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

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 alpha grain fragmentation. For the heterogeneous equiaxed structure, the grain boundaries between large equiaxed and small fragmented alpha 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 alpha 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 alpha 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.