Kinking-induced {1121} twin in Ti-Sn and Ti-Al alloys

Deformation twinning is generally operated via successive gliding of twinning dislocations/disconnections on coherent twin interface. Kinking is associated with the pileup of an array of gliding dislocations. Both twinning and kinking generate shear band and cause crystal rotation. The major difference between two mechanisms is that twinning causes a specific rotation angle while rotation angle associated with kinking is unspecific, related to the density of dislocations. In this work, we investigated shear bands in pure Ti, Ti-Sn and Ti-Al binary alloys (4 at.% Sn and 5 at.% Al) subjected to high strain rate impact. These shear bands are characterized to be {1012}, {1122} and {1121} twins. Especially, the addition of Sn or Al significantly enhances the activation of {1121} twins while inhibiting {1122} twins. More importantly, the {1121} twins in Ti-Sn and Ti-Al binary alloys exhibit the feature of kink bands, i.e., their boundaries are composed of basal dislocation walls and their misorientation angles are lower than the ideal twin misorientation angle. Atomic-resolution characterization reveals that these {1121} twins evolve from kink bands because of the characters of dislocations at the boundary and the lower gliding resistance of basal (a) dislocations in Ti-Sn and Ti-Al binary alloys. First-principles calculations further confirm that Sn and Al promote the activation of basal (a) dislocations. Molecular statics/dynamic simulations confirm that {1121} coherent twin boundary can be well reproduced by piling up and relaxing basal (a) dislocations. We thus conclude that {1121} twinning can occur via kinking mechanism associated with nucleation, gliding and patterning of basal (a) dislocations.