Kinetics and mechanism of low-temperature internal oxidation of Ag–2 and 4 at.% Mg alloys

Abstract

Gravimetry and x-ray diffraction were used to study the kinetics and mechanism of internal oxidation in air at 300–600 °C of Ag–2 and 4 at.% Mg alloys. All the features typical of the non-Wagnerian behavior in more dilute alloys (the slower initial and faster intermediate kinetics, oxygen release from the samples at the late stages, and stabilization of highly hyperstoichiometric oxides with O/Mg up to 1.74 ± 0.02) are also observed in these more concentrated alloys. A linear correlation between the lattice parameter of oxidized alloys, magnesium content, and amount of excess oxygen is found and used for analyzing the published data on the lattice parameters in other oxidized Ag–Mg alloys. Such analysis suggests that, at early stages of low-temperature oxidation, O/Mg ratios could be higher than 4 ± 0.8, which is interpreted as evidence that O–Mg interaction at these stages is the trapping of mobile O atoms by immobile Mg atoms, so that oxidation of Ag–Mg alloys can be considered as formation and subsequent decomposition of Ag–Mg–O solid solutions, and it is the decomposition stage that significantly affects the oxidation kinetics. Possible impact of these findings on optimization of alloy-sheath composition and microstructure in Bi2223/AgMg superconducting tapes is discussed.