Excellent strength-ductility synergy properties of gradient nano-grained structural CrCoNi medium-entropy alloy

Tailoring heterogeneities could bring out excellent strength-ductility synergy properties. A gradient nano-grained (GNG) structure, i.e. grain size range from nanometer (∼50 nm) at topest surface layer to micrometer (∼1.3 μm) at center layer, was successfully introduced into CrCoNi medium-entropy alloy (MEA) by means of high energy shot peening in this work. Experimental results demonstrated that this GNG CrCoNi MEA shows excellent strength and ductility combination properties, exhibiting high yield strength and ultimate tensile strength of ∼1215 MPa and ∼1524 MPa, respectively, while remaining a good ductility of ∼23.0%. The extraordinary hetero-deformation induced (HDI) hardening origins from heterogeneous structure, i. e. GNG structure, which contributes to the majority strength enhancement. Dynamical reinforced heterogeneous structure during tension process results in the enhanced HDI hardening effect, which facilitates excellent ductility and strain hardening capacity at high-level strength. Our work provide not only a feasible and effective way to strengthen the CrCoNi MEA, and other low stacking faults energy (SFE) materials, but also an useful insight to understanding HDI hardening in heterogeneous structure.