Influence of Solutes on Self-Diffusion in the Face-Centered Cubic Lattice

A theory is given for the influence of substitutional solutes on self-diffusion in the face-centered cubic lattice. The theory is limited to cases in which the concentration of solute is low enough so that only one solute atom at a time can interact with a given tracer atom.Two different kinds of approximation are employed, one in which the processes of association and dissociation of vacancies and solute atoms do not themselves contribute to transport, and one in which they do but the frequency of exchange between solute and vacancy is considered to be infinite.From data on the diffusion coefficient of the solute as well as on the self-diffusion coefficient in its dependence on solute concentration the ratio of the frequency with which a vacancy exchanges with a solute atom to that with which it exchanges with a host atom in the first coordination shell of a solute can be estimated. This ratio appears to lie between 0.1 and 0.5 for solutes in silver which increase self-diffusion and for which experimental data are available.An analysis is given which shows that a good estimate of the influence of a given solute on self-diffusion can be made when only the diffusion coefficient of that solute is known.Finally the effect which Pd in silver has on the self-diffusion coefficient (Pd reduces the self-diffusion coefficient) is calculated on the basis of the theory. Agreement between theory and experiment is satisfactory.