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Atomic simulation of the a/2 <110> (110) edge dislocation in the NaCl lattice /Hoagland, Richard Gordon January 1973 (has links)
No description available.
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Computer simulations of elastically strained surfaces and grain boundaries in bcc crystals /Price, Clifford Warren January 1975 (has links)
No description available.
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Texture development in polycrystalline copper during torsional deformationCanova, Gilles R. January 1982 (has links)
No description available.
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Analyse des mécanismes de glissement des dislocations dans l'UO2 à l'aide de la modélisation multi-échelles comparée à l'expérience / Analysis of dislocation gliding mechanisms in UO2 thanks to multi-scale modelling compared to the experiencePortelette, Luc 10 October 2018 (has links)
Dans l'étude des éléments combustibles des réacteurs à eau pressurisée, cette thèse s'inscrit dans la compréhension et la modélisation du comportement viscoplastique du dioxyde d'uranium (UO2) à l'échelle du polycristal. Lors de fonctionnement de type incidentel du réacteur, le combustible subit une forte élévation de la température avec un gradient thermique de la pastille engendrant des déformations viscoplastiques contrôlées par des mouvements de dislocations. D'abord, un modèle de plasticité cristalline a été développé de manière à décrire l’anisotropie viscoplastique du matériau en fonction de la température et de la vitesse de sollicitation. Des simulations par éléments finis (EF) sur monocristaux ont permis d’identifier que les trois modes de glissement généralement observés dans l'UO2 sont importants pour décrire le comportement anisotrope du matériau. Dans un second temps, les coefficients de la matrice d'interactions entre dislocations ont été déterminés spécifiquement pour l’UO2 afin d’améliorer la modélisation des polycristaux. En effet, en calculant par EF les dislocations géométriquement nécessaires, qui sont responsables d’une forte augmentation de la densité de dislocations stockées dans les polycristaux, les interactions entre dislocations permettent de simuler l’effet dé taille de grain et l’écrouissage des pastilles. Finalement, le modèle, adapté pour les polycristaux, a été validé par comparaison avec les essais expérimentaux sur pastille et par comparaison du comportement intra-granulaire simulé avec des mesures EBSD. Grâce à cette dernière comparaison, il est possible de remonter indirectement aux hétérogénéités de déformation dans les grains / This thesis is part of the study of fuel elements of pressurized water reactors and, more specifically, focus on the understanding and modelling of the viscoplastic behavior of uranium dioxide (UO$_2$) at polycrystalline scale. During the incidental operation of the reactor, the fuel undergoes a strong increase of temperature and thermal gradient between the center and the periphery of the pellet leading to viscoplastic strains due to dislocation movement mechanisms. First, a crystal plasticity model was developed in order to describe the viscoplastic anisotropy of the material considering the temperature and the loading rate. Finite element (FE) simulations on single crystals enabled to highlight that the three slip modes generally observed in UO$_2$ are crucial to describe the anisotropic behavior of the material. Secondly, coefficients of the interaction matrix have been identified specifically for UO$_2$ in order to improve the polycrystal modelling. Indeed, by calculating geometrically necessary dislocations (GNDs), which are responsible of the great increase of the stored dislocation density in polycrystals, the interactions between dislocations enable to simulate de grain size sensitivity and hardening of the fuel pellet. Finally, the model adapted for polycrystals, have been validated by comparing FE simulations with pellet compression tests and by comparing the simulated intra-granular behavior with EBSD measurements. Thanks to the latter comparison, it is possible to indirectly compare the strain heterogeneities in the grains
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COMPARATIVE STUDY OF DISLOCATION BEHAVIOR IN SINGLE-CRYSTAL AND RIBBON-TO-RIBBON SILICON.Pinamaneni, Subba Rao. January 1983 (has links)
No description available.
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Properties of low dislocation density metal crystalsBuckley-Golder, I. M. January 1977 (has links)
This thesis describes the growth, X-ray diffraction assessment and tensile deformation properties of dislocation-free copper single crystals. As such it has been possible to conveniently section the work carried out into these three main areas within this thesis. Consequently, each chapter may be read almost independently of the others with references and further work suggestions being incorporated at the end of each chapter. This format, it is felt, does not disjoint the work: rather, it enables the central theme (i.e. the title of this thesis) to be developed in a much more continuously clear way than is normally apparent in a thesis where conclusions, further work suggestions and references are not drawn together until the end of the volume. Chapter I opens with a brief outline of the crystal growth methods which could have been used to produce low dislocation density (< 10<sup>6</sup> cm/cm<sup>3</sup>) crystals by utilising the three fundamental phase transitions, i.e. solid to solid, vapour to solid and liquid to solid. The Czochralski method is then discussed in detail since it was used by the author to produce dislocation-free copper single crystals. The technological problems in obtaining such crystals are extensively enumerated and solutions presented. For instance, melt surface vibrations were eliminated by using a continuous flow of cooling water, by standing the complete crystal puller on a bed of foam, and by rigidly clamping the R.F. coil. It is emphasised that these technological problems must be solved before the scientific aspects of the growth of dislocation-free crystals can be studied. It is further shown that the author's modified crystal growing technique can be reliably used to grow dislocation-free crystals of copper each time, providing adequate care is taken over growth rates (1.2 cm/hr); specimen shape (a long thin "double-neck" must precede the required crystal to eliminate dislocations propagating from the seed and to act as a heat flow resistance); and crystal cooling rates (a long "tail" allowed the crystal to reach the ambient temperature slowly thereby minimising dislocation by thermal stresses and/or vacancy condensation). In the future, it is suggested, an automated crystal pulling system would be advantageous and a study of crystal growth in a synchrotron X-ray beam could be potentially definitive experiment on crystal growth. Chapter II looks at the theoretical aspects of Czochralski crystal growth with the aid of a new model which has been analysed on the Oxford University Computer. The model assumes a crystal-neck-seed configuration to be "sitting" on a liquid and examines the influence of geometric changes of the seed-neck-crystal and of radiation changes on the temperature gradients, primarily at the growth front. Four elements were chosen for study in this way: Si, Ge, Ag, Cu, i.e. four elements which have been grown dislocation-free. It was found that the seed size and shape played a small part in determining the interfacial temperature gradients, DT<sub>c</sub>(0), of all the elements. The neck, however, could have a marked influence on DT<sub>c</sub>(0) values in metals but not so much in semiconductor crystals. By geometric control alone it was found that the best way to reduce DT<sub>c</sub>(0) values was to grow a large diameter crystal. The influence of radiation losses was found to be marked for semiconductor crystals but not for metal crystals. Finally, it is concluded that to minimise DT<sub>c</sub>(0) values then the seed and neck must be long and thin and the crystal fat. These results fit in well with experimental knowledge. Further work to be carried out could consider the influence of a varying ambient temperature and convective heat losses on the interfacial temperature gradients. Chapter III is concerned with the interaction of X-rays with perfect crystals. The Lang-Borrmann X-ray topography technique is examined, and the experimental methods used to obtain X-ray topographs taken throughout this work are discussed. The major part of the chapter takes up the discussion of the theoretical interaction of X-rays with a perfect crystal set to diffract such X-rays. It is demonstrated that for a plane-wave incident on a cylindrical crystal, for the boundary condition to be satisfied the dispersion surface tie-points are displaced as the crystal traverses the incident X-ray beam. Thus the crystal wave-vectors no longer exactly satisfy the Bragg condition. This effect, was never unambiguously monitored experimentally because of the incident beam divergence. It is suggested that a future study could consider this problem in more detail. The Takagi-Taupin-Uragami generalised X-ray diffraction theory is reviewed and then used to calculate the intensity in the diffracted beam of a traverse topograph from a perfect cylindrical copper crystal. This computer simulation is also compared to an experimental condition. In both cases it is found that the Bragg surface of the crystal produces a very intense reflection whilst the remainder of the crystal gives a much reduced diffracted intensity in comparison. This asymmetric profile is interpreted in terms of absorption mechanisms which are strong at the centre of the crystal but not at the surface. The general agreement between theory and experiment is considered to be good although a few discrepancies arose, e.g. the lateral extent of the Bragg surface peak was found to be larger for the experiment than for the theory. Continued research in this area must explore further these small discrepancies; and it is suggested that a possible line of future study would be to examine theoretically and experimentally the X-ray diffraction from dislocation-free cylindrical crystals which possess low absorption coefficients (e.g. aluminium, silicon) for harder radiations (e.g. M<sub>oK<sub>1</sub></sub>, Ag<sub>K<sub>1</sub></sub>). It should then be possible to produce interference fringes and then it should be possible to examine the effects of a strain gradient on fringe spacing and visibility. Chapter IV sets out to discuss the tensile deformation behaviour of [1-2-3] growth axis, dislocation-free, chromium plated copper single crystals. This is done using the results from two sets of coupled experiments: an Instron deformation study and a synchrotron deformation investigation. It is first shown that chromium plating can destroy the perfection of the crystal unless care is exercised over plating temperatures and times. It was finally found that after plating for 10 seconds at 55C (1 μm of Cr deposited) that there was no indication of lattice dislocation. The stress-strain curve of a chromium plated dislocation-free copper crystal (Cr thickness 1 μm) is shown to exhibit a yield point commensurate with that for a non-plated, dislocation-free copper crystal, i.e. 90 g/mm<sup>2</sup>. The work hardening curve is shown to be comprised of three distinct regions: an initial rapid work hardening rate; a transitionary work hardening rate; and a stabilised work hardening rate up to a shear strain of 6%. In straining the crystal through these regions the work hardening rate progressively decreased. For a plated crystal which is dislocation-free initially it is shown that serrations in the flow curve occur, whilst for all other crystals these are shown to be absent. Such serrations are argued to occur by a source suppression and new source operation mechanism during the early deformation stages (1% shear strain), and then by a crack formation mechanism at the copper-chromium interface during the later stages of deformation. The initial work hardening rates (18 kg/mm<sup>2</sup>) were found to be independent of coating thickness (0.5 μm and 1 μm) but in the latter two regions the thicker coating imparted a higher work hardening rate to the crystal than the thinner layer, e.g. for a 0.5 μm chromium layer the work hardening rate was 1.65 kg/mm<sup>2</sup> in region iii and for a 1 μm chromium layer it was 2.97 kg/mm<sup>2</sup> in region iii. It is tentatively suggested that the chromium layer affected mobile dislocation motion rather than dislocation generation. The synchrotron work produced no evidence to support the argument that dislocation motion up the elastic line took place. Yielding was found to occur at a stress level similar to that measured in the Instron work. The complex stress system imposed by the deformation jig and the lack of resolution rendered it impossible to decide where the dislocation sources were located. Further increases in the load on the crystal produced double slip and this was argued to prematurely occur because of the combination of torsion, bending and tension which the crystal experienced. Certain interfacial dislocation activity was registered but this was not readily analysed in terms of surface sources. The yielding behaviour was inhomogeneous and appeared to be remote from the few slip bands induced by specimen transport. After yielding evidence was found for dislocation pile-up at the centre of the crystal. It is pointed out that future studies should use a better design of deformation jig so as to apply only a tensile stress to the crystal. Plating thickness and specimen size could be further explored.
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Modélisation multi-échelle de la déformation plastique de MgO monocristallin : du laboratoire au manteau terrestre / Multi-scale modeling of the plasticity of magnesium oxyde single crystal : from laboratory conditions to the Earth’s mantleAmodeo, Jonathan 15 December 2011 (has links)
Les évènements géologiques de surface, comme le volcanisme ou les séismes, sont le fruit d'une dynamique qui vise à dissiper la chaleur interne de notre planète. Dans le manteau terrestre, les roches sont déformées plastiquement dans des conditions extrêmes de pression, de température et de vitesse de déformation. Malgré les récentes avancées expérimentales, il est impossible de reproduire de telles conditions de déformation en laboratoire. C'est pourquoi nous proposons, dans ce travail de thèse, une approche numérique, basée sur la modélisation multi-échelle de la plasticité, des conditions du laboratoire à celles qui caractérisent le manteau terrestre. Nous avons choisi d'appliquer cette méthode à MgO, phase importante du manteau inférieur.À partir des propriétés de cœur des dislocations, nous avons utilisé la théorie des double-décrochements afin de décrire la mobilité d'une dislocation isolée en fonction de la température et de la contrainte. Nous avons ensuite implémenté, dans un code de Dynamique des Dislocations (DD), les paramètres de mobilité des différents défauts afin de décrire le comportement collectif des dislocations lors d’essais numériques de déformation. Les résultats montrent que les propriétés mécaniques de MgO dépendent fortement de la pression et de la vitesse de déformation. / Surface geological events, like volcanos and earthquakes, are due to the internal dynamics of the Earth which tends to release its internal heat. Inside the Earth's mantle, solid rocks are plastically strained under extreme conditions of pressure, temperature and strainrate. In spite of recent experimental progress, it is still impossible to reach such conditions of deformation. This is why we propose an alternative approach, based on the multi-scale modeling of plasticity, from the laboratory conditions to the Earth's mantle. We have choosen to apply our model to magnesium oxide which is a phase present in the lower mantle.From core properties, we modeled a dislocation thermally activated mobility law based on the kink pair theory. Then, we have incorporated it inside a Dislocation Dynamics code to describe the collective behaviour of dislocations throughout numerical strain experiments. Here we show that MgO mechanical properties depends significantly on pressure and strainrate.
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Modelling helium embrittlement in iron based metals under DEMO conditionsMenzies, Luke January 2018 (has links)
Steel components within fusion reactors will be subject to high transmutation rates due to high energy neutrons. In iron based alloys such as steels, high amounts of helium accumulate through transmutation. This leads to helium embrittlement through helium accumulating on the grain boundaries of metal. Worst case scenario predictions were made for DEMO, estimating that for a grain size of 5 micro-meters, embrittlement could happen within 2 years of the blanket region of DEMO. This thesis elaborates on previous worst case scenario calculations by including inter-granular tapping mechanisms, within rate theory simulations. A rate theory code was developed for the purpose of this work, tailored towards a fusion environment. Calculations were performed using rate theory that predicted the timescales in which helium embrittlement occurred within a conceptual DEMO design in the first wall region and the blanket region. The calculations used several parameter sets, where preliminary simulations were performed using the parameter sets, that were compared with cluster density data determined using Transmission Electron Microscopy (TEM) and Positron Annihilation Spectroscopy (PAS). The simulations showed that the helium embrittlement time was heavily influenced by the chosen dislocation density, parameter set and grain size. The simulations conducted to represent the blanket region, showed an increase as high as 94% from the 2 years that has previously been predicted under certain scenarios. However results also showed that assuming a certain parameter set with a low dislocation density, showed no significant increase in embrittlement time. This was not a concern since it was concluded that advanced steel concepts would be expected to have a small average grain size, that would dramatically increase the embrittlement time. The work in this thesis also focused on defect interaction with dislocations. A model was constructed that made use of elasticity theory and VASP calculations that produced the interaction energy map for various defects with an edge dislocation. The interaction energy map for helium interstitials with an edge dislocation was compared with molecular dynamics (MD) simulations produced for this work. The model and simulations showed good agreement. Temperature effects were then included in the model that allowed the concentration around a dislocation to be temperature dependent. These temperature dependent interaction energy maps were then implemented into the advection-diffusion equation, that were solved numerically to explore the capture efficiencies and bias towards certain defects within iron. These values were then used within the rate theory simulations to produce temperature effects on the dislocation sink strengths for vacancies, SIA and helium interstitials.
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Design, Processing and Characterization of Silicon Carbide DiodesZimmermann, Uwe January 2003 (has links)
Electronic power devices made of silicon carbide promisesuperior performance over today's silicon devices due toinherent material properties. As a result of the material'swide band gap of 3.2eV, high thermal conductivity, itsmechanical and chemical stability and a high critical electricfield, 4H-silicon carbide devices have the potential to be usedat elevated temperatures and in harsh environments. Shortercarrier lifetimes and a reduction in the necessary width of thelow-doped drift zone in silicon carbide devices compared totheir silicon counterparts result in faster switching speedsand lower switching losses and thus in much more efficientpower devices. High-voltage 4H-silicon carbide diodes have been fabricatedin a newly developed processing sequence, using standardsilicon process equipment. Epitaxial layers grown by chemicalvapor deposition (CVD) on commercial 4H-silicon carbidesubstrates were used as starting material for both mesa-etchedepitaxial and implanted p+n-n+ planar diodes, Schottky diodesand merged pn-Schottky (MPS) diodes, together with additionaltest structures. The device metallization was optimized to givea low contact resistivity on implanted and epitaxial layers anda sufficiently high Schottky barrier with a singlemetallization scheme. Different high-field termination designshave been tested and breakdown voltages of up to 4 kV onimplanted, field-ring terminated diodes were achieved,corresponding to 80% of the critical electric field. A 5kVepitaxial diode design with a forward voltage drop of 3.5V at acurrent density of 100Acm-2 equipped with an implanted junctiontermination extension (JTE) was also fabricated. A new measurement setup was designed and built with thecapability of measuring current-voltage and capacitance-voltagecharacteristics of semiconductor devices at reverse biases upto 10kV. Together with these electrical measurements, theresults of other characterization techniques were used toidentify performance limiting defects in the fabricated siliconcarbide diodes. Increased forward voltage drop of bipolardevices during on-state operation was studied and it was shownthat the stacking faults causing forward degradation arevisible in scanning electron microscopy. With the help ofsynchrotron white-beam X-ray diffraction topographs (SWBXT),electron beam induced current (EBIC) and electroluminescencemeasurements of silicon carbide diodes, the role of screwdislocations as a dominant source of device failure in the formof localized microplasma breakdown was identified. Screwdislocations with and without open core have been found tocause a 20-80% reduction in the critical electric field of4H-silicon carbide diodes, both for low-voltage (150V) andhigh-voltage (~5kV) designs. While micropipes have almost beeneliminated from commercial silicon carbide material,closed-core screw dislocations are still abundant withdensities in the order of 10000cm-2 in state-of-the-art siliconcarbide epitaxial layers.
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Atomistic Calculations of Nanoscale Interface Behavior in FCC MetalsSpearot, Douglas Edward 19 July 2005 (has links)
This dissertation focuses on the behavior of homogeneous FCC metallic interfaces on the nanoscale. Specifically, atomistic calculations (molecular statics and molecular dynamics) with embedded-atom method potentials are used to study the fundamental failure processes that occur at a bicrystal interface in Cu and Al as a result of a mechanical deformation. There are four primary objectives to this dissertation. First, molecular statics calculations are used to determine the most appropriate (minimum energy) structure of homogeneous bicrystal interfaces in Cu and Al. Interface structures and energies are reported in this work, with comparison to both theoretical and experimental characterizations of interface configuration. Second, molecular dynamics simulations are performed to provide a characterization of atomic scale inelastic behavior, including both dislocation and void nucleation activities which lead to interfacial failure. Specifically, two types of interfaces are highlighted in this work: a mirror symmetric interface in aluminum and an asymmetrically dissociated interface in copper. Distorted interface structures (after the dislocation nucleation event) are discussed in terms of partial dislocations or disclinations. Third, molecular dynamics simulations are used to investigate potential relationships between interface structure and interface properties or morphology. The orientation of the primary slip planes with respect to the loading direction and the porosity within the interface region are found to be critical factors in defining the strength of the bicrystal interface, for example. Finally, results of the atomistic calculations are utilized to motivate improved forms for continuum interface separation potentials, ultimately increasing the applicability of these relationships to include cohesive failure in ductile crystalline materials.
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