Strengthening and toughening bulk Ni_(2)CoFeV_(0.5) medium-entropy alloy via thermo-mechanical treatment

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 Ni_(2)CoFeV_(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∼4μm)and nonrecrystallized 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℃ 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 heterodeformation-induced strengthening produced by the HL structure.Hall-Petch relationship of Ni_(2)CoFeV_(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 MPaμm1/2.Our work provides optional and controllable solutions for preparing MEAs/HEAs with excellent mechanical properties by low-cost and high-efficiency thermomechanical treatments.