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<p>Crystal growth has been recognized as a paradigm for non-equilibrium pattern formation for decades. Scientific interest in this field has focused on the growth rates and curvature of branches in snow flake-like structures patterned after a solid's crystallographic orientations. Similar patterns have been extensively identified in solidification of metals and organic metal analogues and are known as dendrites, which is originated from a Greek word "dendron" meaning tree.</p> <p>Dendritic spacing and morphology established during casting often sets the final microstructure and second phase formation that develops during manufacturing of alloys. This is particularly true in emerging technologies such as twin belt casting of aluminum alloys, where a reduced amount of thermomechanical processing reduced the possibility of modifying microstructure from that determined at the time of solidification. Predicting and controlling these microstructure of cast alloys has thus been a driving force behind various studies on solidification of materials.</p> <p>Mg-based alloys are another class of materials gaining importance due to the high demand for weight reduction in the transportation industry which accordingly reduces the gas consumption. While the solidified microstructure and its effect on the material properties have been the subject of intensive studies, little is known about the fundamental mechanisms that determine dendritic microstructure in Mg alloys and its evolution under directional growth conditions.</p> <p>This thesis investigates the relationship between the microstructure and cooling conditions in unsteady state upward directional solidification of Al-Cu and Mg-Al alloys. The four-fold symmetry of Al-Cu alloys are used to study the dynamical spacing selection between dendrites, as the growth conditions vary dynamically, whereas, the Mg-Al system with a six-fold symmetry is used to study a competition between neighbouring, misoriented grains and the effect of this as the resulting microstructure. Mg-Al also presents a situation wherein the cooling conditions dynamically vary from the preferred crystallographic growth direction. Analysis of phase field simulations is used to shed some light on the morphological development of dendrite arms during solidification under transient conditions. Our numerical results are compared to new casting experiments.</p> <p>Chapter three studies spacing selection in directional solidification of Al-Cu alloys under transient growth conditions. New experimental results are presented which reveal that the mean dendritic spacing versus solidification front speed exhibits plateau-like regions separated by regions of rapid change, consistent with previous experiments of Losert and co-workers. In fact, The primary spacing of a dendritic array grown under transient growth conditions displays a distribution of wavelengths. As the rate of change in solidification front velocity is decreased, the evolution of the spacing follows the prediction of the geometrical models within a band of spacing fluctuations. The width of the band is shown to highly depend on the rate of the solidification front velocity acceleration, such that the higher the rate, the wider the band of available spacings. Quantitative phase field simulations of directional solidification with dynamical growth conditions approximating those in the experiments confirm this behavior. The mechanism of this type of change in mean dendrite arm spacing is consistent with the notion that a driven periodically modulated interface must overcome an energy barrier before becoming unstable, in accord with a previous analytical stability analysis of Langer and co-workers.</p> <p>In chapter four, it is demonstrated both computationally and experimentally that a material's surface tension anisotropy can compete with anisotropies present in processing conditions during solidification to produce a continuous transition from dendritic microstructure morphology to so-called seaweed and fractal-like solidification microstructures. The phase space of such morphologies is characterized and the selection principles of the various morphologies explored are explained. These results have direct relevance to the microstructure and second phase formation in commercial lightweight metal casting.</p> / Doctor of Philosophy (PhD)

Das Hochtemperaturverhalten beeinflusst viele verschiedene Prozesse von der Materialherstellung bis hin zur technologischen Anwendung. In-situ Transmissionselektronenmikroskopie (TEM) bietet die Möglichkeit, die atomaren Prozesse während struktureller Phasenübergänge direkt und in Realzeit zu beobachten. In dieser Arbeit wurde in-situ TEM angewendet, um die Reversibilität des Schmelz- und Kristallisationsprozesses, sowie das anisotropen Sublimationsverhaltens von Ge-Sb-Te (GST) Dünnschichten zu untersuchen. Die gezielte Probenpräparation für die erfolgreiche Beobachtung der Hochtemperatur-Phasenübergänge wird hervorgehoben. Die notwendige Einkapselung für die Beobachtung der Flüssigphase unter Vakuumbedingungen und die erforderliche sauberer Oberfläche für den Sublimationsprozess werden detailliert beschrieben. Außerdem wird die Elektronenenergieverlustspektroskopie eingesetzt um die lokale chemische Zusammensetzung vor und nach den Übergängen zu bestimmen. Die Untersuchung der Grenzflächenstruktur und Dynamik sowohl beim Phasenübergang fest-flüssig als auch flüssig-fest zeigt Unterschiede zwischen den beiden Vorgängen. Die trigonale Phase von GST weist beim Schmelzen eine teilweise geordnete Übergangszone an der fest-flüssig-Grenzfläche auf, während ein solcher Zwischenzustand bei der Erstarrung nicht entsteht. Außerdem läuft der Schmelzvorgang zeitlich linear ab, während die Kristallisation durch eine Wurzelabhängigkeit von der Zeit mit überlagerter Start-Stopp-Bewegung beschrieben werden kann. Der Einfluss der Substrat-Grenzfläche wird diskutiert und die Oberflächenenergie von GST bestimmt. Die anisotrope Dynamik führt beim Phasenübergang fest-gasförmig der kubischen Phase von GST zur Ausbildung stabiler {111} Facetten. Dies erfolgt über die Bildung von Kinken und Stufen auf stabilen Terrassen. Die Keimbildungsrate und die bevorzugten Keimbildungsorte der Kinken wurden identifiziert und stimmen mit den Voraussagen des Terrassen-Stufen-Kinken Modells überein. / High-temperature behavior influence many different processes ranging from material processing to device applications. In-situ transmission electron microscopy (TEM) provides the means for direct observation of atomic processes during structural phase transitions in real time. In this thesis, in-situ TEM is applied to investigate the reversibility of the melting and solidification processes as well as the anisotropic sublimation behavior of Ge-Sb-Te (GST) thin films. The purposeful sample preparation for the successful observation of the high-temperature phase transitions is emphasized. The required encapsulation for the observation of the liquid phase inside the vacuum conditions and the necessary clean surface for sublimation process are discussed in detail. Additionally electron energy-loss spectroscopy in the TEM is used to determine the local chemical composition before and after the phase transitions. The analysis of the interface structure and dynamic during the solid-to-liquid as well as the liquid-to-solid phase transition shows differences between both processes. The trigonal phase of GST exhibits a partially ordered transition zone at the solid-liquid interface during melting while such an intermediate state does not form during solidification. Additionally the melting process proceeds with linear dependence on time, whereas crystallization can be described as having a square-root time-dependency featuring a superimposed start-stop motion. The influence of the interface is addressed and the surface energies of GST are determined. The anisotropic dynamic of the solid-to-gas phase transition of the cubic GST phase leads to the formation of stable {111} facets. This happens via kink and step nucleation on stable terraces. The nucleation rates and the preferred kink nucleation sites are identified and are in accordance with the predictions of terrace-step-kink model.

This work identifies alloy terminal freezing range, columnar growth, grain coarsening, liquid availability towards the terminal stage of solidification, and segregation towards boundaries as primary factors affecting the hot-cracking susceptibility of fusion-based additive manufacturing (F-BAM) processed alloys. Additionally, an integrated computational materials engineering (ICME)-based approach has been formulated to design novel Al alloys, and high entropy alloys for F-BAM processing. The ICME-based approach has led to heterogeneous nucleation-induced grain refinement, terminal eutectic solidification-enabled liquid availability, and segregation-induced coalescence of solidification boundaries during laser-powder bed fusion (L-PBF) processing. In addition to exhibiting a wide crack-free L-PBF processing window, the designed alloys exhibited microstructural heterogeneity and hierarchy (MHH), and thus could leverage the unique process dynamics of L-PBF to produce a fine-tunable MHH and mechanical behavior. Furthermore, alloy chemistry-based fine tuning of the stacking fault energy has led to transformative damage tolerant alloys. Such alloys can shield defects stemming from the stochastic powder bed in L-PBF, and consequently can prevent catastrophic failure despite the solidification defects. A modified materials systems approach that explicitly includes alloy chemistry as a means to modify the printability, properties and performance with F-BAM is also presented. Overall, this work is expected to facilitate application specific manufacture with F-BAM and eventually facilitate widespread adoption of F-BAM in structural application.

La thèse est centrée sur la modélisation de la physique du comportement d’un alliage binaire et l’implémentation du meilleur modèle mathématique pour simuler le changement de phase liquide solide en tenant compte de la vitesse de refroidissement, la vitesse de solidification, la ségrégation, la convection naturelle et la surfusion afin d’optimiser la capacité de stockage de chaleur d'un tel matériau. Dans le présent travail, les températures pour lesquelles le changement de phase s'opère sont estimées grâce aux diagrammes des phases et la méthodologie CALPHAD qui retraduisent les différentes phases d'un alliage binaire, y compris la transformation isotherme. Pour cela, la minimisation de l'énergie de Gibbs est résolue dans un code de calcul développé à cette occasion et aboutit à l'identification des phases stables du matériau. Pour un intervalle de température souhaite le code permet d'estimer rapidement la décharge de chaleur pour la composition de l'alliage sélectionné en équilibre ou hors équilibre. Dans la méthode proposée, la vitesse de refroidissement du système permet de calculer la vitesse de solidification. Puis,celle-ci établit la relation entre la cinétique globale et la macrostructure. Basé sur le modèle de non-équilibre local, qui dépend de la variation du coefficient de partition, le degré de surfusion est prédit à partir de la vitesse de refroidissement appliquée. Une étude bibliographique a été réalisée pour amener une comparaison numérique et assurer la capacité de notre méthode à reproduire le changement de phase, en incluant des phénomènes spécifiques tels que la surfusion et la recalescence. / Latent Heat Thermal Energy Storage (LHTES) shows high storage density compared to sensible thermal systems. For high temperature applications, the use of alloys as phase change materials presents many advantages. Principally, varying alloy composition allows controlling the storage\discharge of thermal energy through an expected temperature range (defined by the heat source), and the high thermal conductivity givessuitable heat transfer properties to the system that receives/supplies the energy. However, some systems need a specific temperature range to correctly operate. In such conditions, subcooling (also known as undercooling) and segregation are undesirable phenomena in alloys when they are used as PCM. In thepresent work, we propose a method to predict the latent heat release during phase transformation of a binaryalloy submitted to a controlled cooling rate, including subcooling, segregation and variation of composition.This thesis describes the physical models that apply when heat is released from such a material. We takeinto consideration the cooling rate applied to the PCM, the solidification velocity, convective phenomena,melting temperature and subcooling. In the present work, phase diagrams and the CALPHAD methodologyare used to determine the temperature range for phase change (or constant temperature value for isothermal transformation) by minimizing the Gibbs equilibrium energy. The Gibbs free energy minimization has been implemented in a homemade numerical code. The material can be screened with different compositions for equilibrium or off-equilibrium solidification allowing quick selection of the optimal material for the specific heatsource. In the proposed method, the solidification velocity is obtained from the cooling rate. Then, variationin microstructure is driven by the solidification velocity using the local non-equilibrium diffusion model. Based on the local nonequilibrium model that depends on the partition coefficient variation, the subcooling degree, wich is derived from the applied cooling rate is predicted. A bibliographic study has been carried out and anumerical comparison has been undertaken to ensure the capacity of our code to reproduce the phase change of various materials that include phenomena such as subcooling and recalescence. The results highlight that the cooling rate is one of the most important parameters in the performance of the thermal storage, having a large effect on segregation and subcooling degree. Moreover, we show the influence ofpartition coefficient on the time evolution of solid fraction, considering a constant or a composition-dependent value. We can conclude that the latent heat release can be correctly predicted provided that the method correctly predicts the phase diagram and the variable partition coefficient. This work helps to accelerate the design and development of thermal storage systems and lays the foundation to continue exploring other kinds of materials (e.g. paraffins).

Les phénomènes d'auto-assemblage sont au premier plan de la recherche en sciences des matériaux car ils comblent le vide laissé entre les procédés d'assemblage à l'échelle macroscopique et nanoscopique. L'auto-assemblage est basé sur l'organisation spontanée de composants individuels en motifs et structures. Contrôler l’agencement de la matière peut accroître les propriétés de matériaux en introduisant une certaine anisotropie. Cet agencement, comme de nombreux matériaux naturels le prouvent, peut même sous certaines conditions faire émerger de nouvelles caractéristiques. Au cours de ces trois années, nous avons utilisé l’ « ice templating » (texturation à la glace) pour déclencher l’alignement de plaquettes de dimensions microniques, le but final étant de répliquer la microstructure de la nacre. Cette technique induit la ségrégation des constituants d’une suspension à l’échelle du micron tout en obtenant des échantillons de quelques centimètres cubes. Ce procédé a permis la création de matériaux inorganique avec une microstructure semblable à la nacre, en additionnant trois niveaux de contrôles successifs : l’alignement local des plaquettes, l’alignement à longue distance des cristaux de glaces et enfin le contrôle de l’interface entre ces-mêmes plaquettes. L’utilisation d’une modélisation par éléments discrets nous a permis d’étudier la dynamique de l’auto-assemblage des particules anisotropes. Ce modèle, parce qu’il tient compte de la dynamique du procédé, nous a révélé comment l’organisation de ces particules se produit. La tomographie par rayon X a permis de visualiser les structures finales des échantillons et d’attester de la pertinence du modèle. L’alignement local des plaquettes dans les parois générées par la solidification de la glace peuvent accroître les propriétés fonctionnelles et structurales de composites. Dans ce cadres deux applications ont été étudiées : la conduction thermique dans des composites nitrure de bore hexagonal / silicone et les propriétés mécaniques d’alumine macroporeuses. Une adaptation du procédé a permis d’obtenir l’alignement à longue distance (quelques centimètres) des cristaux de glaces. Différents outils ont été développés pour caractériser la réponse fonctionnelle de ce type de composite en fonction de leurs architectures aux deux échelles considérées (celles des macropores et parois). Enfin, après la mise en place de ces deux niveaux de contrôle sur la structure, l’addition d’une phase vitreuse inorganique et de nanoparticules aux joints de grains des plaquettes a introduit, de façon similaire à la nacre, des interfaces pouvant dévier et arrêter la propagation de fissures. / Self-assembly phenomena are of prime interest in materials science, because they fill the gap between assembly of macrostructure and processing of nanomaterials. Self-assembly is based on the spontaneous organization of individual small constituents into patterns and structures. Controlling the spatial arrangement can possibly improve materials properties by maximizing its response in a given direction. Furthermore, particular types of spatial arrangement, such as found in natural structures, can even induce new properties. During the past three years, we have used ice templating process to trigger the assembly of platelet-shaped particles to replicate the hierarchical structure of nacre. Control over this technique allowed structural customization at different length-scales: local orientation of the platelets, ice crystal long range order, and the control if the interfaces between the platelets. This hierarchical process has set the ground for the creation of a new fully mineral nacre-like alumina. The local platelet self-assembly triggered by ice growth was investigated by Discrete Element Modelling which provided new insight into the dynamic phenomenon responsible for the particles alignment. Synchrotron X-ray tomography was used to validated the model results. The different architecture observed in the final samples are not the result of a percolation threshold, as one could expect, but is instead a consequence of the delicate balance between pushing and engulfment at the solidification front. The local alignment of platelets can be beneficial for the functional and structural characteristics of composites and relevant aspects for two potential applications were investigated: the thermal properties of the hexagonal boron nitride/silicon rubber composites and the mechanical properties of macroporous alumina. Further adaptation of the process allowed for long range ordering of the ice crystals (up to the centimeter scale). Different tools have also been developed in order to characterize the response of composites as a function of the architecture at the level of the macropores and particle organisation. Once those two levels of alignment were achieved, the addition of a glassy phase and nanoparticles to the grain boundaries of the platelets introduces, just like in nacre, interfaces capable of deflect and even stopping crack propagation.

Duplex stainless steels can be very attractive alternatives to austenitic grades due to their almost double strength at equal pitting corrosion resistance. When welding, the duplex alloys normally require addition of filler metal, while the commodity austenitic grades can often be welded autogenously. Over-alloyed consumables are used to counteract segregation of important alloying elements and to balance the two phases, ferrite and austenite, in the duplex weld metal. This work focuses on the weldability of the recently-developed lean duplex stainless steel LDX 2101® (EN 1.4162, UNS S32101). The pitting corrosion resistance of this grade is better than that of austenitic AISI 304 (EN 1.4307) and can reach the level of AISI 316L (EN 1.4404). The austenite formation is rapid in LDX 2101 compared to older duplex grades. Pitting resistance tests performed show that 1-2.5 mm thick laser and gas tungsten arc (GTA) welded LDX 2101 can have good corrosion properties even when welding autogenously. Additions of filler metal, nitrogen in the shielding gas, nitrogen-based backing gas and use of laser hybrid welding methods, however, increase the austenite formation. The pitting resistance may also be increased by suppressing formation of chromium nitrides in the weld metal and heat affected zone (HAZ). After thorough post-weld cleaning (pickling), pitting primarily occurred 1-3 mm from the fusion line, in the parent metal rather than in the HAZ. Neither the chromium nitride precipitates found in the HAZ, nor the element depletion along the fusion line that was revealed by electron probe microanalysis (EPMA) were found to locally decrease the pitting resistance. The preferential pitting location is suggested to be controlled by the residual weld oxide composition that varies over the surface. The composition and thickness of weld oxide formed on LDX 2101 and 2304 (EN 1.4362, UNS S32304) were determined using X-ray photoelectron spectroscopy (XPS). The heat tint on these lean duplex grades proved to contain significantly more manganese than what has been reported for standard austenitic stainless steels in the AISI 300 series. A new approach to heat tint formation is presented; whereby evaporation of material from the weld metal and subsequent deposition on the already-formed weld oxide are suggested to contribute to weld oxide formation. This is consistent with manganese loss from the weld metal, and nitrogen additions to the GTA shielding gas enhance the evaporation. The segregation of all elements apart from nitrogen is low in autogenously welded LDX 2101. This means that filler wire additions may not be required as for other duplex grades assuming that there is no large nitrogen loss that could cause excessive ferrite contents. As the nitrogen appears to be controlling the austenite formation, it becomes essential to avoid losing nitrogen during welding by choosing nitrogen-containing shielding and backing gas. / QC 20101213

No description available.

Nous développons dans ce travail de thèse un outil de simulation numérique pour les matériaux à changement de phase (MCP), en tenant compte du phénomène de convection naturelle dans la phase liquide, pour des configurations en deux et trois dimensions. Les équations de Navier-Stokes incompressible avec le modèle de Boussinesq pour la prise en compte des forces de flottabilité liées aux effets thermiques, couplées avec une formulation de l’équation d’énergie suivant la méthode d’enthalpie, sont résolues par une méthode d’éléments finis adaptatifs. Une approche mono-domaine, consistant à résoudre les mêmes systèmes d’équations dans les phases solide et liquide, est utilisée. La vitesse est ramenée à zéro dans la phase solide, en introduisant un terme de pénalisation dans l’équation de quantité de mouvement, suivant le modèle de Carman-Kozeny, consistant à freiner la vitesse à travers un milieu poreux. Une discrétisation spatiale des équations utilisant des éléments finis de Taylor-Hood, éléments finis P2 pour la vitesse et éléments finis P1 pour la pression, est appliquée, avec un schéma d’intégration en temps implicite d’ordre deux (GEAR). Le système d’équations non-linéaires est résolu par un algorithme de Newton. Les méthodes numériques sont implémentées avec le logiciel libre FreeFem++ (www.freefem.org), disponible pour tout système d’exploitation. Les programmes sont distribués sous forme de logiciel libre, sous la forme d’une forme de toolbox simple d’utilisation, permettant à l’utilisateur de rajouter d’autres configurations numériques pour des problèmes avecchangement de phase. Nous présentons dans ce manuscrit des cas de validation du code de calcul, en simulant des cas tests bien connus, présentés par ordre de difficulté croissant : convection naturelle de l’air, fusion d’un MCP, le cycle complet fusion-solidification, chauffage par le bas d’un MCP, et enfin, la solidification de l’eau. / In this thesis we develop a numerical simulation tool for computing two and three-dimensional liquid-solid phase-change systems involving natural convection. It consists of solving the incompressible Navier-Stokes equations with Boussinesq approximation for thermal effects combined with an enthalpy-porosity method for the phase-change modeling, using a finite elements method with mesh adaptivity. A single-domain approach is applied by solving the same set of equations over the whole domain. A Carman-Kozeny-type penalty term is added to the momentum equation to bring to zero the velocity in the solid phase through an artificial mushy region. Model equations are discretized using Galerkin triangular finite elements. Piecewise quadratic (P2) finite-elements are used for the velocity and piecewise linear (P1) for the pressure. The coupled system of equations is integrated in time using a second-order Gear scheme. Non-linearities are treated implicitly and the resulting discrete equations are solved using a Newton algorithm. The numerical method is implemented with the finite elements software FreeFem++ (www.freefem.org), available for all existing operating systems. The programs are written and distributed as an easy-to-use open-source toolbox, allowing the user to code new numerical algorithms for similar problems with phase-change. We present several validations, by simulating classical benchmark cases of increasing difficulty: natural convection of air, melting of a phase-change material, a melting-solidification cycle, a basal melting of a phase-change material, and finally, a water freezing case.