Investigating the micro and nanomechanical properties of CoCrFeNi-C high-entropy alloys containing eutectic carbides

In this study, we systematically investigated the effect of the carbon content on the phase structure evolution and macro and nanomechanical properties of the equiatomic CoCrFeNi alloy. A series of CoCrFeNi-based high-entropy alloys with 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, and 0.9 wt% carbon were fabricated by vacuum arc melting. The results showed that the alloys retained their single face-center-cubic (FCC) structure with large grains at the carbon contents up to 0.2 wt%. However, in the case of the alloys with carbon contents higher than 0.3 wt%, the single phase structure transformed into the dendrite structure. The inter-dendrite regions of these alloys were eutectic carbides containing lath M7C3 carbides and the FCC phase. The tensile yield strength of the CoCrFeNi alloy increased from 295 to 512 MPa with the addition of 0.9 wt% carbon. The strengthening mechanisms of the alloys were investigated, including the solid solution strengthening due to the interstitial carbon atoms and the precipitation strengthening due to the M7C3 carbides. In addition, the nanoindentation results revealed that the M7C3 carbides showed a high nanohardness of ~17.7 GPa, which is almost four times of that of the FCC phase. We also investigated the elasto-plastic relationships, plastic properties and wear resistance of the FCC phase, while elucidating the eutectic carbide regions and M7C3 carbides. The eutectic carbide regions exhibited brittle fracture, whereas the FCC phase showed plastic fracture.