En France, les déchets de Haute Activité à Vie Longue (HAVL) issus du retraitement du combustible nucléaire usé sont confinés à l’échelle atomique dans un verre borosilicaté appelé le verre R7T7. Lors de son élaboration à haute température, des cristaux incorporant les radionucléides se forment de manière transitoire sous différentes conditions dans le liquide avant d’être dissouts. Jusqu’à présent, de nombreuses études se sont portées sur la cristallisation de ces phases mais leur dissolution n’a pas fait l’objet d’étude approfondie. L’objectif de cette thèse est de décrire les cinétiques et les mécanismes de la dissolution des cristaux dans le liquide borosilicaté.Dans ce but, la dissolution de silicates de terres rares de structure apatite, type Ca2TR8(SiO4)6O2, est étudiée en conditions isothermes dans trois compositions borosilicatées différentes en système fermé. Afin de suivre l’évolution des fractions cristallines au cours du temps, une méthodologie basée sur l’acquisition de mosaïques d’images MEB associée à du traitement d’images a été optimisée. En compléments, des profils chimiques aux interfaces cristaux/verre sont également réalisés par microsonde électronique.Le modèle Johnson-Mehl-Avrami-Kolmogorov (JMAK), habituellement employé pour décrire la cristallisation dans les verres, est appliqué avec succès aux fractions cristallines mesurées au cours de la dissolution. L’application de ce modèle pour différentes températures de traitement thermique valide son efficacité à décrire les cinétiques de dissolution des silicates de terres rares dans des systèmes borosilicatés en régime statique. Son application est également étendue à des systèmes plus complexes présentant simultanément plusieurs phases cristallines de nature autre que les silicates de terres rares. De plus, ce modèle permet d’accéder au mécanisme contrôlant la dissolution des cristaux dans les liquides borosilicatés (dans les systèmes étudiés, la diffusion), à l’énergie d’activation de leur dissolution ainsi qu’aux constantes de dissolution et aux durées caractéristiques de la dissolution pour chaque température. / In France, High-Level radioactive Wastes (HLW) coming from nuclear spent fuel treatment are confined at the atomic scale in an homogeneous sodium-borosilicate glass called the R7T7 glass. At different stages of the melting process performed at high temperature, crystallized phases which incorporate the radionuclides are temporarily formed before their dissolution. The formation conditions of these crystals have already been studied but, to date, few studies on their dissolution in nuclear glass melt have been performed. The objective of this thesis is the description of the kinetics and mechanisms of the crystals dissolution in the borosilicate melt.This study focuses on the dissolution of rare earth silicates with an apatite structure (Ca2Nd8(SiO4)6O2 type) in three borosilicate melt compositions. Experiments are conducted in isothermal mode and in closed system. The crystalline fractions are followed according to time thanks to the acquisition of SEM images mosaics coupled with image analysis. In addition, chemical profiles at the crystal/melt interfaces are acquired by microprobe.The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, usually applied to describe the crystallization in melt, is successfully applied to the crystalline fractions measured during the dissolution process. The application of this model for different temperatures of thermal treatment validates its effectiveness to describe the dissolution kinetics of rare earth silicates in borosilicate systems in static mode. Its application is furthermore extended to the crystals dissolution in more complex systems where different types of crystals are simultaneously present. This model allows to determine the mechanism limiting the crystals dissolution in borosilicate melts, i.e. the diffusion in the studied systems, the activation energy of the dissolution as well as the constants of dissolution and the characteristic durations of dissolution for each considered temperature.

No presente trabalho estudou-se a recristalização do aço de alta resistência baixa liga (ARBL) microligado ao nióbio (160 ppm em peso) da Companhia Siderúrgica Nacional 2336 utilizado na fabricação de reforços estruturais para a indústria automobilística. A grande utilização deste tipo de aço pelas indústrias tem motivado o desenvolvimento de novas ligas capazes de atender a requisitos críticos de estampagem combinado com alta resistência mecânica. Amostras deste aço no estado laminado a quente foram deformadas posteriormente a frio em escala piloto com reduções de 50 e 70%, reduções típicas na indústria, e recozidas em banho de sal em temperaturas variando entre 600 e 700ºC em diferentes tempos. A evolução microestrutural das amostras no estado encruado e na condição recozida foi acompanhada utilizando-se as técnicas de microscopia ótica e eletrônica de varredura. As texturas de deformação e de recristalização foram determinadas via difração de raios X. A fração volumétrica recristalizada (Xv) e o tamanho de grão foram determinados com o auxílio de metalografia quantitativa nas amostras recozidas. Medidas de microdureza Vickers foram utilizadas para construir as curvas de encruamento e da cinética de amolecimento em função da temperatura de recozimento e comparadas com curvas construídas pelo primeiro método. Os resultados mostram que a recristalização ocorre em temperaturas acima de 650°C. Com auxílio de um gráfico de Xv contra A (fração amolecida), podemos observar a ocorrência de um desvio da idealidade a 650°C e 700°C. A explicação mais provável é a ocorrência de precipitação, especialmente para o aço menos deformado. Os valores do expoente de Avrami obtidos a partir das duas abordagens (medidas de microdureza e metalografia quantitativa) são apresentados e discutidos. / The recrystallization of a high strength low alloy steel (HSLA) containing niobium (160 wt-ppm) was investigated. The HSLA steel, named CSN 2336, is used in the manufacture of structural parts for automotive industry. The increasing use of this application type of steels in automotive industries has motivated the development of new alloys able to gather the critical requirements for deep drawing combined with high strengths. Samples of this steel in hot-rolled condition were cold-rolled with reductions of 50% and 70%, commonly used in most of applications, and isothermally annealed in salt bath at temperatures between 600 and 700ºC. The microstructure of the samples in both coldworked and annealed states was imaged using light optical (LOM) and scanning electronic microscopies (SEM). Deformation and recrystallization textures were determined by X-ray diffraction. The recrystallized volume fraction (Xv) and the mean grain size were determined by quantitative metallography in the annealed samples. Vickers microhardness testing was used to determine the work hardening and the softening kinetics behaviors. The results show that recrystallization occurs at temperatures above 650°C for the most deformed steel. By plotting Xv against A (softening fraction), we observe the occurrence of a deviation from ideality at 650°C and 700°C, likely caused by solid-state precipitation, especially for the less deformed steel. The values of n (Avrami exponent) obtained from both approaches (softening curves and quantitative metallography) are presented and discussed.

Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2018-12-21T10:53:57Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Giovanne Steudel.pdf: 9400807 bytes, checksum: 8a6ed5c0cfacf8380363dfd4584186c1 (MD5) / Made available in DSpace on 2018-12-21T10:53:57Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Giovanne Steudel.pdf: 9400807 bytes, checksum: 8a6ed5c0cfacf8380363dfd4584186c1 (MD5) Previous issue date: 2018-08-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O objetivo deste trabalho é avaliar os efeitos na cinética de transformação e nas propriedades mecânicas dos aços inoxidáveis ferrítico AISI 430 (16-18% Cr) e superdúplex SAF 2507 (24-26% Cr), quando submetidos a tratamentos térmicos de envelhecimento em 475°C, assim fragilizados pela formação da fase alfa linha. Este fenômeno é conhecido por “fragilização de 475°C” e ocorre na ferrita de aços inoxidáveis ferríticos e dúplex, e nos aços martensíticos envelhecidos entre 300°C e 550°C. O acompanhamento e análise da cinética de formação de α’ nos dois aços inoxidáveis, o ferrítico AISI 430 e superdúplex SAF 2507, foram feitas com base em ajustes matemáticos por regressão não-linear via sete modelos sigmoidais de aplicação geral, os quais serão comparados ao modelo JMAK de transformação de fases difusivas elaborada por Johnson, Mehl, Avrami e Kolmogorov. Amostras do aço ferrítico foram submetidas a tratamentos térmicos de solubilização em forno tubular em 1050 °C e 30 minutos, e em 1150°C durante 30 minutos para o aço dúplex. Tratamentos de envelhecimento posteriores em 475°C por diversos períodos foram realizados. Verificou-se a evolução da “fragilização de 475°C” e, indiretamente, a formação de α’, por ensaios de dureza Vickers, de tração e de impacto instrumentado Charpy. Análises microestruturais e fractográficas foram feitas em microscopias óptica e eletrônica de varredura (MEV-FEG); análises cristalográficas por difração de elétrons retroespalhados (EBSD); e as análises químicas por espectroscopia por dispersão de energia (EDS). O aço superdúplex, com aproximadamente 25,2% Cr, apresentou cinética de fragilização mais rápida do que o ferrítico, com cerca de 16,6% Cr. O modelo sigmoidal que mais se adequou aos dados de dureza de ambos os aços foi o de Boltzmann; o qual também apresentou maior versatilidade de informações e melhor ajuste e capacidade preditiva do que o modelo JMAK. As tendências para tempos de envelhecimento crescentes do aço AISI 430 foram o aumento dos limites de escoamento e de resistência à tração, com dados de tensão na ruptura oscilando em função do tempo de envelhecimento, porém a literatura aponta aumento também da tensão de ruptura. A mudança de comportamento de dúctil para frágil (clivagem) do aço AISI 430, em função do prosseguimento da “fragilização de 475°C”, foi mais acentuada nos corpos de prova de impacto, em relação aos de tração. / The goal of this work is to evaluate the effects in the transformation kinetics and in the mechanical properties of the AISI 430 (16-18% Cr) ferritic and the SAF 2507 (24-26% Cr) superduplex stainless steels, when they undergo aging heat treatments at 475°C, thus embrittled by the formation of the alpha prime phase. This phenomenon is known as “475°C embrittlement” and it occurs in the ferrite of ferritic and duplex stainless steels, and in martensitic stainless steels aged between 300°C and 550°C. The monitoring and analysis of the kinetics of α’ formation in the AISI 430 and SAF 2507 steels was done based on mathematical fitting by means of nonlinear regression of seven sigmoidal models of general application, whose were compared to the JMAK model of diffusive phase transformations elaborated by Johnson, Mehl, Avrami and Kolmogorov. Ferritic steel samples were submitted to solubilization heat tratments in a tubular furnace at 1050°C for 30 minutes, and at 1150°C during 30 minutes for the duplex steel. Posterior aging treatments will hold at 475°C by various periods. The evolution of the “475°C embrittlement” and, indirectly, the formation of α’, were verified by the way of Vickers hardness, tensile and Charpy instrumented impact tests. Microstructural and fractographic analyses were done by optical and scanning electron microscopies (FEG-SEM); crystallographic analyses by electron backscattered diffraction (EBSD); and chemical analyses by energy dispersive spectroscopy (EDS). The superduplex steel, containing approximately 25,2% Cr, showed a faster embrittlement kinetics than the ferritic steel, with about 16,6% Cr. The sigmoidal model that better fitted to the hardness data of both steels was the Boltzmann one; which also showed greater versatility of information and a better fit and predictive ability than the JMAK model. The trends for rising aging times of the AISI 430 steel were the increase in the yield and ultimate tensile stresses, having oscillation in the breaking stress in function of the aging time, yet the literature claims that there is also a increase in the breaking stress. The change in the mechanical behavior from ductile to brittle (cleavage) of the AISI 430 steel, in function of the following of the “475°C embrittlement”, was more severe in the impact test specimens, compared to the tensile specimens.

No presente trabalho estudou-se a recristalização do aço de alta resistência baixa liga (ARBL) microligado ao nióbio (160 ppm em peso) da Companhia Siderúrgica Nacional 2336 utilizado na fabricação de reforços estruturais para a indústria automobilística. A grande utilização deste tipo de aço pelas indústrias tem motivado o desenvolvimento de novas ligas capazes de atender a requisitos críticos de estampagem combinado com alta resistência mecânica. Amostras deste aço no estado laminado a quente foram deformadas posteriormente a frio em escala piloto com reduções de 50 e 70%, reduções típicas na indústria, e recozidas em banho de sal em temperaturas variando entre 600 e 700ºC em diferentes tempos. A evolução microestrutural das amostras no estado encruado e na condição recozida foi acompanhada utilizando-se as técnicas de microscopia ótica e eletrônica de varredura. As texturas de deformação e de recristalização foram determinadas via difração de raios X. A fração volumétrica recristalizada (Xv) e o tamanho de grão foram determinados com o auxílio de metalografia quantitativa nas amostras recozidas. Medidas de microdureza Vickers foram utilizadas para construir as curvas de encruamento e da cinética de amolecimento em função da temperatura de recozimento e comparadas com curvas construídas pelo primeiro método. Os resultados mostram que a recristalização ocorre em temperaturas acima de 650°C. Com auxílio de um gráfico de Xv contra A (fração amolecida), podemos observar a ocorrência de um desvio da idealidade a 650°C e 700°C. A explicação mais provável é a ocorrência de precipitação, especialmente para o aço menos deformado. Os valores do expoente de Avrami obtidos a partir das duas abordagens (medidas de microdureza e metalografia quantitativa) são apresentados e discutidos. / The recrystallization of a high strength low alloy steel (HSLA) containing niobium (160 wt-ppm) was investigated. The HSLA steel, named CSN 2336, is used in the manufacture of structural parts for automotive industry. The increasing use of this application type of steels in automotive industries has motivated the development of new alloys able to gather the critical requirements for deep drawing combined with high strengths. Samples of this steel in hot-rolled condition were cold-rolled with reductions of 50% and 70%, commonly used in most of applications, and isothermally annealed in salt bath at temperatures between 600 and 700ºC. The microstructure of the samples in both coldworked and annealed states was imaged using light optical (LOM) and scanning electronic microscopies (SEM). Deformation and recrystallization textures were determined by X-ray diffraction. The recrystallized volume fraction (Xv) and the mean grain size were determined by quantitative metallography in the annealed samples. Vickers microhardness testing was used to determine the work hardening and the softening kinetics behaviors. The results show that recrystallization occurs at temperatures above 650°C for the most deformed steel. By plotting Xv against A (softening fraction), we observe the occurrence of a deviation from ideality at 650°C and 700°C, likely caused by solid-state precipitation, especially for the less deformed steel. The values of n (Avrami exponent) obtained from both approaches (softening curves and quantitative metallography) are presented and discussed.

Les propriétés d'usage de l'alliage Ti17 dépendent fortement de la taille, de la morphologie et de la répartition des précipités de phase α au sein de la matrice β. Ces paramètres microstructuraux découlent des conditions de traitement thermomécanique et c'est pourquoi la maîtrise des propriétés mécaniques et l'optimisation des procédés nécessitent de connaître les microstructures finales comme leur évolution pendant leur mise en oeuvre. A cet effet, deux approches de modélisation sont développées dans le cas du refroidissement consécutif au forgeage à haute température :<br />La première approche utilise la loi de Johnson – Mehl – Avrami – Kolmogorov (JMAK) qui permet de prédire, à l'échelle macroscopique, la proportion des différentes morphologies de la phase α. Le calcul métallurgique nécessite au préalable la mesure des cinétiques isothermes de transformation afin de déterminer les paramètres de la loi de JMAK. Des refroidissements continus à température contrôlée sont simulés, la comparaison avec l'expérience se révélant concluante. Le cas de pièces massives, sièges de gradients thermiques et microstructuraux est alors envisagé. Les données thermophysiques de l'alliage et l'enthalpie de transformation étant mesurées au préalable, les évolutions couplées thermiques et microstructurales sont prédites à l'aide du logiciel de calcul par Eléments Finis ZeBuLoN dans lequel nous avons introduit le modèle de calcul de cinétique de transformation de phase. Les calculs sont comparés aux résultats obtenus lors d'expériences de refroidissement de cylindres instrumentés. Le module de calcul ainsi obtenu est appliqué au cas d'une pièce industrielle.<br />La deuxième approche repose sur la modélisation de la germination et de la croissance des précipités de phase α aux joints de grain. Ce calcul à l'échelle microscopique, celle des précipités, permet de prédire l'influence de la déformation plastique dans le domaine β sur la transformation de phase. Le modèle s'appuie sur les microstructures de déformation de la phase β à haute température, (par exemple la taille de grain recristallisé), fonction des conditions de déformation. Les cinétiques de transformation sont prédites pour divers traitements à température contrôlée consécutifs à une simple mise en solution ou bien à une déformation plastique dans des conditions données. Concernant la prise en compte d'une pré déformation, la comparaison avec des résultats expérimentaux de la littérature est concluante. Par ailleurs, le modèle est utilisable sur d'autres alliages (connaissance des compositions d ‘équilibre des phases et des coefficients de diffusion) et il permet d'accéder à des paramètres microstructuraux tels que des tailles de précipités.

The process optimization of continuous annealing furnace, RHF 125, for recrystallization annealing of two steel grades, Sandvik 13C26 and Sandvik 20C has been carried out. To recreate the continuous annealing process carried out in the roller hearth furnace in the industry, samples with different cold reduction rates were chosen from ongoing production lines. An experimental heat treatment model was chosen by the ‘Design of Experiments’ approach from MODDE (from U-Metrics). The annealing temperature was chosen below the austenitization temperature for both steel grades and soaking time of 30 seconds to 240 seconds were chosen. Microscopic estimation of fraction recrystallized was performed with the help of Electron Back Scattered Diffraction, accompanied by mechanical testing methods to measure the hardness and yield strength of the steel strips. The experimental output was used to create a model to correlate between the different cold reduction rates and annealing parameters to achieve a higher degree of recrystallization along with desirable mechanical properties. Also, a modified Johnson-Mehl-Avrami-Kolomogrov model, based on hardness values, to determine the transformation kinetics by tracking the progress of recrystallization was developed. The model was verified with EBSD measurements for Sandvik 13C26 strip steels. For 20C, inhomogeneous recrystallization was observed, thus limiting the model’s adaptability to steels which exhibit homogeneous recrystallization behavior and negligible change in precipitation and/or coarsening of secondary phases. / Processoptimering av en kontinuerlig glödgningsugn, RHF 125, för rekristallisationsglödgning av två Sandvik-stål, Sandvik 13C26 och Sandvik 20C, har genomförts. För att återskapa den kontinuerliga glödgningsprocessen som utförs den verkilga processen i valdes prover och olika kallreduktionshastigheter från pågående produktionslinjer. En experimentell värmebehandlingsmodell valdes med metoden 'Design of Experiments' med MODDE (från U-Metrics). Glödgningstemperaturen valdes till en temperatur under austeniseringstemperaturen för båda stålen och hålltider varierade från 30 s till 240 s. Mikroskopisk uppskattning av fraktionen rekristalliserat material utfördes med hjälp av Electron Back Scatter Diffraktion (EBSD), åtföljd av mekaniska testmetoder för att mäta hårdheten och sträckgränsen för stålproverna. De experimentella resultaten användes för att skapa en modell för att korrelera mellan de olika reduktionshastigheterna och glödgningsparametrarna för att uppnå högre grad av rekristallisation tillsammans med önskvärda mekaniska egenskaper. Dessutom utvecklades en modifierad Johnson-Mehl-Avrami-Kolomogrov-modell, baserad på hårdhetsvärden, för att bestämma transformationskinetiken genom att spåra evolutionen för rekristallisation. Modellen verifierades genom jämförelse med EBSD-mätningarna för Sandvik 13C26 bandstål. För 20 °C observerades inhomogen rekristallisation, vilket begränsade modellens användbarhet till stål som uppvisade homogent rekristallisationsbeteende och försumbar förändring i utskiljning och/eller förgrovning av sekundära faser.

<p> The recrystallization behavior of three Al-Mg-Si-Cu alloys with varying iron and manganese additions was studied by differential power scanning calorimetry under nonisothermal annealing conditions. The influence of cold deformation on the precipitation sequence and its interaction with recrystallization was also investigated. The DSC experiments were complemented by hardness measurements and microstructural studies by optical and electron microscopy. The DSC signals, after optimization of the baseline, were used for the calculation of the kinetic parameters of the recrystallization process. Two different modeling approaches based on global JMAK kinetics were implemented. The first approach utilizes the classical isothermal JMAK expression directly, while the second approach introduces a path variable related to the thermal history of the material in the JMAK description. Model-independent estimates of the activation energy were also evaluated using the Flynn-Wall-Ozawa integral isoconversion method. </p> <p> The results show that the initial stages of recrystallization are not affected by the preceding precipitation processes and recrystallization always follows the precipitation of the Q' phase. However, during recrystallization enhanced coarsening of the Q' phase takes place leading to its transformation to the more stable Q phase. The Q phase exerts a Zener pinning pressures on the migrating boundaries preventing the formation of an equilibrium grain structure. Moreover, for high Fe and Mn additions, discontinuous precipitation of Mg2Si overlaps with the end of recrystallization and exerts an additional pinning pressure on the boundaries. Varying the Fe and Mn content significantly affects the recrystallization kinetics. PSN is promoted in alloys with the higher Fe and Mn content and the recrystallization temperature shifts to lower values. The modeling results show that the recrystallization process conforms to the classical JMAK type behavior. The course of the reaction was reproduced successfully by the path variable approach and the evaluated activation energies were in good agreement with the isoconversional model-independent estimates. However, when the classical JMAK expression was applied directly to non-isothermal measurements, a dependency of the recrystallization process on thermal history was observed. </p> / Thesis / Doctor of Philosophy (PhD)

<p>Advanced high strength steels, such as dual-phase and martensitic steels, are increasingly being used by automakers to decrease the thickness of steel sheet used in parts without sacrificing part strength. When welded, the martensite within the dual-phase and martensitic steel microstructures tempers, reducing the heat-affected zone (HAZ) hardness compared to the base material, locally reducing strength. This process is known as HAZ softening. HAZ softening has been well studied; however, the kinetics of this process has not been quantified and the processes responsible for HAZ softening have not been examined. This thesis investigated both of these topics.</p> <p>HAZ softening was modelled using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. As the thermal profile during welding is non-isothermal, the effects of temperature and time on steel tempering kinetics could not be separated by examining post-welded properties. The effects of tempering temperature and time were separated through a series rapid isothermal tempering experiments. Hardness data from these experiments allowed the HAZ softening rate to be empirically quantified through fitting the JMAK equation. This material model was then validated by predicting HAZ softening in laser and resistance spot welds. Although the fitted JMAK constants could be used to predict post-weld HAZ hardness, they did not agree with the classic literature values associated with martensite tempering.</p> <p>To understand why the JMAK coefficients did not match those of the classic martensite tempering literature, the softening data from one of the martensitic steels was re-examined. This study revealed that the softening process was a combination of two processes: carbide nucleation and carbide coarsening. The activation energies calculated for each process matched the classic literature values. Carbide coarsening dominated during tempering, which had a non-linear relation with change in hardness. The relationship between carbide coarsening and hardness was responsible for the softening kinetics measured from the rapid tempering experiments.</p> / Doctor of Philosophy (PhD)

In an effort to minimize the harmful effects of intracellular ice formation (IIF) during cryopreservation of confluent tissues, computer simulations based on Monte Carlo methods were performed to predict the probability of IIF in confluent monolayers during various freezing procedures. To overcome the prohibitive computational costs of such simulations for large tissues, the well-known Johnson-Mehl-Avrami (JMA) model of crystallization kinetics was implemented as a continuum approximation of IIF in tissues. This model, which describes nucleation, growth, and impingement of crystals in a supercooled melt, is analogous to the process of intracellular ice formation and propagation in biological tissues. Based on the work of Weinberg and Kapral (1989), the JMA model was modified to account for finite-size effects, and was shown to predict accurately the results of freezing simulations in 1-D tissue constructs, for various propagation rates and tissue sizes. An initial analysis of IIF kinetics in 2-D tissues is also presented. The probability of IIF in 2-D liver tissue was measured experimentally during freezing of HepG2 cells cultured in monolayers, and compared to Monte Carlo simulations and predictions of the continuum model. The Avrami coefficient and exponent for IIF in HepG2 tissue were estimated to be k = 0.19 and n = 0.45.