Hierarchical Strengthening of Polycrystal-Inspired Lattice Materials
Creators: name: Lertthanasarn, Jedsada
Publisher: name: Imperial College London
Year: 2021

description: Architected lattice materials offer excellent specific properties, ideal for high-performance and weight-critical applications. However, high-strength lattice materials also often exhibit a significant and catastrophic post-yielding collapse due to buckling and plastic yielding of struts, leading to a trade-off between the strength and stability, and compromising the energy absorption capacity. The mimicry of crystalline strengthening features in the lattice architecture at the mesoscale offers effective ways of improving the energy absorption capacity and eliminating the post-yield collapse of high-strength architected lattice materials. Such crystal-inspired lattice materials are called meta-crystals. This PhD thesis aims to establish the relationship between the intrinsic crystalline microstructure, the extrinsic architected mesostructure, and the mechanical behaviour of meta-crystals, in particular the separate and synergistic strengthening of intrinsic and extrinsic hierarchical features at different length scales. The mechanical behaviours of metallic polycrystal-like meta-crystals were studied by quasi-static compression experiments with digital image correlation (DIC) analyses, hardness testing, and finite element analysis (FEA). The as-printed and post-processed meso- and microstructures of the meta-crystals were characterised via X-ray computed tomography (x-CT), scanning electron microscope (SEM) imaging, electron backscatter diffraction (EBSD) analysis, electron dispersive x-ray spectroscopy (EDX) analysis, and in the transmission electron microscope (TEM). The meta-crystals investigated contained varying numbers of meta-grains, different strut diameters, were fabricated from different base materials, and were subjected to different post-processing treatments. The experiments were designed to deconvolute the strengthening contributions from the crystalline microstructure and architected features at different length scales, as well as the synergistic strengthening across the hierarchical structures. The study showed that the presence of defects can overwhelm the strengthening contributions from the crystal-inspired architecture and the intrinsic crystalline microstructure, particularly when the base material has low ductility and work hardening. The influence of surface processing defects such as lack-of-fusions is especially detrimental compared to internal porosities and such defects need to be minimised. Excessive effects from the processing defects lead to premature fracture of struts rendering the strengthening architecture ineffective. The influence of defects can be minimised via optimisation of the processing parameters, altering the microstructure, or increasing the strut diameter. The examination of various base materials such as Ti-6Al-4V, 316L, and IN718 meta-crystals highlighted the role of the base material in minimising the influence of the processing defects. Additionally, the base materials’ properties also affected the efficacy of the polycrystal-like architecture, with precipitation-hardenable alloys such as IN718 shown to be the most ideally suited to enable the combined strengthening induced by both the intrinsic and extrinsic features. Optimising the crystalline microstructure of as-printed IN718 enabled meta-crystals with exceptional strength and energy absorption capacity. A theoretical framework for the strength of polycrystal-like meta-crystals was also developed by characterising the hierarchical features of the IN718 meta-crystals, providing a basis for future designs.