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

The transient thermal cycling characteristics during laser powder bed fusion (LPBF) 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 the α″ phase (from an acicular shape to nanoparticles) to synergistically optimize the uniformity, strength and ductility of LPBF-fabricated Ti-35Nb-5Cu-xMo (Ti355x, x = 0, 1, 2, 4 wt%) alloys. The results demonstrate that LPBF-fabricated Ti355x alloys display equiaxed/columnar 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 an acicular-shaped to nanoparticle. As a result, the interactions between dislocations and α″ nanoparticles promote cross slip, thereby homogenizing plastic flow and delivering high ductility (>25%) for the HT950-Ti3552 alloys during tensile deformation. Moreover, the addition of Mo increases the dislocation density and enhances the solute drag effect, leading to refined β grains and α″ nanoparticles, which in turn increase the yield strength (YS) of the HT950-Ti3552 alloy by about 90 MPa compared to that of the HT950-Ti355 alloy. This work establishes a theoretical basis and technical route for developing LPBF-fabricated β-type titanium alloys with isotropic mechanical properties and an improved balance between strength and ductility.