High-temperature oxidation behavior of multi-phase Mo-containing γ-TiAl-based alloys

Intermetallic titanium aluminides are potential materials for a number of high-temperature components used in aero and automotive engines. In particular, alloys solidifying via the β-phase are of great interest because they possess a significant volume fraction of the disordered body-centered cubic β_o-phase at elevated temperatures ensuring good processing characteristics during hot-working. Nevertheless, the practical use of such alloys at a temperature as high as 800 °C requires improvement of their oxidation resistance. Various attempts have been made including alloying with additional elements such as Nb, Cr, Mo etc. or applying the so-called fluorine effect. However alloying could not provide a sufficient oxidation resistance above 850 °C whereas the fluorine effect protects the base material against environmental degradation up to over 1000 °C. This paper aims to investigate the influence of the phase composition on the oxide scale morphology without and with fluorine effect. The results refer to the oxidation behavior of three β-solidifying γ-TiAl-based alloys in the cast and hot-isostatically pressed condition at 800 °C in air. The behavior of the TNM alloy (Ti–43.5Al–4Nb–1Mo–0.1B, in at.%) was compared with that of two Nb-free TiAl alloys which contain different amounts of Mo (3 and 7 at.%, respectively) and hence a different microstructure (α₂/β_o/γ vs. β_o/γ). During testing in dry synthetic air at 800 °C a mixed oxide scale develops on all three alloys. This behavior was changed via the fluorine effect, as demonstrated for previously investigated TiAl alloys with an Al-content higher than 40 at.% based on α₂/γ and α₂/β_o/γ phases. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. The reason for this is the change in the oxidation mechanism triggered by the small additions of fluorine in the subsurface zone of the investigated alloys. The results of isothermal oxidation tests at 800 °C in air are presented and discussed in view of chemical composition and microstructure, along with the impact of the phase composition on the efficiency of the fluorine effect. From a microstructural perspective the fluorine effect leads to the formation of an even thinner oxide scale on the β-phase compared to the γ-phase.