Abstract: A high-performance Ti-Ni-B alloy with good tensile properties and reduced mechanical anisotropy was developed by promoting the columnar to equiaxed transition (CET) of prior-β grains and modifying α-laths to equiaxed grains. Both Ni and B contributed to the refinement of columnar prior-β grains during the L→β phase transformation by generating constitutional undercooling. Compared with Ni, B had a superior capability of generating constitutional undercooling, which not only replaced a significant amount of Ni with a minor addition to reduce the formation of brittle eutectoid, but also reacted with Ti to form TiB to promote heterogeneous nucleation of α-Ti grains during the β→α phase transformation. Together with the restricted growth of α-laths induced by the refinement of prior-β grains, a fully equiaxed α-Ti structure was obtained. The competition between the negative effect of brittle eutectoid and the positive role of α-lath to equiaxed grain transition on the ductility of as-printed Ti-Ni-B alloys was fundamentally governed by the morphology of eutectoid and technically dependent on the Ni-B content. When the addition was 1.2Ni-0.06B (wt.%) or less, the positive effect of α-lath on equiaxed grain transition can effectively mitigate the issue of reduced ductility caused by brittle eutectoid. In contrast, at 1.8Ni-0.09B or greater, the negative effect of eutectoid dominated. New insights into microstructural design obtained through the aforementioned approach were presented and discussed.

Key words: Laser powder bed fusion, Titanium alloys, Grain refinement, Powder processing, Anisotropy