High temperature oxidation and pesting of Mo(Si,Al)2

Molybdenum aluminosilicide, Mo(Si,Al)2 with the C40 structure is a promising material at elevated temperatures. High temperature oxidation of Mo(Si,Al)2 gives a protective Al2O3 scale at temperatures below 1868 K of which is the eutectic temperature in the SiO2-3Al2O32SiO2 system. Above the eutectic temperature, the scale is a liquid of SiO2–Al2O3 with the Al2O3 content being richer than the eutectic composition. The cooling of the hypereutectic liquid scale suppresses the formation of β-crystobalite and gives an amorphous scale with an excellent adherence to intermetallics. The disintegration of MoSi2 (pesting) due to oxidation is severe around 773 K. The addition of Al markedly reduces pesting. Amorphous Mo–Si–Al–O forms in initial cracks and voids. This amorphous oxide is probably more plastic than the amorphous Mo–Si–O formed on MoSi2 and decreases the stress generation at the crack which leads to disintegration.