Achieving superior strength-ductility balance of Co–Cr–Fe–Ni high entropy alloy via annealing

A novel (Co40Cr25(FeNi)31Mo4)99·5C0.5 HEA (high entropy alloy) was prepared by vacuum arc melting furnace and hot rolled with a reduction of 20 % at 1100 °C, cold rolled with a reduction of ∼50 %, and then annealed at 800, 850, 900, 950, 1000, and 1100 °C for 6 min, respectively. The effect of annealed on the microstructure evolution and mechanical properties of HEAs were systematically analyzed. The results showed that the HEAs still maintains single FCC(Face-centered cubic) crystal structure after different annealing temperatures. With increasing annealed temperature, the area of abnormal grain gradually decreases and forming equiaxed grain finally. Annealing at lower temperature (800 °C) gives rise to the formation of nanoscale and submicron M23C6 carbide, which hindered the recrystallization of grains and resulting to the higher tensile strength of ∼1352 MPa and lower elongation of ∼37 % when comparing with other annealed HEAs. However, the amount of M23C6 precipitates decreases when the annealing temperature was increased to 1100 °C. At this time, the alloy structure was consisting of recrystallized grains and a large number of annealing twins, and the tensile strength decreased to ∼1062 MPa and the elongation increased to ∼85 %. Due to annealing twins effect and grain refinement, the 1100 °C annealed HEA achieved superior strength-ductility balance (TS × EL) of ∼90.3 GPa%, while the strength-ductility balance (TS × EL) of 800 °C annealed HEA was only ∼50 GPa%. The 1100 °C annealed HEA possesses excellent comprehensive mechanical properties, which is superior to a majority of reported HEAs and conventional metal alloys.