Achieving excellent strength-ductility combination in Co_60.8-xNi_39.2Al_x eutectic medium-entropy alloys

In this study, we design and prepare three eutectic medium-entropy alloys (EMEAs) with the composition of Co_60.8-xNi_39.2Al_x (x = 19, 21.6 and 24). All three alloys exhibit a FCC-B2 dual-phase structure, which evolves from hypoeutectic to eutectic and then to hypereutectic with increasing Al content. The Co_39.2Ni_39.2Al_21.6 EMEA demonstrates a complete lamellar eutectic structure with fine and disperse ordered L1₂ precipitations in the FCC phase, while the nano-scale strip L1₀ martensite is present in the B2 phase. The Co_39.2Ni_39.2Al_21.6 EMEA exhibits an exceptional combination of strength and ductility, with high yield strength and elongation that are superior to other FCC-B2 dual-phase EMEA reported in the past five years. The superior ductility was related to the stable K–S interface orientation during the deformation process and the transformation-induced plasticity effect in B2 phases. The high strength is derived from the interface strengthening effect of the FCC-B2 interfaces. Moreover, the hindering of dislocation slipping by the ordered L1₂ precipitates and the Lomer-Cottrell locks in the FCC lamellar can also enhance the strength of the alloy. These findings have important implications for the structural design of eutectic high/medium-entropy alloys.