Enhanced strength-ductility synergy in a gradient hetero-structured CrCoNi medium-entropy alloy

A CrCoNi medium-entropy alloy (MEA) with gradient hetero-structure (GHS) composing of gradient nanostructured surface layer and hetero-structured matrix was prepared via a two-step process: pre-cold rolling (CR) deformation and subsequent annealing followed by surface mechanical grinding treatment (SMGT) and secondary annealing. The GHS CrCoNi exhibits an excellent synergy of strength and ductility, as evidenced by a yield strength of approximately 1.2 GPa and a uniform elongation of about 20%. The GHS displays a pronounced hetero-deformation induced (HDI) hardening effect, whereby the central layer (CL) and surface layer (SL) undergo local strain hardening in a mutually compatible manner. This process facilitates the multiplication and accumulation of geometrically necessary dislocations (GNDs) at intra- and inter-layer hetero-interfaces and also activates the deformation faulting and twinning in both CL and SL, thus enhancing the overall mechanical response. The interlayer compatible deformation observed in the GHS significantly bolsters the sustainability of strain hardening at the microscopic scale. This effect is of significant importance in redistributing stress and strain more rationally across the entire GHS sample, effectively reducing the risk of localized deformation failures. This study offers insight into the correlation between macroscopic strain hardening and micromechanical behavior in gradient-/hetero-structured materials.