| Grain boundary migration is key to materials microstructural processes such as grain growth and recrystallization. Quantitative boundary dynamic data is difficult to obtain, yet important for quantitative prediction of microstructural evolution and understanding migration fundamentals. Our molecular dynamics simulations first focus on curvature driven grain boundary migration to extract the reduced mobility and activation energy for migration as a function of boundary misorientation in aluminum. Simulation results are in good agreement with experimental observations except that the activation energy for migration found is much smaller than in experiment. This discrepancy led to a more systematic study of the absolute mobility and atomistic level mechanism for boundary migration. To study the mobility of a flat, fully defined boundary, we developed a strain-energy-anisotropy-driven migration simulation method. We applied this method to a series of Sigma5 [010] asymmetric tilt grain boundaries and extracted the absolute mobility as a function of temperature and inclination. Simulation results suggest that the mobility is a sensitive function of temperature and inclination. The boundary mobility tends to be minimized when one of the grain boundary planes has low Miller indices. Meanwhile, the comparison between grain boundary mobility, grain boundary self-diffusivity and energy suggests strong correlation at special inclinations, when one of the boundary planes is a high symmetry plane. In addition, we derive the grain boundary stiffness and reduced mobility as a function of boundary inclination. The grain boundary stiffness exhibits a large anisotropy, which is of the same order of magnitude as that of the grain boundary mobility. However, these two anisotropies nearly cancel, leaving the reduced mobility nearly isotropic. Finally, we identify the migration mechanism through frequent quenches and analysis of the atomic displacements, local and global excess volume, and stress. The migration mechanism has the following components: local volume fluctuations precede the displacements of 3--4 linear atomic clusters in the direction parallel to the tilt axis, which are followed by individual atomic hops that are primarily perpendicular to the boundary plane. Further simulations suggest that the presence of free surfaces interferes with the collective motions of the atoms and hence slows the migration. |
