On the reversibility of the α2/ωo phase transformation in a high Nb containing TiAl alloy during high temperature deformation

Abstract The transformations between the phases α2 (Ti3Al) and ωo were investigated in a lamellar multiphase titanium aluminide alloy based on γ (TiAl). The paper complements an earlier investigation performed on the same material in which the importance of deformation-induced twin structures for the α2→ωo transformation was demonstrated. The present study shows that the reverse transformation ωo→α2 can also occur during high-temperature deformation. The transformation is probably triggered by constrained stresses, which exist between the different constituents due to the crystalline mismatch. The combined operation of mechanical twinning of the α2 phase and the reversible transformation fully converts the α2 lamellae into a mixture of α2 and ωo. This conversion greatly reduces the mechanical anisotropy existing in former α2 lamellae. Regarding the technical use of the alloy, the stability of the converted structure with respect to further annealing was also examined. The reported processes occur at the nano-meter and sub nano-meter scale, thus, advanced characterization techniques were applied, such as high-resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT).