Microstructure, mechanical and frictional properties of CrCoNi-xNb alloys prepared through powder metallurgy

Herein, CoCrNi-xNb (x = 0, 1, 3, 5, 7 and 9 wt%, referred to as 0Nb, 1Nb, 3Nb, 5Nb, 7Nb and 9Nb, respectively) alloys were prepared through mechanical alloying and spark plasma sintering techniques. The impact of Nb on the microstructure, mechanical and frictional properties of CoCrNi medium-entropy alloys were investigated. The evolution of the Laves phase (HCP) structure in alloy systems was well predicted using the $\overline{Md}$ (the average energy level of d-orbitals) criterion. The results show that the structure changed from FCC single-phase (x = 0) to FCC + Laves biphase (x > 0). The alloy's hardness, YS and UTS increase as increase of Nb content increases the volume fraction of the Laves phase and reduces the FCC phase. The increase in hardness is beneficial for improving the wear resistance of the material. The wear mechanism of the alloy changes from abrasive to adhesive wear and then to oxidative wear, and it considerably improves the wear resistance of discs and pins. Thus, CoCrNi-xNb/GCr15 steel pairs are expected to achieve excellent tribological properties under high-temperature conditions. Among them, the mechanical and tribological properties of the CoCrNi-5Nb alloy are excellent. These results show that combining plasticity matrix and intermetallic compounds through powder metallurgy is a viable approach for designing high-strength wear-resistant alloys.