Effect of carbon alloying and heat treatment on the microstructure, mechanical properties, and tribological behaviour of a powder-metallurgy CoCrCuFeNi high-entropy alloy (HEA)

Abstract

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.