abstract: Al 7075 alloys are used in a variety of structural applications, such as aircraft wings, automotive components, fuselage, spacecraft, missiles, etc. The mechanical and corrosion behavior of these alloys are dependent on their microstructure and the environment. Therefore, a comprehensive study on microstructural characterization and stress-environment interaction is necessary. Traditionally, 2D techniques have been used to characterize microstructure, which are inaccurate and inadequate since the research has shown that the results obtained in the bulk are different from those obtained on the surface. There now exist several techniques in 3D, which can be used to characterize the microstructure. Al 7075 alloys contain second phase particles which can be classified as Fe-bearing inclusions, Si-bearing inclusions and precipitates. The variation in mechanical and corrosion properties of aluminum alloys has been attributed to the size, shape, distribution, corrosion properties and mechanical behavior of these precipitates and constituent particles. Therefore, in order to understand the performance of Al 7075 alloys, it is critical to investigate the size and distribution of inclusions and precipitates in the alloys along with their mechanical properties, such as Young's modulus, hardness and stress-strain behavior. X-ray tomography and FIB tomography were used to visualize and quantify the microstructure of constituent particles (inclusions) and precipitates, respectively. Microscale mechanical characterization techniques, such as nanoindentation and micropillar compression, were used to obtain mechanical properties of inclusions. Over the years, studies have used surface measurements to understand corrosion behavior of materials. More recently, in situ mechanical testing has become more attractive and advantageous, as it enables visualization and quantification of microstructural changes as a function of time (4D). In this study, in situ X-ray synchrotron tomography was used to study the SCC behavior of Al 7075 alloys in moisture and deionized water. Furthermore, experiments were performed in EXCO solution to study the effect of applied stress on exfoliation behavior in 3D. Contrary to 2D measurements made at the surface which suggest non-uniform crack growth rates, three dimensional measurements of the crack length led to a much more accurate measurement of crack growth rates. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2015

abstract: Energy return in footwear is associated with the damping behavior of midsole foams, which stems from the combination of cellular structure and polymeric material behavior. Recently, traditional ethyl vinyl acetate (EVA) foams have been replaced by BOOST(TM) foams, thereby reducing the energetic cost of running. These are bead foams made from expanded thermoplastic polyurethane (eTPU), which have a multi-scale structure consisting of fused porous beads, at the meso-scale, and thousands of small closed cells within the beads at the micro-scale. Existing predictive models coarsely describe the macroscopic behavior but do not take into account strain localizations and microstructural heterogeneities. Thus, enhancement in material performance and optimization requires a comprehensive understanding of the foam’s cellular structure at all length scales and its influence on mechanical response. This dissertation focused on characterization and deformation behavior of eTPU bead foams with a unique graded cell structure at the micro and meso-scale. The evolution of the foam structure during compression was studied using a combination of in situ lab scale and synchrotron x-ray tomography using a four-dimensional (4D, deformation + time) approach. A digital volume correlation (DVC) method was developed to elucidate the role of cell structure on local deformation mechanisms. The overall mechanical response was also studied ex situ to probe the effect of cell size distribution on the force-deflection behavior. The radial variation in porosity and ligament thickness profoundly influenced the global mechanical behavior. The correlation of changes in void size and shape helped in identifying potentially weak regions in the microstructure. Strain maps showed the initiation of failure in cell structure and it was found to be influenced by the heterogeneities around the immediate neighbors in a cluster of voids. Poisson’s ratio evaluated from DVC was related to the microstructure of the bead foams. The 4D approach taken here provided an in depth and mechanistic understanding of the material behavior, both at the bead and plate levels, that will be invaluable in designing the next generation of high-performance footwear. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

<div>Metallic uranium-10 weight percent zirconium (U-10wt.%Zr) nuclear fuels are classified as potential fuels for fast breeder reactors as they possess a high fissile density and have increased compatibility with sodium, a frequently used reactor coolant. Despite their advantages when exposed to neutron irradiation in reactors, the fuels are subject to damage cascades and microstructural alterations. Fuel constituent re-distribution, phase transformation, fuel swelling, and fuel cladding chemical interactions (FCCI) are a few of the major interdependent microstructural alterations that occur in these fuels at the onset of neutron irradiation. The primary objective of this research is to understand the above-mentioned microstructural alterations in different regions of a neutron irradiated U-10wt.%Zr fuel and HT9 cladding that has achieved a cross-sectional burnup of 5.7 atomic percent (at%.). Additionally, this study also aims to provide a relationship between the microstructural alterations and local mechanical property changes exhibited at different regions of the HT9 cladding as a consequence of neutron irradiation, FCCI, and fission product migration.</div><div>To achieve this goal, a coordinated group of experiments was performed on the neutron irradiated U-10wt.%Zr/HT9 (fuel/cladding) at the nanoscale, microscale, and mesoscale, respectively. The experimental techniques used for microstructural analysis included the following: (1) transmission electron microscopy of focused ion beam (FIB) lamellas for nanoscale assessments, (2) serial sectioning of FIB cuboids for microscale assessments, and (3) synchrotron micro-computed tomography of FIB obelisks for mesoscale assessment. Following the microstructural assessments, nano-indentation experiments were performed on the neutron irradiated HT9 cladding to determine the changes in mechanical properties as a function of distance from cladding edge to FCCI locality, and the changes in mechanical properties as a consequence of several microstructural alterations. Furthermore, the results produced from the various experiments in this study were compared and correlated to existing literature (both in-reactor and out-of-reactor experiments), and new theories to explain the reason for the observed changes were established. This research also revealed several novel observations such as probable radiation induced segregation in fuels, localized fuel swelling and porosity distribution at different regions in the fuel, crystal structure of phases present at different regions in the fuel and their influence on pore morphologies, and nano mechanical properties of a neutron irradiated HT9 cladding.</div>

<p> Monolithic uranium molybdenum alloys (U-Mo) are considered a candidate for converting high-performance research and test reactors from high-enriched uranium to low enrichment alternatives. The metallic fuel is capable of conversion due to the high U loading and favorable radiation performance. During irradiation, the fuel undergoes a three-part swelling behavior, with an initial linear swelling rate followed by an increase in the swelling rate represented by an increase in the nucleation of fission-gas bubbles, and ending with stabilization at the highest fission densities. Understanding the high burnup regime is critical to extending the life of the reactor and creating accurate fuel performance models. To accurately inform swelling models, it is necessary to experimentally characterize the pore evolution as a function of burnup and the influence of diffusion barrier-fuel interaction on the morphology. Therefore, a systematic approach was conducted to experimentally characterize the influence of irradiation and fuel-diffusion barrier interaction on the pore morphology and then empirically model the porosity evolution. Initially, three separate locations in a monolithic U-Mo fuel plate with burnups ranging from 8.9-9.4x1021 fissions/cm3 were investigated using scanning electron micrography (SEM) to characterize the morphological porosity dependence on fission density. To investigate the impact of the Zr-fuel interface on the pore morphology, two specimens were imaged using synchrotron microcomputed tomography (Sr-µCT) from a U-Mo monolithic miniplate irradiated to 9.8x1021 fissions/cm3, one at the diffusion barrier and one in the bulk fuel. Synchrotron microcomputed tomography allows for the characterization of the influence of fuel-Zr diffusion barrier interaction on the pore morphology in three dimensions; however, due to the novelty of this technique applied to nuclear fuels the results were verified with SEM serial sectioning. The multimodal comparison between the Sr-µCT and SEM serial sectioning allows for a direct assessment of the capabilities of each technique for nuclear fuel applications. Due to the complex microstructure and imaging challenges in analyzing these samples, several model-based image processing and characterization tools were developed to aid in the analysis. An empirical model for porosity evolution in high-burnup U-Mo was developed and accurately modeled the porosity behavior. The experimental results from the current work and the empirical model developed can be used to inform mechanistic modeling efforts in the community. </p>

The impact of the end-Devonian mass extinction (the Hangenberg extinction) on tetrapods is not well understood. One issue is that we have very little knowledge of the Devonian tetrapod fauna that immediately preceded, and experienced, the mass extinction. New specimens from an early tetrapod bone bed of the latest Famennian, Stensiö Bjerg Formation of Celsius Bjerg, East Greenland have the potential to shed light on this problem. In this study, five new well-preserved specimens are presented: a partial skull, two humeri, a left pelvis, and a strange vertebral element. The specimens were imaged using propagation phase-contrast synchrotron microtomography (PPC-SRμCT), and then virtually segmented and rendered.  The partial skull and left pelvis share many similarities with Ichthyostega and Acanthostega, but also have some distinctive features. Notably, the epipterygoid does not articulate with the skull roof, the fenestra vestibuli appears to have an anterior lobe, the postorbital has a posteroventral process, and there is a sharp contrast between the rugose dermal ornament of the skull roof and the unornamented cheek. In addition, the partial skull is box-like in shape and has a laterally facing orbit, which are features seen in some Carboniferous tetrapods. Among other characters, the left pelvis has a differently curving posterior iliac process, and differently shaped ischium and anterior pubic margin compared to Ichthyostega and Acanthostega. The acetabulum of the left pelvis is also lacking a posterior notch, a feature seen in many other early tetrapods. The humeri presented in this thesis are curiously similar to the isolated tetrapod humerus from the Catskill Formation, Pennsylvania, USA known as ANSP 21350. In fact, they are more similar to each other and to ANSP 21350 than to any other early tetrapod humeri. This is in large part due to a distinctly distal supinator ridge which was previously only known from ANSP 21350. The mysterious vertebral element does not resemble any known early tetrapod bone due to its large, ventral, concave saddle-shaped projections. It consists of five fused vertebrae and is interpreted here to be a unique sacrum of an early tetrapod. The specimens described and interpreted in this study represent at least two new species, none of which can be assigned to known Devonian early tetrapods from Greenland. It is clear that a new and important faunal assemblage is emerging.

Die Aufklärung des kooperativen Materialtransports während des Sinterns von Metallen zielt auf eine verbesserte Vorhersage und theoretische Beschreibung des Sinterprozesses ab. Dem kooperativen Materialtransport wird in der Literatur ein signifikanter Beitrag zur Gesamtschwindung zugeschrieben. Lose oder gepresste Pulverschüttungen weisen unter Wärmebehandlung eine Volumenabnahme auf. Neben der Zentrumsannäherung und dem Wachstum der Sinterkontakte rührt diese Schwindung von spannungsbedingten Relativbewegungen der Partikel in Form von Translationen und insbesondere von Rotationen her. Die Sinterprozesse wurden bereits seit den 1940er Jahren für Zwei-Teilchen-Modelle, sowie für 1- und 2-dimensionale Versuchsanordnungen ausführlichen Analysen unterzogen. Die gewonnenen Erkenntnisse lassen sich aber nur bedingt auf 3-dimensionale Proben übertragen. Berechnungen der Gesamtschwindung basieren auf dem Zwei-Teilchen-Modell auf und berücksichtigen den kooperativen Materialtransport nur in unzureichendem Maße, so dass eine Diskrepanz zwischen berechneter und an realen Sinterkörpern beobachteter Schwindung auftritt. In dieser Arbeit wurde der kooperative Materialtransport im Sinteranfangsstadium (bis 1050°C) am Modellsystem Kupfer mittels Synchrotron-Computertomographie an 3-dimensionalen Proben untersucht. Die Tomographie bietet als erste Methode die Möglichkeit, Partikelbewegungen in 3-dimensionalen Proben über mehrere Sinterschritte hinweg zerstörungsfrei zu untersuchen und quantitative Daten zu erhalten. Auf diese Weise konnten die Bewegungen eines jeden Partikels in-situ über den Sinterverlauf verfolgt werden. Zur Bestimmung der Orientierungsänderung der sphärischen Partikel war es notwendig, die Partikel mittels Focused Ion Beam mit Markierungen zu versehen. Die Detektion und Auswertung der Partikelbewegung erfolgte über eine eigens entwickelte Analysesoftware basierend auf photogrammetrischen Methoden. Diese Software erlaubt es, Aussagen über Rotationsgeschwindigkeit und Rotationswinkel der Partikel sowie über die Dichte, Zentrumsannäherung, Änderung der Koordinationszahl und Gesamtgröße zu treffen. Neben der bereits in der Literatur dargestellten Rotation der Partikel in Bezug auf dessen Kontaktpartner konnte zusätzlich eine Eigenrotation um den eigenen Schwerpunkt festgestellt werden. Es wurde ein neues Modell entwickelt, das neben Diffusionsmechanismen auch Korngrenzengleiten zulässt und damit Eigenrotationen ermöglicht. / The investigation of the cooperative material transport of metals during sintering aims to improve the theoretical descriptions of the sintering process. In literature the cooperative material transport is ascribed to have a significant contribution to the total shrinkage. Loose or pressed powder packed beds show a volume shrinkage during heat treatment. This shrinkage is caused by centre approach and the growth of sintering necks but also results of particle movements like translations and particularly rotations. The sintering process was investigated in detail since the 1940’s on the basis of the two particle model as well as on 1- and 2- dimensional test arrangements. The obtained data can be transferred to 3-dimensional specimens only to a limited extent. Calculations of the total shrinkage are based on the two particle model and do hardly consider the cooperative material transport. That is why there is a difference between calculated and measured shrinkage in real sinter compacts. In this study the cooperative material transport was investigated in the initial sintering stage (up to 1050°C) in the model system copper. These measurements were conducted on 3-dimensional samples using synchrotron computed tomography. Tomography offers for the first time the opportunity to investigate particle movements in 3-dimensional samples non-destructively during several sintering steps to obtain quantitative data. Using this method it was possible to track the particle movements during the sintering process in-situ. To determine orientation rearrangements of spherical particles it was necessary to mark each particle by focused ion beam with a borehole. The detection and analysis of particle movements was carried out by custom image analysis software using photogrammetric methods. This software is able to determine the rotation speed and angle of particles, the local density, the centre approach, the change of the coordination number and the total size of a specimen. Beside the in literature described particle rotation in relation to contact partners an additional intrinsic rotation around the centre of mass of a particle was discovered. A new model was developed which permits not only diffusion mechanisms but also grain boundary sliding and therefore allows intrinsic rotations.

Dans le contexte actuel, où la préservation des ressources et le développement durable sont devenus des enjeux importants de ce siècle, la production de matériaux tant performants que respectueux de l’environnement est devenue indispensable. Ainsi, ce travail de thèse porte sur l’élaboration de matrices poreuses de biopolymères en utilisant la technologie CO2 supercritique (CO2-SC). L’utilisation du PLA permet de produire des matrices 100% biosourcées et biodégradables, alors que l’utilisation de CO2-SC permet de diminuer l’impact écologique des procédés de mise en forme. Dans un premier temps une étude paramétrique de la mise en forme de matrice de PLA par une méthode de séparation de phase thermique (TIPS) couplée à un séchage par CO2 a été réalisée. Elle a permis de produire des matrices microcellulaires de faible densité (entre 60 et 320 kg/m3) et aux propriétés structurales mécaniques modulables. L’ensemble du procédé a fait l’objet d’une analyse de cycle de vie et il a été démontré que l’utilisation du CO2-SC en remplacement de la lyophilisation a réduit d’entre 50 et 90% l’impact environnemental. Dans un second temps une étude in-situ de la séparation de phase par tomographie-X en rayonnement synchrotron a permis de mieux comprendre la mécanistique de notre procédé. Enfin, la dernière partie de ce travail a été consacré à la mise en forme de matrice de PLA sans solvant, en utilisant le CO2-SC comme agent gonflant. Les résultats obtenus ont servi à réaliser une étude comparative des deux procédés développés. / In the present context, where the preservation of resources and sustainable development became the main issues of this century, the production of more efficient and environmentally friendly materials is essential. Thus, this work deals with thedevelopment of biobased polymeric porous matrix using SC-CO2. The use of PLA makes it possible to produce 100% biosourced and biodegradable matrices, while the use of CO2-SC reduces the ecological impact of the shaping processes. In a first step, a parametric study of PLA matrix shaping by a thermal induced phase separation (TIPS) method coupled to CO2 drying was performed. Low density microcellular matrices were obtained with tunable structural and mechanical properties. The whole process was analyze by life cycle assessment and the results showed that SC-CO2 replacing freeze drying has reduced the environmental impact between 50 and 90%. Secondly, a phase separation in situ study by tomography-X synchrotron radiation tomography allowed us to better understand the mechanics of our process. Finally, the last part of this work was devoted to the implementation of a solvent free process, using SC-CO2 as a blowing agent. The results obtained were used to carry out a comparative study of the two processes developed.

Die Aufklärung des kooperativen Materialtransports während des Sinterns von Metallen zielt auf eine verbesserte Vorhersage und theoretische Beschreibung des Sinterprozesses ab. Dem kooperativen Materialtransport wird in der Literatur ein signifikanter Beitrag zur Gesamtschwindung zugeschrieben. Lose oder gepresste Pulverschüttungen weisen unter Wärmebehandlung eine Volumenabnahme auf. Neben der Zentrumsannäherung und dem Wachstum der Sinterkontakte rührt diese Schwindung von spannungsbedingten Relativbewegungen der Partikel in Form von Translationen und insbesondere von Rotationen her. Die Sinterprozesse wurden bereits seit den 1940er Jahren für Zwei-Teilchen-Modelle, sowie für 1- und 2-dimensionale Versuchsanordnungen ausführlichen Analysen unterzogen. Die gewonnenen Erkenntnisse lassen sich aber nur bedingt auf 3-dimensionale Proben übertragen. Berechnungen der Gesamtschwindung basieren auf dem Zwei-Teilchen-Modell auf und berücksichtigen den kooperativen Materialtransport nur in unzureichendem Maße, so dass eine Diskrepanz zwischen berechneter und an realen Sinterkörpern beobachteter Schwindung auftritt. In dieser Arbeit wurde der kooperative Materialtransport im Sinteranfangsstadium (bis 1050°C) am Modellsystem Kupfer mittels Synchrotron-Computertomographie an 3-dimensionalen Proben untersucht. Die Tomographie bietet als erste Methode die Möglichkeit, Partikelbewegungen in 3-dimensionalen Proben über mehrere Sinterschritte hinweg zerstörungsfrei zu untersuchen und quantitative Daten zu erhalten. Auf diese Weise konnten die Bewegungen eines jeden Partikels in-situ über den Sinterverlauf verfolgt werden. Zur Bestimmung der Orientierungsänderung der sphärischen Partikel war es notwendig, die Partikel mittels Focused Ion Beam mit Markierungen zu versehen. Die Detektion und Auswertung der Partikelbewegung erfolgte über eine eigens entwickelte Analysesoftware basierend auf photogrammetrischen Methoden. Diese Software erlaubt es, Aussagen über Rotationsgeschwindigkeit und Rotationswinkel der Partikel sowie über die Dichte, Zentrumsannäherung, Änderung der Koordinationszahl und Gesamtgröße zu treffen. Neben der bereits in der Literatur dargestellten Rotation der Partikel in Bezug auf dessen Kontaktpartner konnte zusätzlich eine Eigenrotation um den eigenen Schwerpunkt festgestellt werden. Es wurde ein neues Modell entwickelt, das neben Diffusionsmechanismen auch Korngrenzengleiten zulässt und damit Eigenrotationen ermöglicht. / The investigation of the cooperative material transport of metals during sintering aims to improve the theoretical descriptions of the sintering process. In literature the cooperative material transport is ascribed to have a significant contribution to the total shrinkage. Loose or pressed powder packed beds show a volume shrinkage during heat treatment. This shrinkage is caused by centre approach and the growth of sintering necks but also results of particle movements like translations and particularly rotations. The sintering process was investigated in detail since the 1940’s on the basis of the two particle model as well as on 1- and 2- dimensional test arrangements. The obtained data can be transferred to 3-dimensional specimens only to a limited extent. Calculations of the total shrinkage are based on the two particle model and do hardly consider the cooperative material transport. That is why there is a difference between calculated and measured shrinkage in real sinter compacts. In this study the cooperative material transport was investigated in the initial sintering stage (up to 1050°C) in the model system copper. These measurements were conducted on 3-dimensional samples using synchrotron computed tomography. Tomography offers for the first time the opportunity to investigate particle movements in 3-dimensional samples non-destructively during several sintering steps to obtain quantitative data. Using this method it was possible to track the particle movements during the sintering process in-situ. To determine orientation rearrangements of spherical particles it was necessary to mark each particle by focused ion beam with a borehole. The detection and analysis of particle movements was carried out by custom image analysis software using photogrammetric methods. This software is able to determine the rotation speed and angle of particles, the local density, the centre approach, the change of the coordination number and the total size of a specimen. Beside the in literature described particle rotation in relation to contact partners an additional intrinsic rotation around the centre of mass of a particle was discovered. A new model was developed which permits not only diffusion mechanisms but also grain boundary sliding and therefore allows intrinsic rotations.