Oxidation behaviour of a Ti–46Al–1Mo–0.2Si alloy: the effect of Mo addition and alloy microstructure

The influence of molybdenum addition and alloy microstructure on the oxidation behaviour of a Ti–46.8Al–1Mo–0.2Si alloy was studied in air at temperatures ranging from 600 to 900°C. The alloy produced by arc melting exhibited a structure of coarse lamellar grains in as-cast condition that transformed to a duplex microstructure after hot extrusion at 1300°C. Oxidation rate and scale spallation resistance were not affected by the type of microstructure. The effect of molybdenum addition was related to the formation and thickening of a protective continuous nitride layer. The low mass gain measured can be associated with the low oxidation rate of the nitride layer. Finally, the poor spalling resistance of the oxide scale at 900°C was attributed to the mismatch in thermal expansion coefficients of the oxide scale and the nitride layer.