Solvent self-diffusion in dilute b.c.c. solid solutions

A calculation is made for b.c.c. alloys of the ‘solute enhancement factor’ b in the equation D(c)=D(0)(1 + bc +…), relating the solvent self-diffusion coefficients D(0), in pure solvent, and D(c), in an alloy of small concentration c. Account is taken of vacancy-solute interactions at first- and second-nearest-neighbour separations and of the altered frequencies, due to the presence of a solute atom, of all vacancy jumps to and from sites which are first and second neighbours of it. An exact expression for b is obtained only for the case of the impurity providing a weak perturbation of the jump frequencies, but the procedure for treating the general case is indicated. Equations are also derived relating b with the diffusion coefficient and correlation factor for diffusion of the solute. The results derived are compared with previous calculations: these are found either to contain errors or to be unnecessarily approximate.