Microstructure of oxide layers formed on Zircaloy-2 in air at 450°C

Abstract

Oxidation tests were conducted at 450°C in air for Zircaloy-2 and modified alloys. The results showed breakaway oxidation, as has been reported previously. The microstructures of the oxide films were examined by transmission electron microscopy (TEM). Cross-sections were prepared from test samples with various thicknesses of oxide using ion beam thinning. The oxide structure was mainly columnar in both the pre- and post-transition oxides, but a heavily twinned structure was observed in the post-transition oxide. At the metal–oxide interface, ω-Zr was observed in the metal beneath the oxide in both the pre- and post-transition conditions. It is argued that the presence of the high-pressure ω-phase may be evidence of compressive rather than tensile stress close to the interface.Two types of the intermetallic precipitates were observed in Zircaloy-2, one was Zr–Fe–Cr type and the other was Zr–Fe–Ni type. The Zr–Fe–Cr type precipitates were found to survive unoxidised in the oxide near the metal–oxide interface, but they disappeared further away from the metal–oxide interface. No Zr–Fe–Ni type precipitates were observed in the oxide. It is suggested that tensile stresses from expansion during progressive oxidation of the Zr–Fe–Cr type intermetallic particles embedded in the oxide may be the cause of the breakaway in air.