Grain Boundary Migration Velocity Research Articles

Unraveling the electric field effect on the grain‐boundary migration in alumina through phase field modeling

AbstractIt is experimentally demonstrated that the electric field drives the grain‐boundary (GB) migration in ceramics, but this has not been interpreted mechanistically. This work develops a phase field model to study the GB migration in alumina (Al2O3) and validate through the comparison with previous experiments. Results show that the GBs move to the small grain domain. Under an electric field in the positive bias direction, GB migration is enhanced, whereas the migration to the small grain domain is inhibited under the electric field in the negative bias direction. The enhancement or inhibition effect becomes more pronounced with increasing the electric field. The high negative bias induces decrease in the GB migration velocity even with the migration direction altering. It is revealed that GB migrations are dominated by the competitive effect between the curvature and electric field driving forces, and an analytical expression of the critical electric field is derived.

Phase field study of grain boundary migration and preferential growth in non-magnetic materials under magnetic field

This work reports a phase field model to enable a profound insight into the grain boundary (GB) migration of non-magnetic materials based on Bi bicrystals under a magnetic field. Simulation results on GB migration velocity agree well with the experimental results, and the velocity increases linearly with the square of the magnetic strength. It is also found that GB migration direction is affected by the magnetic field direction, and grain with c-axis parallel to the magnetic field direction grows preferentially at the expense of the grain with c-axis perpendicular to the magnetic field direction. It is revealed that the preferential grain growth is induced by the difference in magnetic free energy density of the grains with different orientations.

Interaction between nano-voids and migrating grain boundary by molecular dynamics simulation

Understanding the interaction between void and grain boundary (GB) is important to the design of radiation resistant materials by GB engineering and to achieve high quality metallurgical diffusion joining. In this study, the interaction between nano-voids and GBs has been systematically investigated by molecular dynamics simulations. The bicrystal Cu sample was used throughout the work, and the dynamic GB-void interaction was achieved by GB migration under shear deformation. Both high-angle GBs (Σ5 (310) GB, Σ5 (210) GB) and low-angle GBs (Σ37 (750) GB, Σ61 (650) GB) were investigated, and the effect of void size and temperature on the simulation result was examined. The transition of the deformation mechanism from GB migration to dislocation propagation was observed during the interaction between voids and high-angle GBs at low temperature (T = 10 K). At higher temperature (T = 300 and 600 K), the migrating GB can be pinned to voids, freely traversed voids, or dissolved voids in the process of their interaction. The void-drag effect on GB motion was analyzed based on the Zener-like equation, which indicates that the retarding pressure applied to the migrating GB by a void is closely related to the surface area of the void, the degree of contact between GB and void, and GB energy. By investigating the thermal stability of a void at the stationary GB, it was found that the dissolution of voids at a moving GB cannot be attributed solely to the thermal diffusion mechanism. The dynamic migration of high-angle GBs can significantly accelerate the dissolution time of the void. Atomistic analysis indicated that the migrating GB rearranged the atoms on the void surface by the collective motion of structural units, and the GB structural phase transformation provided an efficient diffusion channel for transporting the vacancies. The low-angle GBs show a reduced ability to dissolve the voids than the high-angle GBs, which can be ascribed to their low GB energy and diffusion coefficient, the fast GB migration velocity, and the discrete GB structure.

Effect of Recovery and Recrystallization on the Hall–Petch Relation Parameters in Submicrocrystalline Metals: III. Model for the Effect of Recovery and Recrystallization on the Hall–Petch Relation Parameters

A theoretical model is developed to describe the influence of recovery and recrystallization on the macroelastic limit, the yield strength, and the grain-boundary hardening coefficient of the submicrocrystalline (SMC) metals fabricated by severe plastic deformation. The anomalous hardening and the increase in the grain-boundary hardening coefficient upon annealing of the SMC metals are shown to be related to defect accumulation in migrating grain boundaries in them. Equations are derived to relate the Hall–Petch relation parameters to the grain-boundary migration velocity, the nonequilibrium state of grain boundaries, the lattice dislocation density, and the annealing temperature and time. The results of the numerical calculations performed using the developed model are compared with the experimental results obtained in part I of this work.

Pinning of grain boundary migration by a coherent particle

We studied single-particle pinning of grain boundary (GB) migration during grain growth. A phase-field model was formulated to simulate the pinning by a coherent particle and validated quantitatively by comparison with analytical prediction. A study of GB migration velocity using this model revealed that second-phase coherent particles have a previously unknown restraining effect over the whole of the GB-particle interaction range, which is qualitatively different from the interaction between GB and incoherent particles.

Time-Resolved Analysis of Domain Growth and Alignment of Cylinder-Forming Block Copolymers Confined within Nanopatterned Substrates

In this Letter, we elucidate the microstructure evolution of asymmetric poly(styrene-b-methyl methacrylate), PS-b-PMMA, block copolymer confined within nanoscale linear channels on silicon nitride substrates. We have directly observed the mechanism for cylinder alignment within the nanochannels using time-resolved AFM imaging and computerized image analysis. The overall alignment occurs via the coarsening of grains that have preferred cylinder orientation along the channel direction and is energetically driven by the interface energies of adjacent diblock domains of differing orientation. Grain coarsening is strongly affected by the annealing temperature and the geometry of the grain boundary itself. Grain coarsening promptly occurs when a domain is bordered by adjacent high-angle grain boundaries. The energetic term that drives this process arises from the fact that high-angle grain boundaries with larger excess energy migrate faster than other lower-angle grain boundaries. The velocity of grain boundary migration is proportional to the boundary tilt angle, while the activation energy depends weakly on this angle.

Formation of Annealing Twins during Recrystallization and Grain Growth in 304L Austenitic Stainless Steel

Understanding of the mechanisms of annealing twin formation is fundamental for grain boundary engineering. In this work, the formation of annealing twins in a 304L austenitic stainless steel is examined in relation to the thermo-mechanical history. The behaviour of annealing twins of various morphologies is analysed using an in-situ annealing device and EBSD. The results confirm that there is a synergistic effect of prior strain level on annealing twin density generated during recrystallization. The higher the prior strain level, the higher the velocity of grain boundary migration and the higher the annealing twin density in the recrystallized grains. This effect decreases as the recrystallization fraction increases. The existing mathematical models (Pande's model and Gleiter's model), which were established to predict annealing twin density in the grain growth regime, can not predict this phenomenon.

Acceleration of grain-boundary diffusion at recrystallization in submicrocrystalline metals produced by the method of equal-channel angular pressing

Results of experimental and theoretical investigations into the effect of grain-boundary diffusion acceleration at recrystallization in submicrocrystalline (SMC) metals by the method of equal-channel angular pressing are described. It is demonstrated that the grain-boundary diffusion coefficient at recrystallization in SMC materials depends on the lattice dislocation density and character and velocity of grain boundary migration.

Grain Boundary Migration in Fe–3mass%Si Alloy Bicrystals under a Magnetic Field

Grain boundary migration under a magnetic field was studied using Fe–3mass%Si alloy bicrystals with ‹001›/Σ5 (φ=36°) and ‹001›/random (φ=45°) tilt boundaries at temperatures between 1223 K and 1323 K. A capillarity method (Sun and Bauer method) was used to observe grain boundary migration. A 6 T magnetic field was applied parallel to the [100] direction, which is one of easy magnetization directions, in a grain whose area increases with occurring grain boundary migration. It was found that the application of a magnetic field observably increased the velocity of grain boundary migration for both bicrystals. The abrupt change in the velocity of grain boundary migration was observed at a critical capillary driving force irrespective of whether the magnetic field was applied. However, this critical driving force was decreased by application of a magnetic field. It was suggested that the magnetic field act as a driving force for grain boundary migration rather than increase its mobility.

Effect of sulphur content and pre-annealing treatments on nickel grain-boundary diffusion in high-purity copper

Abstract Auger electron spectroscopy (AES) and radiotracers were used to study the effects of pre-annealing treatments and grain boundary migration on nickel grain-boundary diffusion in pure 5N copper at 718 K in the B-kinetic regime. The diffusion penetration profiles are complex, which can be interpreted by the copresence of stationary and moving grain boundaries. Attempts to emphasize the grain-boundary migration effect were made by the application of different preannealing treatments before diffusion. It was shown by AES that this preparation procedure induces a modification in the bulk sulphur content which is a residual impurity in 5N copper. It is shown that a few ppm of sulphur dissolved in the bulk has a pronounced influence on the data; the higher the sulphur content, the lower the triple-product and the grain-boundary migration velocity.

Grain boundary migration at surface of austenitic stainless steel weld metal

Summary This paper describes an investigation of the grain boundary migration occurring at the weld metal surface, comparing the actually observed grain boundary migration and the results predicted by the thermal grooving theory of grain boundary migration based on the flow of surface atoms. The relationship between the solidification grain boundary plane angle in relation to the direction normal to the surface and the amount of grain boundary migration are discussed. The results obtained may be summarised as follows. 1. The grain boundary migration occurring at the weld metal surface differs in a number of important respects from that which occurs inside. Grain boundary migration is here basically observed to occur from the columnar crystals towards the axial crystals. In this case, the solidification grain boundary plane is inclined at less than around 40° in relation to the direction normal to the surface. The cross sectional shape of the grain boundary groove qualitatively resembles the analytical solution predicted by the thermal grooving theory of grain boundary migration based on the flow of surface atoms. The direction of grain boundary migration also agrees with the analytical solution. In the weld metal, grain boundary migration therefore occurs through being susceptible to the effect of the flow of surface atoms depending on the angle at which the solidification grain boundary plane is inclined in relation to the direction normal to the surface. 2. As the solidification grain boundary plane becomes increasingly inclined in relation to the direction normal to the surface, the amount of grain boundary migration at the weld metal surface increases. At a solidification grain boundary plane angle of less than around 40°, however, the amount of grain boundary migration at the surface sharply decreases. The grain boundary migration induced by the flow of surface atoms also only ever occurs near the surface. This is due to the fact that the grain boundary plane inside migrates in the wake of the grain boundary groove migration at the surface. That is to say, the grain boundary plane inside acts as a friction coefficient in relation to the velocity of migration itself. This friction coefficient is proportional to the third power of the grain boundary length (corresponding to the number of atoms migrating across the grain boundary plane). Incidentally, fewer atoms migrate across the inside grain boundary plane as the solidification grain boundary plane becomes increasingly inclined in relation to the direction normal to the surface, the velocity of grain boundary migration consequently increases and the amount of grain boundary migration increases.

Grain Growth Behavior of Alumina Compact Processed by Controlled Fracture Forming.

The effect of the green microstructure in the swaged Al2O3 with 98% reduction in area by the controlled fracture forming (CFF) on its grain growth behavior was investigated. The kinetics of grain growth in the Al2O3 compacts fabricated by the CFF process and by conventional process was estimated. The results indicate that both Al2O3 compacts obey a cubic law, but the grain growth rate constant K of the conventionally processed Al2O3 is 6 times as large as that of the swaged Al2O3 with 98% reduction in area. The retardation of grain growth in the swaged Al2O3 is attributed to the pores which are homogeneously distributed in the microstructure due to the flow and rearrangement of particles during the CFF process. The increase in number of pores per grain is responsible for the decrease in velocity of grain boundary migration. The mechanism for retardation of grain growth in the swaged Al2O3 is considered to be the pore-controlled mechanism.

Steady-state flow controlled by the velocity of grain-boundary migration

A model is developed for steady-state plastic flow in which the strain rate is controlled by the rate of grain boundary migration and the strain is due to dislocation slip within the grains.

Effect of austenite grain boundary mobility on hardenability of steels containing vanadium

AbstractA correlation has been established between the rate of grain boundary migration during austenitisation and the hardenability of steels containing 0·2–0·3%C, 1·5–1·7%Mn, up to 0·35% V, and small additions of Al or Ti. Interaction between the austenite grain boundaries and pinning particles was investigated using transmission electron microscopy and segregation to the austenite grain boundaries was examined using Auger electron spectroscopy. It has been concluded that the velocity of grain boundary migration during austenitisation influences the extent of equilibrium segregation to the austenite grain boundaries which, in turn, affects the hardenability. Pinning of the austenite grain boundaries enabled the potential hardenability effect of the alloying elements to be increased. Mechanisms have been discussed for the ways in which segregation, particularly of V, occurs to pinned or immobilised austenite grain boundaries, and the conditions by which most effective grain boundary pinning can be achiev...

The effect of solute distribution coefficient on grain growth in zone refined lead

The effect of k 0 < 1 and k 0 > 1 solutes on grain growth in zone refined lead has been investigated. The results show that the decrease in grain growth rates caused by solute additions arises from the reduction in effective driving force by the impurity drag. By applying Cahn's impurity drag theory to grain growth, the results can be explained successfully in terms of impurity-grain boundary interaction. In k 0 < 1 system the impurity drag is proportional to the velocity of grain boundary migration and in k 0 > 1 system it is inversely proportional to velocity. The grain growth laws have been established for the two solute systems and correlate very well with the experimental results. The activation energy of grain boundary migration is shown not to be affected by solute additions but remains the same as in zone refined lead.