Nanoparticle-mediated ultra grain refinement and reinforcement in additively manufactured titanium alloys

Additive manufacturing, also known as 3D printing, overcomes many design and manufacturing constraints to allow almost direct production of metals into complicated geometries. However, coarse columnar grain structures, up to the millimeter-scale, are commonly produced in titanium and its alloys through the layer-by-layer process and this causes significant anisotropy in mechanical properties. Here we report an innovative approach for microstructure refinement of an additively manufactured Ti-6Al-4V alloy via directed energy deposition of boron nitride nanotube (BNNT) decorated powders. With only 0.4 wt% BNNT, this process results in unprecedented grain refinement down to a few micrometers and over 50% strength enhancement. A unique texture-weakened structure comprising fine equiaxed grains is achieved via a novel nanoparticle-mediated nucleation mechanism enabled by local hypereutectic precipitation in the rapid solidification process. This mechanism is highly suited to the metallurgical environment of metal additive manufacturing and creates a pathway for screening effective grain refiners in titanium and other alloy systems.