Shock-induced two types of {101¯2} sequential twinning in Titanium

Shock loading causes complicated twinning processes in Ti and Ti alloys. In this work, two types of {101¯2} sequential twinning processes are for the first time reported according to electron backscatter diffraction (EBSD) characterization of titanium sheets that are subjected to shock loading. One is described as {112¯1}⇒P→{101¯2} (T_i ^II⇒P→T_j ^I), i.e., {101¯2} sequential twinning is activated in the parent grain (P) along with {112¯1} twins. The other is described as {112¯2}⇒{112¯4}→{101¯2} (C_i ^I⇒C_j ^II→T_k ^I), i.e., {101¯2} secondary twinning is stimulated inside {112¯4} twin by co-zone {112¯2} twin. A statistical analysis according to EBSD characterization reveals the well-defined crystallographic relations between sequential {101¯2} twin variants and the primary/incoming twins, i.e., T_i ^II⇒P→T_i ^IorT_i+1 ^I and C_i ^I⇒C_i+3 ^II → T_i ^I or T_i+1 ^I. We proposed and examined two sequential twinning mechanisms, (i) emissary twinning disconnections at steps along twin boundary for the first case and (ii) shear transformation into the primary twin for the second case, based on dislocation theory and the deformation accommodation ability of the sequential {101¯2} twinning to the shear deformation of the primary/incoming twin. Crystal plasticity modelling is performed to calculate the local stress field associated with the proposed twinning mechanisms. The results demonstrate that the preferred twin variant observed in experiments has the maximum resolved shear stress among the six twin variants. Our work suggests that complicated twinning processes under shock loading obey crystallographic relations according to deformation accommodation ability. The findings from this study can be implemented into meso-/macro-scale crystal plasticity models for predicting mechanical behaviors and texture evolution of polycrystalline aggregates of hexagonal materials.