Diffusion, the Kirkendall effect and vacancy jump frequency ratios in dilute Al-Zn alloys

Abstract

The method of vapour-solid couples was applied in order to investigate interdiffusion and intrinsic diffusion in dilute Al-Zn alloys. Although an aluminium oxide layer could not be avoided in our experiments, typical concentration-penetration curves were measured by electron probe microanalysis. The Boltzmann-Matano evaluation of the profiles yielded interdiffusion coefficients in agreement with previously published results. Contrary to the expected behaviour, bulges were observed by metallographic inspection at those regions of surfaces where zinc diffused into the samples. From results of interdiffusion and from measurements with only 1h diffusion time, it is deduced that the penetration of Zn atoms is nearly unaffected by the surface oxide layer and shows no local differences. Therefore, the reason for the growth of the bulges is the diffusion of the host component Al, which is possible only at preferred sites of the oxide layer. The ratio DZn/DAI was determined at those sites where the oxide layer was penetrated by both Zn and Al atoms. From the value measured for an infinitely dilute solution, (DZn/Dl))0=1.70+or-0.25, the vacancy flow factor (LAlZn/ZnZn)0=-0.19+or-0.13 was derived. Vacancy jump frequency ratios according to the five-frequency model were calculated which differ from unity only slightly and are similar to those obtained for the systems Ag-Zn and Cu-Zn. A weak binding between vacancies and Zn impurity atoms was estimated by means of two different methods.