Overcoming the strength-ductility trade-off in zirconium using twin boundary engineering

Tailoring the strength and ductility of metals through microstructural design has been a longstanding endeavour. Here, we report the development of a hierarchical ultrafine twinned microstructure in zirconium via a novel multi-axial cryo-forging (MACF) process. MACF of Zr resulted in the formation of dense and fine tensile twins, $\left\{10\overline{1}2\right\}<10\overline{1}1>$ (T-1) and $\left\{11\bar{2}1\right\}⟨\bar{1}\bar{1}26⟩$ (T-2) within coarse and equiaxed grains. This microstructure exhibited a remarkable combination of enhanced strength, strain-hardening rate, and ductility, compared to its coarse-grained counterpart. The yield strength and ultimate tensile strength increased by up to 26 % and 30 %, respectively, without compromising material ductility. The improved strength and strain-hardening stemmed from the activation of the harder <c+a> slip system and its consequent interaction with the fine twin boundaries. Notably, the growth of fine T-2 twins during ambient temperature deformation, absent in coarse-grained zirconium, contributed to additional ductility. Crystal plasticity simulations reveal that the activity of pyramidal <c+a> slip within T-2 twins is twice that of prismatic <a> slip, leading to an enhanced strain-hardening rate contributed by the T-2 twins. Subsequent heat treatment of MACF Zr at 500 °C relieved the elastic distortions around twin boundaries, and hence tensile straining led to stress-assisted easier migration of pre-existing twin boundaries. This resulted in a lowering of the strain-hardening rates. It was shown that the dislocation substructure around these typically incoherent tensile twins in hcp metals plays a crucial role in their strain-hardening behaviour.