La rentabilité des cellules à base de silicium est un point essentiel pour le marché photovoltaïque et cela passe notamment par l'amélioration du rendement électrique, la baisse des coûts de production ainsi que le renforcement de la fiabilité/durabilité des wafers. Des procédés innovants émergent, qui permettent d'obtenir des wafers ultra minces avec moins de perte de matière première. Cependant il est nécessaire de mettre en place des méthodes de caractérisation afin d’analyser la rigidité et la tenue mécanique de ces matériaux. Dans ce travail, des essais de flexion ont été effectués pour caractériser à la fois la rigidité et la rupture. Afin d’étudier la rupture fragile, une caméra rapide a été utilisée, des analyses fractographiques ont été menées. La diffraction d'électrons rétrodiffusés et la diffraction par rayon X de Laue ont été utilisées afin d'explorer le lien entre les orientations cristallographiques et les comportements observés. Conjointement, des simulations numériques EF ont été mise en place. Grâce à ce couplage expériences-simulations numériques, une caractérisation fiable de la rigidité des wafers a été effectuée. Une stratégie d'identification de l'origine de la rupture est également proposée. L'étude de la rupture du silicium monocristallin a mis en évidence la stabilité du clivage (110), la grande vitesse d'amorçage de la fissure, la dépendance de la forme du front de fissure à la vitesse de propagation ainsi que l'apparition de "Front Waves" pour les fissures à très grande vitesse. L'étude de la rupture des wafers multi-cristallins démontre une fissuration intra-granulaire. Des éprouvettes jumelles ont permis d’étudier la répétabilité du chemin de fissuration : une attention particulière a été portée à la nature des plans de clivage ainsi que l'effet des joints de grains. Enfin, une modélisation par la méthode des éléments finis étendus est proposée. Elle permet de reproduire le chemin de fissuration expérimentalement observé. / The profitability of silicon solar cells is a critical point for the PV market and it requires improved electrical performance, lower wafer production costs and enhancing reliability and durability of the cells. Innovative processes are emerging that provide thinner wafers with less raw material loss. But the induced crystallinity and distribution of defects compared to the classical wafers are unclear. It is therefore necessary to develop methods of microstructural and mechanical characterization to assess the rigidity and mechanical strength of these materials. In this work, 4-point bending tests were performed under quasi-static loading. This allowed to conduct both the stiffness estimation and the rupture study. A high speed camera was set up in order to track the fracture process thanks to a 45° tilted mirror. Fractographic analysis were performed using confocal optical microscope, scanning electron microscope and atomic force microscope. Electron Back-Scatter Diffraction and Laue X-Ray diffraction were used to explore the relationship between the microstructural grains orientations/textures of our material and the observed mechanical behavior. Jointly, finite element modeling and simulations were carried out to provide auxiliary characterization tools and help to understand the involved fracture mechanism. Thanks to the experiment-simulation coupled method, we have assessed accurately the rigidity of silicon wafers stemming from different manufacturing processes. A fracture origin identification strategy has been proposed combining high speed imaging and post-mortem fractography. Fracture investigations on silicon single crystals have highlighted the deflection free (110) cleavage path, the high initial crack velocity, the velocity dependent crack front shape and the onset of front waves in high velocity crack propagation. The investigations on the fracture of multi-crystalline wafers demonstrate a systematic transgranular cracking. Furthermore, thanks to twin multi-crystalline silicon plates, we have addressed the crack path reproducibility. A special attention has been paid to the nature of the cleavage planes and the grain boundaries barrier effect. Finally, based on these observations, an extended finite element model (XFEM) has been carried out which fairly reproduces the experimental crack path.

O aumento no consumo energético mundial e a perspectiva de esgotamento das reservas de combustíveis fósseis têm estimulado o desenvolvimento de tecnologias e materiais para aplicações nos futuros reatores de fusão nuclear e reatores de fissão a nêutrons rápidos. O foco deste trabalho é avaliar a estabilidade microestrutural de um material tecnologicamente promissor por meio de diferentes técnicas de caracterização. O material estudado, o aço ODS-EUROFER, é um aço ferrítico-martensítico de atividade reduzida de composição 9%Cr-1%W (em massa) endurecido pela dispersão de 0,3% (em massa) de óxido de ítrio. Amostras foram laminadas até 80% de redução e submetidas a tratamentos isotérmicos em 800 °C por até 6 meses (4320 h) de duração para se investigar a ocorrência de fenômenos tais como recuperação, recristalização, crescimento de grão, precipitação e engrossamento de Ostwald. Outro conjunto de amostras foi recozido por 1 hora em temperaturas entre 900 e 1300 °C para se estudar a transformação martensítica neste material. Diversas técnicas de caracterização microestrutural complementares entre si (MEV, MET, DRX, EBSD, APT, ensaios de microdureza Vickers, dilatometria, DTA e magnetização) foram empregadas. Os resultados obtidos indicam que a dispersão de partículas nanométricas de Y2O3 confere uma grande resistência à recristalização primária, favorecendo a recuperação estática como principal mecanismo de amolecimento durante o recozimento prolongado deste aço. De modo similar, o crescimento de grão foi suprimido no campo austenítico em temperaturas tão altas quanto 1200 °C. / The rise in the world energy consumption and the possibility of depletion of the fossil fuel reserves have stimulated the development of new technologies and new materials for applications in the future nuclear fusion reactors and in the fast breeder fission reactors. The aim of this study is to evaluate the microstructural stability of a technologically promising material by using different characterization techniques. The investigated material, the ODS-EUROFER steel, is 9Cr-1W (%wt) reduced-activation ferritic-martensitic steel reinforced with a dispersion of 0.3%wt of yttrium oxide nanoparticles. Samples were cold rolled to 80% thickness reduction and subjected to isothermal annealing at 800 °C for up to 6 months (4,320 h) to investigate the occurrence of phenomena such as recovery, recrystallization, grain growth, precipitation, and Ostwald ripening. Another set of samples was annealed for 1 hour at temperatures between 900 and 1300 °C to study the martensitic transformation in this steel. Several complementary microstructural characterization techniques were employed (SEM, TEM, XRD, EBSD, APT, Vickers hardness, dilatometry, DTA and magnectic measurements). The results suggest that the dispersion of nanoscaled particles of Y2O3 provides a high resistance to discontinous recrystallization and favors static recovery as the main softening mechanism during long-term annealing in this steel. Similarly, grain growth was suppressed even in temperatures as high as 1200 °C in the austenitic field.

Friction Stir Processing (FSP) is a promising thermomechanical technique that is used to modify the microstructure of metals locally, and thereby locally improve mechanical properties of the material. FSP uses a simple and inexpensive tool, and has been shown to eliminate pores and also reduce the sizes of intermetallics in aluminum alloys. This is of great interest for research on solidification, production and performance of aluminum alloy castings because FSP can enhance the structural quality of aluminum casting significantly by minimizing the effect of those structural defects. In the literature, there is evidence that the effectiveness of FSP can change with tool wear of the tool used. Therefore, a study was first conducted to determine the effect of FSP time on the tool life and wear in 6061-T6 extrusions. Results showed the presence of two distinct phases in the tool life and wear. Metallographic analyses confirmed that wear in Phase I was due to fracture of the threads of the tool and Phase II was due to regular wear, mostly without fracture. Moreover, built-up layers of aluminum were observed between threads. The microhardness profile was found to be different from those reported in the literature for 6061-T6, with Vickers hardness increasing continuously from the the stir zone to the base material. To investigate the degree of effectiveness of FSP in improving the structural quality of cast A356 alloys, ingots with different quality (high and low) were friction strir processed with single and multiple passes. Analysis of tensile test results and work hardening characteristics showed that for the high quality ingot, a single pass was sufficient to eliminate the structural defects. Subsequent FSP passes had no effect on the work hardening characteristics. In contrast, tensile results and work hardening characteristics improved with every pass for the low quality ingot, indicating that the effectiveness of FSP was dependent on the initial quality of the metal. The evolution of microstructure, specifically the size and spacing of Silicon (Si) eutectic particles, was investigated after friction stir processing of high quality A356 castings with single and multiple passes. Si particles were found to coarsen with each pass, which was in contrast with previous findings in the literature. The nearest neighbor distance of Si particles also increased with each FSP pass, indicating that microstructure became progressively more homogeneous after each pass. In the literature, the improvement observed after FSP of Al-Si cast alloys was attributed to the refinement of Si particles. Tensile data from high quality A356 ingot showed that there was no correlation between the size of Si particles and ductility. To the author’s knowledge, this is the first time that the absence of a correlation between Si particle size and ductility has been found.

The Cold-Gas Dynamic-Spray process also known as Cold Spray (CS) has been researched for three decades. The CS process is a solid-state deposition technique via supersonic velocity of powder particles at a temperature significantly below the melting point of the spray material. This thesis presents background on the overall CS process parameters, and additional information on the microstructural and mechanical properties of typical Cold Sprayed materials.This Thesis primarily presents a study on the microstructural annealing response of CS Al 6061. It should be noted that for this study, the term “annealing” is used in the sense of the classical metallurgical definition of annealing, and not a specific temper designation for the 6061 alloy. Cross sections of CS Al 6061 were imaged with a scanning electron microscope (SEM) in secondary electron (SE), backscatter electron (BSE), and electron backscatter diffraction (EBSD) imaging mode for quantitative and qualitative information on the grain size and orientation of the CS microstructure. The detailed SE, BSE and EBSD mode images present the grain size and grain orientation of the original powder, as received (AR) state and after heat treating at 200°C for 1 hour, 10 hours, and 100 hours. Three different regions, characterized with distinctly differing microstructures, are labeled as low, medium, and high deformation regions, and their microstructures, and evolving features are discussed. Vickers microhardness testing are performed to examine the differences in hardness values between different heat treatments, and for correlation with the level of deformation and grain refinement in the microstructure. SEM imaging was used in BSE mode to correlate microhardness variation to the different regions within the CS microstructure.

Gelfill (GF) is made of tailings, water, binder and chemical additives (Fillset, sodium silicate gel). The components of GF are combined and mixed on the surface and transported (by gravity and/or pumping) to the underground mine workings, where the GF can be used for both underground mine support and tailings storage. Thermal (T), hydraulic (H), and mechanical (M) properties are important performance criteria of GF. The understanding of these engineering properties and their evolution with time are still limited due to the fact that GF is a new cemented backfill material. In this thesis, the evolution of the thermal, hydraulic, mechanical, and microstructural properties of small GF samples are determined. Various binder contents of Portland cement type I (PCI) are used. The GF is cured for 3, 7, 28, 90, and 120 days. It is found that the thermal, hydraulic and mechanical properties are time-dependent or affected by the degree of binder hydration index. Furthermore, a relationship is found between the compressive strength and the saturated hydraulic conductivity of the GF samples. The unsaturated hydraulic properties of GF samples have also been investigated. The outcomes show that unsaturated hydraulic conductivity is influenced by the degree of binder hydration index and binder content, especially at low suction ranges. Simple functions are proposed to predict the evolution of air-entry values (AEVs), residual water content, and fitting parameters from the van Genuchten model with the degree of hydration index (α). Furthermore, two columns are built to simulate the coupled thermo-hydro-mechanical (THM) behaviour of GF under drained and undrained conditions. The obtained results from the GF columns are compared with the small samples. It is observed that the mechanical properties, hydraulic properties (suction and water content), and temperature development are strongly coupled. The magnitude of these THM coupling factors is affected by the size of the GF. The findings also show that the mechanical, hydraulic and thermal properties of the GF columns are different from samples cured in plastic moulds.

The effect of solidification conditions on microstructural and mechanical properties of eutectic TiFe alloy cast under different conditions was examined. Samples exhibit different ultrafine eutectic structures (β-Ti(Fe) solid solution + TiFe). Different cooling conditions lead to the evolution of ultrafine eutectic oval-shaped colonies or elongated lamellar colonies with preferred orientation. Isotropic as well as anisotropic mechanical properties were obtained. Alloys exhibit compressive strengths between 2200 and 2700 MPa and plastic strains between 7 and 19 pct. in compression.

Gelfill (GF) is made of tailings, water, binder and chemical additives (Fillset, sodium silicate gel). The components of GF are combined and mixed on the surface and transported (by gravity and/or pumping) to the underground mine workings, where the GF can be used for both underground mine support and tailings storage. Thermal (T), hydraulic (H), and mechanical (M) properties are important performance criteria of GF. The understanding of these engineering properties and their evolution with time are still limited due to the fact that GF is a new cemented backfill material. In this thesis, the evolution of the thermal, hydraulic, mechanical, and microstructural properties of small GF samples are determined. Various binder contents of Portland cement type I (PCI) are used. The GF is cured for 3, 7, 28, 90, and 120 days. It is found that the thermal, hydraulic and mechanical properties are time-dependent or affected by the degree of binder hydration index. Furthermore, a relationship is found between the compressive strength and the saturated hydraulic conductivity of the GF samples. The unsaturated hydraulic properties of GF samples have also been investigated. The outcomes show that unsaturated hydraulic conductivity is influenced by the degree of binder hydration index and binder content, especially at low suction ranges. Simple functions are proposed to predict the evolution of air-entry values (AEVs), residual water content, and fitting parameters from the van Genuchten model with the degree of hydration index (α). Furthermore, two columns are built to simulate the coupled thermo-hydro-mechanical (THM) behaviour of GF under drained and undrained conditions. The obtained results from the GF columns are compared with the small samples. It is observed that the mechanical properties, hydraulic properties (suction and water content), and temperature development are strongly coupled. The magnitude of these THM coupling factors is affected by the size of the GF. The findings also show that the mechanical, hydraulic and thermal properties of the GF columns are different from samples cured in plastic moulds.

Gelfill (GF) is made of tailings, water, binder and chemical additives (Fillset, sodium silicate gel). The components of GF are combined and mixed on the surface and transported (by gravity and/or pumping) to the underground mine workings, where the GF can be used for both underground mine support and tailings storage. Thermal (T), hydraulic (H), and mechanical (M) properties are important performance criteria of GF. The understanding of these engineering properties and their evolution with time are still limited due to the fact that GF is a new cemented backfill material. In this thesis, the evolution of the thermal, hydraulic, mechanical, and microstructural properties of small GF samples are determined. Various binder contents of Portland cement type I (PCI) are used. The GF is cured for 3, 7, 28, 90, and 120 days. It is found that the thermal, hydraulic and mechanical properties are time-dependent or affected by the degree of binder hydration index. Furthermore, a relationship is found between the compressive strength and the saturated hydraulic conductivity of the GF samples. The unsaturated hydraulic properties of GF samples have also been investigated. The outcomes show that unsaturated hydraulic conductivity is influenced by the degree of binder hydration index and binder content, especially at low suction ranges. Simple functions are proposed to predict the evolution of air-entry values (AEVs), residual water content, and fitting parameters from the van Genuchten model with the degree of hydration index (α). Furthermore, two columns are built to simulate the coupled thermo-hydro-mechanical (THM) behaviour of GF under drained and undrained conditions. The obtained results from the GF columns are compared with the small samples. It is observed that the mechanical properties, hydraulic properties (suction and water content), and temperature development are strongly coupled. The magnitude of these THM coupling factors is affected by the size of the GF. The findings also show that the mechanical, hydraulic and thermal properties of the GF columns are different from samples cured in plastic moulds.

La present tesi proposa una metodología per a la simulació probabilística de la fallada de la matriu en materials compòsits reforçats amb fibres de carboni, basant-se en l'anàlisi de la distribució aleatòria de les fibres. En els primers capítols es revisa l'estat de l'art sobre modelització matemàtica de materials aleatoris, càlcul de propietats efectives i criteris de fallada transversal en materials compòsits.El primer pas en la metodologia proposada és la definició de la determinació del tamany mínim d'un Element de Volum Representatiu Estadístic (SRVE) . Aquesta determinació es du a terme analitzant el volum de fibra, les propietats elàstiques efectives, la condició de Hill, els estadístics de les components de tensió i defromació, la funció de densitat de probabilitat i les funcions estadístiques de distància entre fibres de models d'elements de la microestructura, de diferent tamany. Un cop s'ha determinat aquest tamany mínim, es comparen un model periòdic i un model aleatori, per constatar la magnitud de les diferències que s'hi observen.Es defineix, també, una metodologia per a l'anàlisi estadístic de la distribució de la fibra en el compòsit, a partir d'imatges digitals de la secció transversal. Aquest anàlisi s'aplica a quatre materials diferents.Finalment, es proposa un mètode computacional de dues escales per a simular la fallada transversal de làmines unidireccionals, que permet obtenir funcions de densitat de probabilitat per a les variables mecàniques. Es descriuen algunes aplicacions i possibilitats d'aquest mètode i es comparen els resultats obtinguts de la simulació amb valors experimentals. / This thesis proposes a methodology for the probabilistic simulation of the transverse failure of Carbon Fibre Reinforced Polymers (CFRP) by analyzing the random distribution of the fibres within the composite. First chapters are devoted to the State-of-the-art review on the modelization of random materials, the computation of effective properties and the transverse failure of fibre reinforced polymers.The first step in the proposed methodology is the definition of a Statistical Representative Volume Element (SRVE). This SRVE has to satisfy criteria based on the analysis of the volume fraction, the effective properties, the Hill Condition, the statistics of the stress and strain components, the probability density function of the stress and strain components and the inter-fibre distance statistical distributions. Once this SRVE has been achieved, a comparison between a periodic model and a random model is performed to quantitatively analyze the differences between the results they provide.Also a methodology for the statistical analysis of the distribution of the fibre within the composite from digital images of the transverse section. This analysis is performed for four different materials.Finally, a two-scale computational method for the transverse failure of unidirectional laminae is proposed. This method is able to provide probability density functions of the mechanical variables in the composite. Some applications and possibilities of the method are given and the simulation results are compared with experimental tests.

Cette thèse porte sur la fonctionnalisation de fils en acier inoxydable via des revêtements colorés base titane, élaborés par pulvérisation cathodique avec un magnétron cylindrique. Ce travail s’intéresse à la caractérisation chimique, morpho-structurale et à l’évaluation de la durabilité mécano-chimique des fils revêtus.Premièrement, la vitesse de dépôt et la composition chimique des films sont déterminées dans des conditions statiques. Les hétérogénéités du plasma dans la cathode sont démontrées et reliées aux paramètres tels que la puissance, la pression et la polarisation d’anodes auxiliaires.Les dépôts monocouches de Ti, TiN et TiOx sont ensuite élaborés en continu. La relation entre la couleur du TiN et sa composition chimique est établie. La couleur dorée est obtenue pour des films stœchiométriques contenant peu d’oxygène (< 5 %at.). Les microstructures sont caractérisées par MET-ASTAR et des cartographies d’orientation sont dressées à l’échelle nanométrique. Tandis que les dépôts de TiN sont colonnaires avec une texturation selon <111>, les grains des films de Ti sont plutôt équiaxes et orientés selon <0001>. Pour une température d’élaboration de 650 °C, les éléments du substrat diffusent dans les films et mènent à la formation de phases de Laves. Les dépôts de TiOx, élaborés en mode métallique, présentent des couleurs d’interférence et une composition proche du monoxyde. Les surfaces revêtues de TiN ont une résistance à la corrosion élevée semblable à l’acier inoxydable 316L, contrairement aux fils revêtus de Ti et TiOx. La ténacité et l’énergie d’adhérence des revêtements sont déterminées par traction in-situ sous MEB : Ti et TiN sont particulièrement adhérents au substrat contrairement à TiOx.Enfin, les dépôts sont architecturés avec l’ajout d’un dépôt de titane entre le substrat et le revêtement céramique. Ainsi, l’adhérence du film Ti-TiOx est largement augmentée par rapport au monocouche TiOx (5 à 200 J/m2). Enfin, les études microstructurales et électrochimiques montrent qu’un paramètre clef de la résistance à la corrosion est la présence de porosité ouverte dans les revêtements. / This thesis treats of the functionalization of stainless steel wires with colored Ti-based coatings, grown by PVD with a cylindrical magnetron, their chemical and morpho-structural characterization, and the evaluation of the chemico-mechanical durability of the coated wires.First, the deposition rate and the chemical composition of the films are determined under static conditions. Cathode plasma heterogeneities are demonstrated and related to parameters such as power, pressure and polarization of auxiliary anodes.Then, Ti, TiN and TiOx monolayer coatings are grown continuously. The relationship between the color of TiN and its chemical composition is established and golden color is obtained for stoichiometric films with low oxygen content (<5% at.). Microstructures are studied with TEM-ASTAR and orientation maps are obtained with a nanometric resolution. While TiN coatings are columnar with <111> texture, Ti grains are rather equiaxed and <0001> oriented. With a 650 ° C substrate temperature, substrate elements diffuse into the films which results in Laves phase formation. TiOx is grown in metallic mode, presents interference colors and a composition close to monoxide. TiN coated surfaces display high corrosion resistance similar to 316L stainless steel, unlike Ti and TiOx coated wires. The toughness and the adhesion energy of the coatings are determined by SEM in-situ tensile tests: Ti and TiN are particularly adherent to the substrate in contrast to TiOx.Finally, coatings are architectured with the addition of a titanium interlayer between the substrate and the ceramic coating. Thus, Ti-TiOx film adhesion is greatly superior compared to the TiOx monolayer (5 to 200 J/m2). Finally, microstructural and electrochemical studies show that a key parameter of corrosion resistance is the presence of open porosity in the coatings.