A review of the metastable omega phase in beta titanium alloys: the phase transformation mechanisms and its effect on mechanical properties

Since its discovery in 1954, the omega (omega) phase in titanium and its alloys has attracted substantial attention from researchers. The beta-to-omega and omega-to-alpha phase transformations are central to beta-titanium alloy design, but the transformation mechanisms have been a subject of debate. With new generations of aberration-corrected transmission electron microscopy and atom probe tomography, both the spatial resolution and compositional sensitivity of phase transformation analysis have been rapidly improving. This review provides a detailed assessment of the new understanding gained and related debates in this field enabled by advanced characterization methods. Specifically, new insights into the possibility of a coupled diffusional-displacive component in the beta-to-omega transformation and key nucleation driving forces for the omega-assisted alpha phase formation are discussed. Additionally, the influence of omega phase on the mechanical properties of beta-titanium alloys is also reviewed. Finally, a perspective on open questions and future direction for research is discussed.