The relationship between t-ZrO2 stability and the crystallization of a Zr-based bulk metallic glass during oxidation

The tetragonal ZrO2 is observed in the oxidation surface of Zr55Cu30Al10Ni5 bulk metallic glass (BMG) while both the tetragonal ZrO2 (t-ZrO2) and monoclinic ZrO2 (m-ZrO2) are determined on the oxidation surface of crystalline counterparts with same chemical composition. In vacuum, the phase transformation of t-ZrO2 into m-ZrO2 in the oxide scale is investigated on the BMG substrate and crystalline substrate, both annealed at different temperatures, respectively. The crystallization of BMG substrate is related to the formation of m-ZrO2. The difference of the diffusion mechanism in the oxidation of bulk metallic glass and its crystalline alloy is characterized by 18O2 isotopic tracer. In the oxidation, the promotion of m-ZrO2 on the crystalline substrate is attributed to the inward diffusion of oxygen.