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David J Srolovitz - One of the best experts on this subject based on the ideXlab platform.
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the grain boundary structural unit model redux
Acta Materialia, 2017Co-Authors: Jian Han, V Vitek, David J SrolovitzAbstract:Abstract Properties of grain boundaries (GBs) and their underlying structures are key to understanding polycrystalline material phenomena. The most widely used model for GB structure is the structural unit model (SUM), introduced ∼ 50 years ago. The SUM represents GB structure as a combination of structural units (SUs); this combination evolves systematically with GB Misorientation. Despite its successes, many observations suggest the SUM does not completely describe the GB structure; its utility for predicting GB properties is limited. There has been a growing realization that, even for fixed Misorientation, multiple stable/metastable structures are common (corresponding to different microscopic degrees of freedom). We generalize the SUM by considering the effect of such metastable structures. While the SUM can describe GB structure evolution between a pair of delimiting boundaries, there will be many such evolutionary paths, corresponding to SUs associated with the metastable structure of the delimiting boundaries. The equilibrium GB energy vs. Misorientation does not necessarily correspond to one of these paths, but will have contributions from many. Recognizing this, we propose a new approach to predict GB structure and energy, allowing for accurate determination of the GB energy vs. Misorientation based on a very small number of atomistic simulations. For example, we predict the GB energy vs. Misorientation for [ 100 ] and [ 111 ] symmetric tilt boundaries in BCC tungsten over the entire Misorientation range to a mean error of 2 % based on atomistic simulations at only three or four Misorientations. Our approach allows for the trade-off between computational cost and prediction accuracy.
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point defect sink efficiency of low angle tilt grain boundaries
Journal of The Mechanics and Physics of Solids, 2017Co-Authors: Jian Han, Shuyang Dai, Yichao Zhu, Yang Xiang, David J SrolovitzAbstract:Abstract We examine the common assumption that grain boundaries (GBs) are ideal (or perfect) sinks for point defects by comparing and contrasting its implications with an explicit model of a low-angle tilt GB described by an array of edge dislocations which annihilate point defects by climbing. We solve the resultant diffusion equation in the absence and presence of irradiation-induced point defects. The GB sink efficiency depends on the physical parameters describing the boundary geometry (i.e., Misorientation), material properties, and/or irradiation conditions (point defect generation and annihilation within the interior of grains). When the constituent dislocation spacing is small (large Misorientation), the GB sink efficiency approaches that of the ideal sink. However, for small Misorientations, the GB sink efficiency drops rapidly to zero and the ideal sink assumption for the GB fails dramatically. We derive a reduced dimension description of GBs where the influence of GB structure is captured in a single parameter in a Robin boundary condition for the diffusion equation. For the case of a low-angle tilt GB, we explicitly relate this parameter to the GB structure. We discuss the generality of this approach for cases where the low-angle GB model applies and parameterize the model so that it accurately reproduces the results of the two-dimensional dislocation model. The applicability of the approach to more general GBs is discussed as well as the implication of these results for predicting grain size effects under irradiation conditions.
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atomic motion during the migration of general 001 tilt grain boundaries in ni
Acta Materialia, 2007Co-Authors: Hao Zhang, David J Srolovitz, Jack F Douglas, James A WarrenAbstract:Abstract We generalize a previous study of the atomic motions governing grain boundary migration to consider arbitrary Misorientations of [0 0 1] tilt boundaries. Our examination of the nature of atomic motions employed three statistical measures of atomic motion: the non-Gaussian parameter, the “dynamic entropy” and the van Hove correlation function. These metrics were previously shown to provide a useful characterization of atomic motions both in glass-forming liquids and strained polycrystalline materials. As before, we find highly cooperative, string-like motion of atoms, but the grain boundary migration itself is a longer timescale process in which atoms move across the grain boundary. These observations are consistent with our previous results for Σ5 [0 0 1] tilt boundaries. It is evident from our work that the grain boundary structure and Misorientation have a significant influence on the rate of grain boundary migration.
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curvature driven grain boundary migration in aluminum molecular dynamics simulations
Acta Materialia, 2005Co-Authors: Hao Zhang, Moneesh Upmanyu, David J SrolovitzAbstract:Abstract Molecular dynamics simulations have been used to study steady-state, capillarity-driven grain boundary migration in three dimensions for a series of 〈1 1 1〉-tilt boundaries in aluminum. The reduced boundary mobility and boundary enthalpy were determined as a function of Misorientation and temperature. For the Misorientations examined, the reduced mobility is a maximum and the activation energy for migration is a minimum at the Σ 7 Misorientation. The reduced mobility is an Arrhenius function of temperature. Excellent agreement between the present three-dimensional simulation results, those obtained earlier in two dimensions and experiment is obtained for a wide variety of features, with the notable exception of the magnitude of the grain boundary mobility. The mobilities from the simulations are much higher than from experiment; the activation energies for migration are much lower. The present results are intrinsic, while the experimental measurements may be limited by extrinsic factors such as impurity drag.
Gregory S. Rohrer - One of the best experts on this subject based on the ideXlab platform.
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effect of manganese on the grain boundary network of lath martensite in precipitation hardenable stainless steels
Journal of Alloys and Compounds, 2021Co-Authors: Vinothkumar Govindaraj, Gregory S. Rohrer, Ehsan Farabi, Peter Hodgson, Sitarama R Kada, Rajkumar Singh, Hossein BeladiAbstract:Abstract The fully martensitic microstructures of a manganese bearing precipitation hardenable (PH) stainless steel and a commercial 17-4 PH steel were compared to investigate the influence of chemical composition on the grain boundary network of the lath martensitic microstructure. The martensitic transformation in both steels led to the bimodal Misorientation angle distribution having multiple maxima at Misorientations in the range of 10–15° and 50–60°. This closely matched with the Misorientations between variants associated with the Kurdjumov-Sachs orientation relationship (K-S OR). However, the Mn addition appeared to relatively reduce the area of boundaries with Misorientations near 60°. This was a result of the reduction in the area of 60°/[011] intervariant boundaries in the Mn bearing steel. The phenomenological theory of martensite revealed that the Mn addition altered the 3-variant clustering (i.e., V 1 V 3 V 5 ) in 17-4PH steel to the 4-varaint clustering (i.e., V 1 V 2 V 3 V 5 ) arrangement to minimize the strain associated with the displacive martensitic transformation, promoting the population of 60°/[111] intervariant boundaries at the expense of 60°/[011] intervariant boundary. The changes in local variant selection affected the connectivity of the grain boundary network, suggesting that grain boundary network characteristics (i.e., population and connectivity) of the lath martensite microstructure can be manipulated by altering the chemical composition of the steel. The five-parameter grain boundary analysis for both martensitic steels, however, revealed similar grain boundary plane distributions for all boundaries associated with the K-S OR, being terminated on {011} planes due to the constraint that results from the displacive phase transformation.
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five parameter intervariant boundary characterization of martensite in commercially pure titanium
Acta Materialia, 2018Co-Authors: Ehsan Farabi, Gregory S. Rohrer, Peter Hodgson, Hossein BeladiAbstract:Abstract The intervariant boundary characteristics of a commercially pure Ti microstructure formed by the β → α martensitic phase transformation were described according to the crystallography of the displacive transformation and the boundary plane orientation. The martensitic transformation created a microstructure whose grain boundary Misorientation angle distribution had four distinct peaks that were consistent with the Misorientations between the variants produced by the Burgers orientation relationship. Interestingly, about 60% of population corresponded to 60°/ [ 11 2 ¯ 0 ] intervariant boundaries. Three-variant clusters with a triangular morphology were observed frequently. This configuration is consistent with the phenomenological theory of martensite, which predicts that these clusters, separated by 60°/ [ 11 2 ¯ 0 ] boundaries, have a lower transformation strain than other possible variant cluster arrangements. Other intervariant boundaries resulted from the impingement of different combinations of distinct three-variant clusters. The five-parameter boundary analysis revealed a strong anisotropy in the plane orientation distribution, showing that boundaries have a tendency to terminate on prismatic { h k i 0 } and pyramidal { 10 1 ¯ 1 } planes, when Misorientation was ignored. The dominant 60°/ [ 11 2 ¯ 0 ] intervariant boundaries had symmetric tilt ( 1 ¯ 101 ) boundary planes, which are a low energy configuration.
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the five parameter grain boundary character distribution of polycrystalline silicon
Journal of Materials Science, 2014Co-Authors: Yohan Yoon, Sutatch Ratanaphan, Gregory S. RohrerAbstract:The purpose of this paper is to describe the five-parameter grain boundary character distribution (GBCD) of polycrystalline silicon and compare it to distributions measured in metals and ceramics. The GBCD was determined from the stereological analysis of electron backscatter diffraction maps. The distribution of grain boundary disorientations is non-random and has peaks at 36°, 39°, 45°, 51°, and 60°. The axis-angle distribution reveals that most of the grain boundaries have Misorientations around the [111], [110], and [100] axes. The most common grain boundary type (30 % number fraction) has a 60° Misorientation around [111] and of these boundaries, the majority are twist boundaries. For other common boundaries, symmetric tilt configurations are preferred. The grain boundary character distribution of Si is distinct from those previously observed for metals and ceramics. The measured grain boundary populations are inversely correlated to calculated grain boundary energies available in the literature.
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the distribution of intervariant crystallographic planes in a lath martensite using five macroscopic parameters
Acta Materialia, 2014Co-Authors: Hossein Beladi, Anthony D Rollett, Gregory S. Rohrer, Vahid TariAbstract:Abstract Electron backscatter diffraction analysis was employed to compute the closest orientation relationship and the distribution of intervariant boundary character in a lath martensitic microstructure. The Misorientations were close to the Kurdjumov–Sachs orientation relationship. The intervariant crystallographic plane distribution exhibited a relatively high anisotropy with a tendency for the lath interfaces to terminate on (1 1 0) planes. This results from the crystallographic constraints associated with the shear transformation rather than a low energy interface configuration. The lath martensite habit plane was determined to be mostly (1 1 0) or near (1 1 0). The relative populations of boundaries with [1 1 1] and [1 1 0] Misorientations were greater than other high index Misorientations, mostly characterized as (1 1 0) symmetric tilt and (1 1 0) twist boundary types, respectively. Analysis with homology metrics of the connectivity in the lath martensitic microstructure revealed the connectivity dominated by population of Misorientation angle and boundary plane type.
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distribution of grain boundaries in magnesia as a function of five macroscopic parameters
Acta Materialia, 2004Co-Authors: David M Saylor, Adam Morawiec, Gregory S. RohrerAbstract:Abstract A semi-automated method has been used to measure all five macroscopically observable parameters of 4.1×10 6 boundary plane segments making up 5.4 mm 2 of boundary area in a hot-pressed magnesia polycrystal. The observations allow a complete description of the distribution of crystal orientations, grain boundary Misorientations, and the crystallographic orientations of grain boundary planes. Among the low Misorientation angle grain boundaries, there is a preference for tilt boundaries, especially those with boundary plane normals in the direction. At all fixed Misorientations, there is a preference for boundaries with a boundary plane normal in the direction. These boundaries are generally asymmetric and occur at least twice as frequently as the average boundary for each fixed Misorientation.
Yudong Zhang - One of the best experts on this subject based on the ideXlab platform.
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grain fragmentation associated continuous dynamic recrystallization cdrx of hexagonal structure during uniaxial isothermal compression high temperature α phase in tial alloys
Intermetallics, 2021Co-Authors: Yudong Zhang, Hongchao Kou, Fengming Qiang, Emmanuel Bouzy, Lingling WangAbstract:Abstract Dynamic recovery (DRV) and dynamic recrystallization (DRX) of hexagonal materials during hot deformation is a sub-domain of restoration processes, which has not well been addressed. In this work, the DRV and DRX mechanisms of high-temperature α phase in TiAl alloys were thoroughly investigated by microstructural characterization and crystallographic analysis with an extended intragranular Misorientation axis (IGMA) analysis method. It was revealed that the nucleation of recrystallized grains experienced three steps: (ⅰ) the occurrence of stress-induced grain boundary bulging and the formation of symmetrical-tilt low-angle boundaries characterized by 〈 0001 〉 disorientation axis induced by prismatic slip; (ⅱ) the evolution of the low-angle boundaries into asymmetrical-tilt boundaries characterized by 〈 10 1 ¯ x 〉 disorientation axis by local basal slip, or tilt-twist boundaries characterized by 〈 11 2 ¯ y 〉 disorientation axis through rotational grain boundary sliding, resulting in the formation of subgrains from the boundary bulges; (ⅲ) the detachment of the subgrains and the mixing of the detached subgrains by grain boundary sliding. These three steps happened continuously and repeatedly from the boundary regions toward grain interior till the completion of recrystallization. This work provided original information of continuous DRX (CDRX) of hexagonal materials. The extended IGMA method developed in this study is helpful for related investigations.
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grain boundary characteristics and texture formation in a medium carbon steel during its austenitic decomposition in a high magnetic field
Acta Materialia, 2005Co-Authors: Yudong Zhang, Xiang Zhao, Claude Esling, Jeansebastien Lecomte, Liang ZuoAbstract:Abstract A 12-T magnetic field has been applied to a medium plain carbon steel during the diffusional decomposition of austenite and the effect of a high magnetic field on the distribution of Misorientation angles, grain boundary characteristics and texture formation in the ferrite produced has been investigated. The results show that a high magnetic field can cause a considerable decrease in the frequency of low-angle Misorientations and an increase in the occurrence of low Σ coincidence boundaries, in particular the Σ3 of ferrite. This may be attributed to the elevation in the transformation temperature caused by the magnetic field and, therefore, the reduction of the transformation stress. The wider temperature range for grain growth offers longer time to the less mobile Σ boundaries to enlarge their areas. Moreover, the magnetic field can enhance the transverse field-direction fiber (〈0 0 1〉∥TFD). It can be assumed that the effects of the field were caused by the dipolar interaction between the magnetic moments of Fe atoms.
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The effects of thermal processing in a magnetic field on grain boundary characters of ferrite in a medium carbon steel
Journal of Materials Science, 2005Co-Authors: Yudong Zhang, G. Vincent, Natanael Dewobroto, Lionel Germain, Xiang Zhao, Liang Zuo, Claude EslingAbstract:Effects of a magnetic field on low-angle Misorientation distribution and CSL boundary occurrence in ferrite in 42CrMo steel during the austenite to ferrite and pearlite transformation were investigated. The results show that a magnetic field can considerably lower the frequency of low-angle Misorientations in ferrite lamellae and raise the occurrence of Σ coincidence boundaries, especially Σ 3 in ferrite. But no obvious effect on crystallographic orientation distribution, or texture, was detected.
Jianbao Gao - One of the best experts on this subject based on the ideXlab platform.
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dewetting of ni silicide thin film on si substrate in situ experimental study and phase field modelling
Acta Materialia, 2021Co-Authors: Jianbao Gao, Annie Malchere, Shenglan Yang, Andrea Campos, Ting Luo, Khalid Quertite, Philippe SteyerAbstract:Abstract In this paper, the in-situ Scanning Electron Microscopy (in-situ SEM) technique and three-dimensional (3-D) phase-field simulation were combined to perform a comprehensive study on the kinetics and mechanisms of dewetting (or agglomeration) of a 30 nm NiSi films on Si(100) substrate at 600°C. The evolution of texture during agglomeration of the polycrystalline NiSi thin film was also studied by ex-situ Electron BackScattered Diffraction (EBSD). The phase-field simulation results showed that abnormal grain growth plays an important role in the dewetting process of polycrystalline films, while the Misorientations between NiSi grains and the Si substrate are the main reason for the agglomeration of NiSi polycrystalline thin film on the monocrystal Si substrate. Moreover, 3-D phase-field simulations coupled with experimental information on Misorientation distribution and initial grain size were also performed, and the simulated Ni silicide grain morphology is in good agreement with the in-situ SEM results during agglomeration. In order to slow down or to suppress the agglomeration, it is highly recommended to either increase the volume fraction of low angle grains, or decrease the Misorientation of the NiSi grain/Si substrate or the NiSi grains.
Liang Zuo - One of the best experts on this subject based on the ideXlab platform.
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grain boundary characteristics and texture formation in a medium carbon steel during its austenitic decomposition in a high magnetic field
Acta Materialia, 2005Co-Authors: Yudong Zhang, Xiang Zhao, Claude Esling, Jeansebastien Lecomte, Liang ZuoAbstract:Abstract A 12-T magnetic field has been applied to a medium plain carbon steel during the diffusional decomposition of austenite and the effect of a high magnetic field on the distribution of Misorientation angles, grain boundary characteristics and texture formation in the ferrite produced has been investigated. The results show that a high magnetic field can cause a considerable decrease in the frequency of low-angle Misorientations and an increase in the occurrence of low Σ coincidence boundaries, in particular the Σ3 of ferrite. This may be attributed to the elevation in the transformation temperature caused by the magnetic field and, therefore, the reduction of the transformation stress. The wider temperature range for grain growth offers longer time to the less mobile Σ boundaries to enlarge their areas. Moreover, the magnetic field can enhance the transverse field-direction fiber (〈0 0 1〉∥TFD). It can be assumed that the effects of the field were caused by the dipolar interaction between the magnetic moments of Fe atoms.
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The effects of thermal processing in a magnetic field on grain boundary characters of ferrite in a medium carbon steel
Journal of Materials Science, 2005Co-Authors: Yudong Zhang, G. Vincent, Natanael Dewobroto, Lionel Germain, Xiang Zhao, Liang Zuo, Claude EslingAbstract:Effects of a magnetic field on low-angle Misorientation distribution and CSL boundary occurrence in ferrite in 42CrMo steel during the austenite to ferrite and pearlite transformation were investigated. The results show that a magnetic field can considerably lower the frequency of low-angle Misorientations in ferrite lamellae and raise the occurrence of Σ coincidence boundaries, especially Σ 3 in ferrite. But no obvious effect on crystallographic orientation distribution, or texture, was detected.