Effects of annealing temperature on microstructures and tensile properties of a single FCC phase CoCuMnNi high-entropy alloy

Copper (Cu) acts as a strong face-centered cubic (FCC) stabilizer as well as an effective element lowering the melting point in FCC-structured high-entropy alloy (HEA) systems. The addition of Cu can reduce the alloy processing temperature; however, there are no studies on the formation of single FCC phase in Cu-added HEAs yet. In this study, a Co₁₀Cu₂₀Mn₃₀Ni₄₀ (at.%) HEA was designed by a thermodynamic calculation using a CALPHAD approach by a software Thermo-Calc 3.0, and effects of annealing temperature on microstructures and room- and cryogenic-temperature tensile properties were investigated. The calculation data showed a very wide region of single FCC phase (532–1073 °C), which was confirmed from the existence of stable single FCC phase at the annealing temperature of 600 °C. The specimen annealed at 600 °C presented the fully-recrystallized FCC phase and the refined grain size of 2.8 μm. As the annealing temperature decreased, thus, the strengths increased while the elongation decreased. In addition, both strength and elongation were improved significantly with decreasing the tensile test temperatures from 298 to 77 K, and the major deformation mechanism was changed from a dislocation slip to a deformation twinning, as confirmed from the specimen annealed at 700 and 900 °C. The present study on grain refinement resulting from the low-temperature annealing would suggest a good method for improving cryogenic mechanical properties of single–FCC–based HEAs.