Effect of minor alloying with Al on oxidation behaviour of MoSi2 at 1200°C

Effect of Al alloying in small concentrations on oxidation behaviour of molybdenum di-silicide (MoSi2) at 1200°C has been investigated. MoSi2–2.8 and 5.5 at.% Al alloys possessed 0.5, and 2.5 at.% Al in solid solution, respectively, and dispersoids of α-Al2O3. On the other hand, MoSi2–9 at.% Al alloy possessed 3.1 at.% in solid solution in MoSi2 and Mo(Si,Al)2 phase, besides α-Al2O3 dispersoids. The kinetics of oxidation of all the alloys followed a parabolic rate law. The oxidation rate was higher in the MoSi2–Al alloys in comparison to MoSi2, with weight gain values varying by an order of magnitude. The MoSi2–5.5 and 9 at.% alloys demonstrated closely related oxidation characteristics and proved to be more resistant to oxidation than MoSi2–2.8 at.% Al alloy. The oxide scale comprised of SiO2 in MoSi2, mixture of SiO2 and α-Al2O3 in MoSi2–2.8 at.% Al alloy, and α-Al2O3 in case of MoSi2–5.5 and 9 at.% Al alloys. The mechanism of oxidation has been analysed using thermodynamic and kinetic considerations.