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Abstract: Metastable β Ti-Nb alloys have potential as biomedical implant materials due to a low elastic modulus and good biocompatibility. However, these alloys are susceptible to the ⍵ phase transformation, which significantly stiffens the alloy. Despite this, there is limited agreement within the literature whether the form of the ⍵ phase is important in governing subsequent mechanical response. Here, this work utilises synchrotron X-ray diffraction data to conclusively demonstrates that ⍵iso significantly inhibits a mechanically driven martensitic transformation, whereas ⍵ath is seen to have a much smaller effect. This work therefore has important consequences for the design of new transforming materials.

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Abstract: This thesis presents a comprehensive investigation into the ultrasound-assisted processing of Al alloys and graphite materials using ultrafast synchrotron X-ray imaging and megahertz (MHz) X-ray free electron laser (XFEL) microscopy techniques, with emphasis on understanding the multiscale and multiphysics mechanisms driving multiphase evolution of Al alloy in solidification and the layer exfoliation of graphite materials. The unifying theme is the exploration of how ultrasonically generated physical phenomena, i.e., cavitation, shockwaves, and acoustic streaming, govern microstructural evolution at multiple scales. This comparative approach provides a comprehensive understanding of ultrasound-matter interactions, revealing universal principles that can be applied to optimize processing parameters for a wide range of materials. The key findings are summarized as follows:(1) In-situ synchrotron X-ray tomography and diffraction revealed, for the first time, the nucleation and co-growth dynamics of multiple Fe-rich intermetallics and their dynamic interactions with the Al dendrites of the recycled Al-5Cu-1.5Fe-1Si alloy in solidification under ultrasound melting processing (USMP). USMP significantly refined the α-Al dendrites, modified the morphologies of α-Fe and β-Fe phases, and altered their spatial distributions, thereby enhancing structural homogeneity and potentially improving mechanical properties. (2) By taking the full advantage of MHz XFEL microscopy, we have imaged and quantified, at ns time scale and μm length scale simultaneously, the local shockwaves produced by the implosion of a single cavitation bubble, multiple bubbles and bubble clouds, as well as the layer exfoliation dynamics under such shockwave impacts. The results confirmed that exfoliation is governed not by a single implosion event, but by repeated cyclic forces induced by shockwave impact, with exfoliation behaviour strongly dependent on local defects and graphite structure. (3) Using quasi-simultaneous synchrotron X-ray tomography and diffraction, we have studied in-situ the complicated peritectic reaction mechanisms involving Al4Mn and Al6Mn phases in an Al-8Mn alloy during the solidification process. The real-time evolution of the spatial and orientation relationships between these intermetallics was quantified for the first time, revealing the highly anisotropic faceted growth and intricate coalescence patterns that deviated from the equilibrium solidification models.

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Abstract: Metastable β Ti alloys have potential for vibration damping and actuation applications within the aerospace industry due to thermal and mechanical hysteresis. However, variations in transformation parameters, which are also seen to change following thermal or mechanical cycling, significantly limit industrial acceptance. There is a widespread belief that these variations are a consequence of ⍵ phase formation. However, here we provide evidence to show that this is not necessarily the case. Instead, we show how residual stresses and defect structures are crucial to the transformation of these alloys and present an understanding of the mechanism that governs their behaviour. Importantly, we highlight the consequences for the design of new transforming alloys and component geometries, and how current design theories may need to be employed in conjunction with other methods to effectively prevent longer-term changes in behaviour. To this end, we demonstrate how functional properties could be periodically recovered by introducing short intercycle heat treatments and suggest possible next steps for advancing our understanding of these materials.

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Abstract: This study presents the first demonstration of the use of X-ray diffraction (XRD) to quantify the radial or transverse deformation in Hexcel IM7 PolyAcryloNitrile (PAN)-based carbon fibres at temperatures as low as 200 K (-70 °C). The Coefficient of Thermal Expansion (CTE) is a critical design parameter that needs to be precisely quantified for the next generation of carbon fibre-based Liquid Hydrogen ( ) storage tanks for net-zero aviation. This variable quantitatively describes the thermal mismatch between the fibre and the resin that is the driver for microcracking and tank leakage. However, quantification of the CTE of the fibres is experimentally challenging. The results provide unique insights, indicating that the microscopic transverse CTE of the fibre ( ) is equal to 26.2 × 10-6 K-1 and is governed by van der Waals forces, similar to those in the basal c-axis (out-of-plane) direction of graphite and the radial direction of multi-wall carbon nanotubes. Taking into account the microcrack-induced relaxation effect reported in polycrystalline graphite, the macroscopic fibre transverse CTE was determined to be 7.86 × 10-6 K-1. XRD data were also collected on Hexcel IM7/8552 Uni-directional (UD) and Quasi-isotropic (QI) composite laminates to investigate the influence of the interaction of the resin matrix with the fibre lattice and the stacking sequence on the development of thermal fibre lattice strain. In the UD laminate, the presence of resin induces an additional transverse strain in the fibres as a result of resin contraction during cooling, leading to the development of a compressive strain in the fibre direction. This behaviour was found to be in good agreement with numerical simulations, with a 13 % error at the lowest measured temperature. In contrast, the fibres in the QI configuration were reinforced in the transverse direction, effectively mitigating the influence of resin contraction. These CTE values, insights, and resulting models are essential for multi-scale modelling, design and certification of carbon fibre composite tanks that are required to achieve net-zero aviation.

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Abstract: This paper demonstrates a new approach that exploits both lattice strain mapping via Wide Angle X-ray Scattering (WAXS) and Digital Volume Correlation (DVC) of Computed Tomography (CT) to understand the material response at different length scales in Carbon Fibre Reinforced Polymers (CFRPs) under in-situ loading, a phenomenon of substantial importance for the modelling, design, and certification of composite structures. WAXS gives insight into fibre lattice strain, while DVC provides sub-laminate response in the CFRP. A detailed numerical simulation was also developed to compare with these novel experimental methods. This approach is the first demonstration that the strain within the crystalline regions of the fibre is distinct from the sub-laminate behaviour, with up to 80 % and 36 % differences in the longitudinal and transverse directions, respectively, as a result of the complex microstructure of the fibres. An improved understanding of composite behaviour is fundamental to understanding how strain accommodation leads to structural failure, providing routes to refine part rejection criteria and reduce the environmental impact of this increasingly widespread material class.