Achieving enhanced strength-ductility synergy in heterogeneous equiaxed structured Ti-6Al-4V alloy sheets by cryogenic pre-stretching

• We designed a heterogeneous equiaxed structure in Ti-6Al-4V alloy via cryogenic pre-stretching. • Back stress strengthening effect and high work hardening rate contributed to the enhanced strength-ductility synergy. • The origin of high back stress strengthening effect was clarified via TEM analysis. Achieving excellent strength-ductility balance in commercial titanium alloys has been recognized as a challenging task. In this study, we designed a heterogeneous equiaxed structure in Ti-6Al-4V alloy via cryogenic pre-stretching at liquid nitrogen temperature. The results show that 7.1% cryogenic pre-stretching of the alloy induces an increased yield/tensile strength of 952 MPa/1005 MPa and maintains a high elongation of 16.1% compared with the as-received alloy (840 MPa/930 MPa and 16.3%, respectively). Compared with ambient deformation, cryogenic deformation can promote the basal slip system activation, dislocation accumulation, and α grain fragmentation. For the heterogeneous equiaxed structure, the grain boundaries between large equiaxed and small fragmented α grains act as barriers for dislocation motion and cause the significant strain gradient and back stress strengthening effect, thus leading to an enhanced strength. On the other hand, the lamellar α fragmentation release the large local stress concentration during cryogenic pre-stretching, which maintains the high work hardening capacity of the alloy. Besides, the parallel lamellar α grains with similar crystal orientation (i.e., macrozone) are destroyed after cryogenic pre-stretching, which is beneficial to hindering the crack propagation during deformation. This study provides an insight into further conquering the strength-ductility paradox of titanium alloys.