Phase fractions, transition and ordering temperatures in TiAl–Nb–Mo alloys: An in- and ex-situ study

Intermetallic γ-TiAl based alloys of the TNM™ alloy family attain their excellent processing characteristics by a high β-phase content present at hot-working temperatures. Subsequent to hot-working the β-phase content is decreased by a heat treatment step performed at temperatures where the β-phase fraction exhibits a minimum. In this study, in- and ex-situ experiments were conducted on three alloys with different contents of β/β0 stabilizing elements. The course of phase fractions as a function of temperature as well as phase transition temperatures were determined by means of in-situ high-energy X-ray diffraction experiments. Additionally, dynamic scanning calorimetry investigations were performed to obtain complementary data on the transition temperatures. Quantitative metallography was conducted on heat treated and quenched specimens to acquire additional information on the dependence of the phase fractions on temperature. By neutron diffraction experiments the ordering temperatures of the constituent phases were determined. It was shown that the experiments yielded consistent results which differ significantly from ThermoCalc simulations for which a commercial TiAl database was used. The differences between the experimental results and the thermodynamic predictions are discussed.