γ′ shearing and deformation twinning enable strength–ductility synergy in high-Co L12 high-entropy alloys at intermediate temperatures

High Al and Ti containing L1₂-strengthened high-entropy alloys (HEAs) show great potential for aero-engine hot-section applications due to their high strength; however, intrinsic brittleness, particularly at intermediate temperatures (800 °C), limits practical use. In this study, a high-Co L1₂-strengthened HEA was designed to suppress brittle phase formation, especially the Ni₃Ti-type η phase. Cold rolling followed by annealing introduced thermally stable deformation twins (DTs). As a result, the alloy exhibits pronounced strength–ductility synergy at both RT and 800 °C, achieving ultimate tensile strengths of 1650 MPa at RT and 966 MPa at 800 °C with elongations exceeding 25%. The typical brittleness of high Al + Ti L1₂-strengthened HEAs is therefore effectively mitigated. TEM observations and strengthening analysis reveal that stacking faults (SFs) and DTs remain active during intermediate-temperature deformation. SF-mediated γ′ shearing governs strength, while DTs traversing γ/γ′ interfaces enhance deformation compatibility. The reduced stacking-fault energy induced by high Co content lowers the critical shear stress for SF and DT activation across a wide temperature range, enabling stable mechanical performance.