Insights into the microstructural design of high-performance Ti alloys for laser powder bed fusion by tailoring columnar prior- grains and -Ti morphology

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 - a grains and modifying alpha- laths to equiaxed grains. Both Ni and B contributed to the refinement of columnar prior - a grains during the L -> fi 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 alpha-Ti grains during the ,B ->alpha phase transformation. Together with the restricted growth of alpha-laths induced by the refinement of prior - a grains, a fully equiaxed alpha-Ti structure was obtained. The competition between the negative effect of brittle eutectoid and the positive role of alpha-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 alpha-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. (c) 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.