Influence of water vapour on the oxidation behaviour of titanium aluminides

The influence of water vapour on the oxidation of a technical γ-TiAl based alloy at temperatures of 700 and 750 °C is discussed. The present paper focuses on the oxidation behaviour in dry air and air containing 10 vol.% H2O with respect to oxidation kinetics and microstructural investigations of the oxide scales and the subsurface zones. The presence of water vapour led to a significant acceleration in the oxidation kinetics as well as to a change in the crystal morphology and microstructure of the oxide scales formed. The results indicate that the kinetics of the reaction of water molecules with the rutile phase and a change in its defect structure are important parameters in the mechanism. The subsurface zone formed in water vapour-containing atmospheres consisted (in addition to the cubic Al-depleted layer) of a lamellar layer of Al2O3 and Ti due to either internal oxidation or decomposition of the cubic phase. Both, the oxide scale and the depletion layer are important regarding the mechanical behaviour of a TiAl component at high temperatures which was also investigated.