Insight into the Ta alloying effects on the oxidation behavior and mechanism of cast TiAl alloy

The effects of Ta alloying on the isothermal oxidation behavior and mechanism of a cast Ti-48Al-3Nb-1.5Ta (at.%) TiAl alloy are investigated. It exhibits excellent oxidation resistance with mass gains of 0.95 and 1.29 mg·cm−2 after 900 °C oxidation for 100/200 h in the air, similar to 8 at.% Nb-containing TiAl alloys. The oxide scale shows a complex multi-layer structure, where the Al2O3 layer and continuous dense nitride layer act as protective layers against diffusion and metal oxidation. The oxidation mechanisms are discussed thermodynamically and kinetically. Compared with Nb, Ta is more significant in inhibiting diffusion and promoting the formation of the protective continuous nitride layer, thus being more effective in improving the oxidation resistance of cast TiAl alloy. Consequently, without considering the influence of microstructure, the oxidation resistance of cast Nb-containing TiAl alloy can be further improved by replacing some Nb with a small amount of Ta. It is supposed to provide new insights into the design of oxidation-resistant TiAl alloys.