Structure and low-temperature micromechanical properties of as-cast and SPD-processed high-entropy Co25−xCr25Fe25Ni25Cx alloys

The effect of carbon additions on the structure and mechanical properties of high-entropy alloys Co25−xCr25Fe25Ni25Cx (x = 0, 1, 3, at. %) in two structural states, as-cast coarse-grained (CG) samples and nanocrystalline (NC) obtained by severe plastic deformation (SPD), was studied. The SPD was performed by high-pressure torsion at room temperature. The mechanical properties were investigated by microindentation in the temperature range of T = 77−300 K. It was found that in the as-cast state, all alloys had a dendritic microstructure and an inhomogeneous distribution of elements. At x = 0 and x = 1, the dendrites were enriched in iron and nickel, and the interdendrite regions were enriched in chromium. At x = 3, in the interdendrite regions, a eutectic consisting of a multicomponent matrix and fine eutectic dendrites of M7C3 carbide, where M is predominantly chromium, was formed. The main phase in alloys had an fcc lattice, while the solubility of carbon in it was about 1 at. %. SPD led to the effective refinement of the microstructure (the size of the coherent scattering regions was about 30−50 nm), to an increase in the dislocation density up to (1−1.5)⋅1015 m−2 and to an increase in the concentration of stacking faults. The microhardness of CG alloys at room temperature increased monotonically with increasing carbon concentration, while in NC alloys the maximum microhardness HV was achieved at 1 at. % of carbon. The reason for this anomalous behavior of the microhardness of NC alloys is an increase in the grain size and a decrease in the dislocation density in the alloy with x = 3 compared to the alloy with x = 1. As the temperature decreased from room temperature to the temperature of liquid nitrogen, the microhardness of CG and NC alloys increased by about 1.5−1.7 and 1.2−1.5 times, respectively, which indicates the thermally-activated nature of plastic deformation under the indenter. The results obtained indicate that the main role in the hardening of the CG alloys Co25−xCr25Fe25Ni25Cx is due to solid solution and dispersion hardening, while in NC alloys it is hardening due to a decrease in the grain size (according to the Hall-Petch relation) and an increase in the dislocation density (according to the Taylor relation).