**Lattice transformation in grain boundary migration via shear coupling
and transition to sliding in face-centered-cubic copper**

B Li and J Leung, ACTA MATERIALIA, 215, 117127 (2021).

DOI: 10.1016/j.actamat.2021.117127

Migration of symmetric tilt grain boundaries (GBs) via shear coupling
has been studied extensively in experiments and simulations. It was
reported that shear coupling transitioned to GB sliding at high
temperatures, but how such transition occurs at low temperatures has not
been investigated. Lattice transformation on the atomic scale during
shear coupling has not been fully understood. In this work, mode of
motion of symmetric tilt GBs with **001** tilting axis in face centered
cubic copper under a shear strain parallel to the boundary plane at 100
K was carefully characterized by tracking the positions of the
corresponding planes in atomistic simulations and new features of GB
motion were observed. The results show that the angles between the two
low-index planes, (110) and (100), and the boundary plane can be used to
define a nominal magnitude of shear s. Approximately, if one of these
two planes has a value of s < 0.5, shear coupling occurs with this plane
being the active invariant plane; if 0.5 < s < 0.6, GB moves by shear
coupling + sliding, i.e. a hybrid mode by which shear coupling
transitions to sliding; if both planes have a value of s > 0.6, only GB
sliding occurs. Careful structural analyses show that, for all the GBs
that undergo shear coupling, some GB atomic planes remain invariant,
very similar to the first invariant plane in deformation twinning,
whereas the other GB atomic planes swap their positions in the GB normal
direction through highly coordinated and complex atomic shuffles. This
behavior allows identification of transformation units that are
reoriented toward the neighboring grain. Rate-limiting factors are
identified for lattice transformation and can be used to infer a
kinetics model for shear coupling. (C) 2021 Acta Materialia Inc.
Published by Elsevier Ltd. All rights reserved.

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