Effect of carbon content and annealing on structure and hardness of the CoCrFeNiMn-based high entropy alloys

A set of CoCrFeNiMn-based high entropy alloys, containing 0, 2.0, 3.4 and 4.8 at.% of carbon (CoCrFeNiMnCx (x = 0; 0.1; 0.175; 0.25) alloys), was prepared by vacuum arc melting. The structure of the alloys was examined in as-solidified conditions and after annealing at 600 °C, 800 °C, 1000 °C and 1200 °C for 14 h. In the as-solidified condition, the CoCrFeNiMn alloy is composed of single fcc (face-centered cubic) solid solution phase, the CoCrFeNiMnC0.1 alloy contains an insignificant fraction of nanoscale M7C3 carbides, and in the CoCrFeNiMnC0.175 and CoCrFeNiMnC0.25 alloys, coarse M7C3 carbides with the volume fraction of 5% and 9.5% respectively are found. The microhardness increases with the increase of carbon content in the alloys from 160 HV of the CoCrFeNiMn alloy to 275 HV of the CoCrFeNiMnC0.25. After annealing at 800 °C nanoscale M7C3 carbides precipitate in the CoCrFeNiMnC0.1, CoCrFeNiMnC0.175 and CoCrFeNiMnC0.25 alloys. Precipitation of carbides is accompanied by a strong hardening of the alloys. For example, the microhardness of the CoCrFeNiMnC0.1 alloy increases from 205 HV in as-solidified condition to 240 HV. Further increase of annealing temperature results in a decrease of hardness. Carbon concentration in the fcc solid solution is estimated and compared with equilibrium thermodynamic predictions. Thermodynamic modeling also demonstrates that increase of the number of components in fcc equiatomic alloys of the Co-Cr-Fe-Ni-Mn system decreases possible carbon solubility level in the fcc solid solution phase, and in the studied alloys Cr is the main element which determines carbon solubility. Hardening mechanisms of the carbon-containing alloys are analyzed quantitatively, and prospects of development of age-hardenable high entropy alloys of Co-Cr-Fe-Ni-Mn-C system are discussed.