ωo phase precipitation in annealed high Nb containing TiAl alloys

The ordered omega phases in high Nb containing TiAl (Nb-TiAl) alloys have been garnering increasing attention in the recent years. However, the investigations on the Nb dependence on the omega(o) precipitation are scarce. In this study, the effect of Nb content on the omega(o) precipitation in high Nb (6-10 at%) containing TiAl alloys after long-time annealing at 850 degrees C has been studied. The results show that small ordered omega particles in the retained beta(o) phase cannot be discerned under scanning electron microscope (SEM) but can be observed using transmission electron microscopy (TEM). Although the NI) segregation can be eliminated after the homogenization heat treatment, the omega(o) phase precipitated in all the alloys studied after annealing at 850 degrees C. TEM examination reveals that the orientation relationship between the omega(o) and alpha(2) phases can be derived as: [0001]omega(o)//[11 (2) over bar0]alpha(2) (11 (2) over bar0)omega(o)//(0001)alpha(2), which indicates that the wo phase is directly transformed from the parent alpha(2) phase Small gamma particles are also observed within the omega(o) areas. The alpha(2) -> + gamma decomposition process is expected during annealing. It is concluded that omega(o) phase is an equilibrium phase in high Nb-TiAl alloys at 850 degrees C. (C) 2015 Chinese Materials Research Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (hup://creativecommons.org/licenses/by-nc-nd/4.0/).