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Abstract: Multi-material structures have shown great versatility in wide applications. However, additive manufacturing of multi-metal mechanical composite structures is challenging. Beyond this, a comprehensive and multi-scale understanding of the fracture mechanisms in such structures has not been sufficiently elucidated. In this study, we exploited synchrotron phase contrast X-ray computed tomography and synchrotron X-ray ptychographic tomography to achieve in situ, continuous observation of the fracturing process in large-scale brick-and-mortar multi-metal composite structures, resolving phenomena spanning from the micro- to nano- scale. Findings suggest that nano-pores prevailingly exist in additively manufactured metals, and interfacial porosity as a transitional geometry between different materials can retard the crack growth and improve fracture toughness. This multi-scale study directly informs the designing, manufacturing, and testing of multi-metal composite structures.

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Abstract: Nanoscale crystals, or ‘nanolites’, are becoming increasingly recognised in both experimental products and natural samples of volcanic eruptions, across a range of magma compositions and explosivity. Nanolites can increase magma viscosity and influence eruptive style, due to the rheological impact of the nanoparticle suspension, by inducing chemical and structural changes in the residual melt and by facilitating heterogeneous bubble nucleation. Due to their large surface area, nanolites are also prone to aggregation. However, their morphology, spatial distribution and interaction in 3D has not yet been investigated. Here we present a 3D, nanometre-scale visualisation and quantification of nanolites within scoriae of highly explosive basaltic volcanic eruptions, obtained using X-ray ptychography, a nanoscale microscopy technique. We find that titanomagnetite nanolites aggregate, forming elongate, irregular structures in 3D. Compositional heterogeneities are also observed within the matrix glass, as extraction of Fe and Ti from the melt during nanolite crystallisation forms differentiated, Si-rich boundary layers surrounding nanolites with higher viscosity. We support our 3D nanoscale observations with images acquired using SEM and STEM, utilising multi-scale imaging methods to visualise nanolite crystallisation in basaltic magmas. We find that syn-eruptive nanolite crystallisation can increase magma viscosity through their aggregation and impact on the composition of the residual melt, increasing the potential of magma fragmentation during ascent. Our results provide insight into the nanoscale structure of volcanic products and also the driving mechanisms of highly explosive basaltic volcanic eruptions.

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Abstract: As its title suggests, this dissertation is about hyperspectral x-ray ptychography. Ptychography is a microscopy technique in which images of a sample are algorithmically reconstructed based on the diffraction patterns that arise from exposure to coherent radiation. An example of coherent radiation in the visible range is laser radiation. However, x-ray ptychography involves illumination with high-energy radiation. This is usually very bright, monochromatic radiation, produced in a synchrotron. The word hyperspectral in the title refers to two things simultaneously. First, the use of radiation that is not monochromatic, namely broadband radiation: radiation with a broad spectrum. Secondly, the use of a hyperspectral x-ray detector, a detector that can measure the energy of individual high-energy photons. Due to the short wavelength of x-rays, x-ray ptychography allows for the imaging of very small structures (the current record is four nanometres). This is not as small as what is possible with, for example, an electron microscope. However, electrons are absorbed relatively quickly in matter. This makes them unsuitable for passing through a sample and can also cause changes in the sample during image acquisition. Due to their lower absorption, x-rays are more suitable for creating images in both of these areas than electrons. Moreover, compared to more conventional x-ray imaging methods, x-ray ptychography is also very efficient in its use of photons. In principle ptychography depends on monochromatic radiation, but there is nothing against repeating a ptychographic acquisition multiple times with different energies, such as done in spectroscopic ptychography. Since the reconstructed images are energy-dependent, this results in a whole spectrum of reconstructed values for each spatial pixel. This wealth of information can then be used to determine the chemical, elemental, and magnetic composition of materials with the same high spatial resolution.

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Abstract: In conventional x-ray ptychography, diffraction data are collected by scanning a sample through a monochromatic and spatially coherent x-ray beam. A high-resolution image is then retrieved using an iterative algorithm. Combined with a scan of the incident photon energy, it is also possible to access chemical and elemental information. Although powerful, the high brilliance required currently constrains the method to third and fourth generation synchrotron sources and long scanning times. An alternative approach is to use broadband illumination in combination with an energy resolving detector. These detectors record the data in a series of energy channels simultaneously, creating stacks of coherent data suitable for a ptychographic reconstruction. This approach promises to unlock the full power of the radiation source and provide spectral imaging at a higher rate and in a single acquisition. However, these detectors currently saturate well below reaching the flux rates produced at synchrotrons, which is preventing the uptake of this approach. Furthermore, current monochromatic synchrotron setups typically employ Fresnel zone plates for pre-sample focusing due to their stability, flexibility, and affordability, but these diffractive optics limit the spectral bandwidth that the setup can accept. In this article, we analyze the problem and consider alternative optics that can both maximize the total photon detection rates and broaden the tolerable bandwidth. Broadband x-ray ptychography has the potential to dramatically reduce data collection times at synchrotron sources but also to harness the full power of lower brilliance sources and transition x-ray ptychography into a laboratory technique.

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Abstract: How pigment distribution influences the cuticle density within a microscopic butterfly wing scale, and how both impact each scale’s final reflected color, remains unknown. We use ptychographic X-ray computed tomography to quantitatively determine, at nanoscale resolutions, the three-dimensional mass density of scales with pigmentation differences. By comparing cuticle densities between two pairs of scales with pigmentation differences, we determine that the density of the lower lamina is inversely correlated with pigmentation. In the upper lamina structure of Junonia orithya and Bicyclus anynana, low pigment levels also correlate with sheet-like chitin structures as opposed to rod-like structures. Within each scale, we determine that the lower lamina in all scales has the highest density, and distinct layers within the lower lamina help explain reflected color. We hypothesize that pigments, in addition to absorbing specific wavelengths, can affect cuticle polymerization, density, and refractive index, thereby impacting reflected wavelengths that produce colors.