New insights into formation mechanism of interfacial twin boundary ω-phase in metastable β-Ti alloys

The omega-phase transformation in metastable beta-titanium (Ti) alloys has attracted great attention in the past decade due to its intrinsic complexity and modifying mechanical properties. Interfacial twin boundary (ITB) omega-phase was reported to appear along {332}(beta) or {112}(beta) twin boundaries in metastable beta-Ti alloys. The formation of such ITB omega-phase was proposed to arise from the reverse alpha '' to beta martensitic transformation and subsequent stress relaxation along the twin boundaries. In this study, a new formation mechanism is revealed in Ti-10wt.%Cr alloy containing co-precipitates in the initial microstructure. It is experimentally found that the formation of ITB cophase is closely correlated with the favored pre-existing one co-variant at the expense of the other three-siblings, i.e., reorientation of omega-variants. The co-reorientation mechanism is further rationalized by first-principles calculation in terms of the energy barrier of transformation pathway between omega-variants. These findings advance our fundamental understanding to the omega-phase transformation and further plastic deformation behavior of Ti alloys.