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 degrees 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 alpha-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 alpha-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 alpha-Al2O3 in MoSi2 - 2.8 at.% Al alloy, and alpha-Al2O3 in case of MoSi2 - 5.5 and 9 at.% Al alloys. The mechanism of oxidation has been analysed using thermodynamic and kinetic considerations. (C) 1999 Elsevier Science S.A. All rights reserved.