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The ductile-to-brittle transition is commonly observed in metastable β -titanium (Ti) alloys containing ω -precipitates, while the fundamental understanding on ω -embrittlement hitherto remains elusive. In this work, the prototypical Ti-20 wt% Mo metastable β -Ti alloy has been systematically investigated by coupling experiments and first-principles calculation to eliminate this puzzle. It is shown that the structural evolution of ω -phase controls the deformation mechanism transition of twinning-to-slip in Ti Mo alloy, being the origin of ductile-to-brittle transition of this alloy. The initial trigonal ω -structure continuously collapses to hexagonal ω -structure (structural collapse) whilst Mo-atoms are rejected out concurrently (stoichiometric varieties), both leading to hardening of ω -precipitates. This self-hardening of ω -precipitates was further rationalized in terms of the enhanced propensity for a covalent character of the atomic bond demonstrated by the electronic density of states (DOS) from first-principles calculation. Specifically, the self-hardening behavior of ω -precipitates promotes dislocation slip on isolated planes in lieu of correlative slip on successive planes inside ω1 -variant, while dislocations are completely blocked ahead ω2/ω3/ω4- variants. This in turn renders the transition from deformation twinning that contributes to great macro-plasticity to ordinary dislocation slip that contributes to localized deformation bands in the present Ti Mo alloy. Image 1 [ABSTRACT FROM AUTHOR]

The ω -phase transformation in metastable β -titanium (Ti) alloys has attracted great attention in the past decade due to its intrinsic complexity and modifying mechanical properties. Interfacial twin boundary (ITB) ω -phase was reported to appear along {332} β or {112} β twin boundaries in metastable β -Ti alloys. The formation of such ITB ω -phase was proposed to arise from the reverse α″ to β 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 ω -precipitates in the initial microstructure. It is experimentally found that the formation of ITB ω -phase is closely correlated with the favored pre-existing one ω -variant at the expense of the other three-siblings, i.e. , reorientation of ω -variants. The ω -reorientation mechanism is further rationalized by first-principles calculation in terms of the energy barrier of transformation pathway between ω -variants. These findings advance our fundamental understanding to the ω -phase transformation and further plastic deformation behavior of Ti alloys. Unlabelled Image • A new formation mechanism of interfacial twin boundary (ITB) ω -phase has been revealed in metastable β -Ti alloys. • The formation of ITB ω -phase is experimentally ascribed to the reorientation of ω -variants. • The ω -reorientation mechanism is further rationalized in terms of the encountered energy barrier by first-principles calculation. [ABSTRACT FROM AUTHOR]