Abstract: Single-phase face-centered cubic (fcc) medium- and high-entropy alloys (MEAs/HEAs) have high ductility but low yield strength. In this work, the microstructures of single-phase fcc Ni₂CoFeV_0.5 MEAs were tailored by cold-rolling and subsequent annealing and typical heterogeneous lamella (HL) structures composed of recrystallized micro-grain lamellae (with an averaged grain size of ∼4 μm) and non-recrystallized nano-/ultrafine-grain lamellae were obtained. Tensile tests revealed that most HL samples exhibited excellent strength and ductility synergy. The HL sample with ∼23 vol% recrystallized grains annealed at 590 °C for 1 h had a high yield strength of 1120 MPa and a good fracture elongation of 12.3%, which increased by 5% and 46%, respectively compared with those of as-rolled sample. Annealing-induced yield strength increase is attributed to high-density annealing twin boundaries (TBs) in the recrystallized grains, the annihilation of mobile dislocations inside the non-recrystallized grains, and extra hetero-deformation-induced strengthening produced by the HL structure. Hall-Petch relationship of Ni₂CoFeV_0.5 MEA can be reasonably described by counting both TBs and grain boundaries, with lattice friction stress of 87.3 MPa and coefficient of 722.8 MPa μm^1/2. Our work provides optional and controllable solutions for preparing MEAs/HEAs with excellent mechanical properties by low-cost and high-efficiency thermo-mechanical treatments.

Key words: Medium-entropy alloys, Microstructures, Strength and ductility, Heterogeneous lamella structure, Cold-rolling, Annealing