Microstructure, mechanical properties and oxidation resistance of β-solidifying γ-TiAl based alloys

In the present work, two β-solidifying γ-TiAl based intermetallic alloys, Ti-43.5Al-4Nb-1Mo-0.1B (TNM) and Ti-43.5Al-6(Nb,Zr,Hf)-0.1B (TNZ) (at.%), have been comparatively studied. Two microstructural conditions per alloy, near duplex and near lamellar, were obtained in the alloys by isothermal upset forging and two-stage heat treatments. Microstructure examination revealed three phases (γ, α₂ and β(β_o)) in both alloys but in the TNZ alloy the β(β_o) phase content was less than 1 vol% in contrast to the TNM alloy, in which the β(β_o) phase content was varied within 4–8 vol%. The near duplex conditions of the alloys were similar, whereas a coarser microstructure was obtained in the TNZ alloy with near lamellar structure. The tensile and creep tests revealed that the brittle-ductile transition in the TNZ alloy occurred at higher temperatures that correlated with an appreciably higher creep resistance of the TNZ alloy as compared to the TNM alloy. In the near lamellar condition, the TNZ alloy showed superior strength at 900–1000 °C as compared to known γ-TiAl alloys (e.g. UTS = 554 MPa at 950 °C). The oxidation resistance of the alloys in the near lamellar conditions was evaluated during exposure at 800 °C for 1000 h. The TNZ alloy showed higher oxidation resistance and thus generally appreciably higher high-temperature capability than the TNM alloy. It has been shown that the fundamental reason for different temperatures of the brittle-ductile transition and different creep and oxidation resistance of the alloys is the enhanced thermal stability of the microstructure of the TNZ alloy compared to the TNM alloy.