Oxidation behavior of TiAl based alloys in a simulated combustion atmosphere

The cyclic oxidation behavior of Ti–48Al–2Cr–2Nb, Ti–48Al–2Cr–2Fe, Ti–48Al–2Cr–2W, and Ti–48Al–2Cr–1W–1Ta (at.%) was studied in a simulated combustion gas, 10O2–7CO2–6H2O–Bal. N2 (vol.%), at 1173 K for up to 1800 ks (500 h, 25 cycles). Metallographic examinations were performed for oxidized specimens using X-ray diffractometry, SEM, and EPMA. All the alloys except Fe-containing one show excellent oxidation resistance for at least 1800 ks in the test gas by forming thin and protective Al2O3-rich scales. In particular, the mass gains of the W-containing alloys are about a third of that of Ti–48Al–2Cr–2Nb. The oxidation of the Fe-containing alloy is fast and its scale repeated partial spallation. All the results are explained mainly in terms of doping effect of the additional elements to TiO2, their influence on the stabilization of the Al-depletion layer formed beneath the scale, and possible β-phase formation in the substrate. The oxidation resistance of the three alloys in the test gas is better than in the laboratory air. This was found to be due to the lower O2 content in the test gas by additional oxidation tests using another test gas containing 15%O2. Both H2O and CO2 in the test gas significantly enhance the oxidation of Ti–50Al, however they have almost no influence on the oxidation behavior of the three alloys. This indicates that these gases are influential to TiO2-rich scales but not to Al2O3-rich scales. Ta seems to behave similarly as Nb with no synergistic effect with W.