Shape functions and kinetics of migrating grain boundaries in nanocrystalline materials

Grain boundary (GB) shapes determine driving force of curvature-driven GB migration and therefore is critical to grain growth kinetics. GB shape functions in nanocrystalline materials are hardly known due to experimental difficulties. This study represents the first one with regard to investigating shape functions of steadily migrating GBs at nanometer scale by using molecular dynamics. It is found that two types of GB migration kinetics occur depending on grain size and temperature, both differing from the one in coarse grain counterparts. The two kinetics correspond to different GB shapes. For the grains in a range of about 10-25 nm (the upper limit is expected to extend to sub-micron) at relatively low temperature, new shape functions have been successfully derived, while for the finest nanograins of around several nm at relatively high temperature their shape functions coincide with the ones developed for coarse grains. In addition, GB faceting occur for some GBs, which causes deviation of boundary shape from the functions proposed. The findings in the present study imply that the boundary shapes and migration kinetics may contribute to the specific grain growth kinetics in nanocrystalline materials.