Calculation of Activation Volumes for Self-Diffusion and Creep at High Temperature

Abstract

It is shown that the activation volume for diffusion or steady‐state creep at high temperature (>0.5T_m) can be estimated for all crystalline solids if the crystal structure, valence, entropy of melting, and volume change on melting are known. The calculated results agree well with the available experimental data for metals and compounds. Calculations performed for a number of crystalline materials show that the activation volume is a function of the crystal structure and that negative activation volumes are obtained for those materials that contract on melting. These results suggest that diffusion in the solid state occurs by a mechanism similar to the ``relaxion'' mechanism as originally proposed by Nachtrieb and Handler. In our proposed model the thermally activated process in diffusion involves the cooperative formation of a liquid‐like cluster of atoms surrounding a vacant lattice site. Such a model predicts negative activation volumes and satisfactorily explains the influence of different crystal structures on the magnitude of the activation volume obtained.