The formation of protective alumina-based scales during high-temperature air oxidation of gamma-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.% A g were tested with respect to oxidation resistance during exposure in air a t 800 degrees C. The exposures up to around 1600 hr showed that suitable Ag additions can promote formation of long-term, protective, alumina scales o n gamma-TiAl alloys. Extensive analyses of the oxidation products using opt ical metalEography SEM, XRD, EPMA, and SIMS revealed that Ag stabilizes the Z-phase (Ti5Al3O2) in the subscale-depletion layer thereby preventing form ation 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 re sults were obtained for the alloy Ti-50Al-2Ag; even during exposures as lon g as around 1600 hr, this alloy still appeared to form a stable alumina lay er. 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.