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Les alliages de plomb-antimoine permettent à 1 milliard de véhicules thermiques de démarrer quotidiennement mais sont également utilisés dans le secteur médical et de l’énergie pour réaliser des équipements de radioprotection. Or la littérature montre une grande disparité des propriétés mécaniques et électrochimiques de ces alliages en fonction des conditions de sollicitation. La cause de cette confusion est liée à une sous-estimation de la sensibilité de ces matériaux à la température et à la vitesse de déformation. La température ambiante représente en effet la moitié de la température de fusion du plomb (en Kelvin) qui subit alors un recuit permanent. Les phénomènes thermiquement activés tels que la restauration et la recristallisation ne sont donc pas négligeable dans la modélisation du comportement mécanique de ces alliages. Après avoir modélisé l’influence de l’antimoine et de la vitesse de solidification sur les propriétés des microstructures de fonderie, une modélisation du comportement mécanique lors de grandes déformations est proposée. Le modèle déterminé est fidèle aux données expérimentales et intègre la sensibilité du matériau aux conditions de déformation sous la forme du paramètre de Zener- Hollomon. Les hétérogénéités de propriétés mécaniques qui sont issues de l’assemblage par soudage TIG sont expliquées par la présence conjointe des deux microstructures : une morphologie de solidification dans le cordon de soudure et une morphologie de déformation dans les laminés. Dans les alliages de plomb-antimoine, une microstructure hybride est créée à l’interface par un phénomène de fusion partielle des joints de grains. / Lead-antimony alloys allow 1 billion thermal vehicles to start daily but are also used in the medical and energy sector to produce radiation protection equipment. However, the literature shows a great disparity in the mechanical and electrochemical properties of these alloys depending on the conditions of stress. The cause of this confusion is related to an underestimation of the sensitivity of these materials to temperature and strain rate. The ambient temperature represents in fact half the melting temperature of the lead (in Kelvin) which then undergoes a permanent annealing. The thermally activated phenomena such as restoration and recrystallization are therefore not negligible in the modeling of the mechanical behavior of these alloys. After modeling the influence of antimony and the rate of solidification on the properties of solidification microstructures, a modeling of the mechanical behavior during large deformations is proposed. The determined model is faithful to the experimental data and incorporates the sensitivity of the material to the deformation conditions thanks to the Zener-Hollomon parameter. The heterogeneities of mechanical properties that arise from the TIG welding are explained by the presence of two microstructures: a solidification morphology in the weld bead and a deformation morphology in the rolls. For leadantimony alloys, a hybrid microstructure is created at the interface by a phenomenon of partial melting of the grain boundaries.

Oxidation damage, in conjunction with fatigue, is a concern for nickel-based superalloys utilised as disc rotors in high pressure compressor and turbine of aero-engines. A combined experimental and numerical study has been carried out for alloy RR1000, developed at Rolls-Royce plc through a powder metallurgy route to meet the demands for higher overall pressure ratios, compressor discharge temperatures and rotational speeds for the latest aero-engines. Cyclic experiments have been carried out for waisted specimens at selected temperatures (700°C-800°C), followed by microscopy examination using Focused Ion Beam (FIB). The results suggest that the major mechanism of oxidation damage consists of the formation of surface oxide scales and internal micro-voids and oxide particles beneath the oxide scales, which becomes more severe with the increase of temperature. Applying a cyclic stress does not change the nature of oxidation damage but tends to enhance the extent of oxidation damage for temperatures at 750°C and 800°C. Further energy dispersive X-ray (EDX) analyses show the enrichment of Cr and Ti, together with lower Ni and Co levels, in the surface oxide scales, suggesting the formation of brittle Cr2O3, TiO2, NiO and Co3O4 oxides on the specimen surface. Penetration of oxygen into the material and associated internal oxidation, which leads to further material embrittlement and associated failure, are evidenced from both secondary ion imaging and EDX analyses. Scanning electron microscopy (SEM) studies of fracture surfaces have been performed for dwell crack growth, and the results confirmed the transition from transgranular to predominant intergranular cracking in alloy RR1000 for increased dwell time. This change in fracture mode supports the oxidation-assisted crack growth mechanism via grain boundary embrittlement. Oxidation embrittlement has also been supported by the FIB analyses of fracture surfaces which confirmed the oxidation reaction for alloy RR1000 at high temperatures. A stress-assisted diffusion approach has been used to model oxygen penetration in the waisted specimen based on Fick’s first and second laws. Grain microstructures were considered explicitly in the model using a finite element submodelling technique, and the grain boundary was taken as the primary path for oxygen diffusion. The material constitutive behaviour was described by a crystal plasticity model to consider the effects of heterogeneous deformation at grain level on oxygen diffusion. Two essential diffusion parameters, i.e., oxygen diffusivity and pressure factor, have been obtained from the simulated oxygen penetration as well as the FIB measurements of internal oxidation depth. Using the obtained diffusion parameters, finite element analyses of a compact tension specimen have been carried out to model oxygen diffusion, coupled with viscoplastic deformation, near a fatigue crack tip. A failure curve for crack growth has been constructed based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The failure curve was then utilized to predict crack growth rates under fatigue-oxidation conditions for selected loading frequencies and dwell periods, with comparison against the experimental results and those predicted from the viscoplastic model alone.

An investigation into the optimisation of magnetic properties of two types of magnetic alloys based on Sm(Co,Cu,Fe,Zr)_z (z = 7.4) and Nd₁₅Fe₇₇B₈ has been carried out. The optimum heat treatment which gave the best magnetic properties for the Sm(Co,Cu,Fe,Zr)_z alloy depended primarily on the sintering and solid solution temperatures. After quenching the alloy from the solid solution temperature, and then ageing isothermally, it was found that the intrinsic coercivity, microhardness and electrical resistance changed with ageing time, and the ageing temperature also affected the rate of these changes. The magnetic properties of specimens produced from the Nd-Fe-B type alloy were also shown to be affected by different heat treatments. The microhardness, microstructure and intrinsic coercivity were affected by an isothermal ageing treatment and the temperature of the treatment. Two routes were used for powder preparation 1n the production of the sintered specimens: i. Mechanical ball milling of crushed ingot material. ii. Hydrogenation followed by ball milling. It was found that higher energy products were achieved in the ball milled material for the two alloys investigated. Two reasons for this were proposed: i. The hydrogenated and ball milled powder had a larger size than the ball milled only powder. ii. The surfaces of the hydrogenated powder were highly reactive in air and some oxidation of the surfaces occurred. Optical and electron microscope studies showed that the variations in the ageing behaviour of both the alloys was concurrent with subtle differences in the microstructure between the samples aged at different isothermal ageing temperatures.

Porous materials possessing random microstructures exist in both organic (e.g. polymer foam, bone) and in-organic (e.g. silica aerogels) forms. Foams and aerogels are two such materials with numerous engineering and scientific applications such as light-weight cores in sandwich structures, packaging, impact and crash structures, filters, catalysts and thermal and electrical insulators. As such, design and manufacture using these materials is an important task that can benefit significantly from the use of computer aided engineering tools. With the increase in computational power, multi-scale modelling is fast becoming a powerful and increasingly relevant computational technique. Ultimately, the aim is to employ this technique to decrease the time and cost of experimental mechanical characterisation and also to optimise material microstructures. Both these goals can be achieved through the use of multi-scale modelling to predict the macro-mechanical behaviour of porous materials from their microstructural morphologies, and the constituent materials from which they are made. The aim of this work is to create novel software capable of generating realistic randomly micro-structured material models, for convenient import into commercial finite element software. An important aspect is computational efficiency and all techniques are developed paying close attention to the computation time required by the final finite element simulations. Existing methods are reviewed and where required, new techniques are devised. The research extensively employs the concept of the Representative Volume Element (RVE), and a Periodic Boundary Condition (PBC) is used in conjunction with the RVEs to obtain a volume-averaged mechanical response of the bulk material from the micro-scale. Numerical methods such as Voronoi, Voronoi-Laguerre and Diffusion Limited Cluster-Cluster Aggregation are all employed in generating the microstructures, and where necessary, enhanced in order to create a wide variety of realistic microstructural morphologies, including mono-disperse, polydisperse and isotropic microstructures (relevant to gas-expanded foam materials) as well as diffusion-based microstructures (relevant for aerogels). Methods of performing large strain simulations of foams microstructures, up to and beyond the onset strain of densification are developed and the dependence of mechanical response on the size of an RVE is considered. Both mechanical and morphological analysis of the RVEs is performed in order to investigate the relationship between mechanical response and internal microstructural morphology of the RVE. The majority of the investigation is limited to 2-d models though the work culminates in extending the methods to consider 3-d microstructures.

Το αντικείμενο της παρούσας διατριβής είναι η πειραματική και θεωρητική ανάλυση με τη χρήση Μοριακής Δυναμικής του μηχανισμού φωτοαποδόμησης μεταλλικών υλικών που προκαλείται από την επίδραση ακτίνων Laser. Η τεχνολογία των μικρο-κατεργασιών με Laser είναι μια νέα τεχνολογία που επιτρέπει τη δημιουργία εξαρτημάτων σε κλίματα μικρομέτρου. Για την υλοποίηση των μικρο-κατεργασιών με Laser χρησιμοποιούνται συστήματα Laser υπερβραχέων παλμών. Φωτοαποδόμηση είναι η διαδικασία αφαίρεσης υλικού, που ακολουθεί την εφαρμογή δέσμης Laser σε αυτό και ουσιαστικά αποτελεί συνδυασμό εξάχνωσης, εξάτμισης και τήξης. Χαρακτηρίζεται από ιδιαίτερα μικρά χρονικά και χωρικά μεγέθη, καθώς και από ακραίες τιμές θερμοκρασίας και πίεσης. Η Μοριακή Δυναμική (ΜΔ) είναι μια αιτιοκρατική μέθοδος προσομοίωσης. Βασίζεται στην επίλυση του δευτέρου νόμου του Νεύτωνα με σκοπό την παρακολούθηση της κίνησης κάθε σωματιδίου σε ένα σύστημα. Η παρούσα διατριβή επικεντρώνεται στην ανάπτυξη μαθηματικών μοντέλων ΜΔ ικανών να προβλέψουν τα διάφορα χαρακτηριστικά της διεργασίας όπως είναι η κρυσταλλική δομή του υλικού πριν την επίδραση της δέσμης Laser, την χρονική και χωρική κατανομή των φωτονίων που μεταφέρονται από τη δέσμη Laser και τη συμπεριφορά των ακτινοβολούμενων σωματιδίων. Τα μοντέλα αυτά συνδυαζόμενα επιτρέπουν τον προσδιορισμό του βάθους φωτοαποδόμησης που προκαλείται σε ένα μεταλλικό υλικό, το οποίο έχει υποστεί ακτινοβολία με υπερβραχείς παλμούς Laser, το παραγόμενο θερμοκρασιακό πεδίο, καθώς και τη χρονική εξέλιξη της θερμοκρασιακής κατανομής. Μετά την πειραματική επιβεβαίωση των θεωρητικών αποτελεσμάτων και της μεθόδου ΜΔ αναλύονται οι μηχανισμοί που οδηγούν στην φωτοαποδόμηση των μεταλλικών υλικών. Ο υπολογιστικός κώδικας παράλληλης επεξεργασίας που αναπτύχθηκε βασίσθηκε πλήρως στη δομή της μεθοδολογίας. Το σημαντικότερο συμπέρασμα που προκύπτει από την συγκεκριμένη διατριβή είναι ότι το βάθος φωτοαποδόμησης καθώς και ο μηχανισμός αυτής εξαρτώνται κυρίως από την πυκνότητα ενέργειας του υπερβραχέου παλμού (J/cm2) που επιδρά στο μεταλλικό υλικό. Τα διάφορα χαρακτηριστικά της διεργασίας μπορούν να προβλεφθούν και να χρησιμοποιηθούν για τον αποδοτικότερο προγραμματισμό της. / The objective of the present thesis is the experimental and theoretical investigation, using Molecular Dynamics, of the mechanisms leading to ablation of metallic materials due to Laser radiation. Laser micro machining is an emerging technology capable of producing parts in the micro and submicron scale. For such application Lasers with pulse duration in the femptosecond range are widely used. A phenomenon called “Laser ablation” is involved in the Laser micromachining. Laser Ablation is the process of material removal after the irradiation of a Laser beam onto the material and causes a combination of sublimation, vaporization and melting. It is commonly characterized by small temporal and spatial scales and extremely high material temperature and pressure. Molecular Dynamics (MD) is a deterministic simulation method. It is based on the solution of Newton’s second law and aims on the monitoring of the movements of an atom within a system. The present work has employed MD models for describing the characteristics and output of the process, i.e. the crystal structure of the metallic material prior to Laser irradiation, the temporal and spatial distribution of the photons produced by the Laser beam and the behavior of the material particles after irradiation. These models when coupled allow the estimation of the ablation depth caused in a metallic material when irradiated with femptosecond Laser pulses, produced temperatures field and the temporal evolution of temperature within the material. Experimental results affirm theoretical MD results and drive to the illustration of the ablation mechanisms. The computational code developed uses parallel processing techniques and is based on the structure of the developed methodology. The main conclusion of this work is that the ablation depth, as well as its mechanisms, are strongly depended to the Laser fluence (J/cm2) of the femptosecond pulse. Process characteristics can be predicted and used for a more efficient process programming.

This thesis describes a study into the optical properties of nano-structured metallic films. Structures are produced by electrochemically depositing metal through a self-assembled template of polymer micro-spheres. This versatile technique allows nano-structured surface made from almost any metal to be produced quickly and cheaply. Geometries ranging from array of shallow dishes, to sharp metallic spikes and encapsulated spherical cavities can all be produced on the same sample. This thesis presents an in-depth study into the properties delocalised and localised surface plasmon polaritons. These plasmons can be tuned in energy by controlling the sample geometry and angle of the incident light. The coupling between these two types of plasmon is also investigated and theories are put forward to understand the observed results. These findings could prove useful in the design of plasmon guiding and computing devices. With an understanding into the plasmonic properties of the metallic nanostructures, research is undertaken to explore how the associate local electric field couples to molecules adsorbed onto a samples surface. A strong correlation between surface plasmons and enhanced Raman scattering is found, leading the observation of the beaming of the Raman scattered light. The nano-structured substrates are also shown to have excellent reproducibility as well as enhancement of the Raman signals, leading to applications such as high sensitivity molecular sensors. Finally, the interaction between organic semiconductor molecules and surface plasmons is explored. A strong interaction between the different states is found and plasmon enhanced fluorescence is also observed. These studies open the way for greater control over the exciton states, which have potential for the use in novel laser systems.

Dans l’industrie de la mise en forme des matériaux métalliques, le frottement joue un rôle très important. Cependant, il est difficile à contrôler totalement, notamment dans les procédés complexes où le taux de nouvelles surfaces générées lors de la déformation plastique est important. La thèse propose d’identifier par analyse inverse les propriétés tribologiques du matériau, directement à partir de la courbe donnant la force en fonction du déplacement pour un procédé d'extrusion directe. La conception d’une filière d’extrusion, avec des dimensions optimales pour maximiser l’influence du frottement a été faite à l’aide de calculs analytiques en se basant sur des critères particuliers tels que: la capacité maximale de la presse, la maximisation de la longueur de frottement, le rapport entre la puissance de frottement et la puissance de déformation et la limite du taux de réduction pour éviter l’endommagement du matériau. Comme dans le procédé d’extrusion il existe un certain couplage entre l’influence des paramètres rhéologiques et des paramètres tribologiques, des tests de compression d’éprouvettes «haltères» ont été réalisés dans une première étape afin d’identifier la loi de comportement rhéologique du matériau. La conception du test proposé a été validée par des simulations numériques avec FORGE2®. Les simulations numériques ont été également effectuées pour analyser la sensibilité des paramètres rhéologiques et tribologiques du matériau sur la force d’extrusion. L’influence de la géométrie de la filière sur la courbe de force a été examinée en détail. Une campagne d’essais a été réalisée pour l’alliage d’aluminium AA5083. La loi de comportement du matériau a été en conséquence identifiée par analyse inverse à partir de la courbe expérimentale donnant la force de compression des éprouvettes «haltères». Dans une deuxième étape, les propriétés tribologiques du matériau pour différents modèles de frottement ont été ensuite identifiées par analyse inverse à partir de la courbe donnant la force d’extrusion. La méthode d’identification par analyse inverse a été enfin validée par la simulation d’un procédé industriel d’extrusion. La campagne d’études expérimentales a été complétée par la caractérisation du matériau par des mesures de dureté, des mesures de diffraction des rayons X et des mesures de diffraction des électrons rétrodiffusés (EBSD) afin de mettre en évidence l’évolution de la surface des pièces après la déformation plastique. Les résultats indiquent que le frottement a une influence importante sur l’évolution des orientations cristallines de la surface du matériau extrudé. / In the industry of metal forming, friction plays a very important role. However, it is difficult to control completely this phenomenon, especially in the complex processes where the rate of the new surface generated during the plastic deformation is important. The thesis proposes to identify by inverse analysis the tribological properties of material, directly starting from the curve of the load versus the displacement of a direct extrusion process. The design of an extrusion die, with optimal dimensions to maximize the influence of friction, was carried out using analytical calculations based on particular criteria such as: the maximum capacity of the press, the maximization of the friction length and of the ratio between the friction power and the deformation power, the limit of the reduction rate to avoid damage of the material. Since in the extrusion process there is a certain coupling between the influence of the rheological parameters and the tribological parameters, compression tests using "dumbbells" specimens were carried out in a first stage in order to identify the rheological behavior law of the material. The design of the proposed test has been validated by numerical simulations using FORGE2®. The numerical simulations were also performed in order to analyze the sensitivity of rheological and tribological parameters of the material on the extrusion load. The influence of the geometry of the die on the extrusion load curve has been examined in detail. A campaign of tests was carried out for the AA5083 aluminum alloy. The behavior law of the material was consequently identified by inverse analysis starting from the experimental load curve of compression tests using "dumbbells" specimens. In a second step, the tribological properties of the material for different friction models were then identified by inverse analysis starting from the extrusion load curve. The method of identification by inverse analysis was finally validated by the simulation of an industrial extrusion process. The campaign of experimental studies has been completed by the characterization of the material by measurements of hardness, X-ray diffraction measurements and electron backscattered diffraction (EBSD) measurements in order to highlight the evolution of the surface of the extruded specimen after the plastic deformation. The results indicate that friction has an important influence on the evolution of crystal orientations of the surface of the extruded material.

Anionic chromate (VI) compounds are inhibitive pigments and have been effectively incorporated into organic coatings to protect metal surfaces from aggressive ions, but their risk as a human carcinogen and being harmful to the environment has led to the search of suitable alternatives. Aluminium alloy, AA2024-T3, is the substrate metal alloy used in the experiments and can be found in aircraft fuselage structures due to their high strength-to-weight ratio. However, the presence of intermetallic particles increases susceptibility to localised corrosion. To investigate the protection mechanisms of primers on light alloys, many different factors must be taken into account; from aluminium alloy corrosion processes, the effects of intermetallic additions to coating chemistry, morphology and inhibitive pigments. The chemical environment in which the samples are tested in will also affect the corrosion mechanisms of the alloy as well as the performance of the coatings and release of pigments. It will be important to consider which factors are operating under particular conditions so that experimental results can then be best interpreted. As part of this project, potentiodynamic polarisation, electrochemical impedance spectroscopy and electrochemical noise analysis have been used to investigate the protective mechanisms in which chromate-based paints protect against corrosion and UV-Visible spectroscopy, scanning acoustic microscopy and optical microscopy have been used to investigate pigment release mechanism to identify what characteristics are important when developing new primers.

Spray forming with eo-injection of a solid particulate phase to form a metal-metal composite has been studied as a new route for manufacture. Two Al-based matrices were investigated: AI-12Si for testing the feasibility of the new manufacturing route and Al-4Cu for providing better mechanical performance. For both composite types, Ti was chosen as the particulate phase and the processing-microstructure-property relationships then studied. At Peak Werkstoff GmbH, Germany 12 wt%Ti particles were eo-injected into an atomised Al alloy droplet spray and eo-deposited to form a rv300 kg billet. The microstructure comprised refined equiaxed a-AI grains (rv5fLm), spherical Si particles (rv5fLm) and uniformly distributed Ti particles (rv80fLm). Sections of the billet were extruded under a range of conditions into long strips 20mm wide and 6mm, 2.5mm and 1mm thickness. At high strains, the Ti particles were deformed into continuous fibres of a few microns in thickness. Accumulative roll bonding was then performed to higher total strains, while maintaining a constant cross-section, reducing the Ti fibres to sub-micron thickness. The fibres were studied by extraction after selective dissolution of the a-AI matrix. There was no interfacial reaction between a-AI and Ti or any measurable oxide formation, thus providing encouragement for the manufacture of metal-metal composites by eo-spray forming. A powder injection pump was successfully integrated and commissioned on the spray forming facility at Oxford University. The pump was calibrated to optimise powder flow rates. Three AI-4Cu+ Ti composite billets were processed with each containing Ti powder with a different processing history. Up to 20vol%Ti was successfully incorporated, however due to the cooling effect from powder injection, porosity was significant. The quenching effect provided a finer AI-4Cu grain structure in the region of Ti injection, and also promoted precipitation of O'-AbCu precipitates. A Ti/ Al-4Cu interfacial reaction was more prominent in the billet spray formed at 850°C than those spray formed at 750°C. Angular Ti processed by a hydride-dehydride route had better deformation characteristics than spherical gas atomised Ti. Deformation processing by extrusion and rolling was investigated for Al-4Cu+20vol%Ti using SEM, EBSD and FIB. After extrusion to a strain of 5, the composite contained elongated reinforcing fibres characteristic of metal-metal composites. The microstructure studied by EBSD revealed equiaxed polygonal Al-4Cu matrix grains. Rolling was not as efficient as extrusion in producing elongated Ti fibres and was attributed to a lower deformation processing temperature. The rolled composites consisted of elongated Al-4Cu grains 1-5J1m in thickness. An UTS of 339MPa at a strain of 3 was attributed to texture strengthening in the Q- AI.