Fracture behavior of laser powder bed fusion fabricated Ti41Nb via in-situ alloying

The microstructures and the mechanical properties, including fatigue crack growth (FCG) and mode I fracture toughness ( K _IC), of in-situ alloyed Ti41Nb (wt.%) fabricated using the laser powder bed fusion (LB-PBF) were investigated. A small hatch spacing - fast scanning strategy was utilized for obtaining parts with minimum porosity and unmelted Nb particle combination. The mesostructure of the fabricated blocks comprises columnar grains with alternating Nb-poor regions (NPRs), which occur due to the short residence time of materials in the melt that limits the melting and diffusion of Nb, and matrix layers. Extensive ω _iso precipitates were noted in the microstructure, which were attributed to the large-build related heat accumulation during the LB-PBF process that stems from the combination of large sample size and high energy density input. Consequently, ductility of the as-built parts was low. The FCG rate at high ∆K was strongly influenced by the layers of NPR and hence depend on the orientation whereas the near-threshold FCG rate was strongly influenced by the local slip systems ahead of the crack tip. Significant anisotropy in K _IC , a result of the crack deflection by the NPR/matrix interfaces, was observed. The mesostructure induced toughening mechanism improves the fracture toughness of the resultant alloy despite its brittle failure response. The potential of in-situ alloying to induce toughening through the mesostructure control was discussed.