The development of microstructure and mechanical properties was investigated in a heavily cold-rolled and annealed AlCoCrFeNi_2.1 high-entropy alloy. The as-cast alloy having a eutectic morphology consisting of alternate bands of ordered L1₂ and B2 phases was 90% cold-rolled. The deformed microstructure showed profuse shear banding and disordering of the L1₂, but no transformation of the B2 phase. A duplex microstructure consisting of ultrafine equiaxed grains (∼0.60 µm) of disordered face centered cubic and B2 was observed after annealing at 800°C. The annealed material showed remarkable strength–ductility combination having ultimate tensile strength ∼1.2 GPa and elongation to failure ∼12%. Impact Statement: AlCoCrFeNi_2.1 eutectic HEA with ordered L1₂ and B2 phases can be thermo-mechanically processed to achieve ultrafine structure and remarkable strength-ductility properties. GRAPHICAL ABSTRACT

Nano-lamellar (L1₂ + B2) AlCoCrFeNi_2.1 eutectic high entropy alloy (EHEA) was processed by cryo-rolling and annealing. The EHEA developed a novel hierarchical microstructure featured by fine lamellar regions consisting of FCC lamellae filled with ultrafine FCC grains (average size ~200–250 nm) and B2 lamellae, and coarse non-lamellar regions consisting of ultrafine FCC (average size ~200–250 nm), few coarse recrystallized FCC grains and rather coarse unrecrystallized B2 phase (~2.5 µm). This complex and hierarchical microstructure originated from differences in strain-partitioning amongst the constituent phases, affecting the driving force for recrystallization. The hierarchical microstructure of the cryo-rolled and annealed material resulted in simultaneous enhancement in strength (Yield Strength/YS: 1437 ± 26 MPa, Ultimate Tensile Strength/UTS: 1562 ± 33 MPa) and ductility (elongation to failure/e_f ~ 14 ± 1%) as compared to the as-cast as well as cold-rolled and annealed materials. The present study for the first time demonstrated that cryo-deformation and annealing could be a novel microstructural design strategy for overcoming strength-ductility trade off in multiphase high entropy alloys.