Designing structures with combined gradients of grain size and precipitation in high entropy alloys for simultaneous improvement of strength and ductility

Suppressing the early strain localization at the nanostructured topmost layer is crucial for achieving better tensile ductility in the gradient structure. Thus, structures with combined gradient distributions along the depth for both grain size and volume fraction of precipitates were designed and introduced in a high entropy alloy by surface mechanical attrition treatment and aging. Yield strength and uniform elongation were observed to be simultaneously improved in the structures with combined gradients as compared to the corresponding structures with only grain size gradient. More severe strain gradients and higher density of geometrically necessary dislocations were observed to be produced at various domain boundaries in the structures with combined gradients, resulting in stronger hetero-deformation-induced (HDI) extra hardening for better tensile properties. Shearing and bowing hardening mechanisms were observed for L12 and B2 precipitates, respectively. Higher volume fractions of B2 and L12 phases at the topmost layer induce stronger precipitation hardening, which compensates the diminished strain hardening due to the reduced grain size at the topmost layer for better tensile ductility in the structures with combined gradients. The observed higher yield strength in the structures with combined gradients have been discussed based on mechanisms of dislocation strengthening, precipitation strengthening and HDI strengthening.