Designing Homogeneous Ti-Nb-Fe-Sn $β$ Titanium Alloys by PBF-LB: A Pre-Alloyed Powder Blend Strategy
Metastable $β$ titanium alloys are attractive for biomedical and structural applications owing to their low elastic modulus, high specific strength, and excellent corrosion resistance. Laser powder bed fusion (PBF-LB) enables complex-shape production and controlled compositional variation through powder blending. However, processing elemental Ti-Nb blends often results in chemical heterogeneity from incomplete dissolution of Nb-rich particles and non-equilibrium phase formation. To address this, low-modulus Ti-Nb-Fe-Sn alloys were produced by PBF-LB using Ti-42Nb, Ti-20Nb-15Fe, and Ti-20Nb-20Sn master-alloy powders blended with commercially pure Ti. Ti-23Nb-3Fe-4Sn, Ti-26Nb-2Fe-4Sn, Ti-29Nb-1Fe-4Sn, and Ti-32Nb-4Sn were fabricated using an uncommonly large 70 $μ$m layer thickness with layer remelting, followed by heat treatment at 1000 $°$C for 2 h and water quenching. After heat treatment, all alloys exhibited low porosity, homogeneous chemical distribution, and single $β$-phase microstructures with predominantly equiaxed grains and weak crystallographic texture. Thermodynamic calculations indicated that solidification descriptors alone could not explain the non-monotonic grain-size evolution, which was attributed to inherited solidification structure, transient TiFe-like phase formation, Nb/Sn partitioning, and/or solute-drag-controlled $β$-grain growth. Hardness and yield strength decreased with decreasing Fe and increasing Nb contents, from 268 to 224 HV and 691 to 468 MPa, respectively. Young's modulus, determined by resonant ultrasound spectroscopy, ranged 63-81 GPa. These results demonstrate that pre-alloyed master-alloy blends combined with remelting and heat treatment provide an effective route for producing chemically homogeneous Ti-Nb-Fe-Sn $β$ alloys while revealing how small compositional changes govern grain-growth behavior and mechanical response.