Isothermal oxidation behaviour of Mo-Si-B and Mo-Si-B-Al alloys in the temperature range of 400-800 °C

Molybdenum silicide based alloys with composition, 76Mo-14Si-10B (MSB), 77Mo-12Si-8B-3Al (MSB3Al), and 73.4Mo-11.2Si-8.1B-7.3Al (MSB7.3Al) processed by reaction hot pressing of elemental powders, possessed α-Mo, Mo3Si, Mo5SiB2 and SiO2 or α-Al2O3 in the microstructures. The alloys were subjected to isothermal oxidation tests in dry air in the temperature range of 400-800 °C for periods of either 24 h or 300 h. The kinetics and composition of the oxide-scales have been found to depend on the alloy composition, temperature and time of exposure. The initial stages of oxidation behaviour at all temperatures have shown segments of time with no change in mass, punctuated by short periods of either gain or loss. Extended exposure at 500 and 600 °C for 300 h has not led to pest disintegration, although a net loss in mass has been observed in almost all cases, except for that of the MSB alloy at 600 °C. On the other hand, continuous and excessive loss in mass has been noticed at 700 °C with the degradation of MSB7.3AL being the worst, having the characteristics of pesting. The trend is similar at 800 °C, with a difference that the MSB alloy develops a protective scale after an initial loss in mass, which survives until about 80 h of exposure. Analyses of the oxide scales have confirmed the presence of Mo5Si3 and oxides of B, Si and Mo for all the alloys. In addition, α-Al2O3 has been found in case of the MSB3Al and MSB7.3Al alloys. The loss in mass has been attributed primarily to the spallation, accompanied by volatilization of MoO3 at and above 700 °C.