Martensite is considered to be the backbone of the high strength of many commercial steels. Martensite is formed by a rapid diffusionless phase transformation, which has been the subject of extensive research studies for more than a century. Despite such extensive studies, martensitic transformation is still considered to be intriguing due to its complex nature. Phase-field method, a computational technique used to simulate phase transformations, could be an aid in understanding the transformation. Moreover, due to the growing interest in the field of “Integrated computational materials engineering (ICME)”, the possibilities to couple the phase-field method with other computational techniques need to be explored. In the present work a three dimensional elastoplastic phase-field model, based on the works of Khachaturyan et al. and Yamanaka et al., is developed to study the athermal and the stress-assisted martensitic transformations occurring in single crystal and polycrystalline steels. The material parameters corresponding to the carbon steels and stainless steels are considered as input data for the simulations. The input data for the simulations is acquired from computational as well as from experimental works. Thus an attempt is made to create a multi-length scale model by coupling the ab-initio method, phase-field method, CALPHAD method, as well as experimental works. The model is used to simulate the microstructure evolution as well as to study various physical concepts associated with the martensitic transformation. The simulation results depict several experimentally observed aspects associated with the martensitic transformation, such as twinned microstructure and autocatalysis. The results indicate that plastic deformation and autocatalysis play a significant role in the martensitic microstructure evolution. The results indicate that the phase-field simulations can be used as tools to study some of the physical concepts associated with martensitic transformation, e.g. embryo potency, driving forces, plastic deformation as well as some aspects of crystallography. The results obtained are in agreement with the experimental results. The effect of stress-states on the stress-assisted martensitic microstructure evolution is studied by performing different simulations under different loading conditions. The results indicate that the microstructure is significantly affected by the loading conditions. The simulations are also used to study several important aspects, such as TRIP effect and Magee effect. The model is also used to predict some of the practically important parameters such as Ms temperature as well as the volume fraction of martensite formed. The results also indicate that it is feasible to build physically based multi-length scale model to study the martensitic transformation. Finally, it is concluded that the phase-field method can be used as a qualitative aid in understanding the complex, yet intriguing, martensitic transformations. / QC 20120525 / Hero-m

In this thesis two kinds of phase change i.e., solid state phase transformation in steels and solid-to-liquid phase transformation in paraffin, have been modeled and numerically simulated. The solid state phase transformation is modeled using the phase field theory while the solid-to-liquid phase transformation is modeled using the Stokes equation and exploiting the viscous nature of the paraffin, by treating it as a liquid in both states.The theoretical base of the solid state, diffusionless phase transformation or the martensitic transformation comes from the Khachaturyan's phase field microelasticity theory. The time evolution of the variable describing the phase transformation is computed using the time dependent Ginzburg-Landau equation. Plasticity is also incorporated into the model by solving another time dependent equation. Simulations are performed both in 2D and 3D, for a single crystal and a polycrystal. Although the model is valid for most iron-carbon alloys, in this research an Fe-0.3\%C alloy is chosen.In order to simulate martensitic transformation in a polycrystal, it is necessary to include the effect of the grain boundary to correctly capture the morphology of the microstructure. One of the important achievements of this research is the incorporation of the grain boundary effect in the Khachaturyan's phase field model. The developed model is also employed to analyze the effect of external stresses on the martensitic transformation, both in 2D and 3D. Results obtained from the numerical simulations show good qualitative agreement with the empirical observations found in the literature.The microactuators are generally used as a micropump or microvalve in various miniaturized industrial and engineering applications. The phase transformation in a paraffin based thermohydraulic membrane microactuator is modeled by treating paraffin as a highly viscous liquid, instead of a solid, below its melting point.  The fluid-solid interaction between paraffin and the enclosing membrane is governed by the ALE technique. The thing which sets apart the presented model from the previous models, is the use of geometry independent and realistic thermal and mechanical properties. Numerical results obtained by treating paraffin as a liquid in both states show better conformity with the experiments, performed on a similar microactuator. The developed model is further employed to analyze the time response of the system, for different input powers and geometries of the microactuator. / <p>QC 20130219</p>

This project is to explore the manufacturing and processing of laser or electron beam, formed on the surface morphology after curing and processing parts, such as surfacefilled, depression, or the formation of ripples; These reactions will directly affect the surface heat treatment and welding quality of thefinished product This study to consider the mass, momentum and energy equations, the introduction of theinterface and boundary conditions to simulate the real process In order to promote quality stability, and a large amount of production capacity and reduce costs, we must understand the institutions of the reaction In this thesis, the phase field method (Phase-field method) (Two-phase flow) two-phase flow simulation of metal surface by a concentrated source of heat melt the transient heat flow behavior

The study applies the phase-field method to simulate the behavior between bubble and liquid-solid front in the solidification. During the process, the two-phase flow module is used to match up with temperature and phase-field function to determine the percentage of- solid, liquid, and gas- in the domain. The governing equations for mass, momentum and energy contain coefficients which are related to percentage of phases.The result show that the surface tension and the temperature difference will influence the shape of bubble and the velocity of solidification.

A crucial aspect of attentional control is the capacity of anticipating a stimulus appearance in order to improve the speed and effectiveness of its subsequent processing. Preparatory attention (PA) is the ability to modulate (enhance) the intensity of attention directed to a selected stimulus prior to its occurrence, preventing subjects from being distracted by interfering stimuli. Some studies propose that PA is lateralized to the right hemisphere (RH) while others suggest that both the left hemisphere (LH) and the RH participate in the modulation of PA. The aim of the present thesis was to examine the role of each brain hemisphere in the modulation of PA directed to a spatial location. We developed a lateralized version of the Attentional Preparatory Test, (APT, proposed par LaBerge, Auclair & Siéroff, 2000), named the Lateralized APT or LAPT. The APT measures the ability of subjects to modulate PA directed to a target location when the probability of a distractor occurrence varies across several blocks of trials. In the APT, the response times increased as the probability of a distractor appearing in the preparatory phase preceding the target presentation increased, showing that PA is modulated by the events occurring prior to the target appearance. We developed the LAPT using the divided visual field paradigm in which stimuli can occur in the left (LVF) or the right (RVF) visual fields. The visual field differences in subjects' performance are assumed to reflect differences in the processing strategies of both hemispheres (RVF/LH vs LVF/RH). In a series of studies we showed that the modulation of PA by the expected probability of events was different in each visual field/hemisphere, depending on task configuration. In the RVF/LH, PA is modulated by the expected probability of distractor events, especially when this probability is explicit. In addition, the LH seems to play a crucial role in modulating PA when the target and the distractor are hard to discriminate. In the LVF/RH, PA is modulated by the temporal probability of events and may depend on the most probable delay in which the target is expected, but only when the discrimination between the target and the distractor is easy. Most importantly, our findings suggest that the differences between RVF/LH and LVF/RH in the modulation of PA take place at the perceptual level of processing because they are independent of the hand use in executing the response, thus also independent of the processes taking place at the motor programming level. Taken together our results, they suggest that each hemisphere uses a different strategy to modulate PA when directed to a spatial location.

This thesis presents work done on the calculation of the effects of anharmonic nuclear motion on the properties of solid materials from first principles. Such anharmonic effects can be significant in many cases. A vibrational self-consistent field (VSCF) method is used as the basis for these calculations, which is then improved and applied to a variety of solid state systems. Firstly, work done to improve the efficiency of the VSCF method is presented. The standard VSCF method involves using density functional theory (DFT) to map the Born-Oppenheimer (BO) energy surface that the nuclei move in, a computationally expensive process. It is shown that the accurate forces available in plane-wave basis DFT can be used to help map the BO surface more accurately and reduce the computational cost. This improved VSCF+f method is tested on molecular and solid hydrogen, as well as lithium and zirconium, and is found to give a speed-up of up to 40%. The VSCF method is then applied to two different systems of physical interest. It is first applied to the case of the neutral vacancy in diamond, in order to resolve a known discrepancy between harmonic ab initio calculations and experiment -- the former predict a static Jahn-Teller distortion, whilst the latter leads to a dynamic Jahn-Teller effect. By including anharmonic corrections to the energy and nuclear wavefunction, we show that the inclusion of these effects results in agreement between first-principles calculations and experiment for the first time. Lastly, the VSCF method is applied to barium titanate, a prototypical ferroelectric material which undergoes a series of phase transitions from around 400 K downwards. The nature of these phase transitions is still unclear, and understanding them is an active area of research. We describe the physics of the phase transitions of barium titanate, including both anharmonicity and the effect of polarisation caused by long wavelength vibrations, to help understand the important physics from first principles.

Investigamos certas propriedades físicas da teoria de campos a temperatura finita. Mostramos que, na presença de campos externos bosonicos, o limite de altas temperaturas das funções de Green pode ser obtido supondo que todas as componentes dos momentos externos sao nulas, para todas as funções de Green 1PI em ordem de um laco e em ordem de dois laços para a auto-energia. No contexto do método do campo de fundo, este resultado implica que a lagrangiana efetiva estática de um campo externo bosonico pode ser obtida calculando a pressão na presença de um campo externo independente do espaço-tempo, no limite de altas temperaturas. Aplicamos este resultado para obter a lagrangiana efetiva estática de um campo de Maxwell interagindo com um plasma da QED em ordem de dois laços. Obtivemos também a lagrangiana efetiva estática de um campo gravitacional interagindo com um campo escalar real livre, e com um plasma da QED em ordem de dois laços. / We have investigated some physical proprieties of nite temperature field theory. We have shown that, in the presence of an external bosonic field, the high temperature limit of the static Green\'s functions can be obtained assuming that all components of the external momenta are zero. This was shown to be the case for all 1PI Green\'s functions, at one-loop order, as well as for the self-energy, at two-loop order. In the context of the background field method, this result implies that the static efective Lagrangian of an external field can be obtained computing the pressure in the presence of an external field which is independent of the space-time, in the high temperature limit. We have applied this result in order to obtain the efective Lagrangian of an external Maxwell field interacting with a QED plasma at two-loop order. We have also obtained the efective Lagrangian of the gravitational field interacting with a free neutral scalar field, as well as with a QED plasma at two-loop order.

O gás de Bose carregado foi estudado em duas e três dimensões, sendo que no caso bidimensional foram considerados dois tipos distintos de interação: l/r e ln(r). Aplicamos a esses sistemas o método do campo auto-consistente que leva em consideração a interação de curto alcance entre os bosons através de uma correção de campo local. Por meio de cálculos numéricos auto-consistentes determinamos o fator de estrutura S(&#8594k) em um amplo intervalo de densidades. A partir de S(&#8594k) obtivemos a função de correlação dos pares, a energia do estado fundamental que é essencialmente a energia de correlação, a pressão do gás e o espectro de excitações elementares. Calculamos ainda a densidade de blindagem induzida por uma impureza carregada fixada no gás. No limite de altas densidades nossos cálculos reproduzem os resultados da teoria de perturbação de Bogoliubov. Na região de densidades intermediárias em que os sistemas são fortemente correlacionados nossos resultados apresentam uma boa concordância com cálculos baseados na aproximação de HNC e no método de Monte Carlo. Nossos resultados são em várias situações confrontados com os de RPA demonstrando que o método que utilizamos é muito mais adequado para tratar o sistema. Os sistemas bidimensionais mostraram-se mais correlacionados que o tridimensional, sendo que o gás com interação l/r é mais correlacionado que o logarítmico a altas densidades, mas na região de densidades baixas essa situação se inverte. Finalmente calculamos as funções termodinâmicas dos sistemas bi e tridimensionais a temperaturas finitas próximas do zero absoluto baseando-nos nos espectros de excitação do gás a temperatura zero. / The two and three-dimensional charged Bose gas have been studied. In the bidimensional case two different types of interaction were considered: l/r and ln(r).We have applied to these systems the self-consistent-field method, which takes into account the short range correlations between the bosons through a local-field correction. By using self-consistent numerical calculations we determinate the structure factor S(&#8594k) in a wide range of densities. From S(&#8594k) we obtained the pair-correlation function, the ground-state energy, the pressure of the gas and the spectrum of elementary excitations. In addition we calculated the screening density induced by a fixed charged impurity. In the high-density limit our calculations reproduce the results given by Bogoliubov\'s perturbation theory. In the intermediate-density region, corresponding to the strongly coupled systems, our results are in very good agreement with calculations based on HNC approximation as well as Monte Carlo method. Our results are compared in several situations with RPA results showing that the self-consistent method is much more accurate. The two-dimensional systems showed to be more correlated than the three-dimensional one; the gas with interaction l/r is also more correlated than the logarithmic one at high densities, but it begins to be less correlated than this one in the low-density region. Finally we calculated the thermodynamic functions of the two and three-dimensional systems at finite temperatures near absolute zero, based upon the gas excitation spectra at zero temperature.

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