Positron annihilation analysis of the atomic scale changes in oxidized Zircaloy-4 samples

The process of oxidation in zirconium alloys is not straightforward. It includes cyclic periods, where the oxidation rate is decelerating as a function of increasing oxide thickness. Transition to the higher oxidation rate happens approximately after every 2 μm oxide growth and each cycle is associated with a layer of oxide with little or no cracking and a similar thickness layer with a large number of micro-scale lateral cracks. This work uses positron annihilation Doppler broadening spectroscopy to investigate the development of atomic-scale defect features in the oxides on Zircaloy-4 samples exposed to alkaline water at 350 °C. Results from the Doppler broadening measurements supported by Density Functional Theory calculations indicate that complexes involving both zirconium-site and oxygen-site vacancies exist in the oxide. The implications of the vacancies in the oxide layer are considered for oxygen diffusion and the oxidation rate.