Oxidation resistance of γ-TiAl based alloys in a simulated combustion atmosphere

The cyclic oxidation behavior of gamma - TiAl based alloys, Ti-48Al-2Cr-2Nb, Ti-48Al-Xr-2Fe, Ti-48Al2Cr-2W and Ti-48Al-2Cr-1W-1Ta (mol.%), was studied in a simulated combustion gas at 1173 K for. up to 25 cycles (500 h). The oxidation products and subsurface areas were examined and characterized by using X-ray diffractometry, SEM and EPMA. All the alloys except Fe-containing one show excellent oxidation resistance for at least up to 25 cycles by forming thin and adherent oxide scales. In particular, the mass gains of W-containing alloys are a third of that of Ti-48Al-2Cr-2Nb, indicating significant effect of W. Ta seems to behave similarly as W. The excellent oxidation resistance is attributed to the formation of a definite and dense Al2O3 layer in the scale during the initial oxidation stage. A stable Al-depleted layer (so-called Z phase) is formed beneath the scale. The influence of Nb, Ta, and W seems to stabilize the Al-depleted layer in addition to the doping effect. However, their mass gains due to the oxidation are smaller than those tested in laboratory air, though the combustion gas was thought more aggressive than air. On the other hand, Ti-48Al-2Cr2Fe oxidizes rapidly with scales consisting of two layers; an outer TiO2 layer with heterogeneously distributed iron and chromium oxides and an inner porous layer which is a mixture of Al2O3 and TiO2 grains.