On the enhancement of diffusion kinetics in nanocrystalline materials

It is well known that volume diffusivity in nanocrystalline materials is or ders of magnitude greater than that in coarse grained materials of identica l chemistry. The present study offers a theoretical explanation of this enh ancement by attempting to correlate the diffusivity with the excess free vo lume of the grain boundary atoms in the nanocrystalline materials. Assuming a simplified geometry of the nanocrystalline aggregate, the excess free vo lume of the grain boundary atoms has been expressed as a function of grain size. Applying the concept of isothermal equation of state, it is shown tha t the effect of negative hydrostatic pressure on the diffusion kinetics in nanocrystalline materials at a given temperature is equivalent to the same obtained for coarse grained materials at an effective elevated temperature. The predicted results have been compared with the earlier published releva nt experimental data from several Cu-based systems. (C) 2001 Elsevier Scien ce B.V. All rights reserved.