Secondary twin variant selection in four types of double twins in titanium

We experimentally characterized four types of double twins in pure titanium and discussed the selection of secondary twin variants based on the apparent Schmid factor (a-SF), the displacement gradient accommodation (DGA), the modified DGA (m-DGA), and the nucleation via dislocation dissociation (NDD). The four types of double twins are classified into two families according to their zone axes: co-family double twins and non-family double twins. Co-family double twins share the zone axis <101¯0>. Non-family double twins have different zone axes with one along <112¯0> and the other along <101¯0>. Experimental observations reveal that the co-family double twins have one secondary twin variant prevailed over others and the non-family double twins have two secondary twin variants preferred than others. The a-SFs associated with secondary twins are always positive once the primary twin is activated, thus predicting no obvious preference for the selection of secondary twin variants. The m-DGA and NDD correctly predict the selection of secondary twin variants, implying that the preferred secondary twin variant would, to the greatest extent, relax plastic deformation associated with the primary twinning and the nucleation of the secondary twin variant is facilitated by the accumulation of available gliding dislocations at the primary twin boundary.