A macrossegregação pode causar problemas sérios em processos de fundição, mas pode ser útil em processos de refino metalúrgico como os empregados na produção de silício de alta pureza. Ela é formada por fluxos convectivos presentes tanto na solidificação plana quanto na solidificação com zona pastosa, enquanto o transporte difusivo é efetivo quando a solidificação direcional é controlada, mantendo-se uma interface plana. A modelagem da solidificação de lingotes em escala macroscópica é atualmente dividida em modelos que assumem a presença de uma zona pastosa e modelos que assumem interface plana como hipótese inicial, exigindo conhecimento prévio da morfologia interfacial durante toda a operação. Um modelo matemático para a macrossegregação causada por transportes difusivos e convectivos de soluto durante a solidificação com crescimento plano ou com zona pastosa foi apresentado e implantado. Uma análise numérica apenas do transporte difusivo revelou a transição da interface plana para uma zona pastosa desenvolvida, que ocorreu em coincidência com critérios de estabilidade clássicos, além de revelar uma região inicial refinada formada mesmo após a formação da zona pastosa e confirmar a ineficiência da difusão sozinha na formação de macrossegregação em maiores velocidades. A convecção demonstrou efeito estabilizador para a interface sólido-líquido plana ao reduzir a camada super-resfriada por soluto, assim como acentuou a macrossegregação, com tendência ao caso de Scheil. Ensaios experimentais para silício presentes na literatura foram avaliados com o modelo proposto, confirmando o efeito estabilizador da interface plana observado e também mostrando o papel fundamental da difusão na formação da interface plana. Os resultados do modelo foram comparados com os de ensaios experimentais conduzidos em um forno do tipo Bridgman com liga Al-1% Cu com velocidades de solidificação crescentes e a estabilidade da interface plana e a macrossegregação correspondente examinadas sem ou com convecção forçada. Os resultados experimentais indicaram uma região com interface sólido-líquido plana que, após a segunda mudança de velocidade, indicou a aparente formação de células, verificadas por análise metalográfica. A velocidade de crescimento crítica foi estimada no experimento, mostrando-se significativamente maior que a calculada através do critério do super-resfriamento constitucional. / Macrosegregation is a source of serious problems in castings, but it can be a useful tool in metallurgical refining processes such as those employed in the production of high-purity silicon. It is often a result of convective liquid flow either at planar solidification or when a mushy zone is present, while diffusive solute transport is effective at controlled directional solidification with planar interfaces. Ingot solidification modeling at macroscopic scale is currently divided in models assuming a mushy zone is always present and planar interfaceassuming models, thus requiring prior knowledge of interface morphology. A mathematical model for macrosegregation resulting from diffusion and convection during solidification with planar interface or mushy zone was presented and implemented. Diffusive transport was evaluated numerically, revealing a transition from planar to mushy-zone solidification conforming to classical stability criterions, with an initial refined zone that grew even after a mushy zone developed and the inefficacy of macrodiffusion-induced macrosegregation at higher growth velocities shown. Convection showed stabilizing effect for planar solid-liquid interface by reducing the solute-undercooled layer and increased macrosegregation, with a tendency towards Scheil model. Silicon experiments publicly available were compared with the present model, confirming the experimentally-observed stabilizing effect and proving the key role of diffusion at macrosegregation formation on these systems. The model was evaluated with experiments made in a Bridgman-type furnace with an Al-1% Cu alloy with increasing growth velocities to study interface stability and macrosegregation with or without liquid stirring. Results indicate a planar growth region followed by a cellular interface one after the second velocity change, as identified by metallographical testing. Critical growth velocities for classical criterions were estimated after the experiments and were smaller than the evaluated ones.

Solidification technology can be an effective process for treating a variety of difficult to manage waste materials containing heavy metals prior to reuse or disposal. There are numerous commercial solidification techniques spanning a spectrum of technical complexity and cost. The most common methods include those based on cement or cement/pozzolanic materials. These materials, which are used in many solidification processes, make the technology appear simple and inexpensive. However, there are significant challenges to the successful application of this technique. The morphology and chemistry of the solidified waste forms are complex, specially when the waste streams used contain components other than the metals that are likely to be effectively immobilised. Also, the selection of the binder, depends upon an understanding of the chemistry of both the contaminants and the binder itself, to ensure efficient and reliable results. Nevertheless,a number of complex interactions are known to cause significant retardation on normal hydraulic reactions of cement-based materials, causing numerous and controversial problems. In recent years there has been renewed interest in elucidating the binding mechanisms responsible for the fixation of waste species. Carbonation, which is known to affect a wide range of cementitious materials, is a phenomenon observed by many scientists and has received very little attention. The aim of this work has been to investigate the effects of natural and accelerated carbonation on the development of mechanical and microstructural properties of solidified products as well as on the binding of metallic waste components. Particular emphasis was paid to examine the influence of different binders on the properties of carbonated solidified waste forms. The kinetics of the carbonation reaction was thoroughly examined, particularly when mix parameters such as binder/waste type and water content were varied. An examination of the resulting products showed that carbonated solidified waste materials had improved mechanical properties and increased metal binding capacity, when compared to specimens cured in nitrogen or normal atmospheric conditions. Microstructural analysis showed that large amounts of calcite where characteristics of carbonated samples. The increased formation of calcite as a result of carbonation appeared to be directly linked with the development of strength and enhanced metals fixation. NMR and FTIR spectroscopy indicated that carbonation has a significant influence on the hydration of waste forms by increasing the degree of polymerisation of the silicate hydration phases, with a consequent acceleration of the hydration of the cement paste. Examination by SEM analysis confirmed an acceleration of C3S hydration, typified by a de-calcified hydration rims and a matrix of dense calcite intergrowth infilling porosity. Some metals appeared to be incorporated in the silica-rich rims and others in the calcite rich matrix, suggesting precipitation of metal as both carbonates, silicates and complex double-salts. An examination of the kinetic of the carbonation reaction revealed that the reactivity of the different cements was different in the presence of carbon dioxide, and that when metal wastes were added the susceptibility of the paste to react with carbon dioxide increased. In general the results of this work indicate the potential of carbon dioxide for incorporation into the treatment of wastes during solidification. However, further work is necessary to establish the long-term performance of these carbonated waste forms as well as the behaviour of carbon dioxide upon different waste streams.

Thesis. 1976. Sc.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / Microfiche copy available in Archives and Science. / Vita. / Includes bibliographical references. / Sc.D.

Semi-solid deformation mechanisms are important in a range of manufacturing and natural phenomena, which range from squeeze casting to magma flows. In this thesis, using high speed synchrotron X-ray tomography and a bespoke precision thermo-mechanical rig, a four dimensional (3D plus time) quantitative investigation was performed to study the mechanical / rheological behavior of semi-solid Al-Cu alloys. Various deformation techniques, namely, isothermal semi-solid compression, extrusion and indentation were used. The time-resolved dynamic 3D images were analyzed with the help of novel image quantification techniques including digital volume correlation and image-based simulations of fluid flow. The quantified dynamics at a microstructural scale was then linked with macroscopic mechanical properties. The qualitative and quantitative analyses revealed a range of important semi-solid micromechanical mechanisms including the occurrence and effects of dilatancy, associated liquid flow through the equiaxed microstructure, intra-dendritic deformation, and strain localization during semi-solid deformation, not only shedding new insights into the mechanisms of deformation-induced solidification defect formation (solute segregation, porosity and hot tearing) of semi-solid alloys at both a macroscopic and microscopic level, but also providing benchmark cases for semi-solid deformation models and theories. The experimental methodology, techniques and analysis procedures developed in this thesis are generic in nature and can be applied to a wide range of research fields.

by Chan Kim Wai. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references. / by Chan Kim Wai. / Chapter Chapter I --- Introduction / Chapter 1.1 --- Rapid solidification / Chapter 1.1.1 --- Rapid quenching --- p.1-1 / Chapter 1.1.2 --- Undercooling --- p.1-2 / Chapter 1.2 --- Grain refinement / Chapter 1.2.1 --- What is grain refinement? --- p.1-5 / Chapter 1.2.2 --- Previous results in grain refinement / Chapter 1.2.2.1 --- Pure metals (or dilute alloys) --- p.1-5 / Chapter 1.2.2.2 --- Alloys --- p.1-9 / Chapter 1.2.2.3 --- Semiconductor --- p.1-10 / Chapter 1.2.3 --- Critical crystal growth velocity V* --- p.1-11 / Chapter 1.2.4 --- Proposed models to grain refinement / Chapter 1.2.4.1 --- Dynamic nucleation and cavitation --- p.1-12 / Chapter 1.2.4.2 --- Remelting (melt-back) --- p.1-14 / Chapter 1.2.4.3 --- Interdendritic fluid flow --- p.1-15 / Chapter 1.2.5 --- Volumetric manifestation of grain refinement --- p.1-15 / Chapter 1.3 --- Aim of this project --- p.1-16 / References / Figures / Chapter Chapter II --- Experimental / Chapter 2.1 --- Pure palladium / Chapter 2.1.1 --- Sample preparation and procedure --- p.2-1 / Chapter 2.1.2 --- Limitation and choice of flux --- p.2-2 / Chapter 2.1.3 --- High temperature furnace --- p.2-3 / Chapter 2.1.4 --- Measurement of specific volume / Chapter 2.1.4.1 --- Theory --- p.2-4 / Chapter 2.1.4.2 --- Setup --- p.2-5 / Chapter 2.1.5 --- Observing internal morphology --- p.2-5 / Chapter 2.2 --- Palladium with insoluble impurity / Chapter 2.2.1 --- Choice of insoluble impurities --- p.2-6 / Chapter 2.2.2 --- Sample preparation --- p.2-7 / References / Figures / Chapter Chapter III --- Results and Discussion / Results / Chapter 3.1 --- Pure palladium / Chapter 3.1.1 --- Specific volume --- p.3-1 / Chapter 3.1.2 --- Grain structure and internal voids --- p.3-2 / Chapter 3.2 --- Palladium with insoluble impurity / Chapter 3.2.1 --- Pinning effect of insoluble impurities --- p.3-3 / Chapter 3.2.2 --- Pd-Ni-S system / Chapter 3.2.2.1 --- Grain refinement in Pd99.9Ni-S)0.1 --- p.3-4 / Chapter 3.2.2.2 --- Change of ΔT* with addition of sulfur --- p.3-5 / Chapter 3.2.2.3 --- Internal voids --- p.3-5 / Discussion / Chapter 3.3 --- Dynamic nucleation of Pd-Ni-S system --- p.3-6 / Chapter 3.4 --- Void formation of pure palladium and Pd-Ni-S --- p.3-6 / Chapter 3.5 --- Grain refinement and specific volume --- p.3-7 / Reference / Figures

Submitted by RODRIGO YOKOYAMA XAVIER null (rodyok@hotmail.com) on 2017-02-16T01:10:07Z No. of bitstreams: 1 Dissertação Mestrado - Rodrigo Yokoyama Xavier.pdf: 6767206 bytes, checksum: 36a0ac2db721a2114f623ea26fe9f582 (MD5) / Approved for entry into archive by Juliano Benedito Ferreira (julianoferreira@reitoria.unesp.br) on 2017-02-22T17:27:06Z (GMT) No. of bitstreams: 1 xavier_ry_me_guara.pdf: 6767206 bytes, checksum: 36a0ac2db721a2114f623ea26fe9f582 (MD5) / Made available in DSpace on 2017-02-22T17:27:06Z (GMT). No. of bitstreams: 1 xavier_ry_me_guara.pdf: 6767206 bytes, checksum: 36a0ac2db721a2114f623ea26fe9f582 (MD5) Previous issue date: 2017-01-30 / O nível de qualidade de peças produzidas a partir de grandes lingotes está intimamente relacionado à qualidade dos lingotes em si. Dentre os diversos defeitos inerentes ao processo de solidificação, destacam-se as microssegregações de elementos de liga, que causam uma deterioração nas propriedades do produto final. Uma maneira de reduzir o dano causado pela microssegregação é através do Tratamento Térmico de Homogeneização, este por sua vez demanda elevados tempos de processo, elevando custos e tempos de fabricação. Uma das formas de reduzir os tempos de homogeneização, uma vez que este apresenta caráter difusional, é através da redução do espaçamento interdendrítico. Neste trabalho foi analisada a influência da deformação plástica como forma de reduzir o espaçamento entre dendritas no tratamento térmico de homogeneização. Para tal fim, utilizou-se um lingote fundido em aço ferramenta de composição química similar ao AISI A2. As amostras foram retiradas do núcleo do lingote no estado bruto de solidificação e sofreram deformações de 0,6 e 1,3 através do processo de laminação a quente, sendo temperadas em água na sequência. Após laminadas as amostras passaram por um tratamento térmico de homogeneização na temperatura de 1200°C por 8h ou 16h e foram novamente temperadas em água. As análises foram feitas através de Microscopia Óptica, Dureza Vickers, Difratometria de Raios-X e Microscopia Eletrônica de Varredura. Foi observado em todas as amostras a presença de microrechupes, e uma microestrutura composta predominantemente por dendritas oriundas da solidificação, identificadas pela fase martensítica, envoltas por uma matriz formada de austenita retida, contendo carbonetos e sulfetos. Com a deformação plástica foi possível quebrar a estrutura dendrítica a aproximar as regiões segregadas das não segregadas. O tratamento térmico por um tempo de 8h não foi suficiente para homogeneizar a microestrutura e reduzir as microssegregações, independentemente do estado de deformação das amostras. O tratamento térmico por 16h apresentou os melhores resultados em relação à homogeneidade química, sendo tanto melhor o resultado quanto maior a deformação imposta às amostras. / The quality of pieces produced from large ingots is closely related to the quality of ingots itself. Among the various defects inherent to the solidification process, there is the microsegregation of alloying elements, causing a deterioration in the properties of the final product. One way to reduce the damage caused by microsegregation is through the homogenization heat treatment, this in turn demands long time of process, increasing costs and lead-times for manufacture. One way to reduce the homogenization time, since it has a diffusive character, is by reducing the interdendritic spacing. In this study was analyzed the influence of plastic deformation as a mean to reduce the spacing between dendrites in the homogenization heat treatment. For this purpose it was used a cast ingot of chemical composition similar to the AISI A2 tool steel. Samples were cut from the ingot center in the as-cast state and suffered deformations of 0.6 and 1.3 through the hot rolling process and quenched in water in the sequence. After rolling the samples passed through a homogenization heat treatment at a temperature of 1200°C for 8h and 16h and again were quenched in water. Analyses were performed by Optical Microscopy, Vickers Hardness, X-Ray Diffractometry and Scanning Electron Microscopy. It was observed in all samples the presence of microcavities, and a microstructure consisting predominantly by solidifications dendrites identified by a martensitic phase, involved by a retained austenite matrix containing carbides and sulfides. The plastic deformation broke the dendritic structure, and approached the segregated regions to the non-segregated regions. The heat treatment for 8h was not sufficient to homogenize the microstructure and reduce the microsegregation, independently of the deformation state of the samples. The heat treatment for 16h presented the best results in relation to the chemical homogeneity, and the better the result as the greater the deformation imposed on the samples.

As energias renováveis têm ganhado destaque como possíveis soluções para a questão da matriz energética mundial. Dentre as diferentes alternativas está a energia solar. As células fotovoltaicas são produzidas majoritariamente com silício metálico de alta pureza que é produzido tradicionalmente por um processo químico de alta complexidade e custo. Alternativas vêm sendo desenvolvidas para produção deste material, sendo a rota metalúrgica uma delas. Nesta rota parte-se de um silício de menor pureza e removem-se as impurezas através de operações metalúrgicas. Um dos elementos a serem removidos é o carbono, que está majoritariamente presente precipitado na forma de carboneto de silício (SiC). Visando o estudo da formação da macrossegregação de partículas de SiC em Si, ensaios de solidificação em um forno do tipo Bridgman com diferentes velocidades de extração e diferentes diâmetros de molde foram realizados. Ensaios para verificar o efeito de decantação destas partículas na macrossegregação também foram conduzidos. Nos ensaios de solidificação, os resultados mostram que um aumento da velocidade de extração do molde para fora da região quente do forno resulta em uma estrutura de grãos menos alinhada na direção axial dos lingotes cilíndricos. Adicionalmente, o aumento do diâmetro do lingote também possui um efeito na macroestrutura, dependo da velocidade de extração. A macrossegregação de carbono para o topo do lingote é mais severa quanto menor for a velocidade de extração e maior for a direcionalidade dos grãos. Esta macrossegregação é causada pelo empurramento das partículas de SiC pela interface sólido/líquido. A decantação de partículas de SiC causou macrossegregação de C para a base dos lingotes de Si, porém menos acentuada que aquela observada nos ensaios de solidificação direcional. A alteração do período de tempo reservado no experimento para ocorrência da decantação de uma para seis horas não influenciou significativamente a macrossegregação. Para um mesmo tempo de processo, a solidificação direcional a uma velocidade de extração de 5 µm/s resultou em uma maior macrossegregação de carbono do que aquela observada após decantação, apresentando um maior potencial para a remoção do carbono contido no silício. / Renewable energies have been considered as possible solutions to the world energy matrix issue. Solar energy is one of the alternatives. Photovoltaic cells are mainly made of high purity silicon, which is produced by a complex and costly chemical process. New alternative processes are under development and one of them is the metallurgical route. In this route, a less pure silicon is refined and impurities are removed by metallurgical operations. One of the elements to be removed is carbon, which is mainly present as silicon carbide (SiC). Aiming at an investigation about the macrosegregation of carbon and SiC particles in Si, solidification experiments were conducted in a Bridgman type furnace using different extraction velocities and different mold diameters. Experiments to investigate the effects of settling of SiC particles were also carried out. The results show that the mold extraction velocity plays an important role in the grain macrostructure formation: higher velocities give rise to a less directional grain structure. Additionally, the change in the ingot diameter also has an effect on the macrostructure depending on the extraction velocity. Carbon macrosegregation to the ingot top is more severe for lower mold extraction velocities. This macrosegregation is a result of SiC particles being pushed by the solid/liquid interface to the ingot top. The settling of SiC particles causes carbon macrosegregation to the ingot bottom, but less intense than that observed in the directional solidification experiments at the ingot top. The change of settling times from one to six hours, have no significant effect on the degree of macrosegregation. For equal processing times, directional solidification at a mold extraction rate of 5 µm/s causes more carbon macrosegregation than settling, representing a higher potential alternative for carbon removal from silicon.

In the field of metal casting, solute composition inhomogeneities at the macroscale are called macrosegregation, and the transition from the elongated grains in the outer portions of a casting to the more rounded grains in the center is termed Columnar to Equiaxed Transition (CET). Simultaneous prediction of macrosegregation and CET is still an important challenge in the field. One of the open questions is the role of melt convection on the CET and the effect of the CET on macrosegregation. A three-phase macroscale model for macrosegregation and CET was developed. The model accounts for numerous phenomena such as columnar dendrite tip undercooling, undercooling behind the columnar tips, and nucleation of equiaxed grains. This three-phase model was used to develop a less complex model that consists of two phases only and disregards undercooling behind the columnar tips and nucleation of equiaxed grains. An in-house parallel computing code on the OpenFOAM platform was developed to solve the equations of these models. The models were used to perform columnar solidification simulations of a numerical benchmark problem. It was found that the predictions of these models are nearly identical. It was also found that the dendrite tip selection parameter, which appears in the constitutive relation for the dendrite tip velocity, plays a key role in these models. With a realistic value for this parameter these models account for columnar dendrite tip undercooling, but as its value is increased in the simulations, predictions of these models converge to predictions of a model that neglects undercooling. Next, the three-phase model was used to perform CET simulations in the numerical solidification benchmark problem in the presence of melt convection. It was found that accounting for stationary equiaxed grains does not change the overall macrosegregation pattern nor the form of channel segregates. Finally, for the first time in the field of solidification, we developed accurate constitutive relations for macroscale solidification models that are based on a formal mesoscale analysis on the scale of a representative elementary volume that is used in developing volume-averaged macroscale models. This upscaling enabled us to present relations that incorporate changes in the shape of grains and solute diffusion conditions around them during growth. The models and constitutive relations we developed can now be used to predict critical phenomena such as macrosegregation, channel segregates, and CET in castings.

Cette thèse a pour objectif d'étudier la modélisation et la résolution numérique de la macroségrégation pendant la solidification de lingots d'alliages métalliques binaires. La macroségrégation est une hétérogénéité de la concentration en éléments d'alliages à l'échelle du lingot. Ces changements de concentration affectent de manière importante les propriétés mécaniques et chimiques du matériau. Il est donc essentiel de pouvoir prédire ces hétérogénéités pour assurer la qualité des lingots. Après avoir décrit le contexte tant industriel que bibliographique de ce travail, nous préciserons les différents modèles implémentés dans le code de calcul THERCAST, un logiciel de solidification développé au CEMEF. La description des équations macroscopiques employées sera précédée d'une discussion sur la manière de valider la modélisation de la macroségrégation. Nous aborderons ensuite la théorie du remaillage adaptatif et nous décrirons les éléments essentiels de la stratégie de remaillage développée dans le cadre de cette thèse. Ces modèles seront appliqués à la simulation de la solidification de plusieurs lingots. La validation se fera sur un lingot de petite taille par comparaison avec des résultats expérimentaux et des simulations avec d'autres logiciels. Ensuite nous analyserons en détail les prédictions de THERCAST sur un lingot plan, un lingot 3D similaire à un lingot industriel et finalement un lingot plus petit qui nous permettra une étude numérique plus complète. Les limitations de notre modélisation et les phénomènes qu'elle a permis de mettre en évidence seront enfin discutés et permettront de définir quelques orientations intéressantes pour poursuivre cette étude de la macroségrégation.

Ce travail de thèse est consacré à l'étude d'un modèle transitoire qui tient compte des effets multidimensionnels dans le procédé de purification par fusion de zone. Nous avons étudié les effets des principaux paramètres gouvernant le processus de purification à savoir: la vitesse de translation des sources de chauffage et de refroidissement, l'intensité de flux de chauffage, l'intensité de flux de refroidissement, la largeur de la zone fondue et en fin le rapport d'aspect géométrique. Ces différents paramètres influent de façon importante sur le processus de purification, sur son efficacité et sur le degré de purification atteint après un ou plusieurs passages des sources. Une application dans le cas du matériau SiGe à très faible concentration a été simulée à l'aide d! u code de calcul par éléments finis Castem 2000. Le résultat de cette simulation a montré la difficulté de résolution numérique pour un nombre de Lewis très grand. Afin d'atteindre l'objectif visé, une étape importante dans cette étude a été consacrée à la détermination d'un couple (nombre de Lewis dans le liquide et rapport des coefficients de diffusion des espèces) pour que le processus de purification soit correctement représenté. Pour ce faire, nous avions étudié la fusion d'une tranche par une source immobile et choisir un couple convenable de ces 2 paramètres déterminants qui nous permet d'atteindre l'objectif visé. Afin d'analyser l'effet de chacune des grandeurs évoquées ci-dessus sur la purification, une deuxième étape a été consacrée à l'étude de la ! phase de purification proprement dite en adoptant une étu! de param étrique.