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Neurons comprise the main information processing cells of the nervous system. To integrate and transmit information, neurons elaborate dendritic structures to receive input and axons to relay that information to other cells. Due to their intricate structures, dendrites and axons are susceptible to damage whether by physical means or via disease mechanisms. Studying responses to axon injury, called Wallerian degeneration, in the neuronal processes of Drosophila melanogaster has allowed the identification of genes that are required for injury responses. Screens in Drosophila have identified dsarm and highwire as two genes required for axon degeneration; when these genes are mutated axons fail to degenerate after injury, even when completely cut off from the neuronal cell body. We found that these genes are also required for dendrite degeneration after injury in vivo. Further, we reveal differences between axon and dendrite injury responses using in vivo timelapse recordings and GCaMP indicators of intracellular and mitochondrial calcium transients. These data provide insights into the neuronal responses to injury, and better define novel targets for the treatment of neurodegenerative diseases.

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<p>Crystal growth has been recognized as a paradigm for non-equilibrium pattern formation for decades. Scientific interest in this field has focused on the growth rates and curvature of branches in snow flake-like structures patterned after a solid's crystallographic orientations. Similar patterns have been extensively identified in solidification of metals and organic metal analogues and are known as dendrites, which is originated from a Greek word "dendron" meaning tree.</p> <p>Dendritic spacing and morphology established during casting often sets the final microstructure and second phase formation that develops during manufacturing of alloys. This is particularly true in emerging technologies such as twin belt casting of aluminum alloys, where a reduced amount of thermomechanical processing reduced the possibility of modifying microstructure from that determined at the time of solidification. Predicting and controlling these microstructure of cast alloys has thus been a driving force behind various studies on solidification of materials.</p> <p>Mg-based alloys are another class of materials gaining importance due to the high demand for weight reduction in the transportation industry which accordingly reduces the gas consumption. While the solidified microstructure and its effect on the material properties have been the subject of intensive studies, little is known about the fundamental mechanisms that determine dendritic microstructure in Mg alloys and its evolution under directional growth conditions.</p> <p>This thesis investigates the relationship between the microstructure and cooling conditions in unsteady state upward directional solidification of Al-Cu and Mg-Al alloys. The four-fold symmetry of Al-Cu alloys are used to study the dynamical spacing selection between dendrites, as the growth conditions vary dynamically, whereas, the Mg-Al system with a six-fold symmetry is used to study a competition between neighbouring, misoriented grains and the effect of this as the resulting microstructure. Mg-Al also presents a situation wherein the cooling conditions dynamically vary from the preferred crystallographic growth direction. Analysis of phase field simulations is used to shed some light on the morphological development of dendrite arms during solidification under transient conditions. Our numerical results are compared to new casting experiments.</p> <p>Chapter three studies spacing selection in directional solidification of Al-Cu alloys under transient growth conditions. New experimental results are presented which reveal that the mean dendritic spacing versus solidification front speed exhibits plateau-like regions separated by regions of rapid change, consistent with previous experiments of Losert and co-workers. In fact, The primary spacing of a dendritic array grown under transient growth conditions displays a distribution of wavelengths. As the rate of change in solidification front velocity is decreased, the evolution of the spacing follows the prediction of the geometrical models within a band of spacing fluctuations. The width of the band is shown to highly depend on the rate of the solidification front velocity acceleration, such that the higher the rate, the wider the band of available spacings. Quantitative phase field simulations of directional solidification with dynamical growth conditions approximating those in the experiments confirm this behavior. The mechanism of this type of change in mean dendrite arm spacing is consistent with the notion that a driven periodically modulated interface must overcome an energy barrier before becoming unstable, in accord with a previous analytical stability analysis of Langer and co-workers.</p> <p>In chapter four, it is demonstrated both computationally and experimentally that a material's surface tension anisotropy can compete with anisotropies present in processing conditions during solidification to produce a continuous transition from dendritic microstructure morphology to so-called seaweed and fractal-like solidification microstructures. The phase space of such morphologies is characterized and the selection principles of the various morphologies explored are explained. These results have direct relevance to the microstructure and second phase formation in commercial lightweight metal casting.</p> / Doctor of Philosophy (PhD)

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Für den Zylinderkopf ist das Schwerkraftgießen ein etabliertes Gießverfahren. Jedoch gehen die Gießereien in der Ausführung des Schwerkraftgusses unterschiedlich vor. Durch die mannigfaltigen Anschnittsysteme und die sich dadurch ergebenden Vor- bzw. Nachteile bei der Herstellung entstehen Unterschiede bei Qualität und Kosten. Ziel dieser Arbeit ist es, Standards und Vereinheitlichungen während der Produktentstehung eines Zylinderkopfes zu etablieren, um eine gleichbleibende Qualität der Zylinderköpfe in den Gießereien zu gewährleisten. Dazu sind vielfältige Ansatzpunkte zu verfolgen. Eine Möglichkeit ist die geometrische Beurteilung des Zylinderkopfs, wie z.B. Wandstärken, Speisungswege und die Außengeometrien. Die nach Lastenheftvorgaben zu erfüllenden Eigenschaften spielen gleichermaßen eine Rolle und haben Einfluss auf die Wahl des Gießverfahrens. Mit Hilfe von speziellen Entscheidungsmethoden ist eine Vorauswahl für ein Gießverfahren möglich. Des Weiteren werden mittels experimentellen Untersuchungen die Entscheidungen gestützt. Die Gießsimulation ist als zusätzliches Auslegungswerkzeug einzusetzen. Hierbei sind Gussfehler im Bauteil zu lokalisieren und zu vermeiden. Unzureichende Speisungswege oder zu geringe Wandstärken durch komplizierte Kerngeometrien sind zu ermitteln. Des Weiteren sind Vorhersagen zu Dendritenarmabständen und Materialausnutzung (Speiserdimensionierung) möglich, die direkt mit der Wahl des Gießverfahrens zusammenhängen. Die Verzahnung von Geometrie- und Metallurgiefaktoren führt idealerweise zur Definition von Standardisierungsaspekten zur Auswahl der Gießtechnologie bei der Zylinderkopfentwicklung. Durch eine parallele Produkt- und Prozessentwicklung ist eine Verkürzung des Produktentstehungsprozesses erreichbar.

Digital image correlation techniques (DICT), a non-contact deformation measuring technique based on gray value digital images, have become increasingly used over the last years. By using the DIC technique during a tensile test, the deformation behavior of different engineering material under an applied load can be determined and analyzed. Digital images, acquired from a tensile test, can be correlated by using DICT software and from that the local or global mechanical properties can be calculated. The local or global mechanical properties determination of a flat test specimens are based on the displacements or changes in a previous stochastic sprayed or natural pattern. The used material for this purpose is cast silicon (Si) based aluminium (Al) component, designated as AlSi7Mg0.3 (Anticorodal-78 dv). The hypoeutectic Al- Si alloy is widely applicable for engine constructions, vehicle and aerospace constructions, shipbuilding, electrical engineering and constructions for food industry. There are many microstructural parameters in a binary system Al- Si alloys, which the mechanical properties can be depended on, for instance phase distribution, Secondary Dendrite Arm Spacing (SDAS), morphology of Si particles (Roundness) and microscopic defects or pores. All these parameters can contribute to enhance the proper mechanical performance (e.g. Strength and ductility) in the Al-Si cast components.