TEM Observation of the Initial Stage Oxidation of TiAl Based Alloys in a Simulated Combustion Atmosphere

Abstract

The oxidation behavior of Ti-48Al-2Cr-2Nb, Ti-48Al-2Cr-2W and Ti-48Al-2Cr-2Fe was studied in a simulated combustion gas, 10O2-7CO2-6H2O-bal.N2 (vol%), at 1173 K and TEM observation was performed for understanding the initial stage of oxidation behavior. Ti-48Al-2Cr-2Nb and Ti-48Al-2Cr-2W show excellent oxidation resistance in the test gas by forming thin and protective Al2O3-rich scales, while Ti-48Al-2Cr-2Fe shows poor oxidation resistance. The superior oxidation resistance of W-containing alloy is explained in terms of the formation of a bcc phase with low Al content in the alloy which was confirmed by TEM observation and also possible enhanced Al diffusion from the substrate to the scale in this phase. The oxidation resistance of the former two alloys in the test gas is better than in laboratory air, due to the lower O2 content in the test gas. The presence of H2O and CO2 in the test gas enhances the oxidation of Ti-50Al, while it has almost no influence on the oxidation behavior of these two alloys, indicating that these gases are influential to a TiO2-rich scale but not to Al2O3-rich scales.