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In this study,a new Al0.9CoFeNi2 eutectic high entropy alloy(EHEA)was designed,and the microstruc-tures as well as the deformation behavior were investigated.The bulk cast Al0.9CoFeNi2 EHEA exhibited an order face-centered cubic FCC(L12)and an order body-centered cubic(B2)dual-phase lamellar eutec-tic microstructure.The volume fractions of FCC(L12)and B2 phases are measured to be 60% and 40%,respectively.The combination of the soft and ductile FCC(L12)phase together with the hard B2 phase resulted in superior strength of 1005 MPa and ductility as high as 6.2% in tension at room temperature.The Al0.9CoFeNi2 EHEA exhibited obvious three-stage work hardening characteristics and high work-hardening ability.The evolving dislocation substructures during uniaxial tensile deformation found that planar slip dominates in both FCC(L12)and B2 phases,and the FCC(L12)phase is easier to deform than the B2 phase.The post-deformation transmission electron microscopy revealed that the sub-structural evolution of the FCC(L12)phase is from planar dislocations to bending dislocations,high-density dislo-cations,dislocation network,and then to dislocation walls,and Taylor lattices,while the sub-structural evolution of the B2 phase is from a very small number of short dislocations to a number of planar dis-locations.Moreover,obvious ductile fracture in the FCC(L12)phase and a brittle-like fracture in the B2 phase were observed on the fracture surface of the Al0.9CoFeNi2 EHEA.The research results provide some insight into the microstructure-property relationship.