Alloying provides an effective approach to designing metallic materials with unique microstructures and enhanced performance. In this work, we developed a series of (CoCrNi)_100−xZr_x (where x = 1, 2, 3, 4, and 5) medium entropy alloys (MEAs) by vacuum arc-melting method. The effects of Zr addition on the microstructures and mechanical properties of (CoCrNi)_100−xZr_x MEAs were systematically investigated. Due to the negative mixing enthalpy of Zr with Co, Cr, and Ni, lamellar C15 Laves-phase precipitates formed within the ductile FCC matrix. As the Zr content increases, the alloys exhibit higher strength but become more brittle at room temperature. Among the (CoCrNi)_100−xZr_x MEAs series, the CoCrNiZr₃ MEA shows an excellent balance between strength and ductility, achieving a compressive yield strength of 610 MPa and a hardness of 249 HV, respectively, while maintaining a good ductility beyond 45%. Microstructural analysis using scanning electron microscope and transmission electron microscope suggests that this outstanding strength-ductility balance of CoCrNiZr₃ MEA arises from the synergistic effect of precipitation strengthening and solid solution strengthening. These findings not only provide deeper insight into the interaction between different strengthening mechanisms but also offer valuable guidance for designing high-performance multi-component alloys through strategic alloying.

In this work, two alloys of Ni₃₅Al₃₀(FeCo)₂₅Cr_10-xMo_x (x = 0, 5) were prepared via the vacuum arc melting method, and the effects of Mo on the microstructure, mechanical properties, and friction and wear properties of the alloys were investigated. The addition of Mo improved the mechanical properties, wear resistance, and corrosion resistance of the alloy system. With the addition of trace amounts of Mo, the precipitate phase of the alloys transformed from spherical to acicular and plate-like. The precipitated phases in a co-lattice relationship with the matrix allow for a substantial increase in the strength of the alloy at both room and elevated temperatures without a significant loss of plasticity. Ni₃₅Al₃₀(FeCo)₂₅Cr₅Mo₅ has excellent mechanical properties, with a hardness of 558.2 HV; a yield strength of 1320 MPa at 600 °C; and a yield strength of 537 MPa at 850 °C. As the temperature increased, the wear mechanism changed from abrasive wear to adhesive wear. At 600 °C, Ni₃₅Al₃₀(FeCo)₂₅Cr₅Mo₅ had the lowest wear rate of 1.78 × 10⁻⁵ (mm³/Nm). The precipitated phases, which have high hardness and maintain a conformal interface with the matrix, play an important role in slowing delamination wear, keeping the wear rate of this alloy low at both room and high temperatures. Electrochemical experiments on the two alloys at room temperature revealed that Ni₃₅Al₃₀(FeCo)₂₅Cr₅Mo₅ exhibited excellent resistance to pitting, with a pitting potential of 0.016 V.

It is essential to develop the optimal hot-working process of the (CoCrNi)₉₄Al₃Ti₃ alloy, a recently developed precipitation-hardened medium-entropy alloy with promising mechanical properties, for its industrial application. In this study, the hot workability of the as-forged (CoCrNi)₉₄Al₃Ti₃ alloy was investigated over a temperature range of 1000 °C to 1150 °C and a strain rate ranging from 0.001 to 1 s⁻¹ using a Gleeble-1500D thermal simulation machine of Dynamic Systems Inc., USA. As a result, the constitutive relationship was established, and the hot deformation activation energy was calculated as 433.2 kJ/mol, suggesting its well-defined plastic flow behavior under low-energy-input conditions. Hot-processing maps were constructed to identify the stable hot-working regions. Microstructure analysis revealed that the hot-forged (CoCrNi)₉₄Al₃Ti₃ alloy exhibited continuous dynamic recrystallization (CDRX) behavior under optimal hot-working conditions. Considering the hot-processing maps and DRX characteristics, the optimal hot-working window of hot-forged (CoCrNi)₉₄Al₃Ti₃ alloy was identified as 1100 °C with a strain rate of 0.1 s⁻¹. This work offers valuable guidance for developing high-efficiency forming processes for (CoCrNi)₉₄Al₃Ti₃ medium-entropy alloy.