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 degrees C, cold rolled with a reduction of -50 %, and then annealed at 800, 850, 900, 950, 1000, and 1100 degrees 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 degrees 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 degrees 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 degrees C annealed HEA achieved superior strength -ductility balance (TS x EL) of -90.3 GPa%, while the strength -ductility balance (TS x EL) of 800 degrees C annealed HEA was only -50 GPa%. The 1100 degrees C annealed HEA possesses excellent comprehensive mechanical properties, which is superior to a majority of reported HEAs and conventional metal alloys.