Durant ces dernières années la déformation plastique est de plus en plus souvent étudiée du point de vue de la dynamique des systèmes complexes, caractérisée par l'auto-organisation et impliquant plusieurs échelles. Le présent travail est une étude multi-échelles de la dynamique collective de dislocations dans un alliage AlMg sujetà l'effet Portevin-Le Chatelier (PLC). Pour atteindre cet objectif nous avons réalisé l'enregistrement simultané des courbes de traction, de l'émission acoustique (EA) et du champ de déformations locales, ainsi que la caractérisation quantitative de la complexité grâce aux analyses statistique et multifractale. Le travail de thèse a permis de montrer que la manifestation apparente des processus de déformation plastique dépend de l'échelle d'observation. L'analyse de l'EA a révélé un comportement intrinsèquement intermittent et invariant d'échelles dans toutes les conditions expérimentales. Ces résultats suggèrent qu'aux échelles associées à l'EA, la plasticité peut être gouvernée par une dynamique universelle, qu'elle soit liée à l'instabilité PLC ou à un écoulement macroscopiquement homogène. Malgré le caractère ubiquitaire de cette observation, le champ de déformations locales montre des processus de déformation ondulatoires, et l'analyse statistique des chutes de contrainte révèle des échelles caractéristiques. Une hypothèse de synchronisation des avalanches des dislocations est avancée afin d'expliquer dans certaines conditions expérimentales le passage de l'échelle mésoscopique à l'échelle macroscopique / In recent years, the plastic deformation more and more often studied in terms of the dynamics of complex systems, which is characterized by self-organization and involves various scales. This work presents a multi-scale investigation of the collective behavior of dislocations in an AlMg alloy prone to the Portevin-Le Chatelier (PLC) effect. To achieve this goal we have performed simultaneous recording of tensile curves, acoustic emission (AE), and local strain field, as well as quantitative characterization of the complexity through statistical and multifractal analyses. The results obtained proved that the apparent manifestations of the plastic deformation processes depend on the scale of observation. The analysis of the AE data revealed an inherently intermittent and scale-invariant behavior in all experimental conditions. These results suggest that at the scales pertaining to the AE, the plasticity may be governed by a universal dynamics, be it related to the PLC instability or macroscopically homogeneous flow. Despite the ubiquitous nature of this observation, the local strain field uncovers wave-like deformation processes, and the statistical analysis of stress serrations reveals characteristic scales. Synchronization of dislocation avalanches is conjectured to explain (under some experimental conditions) the transition from the mesoscopic to the macroscopic scale

No description available.

<p> The galvanomagnetic properties and shape of the Fermi surface of copper single crystals containing different density of dislocations have been studied experimentally and theoretically through magnetoresistivity measurement and using effective medium theory respectively. </p> <p> In experiments, two crystallographic orientations of copper single crystal samples with tensile axis parallel to < 100 > and < 541 > have been plastically strained to various stress levels to introduce different density Nd of dislocations. The experimental data of angle and field-dependent magnetoresistivity measured at temperature T = 2K and in the magnetic filed up to 9 Tesla show that dislocations influence substantially the galvanomagnetic properties of copper crystal samples in the open, extended and closed orbit crystallographic orientations. The results reveal that the pure samples with resistivity ratio RR equal to or larger than 151 show quadratic dependence of transverse magnetoresistivity as a function of the magnetic field in the open-orbit orientation, which changes to linear variation of magnetoresistivity with magnetic fields in highly deformed samples with RR smaller than 138. A quadratic dependence of transverse magnetoresistivity as a function of the magnetic field is significantly suppressed as the density of dislocations increases. The magnetoresistivity decrease with the increase of the density of dislocations was also observed in the closed-orbit crystallographic orientation. Such effect is independent upon the type of dislocations introduced to the crystal lattice.</p> <p> Measurements of the de Hass-van Alphen effect in plastically deformed copper single crystals have been carried out with torque magnetometer and AC susceptibility options of Quantum Design PPMS-9 system. The oscillation frequencies for the extremal orbits normal to the principal crystallographic directions are obtained through Fourier transform of torque versus inversed field characteristics. By comparing these frequencies with the analogous frequencies obtained for undeformed copper crystals, the changes in the cross-sectional area of the Fermi surface corresponding to the extremal orbits are obtained and the shape of the dislocation-distorted Fermi surface is postulated based on measurements performed.</p> <p> The effective-medium approximation and Green's function method are applied to model the magnetoresistivity data and to gain insight into the fundamental material properties responsible for the observed magnetoresistivity behavior. The effective magnetoresistivity calculated using a self-consistent method shows a good agreement with the experimental results.</p> / Thesis / Doctor of Philosophy (PhD)

No description available.

No description available.

Thesis. 1975. Ph.D. cn--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 180-186. / Ph.D.cn

Thesis. 1976. M.S. cn--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Bibliography: leaves 39-40. / by Brenda J. Blake. / M.S.cn

Recently it has been concluded that the SiO₄ silicate tetrahedra in crystals behave as rigid bodies. This conclusion is based on analyses of the atomic displacement factors of Si and O atoms obtained from single crystal diffraction experiments wherein the amplitudes of atomic vibrations are ascribed to translational, librational and screw-correlated modes of motion for the entire SiO₄ group. If the displacement ellipsoids are considered to represent time averaged quadratic surfaces of equal configurational potential energy about the mean position of an atom, then an analysis of the these displacements should provide detailed information about the SiO₄ group and the crystal. The apparent SiO bond lengths recorded for silicates over a range of temperatures are typically either invariant or exhibit a contraction with increasing temperature. A rigid-body thermal analysis was completed for the tetrahedra in nine silicates whose structures have been determined over a range of temperatures from 15 K to 1250 K and whose tetrahedra seem to behave as rigid units. The coordinates provided by the analysis yield bond lengths and polyhedral volumes corrected for the librational motion of each silicate tetrahedron. The bond lengths and volumes estimated for tetrahedra with four bridging oxygens seem to increase with temperature at a faster rate than those with four nonbridging oxygen atoms. Those for tetrahedra with two or three nonbridging oxygen atoms tend to increase at an intermediate rate. An analysis of the rigid-body motion of coordinated polyhedra yields a simple but accurate expression for correcting bond lengths for thermal vibrations. Observed anisotropic displacement parameters for Si and O atoms indicate that the SiO₄ tetrahedra in quartz behave as rigid bodies. A configurational potential energy curve, constructed from the librational components of the rigid body motion of the tetrahedra, shows a double well for α quartz and a single well for β quartz when plotted as a function of the displacement of the O atom with temperature. The configurational energetics of α and β quartz are examined with a theoretical potential energy function based on parameters obtained from molecular orbital calculations. The calculations indicate that the temperature behavior of a quartz is governed by the energetics of the SiOSi angle, in contrast to β quartz which is governed by the energetics of the SiO bond. The mechanism of the α ⇌ β transition is examined in terms of the experimental and modeled configurational potential energy curves. Evidence for the proposal that π bonding is the driving mechanism for the transition is lacking. Structural and volume compressibility data for α-cristobalite were determined by single crystal X-ray diffraction methods for pressures up to ~1.6 GPa, where cristobalite undergoes a reversible phase transition. The bulk modulus was determined to be 11.5(7) GPa with a pressure derivative of 9(2). The SiOSi angle shows a greater decrease than observed for quartz and coesite while the SiO bond lengths and the OSiO angles remain essentially unchanged. The responses of V/V₀ and SiOSi angle to pressure for the silica polymorphs are compared and it is found that the percentage decrease in the volume is linearly correlated with the percentage decrease in the SiOSi angle, regardless of the framework structure type. A mathematical modeling of the energies of the structural changes that are induced by pressure suggests that the contribution to the total energy ascribed to Si0Si angle bending terms is the same in quartz and cristobalite. / Ph. D.

The lack of ductility of the B2 NiAl alloy stands in the way of promising applications. In an effort to understand the dislocation behavior, computer simulation of the planar faults involved in the core spreadings of <100> and <111> dislocations was carried out. Seven γ-surfaces were computed for different crystallographic planes ({110}, {112}, {123}, {210}, {100}, {111} and {122}). Stable APB's are observed in the {110} and {112} planes but they are deviated from the exact 1/2a<111> position. No other stable planar fault was observed. The dislocation core spreadings are explained by the energy balance among the directions of lowest restoring forces observed in the γ-surfaces. The complete <111> screw dislocation was stable in the simulation. According to the stable APB's, two dissociation reactions of the <111> screw dislocation in the {110} and {112} planes are proposed. The simulation of metastable superpartials shows that the dissociation in the {112} planes is very close to a stable dissociation. / Master of Science

Les travaux exposés dans cet ouvrage portent sur le comportement mécanique de l'oxyde de magnésium (MgO) sous nanoindentation. L'étude est plus particulièrement focalisée sur les mécanismes élémentaires de déformation précédant et accompagnant le pphénomène de pop-in. L'observation de la surface autour des zones indentées, et en particulier des lignes de glissement, a été réalisée par microscopie à force atomique (AFM). La structure en volume des dislocations nucléées lors de l'essai de nanoindentation a été explorée par une technique de tomographie couplant polissage mécano-chimique (CMP) et nano-attaque chimique. Ces études expérimentales ont été confrontées à des simulations numériques menées à l'aide d'un code de dynamique de dislocations discrètes (DDD) couplé à un code éléments finis. Une étude bibliographique présentant en particulier les mécanismes de plasticité associés au phénomène de pop-in est tout d'abord proposée. Des expériences menées sur des échantillons présentant une très faible concentration de défauts sont ensuite présentées. Les résultats expérimentaux montrent que contrairement à l'idée communément admise, la déformation précédant le pop-in n'est pas nécessairement purement élastique. Par la suite, l'introduction contrôlée de défauts (défauts ponctuels, dislocations) dans le matériau a permis d'étudier leur influence sur les mécanismes initiaux de plasticité. Enfin, les simulations numériques ont permis d'accéder au comportement des dislocations individuelles dans le champ de contraintes complexe généré par l'indenteur tout au long d'un cycle d'indentation. Ces résultats apportent un éclairage sur les comportements observés expérimentalement.