This thesis entails a detailed microstructure analysis of directionally solidified (DS) Al-7Si alloys processed in microgravity aboard the International Space Station and similar duplicate ground based experiments at Cleveland State University. In recent years, the European Space Agency (ESA) has conducted experiments on alloy solidification in microgravity. NASA and ESA have collaborated for three DS experiments with Al- 7 wt. % Si alloy, aboard the International Space Station (ISS) denoted as MICAST6, MICAST7 and MICAST12. The first two experiments were processed on the ISS in 2009 and 2010. MICAST12 was processed aboard the ISS in the spring of 2014; the resulting experimental results of MICAST12 are not discussed in this thesis. The primary goal of the thesis was to understand the effect of convection in primary dendrite arm spacings (PDAS) and radial macrosegregation within DS aluminum alloys. The MICAST experiments were processed with various solidification speeds and thermal gradients to produce alloy with differences in microstructure features. PDAS and radial macrosegregation were measured in the solidified ingot that developed during the transition from one solidification speed to another. To represent PDAS in DS alloy in the presence of no convection, the Hunt-Lu model was used to represent diffusion-controlled growth. By sectioning cross-sections throughout the entire length of solidified samples, PDAS was measured and calculated. The ground-based (1-g) experiments done at Cleveland State University CSU were also analyzed for comparison to the ISS experiments (0-g). During steady state in the microgravity environment, there was a reasonable agreement between the measured and calculated PDAS. In ground-based experiments, transverse sections exhibited obvious radial macrosegregation caused by thermosolutal convection resulting in a non-agreement with the Hunt- Lu model. Using a combination of image processing techniques and Electron Microprobe Analysis, the extent of radial macrosegregation was found to be a function of processing conditions and PDAS.

This dissertation explores macrosegregation in directionally solidified aluminum castings. Two methods of interpolating thermocouple data are presented. A method using Lagrangian polynomials to interpolate thermocouple profiles is described and gives the best results for steady state furnace conditions. Using cubic splines to interpolate temperatures works best under transient conditions. A simple model, neglecting convection, is presented for predicting macrosegregation during melting, holding, and solidification of a sample and is compared with existing models. The model is able to accurately capture macrosegregation in microgravity experiments and is verified by experimental results. A two dimensional model of solidification, including convection, is presented and used to simulate samples grown in microgravity and terrestrially. The terrestrial samples exhibit steepling convection, while the microgravity samples do not. Causes of the steepling convection are explored and quantitative comparisons are made against experimental samples, with good agreement. The role of the furnace temperature profile is discussed and it is shown how it can be used to manipulate the steepling convection. Simulations of directional solidification through changes in cross section are presented for four experiments in graphite molds and one hypothetical experiment in an alumina mold. When solidifying through a contraction in cross section, the mold material is shown to have a strong influence on the convection and resulting macrosegregation. When solidifying out of an expansion, there is less of a difference between the two mold materials. Qualitative comparisons are made against experimentally obtained microstructures and good agreement is found. Stray grains were found, at the expansion, in some of the experimental samples and an explanation based on the results of the simulations is given.

L'objectif de c e mémoire est d'apporter une contribution à la modélisation et la simulation numérique de la convection thermosolutale de mélanges binaires de gaz parfaits contenus dans des cavités. Un modèle a été élaboré en se basant sur l'approximation de faible compressibilité. Le premier chapitre précise la démarche suivie dans la modélisation et une formulation originale en est déduite afin de traiter les différents types de conditions aux limites et de conditions de références hydrostatiques analysés dans le mémoire. Les variations de masse volumique sont déduites de la loi des gaz parfaits et la pression thermodynamique est calculée à partir de la conservation de la masse totale. La méthode numérique repose sur la méthode des volumes finis mise en uvre sur des maillages décalés. Le couplage vitesse-pression est traité par un nouvel algorithme dont l'efficacité est discutée en détail. La démarche numérique est validée via des comparaisons avec des solutions de références, en régime stationnaire comme en régime transitoire pour des écoulements transitionnels. Dans la seconde partie du mémoire, on considère d'abord la convection thermosolutale dans une cavité rectangulaire verticale dans le cas où les écoulements sont induits par des gradients horizontaux de température et de concentration. On discute en particulier les limites de l'approximation d'extrême dilution. La condensation de vapeur d'eau et l'évaporation d'un film d'eau liquide sur les parois d'une cavité sont ensuite étudiées en régime transitoire. Ces changements de phase surfaciques sont associés à la convection naturelle dans une cavité dont les températures des quatre parois varient au cours du temps / The aim of this dissertation is at modeling and numerically simulating thermosolutal convection within cavities filled by binary gas mixtures of ideal gases. A new problem formulation, based on the weakly compressible approximation, has been derived in order to account for the changes in density as well as thermodynamic pressure. The ideal gas law and global conservation of mass are invoked for the predictions of density field and thermodynamic pressure. The first part of the manuscript is devoted to the mathematical derivation of the governing equations and boundary conditions, numerical procedure implemented and, checks of the accuracy of the results through deeply comparisons with updated benchmark solutions. The emphasis is put on the efficiency of algorithm used for solving the pressure-velocity coupling for unsteady, transitional flow regimes. Thermosolutal convection without phase changes at the cavity surfaces is first considered in the second part of the manuscript. We investigated the case of vertical cavities with horizontal gradients of concentration and temperature. The results clearly show that the dilute approximation fails to be valid as soon as the maximum concentration difference exceeds a critical value function of the flow parameters. Surface condensation of water vapor or evaporation of liquid water film in vertical cavities is then considered. The specific thermal boundary conditions of uniform but time-varying temperatures of the four walls are considered. Periodic variations of the flow, temperature and relative humidity fields are discussed in detail. The evolutions of thicknesses of the water film over the four walls are also predicted

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The semiconducting compounds CdTe and CdZnTe have important applications in high-energy radiation detectors and as substrates for infrared devices. The materials offer large band gaps, high resistivity, and excellent charge transport properties; however all of these properties rely on very precise control of the material composition. Growing bulk crystals by the travelling heater method (THM) offers excellent compositional control and fewer defects compared to gradient freezing, but it is also much slower and more expensive. A particular challenge is the current need to grow new crystals onto existing seeds of similar size and quality. Simulations and experiments are used in this work to investigate the feasibility of growing these materials by THM without the use of large seed crystals. A new fixed-grid, multiphase finite element model was developed based on the level set method and used to calculate the mass transport regime and interface shapes inside the growth ampoule. The diffusivity of CdTe in liquid tellurium was measured through dissolution experiments, which also served to validate the model. Simulations of tapered THM growth find conditions similar to untapered growth with interface shapes that are sensitive to strong thermosolutal convection. Favourable growth conditions are achievable only if convection can be controlled. In preliminary experiments, tapered GaSb crystals were successfully grown by THM and large CdTe grains were produced by gradient freezing. Beginning with this seed material, 25 mm diameter CdTe and CdZnTe crystals were grown on 10 mm diameter seeds, and 65 mm diameter CdTe on 25 mm seeds. Unseeded THM growth was also investigated, as well as ampoule rotation and a range of thermal conditions and ampoule surface coatings. Outward growth beyond one or two centimeters was achieved only at small diameters and included secondary grains and twin defects; however, limited outward growth of larger seeds and agreement between experimental and numerical results suggest that tapered growth may be achievable in the future. This would require active temperature control at the base of the crystal and reduction of convection through thermal design or by rotation of the ampoule or applied magnetic fields. / Graduate / 0346 / 0794 / 0548 / jordan.roszmann@gmail.com