Omega transitional structure associated with {112}<111> deformation twinning in a metastable beta Ti-Nb alloy, revealed by atomic resolution high-angle annular dark-field scanning transmission electron microscopy

The omega structure associated with the {112}<111> deformation twinning in a deformed Ti-50Nb alloy have been examined by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Atomic resolution HAADF-STEM observations reveal that, instead of the ideal omega structure, a kind of omega transitional structure characterized by gradual collapse of {111} atom planes of beta phase can be formed in the twin growth frontier region to accommodate high stress concentration occurring there, while layered omega transitional structure, which can be so thin to have just one-unit-cell layer in thickness, can be formed along the longitudinal twin boundaries to relax relatively weak interfacial stress. Also, we have made it clear that the omega transition induced by compressive stress concentration at the twin growth frontier is accomplished by displacements of atoms in not only the beta matrix but the twinned beta structures as well. These observations offer a conclusive evidence to understand that the {112}<111> twinning shear is critical in assisting the beta-to-omega structural transition locally. On the basis of these results, the formation process of the omega transitional structure associated with the {112}<111> twinning is discussed.