| Summary: | This work investigated the effect of Fe/Mn ratio on the microstructure and mechanical properties of non-equimolar Fe<sub>80−<i>x</i></sub>Mn<i><sub>x</sub></i>Co<sub>10</sub>Cr<sub>10</sub> (<i>x</i> = 30% and 50%) high-entropy alloys (HEAs) fabricated by laser powder bed fusion (LPBF) additive manufacturing. Process optimization was conducted to achieve fully dense Fe<sub>30</sub>Mn<sub>50</sub>Co<sub>10</sub>Cr<sub>10</sub> and Fe<sub>50</sub>Mn<sub>30</sub>Co<sub>10</sub>Cr<sub>10</sub> HEAs using a volumetric energy density of 105.82 J·mm<sup>−3</sup>. The LPBF-printed Fe<sub>30</sub>Mn<sub>50</sub>Co<sub>10</sub>Cr<sub>10</sub> HEA exhibited a single face-centered cubic (FCC) phase, while the Fe<sub>50</sub>Mn<sub>30</sub>Co<sub>10</sub>Cr<sub>10</sub> HEA featured a hexagonal close-packed (HCP) phase within the FCC matrix. Notably, the fraction of HCP phase in the Fe<sub>50</sub>Mn<sub>30</sub>Co<sub>10</sub>Cr<sub>10</sub> HEAs increased from 0.94 to 28.10%, with the deformation strain ranging from 0 to 20%. The single-phase Fe<sub>30</sub>Mn<sub>50</sub>Co<sub>10</sub>Cr<sub>10</sub> HEA demonstrated a remarkable combination of high yield strength (580.65 MPa) and elongation (32.5%), which surpassed those achieved in the FeMnCoCr HEA system. Comparatively, the dual-phase Fe<sub>50</sub>Mn<sub>30</sub>Co<sub>10</sub>Cr<sub>10</sub> HEA exhibited inferior yield strength (487.60 MPa) and elongation (22.3%). However, it displayed superior ultimate tensile strength (744.90 MPa) compared to that in the Fe<sub>30</sub>Mn<sub>50</sub>Co<sub>10</sub>Cr<sub>10</sub> HEA (687.70 MPa). The presence of FCC/HCP interfaces obtained in the Fe<sub>50</sub>Mn<sub>30</sub>Co<sub>10</sub>Cr<sub>10</sub> HEA resulted in stress concentration and crack expansion, thereby leading to reduced ductility but enhanced resistance against grain slip deformation. Consequently, these interfaces facilitated an earlier attainment of yield limit point and contributed to increased ultimate tensile strength in the Fe<sub>50</sub>Mn<sub>30</sub>Co<sub>10</sub>Cr<sub>10</sub> HEA. These findings provide valuable insights into the microstructure evolution and mechanical behavior of LPBF-printed metastable FeMnCoCr HEAs.
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