Vacancy generation during grain boundary migration

We present a molecular dynamics simulation study of vacancy generation and emission from grain boundaries during curvature-driven grain boundary migration. The U-shaped half-loop bicrystal geometry is employed in order to maintain a constant driving force during boundary migration. Nonlinearities in plots of half-loop grain area versus time indicate the onset of non-steady-state behavior and vacancy formation. Such events appear much more frequently at elevated temperature than at low temperature. Detailed observation of the atomic configurations in the vicinity of the grain boundary before and after these events clearly demonstrate that these nonlinearities are associated with the generation and subsequent emission of vacancies. The excess volume associated with the grain boundaries in conjunction with the loss in grain boundary area during curvature-driven migration accounts for the generation of these vacancies. This description is used to predict the rate of vacancy emission, which represents an upper bound on the observed emission rates.