Multiphase structure enables strength-ductility-hardening synergy in additively-manufactured Fe-Mn-Cu alloys

Additively manufactured Fe-Mn alloys hold promise for biodegradable orthopedic load-bearing applications, yet their strength-ductility-strain hardening synergy is constrained by the formation of excessive ε-martensite under rapid cooling. In this study, a copper (Cu)-alloying strategy combined with laser powder bed fusion (LPBF) to construct a multiphase structure comprising γ-austenite (71.9–98.8 vol. %), ε-martensite, α-ferrite, and Cu-rich nano-precipitates. With the increase of Cu content, the texture was gradually weakened, and the greater heat accumulation led to more widely distributed dislocation cells, captured by the low angle grain boundaries. Mn and Cu segregation occurred in the dislocation cell walls, and the SFE (∼ 25.4 mJ/m²) of the Fe-18Mn-1Cu alloy was close to that of Fe-18Mn, resulting in a synergistic effect of TRIP and TWIP during deformation. Molecular dynamics (MD) indicated that, the combined TRIP/TWIP effects persistently form hard martensite and deformation twins, with interphase stress/strain-partitioning induced strengthening, enhancing the dislocation formation kinetics and improving the strength and hardening. In contrast, Fe-18Mn-3Cu primarily undergoes dislocation slip, depleting ε-martensite and diminishing strain hardening and relying on dislocation cells and minimal α′-martensite for strain accommodation. The Fe-18Mn-1Cu alloy shows higher ultimate tensile strength of 837 MPa, improved ductility of 16%, and a superior strain hardening rate of ∼9000 MPa. This work refined the theoretical foundations for the multiphase structures design of Fe-Mn-Cu alloys through multi-scale experiments and simulations, and quantified the contribution of the strengthening mechanism, finally demonstrated the feasibility of their development in orthopedic load-bearing applications.