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Sample records for low-angle grain boundaries

  1. Effect of dislocation structure evolution on low-angle grain boundary formation in 7050 aluminum alloy during aging

    NASA Astrophysics Data System (ADS)

    Gu, Wei; Li, Jing-yuan; Wang, Yi-de

    2015-07-01

    The effect of dislocation structure evolution on low-angle grain boundary formation in 7050 aluminum alloy during aging was studied by using optical microscopy, transmission electron microscopy, and electron backscatter diffraction analysis of misorientation angle distribution, cumulative misorientation and geometrically necessary dislocation (GND) density. Experimental results indicate that coarse spindle-shaped grains with the dimension of 200 µm × 80 µm separate into fine equiaxed grains of 20 µm in size as a result of newborn low-angle grain boundaries formed during the aging process. More specifically, the dislocation arrays, which are rearranged and formed due to scattered dislocations during earlier quenching, transform into low-angle grain boundaries with aging time. The relative frequency of 3°-5° low-angle grain boundaries increases to over 30%. The GND density, which describes low-angle grain boundaries with the misorientation angle under 3°, tends to decrease during initial aging. The inhomogeneous distribution of GNDs is affected by grain orientation. A decrease in GND density mainly occurs from 1.83 × 1013 to 4.40 × 1011 m-2 in grains with <111> fiber texture. This is consistent with a decrease of unit cumulative misorientation. Precipitation on grain boundaries and the formation of a precipitation free zone (PFZ) are facilitated due to the eroding activity of the Graff etchant. Consequently, low-angle grain boundaries could be readily viewed by optical microscopy due to an increase in their electric potential difference.

  2. Influence of coherent nanoinclusions on stress-driven migration of low-angle grain boundaries in nanocomposites

    NASA Astrophysics Data System (ADS)

    Konakov, Ya. V.; Ovid'ko, I. A.; Sheinerman, A. G.

    2016-09-01

    A theoretical model that effectively describes stress-driven migration of low-angle tilt grain boundaries in nanocomposites with nanocrystalline or ultrafine-grained metallic matrices containing ensembles of coherent nanoinclusions has been developed. Within this model, low-angle tilt boundaries have been considered as walls of edge dislocations that, under the influence of stress, slip in the metallic matrix and can penetrate into nanoinclusions. The dislocation dynamics simulation has revealed three main regimes of the stress-driven migration of low-angle grain boundaries. In the first regime, migrating grain boundaries are completely retarded by nanoinclusions and their migration is quickly terminated, while dislocations forming grain boundaries reach equilibrium positions. In the second regime, some segments of the migrating grain boundaries are pinned by nanoinclusions, whereas the other segments continue to migrate over long distances. In the third regime, all segments of grain boundaries (except for the segments located at the boundaries of inclusions) migrate over long distances. The characteristics of these regimes have been investigated, and the critical shear stresses for transitions between the regimes have been calculated.

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

  4. Interplay between intrinsic point defects and low-angle grain boundary in bcc tungsten: effects of local stress field

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

    We have used molecular statics in conjunction with an embedded atom method to explore the interplay between native point defects (vacancies and self-interstitials (SIAs)) and a low-angle grain boundary (GB) in bcc tungsten. The low-angle GB has biased absorption of SIAs over vacancies. We emphasize the significance of phenomena such as vacancy delocalization and SIA instant absorption around the GB dislocation cores in stabilizing the defect structures. Interstitial loading into the GB can dramatically enhance the interaction strength between the point defects and the GB due to SIA clustering (SIA cloud formation) or SIA vacancy recombination. We propose that the ‘maximum atom displacement’ can complement the ‘vacancy formation energy’ in evaluating unstable vacancy sites. Calculations of point defect migration barriers in the vicinity of GB dislocation cores show that vacancies and SIAs preferentially migrate along the pathways in the planes immediately above and below the core, respectively.

  5. Evaluation of vortex pinning across low angle grain boundary in YBa{sub 2}Cu{sub 3}O{sub 7} film

    SciTech Connect

    Horide, Tomoya; Matsumoto, Kaname

    2012-09-10

    Vortex pinning potential across a 5 Degree-Sign tilt low angle grain boundary (GB) was evaluated by measuring angular dependences of critical current density in a bicrystal YBa{sub 2}Cu{sub 3}O{sub 7} film under the variable Lorentz force configuration. Trapping angles of the GB depend on directions in the GB plane (70 Degree-Sign -80 Degree-Sign and 15 Degree-Sign -45 Degree-Sign for the c-axis direction and the ab plane one, respectively), due to anisotropic pinning of GB dislocations. Pinning potential across a GB is as large as that of heavy ion irradiated columnar defects, indicating that density and distribution of GBs should be controlled to improve vortex pinning in coated conductors.

  6. Grain boundaries

    SciTech Connect

    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.

  7. Grain boundary hardening and triple junction hardening in polycrystalline molybdenum

    SciTech Connect

    Kobayashi, Shigeaki . E-mail: skoba@ashitech.ac.jp; Tsurekawa, Sadahiro; Watanabe, Tadao

    2005-02-01

    The grain boundary and triple junction hardenings in molybdenum with different carbon content were studied in connection with the character and the connectivity of grain boundaries at triple junctions by the micro-indentation test. The triple junction hardening is smaller at the junctions composed of low-angle and {sigma} boundaries than at the junctions composed of random boundaries. This difference in the hardening depending on the grain boundary connectivity becomes more significant with a decrease in carbon content in molybdenum.

  8. Grain Boundary Specific Segregation in Nanocrystalline Fe(Cr)

    PubMed Central

    Zhou, Xuyang; Yu, Xiao-xiang; Kaub, Tyler; Martens, Richard L.; Thompson, Gregory B.

    2016-01-01

    A cross-correlative precession electron diffraction – atom probe tomography investigation of Cr segregation in a Fe(Cr) nanocrystalline alloy was undertaken. Solute segregation was found to be dependent on grain boundary type. The results of which were compared to a hybrid Molecular Dynamics and Monte Carlo simulation that predicted the segregation for special character, low angle, and high angle grain boundaries, as well as the angle of inclination of the grain boundary. It was found that the highest segregation concentration was for the high angle grain boundaries and is explained in terms of clustering driven by the onset of phase separation. For special character boundaries, the highest Gibbsain interfacial excess was predicted at the incoherent ∑3 followed by ∑9 and ∑11 boundaries with negligible segregation to the twin and ∑5 boundaries. In addition, the low angle grain boundaries predicted negligible segregation. All of these trends matched well with the experiment. This solute-boundary segregation dependency for the special character grain boundaries is explained in terms of excess volume and the energetic distribution of the solute in the boundary. PMID:27708360

  9. Grain Boundary Specific Segregation in Nanocrystalline Fe(Cr)

    NASA Astrophysics Data System (ADS)

    Zhou, Xuyang; Yu, Xiao-Xiang; Kaub, Tyler; Martens, Richard L.; Thompson, Gregory B.

    2016-10-01

    A cross-correlative precession electron diffraction – atom probe tomography investigation of Cr segregation in a Fe(Cr) nanocrystalline alloy was undertaken. Solute segregation was found to be dependent on grain boundary type. The results of which were compared to a hybrid Molecular Dynamics and Monte Carlo simulation that predicted the segregation for special character, low angle, and high angle grain boundaries, as well as the angle of inclination of the grain boundary. It was found that the highest segregation concentration was for the high angle grain boundaries and is explained in terms of clustering driven by the onset of phase separation. For special character boundaries, the highest Gibbsain interfacial excess was predicted at the incoherent ∑3 followed by ∑9 and ∑11 boundaries with negligible segregation to the twin and ∑5 boundaries. In addition, the low angle grain boundaries predicted negligible segregation. All of these trends matched well with the experiment. This solute-boundary segregation dependency for the special character grain boundaries is explained in terms of excess volume and the energetic distribution of the solute in the boundary.

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

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

  11. Simulation of electron transport across charged grain boundaries

    SciTech Connect

    Srikant, V.; Clarke, D.R.; Evans, P.V.

    1996-09-01

    The {ital I}{endash}{ital V} (current density-electric field) characteristics of low-angle grain boundaries consisting of periodic arrays of charged dislocations are computed using a quasiclassical molecular dynamics approach. Below a critical value of the grain boundary misorientation, the computed {ital I}{endash}{ital V} characteristics are linear whereas above they are nonlinear. The degree of nonlinearity and the voltage onset of nonlinearity are found to be dependent on the grain boundary misorientation. {copyright} {ital 1996 American Institute of Physics.}

  12. Geometry and crystallographic configuration of grain boundaries

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  13. Grain Boundary Character Distribution of TLM Titanium Alloy During Deformation

    NASA Astrophysics Data System (ADS)

    Bai, X. F.; Zhao, Y. Q.; Jia, Z. Q.; Zhang, Y. S.; Li, B.

    2016-06-01

    The grain boundary character distribution of TLM titanium alloy (with a nominal chemical composition of Ti-3Zr-2Sn-3Mo-25Nb) was studied under the deformation condition with different strain rates and compression reductions. The experimental results showed that the evolution and character distribution of grain boundaries structure during deformation were both related to grain boundary coupling, sliding, migration, and the grain rotating in nature. In TLM titanium alloy, the type of grain boundaries under different deformation condition included high-angle boundaries, low-angle boundaries, and the CSL boundaries of Σ3, Σ13, Σ29, and Σ39. Under the strain rate of 1 s-1, the numbers of grain boundaries with misorientation angle of 3°, 30°, and 60° all decreased obviously with the increasing compression reduction to 4.5%, comparing to those obtained under the strain rate of 0.001 s-1. Under the strain rate of 1 s-1, the numbers of Σ29 boundaries greatly increased with the compression reductions of 3 to 4.5% comparing to those obtained under the strain rate of 0.001 s-1, and the numbers of Σ3 boundaries increased firstly and then stabilized with the compression reduction increasing from 0 to 4%, while the numbers of Σ39 boundaries decreased with the compression reduction increasing to 4.5%.

  14. 3D grain boundary migration

    NASA Astrophysics Data System (ADS)

    Becker, J. K.; Bons, P. D.

    2009-04-01

    Microstructures of rocks play an important role in determining rheological properties and help to reveal the processes that lead to their formation. Some of these processes change the microstructure significantly and may thus have the opposite effect in obliterating any fabrics indicative of the previous history of the rocks. One of these processes is grain boundary migration (GBM). During static recrystallisation, GBM may produce a foam texture that completely overprints a pre-existing grain boundary network and GBM actively influences the rheology of a rock, via its influence on grain size and lattice defect concentration. We here present a new numerical simulation software that is capable of simulating a whole range of processes on the grain scale (it is not limited to grain boundary migration). The software is polyhedron-based, meaning that each grain (or phase) is represented by a polyhedron that has discrete boundaries. The boundary (the shell) of the polyhedron is defined by a set of facets which in turn is defined by a set of vertices. Each structural entity (polyhedron, facets and vertices) can have an unlimited number of parameters (depending on the process to be modeled) such as surface energy, concentration, etc. which can be used to calculate changes of the microstructre. We use the processes of grain boundary migration of a "regular" and a partially molten rock to demonstrate the software. Since this software is 3D, the formation of melt networks in a partially molten rock can also be studied. The interconnected melt network is of fundamental importance for melt segregation and migration in the crust and mantle and can help to understand the core-mantle differentiation of large terrestrial planets.

  15. Triple junction and grain boundary character distributions in metallic materials

    SciTech Connect

    Fortier, P.; Miller, W.A.; Aust, K.T.

    1997-08-01

    Triple junction and grain boundary orientations were obtained by electron backscattered diffraction in high purity aluminium and copper, and in copper-bismuth alloys, and were then characterized using the CSL, CAD and I-line (O-lattice) geometrical models. A computer simulation was also performed and compared to the experimental results. Relationships were established between triple junction and grain boundary character distributions using both experimental and computer simulated results. A general trend was observed which shows an increase in special triple junction character density with increasing special grain boundary character content. An increased frequency of low angle and twin boundaries is shown to lead to an increase in the I-line triple junction density.

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

    NASA Astrophysics Data System (ADS)

    Hallberg, Håkan

    2014-12-01

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

  17. The evolution of the grain boundary ensemble of polycrystalline nickel under creep at elevated temperatures

    NASA Astrophysics Data System (ADS)

    Naydenkin, E. V.; Ivanov, K. V.

    2013-03-01

    The evolution of the grain boundary ensemble of polycrystalline nickel under creep at 823 K (~0.5 Тm) has been investigated by the electron backscattered diffraction technique. At low strains, delamination of twin grain boundaries Σ3 is found to take place, giving rise to coincidence site lattice grain boundaries Σ9 and Σ27. Low-angle misorientations in the material are shown to increase considerably with increase in the degree of strain, which causes a decrease in the fraction of the coincidence site lattice grain boundaries due to interaction with lattice dislocations.

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

    PubMed

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

    2016-01-01

    The sensitization and subsequent intergranular corrosion of Al-5.3 wt.% Mg alloy has been shown to be an important factor in stress corrosion cracking of Al-Mg alloys. Understanding sensitization requires the review of grain boundary character on the precipitation process which can assist in developing and designing alloys with improved corrosion resistance. This study shows that the degree of precipitation in Al-Mg alloy is dependent on grain boundary misorientation angle, adjacent grain boundary planes and grain boundary types. The results show that the misorientation angle is the most important factor influencing precipitation in grain boundaries of the Al-Mg alloy. Low angle grain boundaries (≤15°) have better immunity to precipitation and grain boundary acid attack. High angle grain boundaries (>15°) are vulnerable to grain boundary acid attack. Grain boundaries with adjacent plane orientations near to {100} have potential for immunity to precipitation and grain boundary acid attack. This work shows that low Σ (Σ ≤ 29) coincident site lattice (CSL) grain boundaries have thinner β precipitates. Modified nitric acid mass loss test and polarization test demonstrated that the global corrosion resistance of sputtered Al-Mg alloy is enhanced. This may be attributed to the increased fractions of low Σ (Σ ≤ 29) CSL grain boundaries after sputtering. PMID:27230299

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

    PubMed Central

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

    2016-01-01

    The sensitization and subsequent intergranular corrosion of Al-5.3 wt.% Mg alloy has been shown to be an important factor in stress corrosion cracking of Al-Mg alloys. Understanding sensitization requires the review of grain boundary character on the precipitation process which can assist in developing and designing alloys with improved corrosion resistance. This study shows that the degree of precipitation in Al-Mg alloy is dependent on grain boundary misorientation angle, adjacent grain boundary planes and grain boundary types. The results show that the misorientation angle is the most important factor influencing precipitation in grain boundaries of the Al-Mg alloy. Low angle grain boundaries (≤15°) have better immunity to precipitation and grain boundary acid attack. High angle grain boundaries (>15°) are vulnerable to grain boundary acid attack. Grain boundaries with adjacent plane orientations near to {100} have potential for immunity to precipitation and grain boundary acid attack. This work shows that low Σ (Σ ≤ 29) coincident site lattice (CSL) grain boundaries have thinner β precipitates. Modified nitric acid mass loss test and polarization test demonstrated that the global corrosion resistance of sputtered Al-Mg alloy is enhanced. This may be attributed to the increased fractions of low Σ (Σ ≤ 29) CSL grain boundaries after sputtering. PMID:27230299

  20. Grain boundaries and surfaces in polycrystalline photovoltaics

    NASA Astrophysics Data System (ADS)

    Haney, Paul; Yoon, Heayoung; Zhitenev, Nikolai

    Despite the fact that polycrystalline photovoltaics materials such as CdTe and CIGS are an established commercial technology, the precise role of grain boundaries in their performance remains poorly understood. The high defect density at grain boundaries is generally detrimental to carrier lifetime, however the electric fields surrounding charged grain boundaries may separate electrons and holes, effectively passivating the grain boundary. One difficulty in ascertaining the properties of grain boundaries is that high spatial resolution experimental techniques needed to probe individual grain boundaries are generally surface sensitive. For this reason, extracting quantitative grain boundary and other material properties from this data requires a quantitatively accurate model of the exposed surface. Motivated by these considerations, we present a theoretical analysis of the response of a polycrystalline semiconductor to a localized excitation near a grain boundary, and near the surface. We use our analytical results to interpret electron beam induced current (EBIC) data on polycrystalline CdTe solar cells.

  1. Grain-boundary structure effects on intergranular stress corrosion cracking of Alloy X-750

    SciTech Connect

    Pan, Y.; Adams, B.L.; Olson, T.; Panayotou, N.

    1996-12-01

    It is well known that the properties and behavior of grain boundaries are strongly affected by local chemistry and atomic structure. This paper focuses on the mesoscale description of grain boundary structure (i.e., the five crystallographic degrees of freedom) and correlations with intergranular stress corrosion cracking observed in Alloy X-750. Orientation imaging microscopy, coupled with serial polishing, is used to reveal mesoscale structure and the connectivity of the grain boundary network. The propensity for cracking is correlated with the coincident site lattice (CSL) classification of grain boundary geometry, coupled with information about the orientation of the grain boundary plane. The data is interpreted to show that low-CSL boundaries (defined by the more restrictive Palumbo-Aust criterion), low-angle boundaries, and general boundaries with plane normals well off the principal stress axis, have low vulnerability to cracking.

  2. Positron trapping at grain boundaries

    SciTech Connect

    Dupasquier, A. ); Romero, R.; Somoza, A. )

    1993-10-01

    The standard positron trapping model has often been applied, as a simple approximation, to the interpretation of positron lifetime spectra in situations of diffusion-controlled trapping. This paper shows that this approximation is not sufficiently accurate, and presents a model based on the correct solution of the diffusion equation, in the version appropriate for studying positron trapping at grain boundaries. The model is used for the analysis of new experimental data on positron lifetime spectra in a fine-grained Al-Ca-Zn alloy. Previous results on similar systems are also discussed and reinterpreted. The analysis yields effective diffusion coefficients not far from the values known for the base metals of the alloys.

  3. Special Grain Boundaries in Ultrafine-Grained Tungsten.

    PubMed

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

    2016-12-01

    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 <110> 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<110> were revealed and studied at the line of local mismatch of {110} atomic planes of adjacent grains. PMID:27416905

  4. Special Grain Boundaries in Ultrafine-Grained Tungsten

    NASA Astrophysics Data System (ADS)

    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 <110> 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<110> were revealed and studied at the line of local mismatch of {110} atomic planes of adjacent grains.

  5. Mechanism for diffusion induced grain boundary migration

    SciTech Connect

    Balluffi, R.W.; Cahn, J.W.

    1980-08-01

    Grain boundaries are found to migrate under certain conditions when solute atoms are diffused along them. This phenomenon, termed diffusion induced grain boundary migration (DIGM), has now been found in six systems. The observed phenomenon and empirical data are used to discard certain concepts for the driving force and the mechanism. A mechanism is proposed in which differences in the diffusion coefficients of the diffusing species along the grain boundary cause a self-sustaining climb of grain boundary dislocations and motion of their associated grain boundary steps.

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

    SciTech Connect

    Taylor, Seth T.

    2002-05-17

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

  7. Mechanical Behavior of Grain Boundary Engineered Copper

    SciTech Connect

    Carter, S B; Hodge, A M

    2006-08-08

    A grain boundary engineered copper sample previously characterized by Electron Backscatter Diffraction (EBSD) has been selected for nanoindentation tests. Given the fact that grain boundaries have thicknesses in the order of 1 micron or less, it is essential to use nanomechanics to test the properties of individual grain boundaries. The Hysitron nanoindenter was selected over the MTS nanoindenter due to its superior optical capabilities that aid the selection and identification of the areas to be tested. An area of 2mm by 2mm with an average grain size of 50 microns has been selected for the study. Given the EBSD mapping, grains and grain boundaries with similar orientations are tested and the hardness and modulus are compared. These results will give a relationship between the mechanical properties and the engineered grain boundaries. This will provide for the first time a correlation between grain boundary orientation and the mechanical behavior of the sample at the nanoscale.

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

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

  9. Stability of grain boundary texture during isothermal grain growth in UO2 considering anisotropic grain boundary properties

    NASA Astrophysics Data System (ADS)

    Hallberg, Håkan; Zhu, Yaochan

    2015-10-01

    In the present study, mesoscale simulations of grain growth in UO2 are performed using a 2D level set representation of the polycrystal grain boundary network, employed in a finite element setting. Anisotropic grain boundary properties are considered by evaluating how grain boundary energy and mobility varies with local grain boundary character. This is achieved by considering different formulations of the anisotropy of grain boundary properties, for example in terms of coincidence site lattice (CSL) correspondence. Such modeling approaches allow tracing of the stability of a number of characteristic low-Σ boundaries in the material during grain growth. The present simulations indicate that anisotropic grain boundary properties have negligible influence on the grain growth rate. However, considering the evolution of grain boundary character distribution and the grain size distribution, it is found that neglecting anisotropic boundary properties will strongly bias predictions obtained from numerical simulations.

  10. Grain boundary segregation and intergranular failure

    SciTech Connect

    White, C.L.

    1980-01-01

    Trace elements and impurities often segregate strongly to grain boundaries in metals and alloys. Concentrations of these elements at grain boundaries are often 10/sup 3/ to 10/sup 5/ times as great as their overall concentration in the alloy. Because of such segregation, certain trace elements can exert a disproportionate influence on material properties. One frequently observed consequence of trace element segregation to grain boundaries is the occurrence of grain boundary failure and low ductility. Less well known are incidences of improved ductility and inhibition of grain boundary fracture resulting from trace element segregation to grain boundaries in certain systems. An overview of trace element segregation and intergranular failure in a variety of alloy systems as well as preliminary results from studies on Al 3% Li will be presented.

  11. Grain boundary curvature and grain growth kinetics with particle pinning

    NASA Astrophysics Data System (ADS)

    Shahandeh, Sina; Militzer, Matthias

    2013-08-01

    Second-phase particles are used extensively in design of polycrystalline materials to control the grain size. According to Zener's theory, a distribution of particles creates a pinning pressure on a moving grain boundary. As a result, a limiting grain size is observed, but the effect of pinning on the detail of grain growth kinetics is less known. The influence of the particles on the microstructure occurs in multiple length scales, established by particle radius and the grain size. In this article, we use a meso-scale phase-field model that simulates grain growth in the presence of a uniform pinning pressure. The curvature of the grain boundary network is measured to determine the driving pressure of grain growth in 2D and 3D systems. It was observed that the grain growth continues, even under conditions where the average driving pressure is smaller than the pinning pressure. The limiting grain size is reached when the maximum of driving pressure distribution in the structure is equal to the pinning pressure. This results in a limiting grain size, larger than the one predicted by conventional models, and further analysis shows consistency with experimental observations. A physical model is proposed for the kinetics of grain growth using parameters based on the curvature analysis of the grain boundaries. This model can describe the simulated grain growth kinetics.

  12. Characterization of grain boundaries in silicon

    NASA Technical Reports Server (NTRS)

    Cheng, L. J.; Shyu, C. M.; Stika, K. M.; Daud, T.; Crotty, G. T.

    1983-01-01

    Zero-bias conductance and capacitance measurements at various temperatures were used to study trapped charges and potential barrier height at the boundaries. Deep-level transient spectroscopy (DLTS) was applied to measure the density of states at the boundary. A study of photoconductivity of grain boundaries in p-type silicon demonstrated the applicability of the technique in the measurement of minority carrier recombination velocity at the grain boundary. Enhanced diffusion of phosphorus at grain boundaries in three cast polycrystalline photovoltaic materials was studied. Enhancements for the three were the same, indicating that the properties of boundaries are similar, although grown by different techniques. Grain boundaries capable of enhancing the diffusion were found always to have strong recombination activities; the phenomena could be related to dangling bonds at the boundaries. Evidence that incoherent second-order twins of (111)/(115) type are diffusion-active is presented.

  13. Grain boundary wetness of partially molten dunite

    NASA Astrophysics Data System (ADS)

    Mu, S.; Faul, U.

    2013-12-01

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

  14. Detection of solute segregation at grain boundaries

    SciTech Connect

    Briceno-Valero, J.; Gronsky, R.

    1980-03-01

    Studies of grain boundary segregation in metallurgical systems are traditionally based upon the premise that grain boundaries are more likely sites for solute atoms than their surrounding grains. This idea is manifested in experimental studies which distinguish the solute concentration at boundaries from that of grain interiors using various spectroscopic techniques, including more recently, energy dispersive x-ray analysis in TEM/STEM instruments. A typical study consists of spot or line scans across a grain boundary plane in order to detect concentration gradients at the boundary region. It has also been pointed out that there are rather severe problems in quantitatively determining the absolute solute concentration within the grain boundary, and data correction schemes for this situation have been proposed. The present paper is concerned with an alternative study of grain boundary segregation where the distribution of solute atoms along the boundary plane (as opposed to that across the boundary plane) is sought. The interest here is to establish whether or not a relationship exists between the structural defect configuration of the boundary plane and site preference for solute segregation.

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

    PubMed Central

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

    2014-01-01

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

  16. Electronic properties of silicon grain boundaries

    SciTech Connect

    Pike, G.E.; Seager, C.H.

    1980-01-01

    Polycrystalline silicon is a clean and relatively simple prototype of electronic ceramics. The theory of the electrostatic barriers which form at silicon grain boundaries will be discussed. The use of experimental conductance and capacitance measurements to obtain the barrier height and energy density of grain boundary states will be illustrated.

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

    SciTech Connect

    Bojarski, Stephanie A.; Rohrer, Gregory S.

    2014-09-01

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

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

    NASA Astrophysics Data System (ADS)

    Lu, K.

    2016-05-01

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

  19. Molecular dynamics simulations of He bubble nucleation at grain boundaries

    SciTech Connect

    Yongfeng Zhang; Paul C Millett; Michael Tonks; Liangzhe Zhang; Bulent Biner

    2012-08-01

    The nucleation behavior of He bubbles in nano-grained body-centered-cubic (BCC) Mo is simulated using molecular dynamics (MD) simulations with a bicrystal model, focusing on the effect of grain boundary (GB) structure. Three types of GBs, the (100) twist S29, the ?110? symmetrical tilt (tilt angle of 10.1?), and the (112) twin boundaries, are studied as representatives of random GB, low angle GB with misfit dislocations, and special sigma boundaries. With the same amount of He, more He clusters form in nano-grained Mo with smaller average size compared to that in bulk. The effects of the GB structure originate from the excess volume in GBs. Trapping by excess volume results in reduction in mobility of He atoms, which enhances the nucleation with higher density of bubbles, and impedes the growth of He bubbles by absorption of mobile He atoms. Furthermore, the distribution of excess volume in GBs determines the distribution of He clusters. The effect of GBs becomes less pronounced with increasing vacancy concentration in the matrix.

  20. Turbulent boundary layers along straight and curved long thin circular cylinders at low angles-of-incidence

    NASA Astrophysics Data System (ADS)

    Jordan, Stephen A.

    2016-05-01

    Long thin circular cylinders commonly serve as towed sonar tracking devices, where the radius-of-curvature along the longitudinal axis is quite low [ρr = O(10-4)]. Because no understanding presently exists about the direct impact of longitudinal curvature on the turbulent statistics, the long cylinder is simply viewed as a chain of straight segments at various (increasing then decreasing) small inclinations to the freestream direction. Realistically, even our statistical evidence along straight thin cylinders at low incidence angles is inadequate to build solid evidence towards forming reliable empirical models. In the present study, we address these shortcomings by executing Large-Eddy Simulations (LESs) of straight and longitudinally curved thin cylinders at low to moderate turbulent radius-based Reynolds numbers (500 ≤ Rea ≤ 3500) and small angles-of-incidence (α = 0° → 9°). Coupled with the previous experimental measurements and numerical results, the new expanded database (311 ≤ Rea ≤ 56 500) delivered sufficient means to propose power-law expressions for the longitudinal evolution of the skin friction, normal drag, and turbulent boundary layer (TBL) length scales. Surprisingly, the LES computations of the curved cylinders at analogous geometric and kinematic conditions as the straight cylinder showed similar character in terms of the longitudinal skin friction. Beyond incidence 1°-3° (upper end corresponds to the highest simulated Rea), the skin friction was directly proportional to the yaw angle and monotonically shifted downward with higher Rea. Conversely, the flow structure, normal drag, TBL length scales, Reynolds stresses, and the separation state of the transverse shear layers towards regular vortex shedding for the curved cylinder were highly dissimilar than the straight one at equivalent incidence angles.

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

    NASA Astrophysics Data System (ADS)

    Lu, Yan-Li; Hu, Ting-Ting; Mu, Hong; Chen, Zheng; Zhang, Liu-Chao

    2014-07-01

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

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

    NASA Astrophysics Data System (ADS)

    Turi, Maria-Lynn

    1997-10-01

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

  3. Grain boundary wetness of partially molten dunite

    NASA Astrophysics Data System (ADS)

    Mu, Shangshang; Faul, Ulrich H.

    2016-05-01

    Samples of Fo90 olivine and basaltic melt were annealed at a range of temperatures and a pressure of 1 GPa in a piston cylinder apparatus from 1 to 336 h. Post-run samples have melt contents from 0.3 to 6.8 % and mean grain sizes from 4.3 to 84.5 μm. Grain boundary wetness, a measure of the intergranular melt distribution, was determined by analyzing scanning electron microscope images with sufficiently high resolution to detect thin layers wetting grain boundaries, as well as small triple junctions. The measurements show that grain boundary wetness increases with increasing melt content to values well above those predicted by the idealized isotropic equilibrium model for a finite dihedral angle. Additionally, the melt geometry changes with grain size, with grain boundary wetness increasing with increasing grain size at fixed melt content. Grain boundary wetness and dihedral angle of samples annealed at a range of temperatures, but constant melt content does not depend on temperature. These observations emphasize that the dihedral angle alone is not adequate to characterize the melt distribution in partially molten rocks, as the idealized isotropic model does not account for the influence of grain growth. Diffusion creep viscosities calculated from the measured wetness reflect the grain size and melt content dependence. Accordingly, experimentally measured viscosities at small grain sizes underestimate the effect of melt to weaken partially molten rocks for coarser grain sizes. The presence of melt in the mantle may therefore enhance diffusion creep relative to dislocation creep.

  4. Enhanced diffusion of phosphorus at grain boundaries

    NASA Technical Reports Server (NTRS)

    Cheng, L. J.; Shyu, C. M.; Stika, K. M.; Daud, T.; Crotty, G. T.

    1982-01-01

    Enhanced diffusion of phosphorus at grain boundaries in cast polycrystalline photovoltaic materials (Wacker, HEM, and Semix) was studied. It was found that the enhancements for the three materials were the same, indicating that the properties of boundaries are similar, even though they were grown by different techniques. In addition, it was observed that grain boundaries capable of enhancing the diffusion always have strong recombination activities. Both phenomena could be related to dangling bonds existing at the boundaries. The present study gives the first evidence that incoherent second order twins of 111/115-plane type are diffusion-active.

  5. Connectivity in Random Grain Boundary Networks

    SciTech Connect

    Kumar, M; Schuh, C A; King, W E

    2002-10-22

    Mechanical properties of FCC metals and alloys can be improved by exercising control over the population of grain boundary types in the microstructure. The existing studies also suggest that such properties tend to have percolative mechanisms that depend on the topology of the grain boundary network. With the emergence of SEM-based automated electron backscatter diffraction (EBSD), statistically significant datasets of interface crystallography can be analyzed in a routine manner, giving new insight into the topology and percolative properties of grain boundary networks. In this work, we review advanced analysis techniques for EBSD datasets to quantify microstructures in terms of grain boundary character and triple junction distributions, as well as detailed percolation-theory based cluster analysis.

  6. Grain boundary studies of high temperature superconducting materials using electron backscatter Kikuchi diffraction

    SciTech Connect

    Goyal, A.; Specht, E.D.; Wang, Z.L.; Kroeger, D.M.

    1996-12-31

    Grain Orientation and gain boundary misorientation distributions in high critical current density, high temperature superconductors were determined using electron backscatter Kikuchi diffraction. It is found that depending on the type of superconductor and the processing method used to fabricate it, there exist different scales of biaxial texture from no biaxial texture, local biaxial texture, to complete biaxial texture. Experimentally obtained grain boundary misorientation distributions (GBMDs) were found to be skewed significantly to low angles in comparison to what is expected on the basis of macroscopic texture alone, suggesting that minimization of energy may be a driving force during the processing of high critical current density materials. In addition, a higher than expected fraction of coincident-site lattice boundaries is observed. Examination of maps of grain boundary misorientations in spatially correlated gains, i.e. the grain boundary mesotexture, suggests the presence percolative paths of high critical current density. A combination of orientation measurements, theoretical modeling of GBMDs and modeling of percolative current flow through an assemblage of gain boundaries is performed to gain an insight into the important microstructural features dictating the transport properties of high temperature superconductors. It is found that maximization of low energy, in particular, low angle boundaries is essential for higher critical currents. The combination of experimental and analytical techniques employed are applicable to other materials where physical properties are dominated by interganular characteristics.

  7. Grain boundary engineering and superstrength of nanocrystals

    NASA Astrophysics Data System (ADS)

    Glezer, A. M.; Stolyarov, V. L.; Tomchuk, A. A.; Shurygina, N. A.

    2016-01-01

    A new paradigm of hardening of nanocrystals is proposed based on the competing influence of various mechanisms of plastic deformation, i.e., dislocation sliding and grain-boundary slip. It has been confirmed using the results of computer modeling and the experimental data that the use of grain boundary engineering on the basis of the proposed ideas makes it possible to enhance substantially the strength of titaniumbased materials up to ultimate (theoretical) values.

  8. Structure of grain boundaries in iron

    SciTech Connect

    Harrison, R.J.; Spaepen, F.; Voter, A.F.; Chen, Shao-Ping

    1988-01-01

    The present research has been carried out as part of the Steel Research Group's program to improve the properties of ultra-high- strength steels. It is an investigation of the mechanism of cohesion at grain boundaries as a step toward the design of alloy steel with improved resistance against boundary decohesion under stress corrosion conditions. This research involves a theoretical study of grain boundaries in iron alloys, examining the mechanism of cohesion as affected by structure including imperfections, by the average alloy composition and by segregation at the boundaries. This paper summarizes what we have done to date in obtaining approximate grain boundary structures. We have tested the approximate interatomic potential for iron which we have obtained by using it to compute the interplanar spacings at the surface and then comparing the computed results with experimental observations. Results are discussed. 51 refs., 9 figs., 3 tabs.

  9. AB INITIO STUDY OF GRAIN BOUNDARY PROPERTIES OF TUNGSTEN ALLOYS

    SciTech Connect

    Setyawan, Wahyu; Kurtz, Richard J.

    2012-04-17

    Density functional theory was employed to investigate the grain boundary (GB) property of W-TM alloys (TM: fifth and sixth row transition metals). GB strengthening was found for Hf, Ta, Nb, Ru, Re, Os and Ir for 27{l_brace}525{r_brace} and to a lesser degree for 11{l_brace}323{r_brace}. Lower valence solutes strengthen the GB at certain substitutional sites, while higher valence elements enforce it at other positions. For 3{l_brace}112{r_brace}, the alloys exhibit reduced cleavage energies. Hence, allowing with TMs increases the GB cohesion more effectively for large-angle GBs whose cleavage energy is, in general, inherently lower than the low-angle ones. Electron density analysis elucidates the mechanism of charge addition or depletion of the GB bonding region upon TM substitution at various positions leading to stronger or weaker intergranular cohesion, respectively.

  10. Quantitatively Analyzing Strength Contribution vs Grain Boundary Scale Relation in Pure Titanium Subjected to Severe Plastic Deformation

    NASA Astrophysics Data System (ADS)

    Luo, Peng; Hu, Qiaodan; Wu, Xiaolin

    2016-05-01

    Electron backscatter diffraction was used to reveal high- and low-angle grain boundaries (HAGBs, with misorientation ≥15 deg, and LAGBs, <15 deg) in pure titanium (ASTM grade 2) subjected to equal channel angular pressing. Comprehensive paradigms were developed to present relations of yield strength vs HAGB grain diameter, and LAGB contribution vs LAGB linear intercept. Incorporating grain orientations (against loading axis) into the Hall-Petch relation, we quantitatively investigated the strength contributions by HAGBs and LAGBs, respectively.

  11. Grain Boundary Effects in Solid Oxide Electrolytes

    NASA Astrophysics Data System (ADS)

    Ng, Mai

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

  12. Grain boundary character distribution in nanocrystalline metals produced by different processing routes

    DOE PAGESBeta

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

    2015-12-28

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

  13. Grain boundary character distribution in nanocrystalline metals produced by different processing routes

    SciTech Connect

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

    2015-12-28

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

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

    NASA Astrophysics Data System (ADS)

    Farkas, Diana

    1994-07-01

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

  16. Grain Boundary Character Distributions In Isostructural Materials

    NASA Astrophysics Data System (ADS)

    Ratanaphan, Sutatch

    Anisotropic grain boundary character distributions (GBCDs), which influence macroscopic materials properties, are thought to be controlled by the grain boundary energy anisotropy. Structurally, grain boundary could be viewed as two free surfaces joined together. Grain boundary energy could be simply defined by the total excess energy for creating two free surfaces minus the energy gained when new bonds are formed between these surfaces. This implies that different crystal structure should have different GBEDs and GBCDs. It was recently discovered that grain boundary energy distributions (GBED) in isostructural materials, a class of materials that share the same crystal structure, are directly related to one another. This suggests that GBCDs in isostructural materials might also be related in a similar way. To test this hypothesis, electron backscatter diffraction (EBSD) was used to map grain orientations in Ag, Au, Cu, Fe, and Mo. The GBCDs were determined from the stereological interpretation of EBSD maps containing on the order of 100,000 grains. It was found that the GBCDs of face-centered cubic (FCC) metals are statistically correlated, while the GBCDs of body-centered cubic (BCC) Fe and Mo are not correlated to the GBCD of FCC metals. The degree of the correlations among the FCC metals is weaker if there are significant differences in grain shape or texture. For example, Ag has the weakest correlation to the other FCC materials and also has quantitatively different grain shapes and texture. The relationship between the populations and energies of grain boundaries was also studied. By comparing the GBCDs of Al, Au, Cu, and Ni to the energies of 388 grain boundaries previously calculated by the Embedded Atom Method (EAM), we observed a moderately inverse correlation between the relative areas of grain boundaries and their energies. Interestingly, there are strong inverse correlations between the energies and populations of the most common grain boundaries (Sigma

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

    NASA Astrophysics Data System (ADS)

    Yan, M. F.; Wu, Y. Q.; Liu, R. L.

    2013-05-01

    The nanocrystallized 18Ni maraging steel was plasma nitrocarburized at 460 °C for 4 h in a mixture gas of N2, H2 and C2H5OH. 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‧-Fe4N and FeN0.049 phases. Most α-Fe and γ‧-Fe4N 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‧-Fe4N and α-Fe grains both with a dimension of nanometer are 0.395 μm2 and 0.397 μm2, respectively. The γ‧-Fe4N 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.

  18. Colloidal crystal grain boundary formation and motion.

    PubMed

    Edwards, Tara D; Yang, Yuguang; Beltran-Villegas, Daniel J; Bevan, Michael A

    2014-01-01

    The ability to assemble nano- and micro- sized colloidal components into highly ordered configurations is often cited as the basis for developing advanced materials. However, the dynamics of stochastic grain boundary formation and motion have not been quantified, which limits the ability to control and anneal polycrystallinity in colloidal based materials. Here we use optical microscopy, Brownian Dynamic simulations, and a new dynamic analysis to study grain boundary motion in quasi-2D colloidal bicrystals formed within inhomogeneous AC electric fields. We introduce "low-dimensional" models using reaction coordinates for condensation and global order that capture first passage times between critical configurations at each applied voltage. The resulting models reveal that equal sized domains at a maximum misorientation angle show relaxation dominated by friction limited grain boundary diffusion; and in contrast, asymmetrically sized domains with less misorientation display much faster grain boundary migration due to significant thermodynamic driving forces. By quantifying such dynamics vs. compression (voltage), kinetic bottlenecks associated with slow grain boundary relaxation are understood, which can be used to guide the temporal assembly of defect-free single domain colloidal crystals.

  19. Grain boundary engineering of highly deformable ceramics

    SciTech Connect

    Mecartney, M.L.

    2000-07-01

    Highly deformable ceramics can be created with the addition of intergranular silicate phases. These amorphous intergranular phases can assist in superplastic deformation by relieving stress concentrations and minimizing grain growth if the appropriate intergranular compositions are selected. Examples from 3Y-TZP and 8Y-CSZ ceramics are discussed. The grain boundary chemistry is analyzed by high resolution analytical TEM is found to have a strong influence on the cohesion of the grains both at high temperature and at room temperature. Intergranular phases with a high ionic character and containing large ions with a relatively weak bond strength appear to cause premature failure. In contrast, intergranular phases with a high degree of covalent character and similar or smaller ions than the ceramic and a high ionic bond strength are the best for grain boundary adhesion and prevention of both cavitation at high temperatures and intergranular fracture at room temperature.

  20. An Optical Study of Ice Grain Boundaries

    NASA Astrophysics Data System (ADS)

    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

  1. Five-parameter grain boundary analysis of a grain boundary-engineered austenitic stainless steel.

    PubMed

    Jones, R; Randle, V; Engelberg, D; Marrow, T J

    2009-03-01

    Two different grain boundary engineering processing routes for type 304 austenitic stainless steel have been compared. The processing routes involve the application of a small level of strain (5%) through either cold rolling or uni-axial tensile straining followed by high-temperature annealing. Electron backscatter diffraction and orientation mapping have been used to measure the proportions of Sigma3(n) boundary types (in coincidence site lattice notation) and degree of random boundary break-up, in order to gain a measure of the success of the two types of grain boundary engineering treatments. The distribution of grain boundary plane crystallography has also been measured and analyzed in detail using the five-parameter stereological method. There were significant differences between the grain boundary population profiles depending on the type of deformation applied. PMID:19250462

  2. Advantageous grain boundaries in iron pnictide superconductors

    PubMed Central

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

    2011-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Hugo, Richard Charles

    1998-12-01

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

  4. Misorientation-angle-dependent electrical transport across molybdenum disulfide grain boundaries

    PubMed Central

    Ly, Thuc Hue; Perello, David J.; Zhao, Jiong; Deng, Qingming; Kim, Hyun; Han, Gang Hee; Chae, Sang Hoon; Jeong, Hye Yun; Lee, Young Hee

    2016-01-01

    Grain boundaries in monolayer transition metal dichalcogenides have unique atomic defect structures and band dispersion relations that depend on the inter-domain misorientation angle. Here, we explore misorientation angle-dependent electrical transport at grain boundaries in monolayer MoS2 by correlating the atomic defect structures of measured devices analysed with transmission electron microscopy and first-principles calculations. Transmission electron microscopy indicates that grain boundaries are primarily composed of 5–7 dislocation cores with periodicity and additional complex defects formed at high angles, obeying the classical low-angle theory for angles <22°. The inter-domain mobility is minimized for angles <9° and increases nonlinearly by two orders of magnitude before saturating at ∼16 cm2 V−1 s−1 around misorientation angle≈20°. This trend is explained via grain-boundary electrostatic barriers estimated from density functional calculations and experimental tunnelling barrier heights, which are ≈0.5 eV at low angles and ≈0.15 eV at high angles (≥20°). PMID:26813605

  5. Electronic phenomena near semiconductor grain boundaries

    NASA Astrophysics Data System (ADS)

    Pike, G. E.

    Various electronic phenomena which are generally associated with grain boundaries in semiconductors are reviewed. At equilibrium majority carriers are trapped at the boundaries, and a corresponding space charge layer of ionized dopant forms on both sides of the boundary. This creates a potential barrier to free carriers. An applied dc voltage causes the barrier to lower and change shape in an asymmetric way. At high voltages hot majority carriers can produce impact ionized minority carriers which further reduce the barrier height. Small ac voltages cause anomalous apparent capacitances which are either positive or negative.

  6. Mg2SiO4 Forsterite Grain Boundary Structures and Self-diffusion from Classical Molecular Dynamics Simulations

    NASA Astrophysics Data System (ADS)

    Wagner, J.; Adjaoud, O.; Marquardt, K.; Jahn, S.

    2015-12-01

    It is well understood that grain boundaries influence many key physicochemical properties of crystalline materials and earth materials are no exception to this. Grain boundaries in the mineral olivine have reshaped our understanding of geophysical processes in the earth's mantle, e.g. in form of enhanced element transport through grain boundary diffusion. Investigations of the relation between transport rate, energy and geometry of individual grain boundaries is compulsory to understand transport in aggregates with a lattice preferred orientation (LPO) that favours the presence and or alignment of specific grain boundaries over random grain boundaries in an undeformed rock. In this contribution, we perform classical molecular dynamics simulations of a series of symmetric and one asymmetric tilt grain boundaries of Mg2SiO4 (forsterite), ranging from 9.58° to 90° in misorientation and varying surface termination (see 1). Our emphasis lies on unravelling structural characteristics of high and low angle grain boundaries and how these influence grain boundary energy and self-diffusion processes. To obtain diffusion rates for different grain boundary geometries, we equilibrate the respective grain boundary systems at ambient pressure and temperatures from 1900-2200K and trace their evolution for run durations of more than100 ps. Subsequently, we track the mean square displacement of the different atomic species within the grain boundary layer over time to estimate self-diffusion constants for each grain boundary geometry and temperature. First results suggest that diffusion rates decrease with increasing grain boundary energy. We will discuss these results in the light of recent experimental data and show strength and limitations of the method applied. 1. Adjaoud, O., Marquardt, K., Jahn, S., Phys Chem Miner 39, 749-760 (2012)

  7. On the interaction of solutes with grain boundaries

    SciTech Connect

    Dingreville, Remi Philippe Michel; Berbenni, Stephane

    2015-11-01

    Solute segregation to grain boundaries is considered by modeling solute atoms as misfitting inclusions within a disclination structural unit model describing the grain boundary structure and its intrinsic stress field. The solute distribution around grain boundaries is described through Fermi–Dirac statistics of site occupancy. The susceptibility of hydrogen segregation to symmetric tilt grain boundaries is discussed in terms of the misorientation angle, the defect type characteristics at the grain boundary, temperature, and the prescribed bulk hydrogen fraction of occupied sites. Through this formalism, it is found that hydrogen trapping on grain boundaries clearly correlates with the grain boundary structure (i.e. type of structural unit composing the grain boundary), and the associated grain boundary misorientation. Specifically, for symmetric tilt grain boundaries about the [001] axis, grain boundaries composed of both B and C structural units show a lower segregation susceptibility than other grain boundaries. A direct correlation between the segregation susceptibility and the intrinsic net defect density is provided through the Frank–Bilby formalism. Moreover, the present formulation could prove to be a simple and useful model to identify classes of grain boundaries relevant to grain boundary engineering.

  8. On the interaction of solutes with grain boundaries

    DOE PAGESBeta

    Dingreville, Remi Philippe Michel; Berbenni, Stephane

    2015-11-01

    Solute segregation to grain boundaries is considered by modeling solute atoms as misfitting inclusions within a disclination structural unit model describing the grain boundary structure and its intrinsic stress field. The solute distribution around grain boundaries is described through Fermi–Dirac statistics of site occupancy. The susceptibility of hydrogen segregation to symmetric tilt grain boundaries is discussed in terms of the misorientation angle, the defect type characteristics at the grain boundary, temperature, and the prescribed bulk hydrogen fraction of occupied sites. Through this formalism, it is found that hydrogen trapping on grain boundaries clearly correlates with the grain boundary structure (i.e.more » type of structural unit composing the grain boundary), and the associated grain boundary misorientation. Specifically, for symmetric tilt grain boundaries about the [001] axis, grain boundaries composed of both B and C structural units show a lower segregation susceptibility than other grain boundaries. A direct correlation between the segregation susceptibility and the intrinsic net defect density is provided through the Frank–Bilby formalism. Moreover, the present formulation could prove to be a simple and useful model to identify classes of grain boundaries relevant to grain boundary engineering.« less

  9. Grain Boundaries In Thin Film Organic Semiconductors

    NASA Astrophysics Data System (ADS)

    Bougher, Cortney; Huston, Shawn; Lees, Eitan; Ward, Jeremy; Obaid, Abdul; Loth, Marsha; Anthony, John; Jurchescu, Oana; Conrad, Brad

    2012-02-01

    We utilize conductive atomic force microscopy (C-AFM) and tunneling atomic force microscopy (TUNA) to characterize dynamics of electronic transport across fluorinated triethylsilylethynyl anthradithiophene (diF-TES ADT) grain boundaries. The crystallization of diF-TES ADT grown on SiO2 will be discussed and related to comparable molecules. The resulting voltage drop between individual crystals as a function of dopants will be discussed in terms of charge transport models and compared to current device work.

  10. Metal precipitation at grain boundaries in silicon: Dependence on grain boundary character and dislocation decoration

    SciTech Connect

    Buonassisi, T.; Istratov, A. A.; Pickett, M. D.; Marcus, M. A.; Ciszek, T. F.; Weber, E. R.

    2006-07-24

    Synchrotron-based analytical microprobe techniques, electron backscatter diffraction, and defect etching are combined to determine the dependence of metal silicide precipitate formation on grain boundary character and microstructure in multicrystalline silicon (mc-Si). Metal silicide precipitate decoration is observed to increase with decreasing atomic coincidence within the grain boundary plane (increasing {sigma} values). A few low-{sigma} boundaries contain anomalously high metal precipitate concentrations, concomitant with heavy dislocation decoration. These results provide direct experimental evidence that the degree of interaction between metals and structural defects in mc-Si can vary as a function of microstructure, with implications for mc-Si device performance and processing.

  11. The Energy of Olivine Grain Boundaries Deduced from Grain Boundary Frequency Analyses

    NASA Astrophysics Data System (ADS)

    Marquardt, K.; Rohrer, G. S.

    2015-12-01

    The properties of grain boundaries strongly differ from those of the crystal lattice, and there is growing evidence that the presence of grain boundaries influence detected geophysical signals such as electrical conductivity and seismic velocities especially in aggregates with a LPO that favours the alignment of specific grain boundaries. However, neither the anisotropic frequency or energy distribution of grain boundary networks are understood in olivine dominated aggregates, neither with nor without LPO. We used electron backscatter diffraction, EBSD to detect the orientations of over 1.4x104 grains corresponding to roughly 5000mm length of grain boundary separating them. Subsequently we used a stereological approach to determine the grain boundary character distribution, GBCD, defined as the relative areas of grain boundaries of different types, distinguished by their five degrees of freedom (Rohrer, 2007). The grain boundary planes showed a preference for low index planes, which is in agreement with recent findings on other materials (e.g. MgO, TiO2, SrTiO3, MgAl2O4). However, our inferred surface energies are controversial with respect to previously simulated surface energies (Watson et al., 1997; de Leeuw et al., 2000; Gurmani et al., 2011). We find that the principal crystallographic planes have the lowest energies and that at 60° misorientation specific grain boundaries with common [100] axis of misorientation are favored compared to 60° misorientations about random axis of rotation. This seems to support the results of (Faul and Fitz Gerald, 1999), even though our data imply that 90°/[001] (100)(010) should be even less favorable for the propagation of melt films. These differences and similarities will be discussed with respect to the different methods and their limitations. References: Faul U. H. and Fitz Gerald J. D. (1999) Phys. Chem. Miner. 26, 187-197. Gurmani S. F. et al. (2011). J. Geophys. Res. 116, B12209. De Leeuw N. H. et al. (2000) Phys

  12. Experimental studies of the structure of grain boundaries

    SciTech Connect

    Sass, S.L.

    1993-04-01

    Goals are to understand factors affecting structure of grain boundaries in intermetallic compounds, understand how solute segregation affects grain boundary structure and causes embrittlement in Fe-base alloys, and explore control of grain boundary properties. Fe and boron-doped Ni[sub 3]Al and NiAl were studied. 7 figs, 1 tab, 18 refs.

  13. In situ visualization of birth and annihilation of grain boundaries in an Au nanocrystal.

    PubMed

    Zheng, He; Wang, Jianbo; Huang, Jian Yu; Cao, Ajing; Mao, Scott X

    2012-11-30

    The formation and vanishing processes of a low angle grain boundary (GB) in nanosized Au during tension and release of stress, respectively, were obsvered by in situ high resolution transmission electron microscopy. The nucleation of perfect dislocations led to the formation of a 15° low angle GB inside an Au nanocrystal upon off-axial tensile loading (coupled uniaxial tensile and bending stress). Strikingly, the dislocations were completely annihilated accompanied with the disappearance of the GB after the removal of external stress, indicating that plastic bending is recoverable in the nanocrystal. The back force and surface stress played important roles in such a pseudoelastic behavior. This transient GB dynamics cannot be captured in ex situ experimental investigations. Such pseudoelastic bending deformation in nanosized crystals will have an important impact on the designing of nanomechanical devices with ultrahigh bending capability.

  14. In Situ Visualization of Birth and Annihilation of Grain Boundaries in an Au Nanocrystal

    NASA Astrophysics Data System (ADS)

    Zheng, He; Wang, Jianbo; Huang, Jian Yu; Cao, Ajing; Mao, Scott X.

    2012-11-01

    The formation and vanishing processes of a low angle grain boundary (GB) in nanosized Au during tension and release of stress, respectively, were obsvered by in situ high resolution transmission electron microscopy. The nucleation of perfect dislocations led to the formation of a 15° low angle GB inside an Au nanocrystal upon off-axial tensile loading (coupled uniaxial tensile and bending stress). Strikingly, the dislocations were completely annihilated accompanied with the disappearance of the GB after the removal of external stress, indicating that plastic bending is recoverable in the nanocrystal. The back force and surface stress played important roles in such a pseudoelastic behavior. This transient GB dynamics cannot be captured in ex situ experimental investigations. Such pseudoelastic bending deformation in nanosized crystals will have an important impact on the designing of nanomechanical devices with ultrahigh bending capability.

  15. Effect of anisotropic interfacial energy on grain boundary distributions during grain growth

    SciTech Connect

    Gruber, J. A.; George, D. C.; Kuprat, A. P.; Rohrer, Gregory S.; Rollett, A. D.

    2004-01-01

    Through simulations with the moving finite element program GRAIN3D, we have studied the effect of anisotropic grain boundary energy on the distribution of boundary types in a polycrystal during normal grain growth. An energy function similar to that hypothesized for magnesia was used, and the simulated grain boundary distributions were found to agree well with measured distributions. The simulated results suggest that initially random microstructures develop nearly steady state grain boundary distributions that have local maxima and minima corresponding to local minima and maxima, respectively, of the energy function. It is well known that the properties and area fractions of various grain boundary types in polycrystals have a dramatic effect on macroscopic materials properties. The goal of the present study is to examine the quantitative relation between grain boundary energies and the distribution of grain boundary types that result from grain growth. In keeping with the prior work, we parameterize the five-dimensional space of grain boundary types using three parameters to describe the lattice misorientation and two parameters to describe the orientation of the grain boundary plane. Of particular interest is the observation that at fixed misorientations, there is significant texture in the distribution of the grain boundary planes and planes with low surface energies appear more frequently. Here we use simulation to test the idea that the observed distributions arise because of the grain boundary energy anisotropy. In comparison to the experiments, the simulations are advantageous because they make it possible to monitor the time evolution of the distribution and to independently determine the influence of different grain boundary properties on the development of the distribution. A moving finite element program, GRAIN3D, has been developed with the capability to incorporate anisotropic grain boundary energy and mobility functions into grain growth simulations

  16. Quantifying grain boundary damage tolerance with atomistic simulations

    NASA Astrophysics Data System (ADS)

    Foley, Daniel; Tucker, Garritt J.

    2016-10-01

    Grain boundaries play a pivotal role in defect evolution and accommodation within materials. Irradiated metals have been observed to form defect denuded zones in the vicinity of grain boundaries. This is especially apparent in nanocrystalline metals, which have an increased grain boundary concentration, as compared to their polycrystalline counterparts. Importantly, the effect of individual grain boundaries on microstructural damage tolerance is related to the character or structural state of the grain boundary. In this work, the damage accommodation behavior of a variety of copper grain boundaries is studied using atomistic simulations. Damage accumulation behavior is found to reach a saturation point where both the free volume and energy of a grain boundary fluctuate within an elliptical manifold, which varies in size for different boundary characters. Analysis of the grain boundaries shows that extrinsic damage accommodation occurs due to localized atomic shuffling accompanied by free volume rearrangement within the boundary. Continuous damage accumulation leads to altered atomic structural states that oscillate around a mean non-equilibrium state, that is energetically metastable. Our results suggest that variation of grain boundary behavior, both from equilibrium and under saturation, is directly related to grain boundary equilibrium energy and some boundaries have a greater propensity to continually accommodate damage, as compared to others.

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

    NASA Astrophysics Data System (ADS)

    Li, Ruizhi; Chew, Huck Beng

    2016-08-01

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

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

    PubMed

    Li, Ruizhi; Chew, Huck Beng

    2016-08-19

    We introduce the notion of continuum-equivalent traction fields as local quantitative descriptors of the grain boundary interface. These traction-based descriptors are capable of predicting the critical stresses to trigger dislocation emissions from ductile ⟨110⟩ symmetrical-tilt nickel grain boundaries. We show that Shockley partials are emitted when the grain boundary tractions, in combination with external tensile loading, generate a resolved shear stress to cause dislocation slip. The relationship between the local grain boundary tractions and the grain boundary energy is established. PMID:27588865

  19. Computer simulation of grain boundaries in Ni/sub 3/Al: The effect of grain boundary composition

    SciTech Connect

    Chen, S.P.; Voter, A.F.; Srolovitz, D.J.

    1986-01-01

    The authors have performed atomistic simulations on three (001) symmetric tilt grain boundaries: 5(210), (310), and 13(320). Depending on which sub-lattice in each of the two grains is occupied by Al, the grain boundary may have different stoichiometries. All of the simulations show that the Al-rich grain boundaries have the highest grain boundary energies. Thus Al-rich grain boundaries are more likely to fail than those which have the bulk stoichiometry or are Ni-rich. This conclusion is consistent with the observed stoichiometry dependence of the beneficial boron effect. The similarity between the grain boundary energies (cohesive energies) of Ni/sub 3/Al and Ni and the much higher yield stress of Ni/sub 3/Al provides a justification for the ''inherent'' brittleness of Ni/sub 3/Al grain boundaries.

  20. GRAIN BOUNDARY STRENGTHENING PROPERTIES OF TUNGSTEN ALLOYS

    SciTech Connect

    Setyawan, Wahyu; Kurtz, Richard J.

    2012-10-10

    Density functional theory was employed to investigate grain boundary (GB) properties of W alloys. A range of substitutional solutes across the Periodic Table was investigated to understand the behavior of different electronic orbitals in changing the GB cleavage energy in the Σ27a[110]{525} GB. A number of transition metals were predicted to enhance the GB cohesion. This includes Ru, Re, Os, Ir, V, Cr, Mn, Fe, Co, Ti, Hf, Ta and Nb. While lanthanides, s and p elements were tended to cause GB embrittlement.

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

    SciTech Connect

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

    2015-03-13

    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.

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

    PubMed Central

    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

  3. The Hide-and-Seek of Grain Boundaries from Moiré Pattern Fringe of Two-Dimensional Graphene

    PubMed Central

    Kim, Jung Hwa; Kim, Kwanpyo; Lee, Zonghoon

    2015-01-01

    Grain boundaries (GBs) commonly exist in crystalline materials and affect various properties of materials. The facile identification of GBs is one of the significant requirements for systematical study of polycrystalline materials including recently emerging two-dimensional materials. Previous observations of GBs have been performed by various tools including high resolution transmission electron microscopy. However, a method to easily identify GBs, especially in the case of low-angle GBs, has not yet been well established. In this paper, we choose graphene bilayers with a GB as a model system and investigate the effects of interlayer rotations to the identification of GBs. We provide a critical condition between adjacent moiré fringe spacings, which determines the possibility of GB recognition. In addition, for monolayer graphene with a grain boundary, we demonstrate that low-angle GBs can be distinguished easily by inducing moiré patterns deliberately with an artificial reference overlay. PMID:26216628

  4. Mechanical Spectroscopy of Grain Boundaries: Insights into Grain and Phase Boundary Sliding (Invited)

    NASA Astrophysics Data System (ADS)

    Sundberg, M.

    2010-12-01

    Grain boundary sliding has been identified as an important contributor to plastic deformation of polycrystalline rocks. Grain boundary sliding commonly acts in kinetic series with some other, usually rate-limiting, step such as grain boundary diffusion or dislocation propagation. Consequently, the mechanical properties of grain and phase boundaries are not typically measurable during steady-state creep tests. In contrast, measurements of the intrinsic shear attenuation (QG-1 ) of a polycrystalline rock as a function of frequency and temperature hold the potential to provide direct measurements of the grain boundary viscosity. Reciprocating torsion tests can be complemented by small-strain transient creep tests that monitor the short-time transient mechanical response of a polycrystalline solid to an instantaneous increase in stress. To develop and test a viscoelastic model that can describe both time- and frequency-domain mechanical behavior and thus allow extrapolation of experimental results to natural conditions, we have conducted an experimental study of low-frequency (10-2.25grained (d~5μm) aggregates of olivine and orthopyroxene ranging in composition from 6-75 vol % opx. The attenuation spectra reveal “high-temperature background” behavior at low to moderate frequencies. At higher frequencies (f>10-0.5 Hz) the attenuation spectra reveal the onset of an apparent Debye peak in the attenuation spectra, likely due to elastically-accommodated grain boundary sliding. A modified Andrade viscoelastic model that incorporates both the high-temperature background and the Debye

  5. Atomistic simulations of dislocation pileup: Grain boundaries interaction

    DOE PAGESBeta

    Wang, Jian

    2015-05-27

    Here, using molecular dynamics (MD) simulations, we studied the dislocation pileup–grain boundary (GB) interactions. Two Σ11 asymmetrical tilt grain boundaries in Al are studied to explore the influence of orientation relationship and interface structure on dislocation activities at grain boundaries. To mimic the reality of a dislocation pileup in a coarse-grained polycrystalline, we optimized the dislocation population in MD simulations and developed a predict-correct method to create a dislocation pileup in MD simulations. MD simulations explored several kinetic processes of dislocations–GB reactions: grain boundary sliding, grain boundary migration, slip transmission, dislocation reflection, reconstruction of grain boundary, and the correlation ofmore » these kinetic processes with the available slip systems across the GB and atomic structures of the GB.« less

  6. Atomistic simulations of dislocation pileup: Grain boundaries interaction

    SciTech Connect

    Wang, Jian

    2015-05-27

    Here, using molecular dynamics (MD) simulations, we studied the dislocation pileup–grain boundary (GB) interactions. Two Σ11 asymmetrical tilt grain boundaries in Al are studied to explore the influence of orientation relationship and interface structure on dislocation activities at grain boundaries. To mimic the reality of a dislocation pileup in a coarse-grained polycrystalline, we optimized the dislocation population in MD simulations and developed a predict-correct method to create a dislocation pileup in MD simulations. MD simulations explored several kinetic processes of dislocations–GB reactions: grain boundary sliding, grain boundary migration, slip transmission, dislocation reflection, reconstruction of grain boundary, and the correlation of these kinetic processes with the available slip systems across the GB and atomic structures of the GB.

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

    NASA Astrophysics Data System (ADS)

    DeCost, Brian L.; Holm, Elizabeth A.

    2016-07-01

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

  8. Effect of grain boundary on the buckling of graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Neek-Amal, M.; Peeters, F. M.

    2012-03-01

    The buckling of graphene nano-ribbons containing a grain boundary is studied using atomistic simulations where free and supported boundary conditions are invoked. We consider the buckling transition of two kinds of grain boundaries with special symmetry. When graphene contains a large angle grain boundary with θ = 21.8°, the buckling strains are larger than those of perfect graphene when the ribbons with free (supported) boundary condition are subjected to compressive tension parallel (perpendicular) to the grain boundary. This is opposite for the results of θ = 32.2°. The shape of the deformations of the buckled graphene nanoribbons depends on the boundary conditions, the presence of the particular used grain boundaries, and the direction of applied in-plane compressive tension.

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

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

    PubMed

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

    2016-01-01

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

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

    PubMed Central

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

    2016-01-01

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

  12. Grain boundary and triple junction diffusion in nanocrystalline copper

    SciTech Connect

    Wegner, M. Leuthold, J.; Peterlechner, M.; Divinski, S. V.; Song, X.; Wilde, G.

    2014-09-07

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

  13. Correlation between grain boundary misorientation and M{sub 23}C{sub 6} precipitation behaviors in a wrought Ni-based superalloy

    SciTech Connect

    Tang, Bin; Jiang, Li; Hu, Rui; Li, Qi

    2013-04-15

    The correlation between the grain boundary misorientation and the precipitation behaviors of intergranular M{sub 23}C{sub 6} carbides in a wrought Ni–Cr–W superalloy was investigated by using the electron backscattered diffraction (EBSD) technique. It was observed that the grain boundaries with a misorientation angle less than 20°, as well as all coincidence site lattice (CSL) boundaries, are immune to precipitation of the M{sub 23}C{sub 6} carbides; in contrast, the random high-angle grain boundaries with a misorientation angle of 20°–40° provide preferential precipitation sites of the M{sub 23}C{sub 6} carbides at the random high-angle grain boundaries with a higher misorientation angle of 55°–60°/[2 2 3] turn to retard precipitation of M{sub 23}C{sub 6} carbides owing to their nature like the Σ3 grain boundaries and retard the precipitation of M{sub 23}C{sub 6} carbides. The low-angle and certain random grain boundary segments induced by twins were found to interrupt the precipitation of the M{sub 23}C{sub 6} carbides along the high-angle grain boundaries. - Highlights: ► The low angle grain boundaries and CSL boundaries are immune to precipitation. ► M23C6 precipitate preferentially at random grain boundaries within 20°–40°. ► Some certain random grain boundary segments interrupt M23C6 precipitation.

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

    PubMed

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

    2016-06-22

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

  15. Influence of point defects on grain boundary diffusion in oxides

    SciTech Connect

    Stubican, V.S.

    1991-03-15

    The influence of point defects on grain boundary diffusion of Co ions in NiO was studied using polycrystalline films and bicrystals. Grain boundary diffusion was studied at 750 C at oxygen partial pressure. Two diffusion regions were found. At low oxygen pressures, extrinsic diffusion was observed. Above oxygen pressure of 10{sup {minus}7}, influence of intrinsic point defects was detected. It was determined that grain boundary diffusion was > 3 orders of magnitude faster than volume diffusion. However, it seems that grain boundary diffusion is influenced by the point defects in a similar way as the volume diffusion. 4 figs.

  16. Research on electronic states at silicon grain boundaries

    NASA Astrophysics Data System (ADS)

    Werner, J. H.

    1983-02-01

    Electronic states at grain boundaries in artificially grown, boron-doped silicon bicrystals are investigated. The dependence of conductivity and capacitance on temperature, photon energy and density, and frequency of the applied alternating voltage are discussed and quantitatively analyzed. The measurements show that the current transport though the grain boundary is controlled by thermal emission from holes over the grain boundary potential barrier. A formal expression is given for the occupation of interface states in stationary disequilibrium. Measurements of the photoconductivity and photocapacity as a function of light wavelength and intensity are analyzed, and a new spectroscopic method for determining the state density at the grain boundary is developed.

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

    SciTech Connect

    Schoenfelder, B. |; Keblinski, P.; Wolf, D.; Phillpot, S.R.

    1998-07-01

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

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

    NASA Astrophysics Data System (ADS)

    Na, S. M.; Flatau, A. B.

    2013-12-01

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

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

    DOE PAGESBeta

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

    2015-03-13

    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,more » 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.« less

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

    SciTech Connect

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

    2015-06-14

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

  1. Irradiation Assisted Grain Boundary Segregation in Steels

    SciTech Connect

    Lu, Zheng; Faulkner, Roy G.

    2008-07-01

    The understanding of radiation-induced grain boundary segregation (RIS) has considerably improved over the past decade. New models have been introduced and much effort has been devoted to obtaining comprehensive information on segregation from the literature. Analytical techniques have also improved so that chemical analysis of layers 1 nm thick is almost routine. This invited paper will review the major methods used currently for RIS prediction: namely, Rate Theory, Inverse Kirkendall, and Solute Drag approaches. A summary is made of the available data on phosphorus RIS in reactor pressure vessel (RPV) steels. This will be discussed in the light of the predictions of the various models in an effort to show which models are the most reliable and easy to use for forecasting P segregation behaviour in steels. A consequence of RIS in RPV steels is a radiation induced shift in the ductile to brittle transition temperature (DBTT). It will be shown how it is possible to relate radiation-induced P segregation levels to DBTT shift. Examples of this exercise will be given for RPV steels and for ferritic steels being considered for first wall fusion applications. Cr RIS in high alloy stainless steels and associated irradiation-assisted stress corrosion cracking (IASCC) will be briefly discussed. (authors)

  2. Grain-boundary migration in KCl bicrystals

    NASA Technical Reports Server (NTRS)

    Gibbon, C. F.

    1968-01-01

    Boundary migration in melt-grown bicrystals of KCl containing pure twist boundaries was investigated. The experiments involve the use of bicrystal specimens in the shape of right-triangular prisms with the boundary parallel to one side.

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

    NASA Technical Reports Server (NTRS)

    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.

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

    PubMed

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

    2016-10-12

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

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

    SciTech Connect

    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.

  6. Electronic and plasmonic phenomena at graphene grain boundaries

    NASA Astrophysics Data System (ADS)

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

    2013-11-01

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

  7. Electronic and plasmonic phenomena at graphene grain boundaries.

    PubMed

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

    2013-11-01

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

  8. YSZ thin films with minimized grain boundary resistivity.

    PubMed

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

    2016-04-21

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

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

    SciTech Connect

    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.

  10. Kapitza Resistance of the Grain Boundaries in Ceria

    SciTech Connect

    David Bai; Jian Gan; Aleksandr Chernatynskiy

    2014-06-01

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

  11. Thermal transport across symmetric tilt grain boundaries in β-SiC: Effect of dopants and temperature

    NASA Astrophysics Data System (ADS)

    Goel, N.; Webb, E. B.; Rickman, J. M.; Oztekin, A.; Neti, S.

    2016-07-01

    The Kapitza resistance at a segregated, low-angle symmetric tilt grain boundary in β-SiC is investigated using non-equilibrium molecular dynamics simulation. In particular, we assess the role of compositional and thermal disorder on the boundary resistance for various doping scenarios. By examining the local vibrational density of states, we identify a subset of modes that are significant for thermal transport in this system. This analysis is complemented by calculations of the projected density of states and a corresponding eigenmode analysis of the dynamical matrix that highlight important phonon polarizations and propagation directions. We also examine the dependence of the Kapitza resistance on temperature and dopant/matrix interaction strength, the latter parameter affecting grain-boundary structure and, hence, phonon scattering.

  12. Magnetization due to localized states on graphene grain boundary.

    PubMed

    Dutta, Sudipta; Wakabayashi, Katsunori

    2015-01-01

    Magnetism in graphene has been found to originate from various defects, e.g., vacancy, edge formation, add-atoms etc. Here, we discuss about an alternate route of achieving magnetism in graphene via grain boundary. During chemical vapor deposition of graphene, several graphene nucleation centers grow independently and face themselves with unusual bonding environment, giving rise to the formation of grain boundaries. We investigate the origin of magnetism in such grain boundaries within first-principles calculations, by letting two nucleation centers interact with each other at their interface. We observe formation of unprecedented point defect, consisting of fused three-membered and larger carbon rings, which induces net magnetization to graphene quantum dots. In case of periodic lattices, the appearance of array of point defects leads to the formation of magnetic grain boundaries. The net magnetization on these defects arises due to the deviation from bipartite characteristics of pristine graphene. We observe magnetic grain boundary induced dispersion less flat bands near Fermi energy, showing higher localization of electrons. These flat bands can be accessed via small doping, leading to enhanced magnetism. Moreover, the grain boundaries can induce asymmetric spin conduction behavior along the cross boundary direction. These properties can be exploited for sensor and spin-filtering applications.

  13. Magnetization due to localized states on graphene grain boundary

    PubMed Central

    Dutta, Sudipta; Wakabayashi, Katsunori

    2015-01-01

    Magnetism in graphene has been found to originate from various defects, e.g., vacancy, edge formation, add-atoms etc. Here, we discuss about an alternate route of achieving magnetism in graphene via grain boundary. During chemical vapor deposition of graphene, several graphene nucleation centers grow independently and face themselves with unusual bonding environment, giving rise to the formation of grain boundaries. We investigate the origin of magnetism in such grain boundaries within first-principles calculations, by letting two nucleation centers interact with each other at their interface. We observe formation of unprecedented point defect, consisting of fused three-membered and larger carbon rings, which induces net magnetization to graphene quantum dots. In case of periodic lattices, the appearance of array of point defects leads to the formation of magnetic grain boundaries. The net magnetization on these defects arises due to the deviation from bipartite characteristics of pristine graphene. We observe magnetic grain boundary induced dispersion less flat bands near Fermi energy, showing higher localization of electrons. These flat bands can be accessed via small doping, leading to enhanced magnetism. Moreover, the grain boundaries can induce asymmetric spin conduction behavior along the cross boundary direction. These properties can be exploited for sensor and spin-filtering applications. PMID:26145161

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

    NASA Astrophysics Data System (ADS)

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

    2013-06-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  17. Grain boundary characterization in an X750 alloy

    SciTech Connect

    Kevin Fisher; Sebastien Teysseyre; Emmanuelle Marquis

    2012-11-01

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

  18. Towards effective analysis of large grain boundary data sets

    NASA Astrophysics Data System (ADS)

    Glowinski, K.; Morawiec, A.

    2015-04-01

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

  19. Influence of point defects on grain boundary motion.

    SciTech Connect

    Foiles, Stephen Martin

    2010-09-01

    This work addresses the influence of point defects, in particular vacancies, on the motion of grain boundaries. If there is a non-equilibrium concentration of point defects in the vicinity of an interface, such as due to displacement cascades in a radiation environment, motion of the interface to sweep up the defects will lower the energy and provide a driving force for interface motion. Molecular dynamics simulations are employed to examine the process for the case of excess vacancy concentrations in the vicinity of two grain boundaries. It is observed that the efficacy of the presence of the point defects in inducing boundary motion depends on the balance of the mobility of the defects with the mobility of the interfaces. In addition, the extent to which grain boundaries are ideal sinks for vacancies is evaluated by considering the energy of boundaries before and after vacancy absorption.

  20. Defect annihilation at grain boundaries in alpha-Fe

    PubMed Central

    Di Chen; Wang, Jing; Chen, Tianyi; Shao, Lin

    2013-01-01

    Understanding radiation responses of Fe-based metals is essential to develop radiation tolerant steels for longer and safer life cycles in harsh reactor environments. Nanograined metals have been explored as self-healing materials due to point-defect recombination at grain boundaries. The fundamental defect-boundary interactions, however, are not yet well understood. We discover that the interactions are always mediated by formation and annealing of chain-like defects, which consist of alternately positioned interstitials and vacancies. These chain-like defects are closely correlated to the patterns of defect formation energy minima on the grain boundary, which depend on specific boundary configurations. Through chain-like defects, a point defect effectively translates large distances, to annihilate with its opposite, thus grain boundaries act as highly efficient defect sinks that cannot saturate under extreme radiation conditions. PMID:23519086

  1. Defect annihilation at grain boundaries in alpha-Fe.

    PubMed

    Chen, Di; Wang, Jing; Chen, Tianyi; Shao, Lin

    2013-01-01

    Understanding radiation responses of Fe-based metals is essential to develop radiation tolerant steels for longer and safer life cycles in harsh reactor environments. Nanograined metals have been explored as self-healing materials due to point-defect recombination at grain boundaries. The fundamental defect-boundary interactions, however, are not yet well understood. We discover that the interactions are always mediated by formation and annealing of chain-like defects, which consist of alternately positioned interstitials and vacancies. These chain-like defects are closely correlated to the patterns of defect formation energy minima on the grain boundary, which depend on specific boundary configurations. Through chain-like defects, a point defect effectively translates large distances, to annihilate with its opposite, thus grain boundaries act as highly efficient defect sinks that cannot saturate under extreme radiation conditions. PMID:23519086

  2. Defect redistribution within a continuum grain boundary plasticity model

    NASA Astrophysics Data System (ADS)

    van Beers, P. R. M.; Kouznetsova, V. G.; Geers, M. G. D.

    2015-10-01

    The mechanical response of polycrystalline metals is significantly affected by the behaviour of grain boundaries, in particular when these interfaces constitute a relatively large fraction of the material volume. One of the current challenges in the modelling of grain boundaries at a continuum (polycrystalline) scale is the incorporation of the many different interaction mechanisms between dislocations and grain boundaries, as identified from fine-scale experiments and simulations. In this paper, the objective is to develop a model that accounts for the redistribution of the defects along the grain boundary in the context of gradient crystal plasticity. The proposed model incorporates the nonlocal relaxation of the grain boundary net defect density. A numerical study on a bicrystal specimen in simple shear is carried out, showing that the spreading of the defect content has a clear influence on the macroscopic response, as well as on the microscopic fields. This work provides a basis that enables a more thorough analysis of the plasticity of polycrystalline metals at the continuum level, where the plasticity at grain boundaries matters.

  3. Discovering the Role of Grain Boundary Complexions in Materials

    SciTech Connect

    Harmer, Martin P.

    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

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

    NASA Astrophysics Data System (ADS)

    Spiteri, David; Anaya, Julian; Kuball, Martin

    2016-02-01

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

  5. Grain boundary engineering for structure materials of nuclear reactors

    NASA Astrophysics Data System (ADS)

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

    2013-10-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-11-01

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

  7. Electronic structure studies of nanocrystalline diamond grain boundaries

    SciTech Connect

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

    1999-11-29

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

  8. Grain boundary energy in 5 degrees of freedom space

    SciTech Connect

    2012-09-21

    GB5DOF is a program written in MatLab for computing excess energy of an arbitrary grain boundary defined by its 5 geometrical degrees of freedom. The program is written in the form of a single self-contained function callable from within commercially available MatLab software package. The function takes a geometric description of the boundary and material identity as input parameters and returns the predicted boundary energy.

  9. Creep of quartz by dislocation and grain boundary processes

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  10. Structure and composition of pyroxene crystallites formed by grain boundary impurity partitioning and Fe-Mg interdiffusion along grain boundaries

    NASA Astrophysics Data System (ADS)

    Thomas, J. B.; Watson, E. B.; Frey, M. D.

    2010-12-01

    Many rocks are composed of mafic phases dispersed in a polycrystalline feldspathic or quartzose matrix. In response to changes in P or T, isolated grains must ‘communicate’ with one another by chemical exchange along grain boundaries. The above realization encouraged us to develop an experimental method to measure type-c grain boundary diffusion kinetics. Fe-Mg interdiffusion along dry quartz-quartz grain boundaries was evaluated by sandwiching a monomineralic quartzite slab between a Mg-source (MgF2) and an Fe-source comprised of a quartzite containing fayalite 'detector' particles. The only available pathway for Fe-Mg communication between the MgF2 source and the fayalite detector particles was along the grain boundaries in the pure quartzite. The uptake of Mg by the fayalite detector particles in the fayalite-bearing quartzite confirmed that Fe-Mg exchange occurred through the dry grain boundaries of the intervening quartzite. Transport of Mg across ˜6 mm thick slabs of pure quartzite suggests minimum Fe-Mg interdiffusivities of ˜10-10 m2s-1 at 1100oC. Using a similar configuration at 700oC, minimum diffusivities are ~10-11 m2s-1, suggesting a low activation energy for Fe-Mg grain boundary diffusion. During diffusion experiments Mn-rich pyroxene crystallites (up to 5 μ m) grew along the grain boundaries of the initially monomineralic quartzite. The new pyroxene crystallites did not exist prior to the diffusion experiments and must have formed as Fe-Mg exchange along the quartz grain boundaries supplied the necessary components to form the new pyroxene crystallites. The Fe and Mg contents of the newly formed pyroxene crystallites vary linearly throughout the intervening quartzite. A time series study shows that steady state interdiffusion is established in <2 days within the intervening quartzite slab. The newly formed pyroxene crystallites have high Mn contents; however, neither the starting materials nor the Ni capsule contain appreciable Mn. We suggest

  11. Pipe and grain boundary diffusion of He in UO2

    DOE PAGESBeta

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

    2016-08-18

    Molecular dynamics simulations have been conducted to study the effects of dislocations and grain boundaries on He diffusion inmore » $$\\text{U}{{\\text{O}}_{2}}$$ . Calculations were carried out for the {100}, {110} and {111} $$\\langle 1\\,1\\,0\\rangle $$ edge dislocations, the screw $$\\langle 1\\,1\\,0\\rangle $$ dislocation and Σ5, Σ13, Σ19 and Σ25 tilt grain boundaries. He diffusivity as a function of distance from the dislocation core and grain boundaries was investigated for the temperature range 2300–3000 K. An enhancement in diffusivity was predicted within 20 Å of the dislocations or grain boundaries. Further investigation showed that He diffusion in the edge dislocations follows anisotropic behaviour along the dislocation core, suggesting that pipe diffusion occurs. Here, an Arrhenius plot of He diffusivity against the inverse of temperature was also presented and the activation energy calculated for each structure, as a function of distance from the dislocation or grain boundary.« less

  12. Pipe and grain boundary diffusion of He in UO2

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

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

    PubMed

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

    2016-10-12

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

  14. Step Coalescence by Collective Motion at an Incommensurate Grain Boundary.

    PubMed

    Bowers, M L; Ophus, C; Gautam, A; Lançon, F; Dahmen, U

    2016-03-11

    Using extended time series scanning transmission electron microscopy, we investigate structural fluctuations at an incommensurate grain boundary in Au. Atomic-resolution imaging reveals the coalescence of two interfacial steps, or disconnections, of different height via coordinated motion of atoms along close-packed directions. Numerical simulations uncover a transition pathway that involves constriction and expansion of a characteristic stacking fault often associated with grain boundaries in face-centered cubic materials. It is found that local atomic fluctuations by enhanced point defect diffusion may play a critical role in initiating this transition. Our results offer new insights into the collective motion of atoms underlying the lateral advance of steps that control the migration of faceted grain boundaries. PMID:27015493

  15. Grain boundary segregation of boron in INCONEL 718

    NASA Astrophysics Data System (ADS)

    Chen, W.; Chaturvedi, M. C.; Richards, N. L.; McMahon, G.

    1998-07-01

    The segregation behavior of boron at grain boundaries in two INCONEL 718+ based alloys with different B concentrations was studied. The alloys, one containing 11 ppm of B and the other 43 ppm, were homogenized at 1200 °C for 2 hours followed by water quenching and air cooling. A strong segregation of boron at grain boundaries was observed using secondary ion mass spectrometry after the heat treatment in both the alloys. The segregation was found mainly to be of nonequilibrium type. The homogenized samples were also annealed at 1050 °C for various lengths of time. During annealing, boride particles were observed to first form at grain boundaries and then to dissolve on continued annealing at 1050 °C. The mechanisms of segregation and desegregation of B are discussed.

  16. Characterization of deformation near grain boundaries in polycrystalline metals

    NASA Astrophysics Data System (ADS)

    Seal, James Robert

    Understanding and describing plastic deformation in polycrystalline materials is fundamentally challenging due to the complex atomic rearrangements that must occur at grain boundaries. These atomic rearrangements can have long-range and substantial impacts on a material's bulk behavior and material properties. Thus, there is a significant need to develop new techniques to study, correlate, and describe deformation accommodation at grain boundaries. Understanding how grain boundaries accommodate plastic deformation at the microscale will provide new insight into the evolution of heterogeneous deformation, stress concentration, and damage nucleation. A series of comprehensive experiments have been conducted in order to develop a quantitative and crystallographically based understanding of the relationships between deformation behavior, material microstructure, and slip transfer mechanisms across grain boundaries in polycrystalline materials. Slip transfer events in polycrystalline metals were investigated using novel analysis techniques in scanning electron microscopy (SEM). The objective of these experiments was to correlate observations of slip transfer with a geometric parameter m', which can be used to identify and predict crystallographic arrangements that are better suited for slip transfer. An emphasis was placed on understanding how the parameter m' can be correlated with heterogeneities in local lattice orientations and local stresses near grain boundaries. A large population of slip transfer reactions across α/beta phase boundaries in Ti-5Al-2.5Sn were imaged by SEM and slip system activity was characterized using electron backscattered diffraction (EBSD) and slip trace analysis. Statistical correlations identified that slip transfer across the α/beta phase boundary was strongly influenced by slip plane alignment across the interface. Slip direction alignment was not strongly correlated to observations of slip transfer and the parameter m' was not useful

  17. Structure of grain boundaries in nanostructured ZnO

    NASA Astrophysics Data System (ADS)

    Wu, Zhihao; Zhou, Yingxue; Zhang, Xinyi; Wei, Shiqiang; Chen, Dongliang

    2004-05-01

    The grain boundary (GB) of nanocrystalline ZnO films is investigated using the x-ray absorption fine structure technique. With the advantage of the dominant GB volume fraction in our samples, the GB structure is found to be neither simply "gas-like" nor "similar to that of coarse-grained phase," but experiences a transition from the modestly ordered innermost coordination shell around centered atoms to partly disordered second coordination shell and then to completely disordered higher coordination shells.

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

    SciTech Connect

    Barmak, Katayun; Darbal, Amith; Ganesh, Kameswaran J.; Ferreira, Paulo J.; Rickman, Jeffrey M.; Sun, Tik; Yao, Bo; Warren, Andrew P.; Coffey, Kevin R.

    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.

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

    NASA Technical Reports Server (NTRS)

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

    1991-01-01

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

  20. Grain boundary studies of doped yttria-stabilized zirconia

    SciTech Connect

    Evans, N.D. |; Imamura, P.H.; Mecartney, M.L.; Bentley, J.

    1998-03-01

    Achieving superplasticity in fine-grained ceramics is a potential method to lower energy costs associated with ceramic manufacturing via net shape forming. Superplasticity is intrinsic in 3-mol%-yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP), and can be enhanced by addition of glass to form intergranular phases which are thought to both limit grain growth and promote grain boundary sliding. However, superplasticity has not been observed in 8-mol%-yttria-stabilized cubic zirconia (8Y-CSZ), ostensibly due to its larger grain size and high grain growth rates. As part of a larger study, high-spatial-resolution energy-dispersive X-ray spectrometry (EDS) has been performed to characterize intergranular compositions of 3Y-TZP and 8Y-CSZ doped with various glassy phases.

  1. Diffusive Fractionation of Lithium Isotopes in Olivine Grain Boundaries

    NASA Astrophysics Data System (ADS)

    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

  2. The Harrison Diffusion Kinetics Regimes in Solute Grain Boundary Diffusion

    SciTech Connect

    Belova, Irina; Fiedler, T; Kulkarni, Nagraj S; Murch, Prof. Graeme

    2012-01-01

    Knowledge of the limits of the principal Harrison kinetics regimes (Type-A, B and C) for grain boundary diffusion is very important for the correct analysis of the depth profiles in a tracer diffusion experiment. These regimes for self-diffusion have been extensively studied in the past by making use of the phenomenological Lattice Monte Carlo (LMC) method with the result that the limits are now well established. The relationship of those self-diffusion limits to the corresponding ones for solute diffusion in the presence of solute segregation to the grain boundaries remains unclear. In the present study, the influence of solute segregation on the limits is investigated with the LMC method for the well-known parallel grain boundary slab model by showing the equivalence of two diffusion models. It is shown which diffusion parameters are useful for identifying the limits of the Harrison kinetics regimes for solute grain boundary diffusion. It is also shown how the measured segregation factor from the diffusion experiment in the Harrison Type-B kinetics regime may differ from the global segregation factor.

  3. Ab initio modelling of UN grain boundary interfaces

    NASA Astrophysics Data System (ADS)

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

    2012-08-01

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

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

    NASA Astrophysics Data System (ADS)

    Käshammer, Peter; Sinno, Talid

    2015-09-01

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

  5. SIMULATION OF STRAIN INDUCED INTERFACE MIGRATION IN SYMMETRIC TILT GRAIN BOUNDARIES

    SciTech Connect

    Namilae, Sirish; Radhakrishnan, Balasubramaniam; Gorti, Sarma B

    2007-01-01

    Grain boundary migration of flat symmetric tilt grain boundaries is simulated using molecular dynamics. The driving force for migration is achieved by applying uniaxial strain on one of the grains in the bicrystal, enabling the growth of strain free grain at the expense of strained grain. Arrhenius dependence of grain boundary mobility on temperature and a linear relation between mobility and grain boundary velocity are observed. Simulations suggest that the mechanism of migration is dependent on vacancy diffusion combined with local reshuffling of atoms near the grain boundary.

  6. Atomistic studies of grain boundaries in alloys and compounds

    SciTech Connect

    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.

  7. Grain growth and grain boundary segregation in binary alloys: A Monte-Carlo simulation

    SciTech Connect

    Liu, J.M. |; Wu, Z.C.

    1997-08-15

    The authors have presented a Monte-Carlo simulation of grain growth and solute segregation on the grain boundaries (GBs) in binary alloys. A thermodynamic approach of the simulation algorithm has been given. The preliminary simulation of a simplified binary system indicates that pronounced solute segregation on the GBs is achieved, characterized by linearly increasing solute concentration on the GBs and significant spatial correlation of the solute distribution. The authors observe the normal grain growth in the present system by demonstrating the scaling behavior of its size distribution although the kinetics is seriously slowed down, exhibiting an exponent much lower than the 1/2 law.

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

    NASA Astrophysics Data System (ADS)

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

    2012-02-01

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

  9. Grain Boundary Junctions in Microstructure Generated by Multiple Twinning

    SciTech Connect

    Gertsman, Valerii Y.; Henager, Charles H.

    2003-10-01

    The microstructure of a Cu-Ni alloy after static recrystallization was investigated using electron backscatter diffraction in a scanning electron microscope and the existence of orientationally related clusters of crystallites formed by multiple twinning has been established. Grain boundary and triple junction character within the clusters are analyzed. While the outer boundaries of the cluster are crystallographically random, all the inner boundaries have sigma 3^n misorientations. A newly developed crystallographic theory of triple junctions and multicrystallite ensembles consisting of CSL boundaries is used to describe the structure of the cluster. The presence of an alpha not egual to 1 triple junction is confirmed. Apparently, the mesostructure of recrystallized materials susceptible to annealing twinning consists

  10. Characterization of microstructures near grain boundary in hot deformed AA 3104 aluminum alloy

    SciTech Connect

    Liu, W.C. Zhai, P.P.

    2011-01-15

    AA 3104 aluminium alloy was deformed at a temperature of 510 deg. C and at a strain rate of 5 s{sup -1} to strains of 0.14 and 0.53 by plane strain compression. The microstructure and orientation gradient in the vicinity of grain boundaries were investigated using transmission electron microscopy. The results show that there is a significant difference in microstructures between the grain-boundary region and the grain interior, depending on the orientations of the neighbouring grains. The grain-boundary region usually displays smaller cells or subgrains and higher misorientation angles than the grain interior. A high orientation gradient is observed near the grain boundary, which corresponds to the difference in deformation microstructures. - Research Highlights: {yields} Grain-boundary region displays finer microstructure and higher misorientation. {yields} Grain-boundary region has higher orientation gradients than the grain interior. {yields} Additional slip systems affect the orientation of dislocation boundaries.

  11. On the structure of grain/interphase boundaries and interfaces

    PubMed Central

    Gleiter, Herbert

    2014-01-01

    Summary Grain/interphase boundaries/interfaces of varying misorientations, free volume fractions, curvatures and irregularities are present in materials, both 3D and 2D, regardless of whether these materials are crystalline or amorphous/glassy. Therefore, a question arises about the central idea on which a general description of grain/interphase boundaries/interfaces can and should be based. It is suggested that a generalized model of a structural/basic unit (crystalline, non-crystalline or of any scale), which depends on the interatomic (including electronic) interactions, the spatial distribution of the atoms and electrons, the number of atoms and free volume fraction present in the structural/basic unit and the experimental conditions should serve the purpose. As the development of a quantitative model, which reflects the effects of all these variables is difficult, slightly defective material boundaries are often modeled by treating the entire boundary as planar and by using the concepts of crystallography. For highly disordered boundaries, a description in terms of a representative volume, made up of a non-crystalline basic unit or a combination of such units, which depend on interatomic (including electronic) interactions and forces, is advocated. The size, shape, free volume fraction and number of atoms in the representative volume could differ with material composition and experimental conditions. In the latter approach, it is assumed that all processes connected to a problem on hand is contained within this representative volume. The unresolved issues are identified. PMID:25383273

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

    PubMed

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

    2016-02-14

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

  14. Structural Dependence of Grain Boundary Resistivity in Copper Nanowires

    SciTech Connect

    Evans III, Boyd Mccutchen; Kenik, Edward A; Kim, Tae Hwan; Kulkarni, Nagraj S; Li, An-Ping; Meyer III, Harry M; Nicholson, Don M; Radhakrishnan, Bala; Zhang, Xiaoguang

    2011-01-01

    We report the direct measurement of individual grain boundary (GB) resistances and the critical role of GB structure in the increased resistivity in copper nanowires. By measuring both intra- and inter-grain resistance with a four-probe scanning tunneling microscope, large resistance jumps are revealed owing to successive scattering across high-angle random GBs, while the resistance changes at twin and other coincidence boundaries are negligibly small. The impurity distributions in the nanowires are characterized in correlating to the microstructures. The resistance of high symmetry coincidence GBs and the impurity contributions are then calculated using a first-principle method which confirms that the coincidence GBs have orders of magnitude smaller resistance than the high-angle random GBs.

  15. Specific Grain Boundary Resistivity Measurements in Thin Film Copper Bicrystals

    SciTech Connect

    Kulkarni, Nagraj S; Evans III, Boyd Mccutchen; Kim, Tae Hwan; Li, An-Ping; Nicholson, Don M; Kenik, Edward A

    2009-01-01

    A technique to estimate the specific grain boundary resistivity contribution in Cu lines using nanoscale four-probe STM measurements across thin film bamboo grain boundaries is presented. FIB milling of evaporated copper films was utilized to create Cu lines of various widths (400-4 microns). BSE and OIM microscopy was used to characterize the microstructure and texture of annealed Cu films containing these lines. Preliminary measurements using a fixed probe spacing with the four-probe STM technique indicated the sensitivity of the technique to detect variations in resistances along the length of the line though conversion to resistivities resulted in abnormally high values likely caused by material redeposition and/or Ga ion impregnation during FIB milling.

  16. Observation of a Reentrant Twist Grain Boundary Phase

    SciTech Connect

    Shankar Rao, D. S.; Krishna Prasad, S.; Raja, V. N.; Yelamaggad, C. V.; Anitha Nagamani, S.

    2001-08-20

    We report the occurrence of a reentrant twist grain boundary phase, which we designate as Re-TGB{sub A}. Microscopic observations on a nonsymmetric dimer showed the phase sequence Iso--N{sup *} --TGB{sub A} --Sm-A --Re--TGB{sub A} --TGB{sub C{sup *}}. Here N{sup *} and Sm-A stand for the chiral nematic and smectic-A phases, TGB{sub A} is the twist grain boundary phase with smectic-A blocks, and TGB{sub C{sup *}} that with smectic-C{sup *} blocks and exhibiting features of both the smectic-C{sup *} and TGB phases. The reentrance of the TGB{sub A} phase is unambiguously demonstrated using x-ray diffraction, selective reflection, and optical rotation data.

  17. Grain orientations and grain boundaries in tungsten nonotendril fuzz grown under divertor-like conditions

    DOE PAGESBeta

    Parish, Chad M.; Wang, Kun; Doerner, Russel P.; Baldwin, Matthew J.

    2016-09-19

    We grew nanotendril “fuzz” on tungsten via plasma exposure and performed transmission Kikuchi diffraction (tKD) in scanning electron microscopy of isolated nanotendrils. 900 °C, 1023 He/m2sec, 4 × 1026 He/m2 exposure of tungsten produced a deep and fully developed nanotendril mat. tKD of isolated nanotendrils indicated that there was no preferred crystallographic direction oriented along the long axes of the tendrils, and the grain boundary character showed slightly preferential orientations. In conclusion, tendril growth is sufficiently non-equilibrium to prevent any preference of growth direction to manifest measurably, and that new high-angle boundaries (with new grains and grain-growth axes) nucleate randomlymore » along the tendrils during growth.« less

  18. Grain Boundary Strengthening in High Mn Austenitic Steels

    NASA Astrophysics Data System (ADS)

    Kang, Jee-Hyun; Duan, Shanghong; Kim, Sung-Joon; Bleck, Wolfgang

    2016-05-01

    The Hall-Petch relationship is investigated to find the yield strengths of two high Mn austenitic steels. The Hall-Petch coefficient is found to depend on the overall C concentration and cooling rate, which suggests that the C concentration at the grain boundaries is an important factor. The pile-up model suggests that C raises the stress for the dislocation emission, while the ledge model predicts that C increases the density of ledges which act as dislocation sources.

  19. Structurally-Bonded Grain Boundary Water in Forsterite

    NASA Astrophysics Data System (ADS)

    Wang, L.

    2009-12-01

    Water storage capacity of nominally anhydrous olivine has been extensively investigated because of its numerous geophysical and geochemical implications for the Earth’s dynamic mantle. However, grain boundary as potential storage sites for water in the mantle has not been experimentally studied, in part because solubility experiments were always performed under water-saturated condition, rendering the examination of grain boundaries nearly impossible due to the presence of free water. In the present study we have conducted annealing experiments on forsterite at high pressure and temperature, and at water-undersaturated condition. Various materials were employed to buffer the silica activity and the oxygen fugacity. FTIR analyses were performed on both single crystal and polycrystalline olivine in doubly-polished thin section of each experimental charge. The results are as follows: (1) single crystal and polycrystalline olivine in the same charge always yielded similar IR pattern, indicating all absorption peaks are due to similar structurally-bonded water; (2) water content of periclase-buffered sample is at least one order of magnitude higher than those of enstatite-buffered and unbuffered samples; (3) under relatively reducing environment, water content of polycrystalline olivine is always higher than that of single crystal by at lease a factor of 5, regardless of silica activity buffering. It is therefore inferred that large amount of structurally-bonded water is stored at grain boundaries; (4) with decreasing oxygen fugacity, relative intensity of hydroxyl absorption peaks changes systematically, indicating a shift in dominant water incorporation mechanism in olivine. These results strongly suggest that grain boundaries could be significant storage sites for water in the Earth’s mantle, especially at locations where oxygen fugacity and silica activity are low.

  20. Intergranular degradation assessment via random grain boundary network analysis

    DOEpatents

    Kumar, Mukul; Schwartz, Adam J.; King, Wayne E.

    2002-01-01

    A method is disclosed for determining the resistance of polycrystalline materials to intergranular degradation or failure (IGDF), by analyzing the random grain boundary network connectivity (RGBNC) microstructure. Analysis of the disruption of the RGBNC microstructure may be assess the effectiveness of materials processing in increasing IGDF resistance. Comparison of the RGBNC microstructures of materials exposed to extreme operating conditions to unexposed materials may be used to diagnose and predict possible onset of material failure due to

  1. Anomalous low-frequency grain-boundary capacitance in silicon

    NASA Astrophysics Data System (ADS)

    Seager, C. H.; Pike, G. E.

    1980-10-01

    The admittance of silicon bicrystals has been measured as a function of temperature, frequency, and dc voltage. In some cases the low-frequency capacitance is anomalously large. The ac response of a simple double depletion layer structure is calculated. The anomalous capacitive currents are due to an out-of-phase modulation of the barrier height caused by charge injected into grain-boundary traps.

  2. Ineffectiveness of energy filtering at grain boundaries for thermoelectric materials

    NASA Astrophysics Data System (ADS)

    Bachmann, M.; Czerner, M.; Heiliger, C.

    2012-09-01

    We use a one-band effective mass model within the Landauer formalism to investigate the influence of double Schottky barriers on the thermoelectric coefficients. It is assumed that these double Schottky barriers arise due to trapping states in grain boundaries. Such barriers can cause an energy filtering effect, which is widely believed to advance thermoelectric efficiencies. We show that for low doping concentrations the Seebeck coefficient is indeed increased due to energy filtering effects, whereas the electric conductivity is strongly decreased. The resulting power factor is also decreased. For higher doping concentrations, which are necessary for large electric conductivities and thus reasonable ZT values, the double Schottky barriers are very small and have therefore an insignificant impact on the thermoelectric parameters. Consequently, there is no significant influence of grain boundaries on ZT values due to additional electrostatic barriers. This does not preclude that other mechanisms at grain boundaries, such as additional scattering due to disorder, can have a positive impact on the power factor.

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

    SciTech Connect

    Pint, Bruce A; Unocic, Kinga A

    2012-01-01

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

  4. Molecular dynamics simulations of He bubble nucleation at grain boundaries.

    PubMed

    Zhang, Yongfeng; Millett, Paul C; Tonks, Michael; Zhang, Liangzhe; Biner, Bulent

    2012-08-01

    The nucleation behavior of He bubbles in single-crystal (sc) and nano-grain body-centered-cubic (bcc) Mo is simulated using molecular dynamics (MD) simulations, focusing on the effects of the grain boundary (GB) structure. In sc Mo, the nucleation behavior of He bubbles depends on irradiation conditions. He bubbles nucleate by either clustering of He atoms with pre-existing vacancies or self-interstitial-atom (SIA) punching without initial vacancies. In nano-grain Mo, strong precipitation of He at the GBs is observed, and the density, size and spatial distribution of He bubbles vary with the GB structure. The corresponding He bubble density is higher in nano-grain Mo than that in sc Mo and the average bubble size is smaller. In the GB plane, He bubbles distribute along the dislocation cores for GBs consisting of GB dislocations and randomly for those without distinguishable dislocation structures. The simulation results in nano-grain Mo are in agreement with previous experiments in metal nano-layers, and they are further explained by the effect of excess volume associated with the GBs.

  5. Grain Boundary Wetting In The Stressed Rock Salt

    NASA Astrophysics Data System (ADS)

    Traskine, V.; Skvortsova, Z.; Barrallier, L.

    Safety assessment of underground salt-based waste disposal sites requires a detailed study of transport mechanisms in the salt around waste containers. Grain boundaries in dense polycrystalline aggregates are known to act as the main mass transfer path, provided that they form an interconnected network. A two-step model has been pro- posed (V.Traskine et al., Colloids and Surfaces A, 2000) allowing firstly to assess the grain boundary wetting (GBW) probability from the solid-liquid to solid-solid inter- face free energy ratio and from the scatter of the latter parameter among individual GBs, and then to estimate the percolation threshold for GBW (critical GBW proba- bility at which fluid inclusions would become interconnected in a polycrystal of any size). This approach, in spite of being simplified (the only driving force for GBW is assumed to be the minimization of interfacial energy), accounts well enough for a dra- matic change in physical and mechanical properties due to GBW observed in various solid-liquid couples. In this paper, we extend the above outlined model taking into account the role of mechanical stresses which are, reportedly, of a great importance in GBW. Experimental study has been made on disks of coarse-grained pore-free syn- thetic NaCl of a diameter of 18 mm and a thickness of 2 to 3 mm, immersed into the brine and submitted to the Brazilian test at increasing applied loads. The overall num- ber of wetted boundaries increases with rising load from about 30% for unstressed salt up to 60 to 70% for a load of 15 N. As it is known, the Brazilian test provides various combinations of shear and normal (mainly compressive) stress components. The anal- ysis of stress state on individual boundaries shows that the shear components enhance the GBW irrespectively to compressive components over practically the whole range of stress states studied. All the boundaries are wetted for shear stresses > 0.7 MPa or tensile stresses > 0.3 MPa. The compressive

  6. The Structure of Olivine Grain Boundaries Inferred from Transient and Steady State Deformation Experiments

    NASA Astrophysics Data System (ADS)

    Faul, U.; Jackson, I.

    2015-12-01

    A consensus has not been reached regarding the structure of general, high angle grain boundaries in olivine. Published high resolution transmission electron microscope images show either abutting lattice planes of the grains on either side of the boundary, or a distinct grain boundary region, about 1 nm wide, that is more disordered than the grain interiors. However, agreement exists that grain boundary region is enriched in olivine trace elements such as Ti, Ca and Al. These analytical methods can not resolve the thickness of this region. The properties of grain boundaries can be interrogated by experimentation, but the interpretation of the experimental results is tied to microphysical models. Models for diffusion creep predict a square grain size dependence for diffusion through grain interiors, and a cubic grain size dependence for diffusion along grain boundaries. Experimental observations for polycrystalline, Fe-bearing olivine are best fit with a cubic grain size dependence, indicating diffusion along grain boundaries. Similarly, models for small strain, transient creep predict that time-dependent, recoverable deformation involves diffusion along grain boundaries. For this process the models predict a linear grain size dependence. Forced torsional oscillation experiments can be employed to investigate the transient creep behaviour over a range of frequencies, temperatures and grain sizes. The observed grain size dependence for the same materials used for conventional, large strain deformation experiments is near linear, indicating diffusionally assisted grain boundary sliding. Both transient and steady state deformation therefore implicate diffusion along grain boundaries as the rate-controlling mechanism. Diffusion and viscous sliding along grain boundaries imply that they are a separate phase with a less ordered structure, consistent with their interpretation as a (narrow) region that is distinct from grain interiors. This region likely also accommodates

  7. Microstructural changes in Beta-silicon nitride grains upon crystallizing the grain-boundary glass

    NASA Technical Reports Server (NTRS)

    Lee, William E.; Hilmas, Gregory E.; Lange, F. F. (Editor)

    1991-01-01

    Crystallizing the grain boundary glass of a liquid phase sintered Si3N4 ceramic for 2 h or less at 1500 C led to formation of gamma Y2Si2O7. After 5 h at 1500 C, the gamma Y2Si2O7 had transformed to beta Y2Si2O7 with a concurrent dramatic increase in dislocation density within beta Si3N4 grains. Reasons for the increased dislocation density is discussed. Annealing for 20 h at 1500 C reduced dislocation densities to the levels found in as-sintered materials.

  8. Grain Boundary Structurally-Bonded Water in Olivine Aggregates

    NASA Astrophysics Data System (ADS)

    Wang, L.

    2008-12-01

    Water storage capacity of nominally anhydrous olivine has been extensively investigated because of its numerous geophysical and geochemical implications for the Earth's dynamic mantle. However, all previous experimental research has been concentrated on the water solubility in single crystals of olivine. Grain boundary as potential storage sites for water in the mantle has not been experimentally studied, in part because solubility experiments were always performed under water-saturated condition, rendering the examination of grain boundaries nearly impossible due to the presence of free water. In the present study we have conducted annealing experiments on forsterite at 5 - 6 GPa and 1200 °C and at water- undersaturated condition. Duration was typically 2 - 3 hours. A small amount of enstatite or periclase was added to the starting forsterite powder (including a few large olivine grains) to buffer the silica activity, while oxygen fugacity was controlled by using various capsule materials (Re, Fe, or BN). FTIR analyses were performed on both single crystal and polycrystalline olivine in doubly-polished thin section of each experimental charge. The results are as follows: (1) single crystal and polycrystalline olivine in the same charge always yielded similar IR pattern, indicating all absorption peaks are due to similar structurally-bonded water (i.e., hydroxyl); (2) water content of periclase-buffered (i.e., low silica activity) sample is at least one order of magnitude higher than those of enstatite-buffered and unbuffered (pure forsterite) samples; (3) under reducing environment (Fe or BN capsule), water content of polycrystalline olivine is always higher than that of single crystal by at lease a factor of 5, regardless of silica activity buffering. We therefore infer that large amount of structurally-bonded water is stored at grain boundaries; (4) with decreasing oxygen fugacity, IR spectra of olivine are increasingly dominated by an absorption peak centered

  9. Physics of grain boundaries in polycrystalline photovoltaic semiconductors

    NASA Astrophysics Data System (ADS)

    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.

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

    NASA Astrophysics Data System (ADS)

    Huang, Yu Li; Chen, Yifeng; Zhang, Wenjing; Quek, Su Ying; Chen, Chang-Hsiao; Li, Lain-Jong; Hsu, Wei-Ting; Chang, Wen-Hao; Zheng, Yu Jie; Chen, Wei; Wee, Andrew T. S.

    2015-02-01

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

  11. Low-energy, Mobile Grain Boundaries in Magnesium

    PubMed Central

    Liu, Xiangli; Wang, Jian

    2016-01-01

    The strong basal texture that is commonly developed during the rolling of magnesium alloy and can even increase during annealing motivates atomic-level study of dislocation structures of both <0001> tilt and twist grain boundaries (GBs) in Magnesium. Both symmetrical tilt and twist GBs over the entire range of rotation angles θ between 0° and 60° are found to have an ordered atomic structure and can be described with grain boundary dislocation models. In particular, 30° tilt and twist GBs are corresponding to energy minima. The 30° tilt GB is characterized with an array of Shockley partial dislocations bp:-bp on every basal plane and the 30° twist GB is characterized with a stacking faulted structure. More interesting, molecular dynamics simulations explored that both 30° tilt and twist GBs are highly mobile associated with collective glide of Shockley partial dislocations. This could be responsible for the formation of the strong basal texture and a significant number of 30° misorientation GBs in Mg alloy during grain growth. PMID:26891595

  12. Phase-field crystal study of segregation induced grain-boundary premelting in binary alloys

    NASA Astrophysics Data System (ADS)

    Lu, Yan-Li; Hu, Ting-Ting; Lu, Guang-Ming; Chen, Zheng

    2014-10-01

    The segregation-induced grain boundary (GB) premelting is studied by phase-field crystal model. We investigate the microstructure evolution of GBs with different misorientation angles and the variation of concentration distribution during premelting process. The liquid film thickness is calculated by an excessive mass technique. The results show that for low-angle GBs, the liquid phase first appears at individual dislocations region where the solute atoms segregate, and there exist two structural transitions during premelting process, which behave as two inflection points in the curve of w versus ΔB0. For high-angle GBs, the liquid film is rather uniform and its thickness diverges logarithmically when the melting point is approached. Moreover, the higher misorientation angle and segregation the GB has, the lower temperature the liquid film occurs. The concentration variation law indicates that GB segregation and GB premelting promote each other when the liquid phase appears. The critical wetting angle is almost equal to 12.5° and this value is much closer to the reality.

  13. PERCOLATION ON GRAIN BOUNDARY NETWORKS: APPLICATION TO FISSION GAS RELEASE IN NUCLEAR FUELS

    SciTech Connect

    Paul C. Millett

    2012-02-01

    The percolation behavior of grain boundary networks is characterized in two- and three-dimensional lattices with circular macroscale cross-sections that correspond to nuclear fuel elements. The percolation of gas bubbles on grain boundaries, and the subsequent percolation of grain boundary networks is the primary mechanism of fission gas release from nuclear fuels. Both radial cracks and radial gradients in grain boundary property distributions are correlated with the fraction of grain boundaries vented to the free surfaces. Our results show that cracks surprisingly do not significantly increase the percolation of uniform grain boundary networks. However, for networks with radial gradients in boundary properties, the cracks can considerably raise the vented grain boundary content.

  14. Structural Dependence of Grain Boundary Resistance in Copper Nanowires

    SciTech Connect

    Kim, Tae Hwan; Zhang, Xiaoguang; Nicholson, Don M; Radhakrishnan, Bala; Radhakrishnan, Balasubramaniam; Evans III, Boyd Mccutchen; Kulkarni, Nagraj S; Kenik, Edward A; Meyer III, Harry M; Li, An-Ping

    2011-01-01

    The current choice of the interconnect metal in integrated circuits is copper due to its higher electrical conductivity and improved electromigration reliability in comparison with aluminum. However, with reducing feature sizes, the resistance of copper interconnects (lines) increases dramatically. Greater resistance will result in higher energy use, more heat generation, more failure due to electromigration, and slower switching speeds. To keep pace with the projected planar transistor density, the first challenge is to identify the dominant factors that contribute to the high interconnect resistance. Here we directly measure individual grain boundary (GB) resistances in copper nanowires with a one-to-one correspondence to the GB structure. The specific resistivities of particular GBs are measured using four-probe scanning tunneling microscopy (STM) to establish a direct link between GB structure and the resistance. High-angle random GBs contribute to a specific resistivity of about 25 10-12 cm2 for each boundary, while coincidence boundaries are significantly less-resistive than random boundaries. Thus, replacing random boundaries with coincidence ones would be a route to suppress the GB impact to the resistivity of polycrystalline conductors. Acknowledgement: The research was supported by the Division of Scientific User Facilities, U. S. Department of Energy.

  15. Grain boundaries in hybrid two-dimensional materials

    NASA Astrophysics Data System (ADS)

    Zhang, Zhuhua; Yang, Yang; Yakobson, Boris I.

    2014-10-01

    In two-dimensional (2D) materials, bisector grain boundaries (GBs) are energetically favorable as they allow perfect match of neighbor grains. We demonstrate here a contrasting behavior for GBs in hybrid 2D materials, which tend to be non-bisector and obey a universal law to optimally match the heterogeneous grains: the ratio of cosines of the rotation angles of two neighbor grains equals the ratio of constituent's lattice parameters, reminiscent of Snell's law for light refraction. Details of the optimal GB structures are further formulated in terms of tilt angle, lattice mismatch strain and deviation angle from the bisector line, in good agreement with comprehensive numerical analyses. The ground state structures of the GBs manifest as a series of laterally misaligned bisector segments, which are verified by intensive first-principle calculations. Our findings not only provide a general guidance for exploring GBs in various hybrid 2D materials but also serve as an important stepping stone for understanding mechanical and electronic behaviors in these 2D nanoscale patchworks.

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

    NASA Technical Reports Server (NTRS)

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

    2006-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-09-01

    Trapping of interstitial (e.g. carbon) atoms is driven by the reduction in energy in the system. Diffusion of interstitials, together with their trapping in dislocation cores and/or grain boundaries, is studied by the thermodynamic extremal principle. In addition to the total Gibbs energy, a well-established formulation of the total dissipation is applied. Dimension-free evolution equations are derived, whose solution is well approximated by an easy to handle kinetic equation. Cottrell's power law can be verified in the initial stage.

  18. Attenuation of seismic waves by grain boundary relaxation.

    PubMed

    Jackson, D D

    1971-07-01

    Experimental observations of the attenuation of elastic waves in polycrystalline ceramics and rocks reveal an attenuation mechanism, called grain boundary relaxation, which is likely to be predominant cause of seismic attenuation in the earth's mantle. For this mechanism, the internal friction (the reciprocal of the "intrinsic Q" of the material) depends strongly upon frequency and is in good agreement with Walsh's theory of attenuation (J. Geophys. Res., 74, 4333, 1969) in partially melted rock. When Walsh's theory is extended to provide a model of the anelasticity of the earth, using the experimental values of physical parameters reported here, the results are in excellent agreement with seismic observations.

  19. Attenuation of Seismic Waves by Grain Boundary Relaxation

    PubMed Central

    Jackson, David D.

    1971-01-01

    Experimental observations of the attenuation of elastic waves in polycrystalline ceramics and rocks reveal an attenuation mechanism, called grain boundary relaxation, which is likely to be predominant cause of seismic attenuation in the earth's mantle. For this mechanism, the internal friction (the reciprocal of the “intrinsic Q” of the material) depends strongly upon frequency and is in good agreement with Walsh's theory of attenuation (J. Geophys. Res., 74, 4333, 1969) in partially melted rock. When Walsh's theory is extended to provide a model of the anelasticity of the earth, using the experimental values of physical parameters reported here, the results are in excellent agreement with seismic observations. PMID:16591937

  20. Theoretical studies of grain boundaries in Ni/sub 3/Al with boron or sulfur

    SciTech Connect

    Chen, S.P.; Voter, A.F.; Albers, R.C.; Boring, A.M.; Hay, P.J.

    1989-02-01

    It is well known that grain boundaries (GB) can have pronounced effects on the physical properties of materials (mechanical properties, corrosion resistance, fracture path, resistivity, etc.). Accordingly, a great deal of effort has been devoted to trying to understand the structure, energetics, and properties of grain boundaries. Significant experimental and theoretical progress has been made in understanding grain boundaries in pure systems, while the understanding of grain boundaries in alloy systems is much less developed. Also the mechanical properties of the grain boundaries are not well understood. In the present report, the authors summarize recent results on atomistic simulations of grain boundaries in the Ll/sub 2/ ordered alloy Ni/sub 3/Al. Understanding grain boundaries in this material is of particular importance since intergranular fracture limits the applicability of this otherwise promising material. To put these results into perspective, additional simulations were performed on grain boundaries in pure Ni and Al. Many features of grain boundaries in the ordered alloy may be understood in terms of the results on pure Ni and Al grain boundaries. The authors also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni and Ni/sub 3/Al.

  1. The Relation Between Grain-Boundary Structure and Sliding Resistance

    SciTech Connect

    Hoagland, Richard G.; Kurtz, Richard J.

    2002-04-01

    During sliding, the grain boundary (GB) energy depends on the atomic structures produced during relative translation of the two grains. The variation of the GB energy within the two-dimensional boundary unit cell (BUC) constitutes the GB gamma surface. Maxima in the slope of the gamma surface determines the sliding resistance, i.e., the stress required to move the system over the lowest saddle points along a particular path within the BUC. In this paper we present the results of an atomistic study of the gamma surfaces for two types of boundaries in an fcc metal, a, Sigma 11<110>{131} is a low energy boundary and has a simple gamma surface with a single stable configuration located at the corners and center of the BUC. The resistance to sliding was determined by chain-of-states methods along four shear vectors connecting equivalent states within the BUC and is found to be very high in all cases. The asymmetric GB has a higher GB energy and its gamma surface is much more complex, with distinctly different structures appearing at various locations in the BUC. At certain locations more than one structure is found for the asymmetric GB. Although complex, a chain-of-states calculation along one path across the BUC suggests that the shear strength of this GB is also quite high. Extrinsic GB dislocations are found to lower the resistance to shear considerably, and, therefore, perform the same role in shear of GBs as do glide dislocations in slip of the lattice. The existence of multiple configurations has significant implications for the interaction of lattice dislocations with GBs, the core structure of GB dislocations, the temperature dependence of GB properties, and the GB sliding resistance, which we discuss.

  2. Phenomena of solid state grain boundaries phase transition in technology

    SciTech Connect

    Minaev, Y. A.

    2015-03-30

    The results of study the phenomenon, discovered by author (1971), of the phase transition of grain boundary by the formation of two-dimensional liquid or quasi-liquid films have been done. The described phenomena of the first order phase transition (two-dimensional melting) at temperatures 0.6 – 0.9 T{sub S0} (of the solid state melting point) is a fundamental property of solid crystalline materials, which has allowed to revise radically scientific representations about a solid state of substance. Using the mathematical tools of the film thermodynamics it has been obtained the generalized equation of Clausius - Clapeyron type for two-dimensional phase transition. The generalized equation has been used for calculating grain boundary phase transition temperature T{sub Sf} of any metal, which value lies in the range of (0.55…0.86) T{sub S0}. Based on these works conclusions the develop strategies for effective forming of coatings (by thermo-chemical processing) on surface layers of functional alloys and hard metals have been made. The short overview of the results of some graded alloys characterization has been done.

  3. Possible model of an antiferroelectric twist grain boundary phase

    SciTech Connect

    Meier, J. G.; Carlsson, T.; Rudquist, P.; Lagerwall, S. T.; Nobili, M.; Brunet, M.

    2007-07-15

    Using x-ray and optical methods we have probed the structural organization of an antiferroelectric twist grain boundary phase (TGBC{sub a}) lying between the regular antiferroelectric smectic-C (SmC{sub a}{sup *}) and the smectic-Q (SmQ) or isotropic phase. We find that the twist axis is everywhere perpendicular to the local smectic layer normal and that the helical superstructure is incommensurate with the smectic layer structure. The twist grain boundaries consist of a periodic lattice of alternating +1/2 and -1/2 dispirations, i.e., unit screw dislocations in combination with half unit disclinations. The molecular tilt plane is alternatingly parallel and perpendicular to the twist axis. We find that the optically measured tilt angle in the SmC{sub a}{sup *} phase is smaller than that measured by x rays, which is the opposite to what is found in the SmC{sup *} phase. This means that the core part tilts less than the end chains in the SmC{sub a}{sup *} phase, while it tilts more in the SmC{sup *} phase. On entering the TGB phase a clear decrease is measured in the tilt angle. This is explained by the elastic influence from the disclinations, which appear in this phase.

  4. Simultaneous Modeling of Transient Creep and Grain Boundary Sliding

    NASA Astrophysics Data System (ADS)

    Cooper, R. F.; Sundberg, M.

    2009-12-01

    Grain boundary sliding (GBS) has been identified as an important contributor to the plastic deformation of polycrystalline solids. This phenomenon, whether accommodated by grain boundary diffusion or dislocation slip, has implications for rheological behavior and microstructural evolution during creep. Because GBS is not an independent deformation mechanism, but rather acts in kinetic series with some other (typically) rate-limiting process, direct investigation of the precise sliding mechanism(s) is difficult during conventional large-strain creep testing. Direct observations of grain boundary sliding can be obtained, however, by: (1) observing the mechanical response of a polycrystalline solid to an oscillating load as a function of frequency using the internal friction technique, and (2) studying the short duration transient response of a polycrystalline solid to a step-function change in stress. To this end, we have conducted an experimental study of low-frequency (10-2.25grained (d~5μm) aggregate of olivine and orthopyroxene (39 vol%). The attenuation spectra reveal “high-temperature background” behavior at low to moderate frequencies where attenuation diminishes smoothly and mildly with increasing frequency (QG-1 ~ f -0.3). At higher frequencies (f >10-0.5 Hz), the attenuation spectra reveal the onset of an apparent Debye peak in the attenuation spectra, likely due to elastically-accommodated GBS (GBS being rate-limiting). Previous experimental studies have demonstrated that the Andrade viscoelastic model can accurately predict both the transient creep response and

  5. Highly conductive grain boundaries in copper oxide thin films

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  6. Grain boundary mobility in anion doped MgO

    NASA Technical Reports Server (NTRS)

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

    1973-01-01

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

  7. Benefit of Grain Boundaries in Organic-Inorganic Halide Planar Perovskite Solar Cells.

    PubMed

    Yun, Jae S; Ho-Baillie, Anita; Huang, Shujuan; Woo, Sang H; Heo, Yooun; Seidel, Jan; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin A

    2015-03-01

    The past 2 years have seen the uniquely rapid emergence of a new class of solar cell based on mixed organic-inorganic halide perovskite. Grain boundaries are present in polycrystalline thin film solar cell, and they play an important role that could be benign or detrimental to solar-cell performance. Here we present efficient charge separation and collection at the grain boundaries measured by KPFM and c-AFM in CH3NH3PbI3 film in a CH3NH3PbI3/TiO2/FTO/glass heterojunction structure. We observe the presence of a potential barrier along the grain boundaries under dark conditions and higher photovoltage along the grain boundaries compare to grain interior under the illumination. Also, c-AFM measurement presents higher short-circuit current collection near grain boundaries, confirming the beneficial roles grain boundaries play in collecting carriers efficiently.

  8. Benefit of Grain Boundaries in Organic-Inorganic Halide Planar Perovskite Solar Cells.

    PubMed

    Yun, Jae S; Ho-Baillie, Anita; Huang, Shujuan; Woo, Sang H; Heo, Yooun; Seidel, Jan; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin A

    2015-03-01

    The past 2 years have seen the uniquely rapid emergence of a new class of solar cell based on mixed organic-inorganic halide perovskite. Grain boundaries are present in polycrystalline thin film solar cell, and they play an important role that could be benign or detrimental to solar-cell performance. Here we present efficient charge separation and collection at the grain boundaries measured by KPFM and c-AFM in CH3NH3PbI3 film in a CH3NH3PbI3/TiO2/FTO/glass heterojunction structure. We observe the presence of a potential barrier along the grain boundaries under dark conditions and higher photovoltage along the grain boundaries compare to grain interior under the illumination. Also, c-AFM measurement presents higher short-circuit current collection near grain boundaries, confirming the beneficial roles grain boundaries play in collecting carriers efficiently. PMID:26262666

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

    PubMed

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

    2015-11-20

    A novel time-resolved cathodoluminescence method, where a pulsed electron beam is generated via the photoelectric effect, is used to probe individual CdTe grain boundaries. Excitons have a short lifetime (≤100 ps) within the grains and are rapidly quenched at the grain boundary. However, a ~47 meV shallow acceptor, believed to be due to oxygen, can act as a long lifetime hole trap, even at the grain boundaries where their concentration is higher. This provides direct evidence supporting recent observations of hopping conduction across grain boundaries in highly doped CdTe at low temperature. PMID:26636877

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

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

    SciTech Connect

    Kozaczek, K.J.; Sinharoy, A.; Ruud, C.O.; McIlree, A.R.

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  13. Comparison of microstructure of superplastically deformed synthetic materials and ultramylonite: Coalescence of secondary mineral grains via grain boundary sliding

    NASA Astrophysics Data System (ADS)

    Hiraga, T.; Miyazaki, T.; Tasaka, M.; Yoshida, H.

    2011-12-01

    Using very fine-grained aggregates of forsterite containing ~10vol% secondary mineral phase such as periclase and enstatite, we have been able to demonstrate their superplascity, that is, achievement of more than a few 100 % tensile strain (Hiraga et al. 2010). Superplastic deformation is commonly considered to proceed via grain boundary sliding (GBS) which results in grain switching in the samples. Hiraga et al. (2010) succeeded in detecting the operation of GBS from observing the coalescence of grains of secondary phase in superplastically deformed samples. The secondary phase pins the motion of grain boundaries of the primary phase; however, the reduction of the number of the grains of secondary phase due to their coalescence allows grain growth of the primary phase. We analyzed the relationships between grain size of the primary and secondary phases, between strain and grain size, and between strain and the number of coalesced grains in the superplastically deformed samples. The results supports participation of all the grains of the primary phase in grain switching process indicating that the grain boundary sliding accommodates almost entire strain during the deformation. Mechanical properties of these materials such as their stress and grain size exponents of 1-2 do not conflict this conclusion. We applied the relationships obtained from analyzing superplastic materials to the microstructure of the natural samples, which has been considered to have deformed via grain boundary sliding, that is, ultramylonite. The microstructure of greenschist-grade ultramylonite reported by Fliervoet et al. (1997) was analyzed. Distributions of the mineral phases (i.e., quartz, plagioclase, K-feldspar and biotite) show distinct coalescence of the same mineral phases in the direction almost perpendicular to the foliation of the rock. The number of coalesced grains indicates that the strain that rock experienced is > 2. [reference] Hiraga et al. (2010) Nature 468, 1091

  14. Grain boundary plane orientation fundamental zones and structure-property relationships

    DOE PAGESBeta

    Homer, Eric R.; Patala, Srikanth; Priedeman, Jonathan L.

    2015-10-26

    Grain boundary plane orientation is a profoundly important determinant of character in polycrystalline materials that is not well understood. This work demonstrates how boundary plane orientation fundamental zones, which capture the natural crystallographic symmetries of a grain boundary, can be used to establish structure-property relationships. Using the fundamental zone representation, trends in computed energy, excess volume at the grain boundary, and temperature-dependent mobility naturally emerge and show a strong dependence on the boundary plane orientation. Analysis of common misorientation axes even suggests broader trends of grain boundary energy as a function of misorientation angle and plane orientation. Due to themore » strong structure-property relationships that naturally emerge from this work, boundary plane fundamental zones are expected to simplify analysis of both computational and experimental data. This standardized representation has the potential to significantly accelerate research in the topologically complex and vast five-dimensional phase space of grain boundaries.« less

  15. Grain boundary plane orientation fundamental zones and structure-property relationships

    SciTech Connect

    Homer, Eric R.; Patala, Srikanth; Priedeman, Jonathan L.

    2015-10-26

    Grain boundary plane orientation is a profoundly important determinant of character in polycrystalline materials that is not well understood. This work demonstrates how boundary plane orientation fundamental zones, which capture the natural crystallographic symmetries of a grain boundary, can be used to establish structure-property relationships. Using the fundamental zone representation, trends in computed energy, excess volume at the grain boundary, and temperature-dependent mobility naturally emerge and show a strong dependence on the boundary plane orientation. Analysis of common misorientation axes even suggests broader trends of grain boundary energy as a function of misorientation angle and plane orientation. Due to the strong structure-property relationships that naturally emerge from this work, boundary plane fundamental zones are expected to simplify analysis of both computational and experimental data. This standardized representation has the potential to significantly accelerate research in the topologically complex and vast five-dimensional phase space of grain boundaries.

  16. Grain Boundary Plane Orientation Fundamental Zones and Structure-Property Relationships

    PubMed Central

    Homer, Eric R.; Patala, Srikanth; Priedeman, Jonathan L.

    2015-01-01

    Grain boundary plane orientation is a profoundly important determinant of character in polycrystalline materials that is not well understood. This work demonstrates how boundary plane orientation fundamental zones, which capture the natural crystallographic symmetries of a grain boundary, can be used to establish structure-property relationships. Using the fundamental zone representation, trends in computed energy, excess volume at the grain boundary, and temperature-dependent mobility naturally emerge and show a strong dependence on the boundary plane orientation. Analysis of common misorientation axes even suggests broader trends of grain boundary energy as a function of misorientation angle and plane orientation. Due to the strong structure-property relationships that naturally emerge from this work, boundary plane fundamental zones are expected to simplify analysis of both computational and experimental data. This standardized representation has the potential to significantly accelerate research in the topologically complex and vast five-dimensional phase space of grain boundaries. PMID:26498715

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

    PubMed

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

    2015-06-01

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

  18. Multiscale model of metal alloy oxidation at grain boundaries

    SciTech Connect

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

    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

  19. Multiscale model of metal alloy oxidation at grain boundaries

    SciTech Connect

    Sushko, Maria L.; Alexandrov, Vitali Y.; Schreiber, Daniel K.; Rosso, Kevin M.; Bruemmer, Stephen M.

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

  20. Multiscale model of metal alloy oxidation at grain boundaries

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

  1. Investigations on residual strains and the cathodoluminescence and electron beam induced current signal of grain boundaries in silicon

    SciTech Connect

    Nacke, M.; Allardt, M.; Hieckmann, E.; Weber, J.; Chekhonin, P.; Skrotzki, W.

    2014-04-28

    Cathodoluminescence (CL) and electron beam induced current (EBIC) measurements were used to investigate the optical behavior and electrical activity of grain boundaries (GBs) in coarsely grained silicon. Electron backscatter diffraction (EBSD) was applied for a comprehensive characterization of the structural properties of the high angle and low angle GBs (HAGBs and LAGBs) in the sample. It was found that not only the EBIC but also the panchromatic (pan) CL contrast of Σ3 HAGBs strongly depends on the hkl-type of the boundary plane. At room temperature coherent Σ3 GBs exhibit no significant contrast in the CL or EBIC images, whereas at low temperatures the pan-CL contrast is strong. For incoherent Σ3 GBs, a strong pan-CL and EBIC contrast was observed in the entire temperature range. Only on a LAGB (misorientation angle 4.5°) CL investigations at low temperatures revealed a line with peak position at about (0.82 ± 0.01) eV, usually related to the dislocation associated D1 transition. Cross-correlation EBSD was applied to analyze the strain fields of Σ3 HAGBs as well as of the LAGB. All the components of the local strain tensors were quantitatively determined. The relationship between the extension of the strain field at the LAGB and the spatial D1 intensity distribution is discussed.

  2. Investigations on residual strains and the cathodoluminescence and electron beam induced current signal of grain boundaries in silicon

    NASA Astrophysics Data System (ADS)

    Nacke, M.; Allardt, M.; Chekhonin, P.; Hieckmann, E.; Skrotzki, W.; Weber, J.

    2014-04-01

    Cathodoluminescence (CL) and electron beam induced current (EBIC) measurements were used to investigate the optical behavior and electrical activity of grain boundaries (GBs) in coarsely grained silicon. Electron backscatter diffraction (EBSD) was applied for a comprehensive characterization of the structural properties of the high angle and low angle GBs (HAGBs and LAGBs) in the sample. It was found that not only the EBIC but also the panchromatic (pan) CL contrast of Σ3 HAGBs strongly depends on the hkl-type of the boundary plane. At room temperature coherent Σ3 GBs exhibit no significant contrast in the CL or EBIC images, whereas at low temperatures the pan-CL contrast is strong. For incoherent Σ3 GBs, a strong pan-CL and EBIC contrast was observed in the entire temperature range. Only on a LAGB (misorientation angle 4.5°) CL investigations at low temperatures revealed a line with peak position at about (0.82 ± 0.01) eV, usually related to the dislocation associated D1 transition. Cross-correlation EBSD was applied to analyze the strain fields of Σ3 HAGBs as well as of the LAGB. All the components of the local strain tensors were quantitatively determined. The relationship between the extension of the strain field at the LAGB and the spatial D1 intensity distribution is discussed.

  3. Effect of tube processing methods on the texture and grain boundary characteristics of 14YWT nanostructured ferritic alloys

    DOE PAGESBeta

    Aydogan, E.; Pal, S.; Anderoglu, O.; Maloy, S. A.; Vogel, S. C.; Odette, G. R.; Lewandowski, J. J.; Hoelzer, D. T.; Anderson, I. E.; Rieken, J. R.

    2016-03-08

    In this paper, texture and microstructure of tubes and plates fabricated from a nanostructured ferritic alloy (14YWT), produced either by spray forming followed by hydrostatic extrusion (Process I) or hot extrusion and cross-rolling a plate followed by hydrostatic tube extrusion (Process II) have been characterized in terms of their effects on texture and grain boundary character. Hydrostatic extrusion results in a combination of plane strain and shear deformations which generate low intensity α- and γ-fiber components of {001}<110> and {111}<110> together with a weak ζ-fiber component of {011}<211> and {011}<011>. In contrast, multi-step plane strain deformation by hot extrusion andmore » cross-rolling of the plate leads to a strong texture component of {001}<110> together with a weaker {111}<112> component. Although the total strains are similar, shear dominated deformation leads to much lower texture indexes compared to plane strain deformations. Further, the texture intensity decreases after hydrostatic extrusion of the alloy plate formed by plane strain deformation, due to a lower number of activated slip systems during shear dominated deformation. Finally and notably, hot extruded and cross-rolled plate subjected to plane strain deformation to ~50% engineering strain creates only a modest population of low angle grain boundaries, compared to the much larger population observed following the combination of plane strain and shear deformation of ~44% engineering strain resulting from subsequent hydrostatic extrusion.« less

  4. The favourable large misorientation angle grain boundaries in graphene

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  5. Grain-boundary layering transitions and phonon engineering

    NASA Astrophysics Data System (ADS)

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

    2016-09-01

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

  6. Atomic structure of [110] tilt grain boundaries in FCC materials

    SciTech Connect

    Merkle, K.L.; Thompson, L.J.

    1997-04-01

    High-resolution electron microscopy (HREM) has been used to study the atomic-scale structure and localized relaxations at grain boundaries (GBs) in Au, Al, and MgO. The [110] tilt GBs play an important role in polycrystalline fcc metals since among all of the possible GB geometries this series of misorientations as a whole contains the lowest energies, including among others the two lowest energy GBs, the (111) and (113) twins. Therefore, studies of the atomic-scale structure of [110] tilt GBs in fcc metals and systematic investigations of their dependence on misorientation and GB plane is of considerable importance to materials science. [110] tilt GBs in ceramic oxides of the fcc structure are also of considerable interest, since in this misorientation range polar GBs exist, i.e. GBs in which crystallographic planes that are made up of complete layers of cations or anions can join to form a GB.

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

    SciTech Connect

    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.

  8. The growth mechanism of grain boundary carbide in Alloy 690

    SciTech Connect

    Li, Hui; Xia, Shuang; Zhou, Bangxin; Peng, Jianchao

    2013-07-15

    The growth mechanism of grain boundary M{sub 23}C{sub 6} carbides in nickel base Alloy 690 after aging at 715 °C was investigated by high resolution transmission electron microscopy. The grain boundary carbides have coherent orientation relationship with only one side of the matrix. The incoherent phase interface between M{sub 23}C{sub 6} and matrix was curved, and did not lie on any specific crystal plane. The M{sub 23}C{sub 6} carbide transforms from the matrix phase directly at the incoherent interface. The flat coherent phase interface generally lies on low index crystal planes, such as (011) and (111) planes. The M{sub 23}C{sub 6} carbide transforms from a transition phase found at curved coherent phase interface. The transition phase has a complex hexagonal crystal structure, and has coherent orientation relationship with matrix and M{sub 23}C{sub 6}: (111){sub matrix}//(0001){sub transition}//(111){sub carbide}, <112{sup ¯}>{sub matrix}//<21{sup ¯}10>{sub transition}//<112{sup ¯}>{sub carbide}. The crystal lattice constants of transition phase are c{sub transition}=√(3)×a{sub matrix} and a{sub transition}=√(6)/2×a{sub matrix}. Based on the experimental results, the growth mechanism of M{sub 23}C{sub 6} and the formation mechanism of transition phase are discussed. - Highlights: • A transition phase was observed at the coherent interfaces of M{sub 23}C{sub 6} and matrix. • The transition phase has hexagonal structure, and is coherent with matrix and M{sub 23}C{sub 6}. • The M{sub 23}C{sub 6} transforms from the matrix directly at the incoherent phase interface.

  9. First-principles study of the effect of phosphorus on nickel grain boundary

    SciTech Connect

    Liu, Wenguan; Ren, Cuilan; Han, Han E-mail: xuhongjie@sinap.ac.cn; Zou, Yang; Zhou, Xingtai; Huai, Ping; Xu, Hongjie E-mail: xuhongjie@sinap.ac.cn; Tan, Jie

    2014-01-28

    Based on first-principles quantum-mechanical calculations, the impurity-dopant effects of phosphorus on Σ5(012) symmetrical tilt grain boundary in nickel have been studied. The calculated binding energy suggests that phosphorus has a strong tendency to segregate to the grain boundary. Phosphorus forms strong and covalent-like bonding with nickel, which is beneficial to the grain boundary cohesion. However, a too high phosphorus content can result in a thin and fragile zone in the grain boundary, due to the repulsion between phosphorus atoms. As the concentration of phosphorus increases, the strength of the grain boundary increases first and then decreases. Obviously, there exists an optimum concentration for phosphorus segregation, which is consistent with observed segregation behaviors of phosphorus in the grain boundary of nickel. This work is very helpful to understand the comprehensive effects of phosphorus.

  10. Migration mechanism of a GaN bicrystalline grain boundary as a model system

    NASA Astrophysics Data System (ADS)

    Lee, Sung Bo; Yoo, Seung Jo; Kim, Young-Min; Kim, Jin-Gyu; Han, Heung Nam

    2016-05-01

    Using in situ high-resolution transmission electron microscopy, we have explored migration mechanism of a grain boundary in a GaN bicrystal as a model system. During annealing at 500 °C, the grain-boundary region underwent a decrease in thickness, which occurred by decomposition or sublimation of GaN during annealing at 500 °C coupled with electron-beam sputtering. The decrease in thickness corresponds to an increase in the driving force for migration, because the migration of the grain boundary was driven by the surface energy difference. As the driving force increased with annealing time, the grain-boundary morphology turned from atomically smooth to rough, which is characterized by kinetic roughening. The observations indicate that a grain boundary exhibits a nonlinear relationship between driving force for migration and migration velocity, in discord with the general presumption that a grain boundary follows a linear relationship.

  11. Energetics of Cs in 3 grain boundary of 3C-SiC

    NASA Astrophysics Data System (ADS)

    Raghani, Pushpa

    2014-03-01

    Energetics of Cs defects at 3 grain boundaries of 3C-SiC has been studied using density functional theory to understand the role of the grain boundaries in Cs diffusion and its eventual release from the tristructural isotropic fuel particles (TRISO). Cs is shown to be much more stable at the 3 grain boundary than in bulk of SiC with a significant decrease (7 - 17 eV) in the formation energies at grain boundaries than in bulk. It is found to have even lower formation energies than those of Ag at the 3 grain boundaries, while this trend was opposite in the bulk SiC as demonstrated previously from similar density functional theory calculations. Based on these results, a possible route to control Cs release from SiC layer via grain-boundary-engineering is suggested. National Science Foundation.

  12. Migration mechanism of a GaN bicrystalline grain boundary as a model system

    PubMed Central

    Lee, Sung Bo; Yoo, Seung Jo; Kim, Young-Min; Kim, Jin-Gyu; Han, Heung Nam

    2016-01-01

    Using in situ high-resolution transmission electron microscopy, we have explored migration mechanism of a grain boundary in a GaN bicrystal as a model system. During annealing at 500 °C, the grain-boundary region underwent a decrease in thickness, which occurred by decomposition or sublimation of GaN during annealing at 500 °C coupled with electron-beam sputtering. The decrease in thickness corresponds to an increase in the driving force for migration, because the migration of the grain boundary was driven by the surface energy difference. As the driving force increased with annealing time, the grain-boundary morphology turned from atomically smooth to rough, which is characterized by kinetic roughening. The observations indicate that a grain boundary exhibits a nonlinear relationship between driving force for migration and migration velocity, in discord with the general presumption that a grain boundary follows a linear relationship. PMID:27210538

  13. Migration mechanism of a GaN bicrystalline grain boundary as a model system.

    PubMed

    Lee, Sung Bo; Yoo, Seung Jo; Kim, Young-Min; Kim, Jin-Gyu; Han, Heung Nam

    2016-01-01

    Using in situ high-resolution transmission electron microscopy, we have explored migration mechanism of a grain boundary in a GaN bicrystal as a model system. During annealing at 500 °C, the grain-boundary region underwent a decrease in thickness, which occurred by decomposition or sublimation of GaN during annealing at 500 °C coupled with electron-beam sputtering. The decrease in thickness corresponds to an increase in the driving force for migration, because the migration of the grain boundary was driven by the surface energy difference. As the driving force increased with annealing time, the grain-boundary morphology turned from atomically smooth to rough, which is characterized by kinetic roughening. The observations indicate that a grain boundary exhibits a nonlinear relationship between driving force for migration and migration velocity, in discord with the general presumption that a grain boundary follows a linear relationship. PMID:27210538

  14. Microprobe Evaluations of Grain Boundary Segregation in KM4 and IN100

    NASA Technical Reports Server (NTRS)

    Gabb, T. P.; Smith, J. W.

    2001-01-01

    Turbine disk alloys subjected to fatigue cycles with dwells at high temperatures and stresses can fail by cracking along grain boundaries. This could be due to concentrated creep deformation or environmental attack at grain boundaries. It would be important to identify any chemical segregation along grain boundaries to aid understanding of this intergranular failure mode. The objective of this study was to evaluate the degree of chemical segregation present at the grain boundaries of two disk alloys, KM4 and IN 100. An electron microprobe employing wavelength dispersive x-ray chemical analyses was used to characterize the chemistry along multiple grain boundaries in metallographically prepared samples of each alloy. Some degrees of boron, chromium, and cobalt enrichment of grain boundaries were observed in each alloy.

  15. The resistivity of grain boundary of K-doped ruthenates in percolative conduction regime

    NASA Astrophysics Data System (ADS)

    Nhat, Hoang Nam; Chinh, Huynh Dang; Phan, Manh-Huong

    2006-09-01

    Percolation theory has been involved to explain the temperature dependence of conductivity in the K-doped perovskite ruthenates and to estimate the resistivity of grain boundary in the percolative conduction regime. Using the two-layer simple effective medium model [A. Gupta, G.Q. Gong, G. Xiao, P.R. Duncombe, P. Lecoeur, P. Trouilloud, Y.Y. Wang, V.P. Dravis, J.Z. Sun, Phys. Rev. B 54 (1996) R15629] and assuming the scaling property of grain boundary system, we have obtained the new formula for grain boundary resistivity, which contains important factors for the grain size, boundary thickness, and boundary fractal dimension. The numerical results for the system A 0.5K 0.5RuO 3 (A=La, Y, Nd, Pr) are in very good agreement with the experiment. Importantly, it reveals that the percolative conduction plays a significant role in ceramic compounds containing polycrystalline grains and grain boundaries.

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

    SciTech Connect

    Zheng, Zexi; Chen, Xiang; Yang, Shengfeng; Xiong, Liming; Chen, Youping; Deng, Bowen; Chernatynskiy, Aleksandr

    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.

  17. Recombination by grain-boundary type in CdTe

    SciTech Connect

    Moseley, John Ahrenkiel, Richard K.; Metzger, Wyatt K.; Moutinho, Helio R.; Guthrey, Harvey L.; Al-Jassim, Mowafak M.; Paudel, Naba; Yan, Yanfa

    2015-07-14

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

  18. A diffuse interface model of grain boundary faceting

    NASA Astrophysics Data System (ADS)

    Abdeljawad, Fadi; Medlin, Douglas; Zimmerman, Jonathan; Hattar, Khalid; Foiles, Stephen

    Incorporating anisotropy into thermodynamic treatments of interfaces dates back to over a century ago. For a given orientation of two abutting grains in a pure metal, depressions in the grain boundary (GB) energy may exist as a function of GB inclination, defined by the plane normal. Therefore, an initially flat GB may facet resulting in a hill-and-valley structure. Herein, we present a diffuse interface model of GB faceting that is capable of capturing anisotropic GB energies and mobilities, and accounting for the excess energy due to facet junctions and their non-local interactions. The hallmark of our approach is the ability to independently examine the role of each of the interface properties on the faceting behavior. As a demonstration, we consider the Σ 5 < 001 > tilt GB in iron, where faceting along the { 310 } and { 210 } planes was experimentally observed. Linear stability analysis and numerical examples highlight the role of junction energy and associated non-local interactions on the resulting facet length scales. On the whole, our modeling approach provides a general framework to examine the spatio-temporal evolution of highly anisotropic GBs in polycrystalline metals. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE's National Nuclear Security Administration under Contract DE-AC04-94AL85000.

  19. Direct atom-resolved imaging of oxides and their grain boundaries.

    PubMed

    Zhang, Zaoli; Sigle, Wilfried; Phillipp, Fritz; Rühle, Manfred

    2003-10-31

    Using high-resolution transmission electron microscopy, we obtained structure images of strontium titanate (SrTiO3) with a clearly resolved oxygen sublattice along different crystallographic directions in the bulklattice and for a Sigma3 tilt grain boundary. Comparison with image simulations showed that the grain boundary contains oxygen vacancies. Measurements of atom displacements near the grain boundary revealed close correspondence with theoretical calculations.

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

    PubMed

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

    2015-12-21

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

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

    NASA Astrophysics Data System (ADS)

    César, Mathieu; Gall, Daniel; Guo, Hong

    2016-05-01

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

  2. Local and bulk melting of Cu at grain boundaries

    SciTech Connect

    Luo, Shengnian; Han, Li - Bo; An, Qi; Zheng, Lianqing

    2008-01-01

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

  3. Measurement of grain boundary composition for X52 pipeline steel

    SciTech Connect

    Wang, J.Q. |; Atrens, A.; Cousens, D.R.; Kelly, P.M.; Nockolds, C.; Bulcock, S.

    1998-10-09

    Analytical electron microscopy was used to measure the composition of grain boundaries (GBs) and interconstituent boundaries (IBs) of X52 pipeline steel using specimens about 40--60 nm in thickness. All elements of interest were examined with the exception of carbon. With this caveat, there was no segregation at proeutectoid ferrite GBs. This indicated that the commonly expected species S and P are not responsible for preferential corrosion of GBs during intergranular stress corrosion cracking of pipeline steels. Manganese was the only species measured to segregate at the IBs. Manganese segregated to the IBs between proeutectoid ferrite and pearlitic cementite, and desegregated from IBs between proeutectoid ferrite and pearlitic ferrite. The pearlitic cementite was Mn rich. There was no Mn segregation at the IBs between pearlitic ferrite and pearlitic cementite. The pattern of Mn segregation could be explained in terms of diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in the IBs between the proeutectoid ferrite and pearlite.

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

    PubMed

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

    2015-12-21

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

  5. Domain Wall Motion Across Various Grain Boundaries in Ferroelectric Thin Films

    DOE PAGESBeta

    Marincel, Daniel M.; Zhang, Huairuo; Jesse, Stephen; Belianinov, Alex; Okatan, Mahmut B.; Kalinin, Sergei V.; Rainforth, W. Mark; Reaney, Ian M.; Randall, Clive A.; Trolier-McKinstry, Susan

    2015-03-21

    Domain wall movement at and near engineered 10°, 15°, and 24° tilt and 10° and 30° twist grain boundaries was measured by band excitation piezoresponse force microscopy for Pb(Zr,Ti)O3 films with Zr/Ti ratio of 45/55 and 52/48. A minimum in nonlinear response was observed at the grain boundary for the highest angle twist and tilt grain boundaries, while a maximum in nonlinear response was observed at the 10° tilt grain boundaries. Lastly, the observed nonlinear response was correlated to the domain structure imaged in cross section by transmission electron microscopy.

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

    NASA Astrophysics Data System (ADS)

    Major, Jonathan D.

    2016-09-01

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

  7. Pore and grain boundary migration under a temperature gradient: A phase-field model study

    DOE PAGESBeta

    Biner, S. B.

    2016-03-16

    In this study, the collective migration behavior of pores and grain boundaries under a temperature gradient is studied for simple single crystal, bi-crystal and polycrystal configurations with a phase-field model formulism. For simulation of the microstructure of solids, composed of pores and grain boundaries, the results indicate that not only the volume fraction of pores, but also its spatial partitioning between the grain boundary junctions and the grain boundary segments appears to be important. In addition to various physical properties, the evolution kinetics, under given temperature gradients, will be strongly influenced with the initial morphology of a poly-crystalline microstructure.

  8. Chevron Defect at the Intersection of Grain Boundaries with Free Surfaces in Au

    NASA Astrophysics Data System (ADS)

    Radetic, T.; Lançon, F.; Dahmen, U.

    2002-08-01

    We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of 90° <110> tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary.

  9. Diffusion of Ag along Σ3 grain boundaries in 3C-SiC

    NASA Astrophysics Data System (ADS)

    Khalil, Sarah; Swaminathan, Narasimhan; Shrader, David; Heim, Andrew J.; Morgan, Dane D.; Szlufarska, Izabela

    2011-12-01

    Ag defects in Σ3 grain boundary of SiC were analyzed to test the hypothesis that Ag release from tristructural isotropic (TRISO) fuel particles can occur through grain boundary diffusion. Although Σ3 grain boundaries cannot provide a connected path through the crystal, they are studied here to provide guidance for overall trends in grain boundary vs bulk Ag transport. Formation energies of Ag defects are found to be 2-4 eV lower in the grain boundaries than in the bulk, indicating a strong tendency for Ag to segregate to the grain boundaries. Diffusion of Ag along Σ3 was found to be dramatically faster than through the bulk. At 1600∘C, which is a temperature relevant for TRISO accident conditions, Ag diffusion coefficients are predicted to be 3.7×10-18 m2/s and 3.9×10-29 m2/s in the Σ3 grain boundary and bulk, respectively. While at this temperature Σ3 diffusion is still two orders of magnitude slower than diffusion estimated from integral release measurements, the values are close enough to suggest that grain boundary diffusion is a plausible mechanism for release of Ag from intact SiC coatings. The remaining discrepancies in the diffusion coefficients could be possibly bridged by considering high-energy grain boundaries, which are expected to have diffusivity faster than Σ3 and which provide a connected percolating path through polycrystalline SiC.

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

    NASA Astrophysics Data System (ADS)

    Li, Hui; Xia, Shuang; Liu, Wenqing; Liu, Tingguang; Zhou, Bangxin

    2013-08-01

    Three dimensional chemical information concerning grain boundary segregation before carbide nucleation was characterized by atom probe tomography in two Ni-Cr-Fe alloys which were aged at 500 °C for 0.5 h after homogenizing treatment. B, C and Si atoms segregation at grain boundary in Alloy 690 was observed. B, C, N and P atoms segregation at grain boundary in 304 austenitic stainless steel was observed. C atoms co-segregation with Cr atoms at the grain boundaries both in Alloy 690 and 304 austenitic stainless steel was found, and its effect on the carbide nucleation was discussed. The amount of each segregated element at grain boundaries in the two Ni-Cr-Fe alloys were analyzed quantitatively. Comparison of the grain boundary segregation features of the two Ni-Cr-Fe alloys were carried out based on the experimental results. The impurity and solute atoms segregate inhomogeneously in the same grain boundary both in 304 SS and Alloy 690. The grain boundary segregation tendencies (Sav) are B (11.8 ± 1.4) > P (5.4 ± 1.4) > N (4.7 ± 0.3) > C (3.7 ± 0.4) in 304 SS, and B (6.9 ± 0.9) > C (6.7 ± 0.4) > Si (1.5 ± 0.2) in Alloy 690. Cr atoms may co-segregate with C atoms at grain boundaries before carbide nucleation at the grain boundaries both in 304 SS and Alloy 690. Ni atoms generally deplete at grain boundary both in 304 SS and Alloy 690. The literature shows that the Ni atoms may co-segregate with P atoms at grain boundaries [28], but the P atoms segregation do not leads to Ni segregation in the current study. In the current study, Fe atoms may segregate or deplete at grain boundary in Alloy 690. But Fe atoms generally deplete at grain boundary in 304 SS. B atoms have the strongest grain boundary segregation tendency both in 304 SS and Alloy 690. The grain boundary segregation tendency and Gibbs free energy of B in 304 SS is higher than in Alloy 690. C atoms are easy to segregate at grain boundaries both in 304 SS and Alloy 690. The grain boundary segregation

  11. Domain Wall Motion Across Various Grain Boundaries in Ferroelectric Thin Films

    SciTech Connect

    Marincel, Daniel M.; Zhang, Huairuo; Jesse, Stephen; Belianinov, Alex; Okatan, Mahmut B.; Kalinin, Sergei V.; Rainforth, W. Mark; Reaney, Ian M.; Randall, Clive A.; Trolier-McKinstry, Susan

    2015-03-21

    Domain wall movement at and near engineered 10°, 15°, and 24° tilt and 10° and 30° twist grain boundaries was measured by band excitation piezoresponse force microscopy for Pb(Zr,Ti)O3 films with Zr/Ti ratio of 45/55 and 52/48. A minimum in nonlinear response was observed at the grain boundary for the highest angle twist and tilt grain boundaries, while a maximum in nonlinear response was observed at the 10° tilt grain boundaries. Lastly, the observed nonlinear response was correlated to the domain structure imaged in cross section by transmission electron microscopy.

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

    SciTech Connect

    Jones, R.H.

    1992-11-01

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

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

    SciTech Connect

    Jones, R.H.

    1992-11-01

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

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

    SciTech Connect

    Hanning, F. Engelberg, D.L.

    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.

  15. Grain Boundary Percolation Modeling of Fission Gas Release in Oxide Fuels

    SciTech Connect

    Paul C. Millett; Michael R. Tonks; S. B. Biner

    2012-05-01

    We present a new approach to fission gas release modeling in oxide fuels based on grain boundary network percolation. The method accounts for variability in the bubble growth and coalescence rates on individual grain boundaries, and the resulting effect on macroscopic fission gas release. Two-dimensional representa- tions of fuel pellet microstructures are considered, and the resulting gas release rates are compared with traditional two-stage Booth models, which do not account for long-range percolation on grain boundary net- works. The results show that the requirement of percolation of saturated grain boundaries can considerably reduce the total gas release rates, particularly when gas resolution is considered.

  16. S–Te Interdiffusion within Grains and Grain Boundaries in CdTe Solar Cells

    SciTech Connect

    Li, C.; Poplawsky, J.; Paudel, N.; Pennycook, T. J.; Haigh, S. J.; Al-Jassim, M. M.; Yan, Y.; Pennycook, S. J.

    2014-09-19

    At the CdTe/CdS interface, a significant Te-S interdiffusion has been found a few nanometers into the grain interiors with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). S substitution at Te sites has been directly resolved in CdTe with STEM Z-contrast images. Moreover, when enough S substitutes for Te, a structural transformation from zinc-blende to wurtzite has been observed. Cl segregation has also been found at the interface. STEM electron-beam-induced current (EBIC) shows that the p-n junction occurs a few nm into the CdTe grains, which is consistent with the S diffusion range we observe. The shift of the p-n junction suggests a buried homo-junction which would help reduce non-radiative recombination at the junction. Meanwhile, long-range S diffusion in CdTe grain boundaries (GBs) has been detected, as well as Te and Cl diffusion in CdS GBs.

  17. S–Te Interdiffusion within Grains and Grain Boundaries in CdTe Solar Cells

    DOE PAGESBeta

    Li, C.; Poplawsky, J.; Paudel, N.; Pennycook, T. J.; Haigh, S. J.; Al-Jassim, M. M.; Yan, Y.; Pennycook, S. J.

    2014-09-19

    At the CdTe/CdS interface, a significant Te-S interdiffusion has been found a few nanometers into the grain interiors with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). S substitution at Te sites has been directly resolved in CdTe with STEM Z-contrast images. Moreover, when enough S substitutes for Te, a structural transformation from zinc-blende to wurtzite has been observed. Cl segregation has also been found at the interface. STEM electron-beam-induced current (EBIC) shows that the p-n junction occurs a few nm into the CdTe grains, which is consistent with the S diffusion range we observe. The shiftmore » of the p-n junction suggests a buried homo-junction which would help reduce non-radiative recombination at the junction. Meanwhile, long-range S diffusion in CdTe grain boundaries (GBs) has been detected, as well as Te and Cl diffusion in CdS GBs.« less

  18. Ensembles of gliding grain boundary dislocations in ultrafine grained materials produced by severe plastic deformation

    SciTech Connect

    Nazarov, A.A.

    1997-10-15

    Severe plastic deformation which is now widely used to produce an ultrafine grained (UFG) structure in metals and alloys necessarily leads to the accumulation of interfacial dislocations which arise due to the incompatibilities of strains of neighbor misorientated areas. The most important result of the dislocation storage at grain boundaries (GBs) and associated internal stresses is the fragmentation which at extremely large strains is usually followed by the formation of a very fine highly misorientated granular structure. After deformation the UFG structure must preserve residual dislocation arrays in GBs, since recovery at room temperature is not enough to provide their annihilation. Recently, a hypothesis has been put forward that the internal stresses induced by these arrays are the origin of a number of fundamental properties of UFG materials different from those of coarse grained ones. The author suggested that, in addition, the nonequilibrium dislocation ensembles in GBs were characterized by disordered distribution of dislocations. Thus, three main sources of internal stresses can exist in UFG materials prepared by severe plastic deformation: (1) disordered sessile EGBD arrays, (2) excess density of sessile EGBDs, that is junction disclinations, and (3) arrays of tangential EGBDs. Each of these components can contribute to the experimentally observed elastic strain, excess GB energy and volume expansion in UFG materials.

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

    NASA Astrophysics Data System (ADS)

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

    1998-04-01

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

  20. Microstructural Features Leading to Enhanced Resistance to Grain Boundary Creep Cracking in ALLVAC 718Plus

    NASA Astrophysics Data System (ADS)

    Unocic, Kinga A.; Hayes, Robert W.; Mills, Michael J.; Daehn, Glenn S.

    2010-02-01

    This study focuses on the microstructural features that enhance the resistance of ALLVAC 718Plus to grain boundary creep cracking during testing of samples at 704 °C in both dry and moist air. Fully recrystallized structures were found to be susceptible to brittle grain boundary cracking in both environments. Detailed transmission electron microscopy (TEM) microstructural characterization reveals features that are believed to lead to resistance to grain boundary cracking in the resistant microstructures. It is suggested that dislocation substructures found within the grains of resistant structures compete with the high-angle grain boundaries for oxygen, thereby reducing the concentration of oxygen on the grain boundaries and subsequent embrittlement. In addition, electron backscatter diffraction (EBSD) misorientation maps reveal that special boundaries ( i.e., Σ3 boundaries) resist cracking. This is in agreement with previous findings on the superalloy INCONEL 718. Furthermore, it is observed that cracks propagate along high-angle boundaries. This study also shows that in this case, the presence of delta phase at the grain boundaries does not by itself produce materials that are resistant to grain boundary cracking.

  1. Effects of grain, grain boundary, and dc electric field on giant dielectric response in high purity CuO ceramics

    NASA Astrophysics Data System (ADS)

    Thongbai, Prasit; Maensiri, Santi; Yamwong, Teerapon

    2008-08-01

    Giant dielectric constant ɛ' of ˜(2.8-3.7)×104 was observed in high purity CuO (99.999%) ceramics with grain sizes of 4.57±1.71 and 9.57±3.01 μm. The ɛ' and Ea increase with an increase in grain size due to the different electrical properties in the grains. The high dielectric response observed in the CuO ceramics can be described by the internal barrier layer capacitance model. The resistance of grain boundaries (Rgb) and the dielectric constant of the CuO samples decrease with increasing dc bias due to the decrease in grain boundaries capacitance, whereas the resistance of grains (Rg) remains constant.

  2. Low temperature superplasticity of AZ91 magnesium alloy with non-equilibrium grain boundaries

    SciTech Connect

    Mabuchi, M.; Ameyama, K.; Iwasaki, H.; Higashi, K.

    1999-05-28

    The superplastic behavior of a fine-grained AZ91 alloy, processed by equal channel angular extrusion, has been investigated in a low temperature range of 423--523 K. The experimental results showed a stress exponent of 2 and the activation energy for superplastic flow was in agreement with that for grain boundary diffusion of magnesium. The alloy with non-equilibrium grain boundary structures exhibited lower superplastic elongation than the alloy with equilibrium grain boundaries. Furthermore, the strain rate for superplastic flow of the former was lower than that of the latter. These differences probably arise because the accommodation process for grain boundary sliding is hampered by the long-range stresses associated with the non-equilibrium grain boundaries.

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

    SciTech Connect

    Moriani, Andrea; Cleri, Fabrizio

    2006-06-01

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

  4. Investigation of the effects of boron on Ni sub 3 Al grain boundaries by atomistic simulations

    SciTech Connect

    Chen, S.P.; Voter, A.F.; Albers, R.C.; Boring, A.M.; Hay, P.J. )

    1990-05-01

    A series of simulations has been performed on grain boundaries in Ni and Ni{sub 3}Al with and without boron doping using embedded atom-style potentials. A new procedure of obtaining reference'' data for boron related properties from electronic band structure calculations has been employed. Good agreement with existing experimental structural and energetic determinations was obtained. Boron is found to segregate more strongly to grain boundaries than to free surfaces. Adding boron to grain boundaries in Ni and Ni{sub 3}Al increases their cohesive strength and the work required to pull apart the boundary. This effect is much more dramatic for Ni-rich boundaries than for stoichiometric or Al-rich boundaries. In some Ni-rich cases, adding boron increases the cohesive strength of the boundary to such an extent that the boundaries become stronger than the bulk. Bulk Ni{sub 3}Al samples that are Ni-rich produce Ni-rich grain boundaries. The best cohesive properties of Ni{sub 3}Al grain boundaries are obtained when the boundary is Ni saturated and also with boron present. Boron and nickel are found to co-segregate to the grain boundaries.

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

    SciTech Connect

    Muhunthan, N.; Singh, Om Pal; Toutam, Vijaykumar; Singh, V.N.

    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.

  6. Effect of the pre-existing carbides on the grain boundary network during grain boundary engineering in a nickel based alloy

    SciTech Connect

    Liu, Tingguang; Xia, Shuang; Li, Hui; Zhou, Bangxin; Bai, Qin

    2014-05-01

    Grain boundary engineering was carried out on an aging-treated nickel based Alloy 690, which has precipitated carbides at grain boundaries. Electron backscatter diffraction technique was used to investigate the grain boundary networks. Results show that, compared with the solution-annealed samples, the aging-treated samples with pre-existing carbides at grain boundaries need longer duration or higher temperature during annealing after low-strain tensile deformation for forming high proportion of low-Σ coincidence site lattice grain boundaries (more than 75%). The reason is that the primary recrystallization is inhibited or retarded owing to that the pre-existing carbides are barriers to grain boundaries migration. - Highlights: • Study of GBE as function of pre-existing GB carbides, tensile strain and annealing • Recrystallization of GBE is inhibited or retarded by the pre-existing carbides. • Retained carbides after annealing show the original GB positions. • More than 80% of special GBs were formed after the modification of GBE processing. • Multiple twinning during recrystallization is the key process of GBE.

  7. Melting in migmatites associated with sub-grain boundaries in quartz

    NASA Astrophysics Data System (ADS)

    Levine, J.; Mosher, S.

    2010-12-01

    Migmatites from three diverse locations and rock types contain pseudomorphs of melt along sub-grain and grain boundaries, particularly in quartz. This relationship suggests melting preferentially occurs in more strained areas of a crystal lattice, consistent with observations from metallurgy and material science. In metals, ceramics and colloids, it has been demonstrated that melting preferentially occurs on grain boundaries, areas of high dislocation density, and that melting begins in areas that are more highly strained. Molecular dynamics simulations and experiments on colloids show that melting will also occur along sub-grain boundaries if there are no true grain boundaries present. A process known as SIMA (strain-induced melt activation) has been used on Al alloys to create a spherical grain shape, utilizing the fact that melt will preferentially form in high-strain areas, including both grain and sub-grain boundaries. In migmatites, previous work indicates that melt travels along grain boundaries as well as fractures within grains. Experimental work and identification of remnant blebs or pseudomorphs of melt along grain boundaries in natural rocks provides evidence for such transport. Our study of deformed rocks from three areas provides new evidence for melting as well as transport along sub-grain boundaries, particularly in quartz. Granitic gneiss samples from the Llano Uplift, central Texas, are from small-scale shear zones and surrounding country rock. The quartz/K-feldspar grain boundaries commonly have cuspate/serrate edges that are associated with sub-grain boundaries in quartz, and blebs of K-feldspar are found along sub-grain boundaries within quartz grains. The K-feldspar most likely represents pseudomorphs of melt that have migrated into and melted the quartz grain along quartz sub-grain boundaries, creating space for the crystallizing melt. Metapelites from the Wet Mountains of south-central Colorado have undergone partial melting via biotite

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

    PubMed

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

    2013-09-01

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

  9. Disjoining potential and grain boundary premelting in binary alloys

    NASA Astrophysics Data System (ADS)

    Hickman, J.; Mishin, Y.

    2016-06-01

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

  10. Grain boundary resistance to amorphization of nanocrystalline silicon carbide

    PubMed Central

    Chen, Dong; Gao, Fei; Liu, Bo

    2015-01-01

    Under the C displacement condition, we have used molecular dynamics simulation to examine the effects of grain boundaries (GBs) on the amorphization of nanocrystalline silicon carbide (nc-SiC) by point defect accumulation. The results show that the interstitials are preferentially absorbed and accumulated at GBs that provide the sinks for defect annihilation at low doses, but also driving force to initiate amorphization in the nc-SiC at higher doses. The majority of surviving defects are C interstitials, as either C-Si or C-C dumbbells. The concentration of defect clusters increases with increasing dose, and their distributions are mainly observed along the GBs. Especially these small clusters can subsequently coalesce and form amorphous domains at the GBs during the accumulation of carbon defects. A comparison between displacement amorphized nc-SiC and melt-quenched single crystal SiC shows the similar topological features. At a dose of 0.55 displacements per atom (dpa), the pair correlation function lacks long range order, demonstrating that the nc-SiC is fully amorphilized. PMID:26558694

  11. Grain boundary resistance to amorphization of nanocrystalline silicon carbide

    NASA Astrophysics Data System (ADS)

    Chen, Dong; Gao, Fei; Liu, Bo

    2015-11-01

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

  12. Giant-block twist grain boundary smectic phases.

    PubMed

    Fernsler, J; Hough, L; Shao, R-F; Maclennan, J E; Navailles, L; Brunet, M; Madhusudana, N V; Mondain-Monval, O; Boyer, C; Zasadzinski, J; Rego, J A; Walba, D M; Clark, N A

    2005-10-01

    Study of a diverse set of chiral smectic materials, each of which has twist grain boundary (TGB) phases over a broad temperature range and exhibits grid patterns in the Grandjean textures of the TGB helix, shows that these features arise from a common structure: "giant" smectic blocks of planar layers of thickness l(b) > 200 nm terminated by GBs that are sharp, mediating large angular jumps in layer orientation between blocks (60 degrees < Delta < 90 degrees ), and lubricating the thermal contraction of the smectic layers within the blocks. This phenomenology is well described by basic theoretical models applicable in the limit that the ratio of molecular tilt penetration length-to-layer coherence length is large, and featuring GBs in which smectic ordering is weak, approaching thin, melted (nematic-like) walls. In this limit the energy cost of change of the block size is small, leading to a wide variation of block dimension, depending on preparation conditions. The models also account for the temperature dependence of the TGB helix pitch.

  13. Grain boundary resistance to amorphization of nanocrystalline silicon carbide.

    PubMed

    Chen, Dong; Gao, Fei; Liu, Bo

    2015-01-01

    Under the C displacement condition, we have used molecular dynamics simulation to examine the effects of grain boundaries (GBs) on the amorphization of nanocrystalline silicon carbide (nc-SiC) by point defect accumulation. The results show that the interstitials are preferentially absorbed and accumulated at GBs that provide the sinks for defect annihilation at low doses, but also driving force to initiate amorphization in the nc-SiC at higher doses. The majority of surviving defects are C interstitials, as either C-Si or C-C dumbbells. The concentration of defect clusters increases with increasing dose, and their distributions are mainly observed along the GBs. Especially these small clusters can subsequently coalesce and form amorphous domains at the GBs during the accumulation of carbon defects. A comparison between displacement amorphized nc-SiC and melt-quenched single crystal SiC shows the similar topological features. At a dose of 0.55 displacements per atom (dpa), the pair correlation function lacks long range order, demonstrating that the nc-SiC is fully amorphilized. PMID:26558694

  14. Volatiles (H, He, Ar) in olivine grain boundaries

    NASA Astrophysics Data System (ADS)

    Demouchy, S. A.; Burnard, P.

    2015-12-01

    The volatiles are key tracers of the evolution of the terrestrial mantle-atmosphere system: a realistic description of their abundance and isotopic heterogeneities in the Earth's mantle is required to properly understand the mantle-atmosphere interactions. In order to be able to interpret measured volatiles heterogeneities, notably H, He and Ar, we need constraints on the fundamental behaviour of volatiles in the mantle (where are they stored, how do they partition during melting or metasomatism and how might they be transported). At present, these parameters are poorly constrained since it is technically challenging to perform experiments with volatiles at high temperature and high pressure, but also because volatiles behaviour in silicate minerals is complex and can be related to numerous possible different host sites and incorporation mechanisms. Here, we synthetize the results of high temperature - high pressure experiments, which aim to constrain hydrogen and noble gas behaviour in crystalline solids versus mono-mineralic aggregates, including grain boundaries, that mimic mantle compositions and conditions. These experiments provide basics for further thermodynamic modeling.

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

    NASA Astrophysics Data System (ADS)

    Zhang, Lei; Fu, Chu-Chun; Hayward, Erin; Lu, Guang-Hong

    2015-04-01

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

  16. A diffuse interface model of grain boundary faceting

    NASA Astrophysics Data System (ADS)

    Abdeljawad, F.; Medlin, D. L.; Zimmerman, J. A.; Hattar, K.; Foiles, S. M.

    2016-06-01

    Interfaces, free or internal, greatly influence the physical properties and stability of materials microstructures. Of particular interest are the processes that occur due to anisotropic interfacial properties. In the case of grain boundaries (GBs) in metals, several experimental observations revealed that an initially flat GB may facet into hill-and-valley structures with well defined planes and corners/edges connecting them. Herein, we present a diffuse interface model that is capable of accounting for strongly anisotropic GB properties and capturing the formation of hill-and-valley morphologies. The hallmark of our approach is the ability to independently examine the various factors affecting GB faceting and subsequent facet coarsening. More specifically, our formulation incorporates higher order expansions to account for the excess energy due to facet junctions and their non-local interactions. As a demonstration of the modeling capability, we consider the Σ5 <001 > tilt GB in body-centered-cubic iron, where faceting along the {210} and {310} planes was experimentally observed. Atomistic calculations were utilized to determine the inclination-dependent GB energy, which was then used as an input in our model. Linear stability analysis and simulation results highlight the role of junction energy and associated non-local interactions on the resulting facet length scales. Broadly speaking, our modeling approach provides a general framework to examine the microstructural stability of polycrystalline systems with highly anisotropic GBs.

  17. Modeling of grain boundary stresses in Alloy 600

    SciTech Connect

    Kozaczek, K.J.; Sinharoy, A.; Ruud, C.O.; Mcllree, A.R.

    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.

  18. Atomic structures and electronic properties of phosphorene grain boundaries

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  19. An investigation of acceptor-doped grain boundaries in ?

    NASA Astrophysics Data System (ADS)

    Ravikumar, V.; Rodrigues, R. P.; Dravid, V. P.

    1996-07-01

    Grain boundary (GB) doped 0022-3727/29/7/014/img2 exhibits interesting electroceramic phenomena including varistor and barrier layer capacitor behaviour. We present here our investigation of GB acceptor-doped 0022-3727/29/7/014/img2 using analytical electron microscopy including electron holography. Mn was diffused into sintered polycrystalline 0022-3727/29/7/014/img2 to attain GBs which are rich in Mn. The presence and spatial extent of Mn at the GBs were analysed using x-ray emission spectroscopy (XES) and parallel electron energy loss spectroscopy (PEELS). The valence state of Mn was determined using PEELS to be predominantly +2. Finally, transmission high-energy electron holography was utilized to directly image and quantify the electrostatic potential and associated space-charge across the GBs directly. The holography results reveal a negatively charged GB with positive space-charge, indicating that Mn with a valence of +2 resides as an acceptor dopant on the Ti site at the GB core. The barrier height and local charge density distribution, including the Debye length, of the double Schottky barrier at the GB are derived from these holography results. This investigation demonstrates the usefulness of electron holography as a bulk-sensitive technique to probe the statics and dynamics of electrostatic field distribution and electrical charge across interfaces in technologically useful materials, and the need to employ diverse analytical techniques for such an investigation.

  20. Kinetics of grain boundary migration in dilute copper-silicon bicrystals

    NASA Astrophysics Data System (ADS)

    Zordan, R. D.

    The effect of silicon solute addition on the kinetic of grain boundary migration in copper was investigated by the reversed-capillarity technique. Single crystal seeds and the required bicrystals of varying alloy content were grown from the melt in split graphite molds, using an induction heated vertical crystal grower specifically designed and constructed for this purpose. Bicrystal misorientation was maintained at 42 deg 100 tilt, to minimize structural effects on the grain boundary migration behavior. Grain boundary mobility was measured between 275 and 400 C for several silicon contents. Qualitatively, grain boundary mobility increased as temperature was increased or solute content was decreased. For specimens containing 50 to 230 ppm silicon, the apparent activation energy for grain boundary migration was found to increase to increase from 27 to 35 kcal/mol. These values of apparent activation energy are consistent with he diffusion of silicon in the grain boundary as the rate controlling step from grain boundary migration. Grain boundary mobility was found to be universely related to impurity content, in excellent qualitative and quantitative agreement with the low velocity region of solute drag theories.

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

    NASA Astrophysics Data System (ADS)

    Xiao, XiaZi; Chu, HaiJian; Duan, HuiLing

    2016-06-01

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

  2. Polarization-resolved spectroscopy imaging of grain boundaries and optical excitations in crystalline organic thin films

    PubMed Central

    Pan, Z.; Rawat, N.; Cour, I.; Manning, L.; Headrick, R. L.; Furis, M.

    2015-01-01

    Exploration of optical properties of organic crystalline semiconductors thin films is challenging due to submicron grain sizes and the presence of numerous structural defects, disorder and grain boundaries. Here we report on the results of combined linear dichroism (LD)/ polarization-resolved photoluminescence (PL) scanning microscopy experiments that simultaneously probe the excitonic radiative recombination and the molecular ordering in solution-processed metal-free phthalocyanine crystalline thin films with macroscopic grain sizes. LD/PL images reveal the relative orientation of the singlet exciton transition dipoles at the grain boundaries and the presence of a localized electronic state that acts like a barrier for exciton diffusion across the grain boundary. We also show how this energy barrier can be entirely eliminated through the optimization of deposition parameters that results in films with large grain sizes and small-angle boundaries. These studies open an avenue for exploring the influence of long-range order on exciton diffusion and carrier transport. PMID:26365682

  3. Grain boundaries in graphene on SiC(0001̅) substrate.

    PubMed

    Tison, Yann; Lagoute, Jérôme; Repain, Vincent; Chacon, Cyril; Girard, Yann; Joucken, Frédéric; Sporken, Robert; Gargiulo, Fernando; Yazyev, Oleg V; Rousset, Sylvie

    2014-11-12

    Grain boundaries in epitaxial graphene on the SiC(0001̅) substrate are studied using scanning tunneling microscopy and spectroscopy. All investigated small-angle grain boundaries show pronounced out-of-plane buckling induced by the strain fields of constituent dislocations. The ensemble of observations determines the critical misorientation angle of buckling transition θc = 19 ± 2°. Periodic structures are found among the flat large-angle grain boundaries. In particular, the observed θ = 33 ± 2° highly ordered grain boundary is assigned to the previously proposed lowest formation energy structural motif composed of a continuous chain of edge-sharing alternating pentagons and heptagons. This periodic grain boundary defect is predicted to exhibit strong valley filtering of charge carriers thus promising the practical realization of all-electric valleytronic devices. PMID:25330353

  4. Chemical state analysis of grain boundaries in ZnO varistors by Auger electron spectroscopy

    SciTech Connect

    Tanaka, Shigeru ); Akita, Chiyoshi ); Ohashi, Naoki ); Kawai, Jun ); Haneda, Hajime; Tanaka, Junzo )

    1993-07-01

    The chemical state of grain boundaries in Bi[sub 2]O[sub 3]-doped ZnO ceramics was investigated by Auger electron spectroscopy. The additive Bi was segregated into grain boundaries 2 to 3 nm thick, where oxygen deficiency occurred. Auger transitions KL[sub 2,3]L[sub 2,3] for oxygen at the grain boundaries were composed of three peaks whose relative intensities varied with the amount of the segregated Bi. Results calculated using a molecular orbital method suggested that the metal-oxygen bonding state in the grain boundary changed with increased amounts of Bi. The change of the bonding character was considered to be related to the formation of an interfacial state at the grain boundary causing nonlinear current-voltage characteristics. 16 refs., 9 figs.

  5. Grain boundaries in graphene on SiC(0001̅) substrate.

    PubMed

    Tison, Yann; Lagoute, Jérôme; Repain, Vincent; Chacon, Cyril; Girard, Yann; Joucken, Frédéric; Sporken, Robert; Gargiulo, Fernando; Yazyev, Oleg V; Rousset, Sylvie

    2014-11-12

    Grain boundaries in epitaxial graphene on the SiC(0001̅) substrate are studied using scanning tunneling microscopy and spectroscopy. All investigated small-angle grain boundaries show pronounced out-of-plane buckling induced by the strain fields of constituent dislocations. The ensemble of observations determines the critical misorientation angle of buckling transition θc = 19 ± 2°. Periodic structures are found among the flat large-angle grain boundaries. In particular, the observed θ = 33 ± 2° highly ordered grain boundary is assigned to the previously proposed lowest formation energy structural motif composed of a continuous chain of edge-sharing alternating pentagons and heptagons. This periodic grain boundary defect is predicted to exhibit strong valley filtering of charge carriers thus promising the practical realization of all-electric valleytronic devices.

  6. Characterization of extrinsic grain-boundary dislocations and grain-boundary dislocation sources by transmission electron microscopy. Final report, June 1, 1979-May 31, 1981

    SciTech Connect

    Murr, L E

    1981-06-01

    The microstructures attendant to specific peak strains along the strain axis of the stress-strain diagram for type 304 stainless steel and nickel have been examined and compared by transmission electron microscopy from epsilon = 0.05% to 55% in the former and from epsilon = 0.05% to 35% in the latter. The onset of flow is characterized by the emission of dislocations from grain boundary ledge source which form emission profiles resembling dislocation pileups in the stainless steel, and a random distribution of dislocations with evidence for very short emission profiles near the grain boundaries in nickel. At the engineering yield point (0.2%) every grain in the stainless steel shows evidence for dislocation emission profiles, while in the nickel every grain contains some dislocations distributed within the grain interior.

  7. General Schema for [001] Tilt Grain Boundaries in Dense Packing Cubic Crystals

    SciTech Connect

    Tong, Wen; Yang, Hao; Moeck, Peter; Nandasiri, Manjula I.; Browning, Nigel D.

    2013-03-16

    Atomic resolution Z-contrast images from a series of CeO2 [001] tilt grain boundaries at coincident site lattice (CSL) or near-CSL misorientations can all be explained within a structural unit model. These structural units (which cover all boundaries from 0-90 degrees) show striking similarities to comparable CSL boundaries observed in cubic crystal structures that are also derived from dense packing (fcc metal, rocksalt, perovskite etc). A general model for the structure of grain boundaries in such similarly structured materials systems has been developed that is based on the crystallography of the parent structures. Changes away from these predicted grain boundary symmetries can be interpreted as showing the frustration of symmetry caused by the incorporation of point defects (vacancies and impurities). This general model for grain boundary structures can, in principle, provide a means to infer the structure-property relationships in broad classes of materials.

  8. In-situ SEM studies of grain boundary migration during recrystallization of cold-rolled nickel

    SciTech Connect

    Le Gall, R.; Liao, G.; Saindrenan, G.

    1999-07-23

    The grain boundary migration (GBM) rate during primary recrystallization depends on the driving force P exerted on the boundary and on the boundary mobility M. M is composed of two terms: an intrinsic mobility of the boundary depending on its structure and a chemical mobility due to the action exerted on the boundary by impurities. The purpose of the present work is to investigate quantitatively the law V = f(P) during primary recrystallization of nickel.

  9. Exploration of the mechanisms of temperature-dependent grain boundary mobility: Search for the common origin of ultrafast grain boundary motion

    DOE PAGESBeta

    O’Brien, C. J.; Foiles, S. M.

    2016-04-19

    The temperature dependence of grain boundary mobility is complex, varied, and rarely fits ideal Arrhenius behavior. This work presents a series of case studies of planar grain boundaries in a model FCC system that were previously demonstrated to exhibit a variety of temperature-dependent mobility behaviors. It is demonstrated that characterization of the mobility versus temperature plots is not sufficient to predict the atomic motion mechanism of the grain boundaries. Herein, the temperature-dependent motion and atomistic motion mechanisms of planar grain boundaries are driven by a synthetic, orientation-dependent, driving force. The systems studied include CSL boundaries with Σ values of 5,more » 7, and 15, including both symmetric and asymmetric boundaries. These boundaries represent a range of temperature-dependent trends including thermally activated, antithermal, and roughening behaviors. Examining the atomic-level motion mechanisms of the thermally activated boundaries reveals that each involves a complex shuffle, and at least one atom that changes the plane it resides on. The motion mechanism of the antithermal boundary is qualitatively different and involves an in-plane coordinated shuffle that rotates atoms about a fixed atom lying on a point in the coincident site lattice. Furthermore, this provides a mechanistic reason for the observed high mobility, even at low temperatures, which is due to the low activation energy needed for such motion. However, it will be demonstrated that this mechanism is not universal, or even common, to other boundaries exhibiting non-thermally activated motion. This work concludes that no single atomic motion mechanism is sufficient to explain the existence of non-thermally activated boundary motion.« less

  10. Atomistic simulations of dislocation nucleation in single crystals and grain boundaries

    NASA Astrophysics Data System (ADS)

    Tschopp, Mark A., Jr.

    The objective of this research is to use atomistic simulations to investigate dislocation nucleation from grain boundaries in face-centered cubic aluminum and copper. This research primarily focuses on asymmetric tilt grain boundaries and has three main components. First, this research uses molecular statics simulations of the structure and energy of these faceted, dissociated grain boundary structures to show that Sigma3 asymmetric boundaries can be decomposed into the structural units of the Sigma3 symmetric tilt grain boundaries, i.e., the coherent and incoherent twin boundaries. Moreover, the energy for all Sigma3 asymmetric boundaries is predicted with only the energies of the Sigma3 symmetric boundaries and the inclination angle. Understanding the structure of these boundaries provides insight into dislocation nucleation from these boundaries. Further work into the structure and energy of other low order Sigma asymmetric boundaries and the spatial distribution of free volume within the grain boundaries also provides insight into dislocation nucleation mechanisms. Second, this research uses molecular dynamics deformation simulations with uniaxial tension applied perpendicular to these boundaries to show that the dislocation nucleation mechanisms in asymmetric boundaries are highly dependent on the faceted, dissociated structure. Grain boundary dislocation sources can act as perfect sources/sinks for dislocations or may violate this premise by increasing the dislocation content of the boundary during nucleation. Furthermore, simulations under uniaxial tension and uniaxial compression show that nucleation of the second partial dislocation in copper exhibits tension-compression asymmetry. Third, this research explores the development of models that incorporate the resolved stress components on the slip system of dislocation nucleation to predict the atomic stress required for dislocation nucleation from single crystals and grain boundaries. Single crystal

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

    SciTech Connect

    Dragunov, Andrei S.; Weckman, A. V.; Demyanov, B. F.

    2014-10-06

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

  12. In-situ investigations of grain boundary-fluid inclusion interaction in recrystallizing rock analogues

    NASA Astrophysics Data System (ADS)

    Schmatz, J.; Schenk, O.; Urai, J. L.

    2009-12-01

    Investigating the mobility of fluid inclusions and fluid-rock interaction helps characterizing the effect of pore fluid on i) mechanical properties of rocks and ii) transport properties of fluid through rock volume. We present results from deformation and annealing experiments in transmitted light using rock analogues containing a liquid phase. The deformation rig is a high-pressure (up to 30 MPa), high-temperature (up to 200°C) see-through vessel with a controlled pore fluid pressure system. Samples were made of bischofite-brine, camphor-ethanol, and camphor-ethylene mixtures with the solid being polycrystalline with grain sizes ranging from 200 to 600 µm and with varying solubility in the liquid phase. In all systems the liquid phase is predominantly observed in isolated fluid inclusions and fluid pockets along grain boundaries with sizes ranging from 1 to 50 µm. Results show the in-situ pore fluid morphology during grain boundary migration recrystallization with preferential leakage of fluid inclusions along grain boundaries. For grain boundary migration rates ranging from 10-10 to 10-6 ms-1 we observed Zener pinning, drag and drop of fluid inclusions by migrating grain boundaries, but also passage of grain boundaries over fluid inclusions without any noticeable interaction, all being dependent on fluid inclusion size and solubility. Systematic measurements allowed tracing the maximum velocity of dragged fluid inclusions as well as the drag-limiting grain boundary velocity. The effects of these limiting conditions on recrystallized texture and on mobility of isolated fluid inclusions are subject to numerical simulations using the microstructural modeling platform ELLE. Image sequence showing fluid inclusion detaching from migrating grain boundary. Systematic measurements on pore shape evolution through time provide us the base for an empirical solution for pore drag and drop. The dragging time can be described as a function of grain boundary velocity, pore

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

    SciTech Connect

    Babinsky, K.; Weidow, J.; Knabl, W.; Lorich, A.; Leitner, H.; Primig, S.

    2014-01-15

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

  14. Analysis of grain boundary dislocations by large angle convergent beam electron diffraction.

    PubMed

    Morniroli, J P; Cherns, D

    1996-01-01

    Large angle convergent beam electron diffraction (LACBED) is used to analyse secondary dislocations in sigma3 and sigma9 grain boundaries in silicon. By selecting reflections from crystal planes common to the adjoining grains, LACBED images are insensitive to the boundaries except where dislocations are present. The dislocation images are closely similar to those for dislocations in single crystals and can be analysed by standard Cherns-Preston rules. It is shown that, for both boundaries, sufficient common reflections can be selected for a complete analysis, and that dislocations can be analysed assuming integer values of g x b, implying that the Burgers vectors are Displacement Shift Complete (DSC) lattice vectors. For both sigma3 and sigma9 boundaries, DSC dislocations are identified which are specific to these boundaries. The experimental conditions for the analysis of grain boundaries are explained, and the extension of the method to other coincidence boundaries is discussed. PMID:22666917

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

    PubMed

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

    2016-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    PubMed Central

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

    2016-01-01

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

  18. Atomic scale verification of oxide-ion vacancy distribution near a single grain boundary in YSZ.

    PubMed

    An, Jihwan; Park, Joong Sun; Koh, Ai Leen; Lee, Hark B; Jung, Hee Joon; Schoonman, Joop; Sinclair, Robert; Gür, Turgut M; Prinz, Fritz B

    2013-01-01

    This study presents atomic scale characterization of grain boundary defect structure in a functional oxide with implications for a wide range of electrochemical and electronic behavior. Indeed, grain boundary engineering can alter transport and kinetic properties by several orders of magnitude. Here we report experimental observation and determination of oxide-ion vacancy concentration near the Σ13 (510)/[001] symmetric tilt grain-boundary of YSZ bicrystal using aberration-corrected TEM operated under negative spherical aberration coefficient imaging condition. We show significant oxygen deficiency due to segregation of oxide-ion vacancies near the grain-boundary core with half-width < 0.6 nm. Electron energy loss spectroscopy measurements with scanning TEM indicated increased oxide-ion vacancy concentration at the grain boundary core. Oxide-ion density distribution near a grain boundary simulated by molecular dynamics corroborated well with experimental results. Such column-by-column quantification of defect concentration in functional materials can provide new insights that may lead to engineered grain boundaries designed for specific functionalities.

  19. Surface Transition on Ice Induced by the Formation of a Grain Boundary

    PubMed Central

    Pedersen, Christian; Mihranyan, Albert; Strømme, Maria

    2011-01-01

    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. PMID:21915317

  20. Atomic Scale Verification of Oxide-Ion Vacancy Distribution near a Single Grain Boundary in YSZ

    PubMed Central

    An, Jihwan; Park, Joong Sun; Koh, Ai Leen; Lee, Hark B.; Jung, Hee Joon; Schoonman, Joop; Sinclair, Robert; Gür, Turgut M.; Prinz, Fritz B.

    2013-01-01

    This study presents atomic scale characterization of grain boundary defect structure in a functional oxide with implications for a wide range of electrochemical and electronic behavior. Indeed, grain boundary engineering can alter transport and kinetic properties by several orders of magnitude. Here we report experimental observation and determination of oxide-ion vacancy concentration near the Σ13 (510)/[001] symmetric tilt grain-boundary of YSZ bicrystal using aberration-corrected TEM operated under negative spherical aberration coefficient imaging condition. We show significant oxygen deficiency due to segregation of oxide-ion vacancies near the grain-boundary core with half-width < 0.6 nm. Electron energy loss spectroscopy measurements with scanning TEM indicated increased oxide-ion vacancy concentration at the grain boundary core. Oxide-ion density distribution near a grain boundary simulated by molecular dynamics corroborated well with experimental results. Such column-by-column quantification of defect concentration in functional materials can provide new insights that may lead to engineered grain boundaries designed for specific functionalities. PMID:24042150

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  2. Statistical model for grain boundary and grain volume oxidation kinetics in UO{sub 2} spent fuel

    SciTech Connect

    Stout, R.B.; Shaw, H.F.; Einziger, R.E.

    1989-09-01

    This paper addresses statistical characteristics for the simplest case of grain boundary/grain volume oxidation kinetics of UO{sub 2} to U{sub 3}O{sub 7} for a fragment of a spent fuel pellet. It also presents a limited discussion of future extensions to this simple case to represent the more complex cases of oxidation kinetics in spent fuels. 17 refs., 1 fig.

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

    SciTech Connect

    Paggi, A.; Angella, G.; Donnini, R.

    2015-09-15

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

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

    NASA Astrophysics Data System (ADS)

    Muller, David

    2012-02-01

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

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

    USGS Publications Warehouse

    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.

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

    SciTech Connect

    Yin, Wenhong; Wang, Weiguo; Fang, Xiaoying; Qin, Congxiang; Xing, Xiaoguang

    2015-09-15

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

  7. Characterization of a ∑9 grain boundary in Fe-45%Cr

    NASA Astrophysics Data System (ADS)

    Miller, M. K.; Jayaram, Raman

    1993-04-01

    An atom probe field ion microscopy characterization has been performed on a ∑9 grain boundary in an Fe-45at%Cr alloy that was aged for 500 h at 500°C. This boundary was decorated with a semicontinuous film of chromium nitride precipitates. The interior of the grains consisted of an ultrafine mixture of chromium-enriched α phase that were produced by spinodal decomposition. The scale of this isotropic α-α' microstructure was found to be constant up to the grain boundary. A 5 to 10 nm thick chromium-depleted zone that was denuded in the α' phase was observed adjacent to the chromium nitride precipitates.

  8. Grain boundary composition and associated hydrogen cracking of modified 4130 steels

    NASA Astrophysics Data System (ADS)

    Craig, Bruce D.

    1984-03-01

    Earlier work on AISI 4130 steels showed that phosphorus segregation to prior austenite grain boundaries was the primary cause for intergranular fracture of these steels when exposed to hydrogen. Reduction of P segregation to grain boundaries by removing the strong segregation couples of Mn-P and Si-P was expected to increase the hydrogen stress cracking resistance of 4130 type steels. Elimination of Mn and/or Si did reduce the concentration of P at prior austenite grain boundaries, but allowed segregation of S and N which acted in the same manner as P, promoting intergranular hydrogen stress cracking.

  9. [Scanning tunnelling microscopy and spectroscopy of ceramic grain boundaries]. [Annual report, September 1992--September 1993

    SciTech Connect

    Not Available

    1993-10-01

    Objective is to study the local geometric and electronic structure at grain boundaries in oxides; this was motivated by the potential to use STM and tunneling spectroscopy on semiconducting ceramics. In order to understand the imaging of low conductivity materials, a number of transition metal oxides were examined: ZnO, TiO{sub 2}. Spatial resolution limits are considered. Conductance profiles across silicon grain boundaries are compared with those across more complex SrTiO{sub 3} grain boundaries. Calculations of space charge in complex oxides are presented. A SEM/STM was constructed which operates in ultrahigh vacuum and has large scale positioning capability (> 1 cm).

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

    SciTech Connect

    Darnbrough, J. E. Flewitt, P. E. J.; Roebuck, B.

    2015-11-14

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

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

    SciTech Connect

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

    1982-08-01

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

  12. Open grain boundaries of quartz as fluid pathways in metamorphic rocks

    NASA Astrophysics Data System (ADS)

    Kruhl, J. H.; Wirth, R.; Morales, L. F. G.

    2012-04-01

    TEM analyses coupled with SEM/FIB sequential imaging of quartz from a metamorphic contact aureole and from greenschist-facies regional metamorphism shows that quartz grain boundaries are partly open on the nanometre scale. Three different types of voids occur: (i) up to 500 nm wide open zones parallel to the grain boundaries, (ii) cavities of variable shape and up to micrometer size along the open grain boundaries, and (iii) cone-shaped, nanometre-sized depressions at sites where dislocation lines meet the open grain boundaries. From animations generated with a pile of 100 pictures it is obvious that in three dimensions the larger cavities are interconnected and form channel-like structures which 'migrate' along the grain boundaries and change in dimensions and shape. Consequently, the partly connected open grain boundaries and cavities form a pathway for fluid percolation. Comparison between the measurements and the results of semi-quantitative modelling indicates that the partially open grain boundaries most probably result (i) from reduction of cell dimensions during cooling below the diffusion threshold of quartz (~300°C; Voll, 1976) and (ii) from the fact that this reduction is anisotropic, i.e., different for different crystallographic directions (Kihara, 1990). Preliminary imaging of phase boundaries of quartz, plagioclase, K-feldspar, amphibole and pyroxene and of grain boundaries in calcite show similar features: several hundred nanometre wide open zones parallel to the boundaries and cavities of variable shape and size, often with euhedral segmentation. In addition, newly-grown, partly euhedral crystalline matter of similar composition as the neighbouring minerals may cover the open grain boundaries. These observations indicate locally strong dissolution-precipitation processes within a connected network of open grain and phase boundaries. All these minerals are common in rocks of the middle and lower continental crust, which partly cooled from higher

  13. Characterization of grain boundary impedances in fine- and coarse-grained CaCu3Ti4O12 ceramics

    NASA Astrophysics Data System (ADS)

    Adams, Timothy B.; Sinclair, Derek C.; West, Anthony R.

    2006-03-01

    The influence of electrode material, dc bias, and pellet thickness on the electrical properties of fine- and coarse-grained CaCu3Ti4O12 (CCTO) ceramics has been investigated using impedance spectroscopy. The low frequency arc observed in Z* plots near room temperature is independent of the electron work function of the metal electrode. It shows significant variation with dc bias and pellet thickness for coarse-grained ceramics, but no such variations for fine-grained ceramics. The results demonstrate the importance of ceramic microstructure in controlling the electrical properties of CCTO ceramics and support the internal barrier layer capacitor (IBLC) model of Schottky barriers at the grain boundaries between semiconducting grains. The IBLC model explains the high permittivity and nonlinear current-voltage characteristics commonly reported for CCTO ceramics.

  14. The role of atomic structure on grain boundary-defect interactions in Cu

    SciTech Connect

    Xian-Ming Bai; Louis J. Vernon; Richard G. Hoagland; Arthur F. Voter; Michael Nastasi; Blas Pedro Uberuaga

    2012-06-01

    We investigate the role that the atomic structure of grain boundaries in Cu has in the interaction with point defects produced during irradiation. We focus on three aspects of defect-boundary interaction: how defects interact with pristine boundaries, how boundaries modify defect production during collision cascades, and how defects interact with damaged boundaries. We find that there are generic features common to most boundaries, including biased absorption of interstitials over vacancies during collision cascades and strong interactions with vacancies for interstitial-loaded boundaries. However, we find that the magnitude of these behaviors depends strongly on the atomic structure of the boundary. In particular, the biased absorption is much stronger for a high angle twist boundary and smallest for a more general twist-asymmetric tilt boundary. Further, the strength of defect-boundary interactions is also sensitive to the boundary structure. We conclude that the sink strength of grain boundaries for interacting with point defects is not an intrinsic property of the boundary but rather depends on the irradiation condition through the absorbed defect content at the boundary.

  15. Analysis of defect structure in silicon. Effect of grain boundary density on carrier mobility in UCP material

    NASA Technical Reports Server (NTRS)

    Dunn, J.; Stringfellow, G. B.; Natesh, R.

    1982-01-01

    The relationships between hole mobility and grain boundary density were studied. Mobility was measured using the van der Pauw technique, and grain boundary density was measured using a quantitative microscopy technique. Mobility was found to decrease with increasing grain boundary density.

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

    NASA Astrophysics Data System (ADS)

    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

  17. Calculation of grain boundary normals directly from 3D microstructure images

    DOE PAGESBeta

    Lieberman, E. J.; Rollett, A. D.; Lebensohn, R. A.; Kober, E. M.

    2015-03-11

    The determination of grain boundary normals is an integral part of the characterization of grain boundaries in polycrystalline materials. These normal vectors are difficult to quantify due to the discretized nature of available microstructure characterization techniques. The most common method to determine grain boundary normals is by generating a surface mesh from an image of the microstructure, but this process can be slow, and is subject to smoothing issues. A new technique is proposed, utilizing first order Cartesian moments of binary indicator functions, to determine grain boundary normals directly from a voxelized microstructure image. In order to validate the accuracymore » of this technique, the surface normals obtained by the proposed method are compared to those generated by a surface meshing algorithm. Specifically, the local divergence between the surface normals obtained by different variants of the proposed technique and those generated from a surface mesh of a synthetic microstructure constructed using a marching cubes algorithm followed by Laplacian smoothing is quantified. Next, surface normals obtained with the proposed method from a measured 3D microstructure image of a Ni polycrystal are used to generate grain boundary character distributions (GBCD) for Σ3 and Σ9 boundaries, and compared to the GBCD generated using a surface mesh obtained from the same image. Finally, the results show that the proposed technique is an efficient and accurate method to determine voxelized fields of grain boundary normals.« less

  18. Calculation of grain boundary normals directly from 3D microstructure images

    SciTech Connect

    Lieberman, E. J.; Rollett, A. D.; Lebensohn, R. A.; Kober, E. M.

    2015-03-11

    The determination of grain boundary normals is an integral part of the characterization of grain boundaries in polycrystalline materials. These normal vectors are difficult to quantify due to the discretized nature of available microstructure characterization techniques. The most common method to determine grain boundary normals is by generating a surface mesh from an image of the microstructure, but this process can be slow, and is subject to smoothing issues. A new technique is proposed, utilizing first order Cartesian moments of binary indicator functions, to determine grain boundary normals directly from a voxelized microstructure image. In order to validate the accuracy of this technique, the surface normals obtained by the proposed method are compared to those generated by a surface meshing algorithm. Specifically, the local divergence between the surface normals obtained by different variants of the proposed technique and those generated from a surface mesh of a synthetic microstructure constructed using a marching cubes algorithm followed by Laplacian smoothing is quantified. Next, surface normals obtained with the proposed method from a measured 3D microstructure image of a Ni polycrystal are used to generate grain boundary character distributions (GBCD) for Σ3 and Σ9 boundaries, and compared to the GBCD generated using a surface mesh obtained from the same image. Finally, the results show that the proposed technique is an efficient and accurate method to determine voxelized fields of grain boundary normals.

  19. Effect of crystallographic orientation on subcritical grain boundary cracking in a conventionally cast polycrystalline nickel-based superalloy.

    PubMed

    Swaminathan, Kameshwaran; Blendell, John E; Trumble, Kevin P

    2013-08-01

    The role of grain orientation and grain boundary misorientation on the formation of subcritical grain boundary cracks in creep of a conventionally cast nickel-based superalloy has been studied. The crystallographic orientations of the grains adjacent to grain boundaries normal to the tensile axis were measured using electron backscattered diffraction. The difference in the Schmid factor for the {111} <112> slip system between the grains was compared to the occurrence of grain boundary cracking. In addition, the difference in the amount of potential primary creep was calculated. The cracked grain boundaries were found to have a larger difference in Schmid factor, as well as a larger difference in potential primary creep, compared with uncracked grain boundaries. PMID:23718929

  20. Effect of crystallographic orientation on subcritical grain boundary cracking in a conventionally cast polycrystalline nickel-based superalloy.

    PubMed

    Swaminathan, Kameshwaran; Blendell, John E; Trumble, Kevin P

    2013-08-01

    The role of grain orientation and grain boundary misorientation on the formation of subcritical grain boundary cracks in creep of a conventionally cast nickel-based superalloy has been studied. The crystallographic orientations of the grains adjacent to grain boundaries normal to the tensile axis were measured using electron backscattered diffraction. The difference in the Schmid factor for the {111} <112> slip system between the grains was compared to the occurrence of grain boundary cracking. In addition, the difference in the amount of potential primary creep was calculated. The cracked grain boundaries were found to have a larger difference in Schmid factor, as well as a larger difference in potential primary creep, compared with uncracked grain boundaries.

  1. Plate-tectonic boundary formation by grain-damage and pinning

    NASA Astrophysics Data System (ADS)

    Bercovici, David

    2015-04-01

    Shear weakening in the lithosphere is an essential ingredient for understanding how and why plate tectonics is generated from mantle convection on terrestrial planets. I present continued work on a theoretical model for lithospheric shear-localization and plate generation through damage, grain evolution and Zener pinning in two-phase (polycrystalline) lithospheric rocks. Grain size evolves through the competition between coarsening, which drives grain-growth, with damage, which drives grain reduction. The interface between phases controls Zener pinning, which impedes grain growth. Damage to the interface enhances the Zener pinning effect, which then reduces grain-size, forcing the rheology into the grain-size-dependent diffusion creep regime. This process thus allows damage and rheological weakening to co-exist, providing a necessary shear-localizing feedback. Moreover, because pinning inhibits grain-growth it promotes shear-zone longevity and plate-boundary inheritance. This theory has been applied recently to the emergence of plate tectonics in the Archean by transient subduction and accumulation of plate boundaries over 1Gyr, as well as to rapid slab detachment and abrupt tectonic changes. New work explores the saturation of interface damage at low interface curvature (e.g., because it is associated with larger grains that take up more of the damage, and/or because interface area is reduced). This effect allows three possible equilibrium grain-sizes for a given stress; a small-grain-size high-shear state in diffusion creep, a large grain-size low shear state in dislocation creep, and an intermediate state (often near the deformation map phase-boundary). The low and high grain-size states are stable, while the intermediate one is unstable. This implies that a material deformed at a given stress can acquire two stable deformation regimes, a low- and high- shear state; these are indicative of plate-like flows, i.e, the coexistence of both slowly deforming plates

  2. Strain heterogeneity and damage nucleation at grain boundaries during monotonic deformation in commercial purity titanium.

    SciTech Connect

    Bieler, T. R.; Crimp, M. A.; Yang, Y.; Wang, L.; Eisenlohr, P.; Mason, D. E.; Liu, W.; Ice, G. E.; Michigan State Univ.; Air Force Office of Scientific Research

    2009-01-01

    Heterogeneous strain was analyzed in polycrystalline, commercial-purity titanium using many experimental techniques that provide information about microstructure, dislocation arrangement, grain orientation, orientation gradients, surface topography, and local strain gradients. The recrystallized microstructure with 50-200 {micro}m grains was extensively characterized before and after deformation using 4-point bending to strains between 2% and 15%. Extremely heterogeneous deformation occurred along some grain boundaries, leading to orientation gradients exceeding 10{sup o} over 10-20 {micro}m. Patches of highly characterized micro-structure were modeled using crystal plasticity finite element (CPFE) analysis to simulate the deformation to evaluate the ability of the CPFE model to capture local deformation processes. Damage nucleation events were identified that are associated with twin interactions with grain boundaries. Progress toward identifying fracture initiation criteria based upon slip and twin interactions with grain boundaries is illustrated with related CPFE simulations of deformation in a TiAl alloy.

  3. Strain Heterogeneity and Damage Nucleation at Grain Boundaries during Monotonic Deformation in Commercial Purity Titanium

    SciTech Connect

    Bieler, T. R.; Crimp, M. A.; Yang, Y.; Eisenlohr, P.; Mason, D. E.; Liu, W.; Ice, Gene E

    2009-01-01

    Heteroeneous strain was analyzed in polycrystalline, commercial-purity titanium using many experimental techniques that provide information about microstructure, dislocation arrangement, grain orientation, orientation gradients, surface topography, and local strain gradients. The recrystallized microstructure with 50-200 ?m grains was extensively characterized before and after deformation using 4-point bending to strains between 2% and 15%. Extremely heterogeneous deformation occurred along some grain boundaries, leading to orientation gradients exceeding 10{sup o} over 10-20 {micro}m. Patches of highly characterized microstructure were modeled using crystal plasticity finite element (CPFE) analysis to simulate the deformation to evaluate the ability of the CPFE model to capture local deformation processes. Damage nucleation events were identified that are associated with twin interactions with grain boundaries. Progress toward identifying fracture initiation criteria based upon slip and twin interactions with grain boundaries is illustrated with related CPFE simulations of deformation in a TiAl alloy.

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

    NASA Technical Reports Server (NTRS)

    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.

  5. Grain boundary mediated leakage current in polycrystalline HfO2 films

    SciTech Connect

    Mckenna, Keith P.; Shluger, AL; Iglesias, V.; Porti, M.; Nafria, M.; Lanza, M.; Bersuker, G.

    2011-07-01

    In this work, we combine conductive atomic force microscopy (CAFM) and first principles calculations to investigate leakage current in thin polycrystalline HfO2 films. A clear correlation between the presence of grain boundaries and increased leakage current through the film is demonstrated. The effect is a result of a number of related factors, including local reduction in the oxide film thickness near grain boundaries, the intrinsic electronic properties of grain boundaries which enhance direct tunnelling relative to the bulk, and segregation of oxygen vacancy defects which increase trap assisted tunnelling currents. These results highlight the important role of grain boundaries in determining the electrical properties of polycrystalline HfO2 films with relevance to applications in advanced logic and memory devices.

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

    PubMed

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

    2015-11-01

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

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

    PubMed

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

    2016-02-19

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

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

    NASA Technical Reports Server (NTRS)

    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.

  9. Improved toughness of refractory compounds. [with elimination of the grain boundary phase

    NASA Technical Reports Server (NTRS)

    Wright, T. R.; Niesz, D. E.

    1974-01-01

    The concept of grain-boundary-engineering through elimination of the grain-boundary silicate phase in silicon nitride was developed. The process involved removal of the silica from the nitride powder via a thermal treatment coupled with the use of nitride additives to compensate the remaining oxygen. Magnesium and aluminum nitrides are found to be the most effective additive for use as oxygen compensators. Strength decreases at elevated temperatures are not observed in the alumina containing material. The creep rate of a dual additive sialon composition was two orders of magnitude lower at 1400 C than commercial silicon nitride. A cursory analysis of the creep mechanism indicate that grain-boundary sliding is avoided through elimination of the grain-boundary silicate phase.

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

    The study of the temperature dependence of tin and antimony segregation at grain boundaries of binary Fe-Sb and Fe-Sn polycrystalline alloys using Auger Electron Spectroscopy suggests that during the brittle intergranular fracture mainly the general grain boundaries are opened in the samples annealed at low temperatures which are more segregated than the special ones. On the contrary, the special grain boundaries are opened in the samples annealed at higher temperatures because - as a consequence of the enthalpy-entropy compensation effect - they are more enriched by the solute. The limiting temperature between these two states is the so-called compensation temperature. Therefore, the temperature dependence of the grain boundary segregation measured in polycrystals should be described by two sets of the thermodynamic parameters (i.e. segregation enthalpy and entropy): one of them correlating the segregation under the compensation temperature, the other one for the segregation at higher temperatures.

  11. Si grain-boundary diffusion in forsterite as a function of pressure, temperature and water content

    NASA Astrophysics Data System (ADS)

    Katsura, T.; Fei, H.; Koizumi, S.; Hiraga, T.; Sakamoto, N.; Hashiguchi, M.; Yurimoto, H.; Yamazaki, D.

    2013-12-01

    In order to estimate the rate of Coble creep of olivine under various regions in the upper mantle, we have measured Si grain-boundary diffusion coefficients of forsterite aggregates. The grain sizes of the aggregates were 0.6 μm in most runs, and 2 μm in some runs to confirm that the Si diffusion occurs in grain boundaries. Measurement for samples without detectable amount of water by FT-IR spectroscopy was conducted at temperatures of 1200 to 1600 K at a pressure of 8 GPa and at pressures of 0 to 13 GPa at a temperature of 1300 K, which gives the activation energy and volume of 240×10 kJ/mol and 1.8×0.2 cm3/mol, respectively. Thus, the activation energy of the grain-boundary diffusion is much smaller than that of the lattice diffusion (415×10 kJ/mol), although its activation volume is identical to that of the lattice diffusion (1.7×0.2 cm3/mol). Measurement for hydrous samples was also conducted at temperatures of 1200 to 1600 K at a pressure of 8 GPa. The water contents in grain boundaries were estimated by subtracting the contribution of the lattice water from the bulk water in FT-IR spectra and normalizing them by the area of grain boundaries. The range of grain-boundary water was up to 130 wt.ppm μm. In this range of grain-boundary water, grain growth during diffusion annealing was very limited, whereas it became prominent above this range, suggesting that the system with the present range of the grain-boundary water content should be free from fluid phases. It was found that the water-content exponent of the Si grain-boundary diffusion is 0.22×0.05, which is even smaller than that of the lattice diffusion (0.32×0.07). These results suggest that the Coble creep rate will have 1) negligible pressure dependence, 2) much smaller temperature dependence than the grain-interior creep (Nabarro-Herring and dislocation creep), and 3) even smaller water content dependence than the grain-interior creep.

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

    PubMed

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

    2011-04-01

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

  13. Crystallization of the glassy phase of grain boundaries in silicon nitride

    NASA Technical Reports Server (NTRS)

    Jefferson, D. A.; Thomas, J. M.; Wen, S.

    1984-01-01

    Three types of hot-pressed silicon nitride specimens (containing 5wt% Y2O3 and 2wt% Al2O3 additives) which were subjected to different temperature heat treatments were studied by X-ray diffraction, X-ray microanalysis and high resolution electron microscopy. The results indicated that there were phase changes in the grain boundaries after heat treatment and the glassy phase at the grain boundaries was crystallized by heat treatment.

  14. Multiscale modeling approach for calculating grain-boundary energies from first principles

    SciTech Connect

    Shenderova, O.A.; Brenner, D.W.; Nazarov, A.A.; Romanov, A.E.; Yang, L.H.

    1998-02-01

    A multiscale modeling approach is proposed for calculating energies of tilt-grain boundaries in covalent materials from first principles over an entire misorientation range for given tilt axes. The method uses energies from density-functional calculations for a few key structures as input into a disclination structural-units model. This approach is demonstrated by calculating energies of {l_angle}001{r_angle}-symmetrical tilt-grain boundaries in diamond. {copyright} {ital 1998} {ital The American Physical Society}

  15. Misoriented grain boundaries vicinal to the ? twin in Nickel part II: thermodynamics of hydrogen segregation

    NASA Astrophysics Data System (ADS)

    O'Brien, Christopher J.; Foiles, Stephen M.

    2016-05-01

    Grain boundary engineered materials are of immense interest for their resistance to hydrogen embrittlement. This work builds on the work undertaken in Part I on the thermodynamic stability and structure of misoriented grain boundaries vicinal to the ?? (coherent-twin) boundary to examine hydrogen segregation to those boundaries. The segregation of hydrogen reflects the asymmetry of the boundary structure with the sense of rotation of the grains about the coherent-twin boundary, and the temperature-dependent structural transition present in one sense of misorientation. This work also finds that the presence of hydrogen affects a change in structure of the boundaries with increasing concentration. The structural change effects only one sense of misorientation and results in the reduction in length of the emitted stacking faults. Moreover, the structural change results in the generation of occupied sites populated by more strongly bound hydrogen. The improved understanding of misoriented twin grain boundary structure and the effect on hydrogen segregation resulting from this work is relevant to higher length-scale models. To that end, we examine commonly used metrics such as free volume and atomic stress at the boundary. Free volume is found not to be useful as a surrogate for predicting the degree of hydrogen segregation, whereas the volumetric virial stress reliably predicts the locations of hydrogen segregation and exclusion at concentrations below saturation or the point where structural changes are induced by increasing hydrogen concentration.

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

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

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

    NASA Astrophysics Data System (ADS)

    Marquardt, Katharina; Dohmen, Ralf; Wagner, Johannes

    2014-05-01

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

  18. Enhancing grain boundary ionic conductivity in mixed ionic–electronic conductors

    PubMed Central

    Lin, Ye; Fang, Shumin; Su, Dong; Brinkman, Kyle S; Chen, Fanglin

    2015-01-01

    Mixed ionic–electronic conductors are widely used in devices for energy conversion and storage. Grain boundaries in these materials have nanoscale spatial dimensions, which can generate substantial resistance to ionic transport due to dopant segregation. Here, we report the concept of targeted phase formation in a Ce0.8Gd0.2O2−δ–CoFe2O4 composite that serves to enhance the grain boundary ionic conductivity. Using transmission electron microscopy and spectroscopy approaches, we probe the grain boundary charge distribution and chemical environments altered by the phase reaction between the two constituents. The formation of an emergent phase successfully avoids segregation of the Gd dopant and depletion of oxygen vacancies at the Ce0.8Gd0.2O2−δ–Ce0.8Gd0.2O2−δ grain boundary. This results in superior grain boundary ionic conductivity as demonstrated by the enhanced oxygen permeation flux. This work illustrates the control of mesoscale level transport properties in mixed ionic–electronic conductor composites through processing induced modifications of the grain boundary defect distribution. PMID:25857355

  19. Grain boundary diffusion of Ag through polycrystalline SiC in TRISO fuel particles

    NASA Astrophysics Data System (ADS)

    Deng, Jie; Ko, Hyunseok; Demkowicz, Paul; Morgan, Dane; Szlufarska, Izabela

    2015-12-01

    The effective diffusivity and release fraction of Ag in polycrystalline SiC are evaluated using a kinetic Monte Carlo model. The effects of various grain boundary network properties on the transport of Ag across the SiC layer have been examined, including fraction of grain boundary type, spread in grain boundary diffusivities and distribution of grain boundary types. It is shown that the effective diffusivity and release fraction of Ag can exhibit a large variability due to changes in the GB structure of SiC, and this variability is almost independent of temperature fluctuation. The present results suggest that the variation in properties of grain boundary networks in SiC may contribute to the spread in the Ag diffusivity and release fraction measured in TRISO particles. It is also found that the grain boundary diffusion alone may be insufficient to account for the Ag diffusivities and release fractions measured in integral release experiments. Additional factors such as irradiation and temperature distribution may also play an important role in Ag transport across the SiC layer.

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

    NASA Astrophysics Data System (ADS)

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

    2016-08-01

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

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

    SciTech Connect

    Palizdar, Y.; San Martin, D.; Ward, M.; Cochrane, R.C.; Brydson, R.; Scott, A.J.

    2013-10-15

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

  2. Enhancing grain boundary ionic conductivity in mixed ionic–electronic conductors

    SciTech Connect

    Lin, Ye; Fang, Shumin; Su, Dong; Brinkman, Kyle S.; Chen, Fanglin

    2015-04-10

    Mixed ionic–electronic conductors are widely used in devices for energy conversion and storage. Grain boundaries in these materials have nanoscale spatial dimensions, which can generate substantial resistance to ionic transport due to dopant segregation. Here, we report the concept of targeted phase formation in a Ce0.8Gd0.2O2₋δ–CoFe2O4 composite that serves to enhance the grain boundary ionic conductivity. Using transmission electron microscopy and spectroscopy approaches, we probe the grain boundary charge distribution and chemical environments altered by the phase reaction between the two constituents. The formation of an emergent phase successfully avoids segregation of the Gd dopant and depletion of oxygen vacancies at the Ce0.8Gd0.2O2₋δ–Ce0.8Gd0.2O2₋δ grain boundary. This results in superior grain boundary ionic conductivity as demonstrated by the enhanced oxygen permeation flux. Lastly, this work illustrates the control of mesoscale level transport properties in mixed ionic–electronic conductor composites through processing induced modifications of the grain boundary defect distribution.

  3. Enhancing grain boundary ionic conductivity in mixed ionic–electronic conductors

    DOE PAGESBeta

    Lin, Ye; Fang, Shumin; Su, Dong; Brinkman, Kyle S.; Chen, Fanglin

    2015-04-10

    Mixed ionic–electronic conductors are widely used in devices for energy conversion and storage. Grain boundaries in these materials have nanoscale spatial dimensions, which can generate substantial resistance to ionic transport due to dopant segregation. Here, we report the concept of targeted phase formation in a Ce0.8Gd0.2O2₋δ–CoFe2O4 composite that serves to enhance the grain boundary ionic conductivity. Using transmission electron microscopy and spectroscopy approaches, we probe the grain boundary charge distribution and chemical environments altered by the phase reaction between the two constituents. The formation of an emergent phase successfully avoids segregation of the Gd dopant and depletion of oxygen vacanciesmore » at the Ce0.8Gd0.2O2₋δ–Ce0.8Gd0.2O2₋δ grain boundary. This results in superior grain boundary ionic conductivity as demonstrated by the enhanced oxygen permeation flux. Lastly, this work illustrates the control of mesoscale level transport properties in mixed ionic–electronic conductor composites through processing induced modifications of the grain boundary defect distribution.« less

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    PubMed Central

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

    2016-01-01

    Tilt-dominated grain boundaries have been investigated in depth in the deformation of MAX phases. In stark contrast, another important type of grain boundaries, twist grain boundaries, have long been overlooked. Here, we report on the observation of small angle twist sub-grain boundaries in a typical MAX phase Ti3AlC2 compressed at 1200 °C, which comprise hexagonal screw dislocation networks formed by basal dislocation reactions. By first-principles investigations on atomic-scale deformation and general stacking fault energy landscapes, it is unequivocally demonstrated that the twist sub-grain boundaries are most likely located between Al and Ti4f (Ti located at the 4f Wyckoff sites of P63/mmc) layers, with breaking of the weakly bonded Al–Ti4f. The twist angle increases with the increase of deformation and is estimated to be around 0.5° for a deformation of 26%. This work may shed light on sub-grain boundaries of MAX phases, and provide fundamental information for future atomic-scale simulations. PMID:27034075

  6. Conducting grain boundaries in the high-dielectric-constant ceramic CaCu3Ti4O12

    NASA Astrophysics Data System (ADS)

    Chen, K.; Li, G. L.; Gao, F.; Liu, J.; Liu, J. M.; Zhu, J. S.

    2007-04-01

    To clarify the electrical property of grain boundaries, the fine-grained ceramics CaCu3Ti4O12 have been treated with the hydrofluoric acid to remove the parts of grain boundaries. The dielectric response difference between the etched samples and the pristine ones indicates that the ceramic CaCu3Ti4O12 consists of insulating or semiconducting grains with conducting grain boundaries. Therefore, the giant dielectric phenomenon is supposed not to derive from the grain boundary barrier layer capacitance effect. The possible mechanism is discussed.

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

    NASA Astrophysics Data System (ADS)

    Borjan, Z.

    2016-09-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

  9. Grain-boundary capacitance of La0.7Ca0.3MnO3 films

    NASA Astrophysics Data System (ADS)

    Glaser, A.; Ziese, M.

    2002-09-01

    The capacitance of grain boundaries formed at step edges in La0.7Ca0.3MnO3 films was measured using impedance spectroscopy. The grain-boundary capacitance decreases with temperature and vanishes in the paramagnetic phase. The grain-boundary region is modeled as an abrupt metal-semiconductor contact inducing a depletion layer; the built in voltage is expected to be caused by chemical potential shifts due to differences in the ferromagnetic order in the grains and the grain boundaries. This model indicates that the capacitance vanishes with the second power of the difference between grain and grain-boundary magnetization; the data indicate that the capacitance follows more closely the grain-boundary magnetization as determined from the magnetoresistance.

  10. Grain boundary fine structure of ultrananocrystalline diamond thin films measured by Raman scattering

    NASA Astrophysics Data System (ADS)

    Veres, M.; Tóth, S.; Koós, M.

    2007-07-01

    Structural units of the grain boundaries in ultrananocrystalline diamond thin films with different grain sizes were investigated using Raman spectroscopy. Characteristic peaks of well-defined molecular structural building blocks were detected in the near-infrared excited Raman spectra of these materials by limiting the excitation volume to the size of the crystallites using an optical microscope and surface enhanced Raman spectroscopy. The analysis of the spectra provides evidence for the presence of aromatic hydrocarbons and different sp3 CHx groups in grain boundaries of these materials.

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

    SciTech Connect

    Valderrama, Billy; He, Lingfeng; Henderson, Hunter B.; Pakarinen, Janne; Jaques, Brian; Gan, Jian; Butt, Darryl P.; Allen, Todd R.; Manuel, Michele V.

    2014-11-01

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

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

    NASA Astrophysics Data System (ADS)

    Valderrama, Billy; He, Lingfeng; Henderson, Hunter B.; Pakarinen, Janne; Jaques, Brian; Gan, Jian; Butt, Darryl P.; Allen, Todd R.; Manuel, Michele V.

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

  14. Grain boundaries as reservoirs of incompatible elements in the Earth's mantle.

    PubMed

    Hiraga, Takehiko; Anderson, Ian M; Kohlstedt, David L

    2004-02-19

    The concentrations and locations of elements that strongly partition into the fluid phase in rocks provide essential constraints on geochemical and geodynamical processes in Earth's interior. A fundamental question remains, however, as to where these incompatible elements reside before formation of the fluid phase. Here we show that partitioning of calcium between the grain interiors and grain boundaries of olivine in natural and synthetic olivine-rich aggregates follows a thermodynamic model for equilibrium grain-boundary segregation. The model predicts that grain boundaries can be the primary storage sites for elements with large ionic radius--that is, incompatible elements in the Earth's mantle. This observation provides a mechanism for the selective extraction of these elements and gives a framework for interpreting geochemical signatures in mantle rocks.

  15. Effect of Grain Boundary Character Distribution on the Adiabatic Shear Susceptibility

    NASA Astrophysics Data System (ADS)

    Yang, Yang; Jiang, Lihong; Luo, Shuhong; Hu, Haibo; Tang, Tiegang; Zhang, Qingming

    2016-11-01

    The adiabatic shear susceptibility of AISI321 stainless steels with different grain boundary character distributions (GBCDs) was investigated by means of split-Hopkinson pressure bar. The results indicate that the width of the adiabatic shear band of the specimen after thermomechanical processing (TMP) treatment is narrower. The comparison of the stress collapse time, the critical stress, and the adiabatic shear forming energy suggests that the TMP specimens have lower adiabatic shear susceptibility than that of the solution-treated samples under the same loading condition. GBCD and grain size affected the adiabatic shear susceptibility. The high-angle boundary network of the TMP specimens was interrupted or replaced by the special grain boundary, and smaller grain size hindered the adiabatic shearing.

  16. A combined experimental-numerical approach for elasto-plastic fracture of individual grain boundaries

    NASA Astrophysics Data System (ADS)

    Kupka, D.; Huber, N.; Lilleodden, E. T.

    2014-03-01

    The parameters for a crystal plasticity finite element constitutive law were calibrated for the aluminum-lithium alloy 2198 using micro-column compression testing on single crystalline volumes. The calibrated material model was applied to simulations of micro-cantilever deflection tests designed for micro-fracture experiments on single grain boundaries. It was shown that the load-displacement response and the local deformation of the grains, which was measured by digital image correlation, were predicted by the simulations. The fracture properties of individual grain boundaries were then determined in terms of a traction-separation-law associated with a cohesive zone. This combination of experiments and crystal plasticity finite element simulations allows the investigation of the fracture behavior of individual grain boundaries in plastically deforming metals.

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

    NASA Astrophysics Data System (ADS)

    Andersson, David A.; Tonks, Michael R.; Casillas, Luis; Vyas, Shyam; Nerikar, Pankaj; Uberuaga, Blas P.; Stanek, Christopher R.

    2015-07-01

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

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

    SciTech Connect

    Andersson, David A.; Tonks, Michael R.; Casillas, Luis; Vyas, Shyam; Nerikar, Pankaj; Uberuaga, Blas P.; Stanek, Christopher R.

    2015-07-01

    In light water reactor fuel, gaseous fission products segregate to grain boundaries, resulting in the nucleation and growth of large intergranular fission gas bubbles. The segregation rate is controlled by diffusion of fission gas atoms through the grains and interaction with the boundaries. Based on the mechanisms established from earlier density functional theory (DFT) and empirical potential calculations, diffusion models for xenon (Xe), uranium (U) vacancies and U interstitials in UO₂ have been derived for both intrinsic (no irradiation) and irradiation conditions. Segregation of Xe to grain boundaries is described by combining the bulk diffusion model with a model for the interaction between Xe atoms and three different grain boundaries in UO₂ (Σ5 tilt, Σ5 twist and a high angle random boundary), as derived from atomistic calculations. The present model does not attempt to capture nucleation or growth of fission gas bubbles at the grain boundaries. The point defect and Xe diffusion and segregation models are implemented in the MARMOT phase field code, which is used to calculate effective Xe and U diffusivities as well as to simulate Xe redistribution for a few simple microstructures.

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

    DOE PAGESBeta

    Andersson, David A.; Tonks, Michael R.; Casillas, Luis; Vyas, Shyam; Nerikar, Pankaj; Uberuaga, Blas P.; Stanek, Christopher R.

    2015-07-01

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

  20. Computer simulation on surfaces and (001) symmetric tilt grain boundaries in Ni, Al, and Ni/sub 3/Al

    SciTech Connect

    Chen, S.P.; Srolovitz, D.J.; Voter, A.F.

    1989-01-01

    We have used ''local volume'' (embedded atom) type potentials to study the surfaces and grain boundaries of Ni, Al, and Ni/sub 3/Al. The simulations show that with appropriately fit potentials, the surface and grain boundnary structure can be realistically calculated. The surface rippling and relaxation show good agreement with experiments. The energies of most surfaces and grain boundaries also agree with existing data. The structural unit model for grain boundaries in Ni/sub 3/Al shows the same generic units as in pure metals, but with large variations due to distortions and multiplicity. The utility of the structural unit model is thus more limited for alloys. The grain boundary energies were found to be the highest for Al-rich Ni/sub 3/Al grain boundaries, and depend significantly on the local composition of the grain boundary. The cusps in the grain boundary energy as a function of misorientation angle are different for different grain boundary stoichiometries. The Ni/sub 3/Al grain boundaries have approximately the same grain boundary energy and cohesive energy as that of Ni.

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

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

    DOE PAGESBeta

    Wang, J.; Du, A.; Yang, Di; Raj, R.; Conrad, H.

    2013-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  4. Efficient Annealing of Radiation Damage Near Grain Boundaries via Interstitial Emission

    SciTech Connect

    Bai, X. M.; Voter, A. F.; Hoagland, R. G.; Nastasi, Michael; Uberuaga, B. P.

    2010-03-25

    Although grain boundaries can serve as effective sinks for radiation-induced defects such as interstitials and vacancies, the atomistic mechanisms leading to this enhanced tolerance are still not well understood. With the use of three atomistic simulation methods, we investigated defect–grain boundary interaction mechanisms in copper from picosecond to microsecond time scales. We found that grain boundaries have a surprising “loading-unloading” effect. Upon irradiation, interstitials are loaded into the boundary, which then acts as a source, emitting interstitials to annihilate vacancies in the bulk. This unexpected recombination mechanism has a much lower energy barrier than conventional vacancy diffusion and is efficient for annihilating immobile vacancies in the nearby bulk, resulting in self-healing of the radiation-induced damage.

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

    SciTech Connect

    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.

  6. On the structure, stress fields and energy of nonequilibrium grain boundaries

    SciTech Connect

    Nazarov, A.A.; Valiev, R.Z. . Inst. for Metals Superplasticity Problems); Romanov, A.E. . A.F. Ioffe Physico-Technical Inst.)

    1993-04-01

    The changes of grain boundary (GB) structure caused by the absorption of lattice dislocations are analyzed. The typical result of the absorption is the disordering of networks of disclination dipoles/grain boundary dislocations (GBDs). It is shown that the variance of GBD spacings is a good quantitative measure of the degree of GB structure nonequilibrium. By the use of Monte Carlo technique the abating law of x[sup [minus]1/2] for long range stress fields of disordered GBD networks is obtained. Excess energies of nonequilibrium boundaries are also calculated. In strongly exited state of GBs characterized by the maximum value of the variance of GBD spacings the excess energy can exceed the equilibrium GB energy. Such high-nonequilibrium boundaries can exist in ultrafine grained materials.

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

    NASA Astrophysics Data System (ADS)

    Fensin, Saryu Jindal

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

  8. Modelling of porphyroclasts in simple shear and the role of stress variations at grain boundaries

    NASA Astrophysics Data System (ADS)

    Wilson, Christopher J. L.; Evans, Lynn; Delle Piane, Claudio

    2009-11-01

    Grain-scale numerical experiments involving simple shear of a two-phase non-linear viscous material are described and compared with mineral fish or lozenge-shaped porphyroclasts, such as muscovite. Two types of 2D models are considered; either a single elongate grain or two parallel elongate grains, in both cases supported by a weaker polygonal grain matrix. The relative viscosities of the contrasting grain structures were systematically varied, allowing us to observe the effects of non-linear viscous rheology on the resulting microstructure and flow patterns. The results show that the finite rotation of the hard elongate grain was similar within any one experiment, but was largely influenced by viscosity contrast, the geometry of the model and the imposed shear strain. Models involving single elongate hard grains show increasing instability at their ends and less strain compatibility with the deforming matrix grains, as the viscosity contrast is increased. In the paired grain models the greatest variation in the matrix grain microstructure is seen in the region where the two hard grains are oriented at a high-angle to the direction of shear. Finally, we consider the changes in intragranular stress by comparing microstructural observations using different viscosities with the distribution of stress in space and during progressive shear. In the plane approximately parallel to the maximum principal stress direction ( σ1), a localised change of stress occurs across and along the interface between the hard and soft grains. Variations in the mean stress at these boundaries are directly attributable to changes in the minimum principal stress. We propose that with shear strains greater than γ = 2 it is the minimum principal stress that can control diffusion processes at the grain boundary rather than mean stress. In conclusion we suggest that our models have the potential for providing useful insights into why metamorphic reactions can occur at the interface between a

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

    NASA Astrophysics Data System (ADS)

    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

  10. Non-planar grain boundary structures in fcc metals and their role in nano-scale deformation mechanisms

    NASA Astrophysics Data System (ADS)

    Smith, Laura; Farkas, Diana

    2014-01-01

    This work presents the results of a comparative molecular dynamics study showing that relaxed random grain boundary structures can be significantly non-planar at the nano-scale in fcc metals characterized by low stacking fault values. We studied the relaxed structures of random [1 1 0] tilt boundaries in a polycrystal using interatomic potentials describing Cu and Pd. Grain boundaries presenting non-planar features were observed predominantly for the Cu potential but not for the Pd potential, and we relate these differences to the stacking fault values. We also show that these non-planar structures can have a strong influence on dislocation emission from the grain boundaries as well as on grain boundary strain accommodation processes, such as grain boundary sliding. We studied the loading response in polycrystals of 40 nm grain size to a level of 9% strain and found that the non-planar grain boundaries favour dislocation emission as a deformation mechanism and hinder grain boundary sliding. This has strong implications for the mechanical behaviour of nano-crystalline materials, which is determined by the competition between dislocation activity and grain boundary accommodation of the strain. Thus, the two interatomic potentials for Cu and Pd considered in this work resulted in the same overall stress-strain curve, but significantly different fractions of the strain accommodated by the intergranular versus intragranular deformation mechanisms. Strain localization patterns are also influenced by the non-planarity of the grain boundary structures.

  11. Grain boundary character modification employing thermo-mechanical processing in type 304L stainless steel

    NASA Astrophysics Data System (ADS)

    Pradhan, S. K.; Mandal, S.

    2016-02-01

    Grain boundary engineering (GBE) approach has been employed to modify the boundaries character of a type 304L stainless steel through thermo-mechanical processing (TMP) route, which combined a low level of cold deformation (5, 10 and 15%) followed by annealing at 1173K and 1273K for 1hour. Employing Electron Back Scatter Diffraction based Orientation Imaging Microscopy, the fraction and distribution of low ∑ CSL boundaries (∑≤ 29) and its effect on random high-angle grain boundaries connectivity and triple junction distribution of as-received (AR) and GBE specimens were evaluated. It was possible to increase the fraction of low ∑ CSL boundaries up to 75% following GBE treatment (as compared to 50% in AR specimen). The GBE specimens also contained maximum number of percolation resistant triple junctions which could render better resistance against percolation related phenomena.

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  13. Investigation of grain-boundary geometry and pores morphology in dense and porous cubic zirconia polycrystals

    SciTech Connect

    Bobrowski, Piotr; Faryna, Marek; Pędzich, Zbigniew

    2014-09-15

    Highlights: • Cubic zirconia sinters were investigated in three dimensions using dual-beam FEGSEM. • The 3D-EBSD technique was successfully applied to non-conductive ceramics. • New sample preparation approach to automated 3D-EBSD was proposed. • Grain boundary microstructures were reconstructed from inverse pole figure maps. • Pore microstructures were reconstructed from image quality maps. - Abstract: Three-dimensional electron backscatter diffraction technique was used for the visualization of grain boundary geometry and pore morphology in cubic zirconia. A set of four samples sintered under different conditions was investigated. Specimens which were characterized by energy dispersive spectroscopy and X-ray diffraction were entirely composed of cubic phase. Investigations of boundaries and pore structures were carried out in a dual-beam scanning electron microscope. For each sample, a volume of 1000 μm{sup 3} was investigated. The analysis of grain boundary networks reconstructed from inverse pole figure maps revealed a strong dependence between grain boundary density and sample preparation parameters. Sintering also affects the size and distribution of pores. The total number of grains analyzed varied from 17 to 357 and the calculated volume of cavities from 0.01% to 21%. This paper shows the application of three-dimensional crystallographic orientation analysis to characterize the microstructure of yttria stabilized zirconia ceramics.

  14. Testing thermal gradient driving force for grain boundary migration using molecular dynamics simulations

    SciTech Connect

    Bai, Xian-Ming; Zhang, Yongfeng; Tonks, Michael R.

    2015-02-01

    Strong thermal gradients in low-thermal-conductivity ceramics may drive extended defects, such as grain boundaries and voids, to migrate in preferential directions. In this work, molecular dynamics simulations are conducted to study thermal gradient driven grain boundary migration and to verify a previously proposed thermal gradient driving force equation, using uranium dioxide as a model system. It is found that a thermal gradient drives grain boundaries to migrate up the gradient and the migration velocity increases under a constant gradient owing to the increase in mobility with temperature. Different grain boundaries migrate at very different rates due to their different intrinsic mobilities. The extracted mobilities from the thermal gradient driven simulations are compared with those calculated from two other well-established methods and good agreement between the three different methods is found, demonstrating that the theoretical equation of the thermal gradient driving force is valid, although a correction of one input parameter should be made. The discrepancy in the grain boundary mobilities between modeling and experiments is also discussed.

  15. Band gap widening at random CIGS grain boundary detected by valence electron energy loss spectroscopy

    NASA Astrophysics Data System (ADS)

    Keller, Debora; Buecheler, Stephan; Reinhard, Patrick; Pianezzi, Fabian; Bissig, Benjamin; Carron, Romain; Hage, Fredrik; Ramasse, Quentin; Erni, Rolf; Tiwari, Ayodhya N.

    2016-10-01

    Cu(In,Ga) Se2 (CIGS) thin film solar cells have demonstrated very high efficiencies, but still the role of nanoscale inhomogeneities in CIGS and their impact on the solar cell performance are not yet clearly understood. Due to the polycrystalline structure of CIGS, grain boundaries are very common structural defects that are also accompanied by compositional variations. In this work, we apply valence electron energy loss spectroscopy in scanning transmission electron microscopy to study the local band gap energy at a grain boundary in the CIGS absorber layer. Based on this example, we demonstrate the capabilities of a 2nd generation monochromator that provides a very high energy resolution and allows for directly relating the chemical composition and the band gap energy across the grain boundary. A band gap widening of about 20 meV is observed at the grain boundary. Furthermore, the compositional analysis by core-loss EELS reveals an enrichment of In together with a Cu, Ga and Se depletion at the same area. The experimentally obtained results can therefore be well explained by the presence of a valence band barrier at the grain boundary.

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

    NASA Astrophysics Data System (ADS)

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

    2015-03-01

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

  17. Xe precipitates at grain boundaries in Al under 1 MeV electron irradiation.

    SciTech Connect

    Allen, C. W.

    1998-10-23

    Crystalline nanoprecipitates of Xe have been produced by ion implantation into mazed bicrystalline Al at 300 K, in which the matrix grain boundaries are mainly 90 deg tilt boundaries. Within Al grains, Xe nanocrystals are fee, isotactic with the Al and cuboctohedral in shape with {l_brace}111{r_brace} and {l_brace}100{r_brace} facets. With an off-axial imaging technique, the nanocrystals were structure imaged against a relatively featureless matrix background. In contrast to metal precipitates in Al, such as Pb, Xe precipitates straddling a matrix grain boundary are bicrystals as small as approximately 2 nm in diameter. Larger Xe precipitates tend to avoid boundaries which are inclined away from asymmetrical orientation and which thus have a significant twist component. Under the 1 MeV electron irradiation employed for HREM observation, small Xe nanocrystals near a grain boundary may migrate to the boundary and coalesce with other Xe precipitates. The structural observations are rationalized on a simple geometrical interpretation.

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

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

    PubMed Central

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

    2016-01-01

    In the present study, a coarse grained AZ31 plate was refined by twin boundaries (TBs) and grain boundaries (GBs), respectively. A comparative study about the different effects of grain refinements by GBs and by TBs on tension-compression yield asymmetry was performed. Our results show that both the refinements by GBs and by TBs increase the tensile and compressive yield strengths, but to a different degree. TBs are more effective to harden twinning, but yield a lower strengthening against prismatic slip, and a much lower tension-compression yield asymmetry is thus obtained. Both the differences in boundary coherence and misorientation between GBs and TBs affect the hardening. The misorientation of TBs provides a lower geometric compatibility factor (a higher hardening) for both prismatic slip and twinning than that of GBs, which in detail is the result of the much higher angle between c-axes of the two sides of TBs (about 86°) than GBs (0–50°). It is found that, for hardening of prismatic slip, boundary coherence plays a more important role than misorientation. With regard to twinning, the different misorientation of TBs from GBs mainly accounts for their different hardening effects. PMID:27375280

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

    PubMed

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

    2016-01-01

    In the present study, a coarse grained AZ31 plate was refined by twin boundaries (TBs) and grain boundaries (GBs), respectively. A comparative study about the different effects of grain refinements by GBs and by TBs on tension-compression yield asymmetry was performed. Our results show that both the refinements by GBs and by TBs increase the tensile and compressive yield strengths, but to a different degree. TBs are more effective to harden twinning, but yield a lower strengthening against prismatic slip, and a much lower tension-compression yield asymmetry is thus obtained. Both the differences in boundary coherence and misorientation between GBs and TBs affect the hardening. The misorientation of TBs provides a lower geometric compatibility factor (a higher hardening) for both prismatic slip and twinning than that of GBs, which in detail is the result of the much higher angle between c-axes of the two sides of TBs (about 86°) than GBs (0-50°). It is found that, for hardening of prismatic slip, boundary coherence plays a more important role than misorientation. With regard to twinning, the different misorientation of TBs from GBs mainly accounts for their different hardening effects. PMID:27375280

  1. A search for shocked quartz grains in the Allerød-Younger Dryas boundary layer

    NASA Astrophysics Data System (ADS)

    Hoesel, Annelies; Hoek, Wim Z.; Pennock, Gillian M.; Kaiser, Knut; Plümper, Oliver; Jankowski, Michal; Hamers, Maartje F.; Schlaak, Norbert; Küster, Mathias; Andronikov, Alexander V.; Drury, Martyn R.

    2015-03-01

    The Younger Dryas impact hypothesis suggests that multiple airbursts or extraterrestrial impacts occurring at the end of the Allerød interstadial resulted in the Younger Dryas cold period. So far, no reproducible, diagnostic evidence has, however, been reported. Quartz grains containing planar deformation features (known as shocked quartz grains), are considered a reliable indicator for the occurrence of an extraterrestrial impact when found in a geological setting. Although alleged shocked quartz grains have been reported at a possible Allerød-Younger Dryas boundary layer in Venezuela, the identification of shocked quartz in this layer is ambiguous. To test whether shocked quartz is indeed present in the proposed impact layer, we investigated the quartz fraction of multiple Allerød-Younger Dryas boundary layers from Europe and North America, where proposed impact markers have been reported. Grains were analyzed using a combination of light and electron microscopy techniques. All samples contained a variable amount of quartz grains with (sub)planar microstructures, often tectonic deformation lamellae. A total of one quartz grain containing planar deformation features was found in our samples. This shocked quartz grain comes from the Usselo palaeosol at Geldrop Aalsterhut, the Netherlands. Scanning electron microscopy cathodoluminescence imaging and transmission electron microscopy imaging, however, show that the planar deformation features in this grain are healed and thus likely to be older than the Allerød-Younger Dryas boundary. We suggest that this grain was possibly eroded from an older crater or distal ejecta layer and later redeposited in the European sandbelt. The single shocked quartz grain at this moment thus cannot be used to support the Younger Dryas impact hypothesis.

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

    NASA Astrophysics Data System (ADS)

    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

  3. Grain boundary sliding behaviour of copper and alpha brass at intermediate temperatures

    NASA Technical Reports Server (NTRS)

    Raj, S. V.

    1991-01-01

    The role of grain boundary sliding in copper and Cu-30 pct Zn in the temperature range 0.50-0.72 Tm, where Tm is the absolute melting point of the material, is examined. First, sliding data obtained on these materials are presented. The results indicate that the stress exponent for sliding is similar to that for lattice deformation, while the activation energy for sliding varies between 0.5 and 1.6 of the activation energy for creep. Several models proposed for grain boundary sliding are discussed, and it is shown that they do not account for the observed results on copper and alpha brass. A phenomenological model is proposed, where it is assumed that grain boundary sliding results from the glide of dislocations on secondary slip planes.

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

    SciTech Connect

    Andersson, Anders D.; Tonks, Michael; Casillas, Luis; Millett, Paul; Vyas, Shyam; Uberuaga, Blas P.; Nerikar, Pankaj

    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.

  5. Dislocation motion and grain boundary migration in two-dimensional tungsten disulphide.

    PubMed

    Azizi, Amin; Zou, Xiaolong; Ercius, Peter; Zhang, Zhuhua; Elías, Ana Laura; Perea-López, Néstor; Stone, Greg; Terrones, Mauricio; Yakobson, Boris I; Alem, Nasim

    2014-01-01

    Dislocations have a significant effect on mechanical, electronic, magnetic and optical properties of crystals. For a dislocation to migrate in bulk crystals, collective and simultaneous movement of several atoms is needed. In two-dimensional crystals, in contrast, dislocations occur on the surface and can exhibit unique migration dynamics. Dislocation migration has recently been studied in graphene, but no studies have been reported on dislocation dynamics for two-dimensional transition metal dichalcogenides with unique metal-ligand bonding and a three-atom thickness. This study presents dislocation motion, glide and climb, leading to grain boundary migration in a tungsten disulphide monolayer. Direct atomic-scale imaging coupled with atomistic simulations reveals a strikingly low-energy barrier for glide, leading to significant grain boundary reconstruction in tungsten disulphide. The observed dynamics are unique and different from those reported for graphene. Through strain field mapping, we also demonstrate how dislocations introduce considerable strain along the grain boundaries and at the dislocation cores.

  6. Characterization of grain boundaries in multicrystalline silicon with high lateral resolution using conductive atomic force microscopy

    NASA Astrophysics Data System (ADS)

    Rumler, Maximilian; Rommel, Mathias; Erlekampf, Jürgen; Azizi, Maral; Geiger, Tobias; Bauer, Anton J.; Meißner, Elke; Frey, Lothar

    2012-08-01

    In this work, the electrical characteristics of grain boundaries (GBs) in multicrystalline silicon with and without iron contamination are analyzed by fixed voltage current maps and local I/V curves using conductive AFM (cAFM). I/V characteristics reveal the formation of a Schottky contact between the AFM tip and the sample surface. The impact of both, the polarity of the applied voltage and the illumination by the AFM laser on the behavior of GBs was analyzed systematically. Depending on the polarity of the applied voltage and the iron content of the sample, grain boundaries alter significantly the recorded current flow compared to the surrounding material. The results also show a clear influence of the AFM laser light on the electrical behavior of the grain boundaries. Conductive AFM measurements are furthermore compared to data obtained by electron beam induced current (EBIC), indicating that cAFM provides complimentary information.

  7. Three dimensional calculations of the effective Kapitza resistance of UO2 grain boundaries containing intergranular bubbles

    NASA Astrophysics Data System (ADS)

    Millett, Paul C.; Tonks, Michael R.; Chockalingam, K.; Zhang, Yongfeng; Biner, S. B.

    2013-08-01

    A parametric study has been performed that quantifies the effective change in grain boundary Kapitza resistance due to the presence of intergranular bubbles. The steady-state heat conduction equation was solved in three-dimensional space using INL's MOOSE finite element software, with which spacial mesh adaptivity was used to resolve interfacial widths down to several nanometers while investigating bubble sizes up to a micrometer. Three critical parameters were systematically varied: the intergranular bubble radius, the fractional grain boundary bubble coverage, and the Kapitza resistance of the intact grain boundary. Using the simulation results, a mathematical model dependent on each of these parameters was developed to describe the effective Kapitza resistance. Furthermore, we illustrate how this model can be implemented in a fuel performance code to predict the temperature profile of a cylindrical fuel pellet.

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

  9. Grain-boundary component in W-Ga composites: A way towards skeleton structures

    NASA Astrophysics Data System (ADS)

    Krauss, W.; Gleiter, H.

    1998-11-01

    Nanostructured materials consist of crystalline and grain-boundary components. In the simplest case, both components are chemically identical. Here, we present the results of a study of a system consisting of a crystalline component built by A atoms (tungsten) and a grain boundary component of B atoms (gallium). Within this system, component B is in a disordered state. Most likely, it exhibits an amorphouslike structure, and coats the tungsten crystals uniformly with a constant thickness, thus forming a Ga skeleton structure. The noncrystalline gallium seems to undergo no first-order structural phase transitions, e.g., no first-order melting transition was noted when the composite was below, at, or above the equilibrium melting point of Ga. The properties of gallium as a grain-boundary component differ significantly from those of crystalline and amorphous bulk gallium.

  10. Critical-current diffraction patterns of grain-boundary Josephson weak links

    SciTech Connect

    Peterson, R.L.; Ekin, J.W. )

    1990-11-01

    We discuss the diffraction patterns and other characteristics of the critical current as a function of magnetic field in grain-boundary Josephson barriers. Diffraction patterns occur not just for {ital SIS} junctions but for all types of Josephson links, including {ital SNS} junctions, which may be present at grain boundaries in high-{Tc} superconductors. We discuss the generality of the Airy diffraction pattern, which is expected to characterize grain-boundary barriers in bulk material more accurately than the Fraunhofer pattern. The transport critical-current density in many bulk, granular high-{ital T}{sub {ital c}} superconductors has a power-law dependence on very low magnetic fields, characteristic of averaged diffraction patterns, and cannot be fitted by an exponential magnetic-field dependence, which may result from the material properties of the barriers.

  11. Grain boundary Josephson devices by YBaCuO films and 77K operations

    SciTech Connect

    Yamashita, T.; Kawakami, A.; Noge, S.; Xu, W.; Takata, M.; Komatsu, T.; Matusita, K.

    1989-03-01

    Magnetron sputtering and screen printing methods were used to fabricate YBaCuO films with thickness of 5 to 30..mu..m. The annealing of the films at 1000/sup 0/C gave abnormal grain growth. The grain size of the films was about 20 to 70..mu..m. With photolithography and razor cutting techniques, the films were formed to bridge type Josephson junctions having a few grain boundaries in the bridge regions. In the devices, clear Shapiro steps and SQUID patterns were observed at 77K. The experiments show that all currents flowing through grain boundaries are Josephson currents in YBaCuO polycrystalline films. Clear Josephson effects were observed in about 30% of the fabricated devices. The device may hae high potentials for high frequency detectors and SQUID flux sensors operative at 77K.

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

    SciTech Connect

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

    1999-11-29

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  14. Origin of recombination activity at small angle grain boundaries in multicrystalline silicon using multi-seed casting growth method

    NASA Astrophysics Data System (ADS)

    Kojima, Takuto; Tachibana, Tomihisa; Ohshita, Yoshio; Prakash, Ronit R.; Sekiguchi, Takashi; Yamaguchi, Masafumi

    2015-08-01

    The effect of misorientation on the recombination activity of tilt small angle grain boundaries was studied by temperature-dependent electron beam induced current (EBIC) analyses of artificially induced grain boundaries in multicrystal grown by casting from multiple seeds. For small misorientation, there is no significant difference in the recombination of grain boundaries at the middle of a grown ingot, whereas moderate contamination on grain boundaries caused changes in the EBIC contrast, especially at room temperature. The EBIC contrast of moderately contaminated grain boundaries at room temperature has a peak at a misorientation of ∼3°, and for misorientation θ > 6°, the recombinative nature diminishes with increasing misorientation. The results indicate differences in the gettering ability of small angle grain boundaries.

  15. Computer Modeling of Transport of Oxidizing Species in Grain Boundaries during Zirconium Corrosion

    SciTech Connect

    Xian-Ming Bai; Yongfeng Zhang; Michael R. Tonks

    2014-06-01

    Zirconium (Zr) based alloys are widely used as the cladding materials in light-water reactors. The water-side corrosion of these alloys degrades their structural integrity and poses serious safety concerns. During the Zr corrosion process, a thin Zr oxide (ZrO2) layer forms on the alloy surface and serves as a barrier layer for further corrosion. The majority of the oxide has the monoclinic phase. At the transition region between the oxide and the metal, the oxide contains a thin layer of stabilized tetragonal phase. It is found that the texture of the tetragonal layer determines the protectiveness of the oxide for corrosion. The transport of oxidizing species, such as anion defects, cation defects, and electron through the tetragonal oxide layer could be the rate limiting step of the corrosion. The defect diffusion can be affected by the growing stresses and microstructures such as grain boundaries and dislocations. In this work molecular dynamics simulations are used to investigate the anion and cation diffusion in bulk and at grain boundaries in tetragonal ZrO2. The results show that defect diffusion at grain boundaries is complex and the behavior strongly depends on the grain boundary type. For most of the grain boundaries studied the defect diffusion are much slower than in the bulk, implying that grain boundaries may not be fast defect transport paths during corrosion. The connection between the modeling results and published experimental work will also be discussed. This work is funded by the Laboratory Directed Research and Development (LDRD) program at Idaho National Laboratory.

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

    SciTech Connect

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

    1989-02-01

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

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

    NASA Astrophysics Data System (ADS)

    Detrois, Martin

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

  18. Relations and interactions between twinning and grain boundaries in hexagonal close-packed structures

    NASA Astrophysics Data System (ADS)

    Barrett, Christopher Duncan

    Improving the formability and crashworthiness of wrought magnesium alloys are the two biggest challenges in current magnesium technology. Magnesium is the best material candidate for enabling required improvements in fuel economy of combustion engines and increases in ranges of electric vehicles. In hexagonal closed-packed (HCP) structures, effects of grain size/morphology and crystallographic texture are particularly important. Prior research has established a general understanding of the dependences of strength and strain anisotropy on grain morphology and texture. Unfortunately, deformation, recrystallization, and grain growth strategies that control the microstructures and textures of cubic metals and alloys have not generally worked for HCPs. For example, in Magnesium, the deformation texture induced by primary forming operations (rolling, extrusion, etc.) is not randomized by recrystallization and may strengthen during grain growth. A strong texture reduces formability during secondary forming (stamping, bending, hemming etc.) Thus, the inability to randomize texture has impeded the implementation of magnesium alloys in engineering applications. When rare earth solutes are added to magnesium alloys, distinct new textures are derived. However, `rare earth texture' derivation remains insufficiently explained. Currently, it is hypothesized that unknown mechanisms of alloy processing are at work, arising from the effects of grain boundary intrinsic defect structures on microstructural evolution. This dissertation is a comprehensive attempt to identify formal methodologies of analyzing the behavior of grain boundaries in magnesium. We focus particularly on twin boundaries and asymmetric tilt grain boundaries using molecular dynamics. We begin by exploring twin nucleation in magnesium single crystals, elucidating effects of heterogeneities on twin nucleation and their relationships with concurrent slip. These efforts highlighted the necessity of imperfections to

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

    NASA Astrophysics Data System (ADS)

    Hua, Chengyun; Minnich, Austin J.

    2014-12-01

    Nanocrystalline silicon and silicon-germanium alloys are promising thermoelectric (TE) materials that have achieved substantially improved figure of merits compared to their bulk counterparts. This enhancement is typically attributed to a reduction in lattice thermal conductivity by phonon scattering at grain boundaries. However, further improvements are difficult to achieve because grain boundary scattering is poorly understood, with recent experimental observations suggesting that the phonon transmissivity may depend on phonon frequency rather than being constant as in the commonly used gray model. Here, we examine the impact of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium alloys in a realistic 3D geometry using frequency-dependent variance-reduced Monte Carlo simulations. We find that the grain boundary may not be as effective as predicted by the gray model in scattering certain phonons, with a substantial amount of heat being carried by low frequency phonons with mean free paths longer than the grain size. Our result will help guide the design of more efficient TEs.

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

    PubMed

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

    2013-08-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-09-01

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

  2. Phosphorus free energy change associated with segregation to grain boundaries in submerged-arc welds

    SciTech Connect

    Mendes, C.M.

    1999-10-01

    The segregation of phosphorus from the matrix to grain boundaries has been associated with increased brittleness in carbon-manganese submerged-arc weld metals, with the failure mode on the lower shelf and transition regime changing from cleavage to intergranular failure. The quantification of segregation and hence thermal embrittlement requires the knowledge of the energy decrease for an atom in the matrix moving to a grain boundary, {Delta}G. There is some evidence to suggest that the energy decrease depends on the level of alloying constituents and this paper considers a steel with a 1.46 wt% manganese and 0.08% carbon. The value of {Delta}G was calculated as a function of temperature by measuring grain boundary segregation after annealings at constant temperature in the range 763--873 K. The grain refined and columnar regions of the welds were analyzed separately. The phosphorus grain boundary monolayer coverages observed were smaller than those reported for a PWR steel.

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

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

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

  4. SEMICONDUCTOR PHYSICS: Grain boundary layer behavior in ZnO/Si heterostructure

    NASA Astrophysics Data System (ADS)

    Bingce, Liu; Cihui, Liu; Bo, Yi

    2010-03-01

    The grain boundary layer behavior in ZnO/Si heterostucture is investigated. The current-voltage (I-V) curves, deep level transient spectra (DLTS) and capacitance-voltage (C-V) curves are measured. The transport currents of ZnO/Si heterojunction are dominated by grain boundary layer as high densities of interfacial states existed. The interesting phenomenon that the crossing of In I-V curves of ZnO/Si heterojunction at various measurement temperatures and the decrease of its effective barrier height with the decrement of temperature are in contradiction with the ideal heterojunction thermal emission model is observed. The details will be discussed in the following.

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

    PubMed

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

    2016-03-28

    The tribological properties of metal-supported few-layered graphene depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the frictional and wetting characteristics of the FLG system. We discretely measured friction characteristics of FLG covering grains and interfacial grain boundaries of polycrystalline Ni metal substrate via an atomic force microscopy (AFM) probe. The friction coefficient of FLG covered at interfacial grain boundaries is found to be lower than that on grains in vacuum (at 10(-5) Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull-off adhesion forces were addressed in the course of friction measurements to explain the role of the out-of-plane deformation of graphene layer(s). Finite element simulations showed good agreement with experiments and led to a rationalization of the observations. Thus, with interfacial grain boundaries the FLG tribology can be effectively tuned. PMID:26948836

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

    SciTech Connect

    Marincel, Dan M.; Zhang, H. R.; Briston, J.; Belianinov, Alex; Jesse, Stephen; Kalinin, Sergei V.; Chen, L. Q.; Rainforth, William M.; Reaney, Ian M.; Randall, Clive A.; Trolier-McKinstry, Susan

    2015-04-27

    The interaction of grain boundaries with ferroelectric domain walls strongly influences the extrinsic contribution to piezoelectric activity in Pb(Zr,Ti)O3 (PZT), ubiquitous in modern transducers and actuators. However, the fundamental understanding of these phenomena has been limited by complex mechanisms originating from the interplay of atomic-level domain wall pinning, collective domain wall dynamics, and emergent mesoscopic behavior. This contribution utilizes engineered grain boundaries created by depositing epitaxial PZT films with various Zr:Ti ratio onto 24º SrTiO3 tilt bicrystals. The nonlinear piezoelectric response and surface domain structure across the boundary are investigated using piezoresponse force microscopy whilst cross section domain structure is studied using transmission electron microscopy. The grain boundary reduces domain wall motion over a width of 800±70 nm for PZT 45:55 and 450±30 nm for PZT 52:48. Phase field modeling provides an understanding of the elastic and electric fields associated with the grain boundary and local domain configurations. In conclusion, this study demonstrates that complex mesoscopic behaviors can be explored to complement atomic-level pictures of the material system.

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

    DOE PAGESBeta

    Marincel, Dan M.; Zhang, H. R.; Briston, J.; Belianinov, Alex; Jesse, Stephen; Kalinin, Sergei V.; Chen, L. Q.; Rainforth, William M.; Reaney, Ian M.; Randall, Clive A.; et al

    2015-04-27

    The interaction of grain boundaries with ferroelectric domain walls strongly influences the extrinsic contribution to piezoelectric activity in Pb(Zr,Ti)O3 (PZT), ubiquitous in modern transducers and actuators. However, the fundamental understanding of these phenomena has been limited by complex mechanisms originating from the interplay of atomic-level domain wall pinning, collective domain wall dynamics, and emergent mesoscopic behavior. This contribution utilizes engineered grain boundaries created by depositing epitaxial PZT films with various Zr:Ti ratio onto 24º SrTiO3 tilt bicrystals. The nonlinear piezoelectric response and surface domain structure across the boundary are investigated using piezoresponse force microscopy whilst cross section domain structure ismore » studied using transmission electron microscopy. The grain boundary reduces domain wall motion over a width of 800±70 nm for PZT 45:55 and 450±30 nm for PZT 52:48. Phase field modeling provides an understanding of the elastic and electric fields associated with the grain boundary and local domain configurations. In conclusion, this study demonstrates that complex mesoscopic behaviors can be explored to complement atomic-level pictures of the material system.« less

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  9. Fundamental studies of grain boundary passivation in polycrystalline silicon with application to improved photovoltaic devices

    NASA Astrophysics Data System (ADS)

    Seager, C. H.; Ginley, D. S.

    1980-02-01

    Several aspects of the electrical properties of silicon grain boundaries were studied. The temperature dependence of the zero bias conductance and capacitance of single boundaries was measured and shown to be in good agreement with a simple double depletion layer/thermal emission model developed to predict the transport properties of such structures. In addition, it was shown that deconvolution of the I-V properties of some boundaries yields effective one electron densities of trapping states which are in good agreement with estimates obtained by low temperature electron emission measurements. Experiments were also performed which indicate that diffusion of atomic hydrogen into silicon grain boundaries greatly reduces this density of trapping states. In properly prepared, large grained polycrystalline samples, all measurable traces of grain boundary potential barriers can be removed to substantial penetration depths after several hours exposure to a hydrogen plasma at elevated temperatures. Initial experiments on prototype polysilicon solar cells have shown that this passivation process can improve AM1 efficiencies.

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

    PubMed

    Dong, Huicong; Wen, Bin; Melnik, Roderick

    2014-11-13

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

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

    SciTech Connect

    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.

  12. Continuous description of a grain boundary in olivine from atomic scale simulations: the role of disclinations

    NASA Astrophysics Data System (ADS)

    Cordier, P.; Sun, X.; Fressengeas, C.; Taupin, V.

    2015-12-01

    A crossover between atomistic description and continuous representation of grain boundaries in polycrystals is set-up to model the periodic arrays of structural units by using dislocation and disclination dipole arrays along grain boundaries. Continuous modeling of the boundary is built by bottom-up processing, meaning that the strain, rotation, curvature, disclination and dislocation density fields are calculated by using the discrete atomic positions generated by molecular dynamics simulations. Continuous modeling of a 18.9° symmetric tilt boundary in copper [1] is conducted as a benchmark case. Its accuracy is validated by comparison with a similar recent technique [2]. Then, results on the 60.8° Mg2SiO4 tilt boundary [3-4] are presented. By linking the atomistic description with continuum mechanics representations, they provide new insights into the structure of the grain boundary. [1] Fressengeas, C., Taupin, V., Capolungo, L., 2014. Continuous modelling of the structure of symmetric tilt boundaries. Int. J. Solids Struct. 51, 1434-1441. [2] Zimmerman, J.A., Bammann, D.J., Gao, H., 2009. Deformation gradients for continuum mechanical analysis of atomistic simulations. Int. J. Solids Struct. 46, 238-253. [3] Cordier, P., Demouchy, S., Beausir, B., Taupin, V., Barou, F., Fressengeas, C., 2014. Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle. Nature 507, 51-56. [4] Adjaoud, O., Marquardt, K., Jahn, S., 2012. Atomic structures and energies of grain boundaries in Mg2SiO4 forsterite from atomistic modeling. Phys. Chem. Miner. 39, 749-760.

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

    NASA Astrophysics Data System (ADS)

    Sun, J.; Zhang, Y. B.; Dahl, A. B.; Conradsen, K.; Jensen, D. Juul

    2015-04-01

    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 analysis, and in this study the so-called “Minkowski sausage” method is adopted. Hereby, two cube-oriented grains in partly recrystallized microstructures are analyzed and quantitative information regarding the dimensions of protrusions/retrusions is obtained.

  14. Uranium vacancy mobility at the sigma 5 symmetric tilt grain boundary in UO2

    SciTech Connect

    Uberuaga, Blas P.

    2012-05-02

    An important consequence of the fissioning process occurring during burnup is the formation of fission products. These fission products alter the thermo-mechanical properties of the fuel. They also lead to macroscopic changes in the fuel structure, including the formation of bubbles that are connected to swelling of the fuel. Subsequent release of fission gases increase the pressure in the plenum and can cause changes in the properties of the fuel pin itself. It is thus imperative to understand how fission products, and fission gases in particular, behave within the fuel in order to predict the performance of the fuel under operating conditions. Fission gas redistribution within the fuel is governed by mass transport and the presence of sinks such as impurities, dislocations, and grain boundaries. Thus, to understand how the distribution of fission gases evolves in the fuel, we must understand the underlying transport mechanisms, tied to the concentrations and mobilities of defects within the material, and how these gases interact with microstructural features that might act as sinks. Both of these issues have been addressed in previous work under NEAMS. However, once a fission product has reached a sink, such as a grain boundary, its mobility may be different there than in the grain interior and predicting how, for example, bubbles nucleate within grain boundaries necessitates an understanding of how fission gases diffuse within boundaries. That is the goal of the present work. In this report, we describe atomic level simulations of uranium vacancy diffusion in the pressence of a {Sigma}5 symmetric tilt boundary in urania (UO{sub 2}). This boundary was chosen as it is the simplest of the boundaries we considered in previous work on segregation and serves as a starting point for understanding defect mobility at boundaries. We use a combination of molecular statics calculations and kinetic Monte Carlo (kMC) to determine how the mobility of uranium vacancies is

  15. Influence of point defects on grain boundary diffusion in oxides. [Annual report, July 1, 1990--March 1, 1992

    SciTech Connect

    Stubican, V.S.

    1991-03-15

    The influence of point defects on grain boundary diffusion of Co ions in NiO was studied using polycrystalline films and bicrystals. Grain boundary diffusion was studied at 750 C at oxygen partial pressure. Two diffusion regions were found. At low oxygen pressures, extrinsic diffusion was observed. Above oxygen pressure of 10{sup {minus}7}, influence of intrinsic point defects was detected. It was determined that grain boundary diffusion was > 3 orders of magnitude faster than volume diffusion. However, it seems that grain boundary diffusion is influenced by the point defects in a similar way as the volume diffusion. 4 figs.

  16. Asymmetric ability of grain boundaries to generate dislocations under tensile or compressive loadings

    NASA Astrophysics Data System (ADS)

    Shimokawa, T.

    2010-11-01

    In order to investigate the effect of structural units on the ability of grain boundaries to generate dislocations, tensile and compressive loading tests were performed on aluminum bicrystals with ⟨112⟩ tilt grain boundaries using molecular-dynamics simulations. Five favored boundaries were found for ⟨112⟩ boundaries in an energy minimization analysis. Each structure could be identified using only its single structural unit period: A unit for Σ1 with θ=0° , B unit for Σ11 with θ=62.96° , C unit for Σ5 with θ=90° , E unit for Σ7 with θ=120° , and D unit for Σ3 with θ=180° . The favored boundaries all show an energy cusp except for the Σ7 boundary. Other boundaries could be represented by a combination of these five structural units. When Σ11 with |B| period showing a minimum energy cusp was used as a reference structure, the core of 2/11[3¯11¯] or 2/11[31¯1] dislocations calculated for the displacement sift complete lattice of Σ11 were introduced at interfaces having larger or smaller misorientation angles than Σ11 , respectively. Each grain-boundary dislocation corresponds to A and C units, respectively. For example, Σ21 with θ=44.42° and Σ15 with θ=78.46° consists of |BBA| and |BBC| periods, respectively. When the Σ15 bicrystal was loaded in tension, lattice dislocations having the 1/2[11¯0] core were emitted from C units and then the C units change their structure to the B units. Therefore, the Σ15 boundary can change its structure to the energetically stable Σ11 structure by emitting dislocations from C units. For Σ21 , the same structural change occurred when the Σ21 bicrystal was deformed in compression. The simulation results show that there are tilt grain boundaries having the strong anisotropic strength and that the ability of grain boundaries to generate dislocations is closely related to their structural units.

  17. Grain-boundary-controlled impedances of electroceramics: Generalized effective-medium approach and brick-layer model

    NASA Astrophysics Data System (ADS)

    Nan, Ce Wen; Zhou, Xisong; Nan, Jun; Wu, Junbo

    2001-04-01

    In every electroceramic there exist variations throughout the microstructure of both grain size (and shape) and electrical properties of individual grain boundaries. To calculate the effects of such microstructural variations on the frequency-dependent impedance/dielectric behavior, we describe a generalized statistical effective-medium approach based on effective-medium theory. To illustrate the predictions of the approach, the effect of various distributions in the grain boundary conductivity and the grain size, as well as the grain shape and porosity, are considered. The calculations show that such variations lead to depression of the boundary arc and deviations of the impedance spectra from the well-known brick-layer model with an idealized microstructure consisting of identical grains and grain boundaries.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    SciTech Connect

    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.

  20. Piezoelectric response enhancement in the proximity of grain boundaries of relaxor-ferroelectric thin films

    NASA Astrophysics Data System (ADS)

    Brewer, Steven; Deng, Carmen; Callaway, Connor; Kalinin, Sergei V.; Vasudevan, Rama K.; Bassiri-Gharb, Nazanin

    2016-06-01

    The influence of surface morphology on the local piezoelectric response of highly (100)-textured 0.70PbMg2/3Nb1/3O3-0.30PbTiO3 thin films is studied using piezoresponse force microscopy in band-excitation mode. The local electromechanical response is mostly suppressed in direct proximity of the grain boundaries. However, within 100-200 nm of the grain boundary, the piezoresponse is substantially enhanced, before decaying again within a region at the center of the grain itself. Nested piezoresponse hysteresis curves confirm the influence of topography descriptors on parameters affecting the hysteresis loop shape. The enhancement of the electromechanical response is rationalized through reduced lateral clamping in the grains with deep trenched boundaries, as well as an expected lower energy for complex domain wall structures, due to curved ferroelectric surfaces. The lower piezoresponse at the center of the grain is assigned to the lateral clamping by the surrounding piezoelectric material.

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

    NASA Astrophysics Data System (ADS)

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

    2014-08-01

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

  2. Theoretical studies of grain boundaries in Ni, Al, and Ni/sub 3/Al with and without boron

    SciTech Connect

    Chen, S.P.; Voter, A.F.; Albers, R.C.; Boring, A.M.; Hay, P.J.

    1988-01-01

    Atomistic simulations of (001) symmetric tilt grain boundaries in Ni, Al, and Ni/sub 3/Al are presented. The atomistic structures of the simulated grain boundaries have been analyzed in terms of the structural unit model, which is found to be of limited utility for intermetallics. Simulation results show that boron segregates more strongly to grain boundaries than to free surfaces, and strengthens the grain boundary. Good cohesive properties of the grain boundaries occur when both boron and some segregated Ni are present. The Ni and B are found to co-segregate to the Ni/sub 3/Al boundary with an energy advantage of /approximately/0.5 eV. 17 refs., 3 figs.

  3. Twist grain boundaries in three-dimensional lamellar Turing structures

    PubMed Central

    De Wit, A.; Borckmans, P.; Dewel, G.

    1997-01-01

    Steady spatial self-organization of three-dimensional chemical reaction-diffusion systems is discussed with the emphasis put on the possible defects that may alter the Turing patterns. It is shown that one of the stable defects of a three-dimensional lamellar Turing structure is a twist grain boundary embedding a Scherk minimal surface. PMID:11038594

  4. Dynamics of Helium-Loaded Grain Boundaries under Shear Deformation in alpha-Fe

    SciTech Connect

    Gao, Fei; Yang, Li; Heinisch, Howard L.; Kurtz, Richard J.

    2014-03-01

    To simulate the motion and behavior of helium-loaded grain boundaries (GBs) in bcc Fe under shear deformation using the molecular dynamics simulation method. Here, we report preliminary results on the effects of helium on coupled migration and relative translation of a sigma3 GB at 300 K.

  5. Field Emission at Grain Boundaries: Modeling the Conductivity in Highly Doped Polycrystalline Semiconductors

    NASA Astrophysics Data System (ADS)

    Sommer, Nicolas; Hüpkes, Jürgen; Rau, Uwe

    2016-02-01

    In this contribution, we elaborate a conductivity model for highly doped polycrystalline semiconductors. The prominent feature of the model is the description of grain-boundary scattering by field emission, i.e., quantum-mechanical tunneling of electrons through potential barriers at grain boundaries. For this purpose, we adapt a theory of Stratton [Theory of field emission from semiconductors, Phys. Rev. 125, 67 (1962)] to double Schottky barriers at grain boundaries. We provide strong evidence that field emission rather than the predominantly applied thermionic emission is the dominant transport path across grain boundaries in semiconductors with carrier concentrations exceeding approximately 1019 cm-3 . We obtain a comprehensive conductivity model for highly doped polycrystalline semiconductors by combining field emission with two intragrain scattering mechanisms, that are ionized-impurity and electron-phonon scattering. The model is applied to a wide range of literature data in order to show its applicability and explanatory power. The literature data comprise, in particular, transparent conductive oxides with a special emphasis on aluminum-doped ZnO.

  6. Measurements of stress fields near a grain boundary: Exploring blocked arrays of dislocations in 3D

    SciTech Connect

    Guo, Y.; Collins, D. M.; Tarleton, E.; Hofmann, F.; Tischler, J.; Liu, W.; Xu, R.; Wilkinson, A. J.; Britton, T. B.

    2015-06-24

    The interaction between dislocation pile-ups and grain boundaries gives rise to heterogeneous stress distributions when a structural metal is subjected to mechanical loading. Such stress heterogeneity leads to preferential sites for damage nucleation and therefore is intrinsically linked to the strength and ductility of polycrystalline metals. To date the majority of conclusions have been drawn from 2D experimental investigations at the sample surface, allowing only incomplete observations. Our purpose here is to significantly advance the understanding of such problems by providing quantitative measurements of the effects of dislocation pile up and grain boundary interactions in 3D. This is accomplished through the application of differential aperture X-ray Laue micro-diffraction (DAXM) and high angular resolution electron backscatter diffraction (HR-EBSD) techniques. Our analysis demonstrates a similar strain characterization capability between DAXM and HR-EBSD and the variation of stress intensity in 3D reveals that different parts of the same grain boundary may have different strengths in resisting slip transfer, likely due to the local grain boundary curvature.

  7. Effect of Phosphorus on the Grain Boundary Cohesion and γ' Precipitation in IN706 alloy

    NASA Astrophysics Data System (ADS)

    Zhang, Sha; Xin, Xin; Yu, Lianxu; Zhang, Anwen; Sun, Wenru; Sun, Xiaofeng

    2016-08-01

    The present paper explored the effect of P content and cooling rate on the microstructure and mechanical properties of IN706 alloy, which was related to the grain boundary cohesion and the γ' precipitation. It was found that P was mainly dissolved in the γ matrix of IN706 alloy when solutioned at 1463 K (1190 °C), and diffused toward and segregated at grain boundaries with the drop of temperature. When dissolved in the γ matrix, P did not influence the microhardness, tensile properties, and impact toughness of IN706 alloy. When aged after 1463 K (1190 °C) solution, P increased the room-temperature yield and ultimate strength, and reduced the elongation of IN706 alloy. The results suggested that the segregation of P at the grain boundaries did not reduce the grain boundary cohesion of IN706 alloy. In addition, P facilitated the γ' phase precipitation by reducing the mismatch lattice between γ and γ' phases. As a result, the strength and microhardness of IN706 alloy were enhanced when the P content was increased and the cooling rate from high temperature became smaller.

  8. Continuum Description of Atomistics for Nanomechanics of Grain Boundary Embrittlement in FCC Metals

    NASA Astrophysics Data System (ADS)

    Kim, K.-S.; Wang, C.-K.; Cha, M.-H.; Chew, H. B.

    2012-02-01

    A nonlinear field projection method has been developed to study nanometer scale mechanical properties of grain boundaries in nanocrystalline FCC metals [1]. The nonlinear field projection is based on the principle of virtual work, for virtual variations of atomic positions in equilibrium through nonlocal interatomic interactions such as EAM potential interaction, to get field-projected subatomic-resolution traction distributions on various grain boundaries. The analyses show that the field projected traction produces periodic concentrated compression sites on the grain boundary, which act as crack trapping or dislocation nucleation sites. The field projection was also used to assess the nanometer scale failure processes of Cu σ5 and σ9 grain boundaries doped with Pb. It was revealed that the most significant atomic rearrangement is dislocation emission which requires local GB slip, and some Pb locks the local GB slip and in turn, embrittles the GB. Reference: [1] C.-K. Wang, et al., 2011, MRS Proceedings, Vol. 1297, DOI: 10.1557/opl.2011.678.

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

    SciTech Connect

    Xu, Tao; Li, Mo

    2015-10-28

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

  10. ATOMISTIC STUDIES OF HELIUM BUBBLE NUCLEATION AT GRAIN BOUNDARIES IN α-Fe

    SciTech Connect

    Yang, Li; Gao, Fei; Heinisch, Howard L.; Kurtz, Richard J.

    2013-04-12

    To simulate the nucleation and growth of He bubbles at grain boundaries (GBs) in alpha-Fe using the newly developed Fe-He potential and to study the evolution of the GB structures due to the accumulation of He atoms and the nucleation and growth of He bubbles.

  11. Measurements of stress fields near a grain boundary: Exploring blocked arrays of dislocations in 3D

    DOE PAGESBeta

    Guo, Y.; Collins, D. M.; Tarleton, E.; Hofmann, F.; Tischler, J.; Liu, W.; Xu, R.; Wilkinson, A. J.; Britton, T. B.

    2015-06-24

    The interaction between dislocation pile-ups and grain boundaries gives rise to heterogeneous stress distributions when a structural metal is subjected to mechanical loading. Such stress heterogeneity leads to preferential sites for damage nucleation and therefore is intrinsically linked to the strength and ductility of polycrystalline metals. To date the majority of conclusions have been drawn from 2D experimental investigations at the sample surface, allowing only incomplete observations. Our purpose here is to significantly advance the understanding of such problems by providing quantitative measurements of the effects of dislocation pile up and grain boundary interactions in 3D. This is accomplished throughmore » the application of differential aperture X-ray Laue micro-diffraction (DAXM) and high angular resolution electron backscatter diffraction (HR-EBSD) techniques. Our analysis demonstrates a similar strain characterization capability between DAXM and HR-EBSD and the variation of stress intensity in 3D reveals that different parts of the same grain boundary may have different strengths in resisting slip transfer, likely due to the local grain boundary curvature.« less

  12. The role of grain boundaries and transient porosity increase as fluid pathways for reaction front propagation

    NASA Astrophysics Data System (ADS)

    Jonas, Laura; John, Timm; Geisler, Thorsten; Putnis, Andrew

    2013-04-01

    The pseudomorphic replacement of Carrara marble by calcium phosphates was studied as a model system to examine the influence of different fluid pathways for reaction front propagation induced by fluid-rock interaction. In this model system, the grain boundaries present in the rock and the transient porosity structures developing throughout the replacement reaction enable the reaction front to progress further into the rock as well as to the center of each single grain until complete transformation. Hydrothermal treatment of the marble using phosphate bearing solutions at temperature levels of 150° C and 200° C for different durations lead to the formation of two product phases which were identified as hydroxyapatite [Ca5(PO4)3OH] as well as β-tricalcium phosphate [β-Ca3(PO4)2] (β-TCP). The formation of β-TCP was probably favored by the presence of ~0.6wt.% of Mg in the parent phase. Completely transformed single grains show a distinctive zoning, both in composition and texture. Whereas areas next to the grain boundary consist of nearly pure hydroxyapatite and show a coarse porosity, areas close to the center of the single grains show a high amount of β-TCP and a very fine porous microstructure. If F was added as an additional solution component, the formation of β-TCP was avoided and up to 3wt.% of F were incorporated into the product apatite. The use of the isotope 18O as a chronometer for the replacement reaction makes it possible to reconstruct the chronological development of the calcium phosphate reaction front. Raman analysis revealed that the incorporation of 18O in the PO4 tetrahedron of hydroxyapatite results in the development of distinct profiles in the calcium phosphate reaction front perpendicular to the grain boundaries of the marble. Through the use of the 18O chronometer, it is possible to estimate and compare the time effectiveness of the different fluid pathways in this model system. The results show that the grain boundaries serve as a

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

    PubMed Central

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

    2016-01-01

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

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

    PubMed

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

    2016-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

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

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

    SciTech Connect

    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.

  17. The structure and properties of grain boundaries in B2 ordered alloys. Progress report

    SciTech Connect

    Baker, I.

    1990-04-16

    This report covers the period from April 1, 1989 to March 30, 1990. During this period further extrusions of fine-grained alloys of FeAl and NiAl with different aluminum contents were produced. Thus far, researchers have (1) completed room temperature mechanical testing which shows that the yield strength, {sigma}{sub y}, of FeAl shows a minimum near Fe-45Al while that for NiAl increases with increasing (nickel-rich) deviation from stoichiometry; (2) showed that the grain boundary strengthening parameter, k, in the Hall-Petch relationship, {sigma}{sub y} = {sigma}{sub 0} + k d{sup {minus}{1/2}} (where {sigma}{sub 0} is the lattice resistance and d the grain size), in FeAl shows a minimum at Fe-45Al whilst in NiAl it decreases with increasing deviation from stoichiometry; (3) studied grain boundary chemistry in near-stoichiometric NiAl and FeAl using scanning Auger electron spectroscopy at Oak Ridge National Laboratory, with the aid of Mr. R.A. Padgett, Jr.; studied grain boundary chemistry using EDS and Z-contrast imaging on field emission gun high resolution scanning transmission electron microscopes; (5) continued in-situ straining experiments in the transmission electron microscope to examine dislocation/grain boundary interactions; (6) examined dislocation structures in lightly-deformed NiAl and FeAl and, based on observations in the latter, proposed a mechanism for transgranular fracture; (7) completed work which examines the effect of cooling rate following annealing on the strength and hardness of FeAl.

  18. The structure and properties of grain boundaries in B2 ordered alloys

    SciTech Connect

    Baker, I.

    1990-04-16

    This report covers the period from April 1, 1989 to March 30, 1990. During this period further extrusions of fine-grained alloys of FeAl and NiAl with different aluminum contents were produced. Thus far, researchers have (1) completed room temperature mechanical testing which shows that the yield strength, {sigma}{sub y}, of FeAl shows a minimum near Fe-45Al while that for NiAl increases with increasing (nickel-rich) deviation from stoichiometry; (2) showed that the grain boundary strengthening parameter, k, in the Hall-Petch relationship, {sigma}{sub y} = {sigma}{sub 0} + k d{sup {minus}{1/2}} (where {sigma}{sub 0} is the lattice resistance and d the grain size), in FeAl shows a minimum at Fe-45Al whilst in NiAl it decreases with increasing deviation from stoichiometry; (3) studied grain boundary chemistry in near-stoichiometric NiAl and FeAl using scanning Auger electron spectroscopy at Oak Ridge National Laboratory, with the aid of Mr. R.A. Padgett, Jr.; studied grain boundary chemistry using EDS and Z-contrast imaging on field emission gun high resolution scanning transmission electron microscopes; (5) continued in-situ straining experiments in the transmission electron microscope to examine dislocation/grain boundary interactions; (6) examined dislocation structures in lightly-deformed NiAl and FeAl and, based on observations in the latter, proposed a mechanism for transgranular fracture; (7) completed work which examines the effect of cooling rate following annealing on the strength and hardness of FeAl.

  19. Grain boundary sliding in aluminum nano-bi-crystals deformed at room temperature.

    PubMed

    Aitken, Zachary H; Jang, Dongchan; Weinberger, Christopher R; Greer, Julia R

    2014-01-15

    Room-temperature uniaxial compressions of 900-nm-diameter aluminum bi-crystals, each containing a high-angle grain boundary with a plane normal inclined at 24° to the loading direction, revealed frictional sliding along the boundary plane to be the dominant deformation mechanism. The top crystallite sheared off as a single unit in the course of compression instead of crystallographic slip and extensive dislocation activity, as would be expected. Compressive stress strain data of deforming nano bicrystals was continuous, in contrast to single crystalline nano structures that show a stochastic stress strain signature, and displayed a peak in stress at the elastic limit of ~ 176 MPa followed by gradual softening and a plateau centered around ~ 125 MPa. An energetics-based physical model, which may explain observed room-temperature grain boundary sliding, in presented, and observations are discussed within the framework of crystalline nano-plasticity and defect microstructure evolution.

  20. Grain Boundary Segregation Behavior of Boron in Low-Alloy Steel

    NASA Astrophysics Data System (ADS)

    Shigesato, Genichi; Fujishiro, Taishi; Hara, Takuya

    2014-04-01

    The boron concentration profiles around prior austenite grain boundaries in Fe-0.05C-0.5Mo-0.001B (mass pct) are examined using aberration-corrected STEM-EELS. In order to obtain the precise distribution of boron around the boundaries, tilt series measurements with thin specimens (<30 nm) are performed and the EEL spectra are analyzed by principal component analysis (PCA) and multivariate curve resolution (MCR). The boron concentration profile changes with the cooling rate from the solid solution temperature. The concentration at grain boundaries is maximized at a medium rate (30 °C/s), where the concentration reaches 8 at. pct, and it decreases at a larger (250 °C/s) or smaller (5 °C/s) rate. On the other hand, the boron distribution becomes wider as the cooling rate becomes smaller. The current results suggest that the boron segregation in the alloy is formed by the "non-equilibrium segregation mechanism."

  1. Grain boundary sliding in aluminum nano-bi-crystals deformed at room temperature.

    PubMed

    Aitken, Zachary H; Jang, Dongchan; Weinberger, Christopher R; Greer, Julia R

    2014-01-15

    Room-temperature uniaxial compressions of 900-nm-diameter aluminum bi-crystals, each containing a high-angle grain boundary with a plane normal inclined at 24° to the loading direction, revealed frictional sliding along the boundary plane to be the dominant deformation mechanism. The top crystallite sheared off as a single unit in the course of compression instead of crystallographic slip and extensive dislocation activity, as would be expected. Compressive stress strain data of deforming nano bicrystals was continuous, in contrast to single crystalline nano structures that show a stochastic stress strain signature, and displayed a peak in stress at the elastic limit of ~ 176 MPa followed by gradual softening and a plateau centered around ~ 125 MPa. An energetics-based physical model, which may explain observed room-temperature grain boundary sliding, in presented, and observations are discussed within the framework of crystalline nano-plasticity and defect microstructure evolution. PMID:23873787

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

    NASA Astrophysics Data System (ADS)

    Kurpaska, L.; Jagielski, J.

    2016-07-01

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

  3. Grain boundary diffusion of titanium in polycrystalline quartz and its implications for titanium in quartz (TitaniQ) geothermobarometry

    NASA Astrophysics Data System (ADS)

    Bromiley, Geoffrey David; Hiscock, Matthew

    2016-04-01

    We have performed a series of experiments to measure diffusivity of Ti in polycrystalline quartz under high pressure/temperature, nominally anhydrous conditions. Resulting diffusion profiles reveal operation of both slow lattice diffusion and faster grain boundary diffusion. Over the temperature range investigated, 1000-1400 °C, grain boundary diffusion of Ti is between 3 and 4 orders of magnitude faster than lattice diffusion and can be expressed by the following Arrhenius relationship: Grain boundary diffusion is expected to have a considerable influence on Ti mobility in the crust in Si-rich rocks under fluid-absent conditions, especially in fine-grained rocks, with grain boundaries acting as fast conduits for transporting Ti. This has important consequences for the application of Ti in quartz geothermobarometry (TitaniQ). Grain boundary diffusion is a viable mechanism for re-equilibrating Ti contents in quartz-rich rocks to lower values, for example during dynamic recrystallization. This implies that TitaniQ can be applied to relatively low temperatures (below 600 °C) although zonation of Ti contents in larger quartz grains is expected due to the relative sluggishness of lattice diffusion under these conditions and because fast diffusion in grain boundary regions effectively inhibits growth entrapment. Grain boundary diffusion for Ti also has implications for the activity of Ti in quartz-rich rocks and application of the TitaniQ geothermobarometer.

  4. Atomic structures and energies of grain boundaries in Mg2SiO4 forsterite from atomistic modeling

    NASA Astrophysics Data System (ADS)

    Adjaoud, Omar; Marquardt, Katharina; Jahn, Sandro

    2012-10-01

    Grain boundaries influence many physical and chemical properties of crystalline materials. Here, we perform molecular dynamics simulations to study the structure of a series of [100] symmetric tilt grain boundaries in Mg2SiO4 forsterite. The present results show that grain boundary energies depend significantly on misorientation angle. For small misorientation angles (up to 22°), grain boundary structures consist of an array of partial edge dislocations with Burgers vector 1/2[001] associated with stacking faults and their energies can be readily fit with a model which adds the Peach-Koehler equation to the Read-Shockley dislocation model for grain boundaries. The core radius of partial dislocations and the spacing between the partials derived from grain boundary energies show that the transition from low- to high-angle grain boundaries occurs for a misorientation angle between 22° and 32°. For high misorientation angles (32.1° and 60.8°), the cores of dislocations overlap and form repeated structural units. Finally, we use a low energy atomic configuration obtained by molecular dynamics for the misorientation of 12.18° as input to simulate a high-resolution transmission electron microscopy (HRTEM) image. The simulated image is in good agreement with an observed HRTEM image, which indicates the power of the present approach to predict realistic atomic structures of grain boundaries in complex silicates.

  5. Correlation between characteristics of grain boundary carbides and creep-fatigue properties in AISI 321 stainless steel

    NASA Astrophysics Data System (ADS)

    Min, Kyung Seon; Nam, Soo Woo

    2003-11-01

    The effects of the interfacial relationships between grain boundary carbides and neighboring grains on the creep-fatigue behaviors have been investigated in AISI 321 stainless steel. The contacting interfacial planes between grain boundary TiC and neighboring grains are found to have lower Miller indices than those between Cr 23C 6 and neighboring grains. From this observation, it is suggested that the interfacial free energy between grain boundary TiC and grains is lower than that between Cr 23C 6 and grains. Creep-fatigue life of TiC aged AISI 321 stainless steel is observed to be longer than that of Cr 23C 6 aged AISI 321 stainless steel. The differences in creep-fatigue life are based on the stronger cavitation resistance of TiC compared with that of Cr 23C 6. From the interfacial relationships between the grain boundary carbides and the neighboring grains, it is verified that formation and growth of grain boundary cavities at TiC carbides are more retarded than those at Cr 23C 6 carbides, thus extending the creep-fatigue life of the steel.

  6. In-situ investigation of grain boundary and triple junction kinetics in aluminium-10 p.p.m. magnesium.

    PubMed

    Mattissen, D; Waerø, A; Molodov, D A; Shvindlerman, L S; Gottstein, G

    2004-03-01

    A special technique for in-situ observation and recording of triple junction motion is introduced. The described method makes it possible to study the motion of connected grain boundaries, i.e. triple junctions and their geometry. The results of an investigation of the steady-state motion of grain boundary systems with triple junctions in aluminium-magnesium are presented. The analysed triple junctions show a distinctive deviation of contact angle from the equilibrium value 120 degrees as assumed for the deviation of the Von Neumann-Mullins relation. The type of triple junction can have an essential influence on grain boundary motion and thus on grain growth.

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

    DOEpatents

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  9. Microstructure of TRISO Coated Particles from the AGR-1 Experiment I: SiC Grain Size and Grain Boundary Character

    SciTech Connect

    Rita Kirchhofer; John D, Hunn; Paul A. Demkowicz; James I. Cole; Brian P. Gorman

    2013-01-01

    Pre-irradiation SiC microstructures in TRISO coated fuel particles from the AGR-1 experiment were quantitatively characterized using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). From EBSD it was determined that only the cubic polymorph of as-deposited SiC was present and the SiC had a high fraction of CSL S3 grain boundaries. Additionally, the local area misorientation (LAM), which is a qualitative measurement of strain in the SiC lattice, was mapped for each fuel variant. The morphology of the SiC / IPyC interfaces were characterized by TEM following site-specific focused ion beam (FIB) specimen preparation. It was determined that the SiC layer had a heavily faulted microstructure typical of CVD deposited SiC and that the average grain diameter increased from the SiC/IPyC interface for all the fuel variants, except V3 that showed a constant grain size across the layer.

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

    SciTech Connect

    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.

  11. Controlled preferential oxidation of grain boundaries in monolayer tungsten disulfide for direct optical imaging.

    PubMed

    Rong, Youmin; He, Kuang; Pacios, Mercè; Robertson, Alex W; Bhaskaran, Harish; Warner, Jamie H

    2015-04-28

    Synthetic 2D crystal films grown by chemical vapor deposition are typically polycrystalline, and determining grain size within domains and continuous films is crucial for determining their structure. Here we show that grain boundaries in the 2D transition metal dichalcogenide WS2, grown by CVD, can be preferentially oxidized by controlled heating in air. Under our developed conditions, preferential degradation at the grain boundaries causes an increase in their physical size due to oxidation. This increase in size enables their clear and rapid identification using a standard optical microscope. We demonstrate that similar treatments in an Ar environment do no show this effect, confirming that oxidation is the main role in the structural change. Statistical analysis of grain boundary (GB) angles shows dominant mirror formation. Electrical biasing across the GB is shown to lead to changes at the GB and their observation under an optical microscope. Our approach enables high-throughput screening of as-synthesized WS2 domains and continuous films to determine their crystallinity and should enable improvements in future CVD growth of these materials.

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  14. Atomic structural features of dopant segregated grain boundary complexions in alumina by EXAFS

    NASA Astrophysics Data System (ADS)

    Behera, Shantanu Kumar

    The primary objective of this undertaking was to characterize the atomic structural features of dopant-segregated interfaces in a (pseudo) single phase microstructure and relate the same to atomic diffusion in the grain boundaries. Alumina was chosen as a model host system based on prior observations of grain boundary complexions in this system by electron microscopy. Two types of dopant chemistry were selected that are known to produce dramatically different microstructural behavior in alumina. These were, (i) rare earth element doping (Y) and (ii) Y-Si co-doping in alumina. In Y-doped alumina microstructures, different Gibbsian excess of the segregated dopant has been known to produce two distinct types of interface complexions. On the other hand, three distinct types of disordered grain boundary complexions have been observed in Y-Si codoped alumina. A quantitative grain growth study was performed in dense microstructures of these materials and different kinetic regimes of boundary mobility were identified. Subsequently, samples annealed at various temperatures were quenched to preserve the grain boundary structure and characterized using synchrotron extended X-ray absorption fine structure spectroscopy (EXAFS) at the Y K-edge. Distinct local structural features of the dopant segregation induced interface phases were observed and were used to distinguish between each of the complexion types. Computation of EXAFS spectra of theoretical clusters by ab initio methods and fitting the same with experimental data identified several types of interface complexions including: (i) sub-monolayer adsorption, where oversized isovalent dopants (Y) occupy substitutional cation sites at the grain boundary core and reduce the interface energy, (ii) saturation of dopants at the interface leading to bilayer adsorption, where dopants (Y) substitute host cations on both sides of the boundary interpolating into the crystals, (iii) multilayered adsorption, where a pseudo

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

    NASA Astrophysics Data System (ADS)

    Miller, Eric Todd

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

  16. Triple junction transport and the impact of grain boundary width in nanocrystalline Cu.

    PubMed

    Chellali, Mohammed Reda; Balogh, Zoltan; Bouchikhaoui, Houari; Schlesiger, Ralf; Stender, Patrick; Zheng, Lei; Schmitz, Guido

    2012-07-11

    Triple junctions (TJ), singular topological defects of the grain boundary (GB) structure, get a dominant role for grain growth and atomic transport in nanocrystalline matter. Here, we present detailed measurements by atom probe tomography, even of the temperature dependence of TJ transport of Ni in nanocrystalline Cu in the chemical regime of interdiffusion. An unexpected variation of the effective width of merging GBs with temperature is detected. It is demonstrated that proper measurement of TJ transport requires taking into account this remarkable effect. TJ diffusion is found to be a factor of about 200 faster than GB diffusion. Its activation energy amounts to only two-thirds of that of the GB.

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

    NASA Astrophysics Data System (ADS)

    Zientarski, Tomasz; Chocyk, Dariusz

    2016-10-01

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

  18. SEMICONDUCTOR PHYSICS: Tunnelling piezoresistive effect of grain boundary in polysilicon nano-films

    NASA Astrophysics Data System (ADS)

    Rongyan, Chuai; Bin, Liu; Xiaowei, Liu; Xianlong, Sun; Changzhi, Shi; Lijian, Yang

    2010-03-01

    The experiment results indicate that the gauge factor of highly boron doped polysilicon nanofilm is bigger than that of monocrystalline silicon with the same doping concentration, and increases with the grain size decreasing. To apply the unique properties reasonably in the fabrication of piezoresistive devices, it was expounded based on the analysis of energy band structure that the properties were caused by the tunnel current which varies with the strain change forming a tunnelling piezoresistive effect. Finally, a calculation method of piezoresistance coefficients around grain boundaries was presented, and then the experiment results of polysilicon nanofilms were explained theoretically.

  19. Grain boundary grooving of Al-bicrystals in the presence of a liquid Al-In alloy

    NASA Astrophysics Data System (ADS)

    Ratke, L.; Vogel, H. J.

    1987-02-01

    Grain boundary grooving of the Al bicrystal by diffusion of Al through an In-rich liquid was investigated at different temperatures below the monotectic. An instability of the groove profile is observed leading to a completely different groove profile than theoretically predicted by the theory of Mullins. The occurrence of this instability is explained by the existence of a second process disturbing the groove kinetics: grain boundary diffusion of In atoms into the bicrystal boundary.

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

    SciTech Connect

    Gelles, D.S.; Ohnuki, S.; Takahashi, H.

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

  2. Grains and grian boundaries in highly-crystalline monolayer molybdenum disulfide

    NASA Astrophysics Data System (ADS)

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

    2013-03-01

    Recent progress in large-area chemical vapor deposition (CVD) synthesis of monolayer molybdenum disulfide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined CVD synthesis to grow highly crystalline islands of monolayer molybdenum disulfide up to 120 micrometers in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology, and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror boundaries that are stitched together by lines of 8- and 4- membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. The knowledge gained about grain structure enables systematic studies of the optical, mechanical, and electronic properties of grain boundaries.

  3. A Model for Predicting Grain Boundary Cracking in Polycrystalline Viscoplastic Materials Including Scale Effects

    SciTech Connect

    Allen, D.H.; Helms, K.L.E.; Hurtado, L.D.

    1999-04-06

    A model is developed herein for predicting the mechanical response of inelastic crystalline solids. Particular emphasis is given to the development of microstructural damage along grain boundaries, and the interaction of this damage with intragranular inelasticity caused by dislocation dissipation mechanisms. The model is developed within the concepts of continuum mechanics, with special emphasis on the development of internal boundaries in the continuum by utilizing a cohesive zone model based on fracture mechanics. In addition, the crystalline grains are assumed to be characterized by nonlinear viscoplastic mechanical material behavior in order to account for dislocation generation and migration. Due to the nonlinearities introduced by the crack growth and viscoplastic constitution, a numerical algorithm is utilized to solve representative problems. Implementation of the model to a finite element computational algorithm is therefore briefly described. Finally, sample calculations are presented for a polycrystalline titanium alloy with particular focus on effects of scale on the predicted response.

  4. Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics

    NASA Astrophysics Data System (ADS)

    Kim, Sang Il; Lee, Kyu Hyoung; Mun, Hyeon A.; Kim, Hyun Sik; Hwang, Sung Woo; Roh, Jong Wook; Yang, Dae Jin; Shin, Weon Ho; Li, Xiang Shu; Lee, Young Hee; Snyder, G. Jeffrey; Kim, Sung Wng

    2015-04-01

    The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit (zT). The zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi0.5Sb1.5Te3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.

  5. Thermoelectrics. Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics.

    PubMed

    Kim, Sang Il; Lee, Kyu Hyoung; Mun, Hyeon A; Kim, Hyun Sik; Hwang, Sung Woo; Roh, Jong Wook; Yang, Dae Jin; Shin, Weon Ho; Li, Xiang Shu; Lee, Young Hee; Snyder, G Jeffrey; Kim, Sung Wng

    2015-04-01

    The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit (zT). The zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi(0.5)Sb(1.5)Te3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices. PMID:25838382

  6. Thermoelectrics. Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics.

    PubMed

    Kim, Sang Il; Lee, Kyu Hyoung; Mun, Hyeon A; Kim, Hyun Sik; Hwang, Sung Woo; Roh, Jong Wook; Yang, Dae Jin; Shin, Weon Ho; Li, Xiang Shu; Lee, Young Hee; Snyder, G Jeffrey; Kim, Sung Wng

    2015-04-01

    The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit (zT). The zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi(0.5)Sb(1.5)Te3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.

  7. Magnetic patterning: local manipulation of the intergranular exchange coupling via grain boundary engineering

    PubMed Central

    Huang, Kuo-Feng; Liao, Jung-Wei; Hsieh, Cheng-Yu; Wang, Liang-Wei; Huang, Yen-Chun; Wen, Wei-Chih; Chang, Mu-Tung; Lo, Shen-Chuan; Yuan, Jun; Lin, Hsiu-Hau; Lai, Chih-Huang

    2015-01-01

    Magnetic patterning, with designed spatial profile of the desired magnetic properties, has been a rising challenge for developing magnetic devices at nanoscale. Most existing methods rely on locally modifying magnetic anisotropy energy or saturation magnetization, and thus post stringent constraints on the adaptability in diverse applications. We propose an alternative route for magnetic patterning: by manipulating the local intergranular exchange coupling to tune lateral magnetic properties. As demonstration, the grain boundary structure of Co/Pt multilayers is engineered by thermal treatment, where the stress state of the multilayers and thus the intergranular exchange coupling can be modified. With Ag passivation layers on top of the Co/Pt multilayers, we can hinder the stress relaxation and grain boundary modification. Combining the pre-patterned Ag passivation layer with thermal treatment, we can design spatial variations of the magnetic properties by tuning the intergranular exchange coupling, which diversifies the magnetic patterning process and extends its feasibility for varieties of new devices. PMID:26156786

  8. Self-organization of cooperative grain boundary sliding in aluminium tricrystals

    SciTech Connect

    Astanin, V.V.; Sisanbaev, A.V.; Pshenichnyuk, A.I.; Kaibyshev, O.A.

    1997-01-01

    Grain boundary sliding (GBS) being an important mechanism of high temperature deformation of polycrystals acquires a leading role in microcrystalline materials providing a superplasticity effect in them. Self-organization of change (increase) in angles of triple junctions providing CGBS realization was experimentally studied in a number of works. In particular, it was established that long range chains of grain boundaries are formed in structure of different materials. That is due to superplastic deformation. These chains are arranged almost in a straight line. Many well-known models on accommodation of GBS during superplastic deformation suggest the straightening of triple junctions. However, driving force and mechanisms of such transformation remain unclear in many cases. The aim of the present work is to design a transmission of GBS via a triple junction, to show physical origin of the triple junction transformation and to suggest a mechanism of the process. Well attested tricrystals are the best two dimensional model objects for such investigations.

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

    PubMed

    Gordon, Lyle M; Joester, Derk

    2015-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-02-01

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

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

    PubMed Central

    Gordon, Lyle M.; Joester, Derk

    2015-01-01

    Dental enamel has evolved to resist the most grueling conditions of mechanical stress, fatigue, and wear. Adding insult to injury, it is exposed to the frequently corrosive environment of the oral cavity. While its hierarchical structure is unrivaled in its mechanical resilience, heterogeneity in the distribution of magnesium ions and the presence of Mg-substituted amorphous calcium phosphate (Mg-ACP) as an intergranular phase have recently been shown to increase the susceptibility of mouse enamel to acid attack. Herein we investigate the distribution of two important constituents of enamel, residual organic matter and inorganic carbonate. We find that organics, carbonate, and possibly water show distinct distribution patterns in the mouse enamel crystallites, at simple grain boundaries, and in the amorphous interphase at multiple grain boundaries. This has implications for the resistance to acid corrosion, mechanical properties, and the mechanism by which enamel crystals grow during amelogenesis. PMID:25852562

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

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

    SciTech Connect

    Mosley, W.C.

    1982-01-01

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

  14. Grain Boundary Structures and Electronic Properties of Hexagonal Boron Nitride on Cu(111).

    PubMed

    Li, Qiucheng; Zou, Xiaolong; Liu, Mengxi; Sun, Jingyu; Gao, Yabo; Qi, Yue; Zhou, Xiebo; Yakobson, Boris I; Zhang, Yanfeng; Liu, Zhongfan

    2015-09-01

    Grain boundaries (GBs) of hexagonal boron nitride (h-BN) grown on Cu(111) were investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The first experimental evidence of the GBs composed of square-octagon pairs (4|8 GBs) was given, together with those containing pentagon-heptagon pairs (5|7 GBs). Two types of GBs were found to exhibit significantly different electronic properties, where the band gap of the 5|7 GB was dramatically decreased as compared with that of the 4|8 GB, consistent with our obtained result from density functional theory (DFT) calculations. Moreover, the present work may provide a possibility of tuning the inert electronic property of h-BN via grain boundary engineering. PMID:26244850

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

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

  16. The effect of grain boundary sliding on the rheology of polymineralic rocks: Nature and numerical experiments

    NASA Astrophysics Data System (ADS)

    Nevitt, J. M.; Piazolo, S.; Evans, L.; Toy, V. G.

    2013-12-01

    -rich aggregates. EBSD indicates that the quartz grains have an average grain size of 7 μm and possess a weak CPO. The prevalence of very fine grained material within the second sample suggests that grain boundary sliding (GBS) played an important role in the deformation. In this case, the weak CPO of quartz could have developed if one particular crystallographic plane (e.g. rhomb) is also a preferred grain boundary orientation and commonly aligns with the shear plane. This is indicated by prevalence of rhomb-slip axes in other Alpine Fault mylonites (Toy et al., 2008). GBS could be accommodated both by grain boundary diffusion and dislocation creep. Based on these typical microstructures, we introduce a new method for modeling GBS in Elle, allowing accommodation of GBS by either diffusion only or by diffusion and dislocation creep. We apply this model to simplified microstructures with different grain size distributions and evaluate the effect of the resultant dominance of GBS and GSS on the developing microstructure and bulk constitutive behavior.

  17. Uranium vacancy mobility at the Σ5 symmetric tilt and Σ5 twist grain boundaries in UO₂

    DOE PAGESBeta

    Uberuaga, Blas Pedro; Andersson, David A.

    2015-10-01

    Ionic transport at grain boundaries in oxides dictates a number of important phenomena, from ionic conductivity to sintering to creep. For nuclear fuels, it also influences fission gas bubble nucleation and growth. Here, using a combination of atomistic calculations and object kinetic Monte Carlo (okMC) simulations, we examine the kinetic pathways associated with uranium vacancies at two model grain boundaries in UO2. The barriers for vacancy motion were calculated using the nudged elastic band method at all uranium sites at each grain boundary and were used as the basis of the okMC simulations. For both boundaries considered – a simplemore » tilt and a simple twist boundary – the mobility of uranium vacancies is significantly higher than in the bulk. For the tilt boundary, there is clearly preferred migration along the tilt axis as opposed to in the perpendicular direction while, for the twist boundary, migration is essentially isotropic within the boundary plane. These results show that cation defect mobility in fluorite-structured materials is enhanced at certain types of grain boundaries and is dependent on the boundary structure with the tilt boundary exhibiting higher rates of migration than the twist boundary.« less

  18. Uranium vacancy mobility at the Σ5 symmetric tilt and Σ5 twist grain boundaries in UO₂

    SciTech Connect

    Uberuaga, Blas Pedro; Andersson, David A.

    2015-10-01

    Ionic transport at grain boundaries in oxides dictates a number of important phenomena, from ionic conductivity to sintering to creep. For nuclear fuels, it also influences fission gas bubble nucleation and growth. Here, using a combination of atomistic calculations and object kinetic Monte Carlo (okMC) simulations, we examine the kinetic pathways associated with uranium vacancies at two model grain boundaries in UO2. The barriers for vacancy motion were calculated using the nudged elastic band method at all uranium sites at each grain boundary and were used as the basis of the okMC simulations. For both boundaries considered – a simple tilt and a simple twist boundary – the mobility of uranium vacancies is significantly higher than in the bulk. For the tilt boundary, there is clearly preferred migration along the tilt axis as opposed to in the perpendicular direction while, for the twist boundary, migration is essentially isotropic within the boundary plane. These results show that cation defect mobility in fluorite-structured materials is enhanced at certain types of grain boundaries and is dependent on the boundary structure with the tilt boundary exhibiting higher rates of migration than the twist boundary.

  19. Effects of B and S on Ni3Al grain boundaries

    SciTech Connect

    Voter, A.F.; Chen, S.P.; Albers, R.C.; Boring, A.M.; Hay, P.J.

    1988-01-01

    This report describes the procedure used to fit embedded atom style potentials to the pure elements Ni, Al, B and S and the cross potentials necessary for treating alloys and segregants. Grain boundary simulations, and results from molecular statics simulations using the potentials for B and S, and their cross potentials with Ni and Al, which was fit from electronic band structure calculations, are included. 21 refs., 1 fig., 3 tabs. (JL)

  20. Dynamic recrystallization and grain boundary migration in B2 FeAl

    NASA Technical Reports Server (NTRS)

    Baker, I.; Gaydosh, D. J.

    1987-01-01

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

  1. Dielectric anomalies due to grain boundary conduction in chemically substituted BiFeO{sub 3}

    SciTech Connect

    Kumari, Shalini; Ortega, N.; Pavunny, S. P.; Katiyar, Ram S.; Kumar, A.; Hubbard, J. W.; Rinaldi, C.; Srinivasan, G.; Scott, J. F.

    2015-03-21

    We describe systematic studies on Nd and Mn co-doped BiFeO{sub 3}, i.e., (Bi{sub 0.95}Nd{sub 0.05})(Fe{sub 0.97}Mn{sub 0.03})O{sub 3} (BNFM) polycrystalline electroceramics. Raman spectra and X-ray diffraction patterns revealed the formation of rhombohedral crystal structure at room temperature, and ruled out structural changes in BiFeO{sub 3} (BFO) after low percentage chemical substitution. Strong dielectric dispersion and a sharp anomaly around 620 K observed near the Néel temperature (T{sub N} ∼ 643 K of BFO) support strong magneto-dielectric coupling, verified by the exothermic peak in differential thermal data. Impedance spectroscopy disclosed the appearance of grain boundary contributions in the dielectric data in the region, and their disappearance just near the Néel temperature suggests magnetically active grain boundaries. The resistive grain boundary components of the BNFM are mainly responsible for magneto-dielectric coupling. Capacitive grain boundaries are not observed in the modulus spectra and the dielectric behavior deviates from the ideal Debye-type. The ac conduction studies illustrate short-range order with ionic translations assisted by both large and small polaron hopping. Magnetic studies indicate that the weak antiferromagnetic phase of BNFM ceramics is dominated by a strong paramagnetic response (unsaturated magnetization even at applied magnetic field of 7 T). The bulk BNFM sample shows a good in-plane magnetoelectric coupling (ME) coefficient.

  2. THE EFFECT OF INTERSTITIAL N ON GRAIN BOUNDARY COHESIVE STRENGTH IN Fe

    SciTech Connect

    Miyoung, Kim, Geller, Clint B., Freeman, A.F.

    2003-09-22

    Increased nitrogen levels have been correlated with decreased ductility and elevated ductile-to-brittle transition temperature in pressure vessel steels [1]. However, the exact role played by nitrogen in the embrittlement of steels remains unclear. Miller and Burke have reported atom probe ion microscopy findings from neutron-irradiated low-alloy pressure vessel steel showing the presence of a 1 to 2 ruonolayer thick film of Mo, N, and C at prior austenitic grain boundaries (GB's) [2], suggesting a role for nitrogen as an intergranular embrittler. It is of interest for the development of mitigation strategies whether nitrogen must combine with other impurities to form nitride precipitates in order to exert an embrittling effect. Briant et al [1] have associated the embrittling effect of N in steels exclusively with intergranular nitride formation. This association suggests that high nitrogen levels may be acceptable if nitride precipitation at grain boundaries is suppressed. To address whether precipitate formation is indeed essential to the N embrittlement process in pressure vessel steel, a computational study was undertaken to ascertain whether the presence of interstitial nitrogen alone could embrittle an Fe GB. If so, nitrogen in any form must be kept completely away from the grain boundaries, if not out of the material altogether. The effect of interstitial N on the cohesion of an Fe {Sigma}3[110](111) grain boundary (GB) was investigated by ab-initio electronic structure calculations to reveal that free interstitial N produces a large strengthening energy, reduces the magnetic moments of the GB Fe atoms and is embrittling at the GB's.

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

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

    PubMed

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

    2016-01-20

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

  6. Influence of Defect Segregation on the Electrical Properties of Nb-doped SrTiO3 Grain Boundary Layer

    NASA Astrophysics Data System (ADS)

    Kim, Seong-Ho; Suh, Jun-Hyuk; Park, Jae-Gwan; Kim, Yoonho

    2000-04-01

    The influences of defect segregation on electrical properties were investigated for a single grain boundary layer in Nb-doped SrTiO3. The electrical properties were compared with those of a grain embedded with a platinum wire. On the basis of the current-voltage (I-V) characteristics, it was found that the grain boundary of Nb-doped SrTiO3 could be presented as a back-to-back double Schottky model. The segregation of defects, strontium vacancies, in grain boundary was suggested from the results of currnet-time (I-t) measurement, impedance spectroscopy, and capacitance-voltage (C-V) measurement. It was also postulated that the space charge originating from this defect segregation forms the potential barrier. The activation energy for conduction through a grain boundary changed with applied voltage from 1.60 eV to 0.97 eV.

  7. Traction–separation relationships for hydrogen induced grain boundary embrittlement in nickel via molecular dynamics simulations

    DOE PAGESBeta

    Barrows, Wesley; Dingreville, Rémi; Spearot, Douglas

    2015-10-19

    A statistical approach combined with molecular dynamics simulations is used to study the influence of hydrogen on intergranular decohesion. This methodology is applied to a Ni Σ3(112)[11¯0] symmetric tilt grain boundary. Hydrogenated grain boundaries with different H concentrations are constructed using an energy minimization technique with initial H atom positions guided by Monte Carlo simulation results. Decohesion behavior is assessed through extraction of a traction–separation relationship during steady-state crack propagation in a statistically meaningful approach, building upon prior work employing atomistic cohesive zone volume elements (CZVEs). A sensitivity analysis is performed on the numerical approach used to extract the traction–separationmore » relationships, clarifying the role of CZVE size, threshold parameters necessary to differentiate elastic and decohesion responses, and the numerical averaging technique. Results show that increasing H coverage at the Ni Σ3(112)[11¯0] grain boundary asymmetrically influences the crack tip velocity during propagation, leads to a general decrease in the work of separation required for crack propagation, and provides a reduction in the peak stress in the extracted traction–separation relationship. Furthermore the present framework offers a meaningful vehicle to pass atomistically derived interfacial behavior to higher length scale formulations for intergranular fracture.« less

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  9. Traction–separation relationships for hydrogen induced grain boundary embrittlement in nickel via molecular dynamics simulations

    SciTech Connect

    Barrows, Wesley; Dingreville, Rémi; Spearot, Douglas

    2015-10-19

    A statistical approach combined with molecular dynamics simulations is used to study the influence of hydrogen on intergranular decohesion. This methodology is applied to a Ni Σ3(112)[11¯0] symmetric tilt grain boundary. Hydrogenated grain boundaries with different H concentrations are constructed using an energy minimization technique with initial H atom positions guided by Monte Carlo simulation results. Decohesion behavior is assessed through extraction of a traction–separation relationship during steady-state crack propagation in a statistically meaningful approach, building upon prior work employing atomistic cohesive zone volume elements (CZVEs). A sensitivity analysis is performed on the numerical approach used to extract the traction–separation relationships, clarifying the role of CZVE size, threshold parameters necessary to differentiate elastic and decohesion responses, and the numerical averaging technique. Results show that increasing H coverage at the Ni Σ3(112)[11¯0] grain boundary asymmetrically influences the crack tip velocity during propagation, leads to a general decrease in the work of separation required for crack propagation, and provides a reduction in the peak stress in the extracted traction–separation relationship. Furthermore the present framework offers a meaningful vehicle to pass atomistically derived interfacial behavior to higher length scale formulations for intergranular fracture.

  10. Characterization of grain boundary conductivity of spin-sprayed ferrites using scanning microwave microscope

    SciTech Connect

    Myers, J.; Nicodemus, T.; Zhuang, Y.; Watanabe, T.; Matsushita, N.; Yamaguchi, M.

    2014-05-07

    Grain boundary electrical conductivity of ferrite materials has been characterized using scanning microwave microscope. Structural, electrical, and magnetic properties of Fe{sub 3}O{sub 4} spin-sprayed thin films onto glass substrates for different length of growth times were investigated using a scanning microwave microscope, an atomic force microscope, a four-point probe measurement, and a made in house transmission line based magnetic permeameter. The real part of the magnetic permeability shows almost constant between 10 and 300 MHz. As the Fe{sub 3}O{sub 4} film thickness increases, the grain size becomes larger, leading to a higher DC conductivity. However, the loss in the Fe{sub 3}O{sub 4} films at high frequency does not increase correspondingly. By measuring the reflection coefficient s{sub 11} from the scanning microwave microscope, it turns out that the grain boundaries of the Fe{sub 3}O{sub 4} films exhibit higher electric conductivity than the grains, which contributes loss at radio frequencies. This result will provide guidance for further improvement of low loss ferrite materials for high frequency applications.

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

    NASA Technical Reports Server (NTRS)

    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.

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

    NASA Technical Reports Server (NTRS)

    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.

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

    PubMed

    Pineau, André

    2015-03-28

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

  14. Carrier mobility measurement across a single grain boundary in polycrystalline silicon using an organic gate thin-film transistor

    SciTech Connect

    Hashimoto, Masaki; Kanomata, Kensaku; Momiyama, Katsuaki; Kubota, Shigeru; Hirose, Fumihiko

    2012-01-09

    In this study, we developed a measurement method for field-effect-carrier mobility across a single grain boundary in polycrystalline Si (poly Si) used for solar cell production by using an organic gate field-effect transistor (FET). To prevent precipitation and the diffusion of impurities affecting the electronic characteristics of the grain boundary, all the processing temperatures during FET fabrication were held below 150 deg. C. From the grain boundary, the field-effect mobility was measured at around 21.4 cm{sup 2}/Vs at 297 K, and the temperature dependence of the field-effect mobility suggested the presence of a potential barrier of 0.22 eV at the boundary. The technique presented here is applicable for the monitoring of carrier conduction characteristics at the grain boundary in poly Si used for the production of solar cells.

  15. Why Does Ga Addition to CIS Limit Its Cell Performance: The Amazing Physics of Grain-Boundaries and Killer-Defects in Chalcopyrites

    SciTech Connect

    Zunger, A.

    2005-11-01

    New theoretical studies reveal the way that grain boundaries lead to a reduction in electron-hole recombination in CIS, and how Ga addition leads to cell deterioration largely because of grain-interior (not boundary) effects.

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

    SciTech Connect

    Vitek, Vaclav

    1998-08-01

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

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

    NASA Technical Reports Server (NTRS)

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

    2012-01-01

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

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

    PubMed

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

    2016-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-08-01

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

  20. High resolution detection of solute segregation to the cores of dislocations in grain boundaries

    NASA Astrophysics Data System (ADS)

    Michael, J. R.; Lin, C. H.; Sass, S. L.

    1988-03-01

    Thin film Fe -0.8 at. percent Au bicrystals, approximately 60 nm thick, containing (001) twist boundaries were prepared. The average compositon of the bicrystal was determined by Rutherford backscattering spectroscopy (RBS), which also showed that the Au was nonuniformly distributed along the direction normal to the interface, with a large amount concentrated at the (001) twist boundary. This observation constitutes direct evidence that the Au is segregated to the vicinity of the grain boundary. The Au distribution within the plane of the interface was analyzed in a Vacuum Generators HB-501 Scanning Transmission Electron Microscope (STEM) equipped with a field emission electron source and a Link Analytical windowless X-ray detector. The boundary plane was oriented perpendicular to the electron beam. Digital line scans were performed across the dislocation network. Au segregates extensively to the cores of the (100)-type dislocations in small angle (001) twist boundaries in Fe-Au alloys. Our observations demonstrate that the (110) network is favored in pure Fe, which leads to the suggestion that the dislocation core energy plays an important role in determining the stable boundary structure.

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

    PubMed Central

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

    2016-01-01

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

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

    DOE PAGESBeta

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

    2016-08-12

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

  3. Effects of thermal history and microstructure on phosphorus and manganese segregation at grain boundaries in C-Mn welds

    SciTech Connect

    Maier, P.; Faulkner, R.G

    2003-08-15

    This work describes a model development to predict the microstructural behaviour of submerged arc C-Mn weld materials used in pressure vessels. Element segregation can be predicted according to the typical operating conditions and microstructure of the materials. Phosphorus, of great interest because it causes embrittlement, is enhanced at the grain boundary and increases with operating temperature and time. Its interaction with carbon and manganese is described. Site competition has to be applied; its prediction results fit best to the experimental data. Sufficient experimental data have been taken by application of a high-resolution analytical transmission electron microscope. The thermodynamic software package MTDATA to determine the solubility of second phases, such as cementite and alloy carbides, is applied. MTDATA is a software/data package for the calculation of phase equilibria in multicomponent multiphase systems. It uses, as a basis, critically assessed thermodynamic data. By using these data, reduced free amount of solute in the alloy, needed as an input to segregation modelling, is predictable. The final grain boundary segregation consists of various segregation types during the thermal history and is the sum of each single step with a final check on the maximum equilibrium segregation, which cannot be exceeded. Weld materials have a very complex microstructure, resulting in coarse and fine grains. For each grain boundary type, prior austenite and ferrite grain boundaries in coarse grain and ferrite grain boundaries in fine grain, the final segregation level has been predicted and good agreement with practical results has been shown.

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

    Nikuradse's (1933) rough pipe study is enormously influential in the understanding of flow resistance over a sediment bed. However, the rough boundary employed in Nikuradse's study differs from permeable sediment beds in rivers. This implies that the results derived from the rough pipe experiments may not be applicable for flows over a permeable coarse-grained bed. The present study aimed to explore to what extent the flow resistance of a permeable coarse-grained boundary deviates from the Nikuradse's observations. Experiments were conducted with rough pipes, which were prepared by overlaying the inner wall with one to four layers of spherical beads. The single layer roughness resembles the experimental setup reported in Nikuradse's study, while the multilayer of grains allows significant flow to pass through the porous roughness layer. In addition, the ratio of grain diameter, k, to pipe diameter, d, was chosen to be one to two orders greater than the range (0.001 < k/d < 0.033) adopted in Nikuradse's experiments. The data show that the friction factor deviates significantly from the prediction based on Nikuradse's rough pipe relation. For hydraulically rough pipes, the friction factor is found to be proportional to the squared ratio of the grain diameter to nominal pipe diameter. This result is different from the one-third power function as implied by Nikuradse's rough pipe relation or the Manning-Strickler formula but agrees well with laboratory observations of open channel resistance in the presence of large-scale roughness. The measurements also suggest the existence of a laminar flow regime, in which the friction factor is inversely proportional to the Reynolds number. The observed variations in the flow resistance are attributed to both wall permeability and large-scale roughness.

  5. A Nanoindentation Study on Grain-Boundary Contributions to Strengthening and Aging Degradation Mechanisms in Advanced 12Cr Ferritic Steel

    SciTech Connect

    Jang, Jae-il; Shim, Sang Hoon; Komazaki, Shin-ichi; Honda, Tetsuya

    2007-01-01

    Nanoindentation experiments and microstructural analysis were performed on advanced 12% Cr ferritic steel having extremely fine and complex martensitic microstructures, to answer unsolved questions on the contributions of grain boundaries to strengthening and aging degradation mechanisms in both as-tempered and thermally aged steels. Interesting features of the experimental results led us to suggest that among several high angle boundaries, block boundary is most effective in enhancing the macroscopic strength in as-tempered virgin sample, and that a decrease in matrix strength rather than reduction in grain-boundary strengthening effect is primarily responsible for the macroscopic softening behavior observed during thermal exposure.

  6. Heterogeneous Stress Relaxation Processes at Grain Boundaries in High-Sn Solder Films: Effects of Sn Anisotropy and Grain Geometry During Thermal Cycling

    NASA Astrophysics Data System (ADS)

    Chen, Wei-Hsun; Sarobol, Pylin; Handwerker, Carol A.; Blendell, John E.

    2016-08-01

    Four different types of stress relaxation responses have been observed in terms of local microstructural changes along grain boundaries (GBs) in large-grained high-Sn (Sn-3.0Ag-0.5Cu) solder films after thermal cycling. The grain boundaries were characterized using scanning electron microscopy, electron back scattered diffraction, and focused-ion beam (FIB) cross-section imaging. While the anisotropic coefficient of thermal expansion of Sn plays an important role in determining which boundaries have high local stresses relative to the film plane and trace of the grain boundary plane during thermal cycling, the four different relaxation behaviors of specific boundaries (no observable changes, surface defect/whisker formation, GB sliding, or a combination of GB sliding and whisker/defect formation) were determined by a combination of Sn anisotropy, the GB geometry, and crystallographic orientation relative to the film plane. The ability to separate GB sliding from surface defect formation provides insights into how long, straight whiskers form in polycrystalline thin films, particularly with respect to grain rotation at the early stages of their growth.

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

    SciTech Connect

    Vaithyanathan, V.; Patro, L. N. E-mail: kkamalabharathi@gmail.com; Kodam, Ugendar; Tan, H.; Inbanathan, S. S. R.; Kamala Bharathi, K. E-mail: kkamalabharathi@gmail.com

    2015-09-21

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

  8. Mechanisms of devitrification of grain boundary glassy phases in Si3N4 materials

    NASA Technical Reports Server (NTRS)

    Hench, L. L.

    1982-01-01

    Changes in the grain boundary (g.b.) phases of Si3N4 are analyzed, the effects of composition and thermal history on devitrification of the g.b. phases are determined, devitrification of the g.b. phases of Si3N are related to mechanical behavior and oxidation sensitivity of the material. The phase relationships that occur within the grain boundaries of Si3N4 containing various densification aids are reviewed. Comparisons of the effects of MgO, Y2O3, CeO2, and Y2O3 + AL2O3 are made in terms of the phase equilibria of the Si3N4 + SiO2 + additive compositional system. Two new equilibrium phase diagrams for the Si3N4-SiO2 and Y2O3 and Si3N4-SiO2-Ce2O3 systems are preented. The effects of Y2O3 vs CeO2 densification aids on the fracture surfaces of Si3N4 are compared. Auger electron spectroscopy shows that both oxides are concentrated within the fracture surface. Scanning electron microscopy shows evidence that Si3N4 with CeO2 formed an intergranular structure of fine grained oxynitride reaction products, as predicted by phase quilibria, whereas the Y2O3 containing sample shows evidence of an intergranular glassy phase.

  9. Mechanism of core loss and the grain-boundary structure of niobium-doped manganese-zinc ferrite

    SciTech Connect

    Inaba, Hideaki; Abe, Teruyoshi, Kitano, Yoko

    1996-01-05

    The mechanism of iron loss was investigated for Mn-Zn ferrites with and without Nb{sub 2}O{sub 5} addition by observing the grain-boundary structure and measuring the various properties. Without Nb{sub 2}O{sub 5} addition Ca and Si atoms concentrate near the grain boundary and are incorporated in the spinel lattice. With Nb{sub 2}O{sub 5} addition Nb atoms concentrate in the grain boundary and keep Ca atoms from being incorporated in the spinel lattice. Hysteresis loss was reduced in spite of smaller magnetic permeability by the addition of Nb{sub 2}O{sub 5}. Eddy current loss and residual loss were reduced by the addition of Nb{sub 2}O{sub 5}, especially at high frequencies. The origin of the decrease in these losses was discussed on the basis of the data of the grain-boundary structure, permeability, electrical conductivity, and disaccomodation.

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

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

  11. Symmetry dependent optoelectronic properties of grain boundaries in polycrystalline Cu(In,Ga)Se{sub 2} thin films

    SciTech Connect

    Müller, Mathias; Bertram, Frank; Christen, Jürgen; Abou-Ras, Daniel Rissom, Thorsten

    2014-01-14

    In a correlative study applying electron backscatter diffraction as well as spatially and spectrally resolved cathodoluminescence spectroscopy at low temperatures of about 5 K, the symmetry-dependent optoelectronic properties of grain boundaries in Cu(In,Ga)Se{sub 2} thin films have been investigated. We find that grain boundaries with lower symmetries tend to show a distinct spectral red shift of about 10 meV and a weak influence on the emission intensity. These behaviors are not detected at high-symmetry Σ3 grain boundaries, or at least in a strongly reduced way. The investigations in the present work help to clarify the ambivalent properties reported for grain boundaries in Cu(In,Ga)Se{sub 2}.

  12. A systematic approach for the study of radiation-induced segregation/depletion at grain boundaries in steels

    NASA Astrophysics Data System (ADS)

    Marquis, Emmanuelle A.; Hu, Rong; Rousseau, Thomas

    2011-06-01

    High Cr ferritic steels are candidate materials for structural applications in Gen-IV and fusion nuclear reactors. However, the relative contributions of irradiation conditions and materials microstructures on radiation-induced segregation or depletion of Cr at grain boundaries in ferritic steels are unclear. Here, the possibility of systematically analyzing the chemistry of the same grain boundary of known character during irradiation is demonstrated using a combination of electron back-scattered diffraction, atom-probe tomography and focused ion beam specimen preparation. This method provides a dynamic evolution of grain boundary chemistry as function of dose, spatial variations within the grain boundary plane, and quantification of minor solute elements such as carbon otherwise difficult to obtain experimentally.

  13. Grain-boundary and subgrain-boundary effects on the dielectric properties of CaCu3Ti4O12 ceramics

    NASA Astrophysics Data System (ADS)

    Cao, Guanghan; Feng, Lixin; Wang, Cao

    2007-05-01

    CaCu3Ti4O12 ceramic samples sintered at different temperatures were studied by measurements of microstructural and dielectric properties. Electron microscopy for thermally-etched samples confirmed a microstructure with subgrains (SG) buried in a crystalline grain. The detailed dielectric properties were shown to be conformable with the double-barrier-layer capacitance model. The complex electric modulus analysis revealed unambiguously each contribution of the SG interiors, SG boundaries and grain boundaries to the dielectric constant. Our result clarified how the dielectric properties change with increasing sintering temperature. It was concluded that grain boundaries rather than SG boundaries played the key role in the low-frequency giant dielectric constant in CCTO ceramics.

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

    NASA Technical Reports Server (NTRS)

    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.

  15. Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide

    SciTech Connect

    Bao, Wei; Borys, Nicholas J.; Ko, Changhyun; Suh, Joonki; Fan, Wen; Thron, Andrew; Zhang, Yingjie; Buyanin, Alexander; Zhang, Jie; Cabrini, Stefano; Ashby, Paul D.; Weber-Bargioni, Alexander; Tongay, Sefaattin; Aloni, Shaul; Ogletree, D. Frank; Wu, Junqiao; Salmeron, Miquel B.; Schuck, P. James

    2015-08-13

    The ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices are two-dimensional monolayer transition metal dichalcogenide semiconductors. Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties. We use the ‘Campanile’ nano-optical probe to spectroscopically image exciton recombination within monolayer MoS2 with sub-wavelength resolution (60 nm), at the length scale relevant to many critical optoelectronic processes. Moreover, synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ~300-nm wide, energetically disordered edge region. Further, grain boundaries are imaged with sufficient resolution to quantify local exciton-quenching phenomena, and complimentary nano-Auger microscopy reveals that the optically defective grain boundary and edge regions are sulfur deficient. In conclusion, the nanoscale structure–property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices.

  16. Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide

    DOE PAGESBeta

    Bao, Wei; Borys, Nicholas J.; Ko, Changhyun; Suh, Joonki; Fan, Wen; Thron, Andrew; Zhang, Yingjie; Buyanin, Alexander; Zhang, Jie; Cabrini, Stefano; et al

    2015-08-13

    The ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices are two-dimensional monolayer transition metal dichalcogenide semiconductors. Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties. We use the ‘Campanile’ nano-optical probe to spectroscopically image exciton recombination within monolayer MoS2 with sub-wavelength resolution (60 nm), at the length scale relevant to many critical optoelectronic processes. Moreover, synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ~300-nm wide, energetically disorderedmore » edge region. Further, grain boundaries are imaged with sufficient resolution to quantify local exciton-quenching phenomena, and complimentary nano-Auger microscopy reveals that the optically defective grain boundary and edge regions are sulfur deficient. In conclusion, the nanoscale structure–property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices.« less

  17. Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide

    NASA Astrophysics Data System (ADS)

    Bao, Wei; Borys, Nicholas J.; Ko, Changhyun; Suh, Joonki; Fan, Wen; Thron, Andrew; Zhang, Yingjie; Buyanin, Alexander; Zhang, Jie; Cabrini, Stefano; Ashby, Paul D.; Weber-Bargioni, Alexander; Tongay, Sefaattin; Aloni, Shaul; Ogletree, D. Frank; Wu, Junqiao; Salmeron, Miquel B.; Schuck, P. James

    2015-08-01

    Two-dimensional monolayer transition metal dichalcogenide semiconductors are ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices. Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties. Here we use the `Campanile' nano-optical probe to spectroscopically image exciton recombination within monolayer MoS2 with sub-wavelength resolution (60 nm), at the length scale relevant to many critical optoelectronic processes. Synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ~300-nm wide, energetically disordered edge region. Further, grain boundaries are imaged with sufficient resolution to quantify local exciton-quenching phenomena, and complimentary nano-Auger microscopy reveals that the optically defective grain boundary and edge regions are sulfur deficient. The nanoscale structure-property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices.

  18. The Role of Thermomechanical Routes on the Distribution of Grain Boundary and Interface Plane Orientations in Transformed Microstructures

    NASA Astrophysics Data System (ADS)

    Beladi, Hossein; Rohrer, Gregory S.

    2016-07-01

    In the current study, a series of thermomechanical routes were used to produce different microstructures (i.e., ferrite and martensite) in low-carbon low alloy steels. The five-parameter grain boundary character distribution was measured for all microstructures. The thermomechanical processing route altered the texture of the fully ferritic microstructure and significantly influenced the anisotropy of the grain boundary character distribution. Generally, the population of (111) planes increased with an increase in the γ-fiber texture for the ferritic microstructure, but it did not change the shape of the grain boundary plane distribution at specific misorientations. The most commonly observed boundaries in the fully ferritic structures produced through different routes were {112} symmetric tilt boundaries with the Σ3 = 60 deg/[111] misorientation; this boundary also had a low energy. However, the grain boundary plane distribution was significantly changed by the phase transformation path (i.e., ferrite vs martensite) for a given misorientation. In the martensitic steel, the most populous Σ3 boundary was the {110} symmetric tilt boundary. This results from the crystallographic constraints associated with the shear transformation (i.e., martensite) rather than the low-energy interface that dominates in the diffusional phase transformation (i.e., ferrite).

  19. Influence of heat-treatment conditions on the electrical conduction mechanism of grain-boundary in cobalt ferrites

    SciTech Connect

    Na, J.G.; Lee, T.D. ); Kim, M.C. . Dept. of Materials Science and Engineering); Park, S.J. )

    1993-11-01

    The electrical resistance and conduction mechanism of the grain-boundary in Co[sub x]Fe[sub 3[minus]x]O[sub 4] ( x = 0.94, 0.97, 1.00, 1.03 and 1.06) under various heat-treatment conditions have been characterized, using a complex impedance analysis technique. The electrical resistance of cobalt ferrites decreases with increasing quenching temperature, which is mainly attributed to the decrease of R[sub g[center dot]b] in Fe-excess cobalt ferrites and R, in Co-excess ones. It is verified that for the furnace-cooled Co[sub 0.94]Fe[sub 2.06]O[sub 4], the conduction mechanism of the grain and grain-boundary is n-type and p-type, respectively. For furnace-cooled Co[sub 1.06]Fe[sub 1.94]O[sub 4], the conduction mechanism of the grain and grain-boundary is p-type and n-type, respectively. These results indicate that the resistivity in the p-type part of cobalt ferrites decreases more rapidly than that in the n-type part with increasing quenching temperature. The conduction mechanism of the grain-boundary is discussed to relate to the preferential oxidation of cations and the disorder near and at the grain-boundary.

  20. Preparation of high-quality HTS rings for application in the magnetic bearing of cryotanks and pinning in grain boundaries

    NASA Astrophysics Data System (ADS)

    Bringmann, B.; Walter, H.; Jooss, Ch.; Leenders, A.; Freyhardt, H. C.

    2002-08-01

    Seeded melt growth of YBCO high-temperature superconductors is one of the most promising preparation techniques to obtain high-quality HTS tiles for application, e.g. in magnetic bearings. Semi-finished HTSL products of complex shapes have to be developed by different seeding and multi-seeding techniques. To obtain large hollow cylinders designed for application in the magnetic bearing of a cryotank a modified multi-seeded melt growth (MSMG) process was employed. This cryotank will be mounted for testing in a vehicle of a major German car manufacturer. The MSMG process introduces grain boundaries into the HTS tiles. For transport current investigations of [0 0 1]-tilt grain boundaries in melt textured YBCO a series of MSMG bicrystals have been prepared. They exhibit a dependence of the critical current density on misorientation angle which is much weaker than the one observed in thin-film bicrystals. The bulk samples have dimensions larger than the magnetic penetration depth along the grain boundary. Thus, flux pinning has to be taken into account. Different contributions to the longitudinal pinning force have to be considered: vortices at grain boundaries can be pinned by magnetic interaction with Abrikosov vortices in the banks, by defects in the grain boundary itself or by defects which are located next to the grain boundary.