Sequential {11[formula omitted]2} twinning associated with primary {11[formula omitted]4} twins in commercial-purity titanium subjected to cryorolling

Abstract

{112‾2} (C1) and {112‾4} (C2) contraction twins are commonly observed in commercial-purity titanium cryorolled to 4% reduction. In this study, sequential {112‾2} twinning associated with pre-existing primary {112‾4} twin is for the first time identified. The six possible {112‾2} (or {112‾4}) variants are defined as follows: V1: (112‾k), V2: (1‾21‾k), V3: (2‾11k), V4: (1‾1‾2k), V5: (12‾1k), and V6: (21‾1‾k), where k = 2 for {112‾2} twins (or k = 4 for {112‾4} twins). According to electron back-scatter diffraction analysis, sequential {112‾2} twinning can be divided into two types: (Ⅰ) sequential C1Vi twinning nucleated in the matrix grain at primary C2Vi twin boundary (TB); (Ⅱ) sequential C1Vi twinning originated from the intersection of primary C2Vi and C1Vi+3 twins. The Schmid factor cannot explain the preference of C1Vi twinning in two types. Regarding Type Ⅰ, the geometrical compatibility factor (m′) analysis shows that the favored C1Vi twinning has the largest m′ value among the six variants with respect to primary C2Vi twin and can be stimulated by the C2Vi twinning shear transmission. And the nucleation of sequential twinning can be facilitated by the dissociation of basal dislocations at C2Vi TB. Regarding Type Ⅱ, the displacement gradient tensor accommodation analysis reveals that the preferred C1Vi twinning requires the least accommodation in the two neighboring twins among all possible {112‾2} and {112‾4} variants. Additionally, basal dislocation is required in the primary C1Vi+3 twin to accommodate the shear induced by C2Vi twin, and it can be dissociated into three C1Vi twining dislocations at C1Vi+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.