Achieving an excellent synergy of strength and ductility in heat-treated laser powder bed fused niobium-based alloys via ZrO2-induced sub-grains

Additive manufacturing of niobium-based alloys demonstrates great potential for the rapid production of complex components in extreme aerospace applications. However, the ductility of niobium-based alloys fabricated by the laser powder bed fusion (LPBF) method is insufficient, which limits their industrial applications. In this study, the Nb521 (Nb-5W-2Mo-1Zr) alloy with a dense microstructure (99.96 %) was successfully fabricated using LPBF with pre-alloyed powders. The results show that the as-built specimens exhibit a typical body-centered cubic (BCC) crystalline structure, with homogeneous and crack-free microstructures. After heat treatment (HT), the ultimate tensile strength and elongation achieved an excellent synergy, being 564.8 ± 6 MPa and 23.62 ± 2.05 %, respectively. Notably, nanoscale monoclinic ZrO₂ particles precipitated in the HTed specimens, and their precipitation behavior was demonstrated using the Vienna Ab-initio Simulation Package (VASP) code, thermodynamic calculations and diffusion coefficients analysis. The improved ductility is attributed to the decrease in dislocation density, the precipitation of ZrO₂, and the formation of sub-grains induced by the ZrO₂ precipitates, which effectively mitigates stress concentration during tensile testing. The fracture mode changed from transgranular to ductile fracture, characterized by numerous dimples. This work investigates the mechanism by which high-vacuum HT enhances the ductility of LPBF Nb521 alloy and provides a viable pathway for the industrial production of niobium-based alloys with an excellent synergy of strength and ductility.