The influence of tantalum on the high temperature characteristics of lamellar gamma + alpha 2 titanium aluminide

The high temperature behavior of as-cast Ti–46Al, Ti–45Al–7Nb and Ti–45Al–7Ta (in at.%) alloys at 800 °C was investigated in this work. The structure, hardness and bending strength of the as-cast alloys were characterized. During long-term (460 h) annealing of the alloys at 800 °C, the structural changes were examined, and the development of hardness was monitored. The creep behavior at 800 °C was studied in the three-point bending mode at bending stresses of 200, 400 and 600 MPa. The oxidation rate of the alloys in air at 800 °C was monitored by measuring the weight gain as a function of oxidation time. It was found that tantalum did not modify the lamellar γ + α2 structure of titanium aluminides, but it negatively influenced the bending strength at room temperature in comparison to niobium. The formation of metastable Ta-supersaturated γ and α2 phases was also revealed to occur in the Ti–45Al–7Ta alloy. During annealing at 800 °C, the Ta-rich τ phase precipitated, which led to hardening of the Ti–45Al–7Ta alloy. The creep resistance of the Ti–45Al–7Ta alloy was shown to be the best of all the investigated materials, and the influence of Ta is discussed in relation to its diffusivity and creep mechanism.