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Abstract

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.