The thermal decomposition of mixed cation amides, Li\(_3\)Na(NH\(_2\))4 and LiNa\(_2\)(NH\(_2\))\(_3\), with light metal hydrides, lithium hydride, sodium hydride and magnesium hydride, was investigated and hydrogen gas was identified as the major desorption product in all cases. Minimal ammonia was detected and therefore the mixed cation amides could be considered as hydrogen storage materials. The reactions were found to be similar to previously studied light metal amide-hydride systems like lithium amide/lithium hydride and lithium amide/magnesium hydride. Magnesium hydride caused the hydrogen desorption from the mixed cation amides to occur at a lower temperature than when they were heated with lithium hydride. The hydrogen desorption was also at a lower temperature than LiNH\(_2\) + LiH and 2LiNH\(_2\) + MgH\(_2\). Although hydrogen was desorbed when the mixed cation amides were heated with NaH, the amount was much smaller than for LiH and MgH\(_2\), therefore making it less suitable as a hydrogen storage material. Reactions in various ratios between NaNH\(_2\) and MgH\(_2\) were investigated to intermediate temperatures up to 350 °C. Thermal decomposition, Raman spectroscopy and thermogravimetric analysis were all employed to explore the decomposition and reaction pathways of these reactions. It was found that the products were analogous to those formed by lithium amide heated with magnesium hydride in similar ratios. The more hydride included in the reaction, the greater the hydrogen loss for the products. Three new phases were identified and each was attempted to be made pure in order to characterise them. Evidence for the formation of a mixed Na-Mg amide, Na-Mg imide and Na-Mg nitride, under different temperatures and reaction conditions, was obtained. Attempts to rehydrogenate the Na-Mg imide were successful, reforming magnesium amide and sodium hydride. Sodium amide(NaNH\(_2\)) has been relatively neglected in the literature and so an examination of its decomposition products and reaction with sodium hydride, as a comparison to LiNH\(_2\)-LiH, were carried out. Using Raman and thermal decomposition techniques, it was proposed that a sodium imide may be formed, possibly with the same variable stoichiometry as observed for LiNH\(_2\)-Li\(_2\)NH. Rehydrogenation was attempted on this phase, but was unsuccessful.

Alloy compositions near to stoichiometric Nd2Fe14B produced under normal casting conditions, with subsequent isothermal heat treatments and with sintering have been characterised. The possible mechanisms for removing the soft magnetic phase, free α-Fe, via additions of 0.6 at% of Zr or 1.0 at% of ZrB2 have been investigated by EPMA (Electron Probe Microanalysis) and TEM (Transmission Electron Microscopy). 1.0 at% of ZrB2 has been found to be very effective in suppressing the formation of free α-Fe dendrites during solidification, but the excessive ZrB2-type needles formed in the NdFeB alloy may have a deleterious effect on the magnetic properties. However, the coercivity of the NdFeB sintered magnets can be improved to some extent by addition of Zr or ZrB2 to the composition Nd12.68Fe77.91-80.91B6.41-8.41Zr0-1 (at%) due to the smaller and more uniform grains of the Nd2Fe14B phase (~6.5 μm) in the sintered magnets. Both SEM (equipped with WDX) and TEM (equipped with EDX) have been employed to investigate ZrB2 needles in Nd12.68Fe77.91-80.91B6.41-8.41Zr0-1 (at%) alloys. It is found that suppression of free α-Fe dendrites in both Zr-containing NdFeB alloys resulted from different solidification routes. Since the sequences of formation of the ZrB2-type needles in Zr-containing alloys are different, the Nd2Fe14B grain growth is inhibited via different mechanisms. Meanwhile, evidence for the high temperature solubility of Fe in ZrB2 needles is found for both Zr-containing NdFeB alloys. Chemical analysis shows that the ZrB2-type needles in the NdFeB alloys are composed mainly of B (~70 at%) and Zr (~20 at%) with a small but significant amount of Fe (~7 at%), a formula which can be expressed as (Zr1-x, Fex)B2 (x~0.3).

Dans ce travail, des modèles d'amas sur réseau ont été développés pour modéliser le système Fe-Al-Mn-C, sur réseau Cubique Centré (CC) et Cubique à Faces Centrées (CFC). Ces modèles, ajustés sur des données ab initio, ont permis de calculer les propriétés thermostatistiques du système (en champ moyen, CVM et Monte-Carlo), dans le but de produire des diagrammes de phases.Le système a été décomposé, sur différents réseaux pour des raisons méthodologiques, et en différents sous-systèmes de chimie différente pour appréhender le problème par parties. L'influence de la base de données sur laquelle sont ajustés les modèles a été étudiée, ainsi que la contribution non configurationnelle de vibration des atomes à l'énergie libre.L'étude du modèle binaire Fe-Al sur réseau CC a permis de mettre en évidence l'effet des structures complémentaires dans la base d'ajustement, qui peuvent s'avérer indispensables. La prise en compte des phonons a un effet non négligeable sur les températures de transition mais s'avère insuffisante pour reproduire parfaitement les données expérimentales. Ces conclusions restent vraies en présence de manganèse. Les modèles quaternaires du modèle Fe-Al-Mn-C sur réseau CC montrent l'influence de la portée des paires. La mise en ordre du carbone adopte un comportement différent en présence d'interactions des atomes interstitiels entre eux. Les modèles sur réseau CFC n'ont pas permis d'aboutir à des résultats satisfaisants, notamment à cause de la difficulté à trouver une approximation valable pour l'étude de son magnétisme. / In this work, lattice models have been developped to model the Fe-Al-Mn-C system, on Body Centered Cubic (BCC) and Face Centered Cubic (FCC) lattices. These models, fitted on ab initio data, allowed to calculate thermostatistic properties of the system (with mean field, CVM and Monte-Carlo methods), in order to produce phase diagrams. The system has been decomposed, on different lattices for methodology reasons, and on sub-systems to deal with modules that depend on the chemistry. The input set database has been studied, as well as the non configurational vibration contribution to the free energy. Studying the binary Fe-Al model on a BCC lattice allowed to highlight the influence of complementary structures in the input set, which are required to obtain good models. Vibration free energy has a significative effect on transition temperatures but is not sufficient to fully reproduce the experimental results. The ordering of carbon adopts a different behavior depending on the presence or on the absence of interactions between interstitial atoms. The models on an FCC network did not lead to satisfactory results, in particular because of the difficulty to find a valid approximation for the study of its magnetism.

Metal hydride materials have attracted much interest for their potential use as hydrogen storage materials. Complex hydrides are amongst the most promising due to their high gravimetric storage capacities and favourable de/rehydrogenation conditions. Here, mixed anion complex hydrides are investigated both through halide doping of LiNH\(_2\) and Li\(_2\)NH, and though a mixed LiNH\(_2\)-LiBH\(_4\) system. The reaction of LiNH\(_2\) and Li\(_2\)NH with lithium or magnesium chloride, bromide and iodide has been shown to form a series of amide- and imide-halide phases. The structures of these phases were investigated through powder diffraction methods as well as Raman spectroscopy. The hydrogen releasing properties of these materials were investigated through reaction with LiH and MgH\(_2\); while the equivalent hydrogenation reactions were also tested. In both cases these materials performed more favourably than the pure LiNH\(_2\)-LiH system. The lithium ion conductivity of these materials was also investigated; it was shown that the most conducting materials were also the quickest to release and uptake hydrogen. The LiNH\(_2\)-LiBH\(_4\) system was studied, with particular focus on the decomposition product, Li\(_3\)BN\(_2\). All three known polymorphs of this compound were shown to be able to form by the reaction of \(_2\)LiNH\(_2\) + LiBH\(_4\) by carefully changing the reaction conditions. Further stages of this system were investigated through reaction of Li\(_2\)NH and Li\(_3\)N with LiBH\(_4\). Here the products from these reactions were studied along with the thermal desorption properties of the systems.

The aims of this study were to understand and improve the mechanical properties of aluminium Al-Cu-Si-Mg P/M alloy Alumix 123 by application of a warm compaction process. They were achieved by investigating the effect of (a) compaction pressure/temperature, (b) admixed lubricants (e.g. Acrawax C and Kenolube P11) on green/sintered density and mechanical properties of compacts. It was found that compaction at 110ºC led to (1) a reduction in the ejection force up to ~ 40 % and (2) an increase in sintered density up to ~ 98 % of theoretical density. After heat treatment at 200ºC for 5 hours (T6), the tensile strength reached ~ 365 MPa and hardness of ~ 126 HV was achieved. Further improvement of mechanical properties of aluminium alloy Alumix 123 can be achieved by reducing the amount of admixed lubricant to 0.5 wt %, and using Acrawax C as the lubricant rather than Kenolube P11. Taguchi analysis was used to identify which parameter (compaction pressure, temperature and lubricant content) affected the densities and mechanical properties of Alumix 123 specimens with single lubricants the most. It was observed that the most effective parameter in warm compaction of Alumix 123 specimens is the lubricant content.

The thesis describes an investigation into the fundamental phenomena governing the electrochemical machining process. It excludes a detailed investigation of the electrochemistry of the anode surface, work on which is in hand at the University of Nottingham. Photographs have been obtained of both electrode surfaces during machining in a two dimensional channel, showing the important role of gas evolution both at currents below the limit and in limiting the current density achievable. The distribution of electrical potential across the gap has been measured, clearly showing that the limiting current phenomenon is governed by a process occurring very close to the cathode. Measurements have been made of the streamwise current distribution; the distribution is essentially uniform at low currents, but as the limit is approached the current at the downstream end falls, and this fall then propagates upstream to fill about two-thirds of the channel. It has been found that the limiting current is proportional to (absolute pressure)^1/3, that the size of bubbles produced is inversely proportional to (absolute pressure)^1/3, and that reduction of the surface tension of the electrolyte leads to a marked fall in limiting current. The efficiency of the process has been investigated by a technique involving the measurement of the gas evolved during machining. An analysis of these results leads to the formulation of an explanation of the cell voltage-current characteristic, a hypothesis to explain the current limiting process, and a suggestion of the detailed mechanism of the latter. The cell voltage-current curve (above) can be explained as follows: - AB is equilibrium dissolution with etching, BC is caused by the formulation of a solid ( impure oxide? ) film on the anode surface. The rise in current from C to D is caused by the anodic evolution of a gas (oxygen? ), causing better mixing conditions in the diffusion layer near the anode and hence a higher metal dissolution rate. The ratio of current used for metal dissolution to current used for gas evolution appears to be a constant for this region. This process would be expected to continue along DE, but the current is limited by the achievement of a maximum rate of cathodic hydrogen evolution which brings about the reduction in current to F. This limiting current crisis has been analysed in terms of the mechanics of bubble formation, and a detailed explanation in terms of various mechanisms has been attempted. The experimental data is fitted by a model in which the hydrodynamic conditions give a velocity at which bubbles can be removed from a fixed number of nucleation sites. The limiting current is then predicted to be proportional to (surface tension)^2 x (absolute pressure)^1/3. Several proposals are made for further experiments to investigate these proposals, and for data needed to extend the industrial application of the theory developed.

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Les alliages de zirconium sont utilisés pour fabriquer des gaines de combustible ainsi que des assemblages combustibles des réacteurs nucléaires à eau sous pression. Sous irradiation, ils montrent un changement dimensionnel communément appelé croissance. Des observations expérimentales ont montré qu'au-dessus d'une dose seuil, ces alliages sont sujets à une croissance accélérée appelée "breakaway". Il a été bien établi que la formation sous irradiation de boucles de dislocation ‹a› et ‹c› est directement responsables de la croissance des alliages de zirconium sous irradiation et que l’apparition des boucles ‹c› est corrélée avec cette accélération de croissance. Cependant, les mécanismes de germination des boucles qui semblent influencés par la présence d’éléments d’alliage sont encore mal compris. Afin d'améliorer notre compréhension des mécanismes élémentaires, une approche multi-échelle a été utilisée pour simuler l'évolution de la microstructure du zirconium sous irradiation. Des calculs à l’échelle atomique basés sur la théorie de la fonctionnelle de la densité (DFT) et sur des potentiels empiriques sont utilisés dans un premier temps pour déterminer les propriétés des amas de défauts ponctuels (boucles de dislocation, cavités, pyramides de fautes d’empilement). Les résultats obtenus sont ensuite insérés en tant que paramètres d'entrée dans un code Monte Carlo cinétique d'objet (OKMC) qui nous permet de simuler l’évolution de la microstructure du matériau sous irradiation, et donc de prédire la croissance. Nos résultats montrent qu’il est nécessaire de considérer une migration anisotrope de la lacune pour prédire l’accélération de croissance. / Zirconium alloys are used to manufacture fuel cladding as well as fuel assemblies of pressurized water nuclear reactors. Under irradiation, they show a dimensional change commonly called growth. Experimental observations have shown that above a threshold dose, these alloys are subject to accelerated growth called "breakaway". It has been well established that the irradiation formation of <a> and <c> dislocation loops is directly responsible for the growth of irradiated zirconium alloys and that the appearance of <c> loops is correlated with this growth acceleration. However, the nucleation mechanisms of the loops that seem to be influenced by the presence of alloying elements are still poorly understood. In order to improve our understanding, a multi-scale modelling approach has been used to simulate the evolution of zirconium microstructure under irradiation. Atomic-scale calculations based on the density functional theory (DFT) and empirical potentials are used to determine the properties of clusters of point defects (dislocation loops, cavities, pyramids of stacking faults). The results obtained are then used as input parameters of an object kinetic Monte Carlo (OKMC) code which allows us to simulate the microstructure evolution of the material under irradiation. Our results show that it is necessary to consider an anisotropic migration of the vacancies to predict the growth acceleration.

Le but de ce travail était de contribuer à une meilleure compréhension des mécanismes complexes qui sous-tendent la formation et l’évolution des hydrures dans le zirconium α et ses alliages. Dans ce contexte, des approches multi-échelles, mêlant simulations atomiques et par champ de phase, ont été employées pour aborder les propriétés structurales et énergétiques des interfaces hétérophases α|hydrures. Un intérêt particulier a été porté à l'hydrure γ-ZrH, en raison de sa stabilité et ses relations avec les chemins de précipitation controversés, via l’étude des interfaces basales et prismatiques complétée par une étude soignée des plans d’habitat. Une méthodologie originale transposable à d’autres systèmes, fondée sur la théorie microélastique combinée à des calculs ab initio, a été proposée pour estimer les énergies relatives à ces interfaces. Les résultats montrent une forte anisotropie jusque-là négligée dans la littérature. Du fait du caractère semi-cohérent des interfaces prismatiques, une évaluation correcte de leur énergie s'est révélée difficile. Une approche alternative a donc été suggérée, fondée sur un formalisme de champ de phase à même de décrire la structure de cœur des dislocations inhérentes à des interfaces prismatiques hc|cfc. Elle s’est avérée utile pour estimer la stabilité interfaciale de divers systèmes et pour prédire la perte de cohérence d’un germe croissant. Il a été montré que l'hydrure ζ-Zr2H cohérent constituerait un bon précurseur de γ. Enfin, l'emploi de la théorie microélastique aux interfaces α|hydrures a clairement indiqué la préférence pour des plans d'habitat pyramidaux πI. Un effet notable de la teneur en hydrogène a aussi été observé. / The goal of this work was to contribute to a better understanding of the complex mechanisms underpinning the formation and evolution of hydrides in α zirconium and its alloys. In this context, multi-scale approaches combining atomic and phase-field simulations were employed to address the structural and energetic properties of heterophase α|hydrides interfaces. A peculiar attention was focused on the γ-ZrH hydride due to its controversed stability and to its supposed role one many sequences of precipitation. In particular, the study was concentrated on the basal and prismatic interfaces complemented by a meticulous study of the habit planes. An original methodology, transferable to other systems and based on the microelasticity theory combined with ab initio calculations, was proposed to estimate the energies related to these interfaces. The results show a strong anisotropy hitherto neglected in the literature. Due to the semi-coherent nature of prismatic interfaces, a correct evaluation of its energy was proven to be difficult. Thus, an alternative approach, grounded in a phase-field formalism and able to describe the core structure of inherent dislocations in prismatic interfaces, has been suggested. This approach turned out useful to estimate the interfacial stability of various systems as well as to predict the coherency loss of a growing seed. It has been showed that the coherent ζ-Zr2H hydride could constitute a good precursor of γ-hydride. Finally, the application of the microelasticity theory to α|hydrides interfaces has clearly showed a preference for πI pyramidal habit planes where a significant effect of hydrogen content in the hydride has also been observed.

Between 50% and 70% of the total energy used in the extraction process may be attributed to comminution. Microwave pre-treatment has been suggested as a means to decrease the energy requirements. A variety of mineral ores have been investigated and the effects of microwave radiation quantified in terms of the mineralogy, changes in the Bond Work Index, flotability and magnetic separation. It has been shown that microwave pre-treatment is most effective for coarse grained ores with consistent mineralogy consisting of good microwave absorbers in a transparent gangue (up to a 90% decrease in Bond work index for Palabora copper ore) whereas fine grained ores consisting predominantly of good absorbers are not affected as well (a reduction of only 25% in work index for Mambula ore). Although the mineralogy of minerals are affected by exposure to microwave radiation, flotability and magnetic separation characteristics have been shown not to be adversely affected, unless the microstructure is completely destroyed after prolonged microwave exposure. Computer simulations have shown that significant changes to comminution circuits are possible as a result of microwave induced work index reductions (three mills reduced to one). Purpose-built microwave units may hold the solution for more efficient mineral extraction in the near future.