Breaking through the strength-ductility trade-off of LPBF-produced Ti-xNb alloys from mixed powders via ω-phase induced heterostructure

Ti-xNb alloys produced by laser powder bed fusion (LPBF) from mixed powder usually exhibit an inhomogeneous elemental distribution, leading to a deterioration in mechanical properties. To address this issue, we proposed a strategy to achieve heterostructure in laser powder bed fused Ti-xNb alloys from mixed powders through precipitation of ω within β. Moreover, the effect of Nb content on the microstructure and mechanical behavior of Ti-xNb alloys was studied. The results indicated that in-situ laser re-melting can realize the homogeneous elemental distribution in Ti-xNb alloys. When the Nb content increases from 30 wt%, 35 wt% to 40 wt%, Ti-xNb alloys experience a transformation from β + α′ to β + ω and monolithic β. Specifically, ω nano-precipitates in Ti-35Nb alloy are only distributed in some β grains, forming a heterostructure with “soft β” and “hard β + ω” grains. As a result, LPBF-produced Ti-35Nb alloy demonstrates excellent mechanical properties, with yield strength of ~(792 ± 6) MPa, tensile strength of ~(806 ± 7) MPa, Young’s modulus of ~(68 ± 6) GPa, and uniform elongation of ~(18.0 ± 1.1)%. The Frank-Read mechanism induces dislocation proliferation and dislocation cross-slip, and the geometrically necessary dislocations (GNDs) are induced at the heterogeneous interface of “soft β” and “hard β + ω” grains, resulting in an enhancement in the strength-ductility synergy of Ti-35Nb alloy produced by LPBF. This work provides an innovative strategy to improve the strength-ductility synergy of LPBF-produced Ti-xNb alloys from mixed powders by tailoring ω nano-precipitates.