Thermally-driven equiaxation of columnar grains and α'' nanoparticles enabling isotropic β-Ti alloys with strength-ductility synergy manufactured by laser powder bed fusion

Abstract The transient thermal cycling characteristics in the powder bed fusion of metals with a laser beam (PBF-LB/M) process induce elemental segregation and columnar growth, resulting in significant mechanical anisotropy and strength-ductility trade-off in titanium alloys. In order to break through this bottleneck, this work proposes a strategy of inducing equiaxation of columnar grains through recrystallization and regulating the morphology of α″ phase (from acicular-shape to nanoparticles) to synergistically optimize the uniformity, strength and ductility of PBF-LB/M-produced Ti-35Nb-5Cu-xMo (Ti355x, x = 0, 1, 2, 4 wt.%) alloys. The results show that PBF-LB/M-produced Ti355x alloys display columnar/equiaxed microstructures with significant mechanical anisotropy and strength-ductility trade-off. After tailored solution heat treatment at 950 ℃, the Ti3552 alloy (HT950-Ti3552) achieves chemical homogenization and thermally-driven equiaxation of columnar grains, leading to excellent isotropic mechanical properties. Concurrently, the morphology of α″ is transformed from acicular-shape to nanoparticles. As a result, the interaction between dislocations and α″ nanoparticles promote cross slip, thereby homogenizing plastic flow and delivering high ductility (> 25%) for HT950-Ti3552 alloys during tensile deformation. Moreover, the addition of Mo increases dislocation density and enhances the solute drag effect, leading to refined β grains and α″ nanoparticles, which in turn raise the yield strength of the HT950‑Ti3552 alloy by approximately 90 MPa compared to the HT950‑Ti355 alloy. This work provides a theoretical basis and technical route for the development of PBF-LB/M-fabricated β-Ti alloys with isotropic mechanical properties and improved strength-ductility synergy.