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If variety is the spice of life, then abnormal grain growth (AGG) may be the materials processing equivalent of sriracha sauce. Abnormally growing grains can be prismatic, dendritic, or practically any shape in between. When they grow at least an order of magnitude larger than their neighbors in the matrix—a state we call extreme AGG—we can examine the abnormal/matrix interface for clues to the underlying mechanism. Simulating AGG for various formulations of the grain boundary (GB) equation of motion, we show that anisotropies in GB mobility and energy leave a characteristic fingerprint in the abnormal/matrix boundary. Except in the case of prismatic growth, the morphological signature of most reported instances of AGG is consistent with a certain degree of GB mobility variability. Open questions remain, however, concerning the mechanism by which the corresponding growth advantage is established and maintained as the GBs of abnormal grains advance through the matrix. [ABSTRACT FROM AUTHOR]

Abstract—The effect of grain-boundary edges and facets on the mobilities of the individual [11 0] and [10 0] tilt boundaries in zinc of various purities has been experimentally investigated at various temperatures. In the temperature range (0.82–0.98)Tm, where Tm is the melting temperature, the grain boundary mobility is found to depend equally on the temperature, the structure of a boundary, the purity of the material, and grain-boundary features. [ABSTRACT FROM AUTHOR]

Abstract—The specific features of the motion of individual grain boundaries and grain boundaries with triple junctions are analyzed depending on their crystallography and grain-boundary phase transitions. Statistical data on the faceting of boundaries and the hysteresis of their mobility are used to make a conclusion about the possible nature of microstructural heterogeneity in space and time. [ABSTRACT FROM AUTHOR]

The microstructure and texture evolution of tantalum (Ta) during annealing were studied in this work, in addition to simulating and analyzing of the grain boundary motions and their interactions using molecular dynamics simulations (MD). The results showed that the evolution of the recrystallized texture in the surface region of Ta is different from that in the core region. Coincident site lattice (CSL) is significantly related to the grain orientation and thus directly affects the recrystallization texture. MD results showed that the grain boundary mobility rate is higher at low angle grain boundaries (LAGB), which implied that the mobility rate is influenced by the tilt angle. Lower mobility rate of high angle grain boundary (HAGB) possibly caused by a large number of stabilized Σ3 grain boundaries (GBs) between them. It is shown that the inhomogeneous distribution of the recrystallization texture during the annealing process can be possibly attributed to the presence of a large number of Σ3 GBs. • The variation of low-Σ CSL GBs with annealing temperature is similar to the {111} orientation, and the high-Σ CSL GBs change similarly to the {001} orientation. • The formation of {111} texture is mainly controlled by Σ3 and Σ5 GB. • Σ3 GBs of large and stable exist, and even other GBs tend to change towards Σ3 GBs, leading to an increase in the {111} recrystallization texture. [ABSTRACT FROM AUTHOR]

We investigate olivine grain boundary (GB) migration in natural peridotites experimentally annealed at high pressure and high temperature, and couple the experimental observations to full-field grain growth models to provide the distribution of GB mobilities in natural olivine polycrystals. A stack of four slices of natural mylonitic peridotite (Oman ophiolite) was annealed at 1473 K for 5 h under a confining pressure of 300 MPa of argon in a Paterson press. The three sintered interfaces of the stack are characterized using scanning electron microscopy and electron backscatter diffraction (EBSD) to extract a distribution of apparent (2D) GB displacements. Full-field simulations of numerically sintered interfaces are then used to infer the GB mean mobility and distribution in olivine polycrystal allowing forward modelling exercise to reproduce the experimental GB displacement distribution. This yields widely dispersed mobilities, which can be approximated by a log-normal distribution of 1 0 − 15.85 ± 2.58 m 4. J − 1. s − 1. Both the average and the dispersion of GB mobilities can be explained by silicon grain boundary diffusion. Finally, we demonstrate that the high variability of GB mobilities in olivine implies a decrease of the mean growth rate with time. This elucidates the difficulties of extrapolating experimental grain growth rates to geological timescales and observed microstructures in peridotites. • New methodology for extracting grain boundary mobilities in natural mantle rocks from experimental and modelling approaches • We show that distribution of olivine GB mobility in mantle rocks spans several orders of magnitude • We show that the wide distribution of GB mobility leads to a decrease of grain growth rate with time in static context • Our findings explain the difficulties of extrapolating laboratory-based growth rate to geological time scales [ABSTRACT FROM AUTHOR]

Two finite element level-set (FE-LS) formulations are compared for the modeling of grain growth of 316L stainless steel in terms of grain size, mean values, and histograms. Two kinds of microstructures are considered: some are generated statistically from EBSD maps, and the others are generated by the immersion of EBSD data in the FE formulation. Grain boundary (GB) mobility is heterogeneously defined as a function of the GB disorientation. On the other hand, GB energy is considered as heterogeneous or anisotropic, which are, respectively, defined as a function of the disorientation and both the GB misorientation and the GB inclination. In terms of mean grain size value and grain size distribution (GSD), both formulations provide similar responses. However, the anisotropic formulation better respects the experimental disorientation distribution function (DDF) and predicts more realistic grain morphologies. It was also found that the heterogeneous GB mobility described with a sigmoidal function only affects the DDF and the morphology of grains. Thus, a slower evolution of twin boundaries (TBs) is perceived. [ABSTRACT FROM AUTHOR]

Utilizing the data assimilation and multi-phase-field grain growth model, this study proposes a novel framework of measuring anisotropic (nonuniform) grain boundary energy and mobility. The framework can evaluate a large number of boundary properties from typical observations of grain growth without requiring specifically designed experiments or calculations. In this method, by optimizing the multi-phase-field model parameters such that the simulation results are in good agreement with the observation data, the energies and mobilities of multiple individual boundaries are directly and simultaneously estimated. To validate the method, numerical tests on boundary property estimation were performed using synthetic microstructure dataset generated from grain growth simulations with a priori assumed property values. Systematic tests on simple tricrystal systems confirmed that the proposed method accurately estimates each boundary energy and mobility within an error of only several % of their assumed true values even for conditions with strong property anisotropy and grain rotation. Further numerical tests were conducted on a more general multi-grain system, showing that our method can be successfully applied to complicated polycrystalline grain growth. The obtained results demonstrate the potential of the proposed method in extracting a large dataset of grain boundary properties for arbitrary boundaries from actual grain growth observations.

For the research of heat treatment technology of aero-engine turbine discs with dual-performance, the experimental methods are mostly used to establish the database of heat treatment technology of turbine disk alloys. However, the experimental methods have a long research period and high cost. A phase field model that can be used to predict the grain evolution during heat treatment process with variable temperature is proposed. To simulate the grain evolution of superalloy at different heat treatment temperatures, the phase field model is improved by introducing the Arrhenius relationship, which describes the quantitative relationship between the grain boundary movement and temperature. Based on the improved phase field model and fitted model parameters, the grain average size and morphology evolution of superalloy FGH96 during the longer heat treatment process are simulated, and the simulated results are compared with the experimental results. The results show that the calculated data coincide with the experiment data, the evolution law of grain is consistent with the experimental observation and theoretical analysis. These results prove that the grain boundary mobility M in the Arrhenius relationship is suitable for simulating the heat treatment process at the corresponding temperature. At the same time, the feasibility of the improved method and accuracy of the fitted parameters of the phase-field model are verified.

Two finite element level-set (FE-LS) formulations are compared for the modeling of grain growth of 316L stainless steel in terms of grain size, mean values, and histograms. Two kinds of microstructures are considered: some are generated statistically from EBSD maps, and the others are generated by the immersion of EBSD data in the FE formulation. Grain boundary (GB) mobility is heterogeneously defined as a function of the GB disorientation. On the other hand, GB energy is considered as heterogeneous or anisotropic, which are, respectively, defined as a function of the disorientation and both the GB misorientation and the GB inclination. In terms of mean grain size value and grain size distribution (GSD), both formulations provide similar responses. However, the anisotropic formulation better respects the experimental disorientation distribution function (DDF) and predicts more realistic grain morphologies. It was also found that the heterogeneous GB mobility described with a sigmoidal function only affects the DDF and the morphology of grains. Thus, a slower evolution of twin boundaries (TBs) is perceived.

The purity, preferred orientation, microstructure, microhardness, bonding strength, thickness uniformity and thermal stability of rhenium (Re) coatings prepared on graphite wafers by chemical vapor deposition (CVD) and electrodeposition (ED) in molten salts were comparatively studied in this paper. It was found that carbon (0.0140 wt%) and oxygen (0.0067 wt%) were the primary impurities for CVD and ED Re coatings, respectively. The diffusion of carbon into CVD Re coating caused higher microhardness near the substrate and helped to improve the bonding strength at the same time. The preferred orientation, microstructure and microhardness of ED Re coating were all susceptible to oxygen. The coating deposition uniformity of ED Re is obviously better than that of CVD Re coating, due to its intrinsic characteristics. The 〈002〉-oriented, coarse columnar CVD Re coating exhibited better thermal stability compared with that of the 〈110〉-oriented, fiber-like columnar ED Re coating, while the ED Re grains grew remarkably and the microstructure evolved toward the similar structure of CVD Re after annealing treatment. The diversity of Re coatings in microstructure could be attributed to the mobility of grain boundaries (affected by temperature and impurity) during deposition processes. [ABSTRACT FROM AUTHOR]

The equilibrium crystal shape is a convex shape bound by the lowest energy interfaces. In many polycrystalline microstructures created by grain growth, the observed distribution of grain boundary planes appears to be dominated at low driving forces (after long grain growth times) by the planes present in the equilibrium crystal shape. However, at earlier stages of grain growth, it is expected that kinetic effects will play an important role in grain boundary motion and morphology. Analogous to the equilibrium crystal shape, the kinetic crystal shape of seed crystals growing from a liquid at higher supersaturations is bound by the slowest growing orientations. This study presents an equivalent construction for grain boundaries in polycrystals and uses it to determine the kinetic crystal shape for strontium titanate as a function of temperature. Relative grain boundary mobilities for strontium titanate for the low energy crystallographic orientations from seeded polycrystal experiments are used to calculate the kinetic crystal shapes as a function of temperature and annealing atmosphere. The kinetic crystal shapes are then compared to the morphologies and orientations of the interfaces of the growing seed crystals, and to the equilibrium crystal shapes, as well. The conclusions are that (1) the kinetic crystal shape is extremely anisotropic and displays significant transitions as a function of temperature that do not mirror changes in equilibrium crystal shape, (2) the kinetic shapes observed in the microstructures are dominated by the growing side of the interface (single crystal) and not by the dissolving side (polycrystalline matrix), and (3) faster growing orientations break up into macroscopic facets composed of slower growing orientations. The implications for grain growth underscore the applicability of crystal growth models to grain growth in polycrystals. In particular, in strontium titanate, the anisotropy of the grain boundary mobility as represented in the kinetic crystal shape is expected to be reduced from five macroscopic parameters to two (interface normal) allowing for incorporation of growth rate anisotropy in simulations of microstructure evolution at the earliest stages of grain growth, i.e. at the highest driving forces. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Grain boundary energy and mobility are strongly correlated with the grain boundary excess free volume, whose dependence on temperature has yet to be investigated. Using molecular dynamics simulations on face-centered-cubic and body-centered-cubic metals, we found that the boundary free volume decreases with temperature, diminishing the interatomic bond distortion and the grain boundary energy per atom. Interestingly, the free volume shrinks much faster above a critical temperature; meanwhile, the activation energy for grain boundary migration jumps due to collective atomic motion. These insights on the temperature dependence of grain boundary free volume can be instrumental in stabilizing the microstructure of nanocrystals. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Classical theories of grain growth assume that grain boundary curvature and velocity are linearly correlated. However, recent experimental observations seem to imply that the relationship is more complex in polycrystalline materials. Here, we determined the velocity and curvature of approximately 12 000 grain boundaries from molecular dynamics simulations of the annealing of randomly textured nanosized polycrystalline Al. The grain boundary kinetics was studied at different scales and compared with theoretical predictions. At the grain level, that is, attributes averaged over all grain boundaries of a grain, the kinetics behavior showed a linear correlation of curvature and velocity. When studying grain-boundary-level behavior (i.e., attributes of individual grain boundaries are considered), we found a clear correlation between the sign of the grain boundary curvature and the direction of migration. Nevertheless, considering all the grain boundaries, the correlation coefficient between the magnitude of velocity and curvature was found to be low (0.34) although a strong linear correlation was observed for a subset of high angle, long-lived, initially large grain boundaries with uniform curvature distribution and similar 5 crystallographic degrees of freedom. Furthermore, some grain boundaries that are close in crystallographic space showed different kinetics. This elucidates the complexity of grain boundary migration in the nanocrystalline ensemble as a result of complex dependence of grain boundary mobility and energy on grain boundary character, and the existence of other driving forces that affect the behavior of grain boundary migration. Among others, defect distributions and variation in stress distribution between neighboring grains are found to influence grain boundary migration. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, four different finite element level-set (FE-LS) formulations are compared for the modeling of grain growth in the context of polycrystalline structures and, moreover, two of them are presented for the first time using anisotropic grain boundary (GB) energy and mobility. Mean values and distributions are compared using the four formulations. First, we present the strong and weak formulations for the different models and the crystallographic parameters used at the mesoscopic scale. Second, some Grim Reaper analytical cases are presented and compared with the simulation results, and the evolutions of individual multiple junctions are followed. Additionally, large-scale simulations are presented. Anisotropic GB energy and mobility are respectively defined as functions of the mis-orientation/inclination and disorientation. The evolution of the disorientation distribution function (DDF) is computed, and its evolution is in accordance with prior works. We found that the formulation called “Anisotropic” is the more physical one, but it could be replaced at the mesoscopic scale by an isotropic formulation for simple microstructures presenting an initial Mackenzie-type DDF.

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Abstract Plastic strain induced grain boundary migration (SIBM) is investigated by means of in-situ SEM experiments and AFM surface observations in the case of 4N pure aluminum. The study focuses on two polycrystalline samples obtained through different thermo-mechanical treatments that provide different initial (grain size and orientation) microstructures with different evolutions during heating. A total of 77 grain boundaries (GBs) were characterized from both samples. Evaluation of GB displacements was allowed by determining fixed points on initial and final EBSD maps and marks from thermal grooving along GB contours. Some sub-surface final examinations ensured the surface ones being pretty well representative of the bulk behaviour. The GB displacements were related to their geometry and to their initially available migration driving force P , the two main contributions of which were estimated. The boundary curvature contribution P c is estimated from SEM observations and the so-called "stored energy" contribution P Δ ρ (that results from the differential of dislocation densities Δ ρ across the boundary) is estimated using a crystal plasticity modeling within a homogenization scheme for aggregates validated on slip trace identifications from AFM observations. The resulting driving force P , related to the GB velocity V through the widely used law V = M P is compared with the observed displacement during a finite annealing time. Additional effects as thermal grooving and triple junction pinning or pulling are also discussed from complementary SEM and AFM observations of some typical GBs. Some evidences of GB out-of-plane displacements possibly contributing to the migration process are also commented. The quite extensive set of data regarding grain orientation, GB misorientation and curvature, intracrystalline slip activity and evolution during heating constitutes references for future comparisons with mesoscale simulations. [ABSTRACT FROM AUTHOR]

Abstract The influence of CaO on the evolving microstructure of alumina has been studied in a range of concentrations below the solubility limit. The amount of Ca in the alumina was determined by conducting fully standardized wavelength dispersive spectroscopy, and the change in grain boundary mobility as a function of the amount of dopant was characterized using scanning electron microscopy. Unlike segregating dopants which reduce grain boundary mobility by solute-drag, CaO increases the rate of grain growth, and a trend of increased mobility with increasing dopant level was shown. The increased mobility with Ca segregation is believed to be due to an increase in vacancy concentration in the vicinity of the grain boundaries, thus facilitating faster grain boundary motion. [ABSTRACT FROM AUTHOR]

Microstructure and mechanical properties development of extruded Mg-1Y (wt. %) binary alloy during equal channel angular pressing (ECAP) with route Bc at 400 °C, and subsequent annealing treatment between 300-400°C at different holding time of 5-120 min were investigated using an optical and scanning electron microscope (SEM), electron back scattered diffraction (EBSD), tensile test, and hardness test. The grain size of as-extruded material (~10.9μm) was refined significantly by 1-pass ECAP (~5.8μm), and resulted in a remarkably enhanced elongation to failure (EL) (~+62%) with a slightly decreased ultimate tensile strength (UTS) (~-3%) comparing to the as-extruded condition (EL = 11.3%, UTS = 200 MPa). The EL was further increased to 27.3% (~+142%) after four passes of ECAP comparing to the as-extruded condition, which was mainly caused by the much more homogenized microstructure. The split basal poles with about 60° rotations to the extruded direction (ED), the relatively coarsened grain size by static recrystallization (SRX) and post-dynamic recrystallization (PDRX) after four passes of ECAP might be responsible for the decreased strength with increasing ECAP pass. During the annealing treatment, recovery dominantly occurred at 300°C, SRX and grain growth emerged at 350°C and 400°C, respectively. Meanwhile, the grain grew and hardness decreased rapidly even within 5 min for 1-pass ECAPed material at 400°C, indicating a larger grain boundary mobility of ECAPed materials induced by higher deformation energy than the as-extruded ones. [ABSTRACT FROM AUTHOR]

Presented here is the evolution of microstructure of Ta during multi-axial forging, with a specific understanding of how grain boundary movement and their interaction occurs during deformation. Coarse grained (CG) tantalum was multi-axially forged till 3, 6, and 9 passes in a closed channel die to refine grain size and increase its strength. The grain size was refined from 64 μm to 760 nm, the dislocation density increased by an order of magnitude, the Vickers hardness increased from 92 to 223 HV, and the yield strength at room temperature increased from 178 to 653 MPa. It was found that the contribution of GBs to strengthening was higher than that of dislocations. Mechanisms for GB structure evolution during deformation were studied using molecular dynamics (MD) simulations. GB mobility was high for low angle GBs (LAGBs) and reduced with increasing tilt angle, after which it took on low values for high angle GBs (HAGBs). The mobility of specific HAGBs were due to the formation of ledges/disconnections and dissociation into other GB types. MD results showed that the decrease in fraction of HAGBs during initial deformation can be attributed to the dissociation of HAGBs with low mobility into GBs with higher mobility. [Display omitted] • Multiaxial forging of tantalum increased the strength from 178 to 653 MPa. • Grain boundary (GB) spacing was reduced from 64 μm to 760 nm after 9 passes. • GBs contribute to yield strength than that of dislocations. • Mobility of low angle GBs are higher than that of high angle GBs. [ABSTRACT FROM AUTHOR]

Conventional grain growth models do not account for the variability in grain boundary (GB) mobility, GB energy, and solute segregation energy that depend in detail on the GB structure. Instead, these parameters are typically determined empirically from experimental measurements. In this study, we present a systematic quantitative analysis that accounts for anisotropic GB properties, i.e., GB mobility and solute drag pressure to rationalize the average grain size evolution using representative GB properties under normal grain growth conditions in a non–textured system. We perform two–dimensional phase field simulations to analyze the role of (1) anisotropic mobility, (2) anisotropic segregation, and (3) combined anisotropic mobility and segregation on the average grain size evolution. A corresponding "representative GB" is introduced for each case considering a phenomenological grain growth model that fits the simulated average grain size evolution. A relationship is proposed to determine the representative GB properties for an arbitrary distribution of anisotropic GB properties in the initial microstructure. Simulations with the combined variation of GB mobility and segregation energy suggest that the representative GB mobility and segregation energy determined from independent simulations with varying either GB mobility or segregation energy can be superimposed to determine the average grain size evolution in agreement with the phase field simulations. The limits of validity, i.e., the range of moderate anisotropies, are identified where the phenomenological model with representative GB properties may be applicable, i.e., as long as a mean grain size is an appropriate descriptor of the grain structure. [Display omitted] • 2D phase field simulations for moderately anisotropic grain growth. • Determination of representative grain boundary (GB) from GB property distributions. • Superposition of mobility and solute drag anisotropies for representative GB. [ABSTRACT FROM AUTHOR]

The impact of DC electric fields on grain growth in strontium titanate is investigated between 1350 °C and 1550 °C for fields of up to 50 V/mm. To prevent joule heating by electrical currents, insulating Al 2 O 3 plates separate electrodes from samples. The seeded polycrystal technique is used, which allows evaluating gradients induced by electric fields. The growth direction of the single crystalline seeds is perpendicular to the electric field; hence electrostatic forces do not influence its growth. Below 1425 °C, the influence of electric fields is very weak. Above 1425 °C the field results in an increase of the grain boundary mobility at the negative electrode. The enhancement of the boundary mobility at the negative electrode is attributed to electric field induced defect redistribution. Oxygen vacancies migrate towards the negative electrode, while strontium vacancies accumulate at the positive electrode. This defect redistribution is connected to the defect chemistry dependent grain growth in strontium titanate. [ABSTRACT FROM AUTHOR]

One of the key aspects influencing microstructural design pathways in metallic systems is grain boundary motion. The present work introduces a method by means of which direct measurement of grain boundary mobility vs. misorientation dependence is made possible. The technique utilizes datasets acquired by means of serial electron backscatter diffraction (EBSD) measurements. The experimental EBSD measurements are collectively analyzed, whereby datasets were used to obtain grain boundary mobility and grain aspect ratio with respect to grain boundary misorientation. The proposed method is further validated using cellular automata (CA) simulations. Single crystal aluminium was cold rolled and scratched in order to nucleate random orientations. Subsequent annealing at 300 °C resulted in grains growing, in the direction normal to the scratch, into a single deformed orientation. Growth selection was observed, wherein the boundaries with misorientations close to Σ7 CSL orientation relationship (38° 〈111〉) migrated considerably faster. The obtained boundary mobility distribution exhibited a non-monotonic behavior with a maximum corresponding to misorientation of 38° ± 2° about 〈111〉 axes ± 4°, which was 10–100 times higher than the mobility values of random high angle boundaries. Correlation with the grain aspect ratio values indicated a strong growth anisotropy displayed by the fast growing grains. The observations have been discussed in terms of the influence of grain boundary character on grain boundary motion during recrystallization. [ABSTRACT FROM AUTHOR]

In molecular dynamics (MD) simulations of grain boundary (GB) migration it is quite common to find a temperature dependence of GB mobility that deviates strongly from an Arrhenius-type dependence. This usually indicates that more than one mechanism is actually active. With the goal to separate different GB migration mechanisms we investigate a Σ7 <111> 38.2° GB by MD using an EAM potential for aluminium. To drive the GB with a well-known and adjustable force, the e nergy c onserving o rientational d riving f orce (ECO DF) is used that had been introduced recently. The magnitude of the DF and the temperature are varied. This yielded a high and a low temperature range for the GB velocity, with a transition temperature that depends on the magnitude of the DF. A method is introduced which allows both a visual and a statistical characterization of GB motion on a per atom basis. These analyses reveal that two mechanisms are active in this GB, a shuffling mechanism and its initiation. These mechanisms operate in a sequential, coupled manner. Based on this, a simple model is introduced that describes all simulated GB velocities (and hence the mobility) very well, including the transition between the dominating mechanisms. [ABSTRACT FROM AUTHOR]

The impact of the oxygen partial pressure on grain growth in high purity strontium titanate is evaluated by observing microstructures, the grain growth constant k and the relative mobility of strontium titanate. The microstructures indicate a reversible wetting transition between 1460 °C and 1500 °C. The wetting second phase is titania-rich and free from any detectable solutes. Exaggerated grain growth was found close to the wetting transition and is explained by a temperature dependent anisotropy of the grain boundary mobility in conjunction with a high mobility of wetted grain boundaries. The grain growth constant in reducing atmosphere shows two transitions at 1350 °C and at 1460 °C. At the first transition the grain growth constant decreases with temperature. A strong increase of k with temperature at the second transition is attributed to the wetting transition. The relative grain boundary mobility of different orientations ({1 0 0}, {1 1 0}, {1 1 1}, and {3 1 0}) was measured in oxidizing and reducing atmosphere by observing the growth of oriented single crystals into polycrystals. At 1550 °C in reducing atmosphere the wetting liquid phase enhances the mobility by a factor of ∼10 compared to oxidizing atmosphere. Additionally, the atmosphere changes grain growth kinetics: in oxidizing, but not in reducing atmosphere growth stagnation occurs for long dwell times. [ABSTRACT FROM AUTHOR]

The intrinsic mobilities of a series of symmetrical tilt grain boundaries (GBs) in Al were simulated using a modified synthetic driving force method. Statistical analysis of the mobility data reveals that low-angle GBs are on average more mobile and show larger spread in their mobilities than high-angle GBs. GBs with axes broadly near 〈111〉 or boundary planes close to {1 1 0} have relatively high mobility. No visible correlation is found between GB mobility and energy. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations of simple bicrystal systems have been much used as a tool to explore how the migration of grain boundaries varies with their structure and with experimental conditions. In order to permit the exploration of a large parameter space, many studies are forced to rely on a small number of simulations (often a single simulation) for each configuration. The motion of a grain boundary is inherently statistical and any variability in the measured grain boundary velocity should be taken into account in subsequent analysis of trends in grain boundary mobility. Here we present the results of large numbers of simulations of equivalent boundaries, which show that this variability can be large, particularly when small systems are simulated. We show how a bootstrap resampling approach can be used to characterise the statistical uncertainty in boundary velocity using the information present in a single simulation. We show that the approach is robust across a variety of system sizes, temperatures and driving force strengths and types, and provides a good order-of-magnitude measure of the population standard deviation across multiple equivalent simulations. [ABSTRACT FROM AUTHOR]

We present a suite of measurements and combined analyses of grain growth in SrTiO 3 for oriented single crystals into polycrystals. The growth distance and standard deviation and the microstructure evolution along the single crystal–matrix interface are used to locally characterize the change in migration behavior as a function of temperature, time and interface orientation. The relative grain boundary mobility was determined between 1250 °C and 1600 °C for four crystallographic orientations {1 0 0}, {1 1 0}, {1 1 1} and {3 1 0}. An absolute mobility of these orientations is estimated. Under fast growth conditions the morphology of single crystals shows macroscopic stepping with parts of the interface rotating to low mobility orientations. This effect represents a kinetic influence on the grain boundary morphology. The results also indicate dragging effects on microstructure coarsening, which indicate the existence of a critical driving force for grain growth. This critical driving force seems to be related to an ‘intrinsic’ interface drag similar to the solute drag, but based on intrinsic defects. At 1460 °C the growth of single crystals was significantly faster than expected from the mobility of the polycrystal and was identified as exaggerated grain growth. The findings give new insights into the recently published grain growth anomaly of strontium titanate, leading to a hypothesis based on the temperature dependent relative mobility of {1 0 0} oriented grain boundaries. [ABSTRACT FROM AUTHOR]

An in situ annealing stage has been developed in-house and integrated in the chamber of a Scanning Electron Microscope equipped with an Electron BackScattered Diffraction system. Based on the Joule effect, this device can reach the temperature of 1200°C at heating rates up to 100°C/s, avoiding microstructural evolutions during heating. A high-purity tantalum deformed sample has been annealed at variable temperature in the range 750°C-1030°C, and classical mechanisms of microstructural evolutions such as recrystallization and grain coarsening phenomena have been observed. Quantitative measurements of grain growth rates provide an estimate of the mean grain boundary mobility, which is consistent with the value estimated from physical parameters reported for that material. In situ annealing therefore appears to be suited for complementing bulk measurements at relatively high temperatures, in the context of recrystallization and grain growth in such a single-phase material. [ABSTRACT FROM AUTHOR]

Abstract: Curvature driven migration of a series of 〈110〉 tilt grain boundaries in a bcc Fe–20Cr alloy is simulated using molecular dynamics to investigate the relationship between the atomic migration mechanism and mobility at medium to high temperatures. The boundaries studied include low angle boundaries (LAGB), high angle boundaries (HAGB) and singular boundaries, such as coherent twins. The steady-state boundary shape and curvature are compared with a simple analytical model which incorporates the dependence of absolute mobility and free energy on boundary inclination. The comparison indicates that the 109.5° () Σ3 coherent twin boundary will have relatively low energy but high mobility. This result is attributed to a particularly effective repeated shuffle mechanism which occurs on the twinning plane. Two other migration mechanisms are observed, one involving the motion of 〈111〉 glissile dislocations in LAGB and the other involving uncorrelated atomic shuffles in HAGB, sometimes associated with interfacial steps. [Copyright &y& Elsevier]

With a new approach of driving force application, the relationship between grain boundary migration rate and driving force was found to be nonlinear as to be expected from rate theory for extremely large applied driving forces in molecular dynamics simulations. By evaluating grain boundary mobility non-linearly, the simulated mobilities at different temperatures reveal that grain boundary migration is a thermally activated process. For a series of <110> twist grain boundaries, the resultant mobilities demonstrate that high-angle grain boundaries move much faster than low-angle grain boundaries. [ABSTRACT FROM AUTHOR]

The mobility of low-angle grain boundaries in pure metals is reviewed and several theoretical treatments are provided. The approach that provides the best agreement with the available experimental data is one in which the mobility is controlled by vacancy diffusion through the bulk to (and from) the dislocations that comprise the boundary that are bowing out between pinning points. The pinning points are presumed to be extrinsic dislocations swept into the boundaries or grown in during the prior processing of the material. This approach yields a mobility that is constant with respect to misorientation angle, up to the transition to the high-angle regime. For small misorientations of the order 1°, however, the mobility appears to increase with decreasing misorientation angle. [ABSTRACT FROM AUTHOR]

Abstract: Computational experiments are used to show that grain boundary (GB) mobility is independent of driving force for a two-dimensional, square-lattice Ising model with Metropolis kinetics. This is established over the entire Monte Carlo (MC) temperature range. A calibration methodology is also provided which endows the MC algorithm with time and length scales and expresses the MC parameters in terms of experimentally measurable quantities. These links are used to verify that the square-lattice MC paradigm delivers the desired sharp-interface grain boundary kinetics over a wide range of mixed driving forces and MC temperatures using a single mobility function. [Copyright &y& Elsevier]

Abstract: Diffusion induced grain boundary migration (DIGM) has been studied in the Ag–Zn system by exposing polycrystalline Ag to Zn vapor with a Ag-25 wt.% Zn alloy as the source of Zn. The time and temperature dependence of the migration distance has been studied in the temperature range 660 to 810 K. The composition profile was obtained on the sheet cross-section along a line perpendicular to the edge to determine Db δ at each temperature. Similarly, the Zn concentration profile was obtained from the region swept by the migrating grain boundary. The coherency strain energy, the total chemical free energy change and the effective free energy change were calculated. The regular solution model was used for calculating the free energy change. It has been observed that a fraction of the total free energy has been used for volume diffusion in front of the migrating grain boundary. The instantaneous rate of migration has been observed to be directly proportional to the chemical free energy change and the coherency strain energy. The instantaneous rate of migration versus the composition graph has indicated that the driving force for DIGM in the Ag–Zn system is the coherency strain energy. The fine-grained layer formed at the surface follows a parabolic growth behavior. The diffusion coefficients calculated from the composition profile as well as from the rate of growth of the fine-grained layer are of the same order of magnitude. The diffusivity values are four to six orders of magnitude higher than the volume diffusion coefficients. From the activation energy and the diffusivities it is clear that DIGM in the Ag–Zn system occurs by the diffusion of Zn along the grain boundaries of polycrystalline Ag. [Copyright &y& Elsevier]

Abstract: We investigated the migration of a symmetric tilt, low-angle grain boundary (LAGB) under applied shear stress in the presence of extrinsic dislocations. The results demonstrate that there is a threshold stress for the LAGB to depin from extrinsic dislocations. Below the threshold stress, the LAGB remains immobile at zero dislocation climb mobility, while for finite climb mobilities, it migrates at a velocity that is directly proportional to the applied stress, with a proportionality factor that is a function of misorientation, dislocation climb mobility and extrinsic dislocation density. We derive analytical expressions for the LAGB mobility and threshold stress for depinning from extrinsic dislocations. The analytical prediction of the LAGB mobility is in excellent agreement with the simulation as well as experimental results. We discuss the implications of these results for understanding the migration of general grain boundaries. [Copyright &y& Elsevier]

[Display omitted] • A novel estimation method for anisotropic grain boundary energy and mobility was proposed using data assimilation and phase-field simulation. • This method allows for direct estimation of the energies and mobilities of multiple individual grain boundaries from grain growth observations. • The high accuracy of the proposed estimation method was demonstrated via numerical tests using synthetic observation data of grain growth. Utilizing the data assimilation and multi-phase-field grain growth model, this study proposes a novel framework of measuring anisotropic (nonuniform) grain boundary energy and mobility. The framework can evaluate a large number of boundary properties from typical observations of grain growth without requiring specifically designed experiments or calculations. In this method, by optimizing the multi-phase-field model parameters such that the simulation results are in good agreement with the observation data, the energies and mobilities of multiple individual boundaries are directly and simultaneously estimated. To validate the method, numerical tests on boundary property estimation were performed using synthetic microstructure dataset generated from grain growth simulations with a priori assumed property values. Systematic tests on simple tricrystal systems confirmed that the proposed method accurately estimates each boundary energy and mobility within an error of only several % of their assumed true values even for conditions with strong property anisotropy and grain rotation. Further numerical tests were conducted on a more general multi-grain system, showing that our method can be successfully applied to complicated polycrystalline grain growth. The obtained results demonstrate the potential of the proposed method in extracting a large dataset of grain boundary properties for arbitrary boundaries from actual grain growth observations. [ABSTRACT FROM AUTHOR]

Abstract: The annealing behavior of cold rolled (78%) commercially pure titanium was investigated during a heat treatment at 530°C in a magnetic field of 19T. X-ray diffraction and electron backscatter diffraction measurements were utilized to characterize the crystallographic texture and the grain microstructure. The analysis revealed that the magnetic annealing promotes grain growth in the investigated material. This manifests itself in an accelerated development of typical “grain growth” texture components and the distinctly increased mean grain size after magnetic annealing compared to annealing at zero field. The observed phenomenon is attributed to an enhanced grain boundary mobility by the applied magnetic field. [Copyright &y& Elsevier]

The effect of SiO2 addition on densification and grain-growth behavior of 8YSCZ/SiO2 composites was investigated using high purity 8 mol% yttria-stabilized cubic zirconia powders (8YSCZ) doped with 0, 1, 5, 10 wt% SiO2. The specimens were sintered at 1400°C for 1 hour. It was seen that the sintered density increased with SiO2 content up to 1 wt% and further increase in SiO2 content led to a decrease in density. The enhanced density with increasing SiO2 content up to 1 wt% could be mainly attributable to liquid phase sintering. For grain growth measurements, the specimens sintered at 1400°C were annealed at 1400, 1500, and 1600°C for 10, 50, and 100 hours. The experimental results showed that the grain growth in 8YSCZ/SiO2 composites occurred more slowly than that in undoped 8YSCZ. Also, the grain growth rate decreased with increasing SiO2 content. The grain growth exponent value and the activation energy for undoped 8YSCZ were found to be 2 and 289 kJ/mol, respectively. The addition of SiO2 raised the grain growth exponent value to 3, and activation energy for the grain growth process was increased from 289 to 420 kJ/mol for the addition of SiO2 from 0 to 10 wt%. [ABSTRACT FROM AUTHOR]

Abstract: Under certain circumstances grain growth in polycrystals is controlled by the mobility of grain boundary junctions. The resulting microstructure is distinctly different from the granular assembly in the course of normal grain growth. Also, the structure established under junction control is rather stable even under the conditions characteristic for grain growth governed by grain boundary motion. This provides a means of controlling the grain microstructure evolution, in particular of ultrafine grained and nanocrystalline materials. It is demonstrated that such an effect can be expected not only for 2D arrangements but for 3D microstructures with quadruple junctions as well. The latter statement is supported by an assessment of the mobility of quadruple junctions. We propose to introduce a new branch of grain boundary engineering, namely grain boundary junction engineering. [Copyright &y& Elsevier]

Abstract: The influence of Mn on the mobilities of grain boundaries during recrystallization of Al–0.1 and –0.3wt.% Mn alloys has been characterized by in situ SEM annealing experiments. Polycrystals of high purity, single-phase Al–Mn alloys were deformed in channel-die plane strain compression at room temperature to strains of 1.3. The specimens were in situ annealed in an SEM/EBSD in order to measure grain boundary mobilities at temperatures between 200 and 450°C. Stable “loaded” boundary migration was observed in the 0.1 and 0.3% Mn alloys. However, unstable, partially “free”, boundary migration could also be found in the 0.1% alloy. The mobilities, deduced from the migration rates and the stored energies, were consistent with the solute drag theories of Cahn, Lücke and Stüwe. The diffusion rates controlling the solute drag were of the same order for both theories and the activation energy for boundary migration was found to be intermediate between that of solute diffusion in the lattice and along the grain boundaries. [Copyright &y& Elsevier]

Abstract: The effects of anisotropic grain boundary properties on the evolution of boundary plane distributions were studied using three-dimensional finite element simulations of normal grain growth. The distribution of boundary planes was affected by energy anisotropy whereas no effect was observed for comparatively larger mobility anisotropy. [Copyright &y& Elsevier]

a) Schematic of the upwards displacement of a {3 3 2} GB and the nucleation of a {1 1 2} twin at a GB dislocation. b) c) and d) Nucleation and growth of the twin due to the pileup of disconnections at both sites of the GB dislocation. e) and f) Growth of the twin due to the motion of the tip inside the grain. [Display omitted] • The (3 3 - 2) grain boundary absorbs the ½ 〈1 1 1〉 dislocations. • Resultant defects at the GB depend on glide plane, orientation and sign of the Burgers vector. • Dislocations transform into elementary disconnections gliding away and a GB dislocation (GBD). • The GBD are sources of disconnections that facilitates the shear-coupled GB migration. • Interaction of GBD with disconnections produces either motion of GBD or nucleation of a {1 1 2} twin. {3 3 2} 〈1 1 0〉 tilt grain boundaries (GB) move conservatively under a shear stress by the creation and glide of disconnections. When crystal dislocations interact with the GB they are absorbed and transformed into GB dislocations (GBD). The behaviour of GBDs under shear stress depends on the orientation of the Burgers vector and sense of shear stress. There are two possible scenarios: a) the GBD moves together with the GB in a compensated climb, then plastic deformation is accommodated by shear-coupled GB migration; b) the GBD is sessile because it cannot undergo a compensated climb when interacting with the disconnections. If so, the sessile GBD is the nucleus of a {1 1 2} twin. The nucleation of the twin is produced by the pileup of disconnections at both sides of the GBD. Then, plastic deformation is accommodated by the combination of the motion of the {3 3 2} GB and the growth of {1 1 2} twins inside the grain. [ABSTRACT FROM AUTHOR]

In this study, four different finite element level-set (FE-LS) formulations are compared for the modeling of grain growth in the context of polycrystalline structures and, moreover, two of them are presented for the first time using anisotropic grain boundary (GB) energy and mobility. Mean values and distributions are compared using the four formulations. First, we present the strong and weak formulations for the different models and the crystallographic parameters used at the mesoscopic scale. Second, some Grim Reaper analytical cases are presented and compared with the simulation results, and the evolutions of individual multiple junctions are followed. Additionally, large-scale simulations are presented. Anisotropic GB energy and mobility are respectively defined as functions of the mis-orientation/inclination and disorientation. The evolution of the disorientation distribution function (DDF) is computed, and its evolution is in accordance with prior works. We found that the formulation called "Anisotropic" is the more physical one, but it could be replaced at the mesoscopic scale by an isotropic formulation for simple microstructures presenting an initial Mackenzie-type DDF. [ABSTRACT FROM AUTHOR]