Mechanical and tribological performance of CoCrNiHf_x eutectic medium-entropy alloys

The excellent properties of the multi-principal element alloys (e.g., the CoCrNi medium-entropy alloy) make them a perfect candidate for structure materials. Their low strength and poor wear-resistance, however, limit considerably their applications. In this study, a lamellar eutectic microstructure was introduced by addition of Hf into CoCrNi alloy to produce a series of CoCrNiHf_x (x = 0.1, 0.2, 0.3 and 0.4) eutectic medium-entropy alloys. A homogeneous eutectic microstructure with an alternate array of the soft FCC solid-solution phase and the hard Laves phase was identified for the as-cast CoCrNiHf_0.3 alloy. After an investigation of the microstructure, mechanical and tribological properties, it was found that the hardness (plasticity) increases (decreases) with the increasing volume fraction of the Laves phase and the CoCrNiHf_0.3 eutectic alloy exhibits both good plasticity and high strength. The wear behavior is strongly dependent on the applied normal load. For a low normal load, its tribological behavior follows the Archard's equation and a higher hardness due to Hf addition can resist plastic deformation and abrasive wear. When the normal load is high enough, the hypoeutectic or hypereutectic alloy, which possessing either high strength or good ductility but not at the same time, exhibit a poor wear resistance. In comparison, the full eutectic CoCrNiHf_0.3 alloy with a superior combination of strength and toughness shows the best wear performance, as it can significantly reduce fracture during wear.