•A crack-free LPBFed Fe-18 mn alloy is fabricated using pre-alloyed powders.•The exceptional work-hardening rate (8500 MPa) of Fe-Mn alloys is achieved by additive manufacturing and subsequent heat treatment.•Deformation mechanisms are illustrated and discussed in details. Laser-powder-bed-fusion (LPBF) fabricated Fe-Mn biodegradable alloys provide an attractive prospect for orthopedic applications due to their good tensile strength and high degradation rate. Nevertheless, the ε-martensite and heterogeneous microstructures produced by the LPBF processing often lead to premature failure of alloys. Herein, we report a LPBFed multiphase Fe-18Mn alloy (γ-austenite, ε-martensite, and α-ferrite) fabricated from pre-alloyed powders. After annealing at 650 °C, the alloy with a uniform microstructure displays a high 1 GPa tensile strength, a good fracture elongation of 16 %, and an extremely high work-hardening rate of 8500 MPa. The work-hardening rate is higher than that reported in most Fe-Mn steels and Fe-based high entropy alloys. The grain size of a few hundred nanometers provided the excess Gibbs free energy, resulting in an increase in the stacking fault energy (SFE) to 23.9 mJ/m2. The multiple deformation mechanisms, i.e., SFs, the martensitic transformation (γ → ε → α') and nano-deformation twins (DTs), were sequentially activated. We elucidate such unique work-hardening capability, originating from the interaction between the DTs, SFs and transformed martensite. Besides a high-density of dislocations were accumulated between parallel planar defects, the cooperative deformation of the soft and hard phases provided continuous hardening. Our findings highlight the exceptional work-hardening capability of additively-manufactured Fe-Mn alloys achieved by a multiphase material exhibiting multiple deformation mechanisms. The work also provides a straightforward approach for the development of stable-implanted Fe-based bone substitutes. Display omitted