Time: 02:00 p.m.
Place: Materials Day 2013, Ruhr-Universität Bochum, Bochum, Germany
Christopher Race, School of Materials, University of Manchester, Manchester, United Kingdom
Johann von Pezold
Dominique Korbmacher, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
We have studied the velocity, morphology, and mechanisms of moving grain
boundaries below the roughening temperature using molecular dynamics
(MD) simulations. When the area of the grain boundary is small (as it
often is in simulations) the boundary moves via the coordinated
rearrangement of the crystal at the boundary. However, when the boundary
has a larger area, its motion requires the formation of "islands" of
reoriented crystal bounded by disconnections. We show that the kinetics
of this homogeneous nucleation process provide a quantitatively accurate
prediction of the complicated (driving-force- and temperature-dependent)
results of our simulations. As a consequence, it is no longer possible
to identify a driving-force-independent activation energy for grain
boundary migration and the concept of an intrinsic mobility for smooth
boundaries is thus inappropriate.
We further show that the presence of certain types of defects in a grain boundary surface removes the need for homogeneous nucleation. The migration kinetics of such defective boundaries agree with the expected picture of a mobility independent of driving force. This mobility is now, however, a function of the defect content and morphology of the boundary rather than a property of the boundary geometry alone.