Sequential {11 2 ‾ 2} twinning associated with primary {11 2 ‾ 4} twins in commercial-purity titanium subjected to cryorolling

Highlights: • Sequential {11$\overline{2}$2} (C1) twinning associated with primary {11$\overline{2}$4} (C2) twins is investigated in cryorolled titanium. • Sequential twinning generates in the matrix grain at C2 twin boundary or at the intersection of primary C1 and C2 twins. • Local strain accommodation is dominant to determine the preferred sequential twinning variant, rather than Schmid factor. • Sequential twinning can be nucleated by basal dislocation dissociation in its neighboring primary twins at twin boundary. -- Abstract: {11$\bar{2}$2} (C1) and {11$\bar{2}$4} (C2) contraction twins are commonly observed in commercial-purity titanium cryorolled to 4% reduction. In this study, sequential {11$\bar{2}$2} twinning associated with pre-existing primary {11$\bar{2}$4} twin is for the first time identified. The six possible {11$\bar{2}$2} (or {11$\bar{2}$4}) variants are defined as follows: V₁: (11$\bar{2}$k), V₂: ($\bar{1}$2$\bar{1}$k), V₃: ($\bar{2}$11k), V₄: ($\bar{1}\bar{1}$2k), V₅: (1$\bar{2}$1k), and V₆: ($2\bar{1}\bar{1}\text{k}$), where k = 2 for {11$\bar{2}$2} twins (or k = 4 for {11$\bar{2}$4} twins). According to electron back-scatter diffraction analysis, sequential {11$\bar{2}$2} twinning can be divided into two types: (Ⅰ) sequential C1V_i twinning nucleated in the matrix grain at primary C2V_i twin boundary (TB); (Ⅱ) sequential C1V_i twinning originated from the intersection of primary C2V_i and C1V_i+3 twins. The Schmid factor cannot explain the preference of C1V_i twinning in two types. Regarding Type Ⅰ, the geometrical compatibility factor (m′) analysis shows that the favored C1V_i twinning has the largest m′ value among the six variants with respect to primary C2V_i twin and can be stimulated by the C2V_i twinning shear transmission. And the nucleation of sequential twinning can be facilitated by the dissociation of basal dislocations at C2V_i TB. Regarding Type Ⅱ, the displacement gradient tensor accommodation analysis reveals that the preferred C1V_i twinning requires the least accommodation in the two neighboring twins among all possible {11$\bar{2}$2} and {11$\bar{2}$4} variants. Additionally, basal dislocation is required in the primary C1V_i+3 twin to accommodate the shear induced by C2V_i twin, and it can be dissociated into three C1V_i twining dislocations at C1V_i+3 TB, providing another way to accelerate the nucleation of sequential twinning. These findings contribute to the understanding of complicated twinning behavior in cryorolled Ti.