The effect of boron on the grain size and texture in additively manufactured β-Ti alloys

One of the critical microstructural attributes affecting the properties of additively manufactured (AM) alloys is the growth of large columnar grains along the build direction. While most of the work in the reported literature is focused on Ti-6Al-4V and other alpha/beta alloys, there are rather limited investigations on grain growth and texture development in AM beta-Ti alloys. The addition of trace amounts of boron to these AM beta-Ti alloys resulted in significant changes in the microstructure. Depending on the alloy system, a grain refinement of 50-100 times was noted. The change in the grain size has been attributed to a combined effect of constitutional supercooling, caused by boron rejection from the growing beta grains, and the growth restriction factor (Q) of the grains caused by the solute elements. The addition of boron also changed the morphology of the grains from being columnar to more equiaxed, a much more pronounced change than observed in traditional alpha/beta alloys such as Ti-6Al-4V. A change in texture of the b grains along the build direction was also noted, wherein the addition of boron randomized the texture from the typically observed strong (001) b oriented grains in AM Ti alloys. Finally, the addition of boron changed the morphology of the a precipitates in the Ti-Mo system from lath-like to more equiaxed, while preserving the Burgers orientation relationship between the a and b phases.