Les alliages à grains ultrafins semblent prometteurs, au vu de leur forte résistance en traction et de la possibilité d'une mise en forme superplastique à basse température. Toutefois, leurs mécanismes de déformation, qui comportent une part plus ou moins forte de glissement aux joints de grains restent mal connus, et leurs performances en fatigue ont été peu étudiées. Ce travail vise à comparer et analyser le comportement viscoplastique et les mécanismes de déformation et d'endommagement en traction et en fatigue d’un alliage d’aluminium-magnésium "classique" et à grains ultrafins (600nm en moyenne) obtenu par déformation plastique sévère, selon le procédé ECAP.Des essais de relaxation, fluage et traction à diverses vitesses et températures ont permis de mesurer les évolutions des sensibilités à la vitesse en fonction de ces deux paramètres et de montrer: 1) que le raffinement microstructural accroît sensiblement la sensibilité à la vitesse 2) que ce paramètre augmente avec la vitesse de déformation 3) qu'il contrôle la ductilité du matériau à grains ultrafins, qui s'accroît donc à faible vitesse 4) que cette ductilité devient supérieure à celle du matériau classique lorsque la température s'élève. Les domaines de vitesse et température dans lesquels le raffinement microstructural accroît ou diminue la résistance en traction ont été délimités.Les mécanismes de déformation et d'endommagement des deux matériaux ont été étudiés au moyen d'essais de traction sous MEB accompagnés de mesures des champs de déformation par corrélation d'images à plusieurs échelles: méso et microscopique, grâce à des microgrilles d'or et sub-micrométrique, grâce à un mouchetis très fin obtenu par démouillage d'un film d'or. Le glissement aux joints est d'autant plus actif, dans les deux matériaux, que la température augmente et que la vitesse de déformation diminue. Dans l'alliage à grains ultrafins, il a un caractère coopératif et survient préférentiellement aux joints de forte désorientation. Les champs de déformation sont plus hétérogènes dans le matériau à grains ultrafins, où le taux de déformation dépasse 100% dans des bandes localisées.Un modèle éléments finis 2D intégrant, outre la viscoplasticité au sein des grains, un glissement visqueux des joints, a été identifié dans toute la gamme de température explorée et rend assez bien compte du comportement viscoplastique des deux matériaux et de la contribution beaucoup plus forte du glissement aux joints dans l'alliage à grains ultrafins. Il permet également de préciser comment évolue cette contribution au cours de l'écrouissage.Des essais de traction-compression à déformation plastique imposée ont permis d'étudier la plasticité cyclique et les mécanismes d'endommagement en fatigue oligocyclique et des essais à contrainte imposée, d'explorer la fatigue à grand nombre de cycles. Les essais ont été suivis d'observations des surfaces de rupture et d'une analyse statistique de l'endommagement en surface, ainsi que d'observations au MET des arrangements de dislocations. Les deux matériaux manifestent un durcissement cyclique, plus modeste dans l'alliage à grains ultrafins, qui présente, à forte amplitude, une croissance de ses grains. L'écrouissage isotrope prédomine dans l'alliage classique, où la densité de dislocations augmente fortement avec la plasticité cyclique, alors que l'écrouissage cinématique prédomine dans l'alliage à grains ultrafins, en raison de sa moindre aptitude à stocker des dislocations et de la plus grande hétérogénéité de sa déformation plastique. A même amplitude plastique, ce dernier a une durée de vie plus faible, en raison d'un amorçage bien plus rapide des fissures, à partir de particules intermétalliques. A contrainte imposée, le matériau a grain ultrafins a une durée de vie légèrement supérieure, grâce à une propagation plus lente des microfissures, dont le trajet est transgranulaire dans les plus gros grains et intergranulaire dans les plus petits. / Ultrafine grained (UFG) alloys seem promising, based on their high tensile properties and the possibility of superplastic forming at relatively low temperature. However, their deformation mechanisms are not fully understood, and their performance in fatigue has not been thoroughly investigated. This work compares the viscoplastic behavior, and the deformation and damage mechanisms in tension and fatigue of a UFG Al-Mg alloy (600 nm mean grain size) obtained by severe plastic deformation (ECAP process) with that of its coarse-grained (CG) counterpart.The strain rate sensitivity (SRS) of both materials has been measured during creep, relaxation and tensile tests run at various strain rates and temperature. Microstructural refinement is shown to increase the SRS, which rises as the strain rate decreases, and controls the ductility. The UFG material becomes softer and more ductile than the CG material at high temperature. The temperature and strain rate domain for which the UFG alloy is stronger or softer has been determined.Tensile tests run in a SEM, with DIC measurements of strain fields at meso/ micro scales (using gold microgrids printed by electron beam lithography) and at sub-micron scale (using a superfine speckle obtained by film remodelling) have shown that grain boundary sliding is more and more active in both materials as the temperature rises and as the strain rate decreases. Grain boundary sliding is cooperative and occurs mostly at high-angle grain boundaries in the UFG alloy, where the strain field is more heterogeneous, and where very high strain levels (> 100%) are often observed in localized bands.A 2D finite element model taking into account the viscoplastic behaviour inside the grains, and viscous sliding at the grain boundaries has been identified other the whole temperature range investigated. It captures well the observed behaviours and the much larger contribution of grain boundary sliding in the UFG alloy. It also provides the evolution of this contribution during strain hardening.Plastic strain-controlled push-pull tests and stress-controlled push-pull tests were run to investigate the cyclic behaviour and damage mechanisms of the two materials in low and high-cycle fatigue. The tests were followed by fractographic observations, statistical analysis of surface damage, as well as TEM observations of dislocations arrangements. Both materials exhibit cyclic hardening, although it is more modest in the UFG alloy, in which grain growth occurs at high amplitude. While isotropic hardening predominates in the CG alloy where the density of dislocation strongly increases during cyclic tests, kinematic hardening predominates in the UFG alloy, because of its limited capacity to store dislocations and its more heterogeneous plastic deformation. For a given plastic strain range, the UFG alloy has a shorter fatigue life than its CG counterpart, because of a much easier crack initiation, mostly from intermetallic particles. For a given stress range, it has a slightly higher life, due to a slower development of microcracks, which have a transgranular path in the largest grains, with some intergranular growth within the smallest grains.

Dissimilar Channel Angular Pressing (DCAP) method is an effective Severe Plastic Deformation (SPD) technique to improve the mechanical properties of sheets or strips by producing ultrafine grains. The aim of this study is to investigate the evolution of the microstructure and the improvement in mechanical properties of 6061 Al-alloy strips deformed by DCAP up to 5 passes. Mechanical properties such as hardness and strength have been observed to increase up to a certain strain level depending on the microstructural evolution. These microstructural changes were investigated by the characterization studies of XRD, SEM and TEM analysis of the DCAPed samples indicating the subgrain formation, changes in the dislocation density and dislocation behaviors.

Nesta dissertação de mestrado foram utilizadas duas ligas de alumínio (AA1100 e AA5052) para a produção de uma microestrutura final apresentando tamanho médio de grãos menores que um micrometro. Tal fato foi possível através do processo de Accumulated Roll Bonding (ARB). Tais materiais produzidos por ARB, foram caracterizados conforme propriedades mecânicas (microdureza, limite de escoamento e resistência) e microestruturais (tamanho de grão e macrotextura e composição química dos precipitados). O principal resultado para este trabalho foi a produção de tiras de AA 1100 e AA 5052 com tamanho médio de grão de 0,50 ± 0,04 m e 0,42 ± 0,06 m respectivamente. Tal resultado é coerente em comparação à literatura. Com relação aos resultados de textura, tais amostras apresentaram temperatura de recristalização acima de 473K e componente de textura típicas de cisalhamento (tanto para o material oriundo de fábrica como para o material laminado). Tal fato é esperado já que não se utilizou lubrificação durante ensaios de laminação. A propriedade mecânica de microdureza de AA 1100 ficou coerente com a literatura embora as propriedades de limite de escoamento e de resistência possuíram módulos abaixo da literatura. / The present work is related to the production, in a laboratory scale, of a metallic microstructure having a grain size smaller than one micrometer, through the Accumulated Roll Bonding technique(ARB), for two different aluminum alloys, namely AA 1100 and AA 5052. The strips obtained by the ARB technique have been characterized by their mechanical properties (microhardness, yield and tensile strength), and their microstructure (grain size, macrotexture and precipitates chemical composition). The main result of this work has been the rolling of AA1100 and AA5052 strips with an average grain size of 0.5 ± 0.04 m and 0.42 ± 0.06 m respectively. Such results are consistent with the literature. Relative to the texture, the samples presented a recrystallization temperature higher than 473K and typical shear texture components (due to the dry rolling conditions during the ARB). Microhardness of the AA110 was consistent with literature data; however yield and tensile strength presented values that were lower than those observed in the literature.

Nesta dissertação de mestrado foram utilizadas duas ligas de alumínio (AA1100 e AA5052) para a produção de uma microestrutura final apresentando tamanho médio de grãos menores que um micrometro. Tal fato foi possível através do processo de Accumulated Roll Bonding (ARB). Tais materiais produzidos por ARB, foram caracterizados conforme propriedades mecânicas (microdureza, limite de escoamento e resistência) e microestruturais (tamanho de grão e macrotextura e composição química dos precipitados). O principal resultado para este trabalho foi a produção de tiras de AA 1100 e AA 5052 com tamanho médio de grão de 0,50 ± 0,04 m e 0,42 ± 0,06 m respectivamente. Tal resultado é coerente em comparação à literatura. Com relação aos resultados de textura, tais amostras apresentaram temperatura de recristalização acima de 473K e componente de textura típicas de cisalhamento (tanto para o material oriundo de fábrica como para o material laminado). Tal fato é esperado já que não se utilizou lubrificação durante ensaios de laminação. A propriedade mecânica de microdureza de AA 1100 ficou coerente com a literatura embora as propriedades de limite de escoamento e de resistência possuíram módulos abaixo da literatura. / The present work is related to the production, in a laboratory scale, of a metallic microstructure having a grain size smaller than one micrometer, through the Accumulated Roll Bonding technique(ARB), for two different aluminum alloys, namely AA 1100 and AA 5052. The strips obtained by the ARB technique have been characterized by their mechanical properties (microhardness, yield and tensile strength), and their microstructure (grain size, macrotexture and precipitates chemical composition). The main result of this work has been the rolling of AA1100 and AA5052 strips with an average grain size of 0.5 ± 0.04 m and 0.42 ± 0.06 m respectively. Such results are consistent with the literature. Relative to the texture, the samples presented a recrystallization temperature higher than 473K and typical shear texture components (due to the dry rolling conditions during the ARB). Microhardness of the AA110 was consistent with literature data; however yield and tensile strength presented values that were lower than those observed in the literature.