Origin of Aspect Ratio Decreasing and Variant Selection for Alpha Laths in Laser Directed Energy Deposited Tc11-Xb Alloys: Spatial Inhibition and Selective Coarsening in Thermal Cycle

Boron addition is prevalently applied for additively manufactured titanium alloys. The aspect ratio of α laths in titanium alloys tends to decrease with boron addition, while the critical cause is not clear. To reveal the mechanism of this circumstance, Ti-6.5Al-3.5Mo-1.5Zr-0.3Si-xB (x = 0, 0.14, and 0.35) alloys were laser directed energy deposited as multi-layer walls, and the morphology, size, and variant selection of α laths before and after thermal cycle were characterized. It is found that the contribution of heterogeneously nucleated α on TiB to the decrease is limited. The α laths are initially refined due to grain refinement and TiB precipitates from boron addition. However, the layer-by-layer thermal cycle renders the selective coarsening of αp laths, and thus the α laths widen with boron addition. In addition, branching of α also assists to widen the laths. Therefore, spatial inhibition and selective coarsening in thermal cycle predominately account for the aspect ratio decrease of α laths. Boron addition weakens the variant selection in both unstable and stable zones, which assists to prove the mechanism of aspect ratio decrease for α laths.