On the reversibility of the α_(2)/ω_(o) phase transformation in a high Nb containing TiAl alloy during high temperature deformation

The transformations between the phasesα_(2)(Ti_(3)Al)andω_(o)were investigated in a lamellar multiphase titanium aluminide alloy based onγ(TiAl).The paper complements an earlier investigation performed on the same material in which the importance of deformation-induced twin structures for theα_(2)→ω_(o) transformation was demonstrated.The present study shows that the reverse transformationω_(o)→α_(2) can also occur during high-temperature deformation.The transformation is probably triggered by constraint stresses,which exist between the different constituents due to the crystalline mismatch.The combined operation of mechanical twinning of theα_(2) phase and the reversible transformation fully converts theα_(2) lamellae into a mixture ofα_(2) andω_(o).This conversion greatly reduces the mechanical anisotropy existing in formerα_(2) 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).