Formation of { 11 2 ¯ 2 } contraction twins in titanium through reversible martensitic phase transformation

We report the discovery of a non-conventional $\left\{11\overline{2}2\right\}$ twinning mechanism in $\alpha$-titanium through reversible $\alpha$$\to$ $\omega$ $\to$ $\alpha$ martensitic phase transformations. Specifically, the parent $\alpha$-phase first transforms into an intermediate $\omega$-phase, which then quickly transforms into a twin $\alpha$-phase, leading to the formation of $\left\{11\overline{2}2\right\}$ contraction twins. In addition, we prove that the reversible $\alpha$ $\to$ $\omega$ $\to$ $\alpha$ phase transformations follow strict orientation relations between the parent $\alpha$-, intermediate $\omega$-, and twin $\alpha$-phases. Finally, we demonstrate that our mechanism agrees with classical twinning theory in the shuffle, shear, and conjugate twinning plane. This study reveals the important role of the intermediate $\omega$-phase in the twinning process, adding critical details to the existing mechanism of $\left\{11\overline{2}2\right\}$ twinning.