Superior strain-hardening and strength-ductility synergy by hierarchical L12 phases in a highly (Al, Ti)-added medium-entropy alloy

A novel precipitate-strengthened Co–Cr–Ni medium-entropy alloy with a pure “FCC + L1 2 ” dual-phase structure is developed by adding a high concentration (6 at.%) of Al and Ti elements. After aging heat treatment at 700 °C for 4 h, the hierarchical L1 2 phases, composed of primary large-sized L1 2 phases (∼70 nm) and secondary L1 2 particles (∼14 nm), are achieved. The designed alloy featuring hierarchical L1 2 phases exhibits excellent strength-ductility combinations and strain-hardening ability, properties that are much improved over their counterparts with homogeneous dispersed L1 2 phases (∼13 nm) and without L1 2 phases. Especially, the yield strength (1320 MPa), ultimate tensile strength (1886 MPa), and average strain-hardening rate (5.15 GPa) are obtained in the alloy with hierarchical L1 2 precipitates deformed at cryogenic temperature. The Orowan bypass mechanism of the primary L1 2 phases, along with the shearing mechanism of the secondary L1 2 phases, collectively contributes to the exceptional strength of the alloy with hierarchical L1 2 precipitates. The penetration of dislocations and stacking faults (SFs) in the primary L1 2 phases, combined with hetero-deformation-induced (HDI) hardening attributed to these hierarchical L1 2 precipitates, may be a key factor in improving strain-hardening properties of the alloy.