First Stages of Internal Oxidation in Ag-0.29 and 2.89 at. % Mg Single-Crystal Alloys

Abstract

Internal oxidation of Ag-0.29 and 2.89 at.% Mg alloys was studied by in-situ X-ray measurements at 300°C. The kinetics and lattice-parameter changes as a function of time are discussed in terms of the formation of elementary MgO* species and non-stoichiometric clusters, during the first stages of oxidation. Studies were made using synchrotron radiation on single-crystal samples. The distorted zone, formed near the surface, was detected by measuring the position and the intensity of the (022) silver peak in the solid-solution alloy. From the diffraction-peak changes, information is deduced on the growth kinetics of the expanded layer. Oxygen diffusivity in silver, C0D0, is calculated on the basis of Wagner’s law. At an early stage, the oxygen diffusivity is slightly lower than the theoretical value. During a second stage, accelerated kinetics give a very high C0D0 value. We determined the O/Mg ratios at different stages during oxidation.