Effect of carbon on recrystallised microstructures and properties of CoCrFeMnNi-type high-entropy alloys

Abstract

The effect of carbon content (x = 0, 0.5, and 2.0 at.%) on the phase composition, microstructure, and mechanical properties of thermomechanically processed CoCrFeMnNi system high-entropy alloys was studied. The molar fraction of Cr in the program Co1Cr0.25Fe1Mn1Ni1 alloy was reduced to 0.25 compared with the equiatomic alloy to increase the solubility of carbon in the face-centred cubic solid solution. The as-cast alloys were cold rolled to 80% thickness reduction and then annealed at 600–1000 °C for 1 h. The addition of carbon to the CoCrFeMnNi alloys resulted in an increase in the temperature at which recrystallisation starts and in the precipitation of the Cr-rich M23C6 carbide particles. The volume fraction of the second phase increased with an increase in the carbon content and decreased with increasing annealing temperature. The formation of precipitates in the carbon-doped alloys strongly limited the matrix grain growth owing to the pinning effect. The thermomechanical processing of the alloys resulted in a reasonable work-hardening capacity and, therefore, in rather high ultimate tensile strength and ductility. Increasing the carbon content led to a pronounced increase in strength and a weak decrease in the ductility of the CoCrFeMnNi alloys. After annealing at 800 °C, the yield strength of the alloys increased from 313 MPa to 636 MPa, whereas the total elongation decreased from 56% to 43% with an increase in the carbon percentage from 0 at.% to 2.0 at.%. Decreasing the testing temperature to 77 K led to a simultaneous increase in both strength and ductility of the alloys. The alloy with 2.0 at.% of carbon demonstrated an attractive combination of mechanical properties at cryogenic temperature, presenting a yield and ultimate tensile strength of 786 MPa and 1218 MPa, respectively, along with a ductility of 52%. The effect of the carbon content on the microstructure development and strengthening mechanisms was discussed.