This paper focuses on the microstructure, phase composition, mechanical, tribological and corrosion properties of high-entropy alloys (HEAs) in the CoCrCuFeNi system depending on copper content, which was varied from 0 to 20 at. % with an increment of 5%. CoCrCuFeNi alloys were manufactured by powder metallurgy methods: mechanical alloying and hot pressing of element mixtures. The solubility limit of copper in CoCrFeNi solid solution was found to be 9 at. %. Segregation of irregularly shaped copper grains sized 1–30 μm is observed at concentrations above this solubility limit. As copper concentration increases, the phase composition of CoCrCuFeNi alloys changes from the single phase based on FCC1 solid solution (Cu = 0–5 at. %) to the dual-phase FCC1 + FCC2 alloy (Cu = 10–20 at. %), where FCC1 is the main phase and FCC2 is the secondary copper-rich phase. Tribological tests have shown that doping the CoCrFeNi alloy with copper increased wear resistance by 23% due to solid solution hardening. As copper content rises above 20%, the content of the secondary FCC2 phase increases, while wear resistance and alloy hardness decline. An analysis of wear tracks and wear products has shown that abrasion of CoCrCuFeNi alloys occurs via the abrasive-oxidative wear mechanism. The corrosion tests of CoCrCuFeNi HEAs in 3.5% NaCl solution had demonstrated that doping the alloy with copper at low concentrations (5–10%) leads to decreasing of corrosion resistance, possibly due to the formation of undesirable oxide Cu₂O along with protective Cr₂O₃. At high copper concentrations (15–20%) galvanic corrosion is suppressed due to coarsening of FCC2 grains and thus decreasing the specific contact surface area between the cathode (FCC2) and the anode (FCC1).

This paper focuses on the study of the structure and mechanical properties of CoCrCu_xFeNi high-entropy alloys and their adhesion to single diamond crystals. CoCrCu_xFeNi alloys were manufactured by the powder metallurgy route, specifically via mechanical alloying of elemental powders, followed by hot pressing. The addition of copper led to the formation of a dual-phase FCC + FCC2 structure. The CoCrCu_0.5FeNi alloy exhibited the highest ultimate tensile strength (1080 MPa). Reductions in the ductility of the CoCrCu_xFeNi HEAs and the tendency for brittle fracture behavior were observed at high copper concentrations. The equiatomic alloys CoCrFeNi and CoCrCuFeNi demonstrated high adhesion strength to single diamond crystals. The diamond surface at the fracture of the composites having the CoCrFeNi matrix had chromium-rich metal matrix regions, thus indicating that chromium carbide, responsible for adhesion, was formed at the composite–diamond interface. Copper-rich areas were detected on the diamond surface within the composites having the CoCrCuFeNi matrix due to the predominant precipitation of the FCC2 phase at the interfaces or the crack propagation along the FCC/FCC2 interface, resulting in the exposure of the Cu-rich FCC2 phase on the surface.

This research explores the potential to enhance the copper solubility limit in high-entropy alloys (HEAs) within the CoCrCuFeNi system by increasing the nickel content twofold and applying additional heat treatment. The CoCrCu_xFeNi₂ HEAs were synthesized through mechanical alloying of elemental powders followed by hot pressing. The study investigated the microstructure and phase composition of CoCrCu_xFeNi₂ HEAs in relation to varying copper concentrations (x = 0; 0.25; 0.5; 0.75; 1.0). The evaluation of the alloy matrix's chemical composition, which is based on the FCC solid solution, enabled the determination of copper solubility. It was found that doubling the nickel content, relative to the equiatomic ratio, facilitated the formation of HEAs with a homogenous FCC structure for copper concentrations up to x ≤ 0.75. Further heat treatment of these HEAs resulted in an enhanced copper solubility of up to 17.5 at.%. The mechanical and tribological properties of CoCrCuxFeNi_y HEAs were also assessed, revealing significant improvements in tensile strength (ranging from 910 to 1045 MPa) and hardness (285–395 HV) for the CoCrCu_xFeNi₂ alloys. Despite the increased copper solubility limit, the heat treatment process caused a decline in mechanical properties by 35–50 %, attributed to grain size enlargement to 5.5 μm. The CoCrCu_0.75FeNi₂ and CoCrCuFeNi₂ alloys exhibited the lowest wear rates when tested against Al₂O₃ counterbody, with wear rates of 1,58·10^–5 and 1,48·10^–5 mm³/(N·m), respectively.

Introduction. Powder metallurgy high-entropy alloys (HEAs) of the CoCrCuFeNi system exhibit high strength but low ductility due to their ultrafine-grained structure and inhomogeneous distribution of the copper-rich phase. One approach to improve the balance of mechanical properties involves the microaddition of non-metallic elements, particularly carbon, which can enhance both strength and ductility. The purpose of this study is to determine the effect of carbon content (0.2–1.0 wt.%) and heat treatment on the phase composition, microstructure, mechanical properties, and tribological behavior of powder metallurgy CoCrCuFeNi HEAs fabricated by mechanical alloying and hot pressing. Materials and methods. The investigation employed X ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), hardness testing, bending testing, tensile testing, and pin on plate tribological testing (with Si3N4 as the counterbody). Results and discussion. It is established that carbon is bound into Cr23C6 and Cr2(C,N) phases without altering the ratio of the major phases. Hardness increase from 3.27 to 3.56 GPa, and bending strength increased from 1,440 to 1,620 MPa. For the alloy containing 1.0 wt.% C after annealing, a bending strength of 1,780 MPa, a tensile strength of 1,010 MPa, and an elongation of 2.3% were achieved, whereas the base alloy exhibited brittle fracture at 730 MPa. TEM analysis revealed that carbides are predominantly located at grain boundaries of the matrix (grain size 620–840 nm), facilitating a transition from intergranular cleavage to ductile dimple fracture. The friction coefficient (0.75–0.77) is weakly dependent on composition, while the lowest wear is attained after annealing of the carbon alloyed composition. It is found that alloying with carbon (1.0 wt.%) followed by heat treatment enhances both the strength and ductility of powder metallurgy CoCrCuFeNi HEAs through carbide strengthening and optimization of grain boundary structure.