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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, th

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, th

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.

A 3D microscale solidification model, resolving complex interaction of solute partition and diffusion, interdendritic thermosolutal convection, dendrite formation and remelting, was used to study freckle formation for unidirectional solidified Ga-In alloys. The simulated results are validated against real time, in situ X-ray radiographic experiments, highlighting both similarities and differences between model and experiment. The redistribution of solute concentration and the formation of segregated solutal channels were both captured by the numerical model. Effects of thermo-solutal convection due to temperature and solute concentration variations were investigated and the formation of segregated solutal channels (freckles) was also examined in detail. With the assistance of the numerical solution, an improved understanding of the freckle channel formation at the microstructural level was provided.

A 3D microscale solidification model, resolving complex interaction of solute partition and diffusion, interdendritic thermosolutal convection, dendrite formation and remelting, was used to study freckle formation for unidirectional solidified Ga-In alloys. The simulated results are validated against real time, in situ X-ray radiographic experiments, highlighting both similarities and differences between model and experiment. The redistribution of solute concentration and the formation of segregated solutal channels were both captured by the numerical model. Effects of thermo-solutal convection due to temperature and solute concentration variations were investigated and the formation of segregated solutal channels (freckles) was also examined in detail. With the assistance of the numerical solution, an improved understanding of the freckle channel formation at the microstructural level was provided.

The physics of double diffusion and the role that it plays in the ocean provided the central theme for the 1996 summer program in Geophysical Fluid Dynamics at the Woods Hole Oceanographic Institution. Lectures and seminars given by staff and visitors surveyed observations, laboratory experiments and theoretical models of double diffusive phenomena. Several lectures dealt with the related phenomena of thermal convection in layered media, the dynamics of binary fluids, viscoelastic convection and magnetoconvection. Research projects by the fellows included experiments with double diffusion in a slot, work on the formation and evolution of staircases, double diffusion in stars and the interaction between convection and radiation. This volume includes write-ups of the principal lectures, report of the fellows' research projects and abstracts of some of the seminars. A list of presentations and a bibliography may be found at the back of the volume., Funding was provided by the National Science Foundation through Grant No. OCE-9314484.

The physics of double diffusion and the role that it plays in the ocean provided the central theme for the 1996 summer program in Geophysical Fluid Dynamics at the Woods Hole Oceanographic Institution. Lectures and seminars given by staff and visitors surveyed observations, laboratory experiments and theoretical models of double diffusive phenomena. Several lectures dealt with the related phenomena of thermal convection in layered media, the dynamics of binary fluids, viscoelastic convection and magnetoconvection. Research projects by the fellows included experiments with double diffusion in a slot, work on the formation and evolution of staircases, double diffusion in stars and the interaction between convection and radiation. This volume includes write-ups of the principal lectures, report of the fellows' research projects and abstracts of some of the seminars. A list of presentations and a bibliography may be found at the back of the volume., Funding was provided by the National Science Foundation through Grant No. OCE-9314484.

The physics of double diffusion and the role that it plays in the ocean provided the central theme for the 1996 summer program in Geophysical Fluid Dynamics at the Woods Hole Oceanographic Institution. Lectures and seminars given by staff and visitors surveyed observations, laboratory experiments and theoretical models of double diffusive phenomena. Several lectures dealt with the related phenomena of thermal convection in layered media, the dynamics of binary fluids, viscoelastic convection and magnetoconvection. Research projects by the fellows included experiments with double diffusion in a slot, work on the formation and evolution of staircases, double diffusion in stars and the interaction between convection and radiation. This volume includes write-ups of the principal lectures, report of the fellows' research projects and abstracts of some of the seminars. A list of presentations and a bibliography may be found at the back of the volume., Funding was provided by the National Science Foundation through Grant No. OCE-9314484.

The physics of double diffusion and the role that it plays in the ocean provided the central theme for the 1996 summer program in Geophysical Fluid Dynamics at the Woods Hole Oceanographic Institution. Lectures and seminars given by staff and visitors surveyed observations, laboratory experiments and theoretical models of double diffusive phenomena. Several lectures dealt with the related phenomena of thermal convection in layered media, the dynamics of binary fluids, viscoelastic convection and magnetoconvection. Research projects by the fellows included experiments with double diffusion in a slot, work on the formation and evolution of staircases, double diffusion in stars and the interaction between convection and radiation. This volume includes write-ups of the principal lectures, report of the fellows' research projects and abstracts of some of the seminars. A list of presentations and a bibliography may be found at the back of the volume., Funding was provided by the National Science Foundation through Grant No. OCE-9314484.

Oceanic observations [Atmos. Ocean 29, 340 (1991)] have revealed small-scale thermohaline plumes near the surface of a calm sea under warming conditions. The stratification was favorable for the double-diffusive salt finger instability, though a previously unreported up-down asymmetry was found in which narrow downward cells are balanced by a broader, weaker upwelling. The scales of the thermal structures are consistent with asymmetric hexagonal salt-finger modes [J. Phys. Oceanogr. 24, 855 (1994)], but no selection mechanism for the asymmetry has previously been identified. This paper explores the influence of nonlinear profiles of temperature and salinity, as might arise due to surface evaporation or warming, on the linear stability problem in a salt-fingering regime. Three models are considered. In the first, a sharp, nonlinear solute-concentration gradient is applied at the upper boundary, as might arise by surface evaporation. A Benard mode appears, driven by the destabilizing density gradient in the thin boundary layer and influencing motion only within the boundary-layer thickness. In the second model, a weak salinity gradient is introduced below the boundary layer; double-diffusive bulk modes influence the motion across the entire fluid. Nonlinear interaction of the boundary layer and bulk modes provides a mechanism for maintaining salt fingers with up-down asymmetry. The third model contains a large temperature gradient at the surface, as might arise from warming by solar radiation, overlying a quasi-isothermal region above a region of moderate gradient. The largest-growth modes are found to be salt fingers that extend throughout the middle region and disappear in the top and bottom regions. This vertical structure is close to that of the asymmetric salt fingers described in Osborn [Atmos. Ocean 29, 340 (1991)]. The differing length scales of the regions impress an up-down asymmetry on plumes; this is expected to yield a hexagonal pattern at the onset. (C)