Extraordinary combination of strength and ductility in an additively manufactured Fe-based medium entropy alloy through in situ formed ?-nanoprecipitate and heterogeneous microstructure

The current study investigated the mechanical properties of Fe65Ni15Co8Mn8Ti3Si (at%) medium entropy alloy (MEA), printed by laser-based direct energy deposition (DED) in the room and liquid nitrogen environments. The as-printed microstructure contains various levels of heterogeneities, including different grain sizes, dual-phase microstructure, in situ formed eta nanoprecipitate, cellular structure, and elemental segregation generated dur-ing the DED process. The impact of each observed heterogeneity on the mechanical properties and the pre-dominant deformation mechanisms were examined. The corresponding MEA revealed a high ultimate tensile strength (UTS) of-1.02 GPa, total elongation (TE) of-46%, and exceptional cryogenic mechanical properties with a UTS and TE of 1.83 GPa and 40%, respectively. According to microstructural observations, the deformation-induced phase transformation from face-centered cubic (FCC) to body-centered cubic (BCC) is the predominant strengthening mechanism at both room and liquid nitrogen temperatures in addition to solid -solution strengthening and dislocation-mediated plasticity. The non-shearable eta nanoprecipitates also enhanced the mechanical properties through precipitation/stacking fault strengthening. Hetero-deformation-induced (HDI) strengthening also occurred in the presence of dual-phase microstructure, cellular microstruc-ture, and elemental segregation in the FCC phase. This upgraded synergy of tensile strength and ductility at liquid nitrogen temperature can be explained through the phase transformation and additional activation of twinned martensite formation, which results in two-step strain-hardening behavior. The presented results expand possibilities for developing DED-processed ferrous HEAs/MEAs to overcome the strength and ductility trade-off at room and liquid nitrogen temperatures.