A precipitate-free AlCoFeNi eutectic high-entropy alloy with strong strain hardening.

[Display omitted] • Developing a bulk cast Al 19.25 Co 18.86 Fe 18.36 Ni 43.53 EHEA with lamellar eutectics. • The developed as-cast EHEA featuring an extraordinary strain-hardening capability. • Uncovering the deformation and fracture mechanisms at multiple length scales. • A few slip lines and lattice dislocations appearing in the hard B2 lamellae. • But dynamic microstructure refinement is identified in the ductile L1 2 lamellae. Over recent years, eutectic high-entropy alloys (EHEAs) have intrigued substantial research enthusiasms due to their good castability as well as balanced strength–ductility synergy. In this study, a bulk cast Al 19.25 Co 18.86 Fe 18.36 Ni 43.53 EHEA is developed with fine in-situ lamellar eutectics. The eutectics comprise alternating ordered face-centered-cubic (L1 2) and ordered body-centered-cubic (B2) phases with semi-coherent interfaces. The resulting microstructure resembles that of most reported as-cast EHEAs, but the B2 lamellae are devoid of nano-precipitates because of the Cr-element removal in current tailored eutectic composition. Surprisingly, the B2 lamellae still feature much higher deformation resistance than the L1 2 lamellae, so that less lattice defects are detected in the B2 lamellae until the fracture. More interestingly, in the L1 2 lamellae we identify a dynamic microstructure refinement that correlates to extraordinary strain hardening in tension. The precipitate-free EHEA consequently shows excellent tensile ductility of ∼10 % and high ultimate strength up to ∼956 MPa. [ABSTRACT FROM AUTHOR]