Effect of Silver on Diffusion in Zinc

The diffusivities of ${\mathrm{Zn}}^{65}$ and ${\mathrm{Ag}}^{110}$ parallel and perpendicular to the hexagonal axis have been measured in monocrystalline zinc doped with up to 1.4-at.% silver. The standard radiotracer sectioning technique was used over the temperature range 320-417°C. An enhancement of both the solvent and solute diffusion occurs at the same activation energies as for the self-diffusion in the pure matrix. The concentration dependence is well fitted by the exponential relation $D\left(c\right)=D\left(0\right)\mathrm{}{e}^{\mathrm{bc}}$, where $c$ is the solute mole fraction. It is found that, at $T=400\mathrm{}$ °C, $b_{\parallel }^{}{}_{}{}^{\mathrm{Zn}}=13\mathrm{}$, $b_{\perp }^{}{}_{}{}^{\mathrm{Zn}}=18\mathrm{}$, $b_{\parallel }^{}{}_{}{}^{\mathrm{Ag}}=19\mathrm{}$, and $b_{\perp }^{}{}_{}{}^{\mathrm{Ag}}=13\mathrm{}$, and that these factors display little or no temperature dependence. This temperature-independent enhancement is consistent with the observations of self-diffusion in zinc with copper and aluminum doping, as well as with other results in doped multivalent metals. Making use of the results of recent experiments on the effect of pressure on the self-diffusion of zinc, we show that the changes in the diffusion rate in the alloy cannot be attributed to purely dimensional changes in the lattice. A qualitative explanation of the augmented self-diffusion is advanced on the basis of the oscillating-potential model of an impurity in a metallic lattice. This model appears to account for the enhancement and, in the case of the self-diffusion, for the anisotropy.