Oxidation behavior of an Zr₅₃Ni_23.5Al_23.5 bulk metallic glass at 400-600 °C

The oxidation behavior of the Zr₅₃Ni_23.5Al_23.5 bulk metallic glass (BMG) and its crystalline counterpart was investigated over the temperature range of 400-600 °C in dry air and pure oxygen. In general, the oxidation kinetics of BMG followed the single- or two-stage parabolic rate law at T ≤ 500 °C, with rate constants (K_p values) generally increased with temperature. Conversely, three-stage parabolic kinetics were observed for BMG at T ≥ 550 °C, with K_p values decreased with increasing temperature. The oxidation rate constants for the BMG alloy are slightly higher than those for crystalline alloy at T ≤ 500 °C. In addition, K_p values of BMG were nearly independent of partial pressure of oxygen, implying a typical scaling behavior with a n-type semiconductivity. The scales formed on the BMG is temperature-dependent, consisting mainly of tetragonal-ZrO₂ (t-ZrO₂) and minor amounts of Al₂O₃ at T ≤ 475 °C. At higher temperatures (T ≥ 500 °C), some monoclinic-ZrO₂ (m-ZrO₂) were also detected, and its amounts increased with increasing temperature. The BMG substrate began to form the crystalline Zr₂Ni, Zr₂Al, and ZrNiAl phases beneath the scales after oxidation at T ≥ 450 °C.