagnancy of abnormal grain growth by annealing beyond a critical time.

  14. Dynamic grain growth during superplastic deformation

    Energy Technology Data Exchange (ETDEWEB)

    Rabinovich, M.Kh. [Ufa State Aviation-Technical Univ. (Russian Federation); Trifonov, V.G. [Russian Academy of Sciences, Ufa (Russian Federation). Inst. for Metals Superplasticity Problems

    1996-05-01

    Superplastic deformation (SPD) causes the accelerated anisotropic grain growth. This process results in the formation of structure which is quasistable during superplastic deformation and unstable after deformation. The degree of instability is determined by the size of grains, their shape coefficient which depends on the nature of an alloy and is equal to 1.1--1.5 after SPD, and by the unbalance of triple junctions at boundaries. Alloying of metals can affect the thermodynamic force and mechanism of dynamic anisotropic grain growth and correspondingly influence the parameters of superplasticity in alloys.

  15. Prediction of dislocation boundary characteristics

    DEFF Research Database (Denmark)

    Winther, Grethe

    Plastic deformation of both fcc and bcc metals of medium to high stacking fault energy is known to result in dislocation patterning in the form of cells and extended planar dislocation boundaries. The latter align with specific crystallographic planes, which depend on the crystallographic....... Crystal plasticity calculations combined with the hypothesis that these boundaries separate domains with local differences in the slip system activity are introduced to address precise prediction of the experimentally observed boundaries. The presentation will focus on two cases from fcc metals...... orientation of the grain [1]. For selected boundaries it has been experimentally verified that the boundaries consist of fairly regular networks of dislocations, which come from the active slip systems [2]. The networks have been analyzed within the framework of Low-Energy-Dislocation-Structures (LEDS...

  16. The Relevance of Grain Dissection for Grain Size Reduction in Polar Ice: Insights from Numerical Models and Ice Core Microstructure Analysis

    Directory of Open Access Journals (Sweden)

    Florian Steinbach

    2017-09-01

    Full Text Available The flow of ice depends on the properties of the aggregate of individual ice crystals, such as grain size or lattice orientation distributions. Therefore, an understanding of the processes controlling ice micro-dynamics is needed to ultimately develop a physically based macroscopic ice flow law. We investigated the relevance of the process of grain dissection as a grain-size-modifying process in natural ice. For that purpose, we performed numerical multi-process microstructure modeling and analyzed microstructure and crystallographic orientation maps from natural deep ice-core samples from the North Greenland Eemian Ice Drilling (NEEM project. Full crystallographic orientations measured by electron backscatter diffraction (EBSD have been used together with c-axis orientations using an optical technique (Fabric Analyser. Grain dissection is a feature of strain-induced grain boundary migration. During grain dissection, grain boundaries bulge into a neighboring grain in an area of high dislocation energy and merge with the opposite grain boundary. This splits the high dislocation-energy grain into two parts, effectively decreasing the local grain size. Currently, grain size reduction in ice is thought to be achieved by either the progressive transformation from dislocation walls into new high-angle grain boundaries, called subgrain rotation or polygonisation, or bulging nucleation that is assisted by subgrain rotation. Both our time-resolved numerical modeling and NEEM ice core samples show that grain dissection is a common mechanism during ice deformation and can provide an efficient process to reduce grain sizes and counter-act dynamic grain-growth in addition to polygonisation or bulging nucleation. Thus, our results show that solely strain-induced boundary migration, in absence of subgrain rotation, can reduce grain sizes in polar ice, in particular if strain energy gradients are high. We describe the microstructural characteristics that can be

  17. The relevance of grain dissection for grain size reduction in polar ice: insights from numerical models and ice core microstructure analysis

    Science.gov (United States)

    Steinbach, Florian; Kuiper, Ernst-Jan N.; Eichler, Jan; Bons, Paul D.; Drury, Martyn R.; Griera, Albert; Pennock, Gill M.; Weikusat, Ilka

    2017-09-01

    The flow of ice depends on the properties of the aggregate of individual ice crystals, such as grain size or lattice orientation distributions. Therefore, an understanding of the processes controlling ice micro-dynamics is needed to ultimately develop a physically based macroscopic ice flow law. We investigated the relevance of the process of grain dissection as a grain-size-modifying process in natural ice. For that purpose, we performed numerical multi-process microstructure modelling and analysed microstructure and crystallographic orientation maps from natural deep ice-core samples from the North Greenland Eemian Ice Drilling (NEEM) project. Full crystallographic orientations measured by electron backscatter diffraction (EBSD) have been used together with c-axis orientations using an optical technique (Fabric Analyser). Grain dissection is a feature of strain-induced grain boundary migration. During grain dissection, grain boundaries bulge into a neighbouring grain in an area of high dislocation energy and merge with the opposite grain boundary. This splits the high dislocation-energy grain into two parts, effectively decreasing the local grain size. Currently, grain size reduction in ice is thought to be achieved by either the progressive transformation from dislocation walls into new high-angle grain boundaries, called subgrain rotation or polygonisation, or bulging nucleation that is assisted by subgrain rotation. Both our time-resolved numerical modelling and NEEM ice core samples show that grain dissection is a common mechanism during ice deformation and can provide an efficient process to reduce grain sizes and counter-act dynamic grain-growth in addition to polygonisation or bulging nucleation. Thus, our results show that solely strain-induced boundary migration, in absence of subgrain rotation, can reduce grain sizes in polar ice, in particular if strain energy gradients are high. We describe the microstructural characteristics that can be used to

  18. The Mechanism of Inhomogeneous Grain Refinement in a NiTiFe Shape Memory Alloy Subjected to Single-Pass Equal-Channel Angular Extrusion

    National Research Council Canada - National Science Library

    Yanqiu Zhang; Shuyong Jiang

    2017-01-01

    ...) subjected to single-pass equal-channel angular extrusion (ECAE) was investigated. The results show that refined grains are mainly nucleated near grain boundaries and a small fraction of them emerges in the grain interior...

  19. Negotiating boundaries

    DEFF Research Database (Denmark)

    Aarhus, Rikke; Ballegaard, Stinne Aaløkke

    2010-01-01

    to maintain the order of the home when managing disease and adopting new healthcare technology. In our analysis we relate this boundary work to two continuums of visibility-invisibility and integration-segmentation in disease management. We explore five factors that affect the boundary work: objects......To move treatment successfully from the hospital to that of technology assisted self-care at home, it is vital in the design of such technologies to understand the setting in which the health IT should be used. Based on qualitative studies we find that people engage in elaborate boundary work......, activities, places, character of disease, and collaboration. Furthermore, the processes are explored of how boundary objects move between social worlds pushing and shaping boundaries. From this we discuss design implications for future healthcare technologies for the home....

  20. Boundary Spanning

    DEFF Research Database (Denmark)

    Zølner, Mette

    The paper explores how locals span boundaries between corporate and local levels. The aim is to better comprehend potentialities and challenges when MNCs draws on locals’ culture specific knowledge. The study is based on an in-depth, interpretive case study of boundary spanning by local actors...... in the period of post-acquisition when their organization is being integrated into the acquiring MNC. The paper contributes to the literature on boundary spanning in three ways: First, by illustrating that boundary spanning is performed by numerous organizational actors in a variety of positions in MNCs......, inclusively by locals in subsidiaries. Second, by showing that boundary spanning is ‘situated’ in the sense that its result depends on the kind of knowledge to be transmitted and the attitude of the receivers. A third contribution is methodological. The study illustrates that combining bottom-up grounded...