The polycrystalline nickel-base superalloys RR1000 and Udimet 720Li (U720Li) were developed for turbine disc applications. These alloys contain a higher volume fraction of the ordered γ' phase (close to 50%) when compared to previous generation alloys (~ 25%) in order to ensure that they retain high strength at operating temperatures exceeding 700°C. The increased percentage of precipitates in the material leads to higher levels of constraint between matrix and the precipitates, and this will have consequences for the deformation mechanisms of the aggregate. It is therefore important to understand how the increased volume fraction of precipitates affects the deformation behaviour of the material. This is not only crucial for the design of the optimum microstructure, but also for lifing models, which predict the lifetime of a component. It is the aim of the present work to improve the understanding of the deformation behaviour of these alloys by focussing on the influence of the γ' precipitate size. These alloys usually comprise a complex trimodal γ' size distribution, which complicates studies on the dependence of the deformation behaviour on the precipitate size. Hence, simplified model microstructures were generated here with a unimodal γ' size distribution. The model microstructures were subjected to in-situ loading experiments with neutron diffraction at temperatures of 20°C, 500°C and 750°C. Neutron diffraction measurements during loading revealed the elastic lattice strain response of both the γ and the γ' phases, which can indicate changes in their respective deformation behaviour. These measurements showed a load transfer from γ to γ' for some test conditions, which indicated that γ was able to deform with noticeably less deformation in the γ' phase. With a larger γ' precipitate size and/or higher test temperature, the tendency for load transfer increased. A post-mortem analysis of the deformed microstructures using advanced electron microscopy techniques (EBSD, ECCI, TEM) showed that shearing of the γ' precipitates dominated the deformed microstructures at 20°C and 500°C and was also observed after deformation at 750°C. Deformation was less localised in the microstructures with large γ' precipitates, which might be correlated with the increased trend for load transfer. The onset of multiple slip or the activation of Orowan looping as an additional deformation mechanism are suggested as possible explanations for these observations.

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<p>Transverse anisotropy is an important phenomenon of practical and scientific interest. Although the presence of ray tissue explains the high radial modulus in many hardwoods, experimental data in the literature shows that this is not the case for pine. It is possible that anisotropy in softwoods may be explained by the cellular structure and associated deformation mechanisms.</p><p>An experimental approach was developed by which local radial modulus in spruce was determined at sub-annual ring scale. Digital speckle photography (DSP) was used, and the density distribution was carefully characterized using x-ray densitometry and the SilviScan apparatus. A unique set of data was generated for radial modulus versus a wide range of densities. This was possible since earlywood density shows large density variations in spruce. Qualitative comparison was made between data and predictions from stretching and bending honeycomb models. The hypothesis for presence of cell wall stretching was supported by data.</p><p>A model for wood was therefore developed where both cell wall bending and stretching are included. The purpose was a model for predictions of softwood moduli over a wide range of densities. The relative importance of the deformation mechanisms was investigated in a parametric study. A two-phase model was developed and radial and tangential moduli were predicted. Comparison with experimental data showed good agreement considering the nature of the model (density is the only input parameter). Agreement is much better than for a regular honeycomb model. According to the model, cell wall bending dominates at both low and high densities during tangential loading. In radial loading, cell wall stretching dominates at higher densities.</p>

Transverse anisotropy is an important phenomenon of practical and scientific interest. Although the presence of ray tissue explains the high radial modulus in many hardwoods, experimental data in the literature shows that this is not the case for pine. It is possible that anisotropy in softwoods may be explained by the cellular structure and associated deformation mechanisms. An experimental approach was developed by which local radial modulus in spruce was determined at sub-annual ring scale. Digital speckle photography (DSP) was used, and the density distribution was carefully characterized using x-ray densitometry and the SilviScan apparatus. A unique set of data was generated for radial modulus versus a wide range of densities. This was possible since earlywood density shows large density variations in spruce. Qualitative comparison was made between data and predictions from stretching and bending honeycomb models. The hypothesis for presence of cell wall stretching was supported by data. A model for wood was therefore developed where both cell wall bending and stretching are included. The purpose was a model for predictions of softwood moduli over a wide range of densities. The relative importance of the deformation mechanisms was investigated in a parametric study. A two-phase model was developed and radial and tangential moduli were predicted. Comparison with experimental data showed good agreement considering the nature of the model (density is the only input parameter). Agreement is much better than for a regular honeycomb model. According to the model, cell wall bending dominates at both low and high densities during tangential loading. In radial loading, cell wall stretching dominates at higher densities. / QC 20101119

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The current research is part of a wider experimental program on creep modeling of glass mat reinforced polypropylene composites which are increasingly being used in molding automotive parts. This specific study is focused on the dimensional and thermal stability of chopped fibre mat and long fibre mat composites. The objective of the study is two-fold. First, to characterize in-situ the micro-failure mechanisms associated with damage accumulation during creep at room temperature and at service temperature (80°C) for stresses up to 67% of the ultimate tensile strength. Second, to characterize the effects of prolonged exposure at elevated temperature on the crystallinity and chemical degradation of the polypropylene matrix. In the first part of the investigation, micro-failure mechanisms including fibre-matrix interface, matrix yielding and cracking during the creep process have been captured in-situ using reflection microscopy. Specimens with 12 mm gauge length were mounted onto a Minimat tensile tester. The applied stress levels of interest were 33% and 67% of the ultimate tensile strength (UTS) at room temperature (RT) and high temperature (HT), respectively. It was found that the deformation mechanisms do not change with temperature but creep in the chopped fibre material is substantially higher than that in the long-fibre. Creep deformation is typically associated with multiple transverse crack initiation at the fibre-matrix interface, crack crazing and rapid coalescence of the small cracks leading to abrupt fracture. Debonding of the fibres is usually detected at the loading stage of the test but fibre breakage is minimal even at high temperature. The change in creep strain at room temperature is similar for both composites but creep strains are highly sensitive to the fibre-mat type at higher temperature. Long-fibre mat structures offer greater creep resistance. Micro-indentations on the matrix-rich regions showed elongation along the loading direction but shear yielding (distortion of indentations) was not noticeable. Using scanning electron microscopy (SEM), the fibre pullout was observed to be pronounced thus suggesting poor adhesion at the fibre-matrix interface. In the second part of this study, the effects of elevated temperature aging on the microstructural changes of isotactic polypropylene matrix in a composite have been studied using wide-angle X-ray scattering (WAXS) and Fourier-transform infrared spectroscopy (FTIR). The objective was to quantify small and slow changes in crystallinity due to thermal aging. To minimize sample variability, polypropylene resin was extracted from the molded composite plaque. Changes in crystallinity level and crystalline form were detected using WAXS after prolonged aging at 90 and 140 °C. FTIR was utilized to monitor in-situ crystallinity changes and to detect oxidation products due to thermal decomposition. The level of crystallinity was monitored by changes in the absorbance ratio of A997/A973 and A841/A973; the former ratio was found to be more sensitive for detecting crystallinity changes. Aging at 140°C resulted in oxidation. The kinetics of secondary crystallization for the aging conditions studied was characterized using Avrami plots.

The current research is part of a wider experimental program on creep modeling of glass mat reinforced polypropylene composites which are increasingly being used in molding automotive parts. This specific study is focused on the dimensional and thermal stability of chopped fibre mat and long fibre mat composites. The objective of the study is two-fold. First, to characterize in-situ the micro-failure mechanisms associated with damage accumulation during creep at room temperature and at service temperature (80°C) for stresses up to 67% of the ultimate tensile strength. Second, to characterize the effects of prolonged exposure at elevated temperature on the crystallinity and chemical degradation of the polypropylene matrix. In the first part of the investigation, micro-failure mechanisms including fibre-matrix interface, matrix yielding and cracking during the creep process have been captured in-situ using reflection microscopy. Specimens with 12 mm gauge length were mounted onto a Minimat tensile tester. The applied stress levels of interest were 33% and 67% of the ultimate tensile strength (UTS) at room temperature (RT) and high temperature (HT), respectively. It was found that the deformation mechanisms do not change with temperature but creep in the chopped fibre material is substantially higher than that in the long-fibre. Creep deformation is typically associated with multiple transverse crack initiation at the fibre-matrix interface, crack crazing and rapid coalescence of the small cracks leading to abrupt fracture. Debonding of the fibres is usually detected at the loading stage of the test but fibre breakage is minimal even at high temperature. The change in creep strain at room temperature is similar for both composites but creep strains are highly sensitive to the fibre-mat type at higher temperature. Long-fibre mat structures offer greater creep resistance. Micro-indentations on the matrix-rich regions showed elongation along the loading direction but shear yielding (distortion of indentations) was not noticeable. Using scanning electron microscopy (SEM), the fibre pullout was observed to be pronounced thus suggesting poor adhesion at the fibre-matrix interface. In the second part of this study, the effects of elevated temperature aging on the microstructural changes of isotactic polypropylene matrix in a composite have been studied using wide-angle X-ray scattering (WAXS) and Fourier-transform infrared spectroscopy (FTIR). The objective was to quantify small and slow changes in crystallinity due to thermal aging. To minimize sample variability, polypropylene resin was extracted from the molded composite plaque. Changes in crystallinity level and crystalline form were detected using WAXS after prolonged aging at 90 and 140 °C. FTIR was utilized to monitor in-situ crystallinity changes and to detect oxidation products due to thermal decomposition. The level of crystallinity was monitored by changes in the absorbance ratio of A997/A973 and A841/A973; the former ratio was found to be more sensitive for detecting crystallinity changes. Aging at 140°C resulted in oxidation. The kinetics of secondary crystallization for the aging conditions studied was characterized using Avrami plots.

Le «pancréas bioartificiel» (ou MAILPAN pour Macro-encapsulation d’ILots PANcréatiques) en développement par la start-up Defymed est un implant médical destiné aux patients atteints de diabète de type I. L’élément-clé de cet implant est une membrane poreuse qui a pour fonction d’assurer une certaine sélectivité moléculaire. De ce fait, une fissuration ou rupture de cette membrane entrainerait la perte de ses fonctionnalités. Il est par conséquent indispensable d’analyser et de comprendre le comportement mécanique de ce matériau afin de garantir son intégrité tout au long de la période d’implantation. Cette thèse s’inscrit dans le projet FUI MECABARP regroupant plusieurs PMEs et laboratoires lorrains et alsaciens.La membrane est un matériau unique obtenu par lamination de plusieurs matériaux polymères poreux. Elle se compose de films rendus poreux par le procédé de «track-etching» ainsi que de non-tissé consolidé par calandrage à picots. Cette thèse a pour objectif d’en étudier les mécanismes de déformation par l’utilisation de techniques de caractérisation et d’imagerie in situ à un essai de traction. Des campagnes expérimentales de micro-tomographie à rayons X et de diffusion de rayons X aux grands angles et petits angles ont été menées sur lignes haute énergie. Ces résultats sont complétés par des essais en laboratoire de microscopie électronique et spectroscopie Raman in situ à un essai de traction. La complémentarité des techniques choisies permet une approche multi-échelles (du millimètre à l’angström) dans le but d’obtenir l’étude la plus complète possible. Les faiblesses de la membrane d'un point de vue mécanique sont mises en avant et des solutions sont proposées. En parallèle, un essai mécanique en gonflement («bulge test») est développé dans l’optique de fournir un chemin de déformation équibiaxial plus proche des sollicitations réelles / The "bioartificial pancreas" (named MAILPAN for Macro-encapsulation d’ILots PANcréatiques) developed by the startup company Defymed is an implantable device for patients diagnosed with type I diabetes. The core element of the device is a porous membrane providing molecular selectivity. The emergence of cracking in this membrane would lead to the loss of its selective properties. As a consequence, it is crucial to study and understand the mechanical behavior of this material in order to ensure its integrity during the lifetime of the device. This thesis is a part of the FUI MECABARP project, gathering together SMEs and laboratories from the Lorraine and Alsace regions. The membrane is a unique material obtained by laminating several porous polymer materials. It is made of porous “track-etched” films as well as thermal-spot bonded nonwovens. The objective is to study its deformation mechanisms using time-resolved imaging and characterization techniques during a tensile test. X-ray micro-tomography and wide- and small-angle X-ray scattering experiments were performed on high energy beamlines. These results were supplemented with time-resolved scanning electron microscopy and Raman spectroscopy experiments during a tensile test. The synergy of the chosen techniques enables a multi-scale approach (from millimeter to angström) in order to obtain the most comprehensive analysis. Solutions are suggested to improve the mechanical properties of the membrane. Besides, a mechanical testing device by inflation (“bulge test”) has been designed to provide an equibiaxial mechanical path that is closer to the actual demands

Title: Investigation of deformation mechanisms in textured magnesium alloy by advanced in-situ methods Author: Bc. Jan Dittrich Department: Department of Physics of Materials Supervisor: RNDr. Peter Minárik, PhD., Department of Physics of Materials Abstract: This thesis aims to investigate the correlation between texture and the activation of particular deformation mechanisms during the deformation of a rolled magnesium alloy AZ31. A combination of advanced in-situ techniques, providing complementary information about the processes within the material during its deformation, was employed to achieve this goal. The combination of neutron diraction and acoustic emission measurements allowed to investigate both rapid and continuous processes related to changes of the material microstructure resulting from its deformation. The in-situ loading of the sample inside the chamber of a scanning electron microscope, coupled with the electron backscatter diraction analysis, provided more direct observations of the microstructural evolution. Furthermore, the high-speed camera imaging of the deformed sample surface enabled a direct, real-time view of the occurring rapid processes. The results of the experiments conrmed the anisotropy of the mechanical behaviour of samples oriented diversely with respect to the...

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