Relation between grain growth and grain-boundary diffusion in a pure material by molecular dynamics simulations

Traditionally, due to the impurities inevitably present in real materials that limit the grain-boundary (GB) mobility, the processes of grain growth and GB diffusion are thought to involve similar activation barriers. However, recent molecular dynamics (MD) simulations of GB migration in bicrystals have suggested that, in pure materials, GB diffusion and GB migration involve distinct mechanisms and, hence, different activation barriers. Here we report MD simulations of grain growth in an impurity-free model nanocrystalline palladium microstructure containing only high-energy GBs. By contrast with the bicrystal simulations, we observe virtually identical activation energies for grain growth and GB diffusion. We discuss several mechanisms that might be responsible for this difference between the bicrystal and polycrystal results, including accommodation of the curvature-driven GB migration by the elimination of GB area and GB excess free volume and a possible finite mobility of the triple junctions. The qualitative agreement of our observations with the general experimental findings is remarkable given the absence of any impurities in our model system. This suggests that a fundamental, intrinsic process may be responsible for the similar activation energies for GB migration and GB diffusion in polycrystals. In our model system this process seems to involve the accommodation of the curvature-driven GB migration by GB diffusion to eliminate the GB area and related GB free volume at the triple junctions.