Oxidation behavior of a single phase (gamma)-TiAl alloy in low-pressure oxygen and hydrogen

The microchemistry and morphology of the oxide layer produced on a single phase (gamma)-TiAl alloy (Ti-52.1Al-2Ta at.%) following anneals at 823 and 1173 K in low-pressure oxygen and hydrogen environments were studied. The oxide structure on the electropolished surface consisted of two layers, an outer one of Al2O3 and an inner one of TiO2. Annealing in low partial pressures of oxygen retained the same oxide structure but increased the total thickness. After the low-pressure oxygen anneals at 823 K, the oxide surface was smooth, whereas after anneals at 1073 K and above the surface consisted of small, round particles, whose size and density increased with increasing annealing temperature and oxygen partial pressure. In contrast, the oxide structure produced by annealing in a hydrogen environment after the low-pressure oxygen treatment consisted of an outer layer of TiO2 and a sub-oxide of Al2O3. The morphology of this oxide consisted of elongated, rod-like particles, whose size increased with annealing temperature. Absorption of H from gaseous hydrogen atmospheres is discussed in relation to the surface and sub-surface oxide structures.