On the mechanism of nucleation and growth of omega phase in beta titanium alloys

The omega phase forms in many beta titanium alloys following fast quenching. It is generally accepted that the mechanism is displacive, involving the collapse of {111} planes in the beta phase. In contrast, the results of the present research are consistent with a mechanism of atom shuffle and diffusion. Thus, full and partial collapse of {111} planes were characterized in the as-quenched state for various Ti-Mo binary systems using aberration-corrected electron microscopy. Solute diffusion between omega phase and parent beta phase was studied in multi-component systems using 3D atom probe and x-ray energy dispersive spectroscopy. The results, obtained from direct observation of the nucleation and growth of omega, support a mechanism of formation involving the shuffle of every two of three neighboring {222} atom planes along <111> direction. The composition of omega phase is achieved by the solute diffusion across omega/beta interface. The transformation is characterized a mixed mode.