OSMOTIC EFFECT IN SELF-DIFFUSION IN METALS.

Self-diffusion in a binary solution is analyzed on the basis of the Onsager theory with zero cross terms. In contrast with the usual (Darken) description, the gradients of the chemical potentials of the components contain terms proportional to the pressure gradient. This pressure, the ″osmotic pressure,″ arises as a result of the different mobilities of the solution components. Taking account of this osmotic pressure leads to a change in the relation between the self-diffusion coefficient and the partial coefficients from that predicted by the Darken analysis. Analysis of data in the literature reveals that the difference usually lies within the experimental error but is important when there is a large difference between the partial coefficients for alloys having approximately equal component concentrations. In this latter case the present analysis does not lead to negative partial coefficients.