Oxidation of Zr55Cu30Al10Ni5 bulk metallic glass in the glassy state and the supercooled liquid state

The oxidation behavior of Zr55Cu30Al10Ni5 bulk metallic glass in air in the glassy state and the supercooled liquid state was studied using a thermogravimetric analyzer, X-ray diffraction and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. It was found that the isothermal oxidation kinetics of the glass in both states follows a two-step parabolic law. The oxidation process is governed by the inward diffusion of oxygen and the outward diffusion of Cu, with the first being dominant. The faster diffusion of atoms in the supercooled liquid state led to a network precipitation of crystalline Cu, and the crystallization that occurred in this state in the later stage of oxidation caused a reduction in the rate of oxidation. Two types of Zr oxides, i.e. t-ZrO2 and m-ZrO2, were formed in the oxidation process in both the glassy and supercooled liquid states. t-ZrO2 mainly formed in the outer layer of the oxide scale, while m-ZrO2 tended to form in the inner layer. The formation of m-ZrO2 is possibly activated by the crystallization of the glass near the interface of the oxide scale and the substrate alloy. In addition, oxidation also has a substantial effect on the formation of crystallized phases. The formation of a Cu-rich phase of Cu10Zr7 occurred in the oxidizing atmosphere. However, the formation of a Zr-rich phase of Zr-2(Ni, Cu) mainly took place in a vacuum environment.