Abstract: Achieving high yield strength and ductility in alloys remains a significant challenge in structural materi-als. In this study, combined nanoprecipitation and gradient grain structure were introduced into a Co-Cr-Ni-based multi-principal element alloy (MPEA) using surface mechanical attrition treatment (SMAT). The multi-scale composite structure, featuring grain sizes refined from ∼43.6 μm to ∼24.3 nm at the topmost surface and high-density L12 nanoprecipitates within the grains, results in a substantial tensile strength of 1733 MPa and a well-maintained ductility of ∼23 %. The alloy with low local stacking fault energy provides sufficient flow stress to reach the critical value for twinning, a phenomenon rarely observed in MPEAs with high-density L12 nanoprecipitates under quasi-static tensile conditions. The formation of nanotwins further facilitates additional strain hardening, enhancing mechanical performance at ultrahigh strength levels. This work offers significant insights into the deformation behavior of gradient-structured materials with high-density nanoprecipitates.

Key words: Multi-principal element alloy, L1₂ nanoprecipitate, Gradient grain structure, Deformation nanotwinning