Abstract: To improve the yield strength of metallic materials at low temperatures, a strategy of combining the calculation of phase diagrams (CALPHAD) technique with the overall valence electron concentration (OVEC) principle is applied, and a Ni₂CoCrNb_0.2 medium-entropy alloy (MEA) with D0₂₂ superlattice (noted as the γ′′ phase) is designed. Bulk MEA samples without defects were successfully fabricated using laser additive manufacturing (AM), followed by solution treatment at 1200 °C for 1 h and then aging at 650 °C for 120 h. The nanoscale γ′′ phase precipitated. The tensile results indicated that the MEA had superior yield strengths of ∼1180 MPa and ∼1320 MPa and tensile strengths of ∼1335 MPa and ∼1552 MPa at 293 K and 77 K, respectively. The yield strength obtained was superior to that of currently reported medium/high-entropy alloys and typical advanced cryogenic steel. The mechanical properties of the Ni₂CoCrNb_0.2 MEA demonstrated a strong temperature dependence, and the increased yield strength was mainly attributed to the increase in lattice friction stress at low temperatures. This research provides a new strategy for producing materials with ultrastrong cryogenic yield strengths by AM.

Key words: Additive manufacturing, Medium-entropy alloys, D0₂₂ superlattice, Temperature dependence, Cryogenic performance