La compréhension des effets du grenaillage ultrasonore sur l’intégrité de surface des composants métalliques représente un enjeu industriel important. Dans le cadre de cette thèse, une modélisation physique multi-échelle de la plasticité cristalline à la DEM (Discrete Element Model) a été développée. Le grenaillage ultrasonore consiste à mettre en mouvement des billes à l’intérieur d’une enceinte par vibration de la sonotrode. Les impacts répétés sur le matériau entraînent un écrouissage en surface, l’établissement de contraintes résiduelles de compression et la formation d’une couche nanostructurée. L’objectif de cette thèse est d’obtenir une meilleure compréhension des mécanismes conduisant à ces modifications sur un alliage à base nickel. Le mouvement des billes obtenu par DEM est relié aux modifications de la microstructure sous impacts via un modèle éléments finis utilisant une loi de plasticité cristalline. Après validation de chaque étape par des mesures expérimentales, le modèle a permis d’étudier l’effet de la quantité de billes utilisées dans le procédé. Ainsi, un nombre croissant de billes induit une augmentation d’impacts en biais de faible vitesse permettant de concentrer les contraintes résiduelles de compression en extrême surface. De plus, les simulations multi-impacts utilisant une loi de plasticité cristalline ont montré que ces impacts en biais engendraient une densité totale de dislocations et un niveau de désorientations élevées pouvant expliquer la fragmentation des grains et la nanostructuration de la surface. Enfin, ces modifications de la microstructure, visibles jusqu’à 300 µm de profondeur, sont en accord avec les profils de dureté obtenus par nano-indentation et les profils de désorientation issus de l’analyse EBSD (Electron BackScatter Diffraction). / Ultrasonic shot peening is widely used to improve mechanical properties of metallic components. Mastering the effects of this surface treatment is a major industrial issue. A physical multi-scale modelling based on crystal plasticity and DEM (Discrete Element Model) was developed in this PhD thesis. This process is performed in a closed chamber where spherical balls are moved by sonotrode vibration. Thousands of impacts induce hardening, residual compressive stress and microstructure modification leading to a nanostructured layer. The aim of this work was to improve our understanding of the mechanisms occurring during this process on nickel-based alloys. Ball motion was computed by DEM and linked to microstructure modifications induced by impacts through a crystal plasticity finite element model. Experimental analyses were performed in order to validate each step of the multi-scale modelling. Then the model was applied to investigate ball quantity effects on peened surface modification. Increasing the number of balls created a larger quantity of low speed oblique impacts which concentrated the residual compressive stress near the surface. Furthermore multi-impacts performed with a crystal plasticity law showed oblique impacts enhanced dislocation storage and disorientations within grains, which could explain the nanostructuration of the peened surface. Moreover, the numerical microstructure modification, observed up to 300 µm in depth, was in agreement with hardness profiles obtained by nanoindentation and disorientation profiles measured by EBSD (Electron BackScatter Diffraction) analysis.

Les champignons mycorhiziens à arbuscules (AMF) sont des symbiotes nécessaires au développement d’environ 80% des plantes vasculaires (Brundrett 2009). Leur impact sur l’adaptation et la croissance des plantes est essentiel, en particulier dans les milieux pauvres comme les maquis miniers de Nouvelle-Calédonie. Les études réalisées jusqu’à présent en Nouvelle-Calédonie sur ce sujet ont démontré l’importance de ces symbiotes dans les maquis miniers et leur rôle dans la nutrition des plantes endémiques et plus généralement dans l’adaptation aux contraintes multiples de ces milieux (Amir et al. 1997 ; Perrier et al. 2006 ;Amir et al. 2007 ; Amir et al. 2008 ; Lagrange 2009 ; Amir et Ducousso 2010). La maîtrise de la symbiose plante/AMF dans ces écosystèmes est essentielle à la mise au point de méthodes de restauration écologique des terrains miniers dégradés, d’autant que les AMF sont des symbiotes non spécifiques et que le même champignon peut être utilisé pour mycorhizer de nombreuses espèces végétales. Toutefois nos connaissances actuelles restent insuffisantes, notamment en ce qui concerne la biodiversité de ces AMF et les mécanismes de leurs effets positifs sur la plante. / In New Caledonia (NC), restoration of open-cat mining sites is far from being mastered. It still needs work, to be improved, especially in plants - soil - microorganisms relationships. These works have focus on description of arbuscular mycorrhizal fungi (AMF) diversity and role in the adaptation of plants to ultramafic constraint, with plant growth promoting rhibzobacteria (PGPR). The diversity of AMF is analyzed on two plant genera with species on ultramafic soils, nickel hyperaccumulat ing or not, and on volcano-sedimentary soils. The analysis of this diversity shows that community composition is contrasted between the two types of soil. Some of AMF taxa seem to be specific to ultramafic soils and some even to be specific to nickel hyperaccumulator species. For the role in adaptation of plants to ultramafic constraints, greenhouse experiments on three endemic species (Alphitonia meocaledonica, Carpolepis laurifolia and Costularia comosa) have shown that naturally brings mycorrhiza improved growth, mineral nutrition, water and Ca /Mg ration. Phosphorus additions seem to change these effects with different consequences on growth according to the host plant. Finally, the use of PGPR alone or with AMF showed that the effect depends on host plant species and bacterial strain, but usually leads to improved growth and plant nutrition. These studies have shown the importance of AMF in ultramafic soils NC.

M.Tech (Metallurgy) / Nickel in South Africa is scarce. Other than as a by-product from platinum mining, its only source is the Uitkomst Complex, a satelite of the Bushveld Igneous Complex, located between Machadodorp and Barberton and mined by Nkomati Nickel. Pentlandite occurs as the main nickel-bearing mineral, disseminated within a sulfide matrix of pyrrhotite, pyrite and chalcopyrite. Accessories are of chromite and platinum in solid solution with the sulfides. The sulfides are hosted in mainly ultramafic rocks. The presence of talc is of particular nuisance. Head grade is in the decline, from earlier 0.7 to more recently 0.3% or even lower. Standard froth flotation yields a concentrate of 7 – 9% nickel at a recovery of up to 70%. A higher nickel recovery from such low – grade feed stock would be desirable. To achieve these two methods were investigated and reported in the following:  Non-oxidative leaching as an alternative to flotation, using strong hydrochloric and sulfuric acid and also aqueous ammonia solutions at various liquid-to-solid ratios. Only the degree of nickel dissolution over time at room temperature was studied and no attempt was made to recover the dissolved metal from solution.  Froth flotation after pre-treatment with microwaves at various power levels and over various periods of time of irradiation. It was surmised that a certain amount of inter-granular cracking could thus be achieved that would improve the flotation behavior in respect of grade and recovery. An extensive literature study, in particular, of the nature of microwaves, their interaction with matter and possible metallurgical benefits derived as a consequence of such interaction, forms part of the investigation and is reported in detail. A lot of theoretical and experimental work has been done in this respect, although the obtained results were not conclusive.

This study reports on an electrochemical capacitive properties of nickel tetraaminophthalocyanine (NiTAPc), nickel tetraaminophthalocyanine incorporated with Nickel oxide (NiTAPc-NiO) and nickel oxide incorporated with multi-walled carbon nanotubes (NiO-MWCNT), using three different techniques known as successive ionic layer adsorption reaction (SILAR), electrodeposition and dip-dry. This study also reports on the effect of undoped polymer of poly-pyrrole on NiTAPc. The physical properties of the synthesised materials were investigated using SEM and EDX and the electrochemical properties were investigated using cyclic voltammetry (CV), charge-discharge (CD) and electrochemical impedance spectroscopy (EIS). The supercapacitive properties of NiTAPc film on nickel foam showed a maximum specific capacitance of 416.0 Fg-1, a maximum power density of 15.50x103 WKg-1 and a maximum specific energy of 66.0 WhKg-1. The NiO-MWCNT film on nickel foam gave a maximum specific capacitance of 1034.0 Fg-1, a maximum power density of 10.41x103 WKg-1 and a maximum specific energy of 132.0 WhKg-1. The NiTAPc-NiOE film on nickel foam was found to possess a maximum specific capacitance of 1117.0 Fg-1, a maximum power density of 20.48x103 WKg-1 and a maximum specific energy of 119.0 WhKg-1. The NiTAPc-NiOE-S film on nickel foam gave a maximum specific capacitance of 1279.0 Fg-1, a maximum power density of 26.96x103 WKg-1 and a maximum specific energy of 114.0 WhKg-1. Finally, the NiO mixed with an oxidant (NiOS-ox) film on nickel foam gave a maximum specific capacitance of 1403.0 Fg-1, power density of 14.44x103 WKg-1 and a maximum specific energy of 147.0 WhKg<sdup>-1. In addition, the electrodes were found to be very stable even after repetitive cycling. These electrodes have clearly proved that they may be suitable for use as potential supercapacitors. Further research is necessary to fully explore their supercapacitive behaviour in single cell (2-electrode)systems. Copyright / Dissertation (MSc)--University of Pretoria, 2012. / Chemistry / unrestricted

The demand for compact, efficient and high performance electronic devices and sensor systems has become one of the primary driving force for rapid advancement in miniaturization of current technology. However, the attempt to push the limits of component length scales into the nano regime is being challenged by possibly unconventional laws of physics. One of the key design parameters for good performance of any system is its structural stability, defined by the strength of a material. The strength of a material is defined as its resistance to plastic (or permanent) deformation. In conventional metals plastic deformation is carried by the migration of lattice defects such as vacancies and dislocations. The barriers to the motion of these defects provide strength to metals, leading to an inverse power law scaling with inter barrier spacing, l  = Bl q where represents the nominal strength, B is a measure for strengthening capability of barriers and q represents the order of strengthening. The well known Hall Petch relation (q=0.5) expresses this effect for grain boundary strengthening, where grain boundaries (GBs) obstruct motion of dislocations during plastic deformation. Extensive research over past few decades has shown that grain size strengthening may be limited by GB mediated deformation processes in nanocrsytalline (nc) metals with grain sizes of ≤ 20 nm. The strength of nanocrsytalline metals saturates, or in some cases decreases, with a reduction in grain size. Molecular dynamic simulations have provided some indication of atomic scale activities that dominate deformation in nc metals. Although it is difficult to experimentally monitor the atomistic processes, in situ mechanical tests in synchrotron facilities have captured some mesoscopic features of deformation in nanocrsytalline metals. For instance, experiments have shown that the typically extended elastic plastic transition during deformation of nanocrsytalline metals could be classified into two regimes. For initial stages of deformation, in the microplastic regime, the width of various diffracting peaks decreases suggesting the dominance of processes leading to structural relaxation. However, at later stages of deformation, in a manner similar to conventional metals, the diffraction peak widths increased signifying the increasing importance of deformation processes that involve an accumulation of defects. It is well known that thermal annealing also causes relaxation of materials and during high temperature deformation continuous relaxation of stress concentrations could retard premature failure of metals. As the ductility in nanocrsytalline metals is limited to 3-5%, with very limited strain hardening, the processes of structural relaxation are very important. Thus, there is a fundamental need to understand the nature of structural relaxation during microplastic deformation in nanocrystals. It is well known that character of grain boundaries plays an important role in material properties. As the grain boundary area per unit volume varies inversely with grain size, nanocrsytalline metals contain a significant amount of grain boundary area. Moreover, due to small grain sizes, conventional Frank Read sources cannot operate for nucleating dislocations. Dislocations in nc metals are nucleated from GBs, traverse grains and gets absorbed in other GBs. The small volume of grains further restricts dislocation interactions. Thus, dislocation nucleation, propagation and absorption become possible rate controlling mechanisms in nc metals. Molecular dynamic simulations of nanocrsytalline and bicrystalline samples have shown that grain boundary structure could significantly affect these mechanisms. Simulations have shown further that apart from dislocation plasticity other grain boundary mediated process like GB sliding and GB diffusion become important with decreasing grain size. These processes are also influenced by GB character. Thus, it is important to understand the role of GB character in deformation of nc metals. On one hand where the structural need for high strength has encouraged reduction of internal microstructural length scales, miniaturization has also encouraged reduction in external length scale of device components. In modern electronic and sensor devices a typical component size varies from a few hundred microns to few tens of nanometer. Several studies have shown that free surfaces could reduce the constraints on deforming grains. With decreasing sample dimensions, the free surface to volume ratio increases and the internal microstructural length scale may become comparable to the external sample size. An increasing contribution from these two geometrical parameters can introduce external size effects in mechanical properties of materials. In the past, most of the external size effects have been attributed to strain gradient plasticity and deformation source starvation. However, a different external size effect has been observed during uniaxial test of polycrystalline metals where the strength of materials was found to deviate from their bulk values at smaller sample sizes. While most studies have shown a weakening effect, there have also been a few observations of strengthening with a reduction in sample size. In most studies, the external sample size was kept constant and the internal grain size was varied by thermal annealing to produce samples with different external to internal size ratios. As the mechanical properties of metals are sensitive to the internal length scales it is difficult to explicitly follow the external size effect during these experiments. Moreover, compared to internal size effects mechanistic understanding of external size effect is limited, and systematic experimental efforts are required for proper characterization of these effects. The present investigation was undertaken to improve the scientific understanding of internal and external length effects on mechanical properties of nanocrystalline and coarse grained (~16 – 140 µm) polycrystalline nickel. For studying the internal size effects free standing nanocrystalline nickel samples with ~30 nm grain size and two different textures were synthesized using galvanostatic pulsed electrodeposition technique from Watts and Sulfamate baths. The nanocrsytalline deposits from a Watts bath showed a strong <100> fiber texture (NiS) while deposits from a Sulfamate bath were relatively weak textured (NiW), with s and w representing strong and weak texture, respectively. In situ mechanical tests at the PSI synchrotron facility in Switzerland were used to understand the nature of relaxation processes during thermal annealing and deformation of nc metals. The diffraction peak analysis showed that thermal annealing at 423 K of strong textured deposits caused a significant reduction in root mean square strain with limited grain growth. Furthermore, no residual strains developed, suggesting a homogenous distribution of relaxation processes during thermal annealing. In contrast, during deformation, structural relaxation was highly biased due to dislocation activities. The grains contributing to <200> diffraction peak transverse to loading axis showed early yielding and faster relaxation during deformation. The inhomogeneous nature of deformation was also reflected in development of transverse tensile residual stresses in the <200> grains. These experiments showed that relaxation processes during thermal annealing and deformation differ in their respective length scales. Nanocrsytalline deposits with two different textures were also deformed under synchrotron to access the role of GB character. As direct quantification of GB character distribution is difficult in nc metals, texture was taken as a qualitative representative for GB character. Previous studies have shown that the fraction of low angle boundaries increases with increasing sharpness of texture in fiber textured materials. Thus, the two textured deposits represented materials with two different low angle GB fractions. In situ tests showed that during the initial stages of microplastic deformation dislocation mechanisms were favored in strongly textured NiS. The transversely oriented <200> grains showed early yielding, which caused a redistribution of stress among other grain families. However, for weakly textured NiW deposits, smaller length scale atomic activities preceded dislocation activities. All the grains supported larger elastic strains at lower stresses suggesting significant plastic activity at GB regions. At higher stresses transversely oriented <220> grains yielded plastically and transferred elastic loads to <200> grains. Thus, the nature of plastic deformation was observed to depend on the distribution of GB character. For understanding the external size effects on mechanical properties polycrystalline nickel samples with grain sizes of 16, 51 and 140 µm were tested uniaxially at various sample thicknesses. Three different deformation regimes were identified based on the thickness of the samples. At higher thicknesses, in regime I, no significant variation of flow strength was observed. Flow strengths in regime II, at intermediate thicknesses, showed a strengthening effect with a reduction in thickness. However at lower thicknesses in regime III, a weakening trend was observed with decreasing thickness. The cross over from strengthening to weakening was observed to depend on grain size and applied strain. Detailed microstructural analysis with electron back scattered diffraction (EBSD) imaging showed that intragranular lattice rotation increases with a reduction in sample thickness. As lattice rotations may be considered to be accommodated by geometrically necessary dislocations, a semi empirical phenomenological model based on strain gradient plasticity was developed to understand the mechanics of external size effect during uniaxial test of polycrystalline samples. Further application of the model to the present experimental results showed that the characteristic length for strain gradient decreased with increasing grain size and applied strain.

No description available.

Fatigue and dispersion strengthening are reviewed generally. Experiments are described in which thin film transmission electron microscopy, as well as optical and electron fractography were employed to elucidate the mechanism of fatigue failure in dispersion strengthened nickel. It was found that whereas fatigue crack initiation occurs as in conventional materials, the propagation of both fatigue and tensile cracks is abnormal. The material fails in shear wherever possible although this is prevented where triaxial tensile stresses exist. Particle-matrix detachment does not occur during either tensile or fatigue stressing. / Thesis / Master of Science (MS)

The ability of simple oligopeptide complexes of nickel (II) to react with various dioxygen intermediates was investigated. Under physiological conditions, nickel (II)-histidine-containing oligopeptides were found to dismutate superoxide anions and disproportionate hydrogen peroxide. In the latter process, chemiluminescence was generated and a strongly oxidizing intermediate was detected capable of oxidizing uric acid, hydroxylating p-nitrophenol, and damaging 2-deoxy-D-ribose. The generation of this reactive intermediate likely occurs without the involvement of free hydroxyl radicals derived from Haber-Weiss or Fenton-type reactions. In addition, the Ni(II) complex of glycylglycyl-L-histidine (GGH) was found to react with mollecular oxygen resulting in the oxidation of the ligand. An attempt was made to relate these reactions to the involvement: of the nickel(III)/(II) redox couple which was shown to exist under physiological conditions. Similar reactivity was observed for non-histidine-containing oligopeptides but higher pH values were required. The oligopeptides used not only represent biologically relevant ligands but: the histidine containing oligopeptides mimics the specific copper(II)/nickel(II) binding and transport site of human serum albumin. The observations made in this study suggest some novel mechanism for the deleterious effects associated with excessive lifelong exposure to nickel compounds, especially in relation to cancer of the respiratory tract. / Thesis / Master of Science (MS)

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Call number: LD2668 .T4 1954 A5 / Master of Science