The development of ultrafine grain structure in an additively manufactured titanium alloy via high-temperature microscopy

Microstructures dominated by acicular & alpha;' martensitic phase, such as in the case of Ti-6Al-4V fabricated by laser powder-bed fusion (PBF-LB), are known to suffer from reduced ductility and low toughness. The decomposition of such metastable microstructures into & alpha;+& beta; lamellar structures during PBF-LB requires either specific laser regimes that are often challenging to be attained or post-process heat treatments which might lead, instead, to undesirable coarsening of the grain structure. Here we propose a novel route for the formation of ultrafine lamellar & alpha;+& beta; microstructures and demonstrate the associated advantages in terms of tensile strength and ductility. Our approach is based on a suitable modification of constitution of Ti-6Al-4V with additions of Fe, a known potent & beta; stabiliser of high intrinsic diffusivity. After printing, this alloy presents a microstructure dominated by metastable & beta; phase. We investigate the details of its decomposition using a combination of in-situ high-energy synchrotron X-ray diffraction and high temperature microscopy up to the & beta; transus temperature. The microstructure evolution is comprised by homogeneous decomposition of the metastable & beta; phase via co-assisted nucleation of & alpha; phase, & alpha; grain growth sustained by early diffusion of Fe in the & beta; phase followed by a conventional partitioning of V. The understanding of this transformation pathway enables the development of ultrafine grained & alpha;+& beta; lamellar microstructures that exhibit outstanding tensile behaviour. The presented approach is machine-agnostic and offers a novel alloy design strategy for development of high-strength alloys in additive manufacturing.