Optimizing the severe strength-ductility trade-off in additively manufactured Ni-free high-N duplex stainless steels via heterogeneous nanoparticles

Duplex stainless steels (DSSs) are a cost-effective option for various industrial applications. However, Ni-free high-N DSSs fabricated via the laser powder bed fusion (LPBF) exhibits a severe strength-ductility trade-off. In this study, a Ni-free high-N DSS composite with great strength-ductility synergy (uniform elongation (UE): ∼13.0 %; ultimate tensile strength (UTS): ∼1096 MPa) was successfully fabricated using submicron TiN inoculants and the LPBF technique, in comparison to pure DSSs (UE: ∼3.9 %; UTS: ∼1314 MPa). Adding submicron TiN particles facilitated the in-situ formation of spherical MnTi₂O₄ and block-shaped core-shell nanoparticles with a MnTi₂O₄ core and a TiN shell. These nanoparticles acted as nucleation sites for ferrite, refining grains. More austenite was formed in DSS/TiN composites. In DSS/TiN composites, the refinement strengthening induced by the addition of TiN particles improved the strength-ductility synergy, the increased austenite content enhanced the ductility, and the in-situ formed MnTi₂O₄ and core-shell nanoparticles contributed to the improvement of strength and ductility. These factors contributed to an excellent strength-ductility synergy in LPBF-fabricated DSS/TiN composites. This study successfully achieved mechanical property optimization of LPBF-fabricated Ni-free high-N DSSs without post-heat treatment by producing in-situ formed MnTi₂O₄ and core-shell nanoparticles. The successful creation of these nanoparticles sets the stage for producing metallic components with great strength-ductility synergy via LPBF technique and in-situ formation strategy.