Tailoring microstructure of Ni2CoFe medium-entropy alloy to achieve optimal comprehensive mechanical properties

Abstract

Single-phase face-centered cubic (fcc) medium- and high-entropy alloys (MEAs/HEAs) have high ductility but low yield strength and hardness. In this work, the microstructures of single-phase fcc Ni2CoFe MEAs were tuned via thermo-mechanical treatment (cold-rolling plus annealing). Microhardness and tensile tests revealed that the as-rolled Ni2CoFe MEA had a high hardness of 331.2 HV and a high yield strength of 913 MPa but a poor fracture elongation of 7.9%, resulting from high-density lattice defects. With increasing annealing temperature, the hardness and strength gradually decreased and meanwhile the ductility gradually improved, due to the microstructural recovery, recrystallization and grain growth. The heterostructured sample annealed at 530 °C for 1 h exhibited simultaneous improvement in ultimate tensile strength and uniform elongation compared with those of as-rolled sample. For the heterostructured sample annealed at 550 °C for 1 h, a good fracture elongation of 13.6% and a decent ultimate tensile strength of 867 MPa were achieved. Extra hardening and strengthening were produced, benefiting from the construction of heterostructure. Moreover, the Ni2CoFe MEA exhibited more superior combination of strength and ductility compared to pure Ni. The severe lattice distortion existing in the Ni2CoFe MEA enhances the strain hardening and ductility by enhancing the dislocation accumulation capability.