Embedding boron into Ti powder for direct laser deposited titanium matrix composite: Microstructure evolution and the role of nano-TiB network structure

The titanium powder embedded with 2.5 vol% TiB was in-situ fabricated by gas atomization and applied for direct laser deposited (DLD) titanium matrix composites. Consistent with the network distributed boron-rich zone in composite powder, a three-dimensional (3D) in-situ ultrafine network structure, consisting of nano-TiB whiskers (TiBw), was characterized throughout the as-fabricated composite (DLDed TiB–Ti). Both the dendritic and equiaxed network, attributed to the boron-induced constitutional supercooling and subsequent nucleation and growth of primary β-Ti, existed in the composite. The addition of boron (B) had a positive effect on the equiaxial transition and grain refinement of both primary β-Ti grains and α grains. Tensile tests showed an enhancement of strength compared with conventionally fabricated homogeneous TiB–Ti composites and selective laser melted CP-Ti. Analyzing the strength mechanism of the DLDed TiB–Ti, apart from the fine-grain strengthening and load-bearing effect of TiBw, the TiB network was found to have an additional contribution to the improvement of strength. Fracture morphology and in situ tensile observation showed the role of network structure in plastic deformation limitation, crack deflection and blunting, which was mainly attributed to the ultrafine cell size and revealed the significance of network structure.