Effect of Co Addition on Microstructure and Mechanical Properties of Ni_50-xCr₂₀Fe₂₀Al₁₀Co_x Medium Entropy Alloys

Ni_50-xCr₂₀Fe₂₀Al₁₀Co_x（x=0, 5, 10, at%） alloy ingots were prepared by vacuum arc melting in high purity argon atmosphere by changing the content of Ni and Co, the corresponding alloys were denoted as Co0, Co5 and Co10. The effect of Co on the stacking fault energy of alloy was analyzed by thermodynamic calculation and molecular dynamics simulation, the morphology of precipitates in the alloys was analyzed by XRD, SEM and TEM, and the tensile properties of the alloys were tested. The results show that the increase of Co content and the decrease of Ni/Co ratio significantly reduce the stacking fault energy of the alloys, therefore it can provide more strengthening sources during the plastic deformation.The aging heat treatment induces the precipitation of a small amount of BCC-phase precipitates and high volume fraction of L1₂-phase nanoparticles in the alloys. The alloys exhibit excellent comprehensive mechanical properties at room temperature. The Co5 alloy in aging state shows the best mechanical properties, with a yield strength of about 685 MPa, a tensile strength of about1326 MPa and an elongation after fracture of about 24%. The Co5 alloy shows an excellent work hardening ability that maintains a steady-state strain hardening rate of 3400 MPa in the true strain range of 1%-9%.