Scatter observed in the fatigue test results of a cast nickel-based superalloy may arise from its coarse grain microstructure. With only a few grains through the sample cross-section, it has been postulated that the inherent anisotropy of individual grains results in the different surface strain distributions during testing. Crystal plasticity finite element modelling (CPFEM) was used to model the deformation of a fat test piece containing a few grains in the cross-section. The mesh was generated using EBSD maps from the surfaces of samples that were subjected to monotonic and cyclic loading at two different temperatures. Digital image correlation (DIC) was used to study the local strain the same sur- faces. Heterogeneous strain distribution, that could be responsible for scatter in the fatigue test results, was observed both in the model and experimentally. However, they were quantitatively different. These differences are attributed to the simplistic microstructural representation in the model and its inability to accurately represent intergranular deformation. The inherent anisotropy within grains resulted in different surface strain distributions during cyclic loading and it was observed that the fatigue life of the test specimens could be correlated to the maximum plastic strain in the sample at the end of the first cycle. As the CPFE model captured the maximum strain measured experimentally, the maximum strain at the end of the first cycle was determined as a fatigue indicator parameter (FIP) for the number of cycles to failure. Randomly generated synthetic microstructures were then loaded in tension and it was observed that when using local strain as a FIP, the scatter in orientations of individual grains resulted in scatter in the expected fatigue life.

Historically, the ability of crystal plasticity to incorporate the Schmid’s law at each integration point has been a powerful tool to simulate and predict the slip behavior at the grain level and the succeeding heterogeneous stress/strain localization and texture evolution at the macroscopic level. Unfortunately, this remarkable capability has not been replicated for materials where twinning becomes a noticeable deformation mechanism, namely in the case of low-stacking fault energy cubic, orthorhombic, and hexagonal close packed structures. This dissertation is an attempt to gain understanding on the heterogeneous deformation due to twinning through various techniques including micromechanics, discrete dislocation dipole loops, and digital image correlation (DIC) analyses, and then bring the collected small scale information up to the fullield crystal plasticity scale using fast Fourier Trans- forms. Results indicate that the twin spacing depends primarily upon the height of the twin, and the stress relaxation from the twinning depends upon the thickness of the twin. Furthermore, in a homogenous stress state, discrete dislocation dipole loop-based twinning model showed that the lenticular shape has the minimum stable energy rather than the lamellar or ellipsoidal twin morphology. Our study on the evolution of twinning under three-point bending condition in strongly basal textured magnesium alloy allowed us to build a strategy to incorporate characteristic twin spacing parameter in the crystal plasticity framework. Inspired by results from molecular dynamics (MD) simulations stressing the effect of shuffles on twin nucleation and disconnection core width, we developed an explicit twinning nucleation criterion based on hydrostatic stress gradient and volume fraction of twin inside a grain. Characteristic twin spacing parameter is used as a function of twin height to determine site specific nucleation points in case of multiple twinnings. This ap- proach offered a good reproduction of the microstructure evolution as affected by twinning in a tri-crystal system.

La compréhension des mécanismes de déformation dans les matériaux polycristallins est un problème important, qui conditionne notre capacité à concevoir et à produire des pièces de structure plus sures et avec un impact environnemental moindre. Cette compréhension est aujourd'hui limitée par notre capacité à observer à la fois la microstructure du matériau et ses mécanismes de déformation en trois dimensions (3D) aux petites échelles, et à informer les simulations mécaniques à partir des mécanismes physique de déformations du réseau cristallin. Des progrès considérables ont été faits dans les dernières décennies avec les observations de surfaces (i.e. technique EBSD associée a de la corrélation d’image) qui a permis de nombreuses études combinant des observations expérimentales à des simulations, à partir de la surface de la microstructure. Cependant, une comparaison précise sans connaitre la microstructure sous-jacente reste un défi. Dans ce travail, nous proposons une nouvelle méthodologie basée d'une part sur des mesures couplant la tomographie et la diffraction des rayons X, et d'autre part sur des simulations mécaniques de platicité cristalline. Cette approche permet une comparaison quantitative en volume entre les mécanismes de déformation, l’évolution de la courbure du réseau cristallin et les champs mécaniques simulés.Pour ce faire, une machine de traction dédiée aux expériences 4D d’imagerie par diffraction sur grands instruments a été conçue, et utilisée pour déformer en tension un échantillon d’Aluminium Lithium. La cartographie 3D de la microstructure a été obtenue par tomographie par contraste de diffraction, et un agrégat de trois grains dans le volume de l’échantillon a été choisi comme région d’intérêt pour des observation 4D par topotomographie. L’apparition des premières bandes de glissement en volume et leur évolution au cours du chargement ont été observées le long de plans cristallographiques bien définis. Les trois grains ont montré une activité plastique le long de deux familles de plans différents, pas toujours en accord avec une analyse macroscopique du facteur de Schmid, ce qui est attribué à l'influence du voisinage sur l'activation des systèmes de glissement. Les changements d’amplitude et d’orientation de la courbure moyenne des grains ont été mesures avec un niveau de détail sans précédent, par une analyse tridimensionnelle des courbes de reflexions.En parallèle, des simulations de la plasticité cristalline par éléments finis (CPFE) ont été menées utilisant la cartographie tridimensionnelle de la microstructure mesurée expérimentalement. Un chargement uniaxial de traction a été applique pour reproduire numériquement l’expérience, et comparer grain par grain l’activité plastique. L’activité des systèmes de glissement prédite par le modèle est conforme aux observations expérimentales d’une activité plastique le long de deux plans. Un cadre mathématique pour prédire l’angle de Bragg local en fonction des déformations et des rotations du réseau cristallin a été formulé. Un post-traitement des champs intragranulaires de déformation à partir des résultats des simulations CPFE a montré une excellente concordance avec les résultats expérimentaux. Ce résultat confirme que la topotomographie in-situ aux rayons X est un outil prometteur pour l’étude des premiers stades de la plasticité cristalline en volume. / Understanding the intimate details of plastic deformation in polycrystalline materials is an important issue to improve material design and ultimately produce safer structural parts with less impact on the environment. This understanding is presently limited by our ability to observe both the microstructure of the material and the deformation processes in three dimensions (3D) at small length scales and inform mechanical simulations with physical deformation mechanisms of the crystal lattice. Considerable progress has been made in the last decade with surface observation (eg EBSD coupled to digital image correlation) which led to numerous studies combining experimental observations and simulations from the surface microstructure. However, an accurate comparison without knowing the underlying microstructure remain challenging. In this work, we propose a new methodology which allows a quantitative comparison between the observation of deformation mechanisms, the evolution of the grain lattice curvature and the simulated mechanical fields.For that purpose, a mechanical stress rig dedicated for synchrotron 4D diffraction imaging experiments has been designed, and used to deform an Aluminium-Lithium specimen under tension. The 3D grain map has been obtained by diffraction contrast tomography analysis, and a cluster of three grains within the bulk has been selected to be the region of interest of the 4D observation by X-ray topotomography. The appearance and evolution of 3D crystalline defects as a function of the applied load has been observed to be located along well defined crystallographic planes. All three grains showed plastic activity on along two different set of planes, which is not always coherent with a macroscopic Schmid Factor analysis. The change of the amplitude and the orientation of the average grain curvature has been measured with an unprecedented level of detail by means of 3D rocking curve analysis.In parallel, crystal plasticity finite element (CPFE) simulations have been carried using the 3D grain map measured experimentally. Tension loading was applied to reproduce the experiment numerically and compare the plastic activity on a grain by grain basis. The slip system activity predicted by the model matches in most cases the observed two slip system scenario. A mathematical framework to predict the local Bragg angle based on the stretch and rotation of the crystal lattice by the elastic strain tensor was derived. Post-processing the intragranular strains fields from the CPFE results allowed to simulate 3D rocking curves, showing excellent agreement with the experimental measurements. This result confirms that in situ X-ray topotomography is a promising tool to study the early stage of polycrystal plasticity within the bulk of millimetric material specimens.

Many metals used in modern engineering exhibit anisotropy. A common assumption when modelling anisotropic metals is that the level of anisotropy is fixed throughout the calculation. As it is well understood that processes such as cold rolling, forging or shock loading change the level of anisotropy, it is clear that this assumption is not accurate when dealing with large deformations. The aim of this project was to develop a tool capable to predict large deformations of a single crystal or crystalline aggregate of a metal of interest and able to trace an evolution of anisotropy within the material. The outcome of this project is a verified computational tool capable of predicting large deformations in metals. This computational tool is built on the Crystal Plasticity Finite Element Method (CPFEM). The CPFEM in this project is an implementation of an existing constitutive model, based on the crystal plasticity theory (the single crystal strength model), into the framework of the FEA software DYNA3D® . Accuracy of the new tool was validated for a large deformation of a single crystal of an annealed OFHC copper at room temperature. The implementation was also tested for a large deformation of a polycrystalline aggregate comprised of 512 crystals of an annealed anisotropic OFHC copper in a uniaxial compression and tension test. Here sufficient agreement with the experimental data was not achieved and further investigation was proposed in order to find out the cause of the discrepancy. Moreover, the behaviour of anisotropic metals during a large deformation was modelled and it was demonstrated that this tool is able to trace the evolution of anisotropy. The main benefit of having this computational tool lies in virtual material testing. This testing has the advantage over experiments in time and cost expenses. This tool and its future improvements, which were proposed, will allow studying evolution of anisotropy in FCC and BCC materials during dynamic finite deformations, which can lead to current material models improvement.

Zirconium is used inside nuclear reactors as fuel cladding. The in-reactor performance of zirconium alloys is strongly influenced by the properties that develop during thermo-mechanical processing, such as the microstructure and crystallographic texture. Optimising the combination of properties would enable improved reactor efficiency, longer component lifetimes and reductions in nuclear waste. Achieving the desired texture and microstructure requires a mechanistic understanding of the processes that govern them: deformation and recrystallisation. These mechanisms are influenced by numerous variables including temperature, strain-rate, and the initial state of the material. This work aims to clarify how texture develops as a result of the active deformation mechanisms of slip and twinning and how these mechanisms are influenced by temperature. The alloy chosen for this is Zircaloy-4.This work has shown that texture evolution varies with deformation temperature. The activation of {10-12}<10-11> tensile twinning dramatically alters the texture up to at least 300°C. In the absence of much twinning at 500°C prismatic slip appears to govern the texture evolution up to moderately high strain. Prismatic slip is generally considered the easiest slip system in zirconium. This work highlights its distinct effect upon both texture and microstructure evolution. In particular the extent of grain fragmentation by prismatic slip is shown to depend upon the initial grain orientation. As a result the break-up of the microstructure takes place heterogeneously. This then has implications for the microstructure and texture development during subsequent recrystallisation treatments. Experimental data indicates that the slip anisotropy between <c+a> and prismatic <a> slip increases with temperature. Crystal Plasticity simulations suggest that the variation of both the twin variant selection and the grain fragmentation with temperature are consistent with increasing slip anisotropy, in contrast to previous experimental and modelling studies on high purity zirconium alloys. The character of {10-12}<10-11> tensile twins and the texture change they induce is influenced by temperature, strain path and weakly influenced by the neighbouring orientations. Increasing temperature causes twin fraction variation, thicker twins and an increased frequency of less favourable twin variants. Plane strain compression also causes less favourable variants to activate more often. Looking at the twinned orientations highlights the importance of grain orientation. Poorly orientated grains do still twin. This work shows that in these instances neighbouring interactions can play a role. In summary, this work contributes to the current understanding of deformation in hexagonal close packed metals. It is hoped that this aids the development of improved physically based crystal plasticity models.

No description available.

The design techniques of the components on the macro level are established in the scientific community, however are far behind from the real material performance limits. To obtain those limits, the deeper understanding of the material structure is required. The methods of a new comonents production through standard alloying are the basis of the modern material science manufacturing. The design of the materials with expected required performance limits is the next conceptual step for the materials scientist. As results, to make this step, the problem of a precise material structure analyses on the microstructural level is one os the major importance for the next generation materials design. The complexity of the material structure across the scales(macro-micro) requires a new non deterministic methods for better understanding of the connectivity betwen a materials performance and material microstructure features. This work presents a various new research methodologies and techniques of the material microstructure characterization and numerical design with future applications to the anlyses of the material behavior. The focus of the particular research was to analyse a new cross correlation function of the material structure on the micro length scale and develop a novel framework which allows a better understanding of various important material phenomenas such as failure initiation and recrystallization.

No description available.