Variation of the phase distribution in a Ti-44Al-2Mo alloy by annealing: Influence on its strength and ductility

Much of the research concerning two-phase (α2 + γ) TiAl alloys has confirmed that the mechanical properties of these materials are strongly dependent on the initial microstructures. On the other hand, most of these results have been obtained from alloys containing more than 46 at% Al. Only recently, some new alloys containing 46 and 44 at% Al have been developed and they have been confirmed to retain the β (b.c.c.) and B2 phases, respectively, at room temperature. The room temperature high strength of the alloy containing 44% Al has been attributed to the presence of the ordered B2 phase at the same time as the latter appeared to limit its ductility. The 46% Al containing alloy showed superplastic behavior at 1,200 C, apparently due to the existence of the β phase along grain boundaries, although this phase was responsible for the total lack of ductility of the alloy at room temperature. Besides, the β phase was reported to disappear during high temperature testing in a specific alloy containing 3% Cr as a ternary element. However, the retention of the B2 ordered phase during heat treatments at 1,200 C was confirmed in the 44% Al alloy containing 2% Mo. Based on the latter results, the present study has been carried out in order to understand the influence of heat treatments on the variation of the phases present, their distribution and their effect on the mechanical properties of a Ti-44Al-2Mo alloy. The total proportion of the different phases present has been quantified and their sizes and distributions measured