Scale formation on gamma-TiAl during oxidation at 800 and 900 degrees C in air and in He+20%O-2

The oxide scale formation on gamma-TiAl at 800 and 900 degrees C was studied using high-temperature X-ray diffraction as an in situ-method. The experiments were performed in air and in He with 20 vol.% O-2. The formation of alumina in the form of alpha-Al2O3 and of TiO2 in the form of rutile was observed in both atmospheres and the formation of TiN was detected in air. Depending on the atmosphere the diffraction peaks of two different additional phases were detected, which do not exist in any data base nor in the Ti-Al-O phase diagram. One of them, the Z-phase, appears in He with 20 vol.% O-2 and the other, the X-phase, in air. The Z-phase was also found at room temperature after oxidation at 900 degrees C in air. The growth of both phases, X and Z, starts immediately with the oxidation process and follows the parabolic rate law.