First-principles study of shear deformation in TiAl alloys

The stress-induced phase-transition γ-TiAl ↔ α2-Ti3Al in TiAl alloys was systemically studied by density functional theory (DFT) calculations. We found that the key role in such phase-transition is the alloy composition. Firstly through studying the relationship between the formation energy, structure (fct and hcp) and alloy composition (Ti:Al = 1–3), we acquired that the stability of fct or hcp phases depends on the Ti:Al ratios. When the ratio meets about 1.5, the formation energies of fct and hcp phases are very close, which means both structures have the similar stability. Further shear deformation calculations on such alloys with different structures and compositions show that phases with Ti:Al value close to 1.5 have smaller dislocation nucleation energies and lower maximum shear strengths than those of the stoichiometric TiAl and Ti3Al phases. Such results not only reveal the composition's effect on the reversible phase transition, but also exhibit the intrinsic controlling role on the alloying properties.