Microstructural evolution mechanisms and cryogenic mechanical properties of induction-melted CoNiCr2 eutectic medium entropy alloy

The tensile mechanical properties of eutectic multi-principal element alloys consisting of solid solution phases and intermetallic compounds are usually unsatisfactory due to their brittleness, let alone the mechanical properties at cryogenic temperatures. In this work, a novel CoNiCr₂ lamellar eutectic medium entropy alloy, mainly composed of FCC-(Co,Ni) and σ-Co₂Cr₃ phases, was prepared by vacuum induction melting. The σ phase originated from the eutectoid transformation of the BCC-(Cr) phase at the low cooling rate of 28 K/min. In contrast, no σ phase was observed in the arc-remelted alloy, since the rapid cooling rate over 1.67 × 10⁴ K/min restrained the eutectoid decomposition. The compressive and tensile mechanical properties at liquid nitrogen temperature (LNT) were systematically investigated. A remarkable compressive yield strength and ductility synergy of 924 MPa and 21.8 % was obtained at LNT compared to room temperature (RT) values of 313 MPa and 37.1 %. The (Co,Ni)-σ interfacial strengthening and enhanced lattice friction stress were responsible for the improved compressive yield strength, while the more brittle σ phase slightly reduced ductility. Moreover, the deformation mechanisms were dominated by Lomer-Cottrell locks at RT, along with stacking faults and nanoscale deformation twins at LNT. This work may provide new insights for designing eutectic high/medium entropy alloys with superior mechanical properties for cryogenic applications.