Heterogeneous grain structure delivers work hardening and high ductility in a VCoNi alloy with ultrahigh yield strength

Strengthening a metal through cold working or grain refinement can significantly increase its yield strength by several times or even more, while it inevitably leads to a dramatic loss of ductility. The problem is the difficulty
of the dislocation multiplication and accumulation within a uniformly-grained structure under uniaxial loading, resulting in the deficiency of work hardening capability. To address this limitation, we have employed the heterogeneous grain structure (HGS) as a microstructural strategy to enhance work hardening to improve ductility. Through thermo-mechanical processing, two types of the face-centered-cubic-structured single-phase. HGSs are produced in a VCoNi alloy. The first HGS is composed of recrystallized grains, spanning ultrafine grains (<1 μm in grain size) and fine grains (>1 μm), along with twinned grains of ~200 nm in size, while the second
remains a part of deformed structure within the matrix of recrystallized grains. Upon straining, these HGSs exhibit synergistic work hardening, combining the forest dislocation-mediated work hardening with geometrically
necessary dislocations-based hardening, accompanied by strain partitioning among grains of varying sizes.
Furthermore, both HGSs undergo dynamic reinforcement during tensile deformation through grain refinement,
particularly evident in the transformation from twinned grains to ultrafine grains, which is more obvious during
cryogenic deformation. As a result, the first HGS shows uniform ductility of 19 % and 29 % at yield strength of
1.6 and 1.8 GPa during ambient (298 K) and cryogenic (77 K) deformation, respectively. The second HGS
achieves enhanced yield strengths to 1.9 and 2.3 GPa at these temperatures, retaining considerable ductility of
10 % and 14 %. These strength-ductility combinations outstrip those in conventional alloys and multi-principal
element alloys.