Simultaneously enhancing strength and ductility of a high-entropy alloy via gradient hierarchical microstructures

Although many high-entropy alloys (HEAs) possess excellent mechanical properties, they are not exempt from the common dilemma of strength–ductility trade-off in most cases, which limits their potential applications. Herein, rotationally accelerated shot peening was used to introduce different gradient hierarchical microstructures, including gradients in twin and dislocation densities, and hierarchical nanotwin, into a CoCrFeNiMn HEA by adjusting the processing parameters. The resulting gradient structures and their effect on hardening behaviour and mechanical properties were systematically explored. Quantitative analysis indicates that deformation twinning, including hierarchical nanotwinning could be more important than dislocation slip in terms of their contribution to hardness and strain hardening capability, depending on the gradient structure profile. It was found that simultaneous improvement of strength and ductility can be achieved in a gradient structure with a thin deformed layer and an undeformed core. Based on our experimental results, we propose that a gradient structure with a largest possible strength difference between the surface layer and the undeformed core would maximize the strength–ductility synergy. [Display omitted] •Gradient hierarchical microstructures to simultaneously enhance strength and ductility of CoCrFeNiMn high entropy alloy.•This structure include: thin gradient layer of deformation twins with undeformed core, and fully deformed gradient layer.•Larger strain gradient promotes accumulation of geometrically necessary dislocations to sustain strain hardening.•Two-order hierarchical nanotwin contributes to higher strain hardening due to twin-twin and dislocation-twin interactions.•Appropriate gradient thickness for the effectiveness of hierarchical nanotwin induced strength-ductility synergy.