Conselho Nacional de Desenvolvimento Científico e Tecnológico, CNPq, Brasil

Cette thèse étudie l'effet de la ségrégation des solutés d’étain sur la formation de la texture de recristallisation dans les alliages ferritiques. La diffraction d’électrons rétrodiffusés (EBSD) et la sonde atomique tomographique ont été utilisées pour étudier respectivement le développement de la texture et la ségrégation locale des atomes de soluté. Des mesures d’EBSD in situ révèlent que l'hétérogénéité de la déformation dans la microstructure laminée à froid est un facteur crucial pour l’évolution au cours du recuit ultérieur, en particulier dans les alliages ternaires Fe-Si-Sn. L’ajout d’étain s'est avéré avoir un effet profond sur la texture de recuit. Il a été montré que Sn affecte principalement les phénomènes de recuit par interaction soluté-dislocation et ségrégation aux joints de grains. Des observations directes par sonde atomique tomographique à chaque étape de la recristallisation est discuté et un effet fort au stade de la germination de la recristallisation est mis en évidence. La sonde atomique tomographique combinée à la modélisation atomistique de la ségrégation à l’équilibre a permis de conclure que la ségrégation dépend de la désorientation. Néanmoins, la ségrégation du soluté dans les joints de grains à grand angle (joints de grains spéciaux et généraux) s'est avérée indépendante de leurs caractéristiques géométriques. Enfin, le développement de la texture peut s’expliquer par la théorie de la nucléation orientée de la recristallisation, alors que la présence de certaines interfaces mobiles pourrait également contribuer à la croissance orientée de certains grains recristallisés / This Ph.D. thesis investigates the effect of Sn solute segregation on the formation of recrystallisation texture in ferritic alloys. Both electron back-scatter diffraction and atom probe tomography were used to investigate the texture development and the local solute segregation respectively. In-situ electron back-scatter diffraction reveals that the strain heterogeneity in the deformed microstructure is a crucial factor for subsequent annealing, especially in the solute added alloys. Solute was found to have a profound effect on the annealing texture. Mainly, Sn was shown to affect the annealing phenomena via solute-dislocation interaction and grain boundary segregation. Direct observations with atom probe tomography reveal and quantify the levels of segregation at grain boundaries during the development of the recrystallised microstructure. The role of segregation at each stage of recrystallisation is discussed and a strong effect at the recrystallisation nucleation stage is suggested. Atom probe tomography results in combination with atomistic modelling of equilibrium segregation, concluded that the segregation depends on the misorientation. Nonetheless, the solute segregation in high-angle grain boundaries was found to be independent of their geometric characteristics (i.e. general, special grain boundaries). Finally, texture development could be explained in terms of the oriented nucleation theory of recrystallisation, while the presence of some mobile interfaces may subsequently also contribute in the oriented growth of some recrystallised grains

Induced pluripotent stem cells (iPSCs) are an attractive cell source for various applications in regenerative medicine and cell-based therapies given their unique capability to differentiate into any cell type of the human body. However, human iPSCs are highly vulnerable to cryopreservation with post-thaw survival rates of 40-60%; this is due to cryoinjury resulting from ice recrystallization when using conventional slow cooling protocols. Ice recrystallization is a process where the growth of large ice crystals occurs at the expense of small ice crystals. Ice recrystallization inhibitors (IRIs) are designed to inhibit the growth of intracellular ice crystals, increasing post-thaw viability. In this study, we tested a panel of four IRIs to determine if the inhibition of ice recrystallization can decrease cellular damage during freezing and improve viability post-thaw of iPSC colonies. We supplemented commercially available and serum-free cryopreservation medium mFreSR, routinely used for the cryopreservation of iPSCs, with a class of N-aryl-D-ß-gluconamide IRIs. A 2-fold increase in post-thaw viability was observed, in a dose dependent response, for N-(4-methoxyphenyl)-D-gluconamide (PMA) at 15 mM, N-(2-fluorophenyl)-D-gluconamide (2FA) at 10 mM, and N-(4-chlorophenyl)-D-gluconamide (4ClA) at 0.5 mM over mFreSR controls. After testing the panel of four IRIs, 2FA frozen iPSCs showed an increase in cell viability, proliferation, and recovery. The addition of ROCK inhibitor (RI), commonly used to increase iPSC viability post thaw, further enhanced the survival of the iPSCs frozen in the presence of 2FA and is used routinely in research. This additive effect increased cell recovery and colony formation post thaw, resulting in increased proliferation with no adverse effects on iPSC pluripotency or differentiation capabilities. The development of improved cryopreservation strategies for iPSCs is key to establishing master clonal cell banks and limiting cell selection pressures, all while maintaining high post-thaw viability and function. This will help ensure sufficient supplies of high-quality iPSC required to meet the cell demands for cell and regenerative based therapies. Since iPSCs hold promise as a potentially unlimited cell source for a plethora of cell-based therapies, improving cryopreservation is essential to the successful deployment of iPSC-derived therapeutic cell products in the future.

In the current study we developed an internal state variable (ISV) model based on the Bammann inelasticity internal state variable model (BIISV) to include damage, recrystallization, and texture development, which we then implemented into a mantle convection code, TERRA2D, to incorporate higher fidelity material behavior into mantle convection simulations. With experimental stress strain data found in the literature model constants for the BIISV model were determined for a number of geologic materials. The BIISV model was shown to be far superior to the steady state power law model currently used by the geologic community to capture the deformation of geologic materials. Once implemented and verified in TERRA2D the BIISV model revealed locations of hardened material that behaved like diverters in the cold thermal boundary layer that the power law model could never produce. These hardened regions could be a plausible reason for the current subduction zones present on the earth. We then altered the BIISV model equation to include the effects of damage, recrystallization, and texture development in order to model possible weakening mechanisms in the cold thermal boundary layer of the mantle. Inclusion of damage and recrystallization allowed the cold thermal boundary layer to mobilize and plunge downward into the hotter region below. Texture development increased the intensity of rotational flow within the hotter zone as cold boundary material plunged downward which aided in destabilizing the cold upper thermal boundary layer. The inclusion of an internal state variable model with damage, recrystallization, and texture development represents a significant advancement in handling deformational physics for mantle phenomena in a comprehensive, unified, and automatic manner.

Fluid-rock interaction causes an exchange of isotopes or elements through various reactions. The rate of these reactions strongly depends on temperature. The interaction involves dissolution precipitation, chemical exchange reactions, redox reactions, diffusion, and their combinations. The goal of studying fluid-rock interaction is to understand the change in mineral chemistry of the rock materials when in contact with an aqueous solution. These processes occur in all regions of the Earth where aqueous solutions are found. This work is comprised of three independent studies which provide an understanding about crystallization processes under multiple hydrothermal conditions with geological and environmental applications. In the chapter 1, subsurface rock and CO2-saturated brine reactions were evaluated under laboratory hydrothermal conditions when injected carbon dioxide is in contact with sedimentary strata at a planned sequestration sites at Kemper County Mississippi. Five rock samples were taken from different depths using core cuttings for experimentation. The results reveal no reaction of clay particles and CO2-rich fluid; in contrast, in samples from the depth of the unconformity, significant formation of secondary minerals occurred by reaction with the rock sample at the unconformity. The second study focuses on the incorporation of uranium (VI) into the crystal lattice of calcite at hydrothermal conditions. This study was designed to understand uranium (VI) behaviors in a calcite-fluid system at elevated temperatures due to decay of radioactive waste from nuclear power plants. The results showed uranyl hydroxide formation was preferred at hydrothermal conditions, 120 – 350 oC. The incorporation of U6+ in calcite lattices was evaluated, though the data showed a limited amount of U6+ entrapment. The third study focuses on quantification of the retention of Mg/Ca, Sr/Ca, and d18O during the aragonite-calcite transformation process as well as evaluation of the transformation rate. The results show partial retention of Mg and Sr during aragonite transformation to calcite in Mg-, Sr-free solutions, but no retention of d18O. Aragonite oxygen isotope composition was erased during mineral transformation because fractionation was controlled by temperature and the d18O of the bulk solution.

Redesigning heavy components with thinner components is one way to lower automotive weight and improve fuel efficiency. Therefore, replacing thick cast iron exhaust manifolds with thinner heat resistant stainless steel one is a prime example of this approach. Material for a thin exhaust manifold must tolerate cyclic thermal fatigue. In SUS 444, this characteristic is directly related to the influence of microstructure on high temperature strength and the stability of the microstructure at the high operating temperature range. The goal of this research is to identify the cause for the drastic difference in the stress-strain behavior between two potential manufacturer heat treatments that will serve as a simplified model case for high temperature cyclic fatigue.  Transmission electron microscopy (TEM) based microstructure analyses of samples which have been aged at 750"C for 100 hours and then hot-tensile tested at 750"C with a strain rate of   suggest continuous recrystallization as the mechanism responsible for the stable high temperature strength. The initial high temperature strength observed in the unaged sample was due to the precipitation of fine Laves phases which pinned down the motion of dislocations. As deformation progressed the strength increased until a critical precipitate size, volume fraction and dislocation density before Laves phases begun to rapidly coarsen and resulted in the abrupt decrease in strength. Microstructure evidence suggests the absence of precipitation strengthening effect in the aged samples could be a contributing factor to the decrease in peak strength between the aged samples and the unaged samples. / Master of Science

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