The microstructure, and phase selections of La₆₆Al₁₄(Cu, Ni)₂₀ alloy were studied by Bridgman solidifications, and composite materials of dendrites in amorphous matrix or micro- and nano- sized eutectic matrix were formed with different cooling rates. The volume fraction of the dendrite phase reaches a maximum at the cooling rate of about 15 K/s, the secondary dendrite arm spacing λ₂ decreases from 4.3 µm to 0.6 µm with the increasing of cooling rate R, and obeys the equation of λ₂R⁰.⁵&#x2077=1.74µm(K/s)R⁰.⁵&#x2077. The compression strength, as well as the elastic strain limit of the dendrite/amorphous matrix composite are 600 MPa, and 2.3%, respectively. Improved ductility was observed for the dendrite amorphous matrix composites with more dendrite phase by slow cooling rate. / Singapore-MIT Alliance (SMA)

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In this dissertation¡Atwo topics in microbubble systems are investigated¡G1) Pore Formation from Bubble Entrapment by a Solidification Front¡F2) Pore formation in Solid¡C In the first study¡Amechanism of the pore shape in solid resulted from a tiny bubble captured by a solidification front is geometrically and generally investigated¡CPore formation and its shape in solid are one of the most critical factors affecting properties¡Amicrostructure¡Aand stresses in materials¡CFor simplicity without loss of generality, the tiny bubble beyond the solidification front is considered to have a spherical cap in this work¡CIntroducing a geometrical analysis it is found that the contact angle of the bubble cap can be governed by the Abel¡¦s equation of the first kind in terms of displacement of the solidification front¡CThe pore can be elongated, expanded¡Ashrunk and closed¡Adepending on relative variation of the bubble growth rate and solidification rate¡CThe pore can be closed by imposing infinitesimal bubble growth rate-to-solidification rate ratio¡Aand a finite bubble growth-to-solidification rate ratio in order to produce a minimal bubble radius at the contact angle of ¡CA criterion intuitively accepted in the literature¡Astating that closure of a pore is attributed to a greater solidification rate than bubble growth rate¡Ais incorrect¡CThe predicted pore shape and contact angle agree with experimental observations¡CManipulating either bubble growth rate or solidification rate can control pore formation in solid¡C In second study¡Athe shapes of a growing or decaying bubble entrapped by a solidification front are predicted in this work¡CThe bubble results from supersaturation of a dissolved gas in the liquid ahead of the solidification front¡CPore formation and its shape in solid are one of the most critical factors affecting properties¡Amicrostructure, and stresses in materials¡CIn this study¡Athe bubble and pore shapes entrapped in solid can be described by a three-dimensional phase diagram¡Aobtained from perturbation solutions of Young-Laplace equation governing the tiny bubble shape in the literature¡CThe predicted growth and entrapment of a microbubble as a pore in solid are found to agree with experimental data¡CThis work thus provides a realistic prediction of the general growth of the pore shape as a function of different working parameters¡C

The microstructure resulting from the solidification of alloys can greatly affect their properties, making the prediction of solidification phenomena under arbitrary conditions a very important tool in the field of computer-aided design of materials. Although considerable attention has been allocated to the understanding of this phenomenon in cases in which the solidification front advances freely into the liquid, the actual microstructure of solidification is strongly dependent of interfacial interactions. Over the past decade, the phase-field approach has been proved to be a quite effective tool for the simulation of solidification processes. In phase-field models, one or more phase fields ø (conserved and/or non-conserved) are introduced to describe the microstructure of a complex system. The behavior of a given microstructure over time is then simulated by solving evolution equations written in terms of the minimization of the free energy of the entire system, which is written as a functional of the field variables as well as their gradients and materials’ constitutive equations. With the given free energy functional, the governing equations (phase-field equation, diffusion equation, heat equation and so on) are solved throughout the entire space domain without having to track each of the interfaces formed or abrupt changes in the topology of the microstructure. In this work I will present phase-field models for solidification processes, solid/liquid interactions as well as their applications.

A thermodynamical framework is presented that describes the solidification of molten polymers to an amorphous as well as to a semicrystalline solid-like state. This framework fits into a general structure developed for materials undergoing a large class of entropy producing processes. The molten polymers are usually isotropic in nature and certain polymers crystallize, with the exception of largely atactic polymers, which solidify to an amorphous solid, to an anisotropic solid. The symmetry of the crystalline structures in the semicrystalline polymers is dependent upon the thermomechanical process to which the polymer is subjected to. The framework presented takes into account that the natural configurations associated with the polymer melt (associated with the breaking and reforming of the polymer network) and the solid evolve in addition to the evolving material symmetry associated with these natural configurations. The functional form of the various primitives such as how the material stores, dissipates energy and produces entropy are prescribed. Entropy may be produced by a variety of mechanisms such as conduction, dissipation, solidification, rearragement of crystalline structures due to annealing and so forth. The manner in which the natural configurations evolve is dictated by the maximization of the rate of dissipation. Similarly, the crystallization and glass transition kinetics may be obtained by maximization of their corresponding entropy productions. The restrictions placed by the second law of thermodynamics, frame indiference, material symmetry and incompressibility allows for a class of constitutive equations and the maximization of the rate of entropy production is invoked to select a constitutive equation from an allowable class of constitutive equations. Using such an unified thermodynamic approach, the popular crystallization equations such as Avrami equation and its various modifications such as Nakamura and Hillier and Price equations are obtained. The predictions of the model obtained using this framework are compared with the spinline data for amorphous and semicrystalline polymers.

No description available.

<p>Diffusion controlled phase transformations are studied bymeans of the phase-field method. Morphological evolution ofdendrites, grains and Widmanst\"atten plates is modeled andsimulated.</p><p>Growth of dendrites into highly supersaturated liquids ismodeled for binary alloy solidification. Phase-field equationsthat involve both temperature and solute redistribution areformulated. It is demonstrated that while at low undercoolingheat diffusion does not affect the growth of dendrites, i.e.solidification is nearly isothermal, at high cooling rates thesupersaturation is replaced by the thermal undercooling as thedriving force for growth.</p><p>In experiments many crystals with different orientationsnucleate. The growth of randomly oriented dendrites, theirsubsequent impingement ant formation of grain boundaries arestudied in two dimensions using the FEM on adaptive grids.</p><p>The structure of dendrites is determined by growthconditions and physical parameters of the solidifying material.Effects of the undercooling and anisotropic surface energy onthe crystal morphology are investigated. Transition betweenseaweeds, doublons and dendrites solidifying out of puresubstance is studied and compared to experimental data. Two-and three-dimensional simulations are performed in parallel onadaptive and uniform meshes.</p><p>A phase-field method based on the Gibbs energy functional isformulated for ferrite to austenite phase transformation inFe-C. In combination with the solute drag model, transitionbetween diffusion controlled and massive transformations as afunction of C concentration and temperature is established byperforming a large number of one dimensional calculations withreal physical parameters. In two dimensions, growth ofWidmanstaetten plates is governed by the highly anisotropicsurface energy. It is found that the plate tip can beapproximated as sharp, in agreement with experiments.</p><p><b>Keywords:</b>heat and solute diffusion, solidification,solid-solid phase transformation, microstructure, crystalgrowth, dendrite, grain boundary, Widmanstaetten plate,phase-field, adaptive mesh generation, FEM.</p>

Reproduction de : Thèse de doctorat : Biomécanique et bio-ingénierie : Poitiers : 2008. / Titre provenant de l'écran-titre. Bibliogr. 107 réf.

Vacuum Arc Remelting (VAR) is a secondary process used for homogenization of high-melting-point and oxygen-sensitive materials such as superalloys and titanium alloys. The VAR process is carried out with the aim of melting a large consumable electrode in such a way that the resulting ingot has improved homogeneity. The Specialty Metals Processing Consortium (SMPC) has spent the past 20 years developing technology to improve control over the final ingot remelting and solidification processes to alleviate conditions that lead to the formation of inclusions and segregation. Channel segregates are concentration defects arising during the solidification of large-diameter solute-rich alloys. As manufacturers for turbine engines and generators call for larger ingots, it becomes more difficult to produce them without these defects. If, however, liquid pool depth can be controlled precisely to stabilize the solidification zone in the ingot, we could, in principle, produce larger ingots that are defect free. A problem arises because measurements obtained from the VAR furnace do not give enough information to accurately estimate the liquid pool shape in dynamic melting situations. Also, the solidification process in VAR is extremely complex due to the multiple physical domains present and a high-fidelity model is required to give an accurate description of the dynamic process. The Basic Axisymmetric Remelting (BAR) code was initially developed by Lee Bertram at Sandia National Laboratories as a high-fidelity multi-energy model to describe ingot casting in this system. In this work we present a new strategy to improve the accuracy of the estimates used in the control system. This strategy consists of implementing BAR as a new set of measurements to be used by the estimator. This new strategy was used in tests jointly sponsored by SMPC and Los Alamos National Laboratory (LANL) in February 2011 using a laboratory-scale furnace and alloy 718 electrodes. / text

Thermosolutal fluid flow has a strong influence on the evolution of solidification microstructures. While porous media theory and volume-averaged permeability relations give a basis to quantify these phenomena, traditional methods of permeability estimation used for random porous media fail to adequately characterize the full relation of microstructural morphology to volume-average permeability. Most significantly, the link between microstructural parameters and permeability is treated as a deterministic function at all scales, ignoring the variability inherent in porous media.The variation in permeability inherent to random porous media is investigated by the numerical solution of Stokes equations on an ensemble of porous media, which represent of many scales of sampling and morphological character. Based on volume-averaging and statistical treatment, the stochastic character of tensoral permeability in porous media is numerically investigated. Quantification of permeability variation and autocorrelation structure are presented as conditions, which future realistic stochastic permeability fields must respect.