Activity Coefficient and Vacancy-Flow Effects on Diffusion in Silver-Gold Alloys

Ten measurements have been made of the mean atom drift for silver and gold tracer atoms placed at the interface of silver-gold alloy diffusion couples. A new technique for marking the original interface, using radioactive hafnium oxide, was developed. The existence of a broad minimum in the plot of $D^{*}$ versus composition allowed a detailed investigation of the effects of a vacancy flux and activity-coefficient gradient on tracer diffusion, since a Kirkendall vacancy flux could be produced in a couple throughout which $D^{*}$ was essentially constant. The largest contribution to the mean atom drift results from the gradient of activity coefficient. The effect of Kirkendall lattice distortion on the drift was calculated and was found to be comparable to the vacancy-flux effect. Good agreement was found between the experimental drift and the net calculated effect. The existence of an impedance to free diffusion across the welded interface of a diffusion couple was demonstrated. Measurements of tracer diffusion coefficients, interdiffusion coefficients, marker shifts, and void positions are also reported.