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

• Published in: Low temperature physics (Woodbury, N.Y.) N.Y.), 2022-07, Vol.48 (7)
• Main Authors: Levenets, A V, Rusakova, H V, Fomenko, L S, Kolodiy, I V, Vasilenko, R L, Tabachnikova, E D, Tikhonovsky, M A, Langdon, T G
• Format: Article
• Language: English
• Subjects: Alloys; Carbon; Chromium; Coherent scattering; Dislocation density; Dispersion hardening alloys; Grain size; High entropy alloys; Liquid nitrogen; Low temperature; Mechanical properties; Microhardness; Microstructure; Plastic deformation; Room temperature; Solid solutions; Stacking faults
• ISSN: 1063-777X; 1090-6517

The effect of carbon additions on the structure and mechanical properties of high-entropyalloys Co25−xCr25Fe25Ni25Cx (x = 0, 1, 3, at. %) in twostructural states, as-cast coarse-grained (CG) samples and nanocrystalline (NC) obtainedby severe plastic deformation (SPD), was studied. The SPD was performed by high-pressuretorsion at room temperature. The mechanical properties were investigated bymicroindentation 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 aninhomogeneous distribution of elements. At x = 0 and x = 1, the dendrites were enriched in iron and nickel, and theinterdendrite regions were enriched in chromium. At x = 3,in the interdendrite regions, a eutectic consisting of a multicomponent matrix and fineeutectic dendrites of M7C3 carbide, where M is predominantlychromium, was formed. The main phase in alloys had an fcc lattice, while the solubility ofcarbon 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 thedislocation density up to (1−1.5)⋅1015 m−2 and to an increase in theconcentration of stacking faults. The microhardness of CG alloys at room temperatureincreased monotonically with increasing carbon concentration, while in NC alloys themaximum microhardness HV was achieved at 1 at. %of carbon. The reason for this anomalous behavior of the microhardness of NC alloys is anincrease in the grain size and a decrease in the dislocation density in the alloy withx = 3 compared to the alloy with x = 1. As the temperature decreased from room temperature to the temperatureof liquid nitrogen, the microhardness of CG and NC alloys increased by about 1.5−1.7 and1.2−1.5 times, respectively, which indicates the thermally-activated nature of plasticdeformation under the indenter. The results obtained indicate that the main role in thehardening of the CG alloys Co25−xCr25Fe25Ni25Cx is due to solid solution and dispersion hardening, while in NCalloys it is hardening due to a decrease in the grain size (according to the Hall-Petchrelation) and an increase in the dislocation density (according to the Taylorrelation).