Oxidation behavior and passivation mechanism of the T2 phase in Mo-Si-B alloys

Abstract

The oxidation behavior of Mo5SiB2 (T2) phase was studied at 400–900 °C to examine their passivation mechanism which is critical for oxidation resistance in high temperature Mo-Si-B alloys. The oxide layer formation behavior of T2-rich Mo-Si-B alloy prepared by mechanical alloying was characterized using X-ray diffraction, scanning electron microscopy, and atom probe tomography. The results revealed that no effective passivation layer is formed and continuous B loss occurred before thermal oxidation of Si. Efficient passivation borosilicate layer can be formed above 900 °C, which further hinders oxygen penetration into the metal substrate and mass loss even for the temperature high enough for MoO3 volatilization. Si and B content profile in different oxidation temperatures showed that passivation layer formation attributed to the Si diffusion and oxidation towards B-depleted surface region. It is suggested that T2 phase is crucial for effective oxidation resistance property of Mo-Si-B alloys.