Effects of major alloying additions on the microstructure and mechanical properties of γ-TiAl

The microstructure and mechanical properties of eight gamma-TiAl based alloys with compositions in the range Ti-44Al-8(Nb,Ta,Zr,Hf)-(0-0.2)Si-(0-1)B have been investigated to assess the possibility of improving the properties of gamma-TiAl through heavy alloying. It has been shown that the microstructures of these alloys can be significantly different from those of the binary or 48-2-2 type alloys as a result of differences in the phase equilibria. As expected with large additions of beta stabilisers such as Nb, Zr and Ta, the beta phase was stabilised to much lower temperatures than that in the Ti-44Al binary ahoy. In some of the alloys the omega phase, which is a transformed product of the beta phase, is stable at room temperature and up to >900 degrees C. In alloys which contain both beta- and gamma- stabilisers, there is no single alpha phase held in the transformation sequence and instead there is a (alpha + beta + gamma) three phase regime. The mechanical data obtained from these alloys indicate that heavy alloying can be used to increase the strength and creep resistance of gamma-TiAl significantly although ductility generally remains poor. The addition of boron appears to be beneficial in that both strength and ductility are improved, particularly for materials with the duplex microstructure. (C) 1998 Elsevier Science Ltd. All rights reserved.