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The selection of high-entropy alloys (HEAs), which are relatively lightweight and have unique mechanical properties, remains a substantial challenge. In this study, six new HEAs were designed from the relatively low-cost Fe–Mn–Ni–Cr–Al–Si system using Thermo-Calc software, and then manufactured using a casting process. The effects of the atomic ratio of the alloying elements on the microstructures and mechanical properties of these alloys in the as-cast condition were systematically investigated. Brittle body-centered cubic BCC/B2 and silicide phases were found in relatively large amounts in the form of dendritic structure within large equiaxed grains with fine needle-shaped phases in the Fe₃₀Mn₁₅Ni₂₀Cr₁₅Al₁₀Si₁₀ and Fe₃₅Mn₁₅Ni₂₀Cr₁₅Al₁₀Si₅ alloys, in addition to the face-centered cubic (FCC) phase. When the contents of Mn and Ni were increased in the Fe₃₅Mn₂₅Ni₁₅Cr₁₅Al₅Si₅ and Fe₃₅Mn₂₀Ni₂₀Cr₁₅Al₅Si₅ alloys, the amounts of brittle phases were reduced; however, the ductile FCC phase is not significant. The FCC phase amount, which appeared as a honeycombed structure, was more than enough when the Si content was decreased to 3%. Broad relationships between the chemical composition of the alloys, especially the Si content, and the hardness and compression properties’ measurements were established. As the Si content decreased, both the hardness and compression properties of the resulting alloy also decreased. The experimental observation of the six HEAs matched the equilibrium phases predicted by the Thermo-Calc calculations.