Ti-6Al-4V (Ti64) is widely used in the additive manufacturing (AM) industry for its superior mechanical properties; however, severe anisotropy is inevitable. In this work, a Ti64 sample fabricated using laser-directed energy deposition is used for fundamental investigations into the anisotropy of its microstructure, mechanical properties, and fracture behaviors. The microstructure of martensite α and prior β-Ti grains are characterized in both the XOY and XOZ planes. The tensile/compressive properties and microhardness along the building direction (BD) and scanning direction (SD) are tested, and it is found that the sample along the SD has better comprehensive mechanical properties. Due to grain boundary α (GB-α), different fracture behaviors and crack propagation paths are found along the BD and SD. When tensile force is parallel to the growth orientation of GB-α, a much higher density of microcracks caused by fractured GB-α is found to contribute to a prolonged elongation and the weakening of strength. While stretching along the SD, the cracks would propagate along the GB-α easily and straightly, which might lead to lower elongation.

Achieving a balance between high strength and adequate ductility at elevated temperatures is a long-standing challenge in titanium matrix composites (TMCs), primarily attributed to thermal softening and the inherent tendency for strain localization. Here, a (TiB+TiC)/Ti6Al4V composite with a three-dimensional network structure is designed and fabricated via laser-directed energy deposition. The composite exhibits an exceptional strength-ductility synergy at high temperatures. At 400 °C, the ultimate tensile strength is approximately 19% higher than that of the as-printed Ti6Al4V alloy while retaining comparable ductility. At 500 °C, the strength-ductility combination approaches that of the base alloy tested at 400 °C, effectively extending the instantaneous load-bearing temperature window by about 100 °C. The network partitions the strain field into interlinked cells, promoting more stable plastic flow and delaying necking initiation. Strain gradients across network walls activate strain-gradient-assisted hardening and enhanced geometrically necessary dislocation accumulation, which helps sustain work hardening over 200–500 °C. In addition, TiB whiskers progressively rotate toward the loading axis, raising the orientation factor and improving stress transfer. The thermally stable TiB/α interfaces suppress interfacial debonding and enable fractured reinforcements to continue sharing load. During fracture, microcracks are arrested within individual network cells and crack-tip blunting/deflection is promoted. These findings establish a multiscale mechanistic framework that directly correlates the additively manufactured reinforcement network architecture with the elevated-temperature strength–ductility synergy, offering a scalable design strategy for advanced heat-resistant TMCs for high-temperature structural applications.