Melting and melt segregation are key processes in the geochemical evolution of the Earth. However, mechanism and time scale of melt transport from the source to the surface are still not well understood and are dependent on the grain-scale distribution of melt. A related question is the retention of melt in partially molten regions of the Earths upper mantle. Seismic observations from mid-ocean ridges (MOR) and subduction zones are interpreted to show in-situ melt contents up to 3%, while geochemical observations from MOR basalts are inferred to indicate very efficient extraction of melt (porosities of order 0.1%). Earlier theoretical models of the melt distribution were based on the balance of surface tension between melt and uniform crystalline grains, predicting a simple net- work of melt along three-grain edges. Analyses of experimentally produced samples of olivine and basaltic melt show that the melt geometry is much more complex, and includes wetted two-grain boundaries. I reconstructed the melt geometry of two experimentally produced samples by serial sectioning and 3-D rendering of the pore geometry which demonstrates for the first time that melt exists in thin layers on two-grain boundaries. This confirms the inferences from previous 2-D observations and has significant implications for physical properties of partially molten regions, for example seismic velocities and attenuation. The wetted two-grain boundaries are inferred to be a consequence of continuous grain growth. Due to the complexity of the 3-D melt geometry the perme- ability of partially molten rocks can not be predicted from simple models. I therefore investigated the permeability as a function of porosity for both synthetic and ex- perimentally determined pore geometries using a lattice-Boltzmann method. The calculated permeability is not a simple function of porosity, but increases rapidly at a critical fraction of wetted two-grain boundaries. In order to extrapolate the experimentally based findings to grain sizes expected in natural rocks I examined the geometry of secondary phases inferred to represent relict melt in mantle peridotites from the Krivaja massif in Bosnia. These findings corroborate the experimental observations of wetted two-grain boundaries.

The work presented in this master thesis deal with the issue of quenching, investigation regarding different cooling rates and its effect on the material properties of aluminum alloy in the 6xxx series used for crash purposes in cars, such as crash boxes, beams and other crash relevant parts. Precipitation of Mg2Si due to different cooling rates affects the material properties such as crash performance, thus the aluminum alloy used is sensitive to different cooling rates. In order to perform tests with different cooling rates a cooling rig was constructed. In order to evaluate the different cooling rates both mechanical testing such as tensile test and 3-point bending test and compression test were performed. Also analyses with scanning electron microscope/energy-dispersive x-ray spectroscopy were performed to estimate grain boundary decoration of Mg2Si due to the different cooling rates. Furthermore LOM analyses were performed to evaluate if the experimental setup had any effect on material properties such as grain size. The constructed cooling rig produced different cooling rates with reliable repeatability as intended. Cooling rates between 130 ̊C/s and 20 ̊C/s were accomplished. Mg2Si occurred in all investigated test samples with various amounts. Higher cooling rates decreases the precipitation of Mg2Si to the grain boundaries, higher cooling rates also increased the bending angle achieved from the 3-point bending test. Furthermore, extensive solution heat treatment at elevated temperatures leads to grain growth.

The increasing clock speed and the further reduction of the feature size in integrated circuits lead to increasing demands on the interconnecting material. Thus an increasing need for a metallization with low electrical resistance and high electromigration endurance exist. Copper can be count as a material with these properties. Since 1998 Copper interconnections are commercially manufactured for integrated circuits. Electromigration is the most lifetime limiting factor in modern integrated circuits. The main the electromigration behavior influencing parameter and especially the influence of the microstructure is unknown. In this work the influence of the grain boundaries and their properties on the electromigration is examined at sputtered (PVD) and electroplated (ECD) Copper interconnects. For this investigation microstructure mappings produced by electron backscatter diffraction (EBSD) are correlated to in-situ electromigration experiments inside the SEM to research the electromigration behavior and the diffusion paths. Microstructure analysis shows big a difference between the two investigated types of interconnects. In both a strong <111> fibre texture is observed, but the PVD Copper shows a stronger texture than the electroplated one. The texture index of the PVD interconnects is 15,9 whereas the ECD lines show an index of 3,9. The frequency densities of the grain boundary misorientation, which is important for the electromigration behavior, are very different for both films. The ECD lines show a fraction of 55% Sigma 3 twin boundaries and 40% high angle grain boundaries. In contrast the PVD interconnects show a fraction of 5% Sigma 3 twin boundaries, 75% high angle grain boundaries and 20% small angle grain boundaries. This shows that a reduction of the high angle grain boundaries is not related to a strong <111> fibre texture. With in-situ experiments correlated to microstructure analysis it is shown, that voiding at high angle grain boundaries occur in the down wind of blocking grains or sites where only Sigma 3 twin boundaries are present. Hillocks were formed at high angle grain boundaries in the upwind of blocking grains or sites where only small angle grain boundaries or Sigma 3 twin boundaries are found. By a statistical evaluation of the in-situ experiments it is shown that more than 50% of the observed electromigration damages could be ascribed clearly to a grain boundary related local mass flux divergence. At strings of high angle grain boundaries voiding at the cathode side and hillock growth at the anode side is shown. The distance between these voids and hillocks is always higher than the Blech length. As the current density increases the distance between these voids and hillocks decreases according to Blech´s law, whereby it´s valid for local divergence is shown. FIB cuts show, that hillocks on PVD lines grow non-epitaxial in contrast to hillocks on ECD lines, which show epitaxial growth. These differences of hillock´s growth may suggest different underlying growth mechanisms. Reliability testing performed on PVD Copper interconnects lead to an activation energy for electromigration of 0,77eV ± 0,07eV. The confidence interval includes reported values for surface and also grain boundary diffusion. This indicates that the electromigration in these experiments is mainly influenced by surface and grain boundary diffusion. In this work the nucleation of voids and hillocks related to the previous analysed microstructure is observed inside the SEM and correlated to high angle grain boundaries and their misorientation angle. The result of this work show that electromigration damage in Copper interconnects is mostly caused by inhomogeneities of the microstructure. In this process the high angle grain boundaries are the main diffusion path. / Mit steigender Taktrate u. weiter fortschreitender Integrationsdichte in mikroelektr. Schaltungen nehmen d. Anforderungen an d. Metallisierungsmaterial weiter zu. Es besteht d. zunehmende Forderung nach Metallisierungen mit geringem elektrischen Widerstand u. hoher Elektromigra- tionsfestigkeit. Kupfer kann als Material angesehen werden, welches d. Anforderungen erfüllt. Seit 1998 wird Kupfer als Metallisierungsmaterial in höchstintegr. Schaltun- gen eingesetzt. Die Elektromigration (EM) ist der d. Zuver- lässigkeit am meisten begrenzende Faktor in mod. mikro- elektron. Schaltungen. Die Haupteinflußgrößen auf d. Elektromigrationsverhalten u. insbes. d. Einfluß d. Gefüges ist unklar. In d. Arbeit wird an nichtpassivier- ten physikalisch (PVD) u. galvanisch (ECD) abgeschied. Kupferleitbahnen d. Einfluß d. Korngrenzen u. deren Eigenschaften auf d. Elektromigrationsverhalten untersucht. Dazu werden Gefügeanalysen mittels Kikuchi-Rückstreutechnik u. in-situ Elektromigrationsexperimente im Rasterelektron- enmikroskop gekoppelt, um d. Elektromigrationsverhalten u. d. Migrationspfade zu erforschen. Gefügeuntersuchungen zeigen, daß d. untersuchten Leitbahnen sich in ihren Gefügeeigenschaften deutl. unterscheiden. Beide Schichten zeigen e. <111> Fasertextur, wobei d. PVD-Leitbahnen e. deutl. schärfere Textur mit e. Texturfaktor von 15,9 gegenüber den ECD-Leitbahnen d. e. Texturfaktor von 3,9 aufweisen. Die Häufigkeitsverteilungen d. Korngrenz- Misorientierung, sind für d. beiden Schichten unterschiedl. Die ECD-Leitbahnen zeigen e. Anteil von 55% Sigma 3-Korngrenzen und 40% Großwinkelkorngrenzen. Die PVD- Leitbahnen hingegen weisen nur e. Anteil von 5% Sigma 3-Korngrenzen, 75% Großwinkelkorngrenzen u. 20% Kleinwin- kelkorngrenzen auf. Dadurch wird gezeigt, daß e. scharfe <111> Textur keine Reduzierung d. Großwinkelkorngrenzen zur Folge haben muß. Anhand von in-situ Experimenten gekoppelt mit Gefügeanalysen wird gezeigt, daß Porenbildung an Groß- winkelkorngrenzen hinter blockierenden Körnern oder hinter Bereichen auftritt, in d. nur Sigma 3-Korngrenzen o. Kleinwinkelkorngrenzen vorliegen. Hügelbildung tritt an Großwinkelkorngrenzen vor blockierenden Körnern o. Berei- chen auf, in denen nur Kleinwinkelkorngrenzen o. Sigma 3-Korngrenzen vorliegen. Mit e. statist. Auswertung d. in-situ Experimente wird gezeigt, daß mehr als d. Hälfte aller Elektromigrationsschädigungen bei beiden Herstellungsmethoden eindeutig auf e. korngrenzbedingte lokale Divergenz im Massenfluß zurückzuführen sind. An Ketten von Großwinkelkorngrenzen wird verdeutl., daß kathodenseitig Porenbildung und anodenseitig Hügelbildung auftritt. Der Abstand zw. Pore u. Hügel liegt hier immer oberh. d. Blechlänge. Mit zunehmender Stromdichte nimmt d. Pore-Hügel-Abstand entspr. d. Blechtheorie ab, wodurch gezeigt wird, daß d. Blechtheorie auch bei lokalen Flußdivergenzen gilt. FIB-Querschnittsanalysen zeigen, daß Hügel auf PVD-Leitbahnen nicht epitaktisch mit d. darunterliegenden Schicht verwachsen sind im Gegensatz zu Hügeln auf ECD-Leitbahnen, die teilw. e. epitaktische Verwachsung mit d. Leitbahn zeigen. Lebensdauermessungen an PVD-Leitbahnen ergeben e. Aktivierungsenergie von 0,77eV ± 0,07eV. Es ist davon auszugehen, daß das Elektromigrationsverhalten d. hier untersuchten unpassi- vierten Leitbahnen haupts. von Korngrenz- u. von Oberfläch- endiffusion beeinflußt wird. In d. Arbeit wurde zum ersten Mal an Kupferleitbahnen d. Entstehung von eit- bahnschädigungen im Zusammenhang mit dem vorher aufgenomme- nen Gefüge im Rasterelektronenmikroskop direkt beobachtet u. mit d. Korngrenzen u. d. Korngrenzwinkeln in Zusammenhang gebracht. Die Ergebnisse d. Arbeit zeigen, daß Schädigungen durch Elektromigration in Kupferleitbahnen vorw. durch Gefügeinhomogenitäten entstehen. Bei d. Prozeß sind Großwinkelkorngrenzen d. bevorzugte Diffusionspfad.

In the transportation industry, weight reduction is essential in order to reduce fuel consumption. A solution towards lighter structures is to improve the mechanical properties while keeping a sufficient ductility for the forming operations. The aim of the thesis was to investigate the enhancement of the mechanical properties of metallic alloys by the refinement of the grain size while playing with other microstructural features in order to maintain the strain hardening, and thus the ductility, as high as possible. Various fine grained dual phase steels were produced by severe plastic deformation followed by thermal treatment. Nano-indentation and tensile tests have been performed to measure the change of flow properties associated to the grain refinement. A new methodology, based on performing nano-indentation tests at different depth inside each phases of the steels, has been proposed in order to separate the different hardening contributions affecting the behaviour of the material. In order to gain a better understanding of the link between the fine grained microstructure and the flow properties, three models were developed introducing successively a richer and richer description of the microstructure. The first model allows interpreting the nano-indentation data at different depths. The second model predicts the flow curve of dual phase steels by considering the accumulation of dislocations on the grain boundaries with the associated back stress and the saturation of this accumulation of excess dislocations, while introducing the second phase particles through a homogenization scheme. Finally, the third model promotes a new explanation of the Hall-Petch law and the interaction of the grain size and the texture for pure copper using a multigrain crystal plasticity model incorporating grain boundary effects. Guidelines are given to optimize the microstructures towards an improvement of the structural properties and formability.

The main focus of this thesis is in understanding the role of phase separation and microstructure in determining the physical properties of manganites. We also aim to be able to tune certain material properties using appropriate control mechanisms. For this, an understanding of the local electronic properties of manganites is essential. We thus set out to study the local electronic states in manganites using a highly sensitive probe: the scanning tunneling microscope (STM). The chapter 1 of the thesis gives an introduction to manganites, and of how manganites are susceptible to various perturbations due to closely lying ground states and an intricate interplay of their charge, spin and lattice degrees of freedom. Chapter 2 of this thesis gives a detailed account of various experimental methods used in the current investigation. In particular, we describe the design and fabrication of a variable temperature ultra-high vacuum scanning tunneling microscope (UHV-STM) which was used to carry out spatially resolved measurements on various manganite systems. This chapter also describes sample fabrication techniques by which strain and microstructure of thin films can be controlled. Other characterization techiniques, such as tranport and magnetotransport measurements, are also described in detail. Chapter 3 presents our investigation of the role of microstructure and phase separation on the DOS and local electronic properties of manganite thin films. We describe various spatially resolved STM/STS measurements carried out on La0.67Sr0.33MnO3 and La0.67Ca0.33MnO3 films having different micrsotructure and varying degrees of phase separation. We also present a theoretical model used in interpreting STS data to account for finite temperature effects and explain the existing data in this context. We use this model to gain insight into the behaviour of the DOS at EF near the MIT where thermal smearing can often give rise to misleading inferences. Chapter 4 presents our investigation on the density of states in a typical charge ordered manganite system, Pr1-xCaxMnO3. We describe STS measurements carried out on this system to study the occurrence and evolution of the charge ordering (CO) gap as a function if temperature as well as tunneling current. We report the observation of destabilization of the CO gap using tunnel current injection by an STM tip. Chapter 5 presents our investigation into the controlled and localized “nanoscale” phase separation in Pr1-xCaxMnO3 (PCMO) using an STM tip. The investigations were carried out on PCMO single crystal and PCMO epitaxial films. Our results raise the possibility of nano-fabrication of metallic nanoislands in a CO matrix using an STM tip. We demonstrate some examples of this and also raise the relevance of intrinsic phase separation in this context. We show that the “melting” of CO using tunnel current injection by an STM tip is analogous to the magnetic field-induced melting of CO on a microscopic scale. Chapter 6 summarizes the important results of this thesis work and suggests the scope for future experiments.

No description available.

In this dissertation we study free boundary problems that model the evolution of interfaces in the presence of elasticity, such as thin film profiles and material void boundaries. These problems are characterized by the competition between the elastic bulk energy and the anisotropic surface energy. First, we consider the evolution equation with curvature regularization that models the motion of a two-dimensional thin film by evaporation-condensation on a rigid substrate. The film is strained due to the mismatch between the crystalline lattices of the two materials and anisotropy is taken into account. We present the results contained in [62] where the author establishes short time existence, uniqueness and regularity of the solution using De Giorgi’s minimizing movements to exploit the L2 -gradient flow structure of the equation. This seems to be the first analytical result for the evaporation-condensation case in the presence of elasticity. Second, we consider the relaxed energy introduced in [20] that depends on admissible pairs (E, u) of sets E and functions u defined only outside of E. For dimension three this energy appears in the study of the material voids in solids, where the pairs (E, u) are interpreted as the admissible configurations that consist of void regions E in the space and of displacements u of the atoms of the crystal. We provide the precise mathematical framework that guarantees the existence of minimal energy pairs (E, u). Then, we establish that for every minimal configuration (E, u), the function u is C 1,γ loc -regular outside an essentially closed subset of E. No hypothesis of starshapedness is assumed on the voids and all the results that are contained in [18] hold true for every dimension d ≥ 2.

The work presented in this doctoral thesis employs colloidal suspensions to explore key open problems in condensed matter physics. Colloidal suspensions, along with gels, polymers, emulsions and liquid crystals belong to a family of materials that are collectively labelled as soft matter. Compositionally, colloidal suspensions consist of particles whose size ranges from a few nanometers to a few microns, dispersed in a solvent. A hallmark feature of these systems is that they exhibit Brownian motion, which makes them suitable for investigating statistical mechanical phenomena. Over the last fifteen years or so, colloids have been used extensively as model systems to shed light on a wide array of such phenomena typically observed in atomic systems. The chief reason why colloids are good mimics of atomic systems is their large size and slow dynamics. Unlike atomic systems, the dynamics of colloids can be probed in real time with single-particle resolution, which allows one to establish the link between macroscopic behavior and the microscopic processes that give rise to it. Yet another important feature is that colloidal systems exhibit various phases of matter such as crystals, liquids and glasses, which makes them versatile model systems that can probe a broad class of condensed matter physics problems. The work described in this thesis takes advantage of these lucrative features of colloidal suspensions to gain deeper insights into the physics of glass formation as well as shear-induced anisotropic grain growth in polycrystalline materials. The thesis is organized into two preliminary chapters, four work chapters and a concluding chapter, as follows. Chapter 1 provides an introduction to colloidal suspensions and reviews the chief theo-retical concepts regarding glass formation and grain boundary dynamics that form an integral part of subsequent chapters. Chapter 2 describes the experimental methods used for performing the work presented in the thesis and consists of two parts. The first part describes the protocols followed for synthesizing the size-tunable poly (N-isoprolypacrylamide) (PNIPAm) particles used in our study of shear-induced grain growth. The second part describes the instrumentation and techniques, such as holographic optical tweezers, confocal microscopy, rheology and Bragg diffraction microscopy, used to perform the measurements described in the thesis. Chapter 3 deals with our work on the dynamical facilitation (DF) theory of glass forma-tion. Despite decades of research, it remains to be established whether the transformation of a liquid into a glass is fundamentally thermodynamic or dynamic in origin. While obser-vations of growing length scales are consistent with thermodynamic perspectives, the purely dynamic approach of the DF theory has thus far lacked experimental support. Further, for glass transitions induced by randomly freezing a subset of particles in the liquid phase, theory and simulations support the existence of an underlying thermodynamic phase transi-tion, whereas the DF theory remains unexplored. In Chapter 3, using video microscopy and holographic optical tweezers, we show that dynamical facilitation in a colloidal glass-forming liquid grows with density as well as the fraction of pinned particles. In addition, we observe that heterogeneous dynamics in the form of string-like cooperative motion, which is consid-ered to be consistent with thermodynamic theories, can also emerge naturally within the framework of facilitation. These findings suggest that a deeper understanding of the glass transition necessitates an amalgamation of existing theoretical approaches. In Chapter 4, we further explore the question of whether glass formation is an intrinsi-cally thermodynamic or dynamic phenomenon. A major obstacle in answering this question lies in determining whether relaxation close to the glass transition is dominated by activated hopping, as espoused by various thermodynamic theories, or by the correlated motion of localized excitations, as proposed in the Dynamical Facilitation (DF) approach. In Chapter 4, we surmount this central challenge by developing a scheme based on real space micro-scopic analysis of particle dynamics and applying it to ascertain the relative importance of hopping and facilitation in a colloidal glass-former. By analysing the spatial organization of excitations within cooperatively rearranging regions (CRRs) and examining their parti-tioning into shell-like and core-like regions, we establish the existence of a crossover from a facilitation-dominated regime at low area fractions to a hopping-dominated one close to the glass transition. Remarkably, this crossover coincides with the change in morphology of CRRs predicted by the Random First-Order Transition theory (RFOT), a prominent ther-modynamic framework. Further, we analyse the variation of the concentration of excitations with distance from an amorphous wall and find that the evolution of these concentration profiles with area fraction is consistent with the presence of a crossover in the relaxation mechanism. By identifying regimes dominated by distinct dynamical processes, our study offers microscopic insights into the nature of structural relaxation close to the glass transi-tion. In Chapter 5, we extend our investigation of the glass transition to systems composed of anisotropic particles. The primary motivation for this is to bridge a long-standing di-vide between theories and simulations on one hand, and experiments on molecular liquids on the other. In particular, theories and simulations predominantly focus on simple glass-formers composed of spherical particles interacting via isotropic interactions. Indeed, even the prominent theory of Dynamical Facilitation has not even been formulated to account for anisotropic shapes or interactions. On the other hand, an overwhelming majority of liquids possess considerable anisotropy, both in particle shape as well as interactions. In Chapter 5, we mitigate this situation by developing the DF theory further and applying it to systems with orientational degrees of freedom as well as anisotropic attractive interactions. By analyzing data from experiments on colloidal ellipsoids, we show that facilitation plays a pivotal role in translational as well as orientational relaxation. Further, we demonstrate that the introduction of attractive interactions leads to spatial decoupling of translational and rotational facilitation, which subsequently results in the decoupling of dynamical het-erogeneities. Most strikingly, the DF theory can predict the existence of reentrant glass transitions based on the statistics of localized dynamical events, called excitations, whose duration is substantially smaller than the structural relaxation time. Our findings pave the way for systematically testing the DF approach in complex glass-formers and also establish the significance of facilitation in governing structural relaxation in supercooled liquids. In Chapter 6, we turn our attention away from the glass transition and address the problem of grain growth in sheared polycrystalline materials. The fabrication of functional materials via grain growth engineering implicitly relies on altering the mobilities of grain boundaries (GBs) by applying external fields. While computer simulations have alluded to kinetic roughening as a potential mechanism for modifying GB mobilities, its implications for grain growth have remained largely unexplored owing to difficulties in bridging the disparate length and time scales involved. In Chapter 6, by imaging GB particle dynamics as well as grain network evolution under shear, we present direct evidence for kinetic roughening of GBs and unravel its connection to grain growth in driven colloidal polycrystals. The capillary fluctuation method allows us to quantitatively extract shear-dependent effective mobilities. Remarkably, our experiments reveal that for sufficiently large strains, GBs with normals parallel to shear undergo preferential kinetic roughening resulting in anisotropic enhancement of effective mobilities and hence directional grain growth. Single-particle level analysis shows that the anisotropy in mobility emerges from strain-induced directional enhancement of activated particle hops normal to the GB plane. Finally, in Chapter 7, we present our conclusions and discuss possible future directions.

Durant cette thèse, on s’intéresse à l’application et au développement d’une théorie de mécanique des champs de dislocations et de désinclinaisons pour modéliser de façon continue les structures de cœur des dislocations et des joints de grains ainsi que leurs interactions. Le vecteur de Burgers/Frank des dislocations/désinclinaisons est régularisé par l’introduction d’un tenseur densité de dislocations/désinclinaisons. A ces densités de défauts sont associées des déformations et des courbures élastiques et plastiques incompatibles responsables de champs de contraintes et de moments de contraintes internes. Le mouvement des défauts produit de la plasticité et est pris en compte par des équations de transport qui font intervenir des forces motrices agissant sur les densités de défauts. Dans un premier temps, les désinclinaisons sont ignorées et nous appliquons la théorie de champ de dislocations seule pour étudier les structures de cœur de dislocations planaires en comparaison avec le modèle de Peierls-Nabarro. La relaxation d’une structure de cœur de dislocation coin initiale arbitraire révèle un étalement infini des densités de dislocations sous l’action de leur propre champ de contrainte interne. Pour stopper cette relaxation infinie, nous proposons d’ajouter une énergie de misfit dans notre modèle. Cette dernière donne lieu à une contrainte de rappel qui s’oppose à l’étalement des cœurs de dislocations et permet d’obtenir des configurations équilibrées. On retrouve la solution de Peierls-Nabarro si on utilise un potentiel sinusoïdal pour l’énergie. Nous substituons ensuite ce potentiel par des énergies de fautes d’empilement généralisées obtenues à partir de simulations atomistiques pour modéliser la dissociation des dislocations et leur mouvement dans le zirconium et le titane. Dans un deuxième temps, nous considérons la théorie complète et nous développons des lois d’élasticité constitutives qui sont propres aux défauts cristallins. Nous proposons qu’en plus des tenseurs élastiques habituels, des tenseurs d’élasticité additionnels existent au niveau du cœur des défauts et relient respectivement les contraintes aux courbures et les moments de contraintes aux déformations. Ces tenseurs sont de nature non locale par définition à cause des relations cinématiques entre déformations et courbures. Ils sont non nuls au niveau des cœurs des défauts où les hétérogénéités de déformations et de courbures sont fortes et deviennent nuls loin des défauts par centrosymétrie. On applique ces nouvelles lois d’élasticité à des distributions de dislocations et de désinclinaisons. On montre que les termes non locaux donnent lieu à des contraintes/moments de contraintes de rappel qui s’opposent aux parties locales. Dans le cas de la dislocation coin, on montre que sa représentation avec un dipôle de désinclinaison coin permet d’obtenir une configuration équilibrée sans l’ajout d’énergie de misfit. On étudie ensuite les interactions élastiques entre dislocations et joints de grains / In this contribution, we apply and develop a mechanical theory of dislocation and disclination fields, to model in a continuous way the core structure of dislocations and grain boundaries, as well as their interactions. The Burgers/Frank vector of dislocations/disclinations is regularized by the introduction of dislocation/disclination density tensors. Incompatible elastic and plastic strains and curvatures are associated to these defect densities and they lead to internal stress and couple stress fields. The motion of defects yields plasticity. It is accounted for by transport equations, where driving forces act on the defect densities. First, we overlook disclinations and we apply the pure dislocation model to investigate the structure of planar dislocation cores, in comparison with the Peierls-Nabarro model. The self-relaxation of an initially arbitrary core structure of an edge dislocation reveals that an infinite spreading of the dislocation density occurs under its own stress field. To stop this endless relaxation, we propose to add a misfit energy in our model. The latter yields a restoring stress that opposes to the spreading of dislocation cores and allows predicting equilibrium core structures. We retrieve the Peierls-Nabarro solution when we use a sinusoidal potential for the misfit energy. We then substitute this sinusoidal potential for generalized stacking fault energies as obtained from atomistic simulations, in order to model the dissociation and motion of dislocations in zirconium and titanium. Second, we consider the full theory and we develop elastic constitutive laws that are specific to crystal defects. We propose that in addition to standard elasticmoduli tensors, additional elastic tensors exist in the core regions of defects and relate respectively stresses to curvatures and couple stresses to strains. These tensors are nonlocal by definition due to kinematic relations between strains and curvatures. They are non-zero in the core of defects, where strong heterogeneities of strains and curvatures occur, and they become progressively null far from the defects due to centrosymmetry. We apply these new elastic laws to distributions of dislocations and disclinations. We show that the nonlocal elastic tensors lead to restoring stresses and couple stresses that oppose to their local parts. In the framework of edge dislocations, we show that the representation using dipoles of wedge disclination cores allows predicting equilibrium structures without adding a misfit energy. We then investigate elastic interactions between dislocations and grain boundaries

La technique Imagerie par Contraste de Canalisation d'Electron (ECCI) est utilisée en microscopie électronique à balayage (MEB) pour visualiser et caractériser des défauts cristallins tels que les dislocations. L’ECCI nécessite l'orientation, avec grande précision (meilleure que 0,1°), du cristal à analyser par rapport au faisceau d’électrons pour satisfaire les conditions très strictes de canalisation d'électrons. À cause de la limitation en résolution spatiale et angulaire des techniques actuelles permettant de déterminer l’orientation cristallographique, la caractérisation des défauts cristallins par ECCI est actuellement appliquée à des monocristaux (ou des polycristaux possédant des gros grains) et les conditions de canalisation ne sont pas toujours satisfaites. Dans ce projet de thèse, un mode de balayage Précession de faisceau (Rocking Beam en anglais) a été développé dans un microscope électronique à balayage. Il permet l’acquisition de diagrammes pseudo-Kikuchi haute résolution spatiale (500nm) et angulaire (0,04°) (High Resolution Selected Area Channeling Pattern en anglais (HR-SACP)) et de contrôler les conditions de canalisations nécessaire à l’ECCI. Ceci a permis d’améliorer considérablement la précision de l’ECCI (Accurate ECCI A-ECCI) et d’élargir son domaine d’application aux matériaux polycristallins à grains fins. Dans un deuxième temps, l’A-ECCI assistée par HR-SACP a été utilisé pour caractériser des défauts cristallins (dislocations, sous joint de grain, domaine d’ordre) dans des matériaux massifs polycristallins (Acier IF, UO2, TiAl). Des procédures similaires à celles utilisées dans la microscopie électronique en transmission (MET) sont alors appliquées en s’affranchissant de la préparation fastidieuse de lames minces et en profitant des autres avantages du MEB / Electron Channeling Contrast Imaging (ECCI) is a Scanning Electron Microscope (SEM) technique used to observe and characterize crystallographic defects. ECCI requires the crystal to be oriented relative to the electron beam with high accuracy (0.1°) in order to control the electron channeling conditions. The SEM techniques used to determine the crystallographic orientation, such as conventional Electron BackScattered Diffraction (EBSD) or Rocking Beam, don’t satisfy the high accuracy required for ECCI. Therefore, the characterization of crystallographic defects by ECCI is used only in single crystals or polycristals with large grains and channeling conditions are not always satisfied. In this thesis, a development of a new Rocking Beam mode in SEM is presented. It allows the collection of High spatial (500nm) and angular (0.04°) Resolution Selected Area Channeling Pattern (HR-SACP) and the control of channeling conditions required for ECCI with high accuracy (Accurate ECCI A-ECCI). In a second phase of this thesis, A-ECCI assisted by HR-SACP is used to characterize crystallographic defects like dislocation, sub-grains and order domains in fine grained bulk materials (IF-Steel, UO2, TiAl). In order to achieve this, several procedures (invisibility criteria) normally used in Transmission Electron Microscopy are applied. Using A-ECCI in SEM has many advantages over TEM such as the possibility of analyzing large areas and the relative easiness in sample preparation