A novel precipitation-strengthened Co₄₀Ni₃₇Cr₁₆Ti₇ medium entropy alloy exhibiting an excellent strength-ductility synergy

In this paper, we developed a novel Co₄₀Ni₃₇Cr₁₆Ti₇ medium-entropy alloy (MEA) exhibiting a unique triplex-phase "FCC + L1₂ + η" microstructure. The microstructure of the alloy can be tailored through thermo-mechanical processing. The microstructure resulting from the cold rolling and single-step or two-step low-temperature annealing treatment (CA-700, CA-800 and CAA) is categorized into shear band regions and non-shear band regions. The non-shear band regions predominantly consist of lamellar precipitates (η) with sparse short rod-like precipitates (η), whereas the shear-band regions exhibit a reversed morphology dominated by short rod-like precipitates. Simultaneously, a dispersion of fine spherical L1₂ precipitates are formed in both regions. The CA-700, CA-800, and CAA alloys exhibit poor ductility owing to the extensive formation of brittle η-phase precipitates. In contrast, the high-temperature recrystallized CRA alloy develops equiaxed grains with fine lamellar η precipitates preferentially distributed along grain boundaries. Concurrently, a high density of near-spherical L1₂ nano-precipitates is uniformly distributed within the grain interiors, leading to an excellent strength-ductility synergy. The CRA alloy achieves a yield strength of ∼1082 MPa, an ultimate tensile strength of ∼1493 MPa, and a ductility of ∼22.4 %. The high strength of the CRA alloy is predominantly attributed to precipitation strengthening, with the uniformly dispersed L1₂ nano-precipitates (volume fraction: ∼49 %) acting as potent barriers to dislocation motion. Furthermore, the CRA alloy maintains remarkable ductility through a deformation mode involving dense slip traces and stacking faults (SFs), which promote homogeneous plastic deformation. This unique combination of strengthening and deformation mechanisms results in the observed excellent strength-ductility synergy.