Frequency Factor and Activation Energy for the Volume Diffusion of Metals

Abstract

The theoretical equations proposed for calculating D0 of the diffusion-constant-temperature relation D=D0e−E/RT are critically discussed in the light of presently available data. It is concluded that none of the relations gives satisfactory agreement with experiment. Empirical analysis of experimental data covering a wide range of D0 and E values showed that the quantity E/Tm, where Tm is the melting point in °K, is the main factor in determining the value of D0, and that D0 depends approximately exponentially on E/Tm. In addition to many other diffusion constants, it is shown that the anisotropic diffusion of zinc and the widely differing diffusion constants for self-diffusion in α- and γ-iron are included in the proposed empirical correlation. A possible theoretical basis for the empirical correlation is described based on the idea of local melting or disordering. The quantity E/Tm is identified with the entropy of activation.