Understanding the plastic deformation mechanisms in polycrystals is a long-standing fundamental problem and its improvement has significant potential impact on the increase in materials resistance to typical failure modes such as fatigue cracking and stress corrosion cracking and hence the increase in the materials strength. However many deformation models are yet to be validated as quantitative experimental results at mesoscale to correlate dislocations and microstructure features are limited. This thesis furthers the High Resolution EBSD (HR-EBSD) technique in Geometrically Necessary Dislocation (GND) density measurement from qualitative analysis with a typical map size of 100 μm x100 μm to quantitative analysis with a map of 500 μm x500 μm by determining the optimised scanning step size (0.5 μm) and detector binning level (4x4 binning). This allows a statistically large number of grains to be sampled. Combining with obtained crystallographical information from a conventional EBSD system, systematic studies on GNDs behaviours with respect to a range of microstructure features such as grain boundaries and triple junctions were conducted on monotonically deformed polycrystal copper samples under tension. Relatively high GND density points were found near triple junctions and some grain boundaries whereas the low GND density points tend to appear near the grains’ interiors. These tendencies are particularly profound in low and moderately deformed samples. Hence more detailed analyses were performed to investigate the relations of GND density and the properties of grain boundaries and triple junctions. These quantitative analyses were complemented with direct visual assessment. The visual inspection provides interesting findings such as the strong GND structure dependence on grain orientations and GND structure development through increasing deformation; grain-grain interaction influences on GND structure development and GND structures near triple junctions. These GND density studies provide experimental results to validate some of the existing plastic deformation models for instance Ashby’s model of hardening and Hall-Petch relation. However, some of the new observations on GND structures at mesoscale cannot be fully rationalised by existing proposed mechanisms. Hence new models have been proposed that these GND structures might be generated from the intersections of different slip systems which occurred in various parts of a grain, or by the dislocation piling-up at some microstructural features e.g. triple junctions and twin boundaries.

Σ3{111} coherent twin boundary (CTB) in face-centered-cubic (FCC) metals and alloys have been regarded as an efficient way to simultaneously increase strength and ductility at the nanoscale. Extensive study of dislocation-CTB interaction has been carried out by a combination of computer simulations, experiments and continuum theory. Most of them, however, are based on the perfect CTB assumption. A recent study [Wang YM, Sansoz F, LaGrange T, et al. Defective twin boundaries in nanotwinned metals. Nat Mater. 2013;12(8):697-702.] has revealed the existence of intrinsic kink-like defects in CTBs of nanotwinned copper through nanodiffraction mapping technique, and has confirmed the effect of these defects on deformation mechanisms and mechanical behavior. One of the deformation mechanisms proposed therein, i.e. general hard dislocation slip intersecting with kink line is studied here in detail by molecular dynamics (MD) simulation. Simulations are performed using copper bicrystal models with a particular focus on the interaction of a screw dislocation with 0 degree and 60 degree kinked CTBs. It is found that kink-like defects have a profound impact on screw dislocation - CTB interaction mechanisms, resulting in significant strengthening or softening effects.

In the presented thesis we investigate heteroepitaxial growth of one element (one type of adsorbate A with negative or positive misfit relative to substrate S) and static properties of two-component surface alloy, i.e. ternary system (two types of adsorbate A and B with negative and positive misfit relative to different substrat S). We use Monte Carlo simulations for an off-lattice model in (1+1) dimensions with Lennard-Jones interaction. In case of incoherent heteroepitaxial growth we investigate formation of misfit dislocations, their influence on structure of growing film and impurity-induced formation of dislocations. Two different types of formation of dislocations are found, depending on the sign and magnitude of misfit. Simulations of static properties of two-component surface alloys show that morphology is quite different for phase separation (formation of domains consist of one type of particles in direction along and vertically towards to substrate-adsorbate interface) and intermixing regime. The structures, which emerged, depend on relative misfit, interaction and concentration of individual elements.

Dans la première partie, on s'intéresse à la multi-diffusion d'une onde acoustique avec une matrice homogène 2D contenant N inclusions. Dans le cas particulier de deux inclusions, on met alors en évidence l'importance du contraste matrice/inclusion dans les termes d'interactions entre inclusions. Le cas général de la multi-diffusion, pour distribution aléatoire de N inclusions, est ensuite développé dans l'esprit de Foldy-Lax basé sur des moyennes d'ensembles. Ainsi on cherche à déterminer le nombre d'onde effectif de l'onde effective, définie comme la moyenne du champ total, dans le cas d'une onde incidente émise par un point source. La deuxième partie est consacrée au cloaking actif dans une plaque. On détermine ainsi les amplitudes modales des sources multipolaires afin d'éteindre une onde plane ou émise par un point source, dans une région donnée. En outre, cette méthode peut s'appliquer pour éteindre l'onde diffractée par un défaut. Enfin dans la dernière partie, on se propose d'étudier la propagation d'onde au sein d'un milieu comportant des dislocations. On utilise la géométrie de Riemann-Cartan afin de modéliser ce milieu continu. Afin d'illustrer les différences que peuvent induire deux définitions possibles de la déformation (spatiale et matérielle), nous étudions la propagation d'ondes 3D dans l'exemple simple d'un milieu continu avec une densité uniforme et stationnaire de défauts. L'anisotropie et l'atténuation sont présentes dans les deux modèles mais sous forme différente. Enfin la déformation matérielle induit des modes de respiration et, en régime haute fréquence, des ondes transverses qui suivent l'escalier en spirale de Cartan. / In the first part, we investigate the multiple scattering of an acoustic wave within an homogeneous matrix containing N obstacles. In the particular case with 2 obstacles, we show the importance of the contrast matrix /obstacle in the coupling terms between inclusions. The general case of multiple scattering by N obstacles randomly distributed is then developed following the Foldy-Lax theory based on ensemble averaging. We aim to evaluate the effective wavenumber of the effective wave, defined as the average of the total field, in the case where the incoming wave is emitted by a point-like source. The second part is dedicated to the active cloaking in a thin plate. Hence we determine the modal amplitudes of the sources in order to extinct an incoming wave in a given region. This method can be applied to extinct the wave scattered by an obstacle. Finally, in the last part, the Riemann-Cartan geometry is used to model continuum with dislocations. In order to illustrate the differences induced by two possible definitions for the strain (spatial or material) in this framework, propagation of 3D waves is studied for a simple example of infinite continuum with uniform and stationary defects density. Anisotropy and attenuation are caught by both models even if these effects are quite different. Furthermore the material strain uniform breathing modes and, in the high frequency regime, transverse waves which follow the Cartan's spiral staircase.

Silicon carbide as a representative wide band-gap semiconductor has recently received wide attention due to its excellent physical, thermal and especially electrical properties. It becomes a promising material for electronic and optoelectronic device under high-temperature, high-power and high-frequency and intense radiation conditions. During the Silicon Carbide crystal grown by the physical vapor transport process, the temperature gradients induce thermal stresses which is a major cause of the dislocations multiplication. Although large dimension crystal with low dislocation density is required for satisfying the fast development of electronic and optoelectronic device, high dislocation densities always appear in large dimension crystal. Therefore, reducing dislocation density is one of the primary tasks of process optimization. This dissertation aims at developing a transient finite element model based on the Alexander-Haasen model for computing the dislocation densities in a crystal during its growing process. Different key growth parameters such as temperature gradient, crystal size will be used to investigate their influence on dislocation multiplications. The acceptable and optimal crystal diameter and temperature gradient to produce the lowest dislocation density in SiC crystal can be obtained through a thorough numerical investigation using this developed finite element model. The results reveal that the dislocation density multiplication in SiC crystal are easily affected by the crystal diameter and the temperature gradient. Generally, during the iterative calculation for SiC growth, the dislocation density multiples very rapidly in the early growth phase and then turns to a relatively slow multiplication or no multiplication at all. The results also show that larger size and higher temperature gradient causes the dislocation density enters rapid multiplication phase sooner and the final dislocation density in the crystal is higher. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2015. / FAU Electronic Theses and Dissertations Collection

The present PhD project was carried out as part of an EPSRC Leadership Fellowship for the study of irradiation damage in zirconium alloys. The National Nuclear Laboratory (NNL) directly supported the project in terms of additional funding and insightful discussions regarding irradiation damage in zirconium alloys. The research carried out within the project aims to gain a better understanding of both a- and c-loops, formed during irradiation damage in zirconium alloys. A range of techniques have been utilised to assess the morphology and density of the dislocation loops after proton-irradiations. These techniques include transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and line profile analysis (LPA) using synchrotron X-ray diffraction (SXRD) profiles and analysing the data utilizing the extended convolutional multiple whole profile (CMWP) analysis software. The effect of experimental conditions on dislocation loop formation and stability of a-loops during post-irradiation annealing have also been investigated. Proton-irradiations were carried out on the commercial alloys Zircaloy-2, Optimized ZIRLOTM and also on binary Zr-0.1Fe and Zr-0.6Fe alloys. A mechanism has been proposed as to the effect of Fe redistribution on dislocation loop formation. By comparing proton-irradiated Zr-0.1Fe and Zircaloy-2 alloys it was possible to investigate the effect of increased amount of Fe redistribution, which occurs from secondary phase particle (SSP) dissolution, on the microstructural features that develop during irradiation. Zircaloy-2 has a higher density of SPPs and these are more homogenously distributed throughout the matrix in comparison to the Zr3Fe SPPs found in the Zr-0.1Fe alloy. It was found that Fe redistribution facilitates the formation of Fe-rich nano-precipitation. Bright-field STEM imaging has been used to image a- and c-loops and it was found that Zircaloy-2 had a lower dislocation line density compared to Zr-0.1Fe for both types of loops at similar damage levels. Therefore it has been proposed that Fe redistributed from SPPs precipitates in the matrix and the subsequent irradiation-induced precipitates act as annihilation sites for point defects; therefore preventing the formation of new dislocation loops and the growth of existing loops. In order to assess the effect of proton-irradiation temperature on a-loops, Zircaloy-2 and Optimized ZIRLOTM were proton irradiated to 2.3 dpa at 280°C, 350°C and 450°C. It was found that the a-loop density dropped in both alloys as irradiation temperature was increased and the a-loop diameter decreased. The changes in the density and size were more dramatic in Zircaloy-2 and this was explained by the presence of fine irradiation induced clustering of Nb seen in Optimized ZIRLOTM. These trends were calculated from both STEM imaging and CMWP, highlighting the suitability of using CMWP to investigate irradiation-induced dislocations. Finally the stability of the a-loops in proton-irradiated Zr-Fe binary alloys were investigated using novel in-situ SXRD and TEM annealing experiments. From CMWP analysis of the profiles generated during the in-situ annealing of a Zr-0.6Fe 3 dpa sample it was shown that the majority of the annealing takes place between 300°C-400°C. This was highlighted by a period of no change in the dislocation density up to 300°C, after which the density drops dramatically. In-situ annealing of a 1.5 dpa Zr-0.1Fe sample in the TEM allowed for the observation of a-loop gliding along prismatic planes enabling the annealing process taking place between 280°C-450°C, i.e. a similar temperature range at which SXRD analysis indicates the greatest level of annealing.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Bibligraphy: leaves 222-228. / by Paul Joseph Lemaire. / Ph.D.

Thesis. 1978. Sc.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / Sc.D.

Cette étude se doit d’analyser ce que les textes de la Collection hippocratique, ainsi que l’apport des commentateurs d’Hippocrate, tels Celse, Galien, et jusqu’aux chirurgiens actuels permettent de connaître de l’art chirurgical des Vème et IVème siècles avant J.-C., en Grèce. Les textes dits « chirurgicaux » sont ceux qui mentionnent un acte technique sur le corps humain, le plus souvent à l’aide d’un instrument « chirurgical ». Ils nous donnent des indications sur les pathologies traitées. Quelques documents iconographiques et quelques rares vestiges sont un apport intéressant. En confrontant les textes hippocratiques et la technique chirurgicale actuelle, nous verrons dans quelle mesure il est possible de reconstituer les conditions de réalisation de l’acte chirurgical par les praticiens hippocratiques. / The present study focuses on surgery procedures in fifth and fourth century B.-C. Greece. The analysis of these procedures is based on information stemming from the text constituting the Hippocratic Corpus as well as from Hippocrates’ commentators such as Celsus and Galen- and modern surgeons. The texts analyzed particularly those named “surgical “are those describing a manual intervention on the human body, most frequently supported by an instrument considered as “surgical.” They offer indications on the pathologies treated. Certain iconographic documents are of particular interest. The confrontation between Hippocratic texts and modern surgical techniques will enable to estimate the possibility of restitution of the surgical procedures’ conditions by the Hippocratic surgeons

Les aciers austénitiques inoxydables sont des candidats potentiels pour des composants de circuits des réacteurs de génération IV. Ces composants sont conçus pour fonctionner à hautes températures (500-600°C) et seront soumis à des sollicitations cycliques incluant de longs maintiens (~1mois) induisant une relaxation due aux phénomènes de viscoplasticité et de diffusion de lacunes. Ces temps de maintien sont inaccessibles en laboratoire d'où l'intérêt de la modélisation. L'objectif de cette étude a été de proposer des modèles de comportement capables de reproduire les différents mécanismes physiques observés expérimentalement. Dans un premier temps, une étude expérimentale a été menée sur l'acier 316L(N) incluant des essais de fatigue et de fatigue-relaxation à 500°C. Des essais de traction à différentes vitesses de déformation ont également été réalisés afin d'étudier le phénomène du vieillissement dynamique. La démarche de modélisation a été progressive. Nous nous sommes d'abord intéressés à la modélisation du comportement mécanique en fatigue pure et à température ambiante, pour différents matériaux métalliques de structure cubique à faces centrées dont l'acier 316L(N), en adoptant l'homogénéisation à champs moyens validée grâce à des calculs par éléments finis multicristallins. Ensuite, un modèle cristallin basé sur les densités de dislocations a été proposé et identifié pour des chargements de traction simple. Le modèle a ensuite été enrichi afin de prendre en compte les mécanismes de viscoplasticité, de montée et le vieillissement dynamique. le modèle fait appel à trois paramètres ajustables seulement et prédit correctement les courbes de traction et de relaxation. / Austenitic stainless steels are potential candidates for structural components of sodium-cooled fast neutron reactors. Many of these components will be subjected to cyclic loadings including long hold times (~ 1month) under creep or relaxation at high temperature. These hold times are unattainable experimentally. The aim of the present study is to propose mechanical models which take into account the involved mechanisms and their interactions during such complex loadings. First, an experimental study of the pure fatigue and fatigue-relaxation behavior of 316L(N) at 500°C has been carried out with very long hold times (10h and 50h) compared with the ones studied in literature. Tensile tests at 600°C with different applied strain rates have been undertaken in order to study the dynamic strain ageing phenomenon. Before focusing on more complex loadings, the mean field homogenization approach has been used to predict the mechanical behavior of different FCC metals and alloys under low cycle fatigue at room temperature. Both Hill-Hutchinson and Kröner models have been used. Next, a physically-based model based on dislocation densities has been developed and its parameters measured. The model allows predictions in a qualitative agreement with experimental data for tensile loadings. Finally, this model has been enriched to take into account viscoplasticity, dislocation climb and interaction between dislocations and solute atoms, which are influent during creep-fatigue or fatigue relaxation at high temperature. The proposed model uses three adjustable parameters only and allows rather accurate prediction of the behavior of 316L(N) steel under tensile loading and relaxation.