Role of the mesoscale in migration kinetics of flat grain boundaries

Classical molecular dynamics simulations of bicrystalline systems are a commonly used tool for exploring the migration of grain boundaries. Most simulation work to date has focused on measuring the mobility of grain boundaries, assuming it to be an intrinsic property of a boundary of a given geometry. Here we present results from simulations of the migration of a typical high-angle grain boundary that show that the concept of intrinsic mobility fails for defect-free, flat boundaries of the type frequently simulated and that key assumptions often made in analyzing the kinetics of migration do not hold. Our dynamical simulations of grain boundary migration show that the grain boundary velocity is not simply proportional to the driving force for grain boundary motion, as commonly assumed, and shows a strong and complex dependence on the system size. By analyzing the migration mechanism at the larger mesoscale we show that defect-free, flat boundaries must migrate via the homogeneous nucleation and growth of islands of transformed crystal volume on the grain boundary surface. We present a detailed analysis of the kinetics of this process, which only emerges in simulations of large grain boundary areas. An island-based mesoscale mechanism implies an energy barrier for migration that is inversely proportional to the driving force for migration-in the experimental (zero-force) limit such boundaries must be immobile. This calls into question the concept of an intrinsic mobility for defect-free, flat grain boundaries and suggests that mobility of real boundaries at low temperatures is rather a function of their morphology and defect content and at high temperatures is a result of thermal roughening.