Abstract: The microstructure of drop-cast Co_31.5Fe_18.5Ni_31.5Al_18.5 eutectic high-entropy alloy (EHEA) consists of two-phase eutectic dendrites and eutectic cells. Within the eutectic dendrites/cells, the two-phase eutectic primarily displays a lamellar structure consisting of NiAl phase (BCC) and CoFeNi phase (FCC). The drop-cast Co_31.5Fe_18.5Ni_31.5Al_18.5 EHEA exhibits a moderate yield strength of 599 ± 5 MPa with an acceptable ductility of 7.8% ± 0.4%. Thermo-mechanical processing, specifically cold rolling and annealing (CRA), is employed to enhance the mechanical properties of the drop-cast EHEA. Following CRA treatment, partial recrystallization occurs within both the BCC and FCC phases. Notably, the FCC phase exhibits a higher degree of recrystallization compared to the BCC phase. Thus, the CRA EHEA is regarded as a dual heterostructured material, achieving a high yield strength of ∼1231±8 MPa while retaining acceptable ductility (7.8% ± 0.3%). Subsequent analysis of tensile deformation behavior, including fracture surface morphology, side-surface observations and deformation substructure, reveals pronounced plastic deformation in the FCC phase, while the BCC phase exhibits rare deformation. Subsequently, the strengthening mechanisms are systematically analyzed from two distinct perspectives. Firstly, the high strength of CRA EHEA is mainly attributed to dislocation strengthening, precipitation strengthening, grain-boundary strengthening and interface strengthening. In particular, the investigation of FCC individual phase property is novel and meaningful, providing critical insights into the understanding of the strengthening mechanism. From another perspective, the high strength of the CRA EHEA can be attributed to hetero-deformation-induced (HDI) stress strengthening. In conclusion, this paper will provide the implications for microstructural optimization and mechanical property improvement of EHEAs.

Key words: Eutectic high-entropy alloy, Thermo-mechanical processing, Partial recrystallization, Tensile behavior, Strengthening mechanism