Achieving strength-ductility synergy in zirconium via ultra-dense twin-twin networks

Hierarchical nano-twinned structures have been proven to be an effective way in breaking the strength-ductility dilemma in face-centered cubic metals. However, it is challenging to introduce high-density of twinned structures in hexagonal-close packed (HCP) metals because deformation twinning are limited in fine-grained structures. Here, we propose an approach to introduce ultra-dense twin-twin networks in high purity HCP Zr with ultra-large grain size (>200 mu m) via cryogenic biaxial-rolling. Tensile tests show that the yield strength and the elongation of the densely twinned Zr are increased by 76.5 % and 28.6 % compared with its coarse-grained counterparts, and reaches 360 MPa and 13.5 %, respectively. The strength of twinned Zr is enhanced by the synergy effect of grain refinement (from 241 mu m to 1.2 mu m) and the obstruction of dislocations by high-density intersecting twin boundaries (TBs). The ductility and strain-hardening ability are improved by the activation of massive prismatic (a) and pyramidal (c + a) dislocations. The three-dimensional TB networks are effective (c + a) dislocation sources which enhance the plasticity along (c) direction. Our work provides a new strategy to design ultra-dense twinned structures and sheds light on the twinning-induced plasticity in HCP metals.