Ordered α2 to ωo phase transformations in high Nb-containing TiAl alloys

The transformation of the D019-α2 phase to the B82-ωo phase is a new type of phase transformation in high Nb-containing TiAl alloys; experimental results concerning this phase transformation are still lacking. Moreover, studies on the orientation relationships (ORs) between these two phases are scarce compared with those reported between the disordered ω and α phases, which can be more complicated due to their ordered structures. In this study, the direct α2 to ωo phase transformation is observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The ωo phase is transformed from the α2 laths in the lamellar structure after annealing over long periods at 850 °C. The various ORs observed between these two phases can be merged into two typical ORs: [112¯0]α2//[0001]ωo,(0002)α2//(112¯0)ωo and [11¯00]α2//[224¯3¯]ωo,(0002)α2//(011¯2)ωo. The other ORs observed are subsets of these two ORs. The edge-to-edge matching model is applied to predict the possible ORs between the ordered α2 and ωo phases based on the calculated close-packed planes of the two phases. The simulation results agree well with the experimental results.