Mechanisms and kinetics of grain boundary migration
Role of the mesoscale in migration kinetics of flat grain boundaries.
Figure: Snapshots of a migrating boundary in a bicrystal. Crystal in the thermodynamically favoured orientation is shown in blue, in the disfavoured orientation, in red (part of the red crystal is cut away to reveal the grain boundary surface. Top to bottom: Islands of crystal with the favoured (blue) orientation spontaneously form within the disfavoured grain (red) at the grain boundary surface. These islands grow and coalesce to effect grain boundary migration.
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We have shown that when grain boundaries are flat (i.e. when they do not contain structural defects) and smooth (i.e. they are below their roughening transition temperature) then their motion is controlled by the kinetics of homogeneous nucleation. This gives rise to a driving force dependence of the mobility. The need for nucleation also introduces a characteristic length scale in the migration mechanism at a given driving force - this can interact with the size of the grain boundary and so care must be taken to ensure simulations are properly converged with respect to system size.
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Published in: C P Race, J von Pezold, and J Neugebauer. “Role of the mesoscale in migration kinetics of flat grain boundaries.” Physical Review B, 89, 214110 (11pp), 2014.
Mechanisms and kinetics of the migration of grain boundaries containing extended defects.
In a detailed study of the kinetics of a [111] Sigma-7 symmetric tilt boundary, we have previously shown that defect-free, flat grain boundaries, below their roughening temperature, can be strictly immobile in the experimental limit. In a recent publication in Physical Review B we present the results of molecular dynamics simulations of grain boundaries containing a variety of “defects.” These simulations show that the presence of some of these defects restores the mobility of flat boundaries, even well below the roughening transition temperature. These defects fundamentally alter the mesoscale mechanism of grain boundary migration from one involving homogeneous nucleation to a heterogenous process. At the atomistic level, the crystal lattice reorients via coordinated shuffling of groups of atoms. In the case of flat boundaries, these shuffles must accumulate to form critically stable nuclei, but in the case of boundaries containing defects the shuffling of a small number of atoms at the defects can be sufficient, fundamentally altering the mechanism and kinetics of migration.
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Published in: C P Race, R Hadian, J von Pezold, B Grabowski, and J Neugebauer. “Mechanisms and kinetics of the migration of grain boundaries containing extended defects.” Physical Review B, 92, 174115 (8pp), 2015.