The formation of protective alumina-based scales during high-temperature air oxidation of γ-TiAl alloys

The effect of Ag additions on the oxidation behavior of gamma-TiAl has been studied The materials investigated containing 47-50 at.% Al and 0-5 at.% Ag were tested with respect to oxidation resistance during exposure in air at 800 degrees C. The exposures up to around 1600 hr showed that suitable Ag additions can promote formation of long-term, protective, alumina scales on gamma-TiAl alloys. Extensive analyses of the oxidation products using optical metalEography SEM, XRD, EPMA, and SIMS revealed that Ag stabilizes the Z-phase (Ti5Al3O2) in the subscale-depletion layer thereby preventing formation alpha(2)-Ti3Al as well as Ti-rich nitrides, which are responsible for the destruction of alumina scales in common gamma-TiAl alloys. The best results were obtained for the alloy Ti-50Al-2Ag; even during exposures as long as around 1600 hr, this alloy still appeared to form a stable alumina layer. It was found that high Ag additions of 5% were detrimental after longer exposure times due to extensive Ag precipitation at the interface between the alloy and depletion layer, resulting in localized formation of vapidly growing, mixed-oxide scales.