Ces travaux ont pour but d'évaluer l'apport de la modélisation numérique pour étudier la modification de la plasticité des polycristaux métalliques par l'hydrogène absorbé. De précédents travaux ont proposé une quantification expérimentale de cet effet, grâce à des mesures par microscopie à force atomique (AFM) des marches de glissement émergeant à la surface d'agrégats polycristallins (316L), chargés ou non en hydrogène.Après avoir étudié l'impact de la modélisation géométrique sur la précision des résultats numériques, nous proposons une méthode permettant d'analyser les résultats AFM grâce à la modélisation numérique, en prenant en compte le niveau de déformation plastique à l'échelle du grain et le fait que les mesures AFM sont des projections des dimensions "réelles" des marches de glissement: le nombre de marches de glissement émises et l'espacement inter-marche. Ces quantités permettent alors de comparer les comportement plastiques observés expérimentalement sur différents agrégats, et donc de quantifier l'impact de l'hydrogène absorbé sur le développement de la plasticité.Nous étudions ensuite la capacité du modèle numérique pour modéliser une modification de la plasticité à l'échelle intragranulaire: des hétérogénéités sont introduites au sein d'un modèle de grain et l'impact sur la distribution de la déformation plastique résultante est analysée. / The modification of plasticity observed in hydrogen-charged metalic polycristals has been studied using numerical modeling (Finite Element Method). This effect has been quantified by a previous study using Atomic Force Microscopy (AFM), by measuring the slip steps forming at the surface of (hydrogen-)charged or uncharged 316L polycristals. However the heterogeneity of the strain field in a polycristal makes it difficult to compare precisely the results from different grains and aggregates.After analyzing the impact of the geometrical modelling on the numerical results, this present study porposes a method using numerical simulations (Crystal Plasticity model) to access the local plastic strain field at grain scale, and improve the analysis of the AFM results. The projections of the slip step "real" dimensions into AFM measures (heights and spacings) are taken into consideration in order to convert AFM data into data that are directly linked to plastic activity: the average number of dislocations and slip step spacing. This quantities make it possible to compare the experimental plastic behaviours of the differents agregates in order to quantify the impact of the hydrogen absorption.The capacity of the crystal plasticity model to simulate plasticity modification at intragranulare scale is then studied by implementing material heterogeneities within a grain model, and the resulting modification of the slip developpement within the grain is then analyzed.

Made available in DSpace on 2017-07-21T20:42:39Z (GMT). No. of bitstreams: 1 Ederson Pauletti.pdf: 5739200 bytes, checksum: 362611be295a3342251b72d168512c61 (MD5) Previous issue date: 2013-03-22 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work the ductile crack growth resistance curves (J-R curves) was evaluated for the steels used in the oil and gas pipelines from the X60 and X70 API 5L grades using the unloading compliance technique. The non standardized SE (T) specimens were used due to the same has low constraint, it is feature of pipelines. The plastic area for the obtaining the J integral was performed by measuring from the crack mouth open displacement, (CMOD). The central emphasis this research was evaluated the delamination phenomenon influence from the fracture toughness and along the J-R curve. As also the microtexture in close to delamination and ductile fracture regions, by using the Electron Backscatter Diffraction (EBSD) technique. The results have shown that value average of 355kJ/m2 was similar for both the shallow and deep crack specimens. The J-R curve values average from the shallow crack specimens have displayed most steep J-R curves, namely, it obtained higher energy values along the ductile crack propagation. Specimens that showed delaminations, during fracture process, obtained higher energy values along the ductile crack propagation. The occurrence of delaminations was more common in deep crack specimens. The microtexture results have shown a random grain sizes distribution. The components with higher intensity in the delamination region were α-fiber textures: {113} <110>, {112} <110>, {223} <110> the same cause anisotropy of mechanical properties and can be influenced out-of-plane constraint during the test. Moreover, there was the occurrence of the {100} <110> recrystallization texture component, which have deleterious effect in occurrence of delamination in microalloyed steels. Whereas in the ductile fracture region occurred the {111} <231>, {111} <132> and {111} <123> γ-fiber, which it can be influenced the ductile crack propagation process. / Neste trabalho foram investigadas as curvas de resistência ao crescimento de trinca dúctil para aços utilizados em tubulações de petróleo e gás, das classes API 5L X60 e X70, utilizando-se da técnica de variação da flexibilidade elástica. Espécimes SE(T) não padronizados foram utilizados, pois o mesmo possui baixa restrição ao fluxo plástico, característica de tubulações. A área plástica para obtenção da integral J foi obtida através da medida de deslocamento de abertura de trinca (crack mouth open displacement, CMOD). A ênfase central desta pesquisa foi avaliar a influência do fenômeno da delaminação na tenacidade à fratura (JIC) e na curva J-R em geral. Como também a obtenção da microtextura em regiões próximas à delaminação e fratura dúctil, que ocorrem durante o processo de fratura no ensaio de tenacidade à fratura, pelo uso da técnica de difração de elétrons retroespalhados (EBSD). Os resultados mostraram que o valor médio de tenacidade à fratura, de 355 kJ/m2 foi equivalente para espécimes com tamanho de trinca rasa e profunda. A média dos valores de curva J-R para espécimes com tamanho de trinca rasa mostrou curvas J-R mais íngremes, ou seja, obtiveram maiores valores energia ao longo da propagação dúctil de trinca. Os espécimes que apresentaram delaminações, durante o processo de fratura, obtiveram maior valor de energia ao longo do crescimento de trinca dúctil. A ocorrência das delaminações foi mais comum em espécimes com tamanho de trinca profunda. Os resultados de microtextura mostraram uma distribuição aleatória de tamanhos de grão. As componentes com maior intensidade na região próxima à delaminação foram as texturas de fibras α: {113}<110>, {112}<110>, {223}<110> que causam anisotropia das propriedades mecânicas e podem ter influenciado no estado triaxial de tensões durante o ensaio. Além disso, houve ocorrência da componente de textura de recristalização {100}<110>, a qual tem efeito deletério na ocorrência de delaminações em aços microligados. As componentes de fibra mais intensas na região próxima à fratura dúctil foram as texturas de fibra γ: {111}<231>, {111}<132> e {111}<123>, as quais podem influenciar no processo de propagação de trinca dúctil.

Este trabalho consiste no estudo das propriedades mecânicas de tração e impacto do aço Hadfield refinado com Hf em relação ao aço Hadfield sem refino. Foram realizadas simulações no software Thermo-Calc para caracterizar o caminho de solidificação e a evolução microestrutural do aço Hadfield (composição química de 13% de Mn, 1,2% de C e 0,65% de Si). Foram fundidos os corpos de prova de aço Hadfield sem refino e refinado pela adição de Hf, ambos com a composição química de acordo com a norma ASTM A128 - grau B2. Os corpos de prova foram submetidos ao tratamento térmico de solubilização à temperatura de 1120oC por 10 horas. A caracterização macroestrutural mostrou que o aço Hadfield com adição de Hf apresenta tamanho do grão austenítico cinco vezes menor (600 µm) do que o aço Hadfield sem refino (de 3000 µm para 600 µm), sendo que este resultado foi confirmado pela técnica de EBSD (Electron backscatter diffraction). Os resultados do ensaio de tração dos corpos de prova para as duas condições mostraram que as propriedades de limite de escoamento (6%), tenacidade (88%), resistência à tração (37%), e do coeficiente de encruamento (30%) do aço Hadfield refinado aumentaram em relação ao aço Hadfield sem refino. A energia absorvida no impacto aumentou de 156 J/cm2 para 179 J/cm2 com o refino do tamanho de grão austenítico. A lei de Hall-Petch foi utilizada para explicar o efeito do refino de grão no limite de escoamento, mas não pode ser usada para explicar o aumento na resistência a tração, tenacidade, e as mudanças no coeficiente de encruamento. A caracterização microestrutural das amostras rompidas em tração e impacto mostrou que a fração de maclas aumentou em 46% (em tração) e 45% (no impacto) para o aço Hadfield refinado em comparação com o aço Hadfield sem refino. Os resultados da caracterização microestrutural dos corpos de prova rompidos no ensaio de tração e impacto indicam que o mecanismo de deformação plástica por maclação foi mais atuante com o refino do grão austenítico. / The tensile and impact properties of Hf-refined Hadfield steel were compared to the properties of non-refined Hadfield steel. Simulations in Thermo-Calc software were made in order to study the solidification and microstructural evolution of a Hadfield steel (13% Mn, 1.2% C and 0.65% Si). Samples of non-refined and refined Hadfield steel (with addition of 0.1% of H), both with chemical composition as described in ASTM A128- grade B2, were cast and subjected to a solution heat treatment at 1100oC for 10 hours, followed by water quenching. Macrostructural characterization showed that the refined Hadfield steel featured an austenitic grain size equal to 600 ?m, five times smaller than the grain size of the non-refined Hadfield steel (3000 µm). These results were confirmed by EBSD (Electron backscatter diffraction) analysis. The tensile test results showed an increase in the values of the yield stress (6%), the fracture toughness (88%), the tensile strength (37%) and the strain hardening coefficient (30%) with the reduction on the austenitic grain size from 3000 µm to 600 µm. Additionally, the value of the absorbed energy of the impact test was improved from 156 J/cm2 to 179J/cm2 with reduction on the grain size. The Hall-Petch law was used to explain the small increase of 8% in the value of the yield stress with the grain refinement, but this law could not be used to explain the increase in the values of the fracture toughness, the tensile strength and the strain hardening coefficient. The microstructural characterization of the fractured test-pieces after tensile and impact testing showed that the values of the fraction of mechanical twins (definer) in the plastically deformed microstructure near the fracture surfaces of the refined condition increased by 45% when compared to the non-refined condition. These results indicated that the austenitic grain refinement of the Hadfield steel changed the plastic deformation mechanism, favoring the activation of the plastic deformation by twinning mechanism and, therefore, increasing the values of the fracture toughness (~88%), the ultimate tensile strength (~30%), the strain-hardening coefficient (~30%) and the absorbed energy during Charpy-test (~15%).

Les travaux portent sur deux nuances d’alliages Ni-Cr-W, l’une contenant des précipités de W (EM722) et l’autre non (EM721). Les matériaux forgés, fournis par Aubert & Duval, présentaient des microstructures hétérogènes avec de très gros grains. Dans un premier temps, une procédure thermomécanique a été développée afin d’obtenir un état de référence avec une microstructure homogène avec une faible taille de grains et une texture relativement peu accusée. A partir de cet état de référence, l’évolution de la texture, au cours du laminage à froid (différents taux de réduction), a été analysée par diffraction des neutrons et a montré un renforcement de la texture de type Laiton. Les mécanismes de déformation ont été étudiés en microscopie électronique en transmission et par EBSD (Electron BackScatter Diffraction) afin de comprendre la formation de ce type de texture. Les bandes de cisaillement sont supposées être un facteur important pour la formation de ce type de texture. A partir des états déformés, la cinétique de recristallisation a été déterminée pour deux températures (700°C et 900°C) à partir de mesures de microdureté et par EBSD. Les mécanismes de recristallisation ont été étudiés par MET. Le mécanisme de SIBM (Strain Induced grain Boundary Migration) est le mécanisme prépondérant pour la nuance EM721. Pour l’autre nuance, EM722, possédant des précipités de W, le mécanisme de SIBM est présent, mais en plus, un second mécanisme de PSN (Particle Stimulated Nucleation) a été mis en évidence. De plus, quel que soit le mécanisme de recristallisation, une fois que les germes sont formés, ils maclent très rapidement. Par conséquent, une étude complémentaire sur l’évolution des joints de macle pendant la recristallisation primaire a été réalisée, en employant l’EBSD et surtout le système ASTAR installé dans un MET. Il a alors été montré que cette évolution dépend à la fois de la taille des grains et de l’énergie stockée par les grains pendant la déformation. Enfin, une simulation de la recristallisation primaire de type Monte-Carlo a également été mise en œuvre. Son originalité réside notamment dans sa capacité à rendre compte de la formation des joints de macles cohérents et rectilignes. / This work focuses on two grades of Ni-Cr-W alloys, one containing W precipitates (EM722) and the other not (EM721). The forged materials, provided by Aubert & Duval, had heterogeneous microstructures with very large grains. At first, a thermo-mechanical procedure was developed to obtain a reference state with a homogeneous microstructure with a small grain size and a texture relatively weak. From this reference state, the texture evolution during cold rolling (different reduction rates), was analyzed from neutron diffraction measurements and showed the strengthening of the Brass-type texture. The deformation mechanisms were studied by TEM (transmission electron microscopy) and EBSD (Electron Backscatter Diffraction) to understand the formation of this type of texture. The formation of shear bands is assumed to be the most important factor for the formation of such a texture. From the deformed states, the recrystallization kinetics was determined for two temperatures (700°C and 900°C) from micro-hardness and EBSD measurements. The recrystallization mechanisms were investigated by TEM. The SIBM (Strain Induced Grain Boundary Migration) mechanism is the predominant mechanism for the EM721 material. For the other one (EM722), which has W precipitates, the SIBM mechanism is present, but a second one (PSN: Particle Stimulated Nucleation) has also been evidenced. Furthermore, regardless of the mechanism of recrystallization, once the nuclei are formed, they twin quickly. Therefore, a further study on the evolution of twin boundaries during primary recrystallization was performed, using EBSD and especially the ASTAR system installed in a TEM. It has been shown that the twin development depends both on the grain size and the stored energy during the deformation. Finally, a Monte Carlo simulation of the primary recrystallization was also implemented. Its originality lies especially on its ability to create coherent and straight twin boundaries.

Les alliages de titane β-métastables comme le Ti-10V-2Fe-3Al se substituent progressivement aux alliages α/β dans les applications aéronautiques du fait de leur résistance spécifique améliorée. Leurs microstructures d'emploi sont cependant complexes et multi-échelles, constituées d'une matrice β (de grains millimétriques) partiellement transformée en nodules primaires αp (micrométriques) et en lamelles secondaires αs (sub-micrométriques). Les propriétés finales peuvent être très sensibles aux variations locales de microstructures et sont souvent non maîtrisées lors du forgeage de pièces massives. De plus la matrice β qui représente ~40% du volume et qui a un comportement élastique et plastique fortement anisotrope, comme la phase α, complique la compréhension des mécanismes de déformation en jeu. Le premier objectif de cette thèse est de mettre en œuvre des techniques de caractérisation multi-échelles (la diffraction des neutrons, l'imagerie électronique couplée à l'analyse d'image et l'EBSD, la reconstruction des microtextures de haute température β/αp) pour analyser efficacement la microstructure/texture des constituants β/αp/αs et caractériser leurs hétérogénéités au sein de demi-produits et de pièces obtenues par matriçage. Les résultats permettent d'analyser la fragmentation des grains β en sous-grains, les macrozones αp, le maintien de relation d'orientation entre β/αp et l'organisation des lamelles αs en colonies ou paniers tressés, en pointant les différences de taille de domaines révélés par la cristallographie et l'imagerie standard. Le second objectif est d'appliquer cette méthodologie à l'analyse de facies de rupture d'éprouvettes présentant un comportement singulier (en traction ou en fatigue) pour caractériser les configurations microstructurales à l'origine de l'amorçage de fissures. Cette analyse a principalement été réalisée par polissage manuel du faciès couplé à des acquisitions EBSD mais également en exploitant le potentiel de l'imagerie 3D par MEB-FIB (Focus Ion Beam) et la technique TKD (Transmission Kikuchi Diffraction) sur lame mince prélevée au niveau d'un site d'amorçage par FIB. Enfin, cette étude expérimentale a été complétée par une première approche en simulation micromécanique sur une microstructure modèle 100% β. L'objectif était d'évaluer l'influence de l’anisotropie élastique de la phase β sur la genèse de contraintes d'incompatibilités dans les régimes élastique et élasto-plastique. L'ensemble des résultats contribue à une meilleure compréhension des variations de propriétés mécaniques en lien avec la microstructure locale / The β-metastable titanium alloys such as Ti-10V-2Fe-3Al are gradually replacing α/β alloys in aeronautical applications thanks to their improved specific strength. However, their microstructures are complex and multi-scale, consisting of a β matrix (of millimetric grains) partially transformed into primary αp nodules (micrometric) and secondary αs lamellae (sub-micrometric). The final mechanical properties are very sensitive to local variations of the microstructure, which are not always fully controlled during forging of massive parts. Moreover, the β matrix, which represent 40% of the volume and whose elastic and plastic behavior is strongly anisotropic (like the α phase) complicates the understanding of the mechanisms of deformation. The first objective of this thesis was to efficiently characterize the microstructure/texture of the different constituents (β/αp/αs) and their heterogeneities within half-finished products and forged parts by using techniques of multi-scale characterization (neutron diffraction, electronic imaging coupled with image analysis and EBSD, reconstruction of high temperature microtextures β/αp). As a result the fragmentation of the β grains into subgrains, the αp macrozones, the destruction of the orientation relation between β/αp and the organization of the αs lamellae in colonies or basket weave was quantified and the differences in size of domains revealed by crystallography and by standard imaging were pointed out. The second objective is to apply this methodology to the analysis of fracture surfaces of samples exhibiting singular behavior (in tension or in fatigue) in order to characterize the microstructural configurations leading to early cracking. This analysis was mainly performed by manual polishing coupled with EBSD acquisitions but also by using 3D imaging by SEM-FIB (Focus Ion Beam) and TKD (Transmission Kikuchi Diffraction) technique on a thin foil FIB-extracted from the crack initiation site. Finally, this experimental study was completed by a micromechanical simulation on a 100% β model microstructure. The objective was to evaluate the influence of the elastic anisotropy of the β phase on the genesis of incompatibility stresses in the elastic and elasto-plastic regimes. The overall results contribute to a better understanding of the variations of mechanical properties related to the local microstructure

In this study polycrystalline NiAl has been subjected to torsion deformation. Torsion has been used because of its characteristics. By this deformation mode high shear strains (gamma = 18 in this study) can be imposed on the sample. The deformation conditions are well-defined because of the local deformation mode, which is simple shear. Due to the monoclinic sample symmetry one half of the pole figure is needed in order to obtain the complete texture information, which is more than is needed e.g. by extrusion or rolling. Therefore, texture analysis might be more sensitive with respect to texture components. Furthermore, torsion deformation is characterized by being inhomogeneous in terms of the amount of shear strain and shear strain rate along the sample radius. The shear strain gradient makes the analysis of different stages of deformation on the same sample (i.e. under the same deformation conditions) possible. Another characteristic being special for torsion is that samples change their length, although no axial stress is applied. This effect is known as Swift effect and will be analyzed in detail. The deformation, microstructure and texture development subject to the shear strain are studied by different techniques (Electron Back-Scatter and High Energy Synchrotron Radiation). Beside the development of microstructure and texture with shear strain, the effect of an initial texture as well as the deformation temperature on the development of texture and microstructure constitute an important part of this study. Therefore, samples with three different initial textures were deformed in the temperature range T = 700K – 1300K. The development of the microstructure is characterized by two different regimes depending on the deformation temperature T. For T up to 1000K, continuous dynamic recrystallization (CDRX) takes place. This mechanism leads to the deformation-induced dislocations forming low angle grain boundaries (LAGBs) or being incorporated into them and the successive transformation of these boundaries into high angle grain boundaries (HAGBs) by a further increase of their misorientation. The predictions of this model were compared with the experimental results. The shear stress – shear strain curves are characterized by a peak at low strains, which is followed by softening and a steady state at high strains. This condition is fulfilled for a number of samples, but especially &amp;lt;111&amp;gt; oriented samples do not show a softening stage at low temperatures. Grain refinement takes place for all samples and the average grain size decreases with temperature. The predicted LAGB decrease is in best agreement with the experiments at the lowest temperatures (T = 700K and 800K). Deviations from the model can be explained by the temperature dependence of the grain boundary mobility. For temperatures T &amp;gt; 1000K, discontinuous dynamic recrystallization (DDRX) occurs, by which new grains form by nucleation and subsequent growth. The texture is characterized by two components, {100}&amp;lt;100&amp;gt; (cube, C) and {110}&amp;lt;100&amp;gt; (Goss, G). The intensity of G increases with temperature, while that of C decreases independent of the initial orientation. Both components have their maximum deviated about the 1 axis. The deviation is larger for grains containing the C component and decreases with temperature. Grains containing the G component have the smaller deviation, which decreases with temperature and strain. Texture simulations based on the full constraint Taylor model under the assumption of {110}&amp;lt;100&amp;gt; and {110}&amp;lt;110&amp;gt; slip were done with the experimental &amp;lt;110&amp;gt; and &amp;lt;111&amp;gt; fibres as well as a theoretical &amp;lt;100&amp;gt; fibre and a {100}&amp;lt;100&amp;gt; single orientation (ideal as well as rotated about the torsion axis). The G component is predicted by the simulations and is therefore a deformation texture. However the C component does not appear in the simulation. It therefore must originate by different mechanisms. For the non-&amp;lt;100&amp;gt; oriented samples, possibly nucleation is responsible for the formation of C oriented nuclei. Simulations with single orientations lead to the conclusion, that the ideal C orientation rotates about the 1 axis, while other C orientations, which are rotated about the torsion axis, increasingly converge towards the G component with strain. A single G orientation on the other hand is stable against such a rotation and is therefore the most likely steady state texture. Based on these results it is proposed, that ideally C oriented nuclei rotate until an orientation is reached into which they grow. These new grains are further rotated up to a critical angle, at which a part of them disappears either by adjacent grains or new C oriented nuclei. The recrystallization texture for T &amp;gt; 1000K is most likely the C component as well. Torsional creep of NiAl is characterized by a stress exponent, which depends on temperature and an activation energy, which is stress dependent. A model incorporating both dependencies is proposed and applied to the creep data. It is shown that these equations are able to describe the experimental findings. Thus creep of NiAl based on this model is dominated by non-diffusional processes such as cross slip of &amp;lt;100&amp;gt; screw dislocations for T  1000K. For T &amp;gt; 1000K the stress exponent and the activation energy are in a region, which according to previous reports is rather dominated by dislocation-climb controlled creep. The Swift effect, due to which samples change their axial dimension during torsion without applied axial stress, is observed for NiAl. It is strongly related to the texture development and in the case of NiAl the C component is identified as being responsible for shortening, whereas the G component leads to lengthening as long as it is not aligned with the shear system. Both tendencies can be explained based on the active slip systems. Simulations fail to predict the experimental observation, because the C component is not present. HESR and EBSD were compared with respect to local texture measurements. It was concluded depending on the average grain size HESR has an advantage in terms of grain statistics. For DDRX samples however, both methods are limited. Local texture inhomogeneities can be better detected using EBSD, whereas for an overall local texture information HESR is better suited.

<p>Cemented carbides (WC-Co) are powder metallurgical products produced by liquid phase sintering. WC-Co is widely used for making a large variety of cutting tools, such as drills and inserts turning applications, due to its great mechanical properties, where the hardness of the WC grains is combined with the toughness of the of the Co binder. WC grain size and grain size distribution are the two most important factors to control the mechanical properties of the products.</p><p>This study examined the grain growth dependence of different milling and sintering times. The resulting grain size and grain size distribution were measured using image analysis on scanning electron microscopy images (SEM) and by using electron backscatter diffraction (EBSD). In addition, the correlation between hardness and coercivity, the most common indirect measures of grain size, and different methods of calculating average grain radius were investigated. An attempt was also made to study the contribution of defects to grain growth. This work also includes an overview of various grain growth equations and a numerical implementation of these.</p><p>Experimental results show that for shorter sintering times, powders milled for short times (15 min and 1 h) have larger average grain radii. There is a crossover after 6 to 8 h of sintering, where the powders milled for a long time (40 h and 200 h), have larger average radii. The measured hardness values correlate well with the average grain radius calculated from the grain surface area and the coercivity correlates with the established equations. EBSD measurements detected boundaries that could not be detected by image analysis, and that were not Sigma 2 boundaries. It is likely that these boundaries are either low energy boundaries or boundaries between grains that are very closely oriented. Comparing heat-treated powder with the untreated resulted in a lower average grain size after sintering for the heat-treated powder. None of the growth equations investigated in this work could fully describe the experimental grain growth.</p><p>Through increased understanding of the grain growth, the growth can be controlled and the end product can have the desired tool properties. The occurrence of abnormal grains in cutting tool applications can cause breakage, which is especially important to avoid in applications such as PCB drills. A correlation between hardness and grain size provides further means for cheap and fast indirect measures of the grain size in production.</p> / <p>Hårdmetall är ett pulvermetallurgiskt material som tillverkas genom smältfassintring och som kännetecknas av hårdhet, styvhet och god slitstyrka. Volframkarbidens (WC) kornstorlek och kornstorleksfördelning är två viktiga faktorer för att kontrollera de mekaniska egenskaperna i hårdmetall.</p><p>I den här studien har korntillväxtens beroende på malning och sintring undersökts. WC-Co maldes och sintrades fyra olika tider och kornstorleksfördelningen mättes med bildanalys på svepelektronmikroskopbilder samt med ``electron backscatter diffraction'' (EBSD). I arbetet har även korrelationen mellan hårdhet, koercivitet och olika sätt att beräkna medelkornstorleken undersökts. Ett försök har också genomförts för att studera hur defekterna i det malda pulvret påverkar korntillväxten. I arbetet har även ett flertal olika tillväxtekvationer modellerats numeriskt och för och nackdelar med de olika tillväxtekvationerna har vägts mot varandra.</p><p>En lång maltid (40 h och 200 h) visade sig ge liten kornstorlek för sintring kortare än 6 h, men för sintringar längre än 8 h gav istället kort malning (15 min och 1 h) den mindre kornstorleken. Det visade sig att uppmätt hårdhet korrelerar bäst med den medelkornstorleksradie som räknats fram från kornytan. I EBSD mätningarna kunde man observera ett flertal korngränser, utöver Sigma 2 korngränser, som inte hade detekterats med bildanalys. Värmebehandlingen av det malda pulvret minskade korntillväxten under efterföljande sintring. Ingen av de undersökta tillväxtekvationerna kunde beskriva de experimentella resultaten fullt ut.</p><p>Genom ökad förståelse för korntillväxt kan man kontrollera tillväxten och slutprodukten kan få önskade egenskaper. Förekomsten av abnorm korntillväxt i skärverktyg i hårdmetall är en av de vanligaste kritiska defekterna och det är speciellt viktigt är undvika korntillväxt i tillverkning av små verktyg, som till exempel kretskortsborrar. Hårdhet och koercivitet är de vanligaste indirekta mätmetoderna för att mäta kornstorlek i produktion. En bra korrelation mellan kornstorlek och indirekta mätmetoder ger utökade verktyg för snabba och billiga mätningar.</p>

Ce travail de thèse concerne l’acier 16MND5 des cuves des Réacteurs à Eau Préssurisée (REP).L’étude vise à caractériser de manière quantitative la microstructure et à comprendre l’effet desparamètres influents (vitesse de refroidissement, température de revenu …) sur sa formation. D’unpoint de vue expérimental, l’étude est menée grâce à l’utilisation des techniques d’observationcomme la métallographie, le MET, MEB et l’EBSD, et la réalisation de traitements thermiquescontrôlés (dilatométrie notamment).Ces observations visent à identifier les différents constituants de l’acier et de déterminer lesgrandeurs quantitatives les caractériser (tailles, fractions volumiques des différentes phases ainsique leurs orientations cristallographiques).Nous avons dans un premier temps caractérisé de manière complète et quantitative, lamicrostructure de l’acier 16MND5 à l’état de réception (trempé et revenu). L’étude a permis demontrer qu’il existe un gradient de microstructure entre la peau interne et le coeur de l’épaisseur dela virole. La microstructure à coeur est constituée de ferrite proeutectoïde, de bainite et d’amas decarbures, tandis qu’en peau interne on est en présence de bainite et d’amas de carbures.Une campagne d’essais de dilatométrie a été réalisée afin de simuler la microstructure brute detrempe de l’acier 16MND5 et d’observer l’effet de la vitesse de refroidissement, des températuresde transformation sur la microstructure des aciers de cuve. Ces essais ont révélé que lamicrostructure de l’acier 16MND5 après refroidissement est constituée de bainite, d’îlots demartensite et d’austénite résiduelle (M-A) enrichie en carbone. Ce sont ces îlots M-A qui donnentnaissance aux amas de carbures lors du revenu final.L’effet de la composition chimique de cet acier a été étudié grâce à l’utilisation d’une tôle ayant unecomposition chimique plus riche en carbone et en éléments d’alliages. Ce travail a montré quel’augmentation des teneurs en carbone et en éléments d’alliages modifiaient la morphologie et lacristallographie de la ferrite.L’étude des comportements mécaniques des constituants présents en peau interne, quart épaisseur etmi épaisseur a été réalisée grâce à des essais de résilience sur des éprouvettes de mini charpy. Cetteétude a permis de montrer qu’à basse température (-120°C) la peau interne présente de meilleurespropriétés de résilience que le quart et la mi épaisseur de la virole. Des examens de la surfacelatérale des éprouvettes de Charpy ont montré que la présence de ferrite proeutectoïde favorise lagermination de micro-fissures de clivage. / Reactor pressure vessel is the second security barrier of the nuclear reactor and it is elaboratedwith a low carbon steel ( C-0.16%). Due to the large size of the ferrule there is temperatureand cooling rate gradient, which lead to microstructure gradient. To develop predictive modelit is necessary to well describe the microstructure which depend to the processing parameters.We focuse our work in investigating the effect of the processing parameters (cooling rate,chemical composition...) on the final microstructure during phase transformation.To get these informations and better describe the microstructure, we have used someexperimental technics like SEM, EBSD, TEM and Optical Microscopy. The steel used is alow carbon steel with a composition of C-0.16%, Mn-1.32%, Ni-0.72%, Mo-0.49%, Si-0.23%, Cr-0.23%, P-0.010%, S-0.004%. The material has been tempered in the range 635°C-660°C after cooling. Three positions have been chosen for examinations, because the coolingrate is not the same between the center and the edge of the material. The results of theobservations made in the different scale, indicate that the microstructure is mainly baniticwith bainitic ferrite and cementite precipitates. Examinations of the precipitates withextractive replicas in TEM reveal that morphology of cementite particles is complex they arecylinder-shaped particles, short bars particles and skeletal particles. OrientationsRelationships (OR) have been determined between ferrite and cementite particles with thinfoils in TEM by using Selected Area Diffraction, in a large number areas the Isaichev andBagaryatskii OR have been observed, a little Pitsch Petch OR have been obtained. The EBSDmap shows that the bainitic ferrite morphologies are both lath like and polygon. Themisorientations inside the laths are very small (0.5° misorientation point to point) andbetween laths we have the range 49-60° misorientation. The profile of misorientationsbetween point to point indicates higher frequency for the range 49-60°. Charpy test have beenperformed to analyze the effect of the microstructure on the fracture energy at lowtemperature. The results show that the fracture energy decreases when the content ofproeutectoid ferrite is high.

Les alliages aluminium-silicium (Al-Si) ont attiré une attention considérable en raison de leur importance pour les applications industrielles. Dans le présent travail, des alliages à haute pureté (Al-12.7 wt. % Si) avec et sans ajout de strontium (400 ppm), solidifiés lentement en creuset ou de façon dirigée (DS), ont été préparés et traités thermiquement. L'influence de l'ajout de strontium et des post-traitements thermiques sur les caractéristiques microstructuraux et cristallographiques des phases eutectiques a été étudiée de façon systématique. Les caractéristiques de croissance du silicium eutectique (Si) dans l'alliage non modifié ainsi que dans l'Al-12.7Si Sr-modifié ont été étudiés. Pour le cas du non-modifié, la formation répétée de variantes de macles mono-orientées permet une croissance rapide du silicium eutectique selon le mécanisme twin plane re-entrant (TPRE). Microscopiquement, les cristaux de silicium ont une forme de plaque allongée dans la direction <1 1 0> non conforme à la croissance selon <1 1 2> présumée par le modèle TPRE. L'élongation selon <1 1 0> est réalisée par des paires en zigzag <1 1 2> sur des plans de maclage parallèles, conduisant à une disparition alternative et à la création de macles rentrantes à 141°. Ce mécanisme de croissance permet aux cristaux de silicium de n'exposer que les plans {1 1 1} à faible consommation d'énergie à la consolidation. Pour les alliages modifiés au strontium, des changements importants de morphologie apparaissent dans le silicium eutectique, attribuable à la croissance de TPRE restreinte et au maclage induit par les impuretés (IIT). Ce dernier améliore la croissance latérale en formant de nouvelles macles avec des plans de macles parallèles, tandis que le second conduit à une croissance isotrope en formant des macles orientées différemment. Le traitement thermique provoque l'affinement des grains des deux phases eutectiques. L'affinement de l'α-Al se produit en même temps que la fragmentation et la sphéroïdisation du silicium et est principalement lié à la fracture des grains de silicium en raison de leur capacité limitée à accommoder la très grande dilatation thermique l'α-Al, ainsi qu'à la diffusion des atomes d'aluminium au cours du traitement thermique. La rupture du silicium génère une force de "capillarité" qui active la diffusion d'atomes d'aluminium dans la fissure. En raison du caractère de substitution de la diffusion de l'aluminium, la migration des lacunes vers l'intérieur de l'α-Al est induite lorsque l'aluminium se déplace dans les fissures, ainsi les vides de la fracture du silicium sont transférés à l'α-Al. De cette façon, les cristaux d'α-Al sont altérés et déformés. Les défauts cristallins produits, à leur tour, initient la restauration et même la recristallisation du α-Al, ce qui entraîne une diminution de taille de grain. La phase α-Al dans l'alliage de Al-12.7Si-0.04Sr solidifiée directionnellement, affiche une forte texture de fibre <1 0 0> parallèle à la direction de solidification. De très gros grains <1 0 0> α-Al sont principalement formés à la périphérie de l'échantillon cylindrique en raison des directions d'évacuation de chaleur favorables disponibles pour les trois directions [1 0 0]. Après traitement thermique, l'intensité de la texture de la phase α-Al diminue en raison de la restauration et de la recristallisation, mais le type de texture ne change pas. Pour la phase de silicium eutectique dans l'alliage de coulée, il y a deux fibres principales de texture, <1 0 0> et <1 1 0> parallèles à la direction de solidification, accompagnées de deux composantes faibles, <2 2 1> et <1 1 3> dans la même direction. Les fibres <1 0 0> et <1 1 0> correspondent à des grains de silicium situés sur la périphérie et dans le centre de l'échantillon. Les composantes <2 2 1> et <1 1 3> proviennent de plusieurs macles de grains orientés <1 1 0> et <1 0 0>. Les faibles intensités de ces deux composantes sont liées à leur fraction volumique mineure [...] / Al-Si alloys have attracted considerable attention due to their importance to industrial applications. In the present work, both crucible slowly solidified and slowly directionally solidified (DS) high-purity Al-12.7 wt. % Si alloys with and without 400 ppm Sr addition have been prepared and heat treated. The influence of Sr addition and post heat treatments on the microstructural and crystallographic features of the eutectic phases has been systematically studied. The growth characteristics of eutectic Si in the unmodified and the Sr-modified Al-12.7Si alloys were investigated. For the non-modification case, the formation of repeated single-orientation twin variants enables rapid growth of eutectic Si according to the twin plane re-entrant (TPRE) mechanism. Microscopically, Si crystals are plate-like elongated in one <1 1 0> direction that is not in accordance with the <1 1 2> growth assumed by the TPRE model. The <1 1 0> extension is realized by paired <1 1 2> zigzag growth on parallel twinning planes, leading to alternative disappearance and creation of 141° re-entrants. This growth manner ensures Si crystals to expose only their low-energy {1 1 1} planes to the melt. For the Sr-modification case, substantial changes appear in eutectic Si morphology, attributable to the restricted TPRE growth and the impurity induced twinning (IIT) growth. The first enhances lateral growth by forming new twins with parallel twinning planes, while the second leads to isotropic growth by forming differently oriented twins. Heat treatment brings about refinement of both eutectic phases. The refinement of the α-Al occurs concomitantly with the fragmentation and spheroidization of Si and is mainly related to the fracture of the Si crystals due to their limited capacity to accommodate the giant thermal expansion of the α-Al and the diffusion of Al atoms to the cracks during the heat treatment. The Si fracture generates “capillarity” force that activates the diffusion of Al atoms to the gap of the crack. Due to the substitutional feature of Al diffusion, the migration of vacancies toward the interior of the α-Al is induced when Al moves to the gaps, thus the voids of the Si fracture are transferred to the α-Al. In this way, the crystals of α-Al are distorted and defected. The produced crystal defects, in turn, initiate recovery and even recrystallization of the α-Al, resulting in grain refinement. The α-Al phase in the directionally solidified Al-12.7Si-0.04Sr alloy, displays a strong <1 0 0> fiber texture in the solidification direction. Giant <1 0 0> α-Al grains are mainly formed in the outer circle region of the cylindrical specimen due to the favorable heat evacuation directions available for the three <1 0 0> directions. After heat treatment, the texture intensity of the α-Al phase decreases due to the recovery and recrystallization, but the texture type does not change. For the eutectic Si phase in the as-cast alloy, there are two main fiber texture components, <1 0 0> and <1 1 0> in the DS direction, accompanied by two weak components, <2 2 1> and <1 1 3> in the same direction. The <1 0 0> and <1 1 0> components are from Si crystals located in the outer circle and center regions of the cylindrical specimen. The <2 2 1> and the <1 1 3> components are from multiple twins of the <1 1 0> and <1 0 0> oriented crystals. The weak intensities of these two components are related to their minor volume fraction. Once heat treated, the twinned parts with minor volume fractions enlarge at the expense of their twin related matrix, thus the <1 1 0> component is weakened and accompanied by the intensification of the components from the twins. The disappearance of the <1 1 3> component and the appearance of the <1 1 5> component are due to crystallographic rotation of Si crystals during their fragmentation

Le dioxyde d’uranium (UO2) est utilisé en tant que combustible, sous forme de pastilles élaborées par métallurgie des poudres, dans les réacteurs nucléaires à eau pressurisée. Lors de transitoires de puissance, le centre des pastilles est le siège de mécanismes de déformation visco-plastique qui peuvent être partiellement reproduits, hors irradiation, par des essais de compression uniaxiale à haute température (typiquement 1500°C). Les conditions de sollicitation et la microstructure initiale des pastilles d’UO2 ont une influence sur leur comportement mécanique macroscopique. A l’échelle des grains, des mécanismes de sous-structuration interviennent mais, à ce jour, la sous-structure n’est pas quantifiée et le rôle des pores sur ces mécanismes n’est pas connu. Afin d’apporter des réponses sur ces points, deux lots de pastilles (L1 et L2) de taille de grains similaires, de même fraction volumique de pores, mais ceux-ci étant distribués différemment (2,5 fois plus de pores intra-granulaires dans L1 que dans L2), ont été fabriqués. Ils ont ensuite été soumis à des essais mécaniques dans différentes conditions. Le résultat montre que le lot L2 présente une vitesse de fluage plus élevée que le lot L1. Les techniques Electron BackScatter Diffraction (EBSD) et Electron Channeling Contrast Imaging (ECCI) ont été mises en œuvre et optimisées pour suivre l’évolution de la microstructure après déformation. En EBSD, le développement d’une procédure adaptée aux matériaux poreux a permis de détecter des sous-joints de grains (S-JG) de très faible désorientation (jusqu’à 0,1°), de mener une étude statistique de l'évolution de la sous-structuration des grains et d'évaluer la densité de dislocations géométriquement nécessaires générées. Différents types d’arrangements de dislocations formant les S-JG ont été révélés et analysés par ECCI. Grâce à la complémentarité de l’EBSD et de l’ECCI, la répartition des pores dans les grains et la localisation des S-JG ont pu être mises en regard. Les résultats montrent que le nombre ainsi que la fraction linéaire des S-JG et leur désorientation augmente avec le taux et la vitesse de déformation. Aux forts taux de déformation, cela conduit à la formation de nouveaux grains par un mécanisme de restauration/recristallisation dynamique par rotation de sous-grains. Pour des conditions de sollicitation identiques, les échantillons du lot L1 présentent un nombre et une fraction linéaire de S-JG nettement supérieurs à ceux du lot L2. De plus, dans le lot L1, les S-JG se localisent essentiellement à proximité des joints de grains alors qu’ils sont répartis dans l’ensemble du grain pour le lot L2. Ces différences seraient liées à une réduction du libre parcours moyen des dislocations du fait de la présence des pores intra-granulaires / Uranium dioxide (UO2) is used as a fuel, in pressurized water nuclear reactors, in the form of pellets produced by powder metallurgy. During power transients, the center part of pellets undergoes visco-plastic deformation by creep mechanisms. These mechanisms can be partially reproduced, out of irradiation, by uniaxial compression tests at high temperature (typically 1500°C). Testing conditions and initial microstructure of the UO2 pellets influence their macroscopic mechanical behavior. At the grain scale, sub-structuring mechanisms are involved, but, up to now, the sub-structure is not quantified and the role of pores on these mechanisms is unknown. In order to provide answers to these points, two batches of pellets (L1 and L2), characterized by a similar grain size, a same volume fraction of pores, but different pores distribution (2.5 times more intra-granular pores in L1 than in L2), were elaborated. They were submitted to mechanical tests under different conditions. The result shows that L1 has as a lower creep rate than L2. Electron Backscatter Diffraction (EBSD) and Electron Channeling Contrast Imaging (ECCI) techniques were used and optimized for porous materials to analyze the evolution of the microstructure after deformation. An original EBSD methodology was implemented to detect Sub-Grain Boundaries (S-GB) with very low disorientation angles (down to 0.1°), study statistically the grain fragmentation into sub-grains and evaluate the average density of the geometrically necessary dislocations. Thanks to ECCI, the arrangement of dislocations in some S-GB was evidenced and analyzed. EBSD and ECCI complementarity allowed relating the distribution of pores within the grains and the S-GB location. The results obtained on the two batches show that the number and the linear fraction of S-GB increases with the deformation level and rate. At high deformation rates, new grains appear by a mechanism of dynamic recovery/recrystallization by rotation of sub-grains. For identical loading conditions and strain rates, the samples of batch L1 have a number and a linear fraction of S-GB that are significantly higher than those of batch L2. Furthermore, in batch L1, S-GB are located essentially in the vicinity of the grain boundaries while they are distributed throughout the grain for batch L2. These microstructural differences seem to be related to a dislocation's mean free path reduction due to the presence of intra-granular pores