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(2023) Muniandy, YokasunderyThesisHigh-entropy alloys (HEAs) are a new class of metallic materials that contain five or more elements in near-equiatomic compositions and that have received significant research attention in the past decade. The reason for this is that some alloy systems have been reported to crystallise as single-phase materials with face-centred cubic (FCC) crystal structure despite their individual elements often having very different crystal structures. One such example, the CrMnFeCoNi alloy, one of the first HEAs reported by Cantor et al. in 2004, has, furthermore, excellent combinations of mechanical properties such as high strength, excellent ductility, and outstanding fracture toughness at room temperature; interestingly, in contrast to many other materials, these properties improve with decreasing temperature down to liquid nitrogen. The reason for this is the staggered activation of deformation mechanisms such as dislocation glide and nano-scale deformation twinning. While numerous studies have reported the CrMnFeCoNi alloy as well as many other multi-component alloy systems to be chemically homogeneous, little attention has been drawn to the impact of processing history on chemical complexity and as such clustering and ordering phenomena that may impact mechanical performance. In this work, CrMnFeCoNi alloys have been fabricated using different processing routes resulting in a chemically homogeneous and a chemically heterogeneous versions of the material. Using various characterisation methods such as a wavelength-dispersive x-ray spectroscopy-based electron micro-probe analyser (WDXS-EPMA) in combination with energy-dispersive x-ray spectroscopy (EDXS) and atom probe tomography (APT), methods that can be utilised across multiple length scales, element composition as well as spatial distribution of the materials have been investigated. The two materials have furthermore been utilised for an APT parameter study to tailor data acquisition parameter for multi-component alloy systems to obtain high quality atom probe data. Based on the obtained results, a generalised multi-component short-range order (GM-SRO) parameter study was conducted to analyse ordering phenomena in the APT data of the HEAs and compared with medium-entropy alloy subsystems containing 3-4 elements and that can be associated with the HEAs in terms of their mechanical and/or elastic properties. Finally, two non-equiatomic alloys from the chemically distinct regions of the heterogeneous material were fabricated and compared in terms of microstructure development and associated mechanical performance to each other and the equiatomic HEA.
(2023) Luo, XiaoxuanThesisComplex borohydrides have the potential to act as solid-state electrolytes for all-solid-state batteries. In this respect, sodium borohydride (NaBH4) is of high interest because it is thermally stable (up to 500 degrees celsius), and it has a high deformability and electrochemical stability against sodium anodes. However, its ionic conductivity at room temperature is extremely low ( ~ 10-10 S cm-1). Accordingly, this thesis aimed at investigating means to create defective NaBH4 structures with the intent to significantly enhance its ionic conductivity. To this aim, several strategies were investigated including the creation of intermediate interfaces, partial anionic substitution, the generation of defects and conducting interfaces through partial hydrolysis. By converting the surface of NaBH4 particles into Na2B12H12 of higher Na+ conductivity, to form NaBH4@Na2B12H12 core-shell structures, the resulting interface was found to lead to an ionic conductivity of 4 × 10-4 S cm-1at 115 degrees celsius, i.e., significantly higher to that of pristine Na2B12H12 (10-7 S cm-1). This demonstrates that it was possible to generate disordered interfaces trough anion mixing. The results suggested that the creation of defects may be more prone to lead to high ionic conductivity. Through partial substitution of BH4- anion by I- in NaBH4, defective NaBH4 structures with varied lattice constants could be created. This anion substitution strategy enhanced the ionic conductivity of NaBH4 doped with NaI to 1.6 × 10-3 S cm-1 at 65 degrees celsius. To further improve upon this, the idea of partial hydrolysis was also investigated with the idea to create both conductivity interfaces and defective NaBH4 structures by exposing NaBH4 to controlled amount of water. The disordered trapped interface located between alpha-NaBH4 and NaB(OH)4 showed fast Na+ dynamics, which led to a Na+ conductivity of 2.6 × 10-3 S cm-1 at 75 degrees celsius. Further addition of poly(ethylene oxide) (PEO) was found to help better control the levels of hydrolysis and the hydrolysed NaBH4-PEO composite electrolyte reached an ionic conductivity of 1.6 × 10-3 S cm-1 at 45 degrees celsius. These results indicate that the controlled formation of defects within NaBH4 is key to the conversion of such hydrides into superionic Na conductors.