Shock-induced {11 ^₂ 1}→{11 ^₂ 2} double twinning in titanium

Six twinning modes have been reported in α-titanium, including three extension twinning modes {10 ^₁ 2}, {11 ^₂ 1} and {11 ^₂ 3} and three compression twinning modes {11 ^₂ 2}, {11 ^₂ 4} and {10 ^₁ 1}. {10 ^₁ 2} and {11 ^₂ 2} twins are frequently observed without strong dependence on strain rate, while {11 ^₂ 1} and {11 ^₂ 4} twins are observed at high strain rate. These twinning modes and their interactions such as double twinning play significant roles in determining mechanical properties and texture evolution of α-titanium. In this work, we study double twinning associated with {11 ^₂ 1} primary twin. In order to activate {11 ^₂ 1} twinning, a split Hopkinson pressure bar (SHPB) device was adopted to conduct high strain rate (∼2600 s^-1) compression of high purity titanium along the extrusion direction. We observed 453 {11 ^₂ 1}→{11 ^₂ 2} double twins but zero {11 ^₂ 1}→{11 ^₂ 4} double twins in six grains. Crystallographic analysis enables the classification of {11 ^₂ 1}→{11 ^₂ 2} double twins into Group I (29.5° < ^₁ 100>), Group II (55°<5 5 ^₁₀ 3>), Group III (80.6°<1 ^₁ 00>) and Group IV(86.8°< ^₅ 15 ^₁₀ 3>) according to the misorientation angle and axis pair. Groups I and II dominate in the proportion of experimentally detected double twins while Groups III and IV take a small proportion. We account for these phenomena according to apparent Schmid factor, modified deformation gradient accommodation, and twin nucleation via dislocation dissociation. The results demonstrate that the preferred secondary twinning mode and corresponding variant would, to the greatest extent, relax plastic deformation associated with the primary twinning.