Origin of shear induced β to ω transition in Ti–Nb-based alloys

Ti–Nb-based alloys are essential materials for biomedical implant and aerospace applications. They reveal complex phase transformation behavior. Here, a {2 1 1}β〈1 1 1〉β twinning induced β (body-centered cubic phase) to ω (hexagonal phase) transition in Ti–Nb-based alloys is demonstrated by transmission electron microscopy and analyzed employing ab initio calculations and the linear elastic inclusion theory. Our theoretical results reveal a distinct energy barrier for the β to ω transition, where the contribution from lattice rearrangement, rather than the elastic contribution associated with lattice parameter mismatch, plays the major role. It is shown that this energy barrier can be overcome by {2 1 1}β〈1 1 1〉β shear, explaining why {2 1 1}β〈1 1 1〉β twinning or, alternatively, the β to α″ (orthorhombic phase) transition promotes local formation of the ω phase.