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

Simonelli, Marco; Zou, Zhiyi; Barriobero-Vila, Pere; Tse, Yau Yau

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

Citation

Simonelli, M., Zou, Z., Barriobero-Vila, P., & Tse, Y. Y. (2023). The development of ultrafine grain structure in an additively manufactured titanium alloy via high-temperature microscopy. Materialia, 30, Article 101856. https://doi.org/10.1016/j.mtla.2023.101856

• Journal Article Type: Article
• Acceptance Date: Jul 20, 2023
• Online Publication Date: Jul 21, 2023
• Publication Date: 2023-08
• Deposit Date: Aug 2, 2023
• Publicly Available Date: Oct 9, 2023
• Journal: Materialia
• Print ISSN: 2589-1529
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 30
• Article Number: 101856
• DOI: https://doi.org/10.1016/j.mtla.2023.101856
• Public URL: https://nottingham-repository.worktribe.com/output/23731710
• Publisher URL: https://www.sciencedirect.com/science/article/pii/S2589152923001837