Synergistic effects of crystalline microstructure, architected mesostructure, and processing defects on the mechanical behaviour of Ti6Al4V meta-crystals

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2021-06, Vol.818, p.141436, Article 141436
• Main Authors: Lertthanasarn, J., Liu, C., Pham, M.S.
• Format: Article
• Language: English
• Subjects: Additive manufacturing; Alternations; Crystal defects; Crystal lattices; Crystal structure; Crystallinity; Crystals; Ductility; Energy absorption; Free surfaces; Heat treatment; Mechanical properties; Meta-materials; Microstructure; Mimicry; Polycrystals; Powder beds; Processing defects; Structural design; Structural hierarchy; Struts; Synergistic effect; Ti6Al4V; Titanium base alloys; Toughness
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2021.141436

The mimicry of crystalline microstructure at the meso-scale creates a new class of architected materials, termed meta-crystals, and offers effective ways of significantly improving the toughness and eliminating the post-yield collapse of architected materials. The application of meta-crystal approach to crystalline alloys provides exciting opportunities for high strength structural components. This study investigated the mechanical behaviour of polycrystal-like meta-crystals fabricated from a widely used alloy, Ti6Al4V, by laser powder bed fusion (LPBF). The use of Ti6Al4V in fabricating meta-crystals created materials containing hierarchical lattice structures across length-scales: the atomic lattice, the intrinsic crystalline microstructure, and the architected polycrystal-like mesostructure, with each hierarchical feature playing an influential role in the mechanical behaviour of meta-crystals. This present study examined the hierarchical lattice structures at different lengthscales and their contribution to the behaviour of meta-crystals. In particular, the presence of acicular α’ martensitic microstructure was responsible for low ductility in the as-printed meta-crystals. Although heat-treatment was able to transform the martensitic microstructure to a typical α+β microstructure thus increasing the ductility, it was found that notch-like defects from lack of fusion at the free surface of struts were significantly detrimental. The study subsequently altered the meso-structural parameters to reduce the influence of the process defects and explored the effects of the heat treatment on the altered meta-crystals. Such alternations of structural design and crystalline microstructure appeared to be successful in minimising the processing effect, enabling the crystal-mimicry approach to effectively improve the toughness of Ti6Al4V meta-crystals. •Polycrystal-inspired meta-materials, termed meta-crystals, were created.•3D printed Ti6Al4V meta-crystals contain multiscale lattice structures.•Effect of the structural hierarchy on mechanical performance was studied.•High performance meta-crystals achievable with appropriate hierarchical designs.