On the Determination of Enhancement Factors of Solvent Diffusion

Abstract

AbstractIn dilute binary f.c.c. alloys of the substitutional type a nonlinear increase of the solvent diffusion coefficient D by alloying solutes with positive excess charge is frequently observed. The formation of vacancy–impurity complexes leads to this increase, which is usually described in terms of the enhancement factors bi in the following manner: D(NB) = D(0) (1 + b1NB + b2NB2 + + b3NB3 + …). The determination of enhancement factors by using this polynomial is critically discussed. From the complex model according to Dorn and Mitchell and more generally according to Bérces and Kovács and its application to solvent diffusion by Hehenkamp et al. it becomes reasonable, that the dependence of the solvent diffusion coefficient on the solute concentration is approximately given by: D(NB) = D(0) exp (b1NB). This relation, which is already known in the literature for a long time, yields an alternative method for the detemination of enhancement factors. Results are presented for many noble metal systems and discussed in the frame of two different estimation methods of the Gibbs free energy of binding δg1 of a vacancy–impurity pair calculated from the enhancement factors b1 and b2.