Gradient recrystallization to improve strength and ductility of medium-entropy alloy

Highlights: • A new strategy was proposed to process MEAs with high strength and good ductility. • The excellent properties are attributed to the gradient recrystallization. • A new model considering the effect of gradient recrystallization was proposed. • The experimental data validate the rationality of the presented model. -- Abstract: Gradient recrystallized CrCoNi medium-entropy alloy (MEA) was processed by vacuum arc melting, pure torsion and subsequent annealing. MEAs with different gradient recrystallizations (GRs) were produced by controlling the annealing time and the cooling type. Uniaxial tensile tests show that the gradient recrystallized MEA annealed at 1073 K for 2.5 min exhibits high yield strength and good ductility. Compared with the homogeneous coarse-grain MEA, the yield strength in the gradient recrystallized MEA is increased from 245 MPa to 930 MPa while a considerable elongation of ∼27% is obtained. Even for higher-strength gradient recrystallized MEA annealed at 1073 K for 2 min, the yield strength is above 1 GPa and the elongation still maintains ∼17%. Microstructural characterizations and theoretical analysis were employed to explain the mechanism for the excellent mechanical properties in the gradient recrystallized MEA. The forest dislocations and nanotwins resulting from pure torsion strengthen the gradient recrystallized MEA. The gradient recrystallization and nanotwins resulting from heat treatment provide extra strain hardening capability, which contribute to the improvement of ductility.