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CoNi-based superalloys offer excellent high-temperature properties; yet, Co is also a strategic alloying element, and its content should only be as high as necessary. This study investigates Fe as a partial substitute for Co to reduce costs while evaluating its impact on mechanical properties. To evaluate this, we systematically investigate the effect of Fe substitutions on thermophysical properties, microstructure, partitioning behavior, lattice misfit, yield strength, and creep performance of three polycrystalline CoNi-based superalloys derived from CoWAlloy1 (Co–32Ni–12Cr–6Al–3W–2.5Ti–1.5Ta–0.4Si–0.1Hf–0.08B all in at. %). In these alloys, 4, 8, and 12 at. % Co is replaced with Fe. Increasing Fe content results in a gradual reduction in the solvus, solidus, and liquidus temperatures by 3.0, 1.9, and 1.4 °C per at. % Fe, respectively. The γ′ volume fraction and the lattice misfit decrease by about 0.7% and 0.01%, respectively, per at. % Fe substitution for Co. Fe predominantly partitions to the γ matrix, enhancing the partitioning of Co and Ni while reducing that of Al, Cr, and Ta, with no significant effect on Ti and W. Substituting Co with Fe moderately reduces yield and creep strength, primarily due to the decreasing γ′ volume fraction and a transition in the dominant deformation mechanisms from stacking fault shearing and microtwinning to matrix-based deformation as Fe content increases. Beneficial elemental segregation behaviors and localized phase transformations along creep-induced stacking faults remain active in alloys with high Fe content. These findings highlight the potential of Fe alloying to reduce costs while maintaining high-temperature strength in CoNi-based superalloys.