Self-Diffusion in Near-Equiatomic Ordered Au-Cd Alloys

Self-diffusion measurements in near-equiatomic ordered Au-Cd alloy single-crystal specimens having 47.5, 49.0, and 50.5 at.% Cd nominal composition have been made over a temperature range of 353-598°C, using ${\mathrm{Cd}}^{109}$, ${\mathrm{Cd}}^{115}$, and ${\mathrm{Au}}^{195}$ radioactive tracers and sectioning techniques. A differential counting method made possible direct measurement of the ratio of diffusivities $\frac{D_{\mathrm{cd}}}{D_{\mathrm{Au}}}$ to within a few percent, following simultaneous diffusion of ${\mathrm{Au}}^{195}$ and ${\mathrm{Cd}}^{109}$ tracers in the same specimen. The experimental determination of this ratio as a function of composition and temperature furnishes strong evidence for a diffusion mechanism in $\beta$ ordered alloys by a correlated sequence of six atom-vacancy jumps. This six-jump unit process is apparently controlled by the relative segregation of vacancies on the two sublattices. In the $\beta$ Au-Cd ordered alloys, segregation of vacancies on the two sublattices appears to be conditioned by the relative size of the alloy constituents and the paramount requirement of maintaining long-range order. Thus there are more vacancies on the Cd sublattice in Au-rich alloys and on the Au sublattice in equiatomic and Cd-rich alloys. The diffusion coefficients obey simple Arrhenius relationships in the various alloys over the whole temperature range. The activation energies and frequency factors for both species show maximum values at approximately 49 at.% Cd composition. The observations are consistent with the presence of a nonequilibrium vacancy defect structure on the Cd-rich side only, as revealed by a measurable difference between x-ray and macroscopic densities of the alloys.