Bulging of grain boundaries and core-shell dislocation structures enhance mechanical properties of equiatomic high-entropy alloys

Regulating elemental compositions of structural materials has been at the heart of interests for metallurgists to ensure target properties under harsh environments. For instance, metastability engineering that exploits phase transformation or deformation twinning depends on a minor modification in atomic compositions. Distinct from the well-studied control of elemental compositions, this work centers on a straightforward thermomechanical process of hot rolling to induce bulging of grain boundaries and core-shell dislocation cell structures. During the hot rolling, the bulging of grain boundaries releases high-density dislocation walls and more dislocations are distributed around the grain boundaries in equiatomic CoCrFeMnNi, one of the most studied high-entropy alloys. Under the tensile deformation at cryogenic temperatures with decreased stacking fault energy, the less stable grain boundaries promote the emanation of partial dislocations and the consequent formation of deformation twinning. As a result, the hot-rolled alloy exhibits an enhanced combination of yield strength of ∼941 MPa and uniform elongation of ∼54% at –196 °C, which is counterintuitive to low ductility of as-rolled metallic materials. This lies at the upper bound in comparison with tensile responses of precipitation-strengthened high-entropy alloys and high-strength steels. The higher propensity of deformation twins in hot-rolled alloy compared to that of cold-rolled and annealed one enhances strain hardening despite the hot-rolled state. Regarding the benefits of the streamlined thermomechanical history, this study validates the academic and industrial worth of hot-rolled metallic materials to develop the alloy science and fabricating technology.