This thesis apply High Resolution Electron Back Scatter Diffraction (HR-EBSD) technique to a variety of microstructure features and their interactions in pure h.c.p polycrystals. By correlating high quality Kikuchi patterns with a reference pattern, the relative state and distribution of strain, stress, and geometrically necessary dislocation (GND) density can be obtained with high strain sensitivity (10^-4) and angular resolution (10<sup-4</sup> radian). This technique is companied by a further investigation of subsurface features using Differential Aperture X-ray Micro-diffraction (DAXM) technique. The two technique have shown excellent agreement in capturing the magnitude and distribution of stress and GND. Stress field and GND distribution induced by slip band and grain boundary interactions, including blocked slip band with no observable slip transfer in SEM and slip transfer, were characterised. It was found that some blocked slip bands lead to high and localised stress concentration in the neighbouring grain while others did not, and no stress concentration were correlated with transferred slip bands. These three categories of interactions were rationalised using a slip transfer criteria (called LRB criteria) by investigating the geometric alignments between the impinging slip system and all possible slip systems in the neighbouring grain. The level of stress concentration were quantified into a stress intensity factor K, following the Frank, Eshelby, and Nabarro (FEN) model. It was found that the level of stress intensity correlates well with the number of dislocations within the pile up plane. The slip band and grain boundary interaction case that led to the highest magnitude of stress intensity factor was further investigated using DAXM experiments. The 3D data set informed us additional information hidden below the sample surface. The distribution of stress concentration in 3D is a ribbon conforming to the line of intersection between slip plane and grain boundary. Stress intensity factor calculation along this ribbon have shown large variations which led to a concern that sometimes 2D results might not be conclusive. For example, if damage is observed in sample surface, there is a possibility that large populations of damage already exist below sample surface as a result of the stress fluctuations. The level of stress concentration and distribution measured by both HR-EBSD and DAXM agree with each other and 3D lattice rotation gradient used in DAXM GND calculation was found to affect the range of GND distribution and how fast it decays away from grain boundary. Twinning is a deformation mechanism in HCP metal that is equally important as dislocation slip. The stress concentrations associated with twin propagation, approaching grain boundary, and thickening were characterised using HR-EBSD, from which the calculated stress tensor were used to generate a local Schmid factor (LSF) map. It was found that during twin propagation, local positive shear provides a favourable LSF condition that promote twin tip extension while supress it from thicken. When twin tip is approaching the grain boundary, the positive shear stress field no longer favour twin propagation, a narrow positive LSF field still exist at the tip of twin, promoting it to grow thick. During propagation and thickening process, the LSF seem to only affect the tip of twins and therefore these processes are possibly tip controlled.

Os aços elétricos apresentam uma enorme relevância para o mundo moderno, pois estão presentes na maioria das máquinas e aparelhos que funcionam a partir de energia elétrica. Existem duas classes de aços elétricos: Os aços elétricos de grão orientado (GO) e os aços elétricos de grão-não-orientado (GNO). Aços elétricos GNO são utilizados em motores elétricos. O rendimento energético dessas máquinas depende da permeabilidade e das perdas magnéticas. Durante a laminação a frio as propriedades magnéticas dos aços se modificam em conseqüência de mudanças estruturais e de textura que acompanham a deformação plástica. A evolução da textura e da microestrutura de deformação do aço elétrico de grão não orientado com 1,25% Si é descrita para graus de deformação entre 10 e 90%. A principal técnica de caracterização empregada foi à difração de elétrons retroespalhados (EBSD), acoplado ao microscópio eletrônico de varredura, que se mostrou bastante eficaz na caracterização da textura e da microestrutura de deformação, mesmo para altas taxas de deformação por laminação a frio. Uma mesma área selecionada também foi analisada, grão a grão, pela técnica de EBSD com vários graus de deformação. A distribuição de deformação na microestrutura em função do nível de deformação foi analisada qualitativamente. Regiões dos contornos de grãos apresentam altos níveis de deformação devido a suas altas densidades de discordâncias. Os grãos não rotacionam uniformemente quando submetidos à deformação. Subestruturas de deformação caracterizadas pela heterogeneidade de cores dentro dos grãos mostram a fragmentação do grão como resultado da operação de diferentes sistemas de deslizamento. Avaliou-se que possíveis efeitos da vizinhança não foram significativos e que orientação inicial do grão pode ser determinante na escolha da trajetória de rotação durante a deformação. / Electric steels are used in most of the machine and equipment using electrical energy. There are two classes of electric steels: Oriented grain electric steel (GO) and non-oriented electric steels (GNO). GNO electric steels are utilized in electrical motors. The energetic yield of these machines depends on magnetic permeability of the steels. This is a consequence of structural and textural changes which results from plastic deformation during manufacturing. Deformation texture and microstructure evolution for Fe-1,25% Si non oriented grain electric steel is presented for deformation levels between 10 and 90%. The main characterization technique utilized was Electron Backscattering Diffraction (EBSD) in a Scanning Electron Microscopy (SEM) which was very efficient in the characterization of deformation microstructure and texture, even for high deformation levels in cold rolling. A selected area was also analyzed, observing individual grains, by EBSD technique with different deformation levels. Grain boundaries show high deformation level due to these high deformation densities. Grains do not rotate uniformly when deformed. Deformation substructures characterized as different colors inside the grains show grain fragmentation due to the activation of different slip system. Possible neighborhood effects were evaluated, but they seemed to be not significant and the initial grain orientation was the main ground for the choice of the rotation trajectory during deformation.

Os efeitos da textura e do tamanho de grão de bobina a quente (BQ) na evolução da textura durante o processamento de aços elétricos semiprocessados foram estudados no presente trabalho. Observou-se que o aumento do tamanho de grão da BQ não é eficiente para obtenção de altas intensidades de componente Goss () após recozimento intermediário quando a bobina a quente apresenta intensidades muito elevadas (em torno de 14 vezes o aleatório) de cubo rodado (). Foi confirmada a hipótese de que ocorre nucleação de grãos com orientação Goss e cubo na face () em bandas de transição de grãos de fibra gama (//DN). Além disso, os resultados sugerem que a textura da BQ é importante para a evolução da textura durante o processo, levando à texturas finais distintas. Observou-se também forte relação entre tamanho de grão inicial (da BQ) e após laminação a frio e recozimento intermediário. Quanto maior o tamanho de grão inicial, maior o tamanho de grão recristalizado. O mesmo comportamento ocorreu durante o recozimento final, etapa em que ocorre migração de contornos induzida por deformação. Quanto maior o tamanho de grão após recozimento intermediário, maior o tamanho de grão após laminação de encruamento e recozimento final. Resultados interessantes foram obtidos a partir de chapas produzidas por solidificação direcional, encontrando-se texturas finais próximas a ideal, principalmente quando se utilizaram laminações a 800ºC. / The grain size and hot band texture effects on the texture evolution during the semi processed electrical steels were studied in this work. The results show that the increase of the hot band grain size is not efficient to obtain high Goss intensities () after intermediate annealing, when the hot band texture presents high intensities (around 14 times random) of rotate cube (). The nucleation of Goss and cube grains () in transition bands of gama fiber grains (<111//DN) was observed. Moreover, the results found in this work suggest that hot band texture is determinant to the texture evolution during the process. Strong influences of hot band grain size in recrystallized grain size after intermediate annealing was found. Samples produced by coarse grains presented high recrystallized grain sizes. The same comportment after temper rolling and final annealing was found, when strain induced boundary migration occurs. Coarse grains after intermediate recrystallization let to high grain sizes after final annealing. Excellent textures for samples produced from directional casting were found, mainly when the sheets were rolled at 800ºC.

Le maclage thermique est un défaut cristallographique largement discuté dans les métaux de type CFC à faible énergie de faute d'empilement. Malgré une importante littérature scientifique dédiée à ce sujet, les mécanismes expliquant précisément la formation de ces macles thermiques ne sont pas totalement élucidés à ce jour. Dans ce travail, nous avons cherché à améliorer notre compréhension de ce phénomène fondamental en métallurgie physique. Différents matériaux de type CFC (acier inoxydable 304L, nickel pur et Inconel 718) ont été considérés. Nous avons confirmé, grâce à des expériences de traitement thermique in situ couplées à des cartographies d'orientation, que la majorité des macles thermiques sont générées durant la recristallisation. De la même manière, par une expérience réalisée sur l'Inconel 718, nous avons mis en évidence que la croissance de grains pure n'était pas source de joints de macle. Par conséquent, il semble évident que les phénomènes de recristallisation et de croissance de grains ont des régimes totalement distincts associés à des mécanismes spécifiques du point de vue de la formation des macles thermiques, et doivent donc absolument être étudiés séparément. Nous avons ainsi proposé un nouveau modèle, dans lequel l'effet du signe de la courbure moyenne du front de recristallisation est pris en compte. Les influences de différents facteurs thermomécaniques, y compris le niveau de déformation, la taille de grains initiale, la température de recuit et la vitesse de montée en température, ont été étudiées à travers deux séries d'expériences. Suite à l'effet du signe de la courbure moyenne du joint de grain, nous avons proposé une méthode pour quantifier la tortuosité du front de recristallisation. Dans cette étude, nous montrons que cette quantité est corrélée à la densité de macles post-recristallisation. En sus des analyses expérimentales, des outils numériques de type champ moyen et champ complet ont également été développés dans cette étude afin de modéliser l'évolution des macles thermiques tout en tenant en compte des mécanismes physiques mis en évidence expérimentalement. Les bases d'un nouveau modèle de type champ moyen ont été proposées afin de modéliser l'évolution de la densité de macles moyenne durant le phénomène de croissance de grains. Ce modèle, dans lequel seulement un paramètre doit être identifié par des donnés expérimentales, semble mieux décrire les résultats expérimentaux obtenus pour l'inconel 718 comparé au modèle de Pande, référence en la matière. Deux méthodes implicites i.e. la méthode level-set et la méthode champ de phase ont été comparées au niveau de leurs formulations et de leurs performances numériques pour des simulations de croissance de grains anisotrope. C'est la première fois que ces deux méthodes sont comparées dans le contexte de l'utilisation de maillages éléments finis non stucturés et hétérogènes en terme de taille de maille. Une nouvelle méthodologie a été ainsi proposée dans le cadre de l'approche level-set pour simuler l'évolution de macles thermiques durant le phénomène de croissance de grains. Dans cette nouvelle méthodologie, les joints de macles peuvent être insérés dans des microstructures synthétiques. De plus, les joints de macles peuvent être distingués selon leur nature cohérente ou incohérente. Nous avons montré à travers les différentes simulations réalisées que les propriétés spéciales des joints de macles peuvent être prises en compte avec ce nouveau formalisme. / Annealing twin is a crystallographic defect that is largely reported in F.C.C. metals especially those with low stacking fault energy. Despite the amount of work dedicated to the subject, the understanding of annealing twin formation mechansims is not complete in the literature. In the present work, by applying both experimental and numerical tools, we tried to have a more profound understanding of this phenomenon, which is essential to Physical Metallurgy. For this purpose, different F.C.C. Materials including 304L stainless steel, commercially pure nickel and nickel based superalloy Inconel 718 are investigated. We confirmed that annealing twins are mainly formed in the recrystallization regime, especially driven by the migration of recrystallization front into deformed regions by using in situ EBSD technique. In addition, we found in the in situ observations that there are almost no twins generated in the grain growth regime. This observation is confirmed by another grain growth experiment performed on Inconel 718. Therefore, curvature driven grain boundary migration by itself is not sufficient to generate annealing twins. A new atomistic model to explain annealing twin formation mechanism, in which the effect of migrating boundary curvature is considered, is proposed. The effects of different thermo-mechanical factors, including prior deformation level, initial grain size, annealing temperature and the heating velocity, on annealing twin formation are determined via two experiments performed on commercially pure nickel. Based on the idea of grain boundary curvature, we proposed a method to quantify recrsytallization front tortuosity. In the present study, we show evidence that this quantity is positively correlated with the twin density at the end of the recrystallization regime. In addition to experimental studies, numerical tools including both mean field and full field approaches are applied to model annealing twin evolution during grain growth by taking into account the revealed mechanisms. A basis of a new mean field model is proposed to model annealing twin density evolution during grain growth. This model, which has only one parameter to be identified, provides a better consistency with the experimental data of Inconel 718 compared to the Pande's model. Besides, full field approaches are also applied to simulate the overall microstructure evolution during grain growth. Two implicit methods i.e. the level set and the multi-phase-field methods are compared in terms of their formulations and their numerical performance in anisotropic grain growth simulations. It is the first time that these two methods are compared in the finite element context with non-structural mesh. In the present numerical context, the level set method is more suitable to describe strong anisotropy in grain boundary energy. A new methodology is thus developed in the level set framework to simulate annealing twin evolution during grain growth. This methodology, in which we can insert annealing twin boundaries into synthetic microstructures and distinguish coherent and incoherent twin boundaries, is proven to be able to counting for the strong anisotropy introduced by coherent annealing twin boundaries.

Les tubes d’alliage Ti-3Al-2.5V laminés à pas de pèlerin sont utilisés à l’état dit « détendu » dans l’aérospatiale commerciale et militaire depuis la deuxième moitié du 20ème siècle. Malgré leur application très répandue, certains points de la relation entre le procédé de fabrication et les propriétés des tubes sont encore méconnus. Les propriétés à maîtriser dans ces produits sont la limite d’élasticité et l’allongement en traction, le CSR (Contractile Strain Ratio) et la résistance à la fatigue. Les premiers fabricants de ce type de tubes ont réalisé dès très tôt l’influence du procédé sur la texture cristallographique, puis des études plus approfondies ont montré l’influence des textures sur les propriétés clés. Cependant, la littérature ouverte au publique omets l’influence de la restauration et de la recristallisation, phénomènes que surviennent à plusieurs reprises au cours du procédé. Afin de répondre au besoin de comprendre l’influence de ces phénomènes, cette thèse se focalise sur la caractérisation fine des microstructures et textures locales par EBSD couplée aux analyses globales par diffraction de rayons X. Les résultats de ces travaux montrent que la recristallisation produite lors du recuit final est de nature discontinue et suit une cinétique locale variable en fonction des hétérogénéités de l’état écroui, qui sont quant à elles une conséquence de l’anisotropie de la phase alpha. La fraction volumique recristallisée influence le comportement mécanique en traction des tubes. Il a été montré que l’apparition d’un faible adoucissement suivi d’un plateau de Lüders sur les courbes de traction est accentuée par l’avancement de la recristallisation. De même, il a été montré que les composantes principales des textures de déformation restent stables jusqu’à des états très avancés de la recristallisation. On observe cependant un affaiblissement de l’intensité de la texture et une microstructure de grains fins et équiaxes. Ce résultat suggère que la phase beta inhibe la croissance de grains et les évolutions de texture qu’y seraient associées. Ce travail analyse aussi l’effet des séquences de déformation et recristallisation sur les textures et microstructures finales. Il a été confirmé que la texture peut être modifiée par le rapport de réduction au laminage et que les textures déterminent la valeur du CSR. Ce dernier résultat a été bien reproduit par des simulations de plasticité cristalline. Quant aux microstructures, l’analyse de tubes de dimensions variées suggère que la taille des grains recristallisés peut être contrôlée par le choix du chemin de déformation de toute la gamme de transformation. Enfin, la résistance à la fatigue des tubes a été étudiée par des tests de fatigue à pression pulsée. Les résultats indiquent que les tubes à texture tangentielle ont une meilleure tolérance aux défauts de surface et à la propagation des fissures. Il apparait cependant que la texture radiale améliore la limite d’endurance sous les conditions de ces essais / Cold pilgered Ti-3Al-2.5V alloy tubes are used in a stress relieved condition in commercial and military aircraft since the late 20th century. Despite the extensive use of these tubes, some aspects of the relations between processing and properties are still misunderstood, restricting the possibilities for the development of new products. The key properties are the tensile yield stress and elongation, the Contractile Strain Ratio (CSR) and the fatigue resistance. While early studies have shown how the pilgering process impacts texture and some of the effects of texture on these properties, public literature dismisses the effects of recovery and recrystallization, which both occur during the various annealing treatments of the transformation schedule. To fill this gap, this work focuses on the fine characterization of the local microstructures and textures of tubes produced in industrial trials. EBSD maps presented in this work show that the stress relieving treatment produces discontinuous recrystallization with variable local kinetics depending on the heterogeneity of the deformed condition, which is caused by the high anisotropy of the hexagonal alpha phase. The recrystallized fraction Rx is shown to influence the tensile deformation behavior, as anomalous yield point phenomena are more pronounced when Rx increases. Under the studied conditions, strong recrystallization is shown to produce fine equiaxe grains and a weak texture although without significant modification of the deformation texture components. This result suggests that grain growth is inhibited by the beta phase present in the microstructure. This work also analyses the influence of the sequences of deformation and recrystallization on the final texture and microstructure. It was confirmed that texture can be modified by the ratio of thickness to diameter reduction and that texture determines the CSR. The latter result is well reproduced in crystal plasticity simulations. The microstructural analysis of a wide range of tube dimensions suggested that the sizes of recrystallized grains in the final microstructures can be tailored through the choice of the overall strain path. Finally, the fatigue behavior of tubes with similar microstructure and different texture were studied under internal pulsed pressure tests. The results suggest that tangentially textured tubes are more tolerant to surface damages and crack propagation at high and medium stress amplitudes. The radial textures, however, improve the endurance limit under the conditions of these trials

Les météorites les plus primitives, appelées chondrites, sont les témoins privilégiés de la formation des premiers solides du système solaire et des processus qui ont conduit à la naissance des premiers corps planétaires. Les chondres, qui sont des sphérules ferromagnésiennes ignées, et leur matrice finement cristallisée constituent l’essentiel du volume des chondrites primitives. L’origine des chondres comme celle de la matrice est encore largement débattue. Cette thèse interroge les relations qui existent entre les chondres et la matrice à grains fins qui les auréole fréquemment, notamment dans les chondrites carbonées. En prenant l’exemple de la chondrite Vigarano (CV3) nous explorons la piste d’un continuum des conditions physico-chimiques pour former les chondres et leurs auréoles de matrice à grain fin. La thèse aborde cette problématique par une double approche : une approche expérimentale consacrée à la quantification des vitesses de dissolution de l’olivine dans des liquides silicatés caractéristiques des chondres de type I et une approche de caractérisation détaillée de la minéralogie, des fabriques et de la composition isotopique de l’oxygène des chondres et de leurs auréoles de matrice à grains fins dans Vigarano. Ces travaux suggèrent que chondre et matrice pourraient enregistrer une continuité d’interaction avec le gaz environnant, d’abord de type liquide/gaz (solubilité) pour la formation des chondres, puis de type solide/gaz (condensation) à plus basse température pour la formation des auréoles de matrice à grains fins / The most primitive meteorites, the chondrites, are our best witnesses of solar system birth and formation of first solids as well as processes that formed the first planetary bodies. Both chondrules, which are ferromagnesian igneous spherules, and fine-grained matrix, constitute the largest volume fraction of primitive chondrites. Their origin still constitutes a matter of debate. This thesis questions the relationship between chondrules and fine-grained matrix, called fine-grained rims, that frequently overlay chondrules in primitive carbonaceous chondrites. By taking the example of Vigarano (CV3), we explore the idea that chondrules and fine-grained rims could form in a continuum of physical and chemical conditions. This thesis combines both experimental and characterization approaches, with an experimental determination of olivine dissolution rates in molten silicates representative of type I chondrule melts and a detailed petrographic characterization of mineralogy, fabrics and oxygen isotopic composition of chondrules and their fine-grained rims in Vigarano. This work suggests that chondrules and their fine-grained rims could register a continuous interaction with surrounding gas, with first, chondrule formation from liquid/gas interaction (solubility) and then fine-grained rim formation, at lower temperature and from solid/gas interaction (condensation)

L’objet de cette thèse consiste en des développements de méthodes en spectroscopie et imagerie Raman. Après une revue des outils statistiques permettant de traiter de gros volumes de données (analyse multivariée, classification), l’étude est appliquée à deux familles de matériaux déjà bien connus, utilisés comme modèles pour tester les limites des développements mis en oeuvre. La première famille est une série de matériaux carbonés, traités à différentes températures, avec des inhomogénéités à l’échelle du nm, dont la taille est déduite d’une approche conjointe Raman-diffraction des rayons X. Un autre résultat concerne l’effet du polissage, qui induit des artefacts Raman conduisant à surestimer le désordre structural local, et une méthode basée sur la largeur de la bande G est proposée. L’autre classe de matériaux présente cette fois des inhomogénéités à des échelles supérieures au micromètre, il s’agit de céramiques d’oxyde d’uranium appauvri. L’imagerie Raman, particulièrement bien adaptée en termes d’échelle spatiale, est utilisée pour sonder les surfaces obtenues, Le traitement des données est effectué par un approche mêlant analyse multivariée (analyse en composantes principales), et ajustement classique par Lorentziennes. L’interprétation des données obtenues est soutenue par une analyse par electron backscattering diffraction (EBSD), permettant de séparer l’effet d’orientation des grains de céramiques, d’autres effets sous-jacents. Ceux-ci, principalement localisés aux joints de grains, avec l’existence d’un mode Raman caractéristique, paraissent liés à des variations de stoechiométrie en oxygène ou d’impuretés, ou à des inhomogénéités de contrainte. Les perspectives de ce travail portent notamment sur la mise en oeuvre d’autres méthodes mathématiques, et pour l’aspect matériaux, sur l’analyse fine d’endommagements par irradiation des UO2, (effets d’anisotropie, rôle des joints de grains. / This thesis is aimed at developing methodologies in Raman spectroscopy and imaging. After reviewing the statistical instruments which allow treating giant amount of data (multivariate analysis and classification), the study is applied to two families of well-known materials which are used as models for testing the limits of the implemented developments. The first family is a series of carbon materials pyrolyzed at various temperatures and exhibiting inhomogeneities at a nm scale which is suitable for Raman--X-ray diffraction combination. Another results concern the polishing effect on carbon structure. Since it is found to induce Raman artifacts leading to the overestimation of the local structural disorder, a method based on the use of the G band width is therefore proposed in order to evaluate the crystallite size in both unpolished and polished nanographites. The second class of materials presents inhomogeneities at higher (micrometric) scales by the example of uranium dioxide ceramics. Being well adapted in terms of spatial scale, Raman imaging is thus used for probing their surfaces. Data processing is implemented via an approach combining the multivariate (principal component) analysis and the classical fitting procedure with Lorentzian profiles. The interpretation of results is supported via electron backscattering diffraction (EBSD) analysis which enables us to distinguish the orientation effects of ceramic grains from other underlying contributions. The last ones are mainly localized at the grain boundaries, that is testified by the appearance of a specific Raman mode. Their origin seems to be caused by stoichiometric oxygen variations or impurities, as well as strain inhomogeneities. The perspectives of this work include both the implementation of other mathematical methods and in-depth analysis of UO2 structure damaged by irradiation (anisotropic effects, role of grain boundaries).

Les alliages de zirconium sont utilisés notamment dans les assemblages de combustible nucléaire pour leur transparence aux neutrons ainsi que pour leur tenue mécanique et leur résistance à la corrosion. La connaissance de leur microstructure et de son évolution est nécessaire pour maîtriser les différents traitements thermomécaniques de la gamme de transformation qui comporte plusieurs trempes depuis le domaine bêta. Cette microstructure présente, à l’issue d’une trempe, des lamelles dites de Widmanstätten. Ces dernières soit se disposent parallèlement entre elles (platelets parallèles), soit se croisent en vannerie. Ces morphologies jouent sur l’étape suivante de filage ; en effet, les platelets parallèles défavorisent la fragmentation des lamelles. Une méthode a été mise en place pour quantifier ces morphologies.Lors de la transformation bêta vers alpha, un grain peut générer 12 orientations alpha (variants). Les paramètres qui influencent leur sélection sont encore peu connus. Le modèle proposé minimise la déformation moyenne lors de la transformation. D’abord analytique, il a été ensuite implémenté numériquement afin d’aborder des effets tels que la relaxation d’Eshelby, l’anisotropie élastique, une contrainte extérieure ou le voisinage. En parallèle, la sélection expérimentale a été quantifiée au moyen original de l’EBSD et des fractions des variants locales dans un ex-grain bêta. La confrontation des résultats expérimentaux avec le modèle a permis de le valider en partie et de déterminer la contrainte de trempe à la surface des éprouvettes ainsi que son effet sur la sélection de variants. / Zirconium alloys are frequently used in nuclear fuel assemblies. They are chosen for their low neutron absorption, their mechanical properties and their corrosion resistance. A better understanding of the microstructure evolution of these alloys should allow a better control of their process of fabrication. During processing, several quenches, from the beta to the alpha domain take place. The resulting microstructures are lamellar and are called Widmanstätten microstructures. These lamellae are either disposed in parallel or in crisscross and are named “parallel platelets” and “basketweaves”, respectively. These various morphologies have a significant impact on the extrusion; basketweaves facilitate grain fragmentation unlike parallel platelets. In this thesis project, a methodology was developed in order to quantify these morphologies.During the phase transformation, one beta grain can generate 12 different orientations of new alpha grains. The parameters which can influence variants selection are not yet well-known. The model proposed in the present study is based on the minimization of the mean elastic energy of the system during the phase transformation. First results were obtained analytically. Then, additional effects such as the Eshelby relaxation, the elastic anisotropy and the external strain were implemented numerically. In parallel, each alpha variant was quantified within a former beta grain by EBSD analysis. The comparison between the model and the experiments helped to partially validate the model as well as determine the quench strain on the surface of the sample. It was then possible to study the effect of quench strain on the variant selection.

Bulk commercially pure titanium was prepared by powder metallurgy, namely by cryogenic milling and spark plasma sintering, with aim to produce ultra-fine grained material with enhanced strength. The microstructure of milled powders was investigated in detail by a novel method called transmission EBSD, which allowed the first direct observation of texture within the powder particles. This texture is similar to rolling texture, because of the similar nature of the defor- mation during milling. Microstructure observations revealed grains with the size under 100 nm. The influence of sintering parameters on material properties were studied by scan- ning electron microscopy including EBSD, X-ray diffraction and by microhardness measurements. The trade-off relationship between porosity and grain size was identified, fully dense material with ultra-fine grained microstructure could not be produced. Increased oxygen content was identified as a main strengthening factor, while porosity has significant deteriorating effect on mechanical properties. The texture of powder was retained in the bulk material. The possibility of stabilizing the microstructure by mechanical alloying of Ti with yttrium oxide nanoparticles was investigated with mixed results. The stabiliza- tion was successful, but several issues...

[pt] Os aços inoxidáveis duplex (AID) possuem microestrutura com o balanço de fase aproximadamente iguais de ferrita (delta) e austenita (gama). A exposição desses materiais a elevadas temperaturas, em torno de 1000 graus C e 1100 graus C resultam em mudanças microestruturais significativas. Mudanças essas que podem variar desde a fração volumétrica das fases presentes, composição química, até precipitação de fases intermetálicas e relações de orientação presentes no material. Dependendo da fração volumétrica e morfologia das fases, haverá diferentes respostas a um determinado meio corrosivo, dentre eles o clorídrico. A composição química do material pode influenciar na formação das fases intermetálicas. E o empobrecimento local de determinados elementos influenciam no processo de repassivação do material. Em condições práticas de soldagem e tratamento térmico algumas propriedades microestruturais do material podem ficar sujeitas a alterações, dentre elas a resistência à corrosão por pites. Os materiais estudados no presente trabalho foram os AID UNS S32304 (lean Duplex) e o UNS S32750 (super duplex), ambos submetidos a tratamentos térmicos a 1000 graus C e 1100 graus C por 24 h seguidos de têmpera em água, os mesmos passaram por ensaio de corrosão segundo a norma ASTM G48-A. A presente dissertação utiliza como método de estudo a caracterização microestrutural através da técnica de microscopia eletrônica de varredura por meio da difração de elétrons retroespalhados (MEV/EBSD), para determinação das relações de orientação Kurdjumov-Sachs (K-S) e Nishiyama-Wassermann (N-W), correlacionando-as com o efeito do tratamento térmico nos aços estudados. Foi realizada a determinação da fração volumétrica das fases em função da temperatura de tratamento térmico. Foi identificada através da espectroscopia de energia dispersiva de raios-X a influência da composição química no material para iniciação da corrosão. A microscopia óptica foi realizada para análise da evolução morfológica e identificação das fases principais e intermetálicas. Foi verificada a precipitação da fase Sigma (sigma) no UNS S32750, e a constatação que os pites iniciaram na interface austenita secundária e sigma. Através da técnica de MEV/EBSD utilizando de uma análise qualitativa e semi-quantitativa verificou-se que o pite tem como local preferencial de iniciação junções triplas e/ou múltiplas de contorno de grão quando o PREN (Pitting Resistance Equivalent Number) das fases austenita e ferrita são próximos. Entretanto para grande variação no valor do PREN entre austenita e ferrita alguns pites iniciaram na fase com menor valor de PREN. / [en] Duplex stainless steels (DSS) have microstructure with approximately equal phase balance of ferrite (delta) and austenite (gamma). Exposure of these materials to high temperatures around 1000 degrees C and 1100 degrees C results in significant microstructural changes. These changes can vary from the volume fraction of the phases present, chemical composition, to precipitation of intermetallic phases and orientation relationship present in the material. Depending on the volume fraction and phase morphology, there will be different responses to a particular corrosive environment, including hydrochloric. The chemical composition of the material can influence the formation of the intermetallic phases. And the local impoverishment of certain elements influences the process of repassivation of the material. Under practical conditions of welding and heat treatment, some microstructural properties of the material may be subject to changes, including resistance to pitting corrosion. The materials studied in the present work were UNS S32304 (lean Duplex) and UNS S32750 (super duplex), both subjected to heat treatments at 1000 degrees C and 1100 degrees C for 24 h followed by quenching in water, the same underwent corrosion testing according to the ASTM G48-A standard. The present dissertation uses as a method of study the microstructural characterization through the technique of Scanning Electron Microscopy by means of electron backscattered diffraction (SEM/EBSD) to determine the Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) orientation relationships, correlating them with the effect of heat treatment on the studied steels. The determination of the volumetric fraction of the phases as a function of the heat treatment temperature was carried out. It was identified through energy dispersive spectroscopy X-ray the influence of the chemical composition on the material for corrosion initiation. light optical microscopy was performed to analyze the morphological evolution and identify the main and intermetallic phases. It was verified the precipitation of the Sigma phase (sigma) in UNS S32750, and the verification that the pits started at the secondary austenite and σ interface. Through the SEM/EBSD using a qualitative and semi-quantitative analysis, it was found that the pit has as its preferred initiation site triple and/or multiple grain boundary junctions when the PREN (Pitting Resistance Equivalent Number) of the austenite and ferrite phases are close. However, due to the large variation in the PREN value between austenite and ferrite, some pits started in the phase with the lowest PREN value.