Mechanical twinning and omega transition by ⟨111⟩ {112} shear in a metastable β titanium alloy

{112} < 111 > mechanical twinning and stress-induced omega transition were observed by high-resolution transmission electron microscope in a metastable beta titanium alloy with chemical composition of Ti-23Nb-0.7Ta-2Zr-1.2O at. % after deformation. The orientation relationships between the omega phase and beta parent matrix are ((1) over bar 010)(omega) parallel to (211)(beta), [1 (2) over bar 10](omega) parallel to [(1) over bar 11](beta), (0001)(omega) parallel to (111)(beta) often observed for the thermal omega transition. Both mechanical twinning and omega transition arise from the shear along < 111 > {112}. A dislocation mechanism for mechanical twinning and stress-induced omega transition was discussed additionally. (C) 2008 American Institute of Physics.