Microstructure, Mechanical and Tribological Properties of CoCrFeMoNiC_x Medium-Entropy Alloys

High-entropy alloys (HEA) and medium-entropy alloys (MEA) have become one of the research focus in the field of materials science over the past decades. HEAs and MEAs usually consist of multi-principal metal elements with equal or near equal atomic ratio, and they have simple phase structures such as face centered cubic (FCC), body centered cubic (BCC) and hexagonal close packed (HCP) solid solutions. Compared with the HEAs and the conventional alloys, the MEAs exhibited superior properties such as high strength and toughness, excellent corrosion and oxidation resistance, high thermal stability, which make them have good application prospects in the industrial field. However, most of the MEAs usually contain a lot of expensive elements such as Co, Cr, Ni and Mo, which result in high cost and limit their application.　Therefore, in this study, a new type of CoCrFeMoNiC_x MEAs with low cost and good performance has been designed and prepared. The design strategies of the MEAs were as follows: firstly, in order to achieve low-cost, the Fe content of the MEAs was more than 60% and the total content of Co, Cr, Ni and Mo was less than 40%; secondly, for improving the strength and the hardness of the MEAs, C was used for interstitial solution strengthening. The CoCrFeMoNiC_x (x=0, 1, 2, 3, 4 and 5) MEAs were prepared by vacuum induction melting technology, and the effects of carbon content on the microstructures, mechanical and tribological properties of the MEAs were systematically studied.　The CoCrFeMoNiC_x MEAs with different C content mainly consisted of BCC phase. The lattice constant of the MEAs was increased by the interstitial solution of C, which caused the diffraction peaks shift to small angles in XRD spectrum with increase C content. The interstitial solid solution of C induced severe lattice-distortion in BCC phase, which could effectively improve the mechanical properties of the MEAs. In the MEA without C, all the elements were homogeneously distributed in the alloy. By comparison, when the C content was higher than 2%, small amounts of stripe carbides were formed in BCC grains. With the increase of C content, the mechanical properties of the CoCrFeMoNiC_x MEAs were significantly improved by the combination of the interstitial solution strengthening of C and the second phase strengthening of strip carbides. Among the CoCrFeMoNiC_x MEAs, the MEA with 5% C content (C₅ MEA) exhibited good comprehensive mechanical properties. The Rockwell hardness, yield strength, compression strength, fracture strain and fracture toughness of the C₅ MEA were 34.1 HRC, 997 MPa, 2088 MPa, 43.2% and 41.2 MPa·m^0.5, respectively.　The addition of C had little effect on the friction coefficient of the MEAs, which always remained between 0.58 ~ 0.71. However, the addition of C could significantly improve the wear resistance of the MEAs. The wear rates of the CoCrFeMoNiC_x MEAs decreased obviously with the increase of C content. The wear mechanisms of the MEAs included abrasive wear, plastic deformation and fatigue wear at room temperature. The good wear resistance of the MEAs with high C content was mainly attributed to as follows. On the one hand, with the increase of C content, the strength and hardness of the MEAs were improved obviously. It was difficult for the Si₃N₄ counterpart to plough the MEAs with high strength and hardness, and thus the effect of abrasive wear and plastic deformation were reduced. On the other hand, the formation of the tribo-oxide films on the worn surface during sliding could isolate the direct contact between the Si₃N₄ counterpart and the MEAs, which played a key role in improving the wear resistance of the alloys. Therefore, the wear resistance of the alloy was significantly improved with the increase of C content.