Influence of gaseous species on the oxidation behaviour of TiAl at high temperatures

TiAl coupon specimens were oxidised in various gas mixtures, O-2-H2O, O-2-CO2 and O-2-N-2, and in the constituent single gases under atmospheric pressure at 1100 K and 1200 K for up to 100 ks. Conventional metallographic examinations were performed for characterising the oxidation products, using X-ray diffractometry, optical microscopy and SEM combined with EDS and WDS. All the gases added to oxygen significantly enhance the oxidation, with the influence decreasing in the following order: H2O, CO2 and N-2. The presence of H2O and CO2 modifies the morphology of TiO2 grains of the outer scale layer, implying enhanced anisotropic mass transport in them. The addition of H2O and CO2 disturbs the merging of oxide grains resulting in a fine porous structure in the inner scale layer. When CO2 was added, TiC was found in the scale near the scale/substrate interface. The mechanisms proposed in the literature may apply to the influence of N-2, though no nitrides were identified by the analytical methods used.