It was recalled that volume diffusivity in nanocrystalline materials was some orders of magnitude greater than that in coarse-grained materials of identical composition. The present work suggested a theoretical explanation for this enhancement by attempting to relate the diffusivity to the excess free volume of the grain-boundary atoms in nanocrystalline materials. By assuming a simplified geometry for the nanocrystalline aggregate, the excess free volume of the grain boundary atoms was expressed as a function of grain size. By using the concept of an isothermal equation of state, it was shown that the effect of a negative hydrostatic pressure, on the diffusion kinetics in nanocrystalline materials at a given temperature, was equivalent to the same effect for coarse-grained material at a high temperature.

On the Enhancement of Diffusion Kinetics in Nanocrystalline Materials. P.P.Chattopadhyay, S.K.Pabi, I.Manna: Materials Chemistry and Physics, 2001, 68[1-3], 80-4