Mechanisms of oxide scale formation on yttrium-alloyed Mo-Si-B containing fine-grained microstructure

A significant improvement in the oxidation resistance of Mo-9Si-8B (at.%) alloys in 750-1000 °C is observed with the alloying addition of yttrium up to 2 at.%. Being a reactive element, yttrium promotes the formation of stable yttrium-molybdate (Y5Mo2O12 and Y6MoO12) phases at the early stages of oxidation. Formation and volatilization of MoO3 is suppressed, and a silica-rich outer scale is formed. The growth kinetics of the inner MoO2 layer is found to reduce drastically for higher yttrium containing alloys. Finite element analysis proves that higher tensile stresses are generated in the silica-rich layer with increasing growth rate of the MoO2 layer.