It was recalled that molecular dynamics simulations of grain-boundary migration in bicrystals had suggested that, in pure materials, grain-boundary diffusion and grain-boundary migration involved distinct mechanisms, and therefore different activation barriers. Molecular dynamics simulations were here made of grain growth in an impurity-free model nanocrystalline Pd microstructure which contained only high-energy grain boundaries. By contrast with bicrystal simulations, essentially identical activation energies for grain growth and grain-boundary diffusion were observed. Several mechanisms were considered which might be responsible for the difference between bicrystal and polycrystal results. These included accommodation of curvature-driven grain-boundary migration by the elimination of grain-boundary area and grain-boundary excess free volume, and a possible finite mobility of the triple junctions. The qualitative agreement of the observations with the overall experimental results was remarkable, in the absence of any impurities in the model system. This suggested that a fundamental intrinsic process was responsible for the similar activation energies for grain-boundary migration and grain-boundary diffusion in polycrystals. In the model system, this process seemed to involve accommodation of the curvature-driven grain-boundary migration by grain-boundary diffusion. This eliminated the grain-boundary area and related grain-boundary free volume at the triple junctions.

Relation between Grain Growth and Grain-Boundary Diffusion in a Pure Material by Molecular Dynamics Simulations. V.Yamakov, D.Moldovan, K.Rastogi, D.Wolf: Acta Materialia, 2006, 54[15], 4053-61