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We present first-principles density functional theory calculations to study the α - ω phase transformation in Ti and Zr and its coupling to slip modes of the two phases. We first investigate the relative energetics of all possible slip systems in the α and ω phases to predict the dominant slip system that is activated during a plastic deformation under an arbitrary load. Using this and the crystallographic orientation relationships between α and ω phases, we construct low energy α / ω interfaces and study the energetics of the slip system at the interface between α and ω to compare to the slip systems in the bulk phases. We find that for a particular crystallographic orientation relationship, where ( basal ) α ∥ ( prismatic - II ) ω , and [ a ] α ∥ [ c ] ω , the slip at the interface is preferred compared to its bulk counterparts. This implies that the plastically deformed α / ω phase with this orientation relationship prefers to retain the interface (or coexisting phases) than transforming back to the pure phase after unloading. This is consistent with the observation that the ω -phase is retained in samples loaded in flyer plate experiments or under high-pressure torsion. Furthermore, calculation of the energy barrier for α to ω phase transformation as a function of glide at the α / ω interface shows significant coupling between the α - ω phase transformation and slip modes in Ti and Zr.
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