Influence of water vapour on the isothermal oxidation behaviour of TiAl at high temperatures

The isothermal oxidation behaviour of TiAl coupons has been studied at 1100 and 1200 K in a flow of O2, mixtures of O2 and H2O (water vapour) or Ar–H2O under atmospheric pressure. Metallographic examinations were performed for the specimens oxidised under specified conditions of temperature, time and oxidant, using X-ray diffractometry, optical microscopy, scanning electron microscopy combined with energy dispersive spectrometry and microprobe analysis. The mass gain due to the oxidation increases as the H2O content increases at 1100 K. At 1200 K, the mass gain increases significantly as the H2O content increases for up to ≈75% H2O over which it decreases to some degree. The oxidation follows approximately linear kinetic laws at 1200 K. The oxide scales formed in the H2O-containing gases are characterised by a two-layer structure: an outer layer consisting mainly of TiO2 (rutile) grains showing very directional growth and an inner layer which is a porous mixture of very fine TiO2 and α-Al2O3 grains. A thin Al2O3-rich layer, which is usually formed at the interface between the two layers in O2 or air, is not formed.