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Research highlight
Figure: Relaxed structures of the investigated grain boundaries [(a) to (c)]. (a) Pure symmetric 7 tilt boundary with a misorientation angle of 38.21◦ around the [1 1 ̄ 1] axis, relaxing to an atomically flat structure. (b) Mixed boundary obtained by introducing a small rotation angle around the [121 ̄] direction in the grain boundary plane, relaxing to a structure with straight steps. (c) Mixed boundary obtained by introducing a small inclination angle to the symmetric boundary, around an arbitrary low symmetry rotation axis in the grain boundary plane as indicated by the black arrow. As a specific representation the [541] rotation axis has been chosen. This boundary relaxes to a structure with kinked steps as shown in (f). In the figures perfect bulk atoms have been removed for clarity, and the color indicates the position of the grain boundary atoms along the boundary plane normal as indicated by the color bar.
For more details see:
R Hadian, B Grabowski, C P Race, and J Neugebauer. “ Atomistic migration mechanisms of atomically flat, stepped, and kinked grain boundaries. ” Physical Review B, 94, 165413 (10pp), 2016. |
PublicationAtomistic migration mechanisms of atomically flat, stepped, and kinked grain boundaries.
We studied the migration behavior of mixed tilt and twist grain boundaries in the vicinity of a symmetric tilt ⟨111⟩ Σ7 grain boundary in aluminum. We show that these grain boundaries fall into two main categories of stepped and kinked grain boundaries around the atomically flat symmetric tilt boundary. Using these structures together with size converged molecular dynamics simulations and investigating snapshots of boundary surfaces during migration, we obtain an intuitive and quantitative description of the kinetic and atomistic mechanisms of the migration of general mixed grain boundaries. This description is closely related to well-known concepts in surface growth such as , step and kink-flow mechanisms and allows us to derive analytical kinetic models that explain the dependence of the migration barrier on the driving force. Using this insight we are able to extract energy barrier data for the experimentally relevant case of vanishing driving forces that are not accessible from direct molecular dynamics simulations and to classify arbitrary boundaries based on their mesoscopic structures. |
Previous highlights
Figure: Snapshots of migrating boundaries containing structural defects associated with small departures from a perfect symmetric tilt boundary: (Step) a pesistent pure step, (Asymmetric) facets due to a slight asymmetry in the boundary plane, and (SGBD) intrinsic secondary grain boundary dislocations arising from a slight change in the misorientation angle. Atoms in one half of the bicrystal are not shown. The atoms are coloured according to their position perpendicular to the grain boundary surface to emphasize the role of out-of-plane features in the migration process (color scales are arbitrary and vary between defects). Time scales for the migration process along with the temperatures and driving forces used are marked in the figure.
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PublicationMechanisms 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. For more details see:
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 (11pp), 2015. |
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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Publication
Role of the mesoscale in migration kinetics of flat grain boundaries.
A recent publication in Physical Review B shows 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. |