Oxidation behavior of a Zr–Cu–Al–Ni amorphous alloy in air at 300–425 °C

The oxidation behavior of Zr–30Cu–10Al–5Ni bulk metallic glass and its crystalline counterpart was studied over the temperature range of 300–425 °C in dry air. In general, the oxidation kinetics of both amorphous and crystalline alloys followed a two- or three-stage parabolic rate law at T⩾350 °C, while at 300 °C the amorphous alloy oxidized following a linear behavior. The oxidation rate constants for the amorphous alloy are slightly higher than those for the crystalline alloy at 350–400 °C. The scale formed on the amorphous alloy consists of mainly tetragonal-ZrO2 at 300 °C, while a mixture of monoclinic-ZrO2 (m-ZrO2) and tetragonal-ZrO2 (t-ZrO2) and some CuO were detected at higher temperatures. The scale formed on the crystalline alloy, on the other hand, consists of mainly Al2O3, some tetragonal-ZrO2, and a slight amount of monoclinic-ZrO2 at 300 °C. At higher temperatures, the crystalline alloy consists of mainly monoclinic-ZrO2, some CuO and Cu2O, and limited tetragonal-ZrO2. It is suggested that the formation of Al2O3 (at 300 °C) and CuO/Cu2O (at 350-400 °C) on the crystalline alloy is responsible for the reduced oxidation rates as compared with those of amorphous alloy.