Achieving excellent strength-ductility synergy in CoCrFeNi2Al0.3Ti0.25 high entropy alloy via analogous harmonic structure and dual-morphology L12 precipitates strengthening

Achieving a superior combination of high strength and ductility in metallic materials remains a key challenge in materials science. In this study, we improve the strength-ductility synergy of a CoCrFeNi-based high entropy alloy (HEA) by designing an analogous harmonic structure via thermomechanical processing. Microstructural analysis reveals that a soft FCC matrix phase is surrounded by a hard L1₂ precipitate phase, forming a typical core-shell heterogeneous structure. The volume fractions of the core and shell regions are 62.8 % and 37.2 %, respectively. Tensile tests at room temperature demonstrate that the HS-HEA exhibits a yield strength of 1018 MPa, an ultimate tensile strength of 1433 MPa, and a total elongation of 29.0 %. The high yield strength arises from significant strain gradients in the core and shell regions, which induced hetero-deformation induced strengthening, grain boundary strengthening, and precipitation strengthening. Meanwhile, the excellent ductility is attributed to dislocation slip, deformation-induced stacking faults, and Lomer-Cottrel locks, with a minor contribution from deformation twins. These mechanisms collectively enhance work-hardening capacity, delaying plastic instability and enabling an exceptional strength-ductility balance. This research presents a novel approach to strengthening HEAs through analogous harmonic structure, which enhances hetero-deformation induced hardening in addition to traditional mechanisms. These findings provide critical insights for designing heterogeneous structural alloys with outstanding mechanical properties.