Influence of different annealing processes under various atmospheres on the oxidation behaviour of γ-TiAl

A detailed study of the oxidation behaviour of bare gamma-TiAl based alloy Ti-45Al-8Nb under various conditions, such as different atmospheres, pressures, temperatures (900 degrees C, 1000 degrees C) and times (100-200 h) is presented. Under high vacuum conditions (10(-6) mbar) a continuous zone of alpha(2)-Ti3Al was formed at the surface with an oxygen-enriched phase on top. No oxide scale formation was obvious. During thermal treatment under Ar-atmosphere at low vacuum pressure (approximately 50 mbar) mainly nitrides (TiN, Ti2AlN) and Al2O3 particles were formed at the surface with an alpha(2)-layer below. Annealing gamma-TiAl in hydrogen atmosphere (about 1040 mbar) led to the formation of a thick reaction zone. A TiO2 layer was formed on top, followed by a mixed oxide scale. Beneath that scale a thick region with alumina, sigma-Nb2Al and alpha(2)-Ti3Al was observed. Moreover, the oxidation behaviour of several thermally pre-treated samples was tested by cyclic oxidation at 900 degrees C in air. The microstructure of the oxide scale formed after testing can be compared with that of non pre-treated material. gamma-TiAl annealed under high vacuum conditions exhibits the lowest oxidation rate, while the mass gain of specimens pre-treated under Ar-atmosphere increased rapidly in the first cycles. All pre-treated specimens exceeded a lifetime of 600 cycles at minimum. The reference material failed after 520 cycles.