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

Abstract

Microstructures dominated by acicular α' 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 α+β 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 α+β 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 β stabiliser of high intrinsic diffusivity. After printing, this alloy presents a microstructure dominated by metastable β 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 β transus temperature. The microstructure evolution is comprised by homogeneous decomposition of the metastable β phase via ω-assisted nucleation of α phase, α grain growth sustained by early diffusion of Fe in the β phase followed by a conventional partitioning of V. The understanding of this transformation pathway enables the development of ultrafine grained α+β 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.