Interstitial doping enhances the strength-ductility synergy in a CoCrNi medium entropy alloy

An equiatomic CoCrNi medium entropy alloy (MEA) with face-centered cubic (FCC) structure exhibits excellent combination of strength and ductility. Here we employ interstitial doping to enhance its mechanical performance. Interstitial CoCrNi MEAs with two different carbon contents, i.e., 0.5 at. % and 1 at. %, as well as a carbon-free CoCrNi reference MEA have been studied. The results show that up to 1 at. % carbon can be fully dissolved into the homogenized plus water-quenched FCC solid solution structure. Subsequent annealing leads to precipitation of nano-sized M23C6 type carbides which provide dispersion strengthening and enhanced strain hardening. The best combination of ultimate tensile strength of 1180 MPa at an elongation above 60% was obtained in fine grained CoCrNi doped with 0.5 at. % of carbon. Carbon alloying is also shown to significantly increase the lattice friction stress. Dislocation glide and mechanical twinning act as main deformation mechanisms. Thus, the joint contribution of multiple deformation mechanisms in the carbon-doped MEAs leads to significantly enhanced strength-ductility combinations compared to the carbon-free reference alloy, demonstrating that interstitial alloying can enhance the mechanical properties of MEAs.