Strong and ductile metastable β titanium alloy via laser powder bed fusion through nitrogen interstitials and tailored microstructure

Owing to the excellent mechanical properties, the laser powder bed fusion (L-PBF) of metastable β-type Ti-10 V-2Fe-3Al (Ti1023) alloys has attracted increasing attention. In this study, an Ar and 10 vol% N₂ reactive atmosphere was employed to tailor the microstructure and achieve solid solution strengthening via interstitial nitrogen atoms, thereby significantly enhancing the mechanical properties of metastable β‑titanium alloys. Interstitial nitrogen atoms in the titanium matrix induce lattice distortion, as determined by XRD (X-ray diffraction), whereas the nitrogen content can reach ∼1900 ppm (parts per million). Under an Ar-10 vol% N₂ reactive atmosphere, various scanning speeds can notably promote the columnar to equiaxed transition (CET) and grain size refinement in Ti1023 alloys. Transmission electron microscopy (TEM) reveals extensive nucleation and growth of nanoscale, dot-like α phases, which correlates with the increased T_β temperature and more intense thermal cycling induced by interstitial nitrogen atoms. Mechanical testing shows that, the increasing scanning speed under a pure Ar atmosphere reduces yield strength from 935.5 ± 33.2 MPa to 903.0 ± 13.0 MPa and increases elongation from 6.4 ± 3.1 % to 13.8 ± 1.5 %. Conversely, under a 10 vol% N₂ atmosphere, yield strength decreases from 1053.0 ± 18.0 MPa to 986.3 ± 7.6 MPa and ductility increases from 5.8 ± 1.1 % to 10.1 ± 1.7 %. Interstitial nitrogen atoms contribute ∼100 MPa to the yield strength. Nanoscale ω and α phases suppress the TRIP (transformation-induced plasticity) effect, thereby preventing the emergence of low yield strength. The promotion of fine equiaxed grain formation preserved the alloy's ductility. This balance results in an optimal combination of strength and ductility. In summary, this study presents a pathway to tailor the microstructure and enhance the mechanical properties of metastable β‑titanium alloys.