The origin of texture components associated with rare-earth (RE) element additions in wrought magnesium (Mg) alloys is a long-standing problem in magnesium technology. The objective of this research is to identify the mechanisms accountable for rare-earth texture during dynamic recrystallization (DRX). Towards this end, we designed binary Mg-Cerium and Mg-Gadolinium alloys along with complex alloy compositions containing zinc, yttrium and Mischmetal. Binary alloys along with pure Mg were designed to individually investigate their effects on texture evolutions, while complex compositions are designed to develop randomized texture, and be used in automotive and aerospace applications. We selected indirect extrusion to thermomechanically process our materials. Different extrusion ratios and speeds were designed to produce partially and fully recrystallized microstructures, allowing us to analyze DRX from its early stages to completion. X-ray diffraction, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were used to conduct microstructure and texture analyses Our analyses revealed that rare-earth elements in zinc-containing magnesium alloys promote discontinuous dynamic recrystallization at the grain boundaries. During nucleation, the effect of rare earth elements on orientation selection was explained by the concomitant actions of multiple Taylor axes in the same grain. Isotropic grain growth was observed due to rare earth elements segregating to grain boundaries, which lead to texture randomization. The nucleation in binary Mg-RE alloys took place by continuous formation of necklace structures. Stochastic relaxation of basal and non-basal dislocations into lowangle grain boundaries produced chains of embryos with nearly random orientations. Schmid factor analysis showed a lower net activation of dislocations in RE textured grains compared to ones on the other side of the stereographic triangle. Lower dislocation densities within RE grains favored their growth by setting the boundary migration direction toward grains with higher dislocation density, thereby decreasing the system energy. We investigated the influence of RE elements on extension twinning induced hardening. RE addition enhanced tensile twinning induced hardening significantly. EBSD analysis illustrated that tensile twins cross low angle grain boundaries in Mg-RE alloys, which produced large twins and facilitated transmutation of basal to prismatic dislocations. Higher activity of pyramidal II dislocations in Mg-RE alloys resulted in higher twinning induced hardening.

At low temperatures, glide twinning activates in HCP structures easier than non-close packed slip necessary to accommodate strain along the c-axis. In contrast to slip, twinning occurs as an accumulation of successive stacking faults that properly report reconstruction of the stacking sequence in a new crystal-reorientated lenticular lamella. These faults are spread by partial dislocations known as twinning dislocations, forcing atoms to switch positions by shear into new crystal planes. As the twinning dislocations thread the faults, the new crystal lamella grows at the expense of the parent. Grain texture changes upon strain, and a strong non-linear trend marks the strain hardening rate. The strain hardening rate changes to a point where it switches sign upon strain. Since activation of these twinning dislocations obey Schmid’s law, twinning could be precluded or exhaustively promoted in sharp textures upon slight changes in loading orientations, so strong anisotropy arises. Moreover, a twinning shear can only reproduce the stacking sequence in one direction, unless the twin mode changes or the c/a ratio crosses a certain ratio. When a twin mode arises with reversed sign, the reorientation is different and more importantly, the strength is different and also the growth rate. Therefore, in addition to strain anisotropy, twin polarity induces a strong asymmetry in textured HCP structures, e.g. wrought HCP metals. This anisotropy/asymmetry is still a barrier to the great economic gain expected from the industrialization of low density, high specific strength and stiffness, HCP Magnesium. This barrier has stimulated efforts to identify the missing links in current scientific knowledge to proper prediction of Magnesium anisotropy. The effect of twinninginduced texture change on the mechanical response is of a major concern. Mesoscale modelers still struggle, without success to predict simultaneously twinning and strain hardening rates upon arbitrary loading directions. We propose a new mechanism that relies on admitting dislocation populations of the twin by dislocations transmuted from the parent when they intersect twinning disconnections. These dislocations interact with original dislocations created in the twin to cause hardening able to faithfully capture anisotropy upon any loading orientation and any initial texture.

The development and example applications of a new EBSD-based texture analysis system are presented. This new system uses the cross-correlation function to compare two EBSD patterns at a number of corresponding regions in each pattern to calculate the deformation gradient tensor. Bragg's Law-based simulated EBSD patterns are used as reference patterns in the cross-correlation method to enable the measurements of absolute elastic strain and lattice orientation at discrete points in a crystalline sample. The resolution limits of this new method are explored using a variety of computational and physical experiments. The simulated pattern method is estimated to be able to measure lattice orientations to within +/-0.02° and elastic strains to within +/-3.6x10-4 for small strains and +/-1x10-3 for large strains. Two example applications are demonstrated. The first demonstration is estimating the dislocation density in a 5.5% compressed Mg-based AZ91 alloy. Nye's and Kröner's formulations are used to estimate the dislocation density. Comparisons are made with traditional OIM measurements and it is found that the simulated pattern method offers an order of magnitude improvement in dislocation density estimations over OIM. The second demonstration is tetragonality measurements of HSLA 65 steel along the weld line of a friction stir welded plate. Accurate tetragonality measurements in the bainite phase of the steel can be made using information from the diagonal components of the elastic strain tensor. The measured tetragonality can be related to the concentration of interstitial carbon atoms in the iron lattice to find the carbon distribution in the sample. From these experiments, it is demonstrated that the simulated pattern method presents a new and powerful methodology for texture analysis that exhibits both ease of use and access to high resolution orientation and elastic strain data.

L'objectif de ce travail était de comprendre les mécanismes menant à la formation de SiC-3C sur substrats de SiC hexagonaux lors de la croissance par mécanisme Vapeur-Liquide-Solide (VLS) à partir d'une phase liquide Si-Ge. Notre étude s'est concentrée sur les premiers instants de l'interaction liquide/SiC, c'est-a-dire sans ajout de phase gazeuse réactive (propane). Nous avons montré qu'il se formait très rapidement des îlots de SiC-3C à la surface des germes. L'étude en fonction de divers paramètres (température et durée de plateau, vitesse de chauffage, nature du germe) a permis de mettre en évidence l'influence prononcée du creuset en graphite dans lequel est confinée la réaction. Les observations expérimentales associées à des calculs thermodynamiques ont permis de montrer que l'étape prépondérante, pour la formation du 3C, est sans doute la réaction transitoire entre un liquide très riche en germanium et le germe de SiC. Des considérations d'ordre cinétiques sont cependant à prendre en compte pour expliquer le caractère hors équilibre de la réaction / The aim of this work was to understand the mechanisms that lead to the 3C-SiC formation on hexagonal SiC substrates during the Vapor-Liquid-Solid (VLS) growth from a Si-Ge liquid phase. Our study focused on the early stages of the liquid/SiC interaction, i.e. without reactive gaseous phase (propane) addition. We have shown that 3C-SiC islands were very rapidly formed upon seeds surface. The study of several parameters (such as temperature and duration of the plateau, heating rate, nature of the seed) evidenced the huge influence of the graphite crucible that contains the reaction. Experimental observations combined with thermodynamic calculations show that the most important step for the 3C formation, is the transient reaction between a germanium very rich liquid and the SiC seed. Kinetic effects have to be taken into account to explain the out-of-equilibrium nature of the reaction.

Les modèles basés sur la mécanique des milieux continus prévoient généralement une déformation à la rupture plus élevée, ainsi qu'une durée de vie en fluage beaucoup plus longue que les valeurs observées expérimentalement. Cette thèse met en évidence deux aspects de cette problématique en analysant l’endommagement à l'aide de tomographie in situ à rayons X de synchrotron et reconstruction 3D de la structure polycristalline par polissages successifs.L’endommagement en termes de fraction surfacique des cavités a été identifié dans les couches de reconstructions tomographiques perpendiculairement à l’axe de déformation. L'évolution de la fraction surfacique des cavités a été comparée avec le modèle de prédiction de Cocks et Ashby. Ce dernier surestime la durée de vie en fluage et sous-estime l’état de l’endommagement. L'importance de l'hétérogénéité initiale de l’endommagement et l’effet de localisation de l’endommagement est également souligné. L'amplitude de la plus grande fluctuation surfacique augmente de façon parabolique en fonction de la fraction surfacique moyenne.Une méthode de sectionnement sériel améliorée basée sur la profilométrie de surface a été développée. Elle permet la mesure précise de l'épaisseur du matériau enlevée localement. Les analyses ont montré que l'emplacement des cavités par rapport aux joints de grains et l’orientation cristallographique des grains au voisinage est similaire pour les échantillons déformé par différents mécanismes de fluage. La population relative des cavités de fluage présente aux joints de grains simples est supérieure à celle présente aux joints triples. Les cavités trouvées aux joints triples, cependant, sont plus grandes. / Power law creep damage is one of the most intriguing unsolved phenomena of materials science. Models based on continuum mechanics generally predict a much higher strain to failure, as well as a much longer creep lifetime than experimentally observed values. This thesis highlights two aspects of this problematic by analyzing creep damage in copper using in situ synchrotron tomography and 3D reconstruction of the damaged polycrystal structure by serial sectioning.Damage in terms of the area fraction of voids was first identified in slices of tomographic reconstructions of creep deformed copper. The local and global evolution of cavities area fraction was checked against the Cocks and Ashby model and it was found that the model overestimates creep lifetime and underestimates damage development. The importance of the initial damage heterogeneity and the role of damage localization are also emphasized. It was found that the amplitude of the largest damage fluctuation increases parabolically as a function of cavity’s mean area fraction.An improved serial sectioning method based on surface profilometry was developed, which allows the accurate measurement of the removed local material thickness. The 3D reconstructions enabled identifying the creep voids and the grains of the polycrystal. It was shown that with the exception of the void shape, the relationship between void location at a given grain boundary and crystallographic orientation of the neighbor grains is similar in samples deformed by different creep mechanisms. The relative population of creep voids is higher at simple grain boundaries than at triple junctions. Voids found at a triple boundary, however, are larger.

O presente trabalho teve como objetivo principal estudar a evolução da microestrutura e da textura cristalográfica de três alumínios de pureza comercial com diferentes níveis de pureza, AA1100 (99,00%), AA1050 (99,50%) e AA1070 (99,70%), e do alumínio super puro, AA1199 (99,995%). A obtenção das placas de alumínio de pureza comercial foi feita pelo processo Direct Chill (DC) seguido de homogeneização e laminação a quente até a espessura final de 10 mm e o alumínio super puro foi produzido em laboratório e encontrava-se na forma de blocos fundidos. Foram utilizadas várias técnicas de análise microestrutural: microscopia óptica convencional, microscopia óptica com luz polarizada, microscopia eletrônica de varredura, microscopia eletrônica de transmissão, medidas de condutividade elétrica e dureza Vickers. Já a evolução da textura do material foi analisada pela técnica de difração de raios X, nos alumínios de pureza comercial e pela técnica de difração de elétrons retroespalhados (EBSD), no alumínio super puro. No estado como recebido e recozido a 400 ºC por 1 e 24 horas foi possível observar que nos alumínios de pureza comercial produzidos pelo processo DC não há variações na composição química ao longo da espessura. Os precipitados nos alumínios AA1100 e AA1070, no estado como recebido estavam distribuídos na direção de laminação, porém com vários locais de aglomeração dos mesmos, e no alumínio AA1050 os mesmos estavam finamente dispersos na direção de laminação. Após recozimento a 400 ºC por 1 e 24 horas as mesmas características foram mantidas, porém ocorreu um aumento na fração volumétrica do estado como recebido para os recozidos a 400 ºC, ou seja, ocorreu precipitação durante o recozimento. Quanto a análise da textura dos alumínios de pureza comercial, na superfície encontrou-se a textura de cisalhamento, devido ao esforço mecânico entre o cilindro de laminação e as amostras, tanto no estado como recebido como após os recozimentos. A ¼ da espessura e no centro do material após recozimentos ainda observou-se a textura de cisalhamento, mas também as texturas tipo cubo, cubo rodado e latão. No centro do material permaneceram a textura tipo cubo e latão. Já o alumínio AA1199 possui uma estrutura grosseira (grãos oligocristalinos), sem a ocorrência de precipitados e não foi possível obter resultados nas análises de textura, devido ao tamanho de grão grande. Após a laminação a frio com aproximadamente 70% de redução em espessura, seguido de tratamentos isócronos no tempo de 1 hora e nas temperaturas de 50, 100, 150, 200, 250, 300, 350, 400 e 500 ºC, o alumínio AA1199 apresentou uma menor temperatura para completa recristalização, em torno de 280 ºC, seguido do alumínio AA1050 (375 ºC) e dos alumínios AA1100 e AA1070 (ambos, 400 ºC). Como as composições químicas das soluções sólidas matrizes são praticamente idênticas, a variação na temperatura de recristalização deve ser atribuída aos precipitados presentes no alumínio que quando finamente dispersos na matriz impedem o movimento dos subcontornos, retardando o início da recristalização. O alumínio AA1199 possui uma temperatura para completa recristalização, menor, em relação aos outros alumínios, uma vez que não possui precipitados em sua microestrutura. Quanto a textura, na superfície e a ¼ da espessura, os alumínios de pureza comercial laminados a frio sem tratamento térmico, permaneceram com a textura de cisalhamento encontrada na superfície no estado como recebido com o surgimento da textura S. Após o tratamento térmico a 250 ºC por 1 hora apareceu a textura Goss e a 350 ºC por 1 hora a textura tipo cubo, cubo rodado e cubo ND, lembrando que em ambos os casos a textura de cisalhamento e S permaneceram. Com o auxílio da utilização da técnica de difração de elétrons retroespalhados (EBSD) encontrou-se no alumínio AA1199, a textura tipo cubo, tanto laminado a frio sem tratamento térmico quanto após os tratamentos. Após a laminação a frio com aproximadamente 80% de redução em espessura, seguida de tratamentos isotérmicos, nas temperaturas de 150, 250 e 350 ºC e nos tempos de ½, 1, 2, 3 e 4 horas, os alumínios de pureza comercial apresentaram uma temperatura para completa recristalização em torno de 350 ºC e por meio da técnica de microscopia eletrônica de transmissão observou-se arranjos celulares de discordâncias, mas não foram detectadas diferenças significativas entre esses arranjos apresentados pelos três alumínios. Diferenças certamente existem, mas para serem caracterizadas é necessário um estudo detalhado com microscopia eletrônica de transmissão. A presença de partículas de compostos intermetálicos foi observada em todas as amostras analisadas, essas partículas são resultado da baixa solubilidade do ferro e do silício no alumínio e se formaram durante as diversas etapas do processamento dos materiais. / The present work had as main objective to study the evolution of the microstructure and crystallographic texture of three commercial purity aluminums with different levels of purity, AA1100 (99.00%), AA1050 (99.50%) and AA1070 (99.70%), and of the super pure aluminum, AA1199 (99.995%). The plates of commercial purity aluminum were obtained through direct chill (DC) process followed by homogenization and hot rolling until the final thickness of 10 mm. The super pure aluminum was produced in laboratory and was in the ascast condition. Several techniques of microstructural analysis were used: conventional optical microscopy, optical microscopy using polarized light, scanning electron microscopy, transmission electron microscopy, measurements of electrical conductivity and Vickers hardness. The evolution of the texture of the commercial purity aluminum was analyzed by Xray diffraction and the super pure aluminum was analyzed using electron backscatter diffraction. Variations in the chemical composition along thickness in the commercial purity aluminum produced by the DC process in the as-received condition and after annealing at 400 ºC for 1 and 24 hours were possible to observe. The precipitates in the aluminum AA1100 and AA1070, in the as-received condition were distributed in the rolling direction, including several sites of clusters of the same precipitates. In the aluminum AA1050 the precipitates were finely dispersed in the rolling direction. After annealing at 400 ºC for 1 and 24 hours the precipitate characteristics were maintained, and there was an increase in the volumetric fraction of the as-received condition in comparison to samples annealed at 400 ºC, showing that precipitation occurred during annealing. The shear texture was observed in the commercial purity aluminums, either in the as-received condition or after annealing, and this effect was due to the mechanical effort between the rolling mill and the samples. At ¼ of the thickness and in the center of the material after annealing the shear texture was still observed, and also the type cube, rotated cube and brass textures. The center of the material continues showing the type cube and brass textures. On the other hand, the aluminum AA1199 has a coarse structure (oligocrystalline grains), without the occurrence of precipitates, and texture results were not obtained due to the fact the grains are much coarser, due to the size of big grain. After cold rolling with approximately 70% of reduction in thickness, followed by isochronous treatments for 1 hour at temperatures of 50, 100, 150, 200, 250, 300, 350, 400 and 500 ºC, the aluminum AA1199 presented a smaller temperature for complete recrystallization, around 280 ºC, followed by the aluminum AA1050 (375 ºC) and of the aluminum AA1100 and AA1070 (both, 400 ºC). As the chemical compositions of the solid solutions matrixes are almost identical, the variation in the recrystallization temperature should be attributed to the presence of precipitates in the aluminum, and when finely dispersed in the matrix they impede the movement of the sub-boundary, delaying the beginning of the recrystallization. In comparison to the other aluminums, the aluminum AA1199 has a smaller temperature for complete recrystallization, since there are not precipitates in its microstructure. Observing the texture, in the surface and the ¼ of the thickness, the commercial purity aluminums, after cold rolling and without heat treatment, showed the shear texture found in the surface in the as-received condition with the appearance of the S texture. After heat treatment at 250 ºC for 1 hour the Goss texture appeared and at 350 ºC for 1 hour the type cube, rotated cube and cube ND textures; notice that in both cases the shear and S texture were still present. Energy dispersive X-ray spectroscopy (microanalysis) showed in the aluminum AA1199, the texture type cube, so cold rolled without heat treatment as after the treatments. After cold rolling with approximately 80% of reduction in thickness, followed by isothermal treatments, at temperatures of 150, 250 and 350 ºC and times of ½, 1, 2, 3 and 4 hours, the commercial purity aluminum presented a temperature for complete recrystallization around 350 ºC. Transmission electron microscopy technique showed dislocations cellular arrangements, but significant differences were not detected among those arrangements presented by the three aluminums. Differences certainly exist, but for their characterization detailed study with transmission electron microscopy is necessary. The presence of particles of intermetallic compounds were observed in all analyzed samples, those particles resulted of the low solubility of iron and silicon in the aluminum and were formed during the several stages of the processing of the materials.

Aços inoxidáveis dúplex apresentam uma estrutura composta por ferrita e austenita. O fato de a austenita ser estável à temperatura ambiente possibilita que esses aços sejam usados no estudo da reação de formação da austenita a partir da ferrita, podendo os resultados ser aplicados a aços de baixa liga, em que a austenita se transforma em martensita no resfriamento brusco, e a outros sistemas com transformações entre fases cúbicas de corpo centrado (CCC) e cúbicas de face centrada (CFC). Foram realizados estudos em um aço inoxidável dúplex do tipo DIN W.Nr. 1.4462 (UNS 31803). As amostras foram solubilizadas no campo ferrítico a 1325°C e resfriadas em água. As amostras foram ainda tratadas isotermicamente em temperaturas entre 700°C e 1100°C por tempos entre 5.000 e 30.000 segundos. Deste modo, a formação de austenita a partir da ferrita foi estudada em seu estágio inicial e em estágios avançados da reação, com relação aos seus aspectos morfológicos e cristalográficos. As morfologias encontradas apresentaram variações dependentes dos segmentos de contorno de grão em que as partículas se formam. As partículas nucleadas nos contornos de grão podem ser adequadamente descritas pela classificação morfológica de Dubé. Além disso, essas partículas apresentaram, em geral, relações de orientação do tipo Kurdjumov-Sachs e Nishyiama-Wassermann com pelo menos um dos dois grãos, podendo ser encontradas relações intermediárias entre essas duas. . Em alguns casos, as partículas mantêm relações de orientação com os dois grãos adjacentes, apresentando pequenos desvios das relações exatas relatadas na literatura. As partículas de austenita intragranulares apresentam desvios em relação à relações exatas maiores do que os encontrados nas partículas de contornos de grão. Em alguns casos, as partículas intragranulares aparentam não apresentar relações de orientação com a matriz ferrítica. / Duplex Stainless Steels have a structure composed by ferrite and austenite. The fact that austenite, in these steels, is stable at low temperatures, allows the use of these steels in studies of austenite formation from ferrite, in such way that the results can be applied to low alloy steels, in which austenite transforms to martensite upon rapid cooling, and to other systems containing transformations between body-centred cubic (BCC) and face-centred cubic (FCC) phases. Studies were performed on a DIN W.Nr. 1.4462 (UNS 31803) duplex stainless steel. The samples were solution treated in the ferrite region at 1325°C and water cooled. Samples were then submitted to isothermal treatments at temperatures between 700°C and 1100°C for up to 30.000 seconds. In this way, austenite formation from ferrite was studied on initial and advanced stages of the reaction, concerning morphological and crystallographic aspects. The morphologies observed varied with the grain boundary segment in which the particles were formed. The grain boundary particles may be adequately described by the Dubé classification. Moreover, these particles presented Kurdjumov-Sachs and Nishyiama-Wassermann orientation relationships with at least one of the adjacent grains, with possibilities of occurrence of intermediate relationships between K-S and N-W relationships. In some cases, the particles show orientation relationships with both adjacent grains, with small deviations form the exact relationships reported in literature. Intragranular austenite particles show higher deviations from the exact relationships than grain boundary particles. In some cases, intragranular particles have no apparent orientation relationships with the ferrite matrix.

A principal motivação para o desenvolvimento do presente projeto de doutorado foi comparar as microestruturas e texturas resultantes da liga de alumínio 3003 obtida por lingotamento contínuo e semicontínuo. O processo “roll caster" (lingotamento contínuo em cilindros) tem sido crescentemente utilizado na indústria de laminação de alumínio. Produtos que antes sofriam altos graus de deformação durante seu processamento são hoje obtidos nas dimensões próximas da espessura final. Com o intuito de entender este processo relativamente novo, as microestruturas e as texturas (macrotextura e microtextura), ao longo da espessura, das chapas produzidas por lingotamento contínuo e por lingotamento semicontínuo (placas fundidas com 250 mm de espessura, seguido de homogeneização e laminação a quente) foram analisadas. Foi possível constatar que os materiais provenientes dos dois processos diferem bastante e que ao longo da espessura também há grandes diferenças. Foi possível observar que durante o processamento “roll caster" há considerável deformação plástica, com a formação de células de discordâncias e subgrãos. Foi possível observar, também, que a microestrutura, a morfologia e o tamanho de grão, da amostra “caster" bruta de fundição são mais homogêneos que na amostra laminada a quente. As duas chapas, a laminada a quente e a obtida por lingotamento contínuo, exibem forte gradiente de textura ao longo da espessura. A textura predominante nos dois processos, ao longo da espessura, é a do tipo latão {011}. A evolução da textura do material proveniente do processo “roll caster" também foi caracterizada após laminação e posterior recristalização, revelando a presença de componentes típicas de laminação e de recristalização de alumínio. Após laminação a frio com redução de 91% e posterior tratamento térmico de 400°C por 1 h, a chapa proveniente do processo “caster" apresentou uma textura de recristalização caracterizada pela presença da componente cubo (recristalização) somada à parcela de textura de deformação. No caso do alumínio e suas ligas, sabe-se que esta textura proporciona os melhores resultados para minimizar o efeito de orelhamento durante a estampagem. Outro ponto relevante do trabalho foi o aperfeiçoamento da técnica de extração de precipitados para a liga 3003, uma vez que, a extração de precipitados em ligas de alumínio é especialmente problemática, pois a maioria das partículas presentes dissolve-se até mais facilmente que a matriz de alumínio. Algumas técnicas foram testadas: 1. dissolução química em solução de iodo em metanol; 2. dissolução eletrolítica em solução de ácido benzóico e hidroxiquinolina em clorofórmio e metanol; 3. dissolução eletrolítica em solução de ácido perclórico, butil glicol, álcool etílico e água; 4. dissolução química em solução de fenol. A técnica que apresentou melhores resultados foi a dissolução química com fenol, onde foi possível separar os precipitados da matriz alumínio. Com o auxílio desta técnica foi possível analisar as transformações de fase do composto intermetálico Al6Mn durante tratamentos térmicos. Obtiveram-se dados da completa transformação da fase Al6Mn em alfa-AlMnSi, também conhecida na literatura como, transformação "six to alfa". A precipitação e os precipitados desempenham um papel importante na cinética de recristalização e no tamanho de grão final. Para se entender este comportamento, comparou-se uma chapa que sofreu tratamento térmico antes da laminação com outra chapa que foi laminada no estado bruto de fundição. Os resultados revelaram que a recristalização foi retardada na amostra, inicialmente, bruta de fundição. A precipitação ocorreu simultaneamente com a recristalização, com isso, os dispersóides precipitaram preferencialmente na microestrutura deformada, em subcontornos ou nas discordâncias isoladas, levando a um considerável atraso no rearranjo das discordâncias e na nucleação da recristalização. / The main reason for the development of this PhD Thesis was to compare the microstructures and textures of the 3003 aluminum alloy produced from continuous and semi-continuous casting processes. The roll caster process (continuous casting) has been increasingly used in the aluminum industry. Products usually obtained by means of heavy rolling operations in the past are nowadays obtained with dimensions close to the final ones using the roll caster technology. To understand this new process, microstructures and textures (macrotexture and microtexture) along the thickness of the sheet produced by continuos casting and semi-continuous casting (plates with 250 mm thickness, followed by homogenization and hot rolling) have been investigated. It was possible to verify that materials coming from these two processes did differ each other a lot mainly across the thickness. During roll casting the plastic deformation was large enough to promote the formation of dislocation cells and subgrains. It was also possible to notice that the microstructure, morphology, and grain size from as-cast sample are more homogenous than the ones found in hot rolled samples. Both the sheets obtained by hot rolling and continuous casting have shown a strong texture gradient across the thickness. The predominant texture in both processes is the Brass component {011} . The texture evolution from sheets obtained by roll casting was also determined after rolling and subsequent recrystallization. It shows the presence of typical components of rolling and recrystallization of aluminum. After 91% cold rolling and subsequent recrystallization at 400°C for 1 h, the cube component (recrystallization texture) and the deformation texture were observed. It is well known that this texture minimizes earing effects during deep drawing of aluminum products. Another relevant point in this work was the development of the technique for the extraction of precipitates for the aluminum 3003 alloy. The extraction of precipitates extraction is particularly problematic in aluminum because most of the particles tend to dissolve more readily than the aluminum matrix. Some of the techniques performed are the following: 1. chemical dissolution with iodine in methanol solution; 2. electrolytic dissolution with benzoic acid and hydroxyquinoline in chloroform and methanol; 3. electrolytic dissolution with perchloric acid, butyl-glycol and ethanol; 4. chemical dissolution with phenol solution. Chemical dissolution with the phenol solution was the technique that provided the best results. It was possible to separate precipitates from the aluminum matrix. With this extraction technique it was possible to analyze phase transformations of the intermetallic compound Al6Mn during heat treating of this alloy. The phase transformation of the Al6Mn compound into alfa-AlMnSi, also known as 6-to-alfa transformation, could be followed in detail by means of this technique. Precipitation plays an important role in the recrystallization kinetics and final grain size. Therefore, to understand this behavior, the roll cast aluminum alloy was cold rolled from two distinct starting conditions: as-cast and heat-treated (homogeneized) conditions. It was shown that recrystallization was delayed in the sheet rolled from the as-cast condition. Precipitation has occurred simultaneously with recrystallization, in such a manner that dispersoids did precipitate in the deformed microstructure, preferentially, at subgrain boundaries or at free dislocations. As a result, the rearrangement of the dislocations and further recrystallization nucleation has been significantly retarded.

Amostras de nióbio de alta pureza na forma de monocristais, bicristais e policristais foram retiradas de seções longitudinais de lingotes fundidos por feixe eletrônico. As amostras foram deformadas via extrusão em canal angular (ECAE - Equal Channel Angular Extrusion) em temperatura ambiente até 8 passes, utilizando a rota Bc numa matriz com ângulo de intersecção entre os canais de  = 90º. As amostras foram caracterizadas em termos da evolução microestrutural e da textura de deformação. A caracterização microestrutural foi realizada com o auxílio de microscopias ótica, eletrônica de varredura e eletrônica de transmissão, além de medidas de difração de elétrons retroespalhados (EBSD) para determinação da microtextura e da mesotextura. Medidas de microdureza Vickers foram realizadas para acompanhar o encruamento e o amolecimento das amostras. Um outro monocristal de nióbio foi deformado em 1 passe via ensaio interrompido, utilizando uma matriz com ângulo  = 120º, a fim de estudar a evolução da textura durante a passagem pelo canal de ECAE. A textura foi determinada por meio de difração de raios X e comparada com os dados da literatura para materiais deformados via ECAE com estrutura CCC e também com as texturas simuladas pelo modelo VPSC (visco-plastic self-consistent). No estudo comparativo numa escala maior (monocristal e policristal), verificou-se que houve um refinamento microestrutural significativo após 8 passes. O espaçamento médio entre os contornos de alto ângulo medido perpendicular à direção de extrusão foi próximo nos dois casos (500 nm), maior que o observado para o monocristal deformado numa escala menor (440 nm). Os resultados mostram ainda que os grãos do policristal deformado são mais equiaxiais que os do monocristal. Amostras foram recozidas isotermicamente para avaliar o comportamento frente ao engrossamento microestrutural. Os resultados mostram que o engrossamento torna-se apreciável, em geral, a partir de 500oC com a ocorrência de recristalização descontínua. Acima de 700oC, o crescimento normal de grão passa a ser o principal mecanismo de engrossamento microestrutural. Efeitos de orientação importantes foram observados no bicristal nos estados encruado e recozido. / High-purity niobium single crystals, bicrystals and polycrystals were cut out from longitudinal sections of ingots processed by electron beam melting. Samples were deformed by Equal Angular Channel Extrusion (ECAE) at room temperature up to 8 passes, using the route Bc with a die angle  = 90o. Samples were characterized in terms of their microstructural evolution and deformation textures. Microstructural characterization was performed using optical, scanning electron, and transmission electron microscopies, as well as electron-backscatter diffraction measurements (EBSD) to determine both microtexture and mesotexture. Vickers microhardness testing was performed to follow hardening and softening behaviors in the samples. Another single crystal was deformed by 1 pass in an interrupted ECAE experiment using a die angle  = 120o to follow the changes in texture through the extrusion channel. Texture was determined by X ray diffraction and compared with those reported in the literature for deformed bcc materials and also with those predicted using the viscoplastic self-consistent model (VPSC). A comparative study in a larger scale (single and polycrystals) was also performed. It was observed that there is a significant refinement of the microstructure after 8 passes. The average spacing between high angle boundaries perpendicular to extrusion direction was close in the two cases (500 nm), larger than observed in the single crystal deformed in a smaller scale (440 nm). Results also show that ultrafine grains of the deformed polycrystal are more equiaxial compared to those found in the deformed single crystal. Samples were annealed to evaluate their behavior regarding microstructural coarsening. Results show that coarsening becomes noticeable at temperatures higher than 500oC by means of discontinuous recrystallization. Above 700oC, normal grain growth becomes the main microstructure coarsening mechanism. Important orientation effects were observed in the bicrystal in both deformed and annealed states.

La glace est un matériau de structure cristallographique hexagonale ayant une anisotropie plastique très importante. La déformation est principalement accommodée par le glissement des dislocations dans le plan basal. Cette forte anisotropie du monocristal de glace conduit lors de la déformation d’un polycristal, à de fortes hétérogénéités de déformation et de contrainte. Lors de la déformation à haute température, les mécanismes de recristallisation dynamique permettent, par le développement d’une nouvelle microstructure et d’une nouvelle texture, d’augmenter la ductilité du matériau. L’objet de cette étude est de mieux caractériser les mécanismes de recristallisation et leurs liens avec les hétérogénéités de déformation afin de mieux comprendre le développement des nouvelles microstrutures et textures ainsi que leurs impacts sur le comportement du matériaux.Ce travail est basé sur des essais de type fluage dans les conditions où la recristallisation dynamique est importante (T > 0.95T f et 0.5 < σ < 1 M P a). Deux types de polycristaux sont étudiés : la glace granulaire, considérée comme un volume élémentaire représentatif, qui permet d’appréhender d’un point de vue statistique l’impact de la recristallisation sur le développement des microstructures et des textures ; et la glace colonnaire, qui permet d’étudier les différents mécanismes de germination et leurs liens avec les hétérogénéités de déformation à l’échelle inter et intra-granulaire. Les outils de caractérisation utilisés sont la mesure in situ de l’évolution du champ de déformation par corrélation d’images numériques et la caractérisation des microstructures et des textures pré- et post- déformation par mesure optique et Electron BackScatter Diffraction (EBSD).Étant donné les fortes hétérogénéités de déformation, les mécanismes de recristallisation dynamique continue et discontinue sont actifs au cours de la déformation. La formation de sous-joints de grains, la germination par gonflement (bulging), où la migration de joints de grains ont été mises en évidence. Des germes fortement désorientés par rapport aux grains parents ont également été observés. Ces observations impliquent un mécanisme de germi- nation différent de ceux mentionnés ci-dessus. De plus, les dislocations géométriquement nécessaires composants les sous-joints de grains ont été caractérisées à l’aide du tenseur de Nye extrait des mesures EBSD. Cela a permis d’observer des sous-structures de dislocations composées de dislocations c. Ces dislocations c étaient jusqu’alors très rarement observées et seulement dans des conditions très spécifiques.Une forte corrélation entre les hétérogénéités de déformation et les mécanismes de recristallisation a été mise en évidence grâce aux mesures de champs de déformation. Les mécanismes de recristallisation se concentrent dans les zones où la déformation est importante et ont tendance à réduire les hétérogénéités de déformation. De plus, ces bandes de déformation, où la recristallisation est active, sont orientées à environ ±45° et ont une dimension supérieure à la taille moyenne des grains, ce qui montre une organisation des hétérogénéités de déformation et de contrainte à grande distance.Un modèle théorique pouvant contraindre l’orientation des germes crées par la germination spontanée est proposé. Ce modèle, basé sur la relaxation de l’énergie élastique, du fait de l’anisotropie élastique du mono-cristal de glace, permettrait de favoriser les cristaux dont l’axe c est aligné avec la direction de la contrainte principale locale. Cette étude a été complétée par des expériences numériques, basées sur un modèle à champ complet et une loi de plasticité cristalline élasto-viscoplastique qui permet de reproduire les hétérogénéités de déformation et de contrainte. Ces expériences numériques ont montré que ce mécanisme de germination orienté pourrait permettre d’expliquer le développement des textures de recristallisation. / Ice is an hexagonal material in which deformation mainly occurs by dislocation glide along the basal plane conferring a strong viscoplastic anisotropy to the single crystal. Hence, during polycrystalline ice deformation the incompatibility between grains lead to highly heterogeneous strain. During ice creep at high temperature, dynamic recrystallisation occurs, leading to the development of a new microstructure and strong recrystallisation textures.These new microstructure and texture increase the ductility of the material. The aims of this study is to investigate dynamic recrystallisation mechanisms and their links with strain heterogeneities to better understand the development of these microstructure and texture.Creep experiments are carried out in conditions where dynamic recrystallisation is important (T > 0.95T f and 0.5 < σ < 1 M P a). Two kinds of polycrystalline samples are used: granular ice, which can be considered as a representative elementary volume and enable to understand the global impact of dynamic recrystallisation mechanisms on texture and microstructure; and columnar ice which enable to better constrain the dynamic recrystallisation mechanism such as nucleation and their link with the strain heterogeneities down to the intra and inter-granular scales. Strain field evolution is measured in situ using digital images correlation and pre- and post- deformation microstructures and textures are measured using optical imaging and Electron BackScatter Diffraction (EBSD).Due to the strong strain heterogeneities, both continuous and discontinuous recrystallisation mechanisms occured. Sub-grain boundary formation, nucleation by bulging and grain boundary migration are mechanisms very active during dynamic recrystallisation in ice. On top of that, we also observed new grains with orientations highly disoriented compared to the neighbouring grains. This observation implies a nucleation mechanism different compare to the one mentioned above. Using Nye theory on EBSD measurements constrains the geometrically necessary dislocations of the sub-grain boundaries. Tilt sub-grain boundaries made of non-basal c dislocations have been observed. Only few observations of c dislocations existed so far, all of them made in very specific conditions.A strong correlation between recrystallisation mechanisms and strain field heterogeneities have been observed. Recrystallisation mechanisms lead to a decrease and spread of strain heterogeneities. The strain localise into bands of deformation oriented at around ±45° from the compression axis and with a typical length higher than the mean grain size. These bands of high deformation localise most of the dynamic recrystallisation mechanisms. This observation shows that the long range interaction of the strain and stress heterogeneities.We propose a new model which could be able to constrain the nucleus orientation for spontaneous nucleation. This model, based on the elastic energy relaxation during nucleation tanks to the elastic anisotropy of ice single crystal, should constrain the c axis to be aligned with the locale principal stress direction. To investigate the impact of such oriented nucleation mechanism on the texture development, a numerical experiment has been proposed using full field simulation with an elasto-viscoplastic law able to simulate the strain and stress fields heterogeneities. These numerical experiments show that the oriented nucleation mechanism might be able to explain the development of recrystallisation texture.