The present investigation, which forms part of a general programme of research into the effects of departures from the stoichiometric composition on the physical and mechanical properties of ceramic compounds, is concerned with the defect structure of non-stoichiometric rutile. The physical properties of rutile, with particular reference to the effects of departure from the stoichiometric composition, have been briefly reviewed. The development and design of an apparatus, incorporating a sensitive microbalance, capable of measuring departures from the stoichiometric composition in rutile at low pressures of oxygen and at temperatures up to a maximum of 1300\(^o\)C is described in detail. The defect structure of non-stoichiometric rutile has been shown to alter as departure from the ideal composition increases. It is concluded that at small departures from the stoichiometric composition free anion vacancies are present, whereas at large departures the vacancies cluster on planes of the {100}, {101} and {110} types, which subsequently collapse by displacement of ½<101> forming interstitial cations. The departures from stoichiometric composition which have been observed in high-purity rutile are very small - much smaller than those previously reported in the literature. Larger departures found to occur in impure rutile can be attributed directly to electronic effects associated with impurity atoms. Much further work of a systematic and long-term nature will have to be carried out before the role of impurity atoms on the defect structure of non-stoichiometric rutile is understood.

As-cast Mg\(_9\)\(_4\)Zn\(_2\)Y\(_4\) alloy has been subjected to compression and equal channel angular pressing (ECAP) separately. The as-cast alloy contains mainly a long-period stacking ordered (LPSO) phase and a Mg\(_2\)\(_4\)Y\(_5\) phase as secondary phases. During compression, kinking occurs in the LPSO phase and LPSO/Mg mixture. Most kink boundaries of LPSO are composed of basal < a > type dislocations. The rotation axes of the kink boundaries in LPSO/Mg are preferentially located in the (0001) plane. ECAP processing develops a bimodal microstructure consisting of large deformed grains and sub-micron dynamically recrystallised (DRXed) grains. The DRXed grains are mainly located along the original grain boundaries. Kink boundaries also acts as DRX nucleation sites. The ECAP processing increased significantly the strength of the alloy. In the as-cast Mg-Zn-Y alloys, the main secondary phase changes when different ratios of Zn/Y are applied: LPSO (Zn/Y ratio is 0.5) → LPSO+W (Zn/Y ratio is 1, W is Mg\(_3\)Zn\(_3\)Y\(_2\)) → W (Zn/Y ratio is 2.33). When Y is half replaced by Gd, the types of phases are similar. When Y is replaced fully by Gd, W phase becomes the main secondary phase. The structure of the LPSO also changes with different Zn/Y ratios and the presence of Y or Gd.

Semi-crystalline polyhydroxybutyrate-co-hydroxyvalerate samples were stored at a range of temperatures between 22 and 150 °C, for up to 672 hours. The secondary crystallisation process was monitored using differential scanning calorimetry (DSC) and infrared spectroscopy, and crystallinity was observed to increase over time at storage temperatures of 20 °C and upwards. Higher storage temperatures produced a greater extent of change within the timescales of this work, indicating a rate effect of temperature on secondary crystallisation. DSC studies also revealed an increase in the melting temperature (Tm), and dynamic mechanical thermal analysis revealed an increase in the glass transition temperature (Tg), in accordance with increasing crystallinity. The corresponding effects on the mechanical properties were analysed using tensile testing. Generally, it was found that with increases in crystallinity there was an increase in the ultimate tensile strength, a decrease in the elongation to break, and therefore an increase in the Young’s Modulus. The increasing embrittlement of samples was most evident during storage at 150 °C, where subsequent decreases in crystallinity, Tm, Tg, mass, molecular weight (Mw), and sample discolouration over time were evidence of an interplay between the secondary crystallisation process and the thermal degradation process, active below the Tm.

Friction Stir Welding (FSW) is a suitable technology for aerospace structure development and is a possible replacement for mechanical fastening. To achieve the application of FSW, pre and post-weld treatments are required. A pre-weld treatment of surface preparation was required to ensure a weld with good mechanical properties. The surface preparation necessary is the removal of paint and anodising layers from aerospace alloys. Laser paint removal was assessed and designed to remove these layers with only an oxide remaining, which welded to produce high quality welds. The post-weld treatment was essential to increase the corrosion resistance of the welded area. The improvement in corrosion resistance was achieved with High Power Beam Surface Modification (HPBSM), which created a homogeneous surface through rapid surface melting and solidification. The rapid thermal processing dissolved and dispersed the precipitate solute atoms which were retained in solid solution through planar solidification. Electron beams and various lasers were used in the HPBSM processing. Excimer Laser Surface Melting (LSM) improved corrosion resistance but the layer depth was restricted by processing parameters. HPBSM processing studies using an electron beam, USP-CO\(_2\) laser and Nd:YAG laser provided understanding on how processing parameters controlled the modified layer characteristics.

The effect of cold-rolling prior to shock loading was investigated in copper and tantalum. Annealed copper was shocked at a peak pressure of 5.08GPa; cold-rolled copper was shocked at peak pressures of 5.87GPa, 5.96GPa and 9.60GPa; as-received tantalum was shock loaded at a peak pressure of 7.20GPa, and cold-rolled tantalum was shocked at a peak pressure of 7.20GPa. The microstructures of the materials were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the mechanical responses were investigated using compression and hardness testing. The effect of varying temperature and strain rate on tantalum during compression was investigated. Tantalum was compressed at 20°C at 10⁻³s⁻¹, 10⁻¹s⁻¹ and 2x10³s⁻¹, and at 10⁻¹s⁻¹ at -40°C and 170°C. Quasi-static compression tests applied 70% strain to the samples and the higher strain rate sample, compressed by Split Hopkinson Pressure Bar (SHPB), was compressed to 19% strain. The microstructures of the materials were investigated using (SEM) and (TEM), and the mechanical responses were investigated using hardness and compression testing. The microstructures of adiabatic shear bands (ASBs) produced by firing a shaped projectile from a single stage gas gun to cause the collapse of a thick-walled cylinder (TWC). The propagation of the ASBs along the cylindrical axis of the TWC was Also investigated. The microstructure was investigated using (SEM), (TEM) and scanning transmission electron microscopy (STEM).

An investigation has been carried out into the mechanisms of failure of silica bonded investment casting moulds, with the aim of reducing failure rates for larger components. The analysis begins with a detailed microstructural examination of a current commercial mould system using scanning electron microscopy and other allied analysis techniques. The mould structure is shown to be nonuniform and unpredictable, with poor packing of the ceramic constituents leading to a non-uniform porosity network. The structure and distribution of the binder is also established and this indicates that the major load bearing points within the mould consist of thin areas of silica. As such, the overall performance of the mould is directly related to that of the silica itself. This binder is shown to contain impurity elements leached from the ceramic filler at various stages during mould manufacture. These elements alter the phase composition and thermal properties of the binder. Mould temperature profiles for a range of casting sizes have been measured and used to illustrate the relationship between metal weight and maximum temperature attained within the mould section. The profiles are also used to investigate the creep response of silica based binders. It is found that a combination of high temperatures and creep within the silica phase is responsible for the casting defects found with larger components. From these observations, a prediction is made regarding failure mechanisms and suggested remedial action. The thermal properties of several commercial water based binders are evaluated and the suitability of each as an alternative binder is discussed.

A knowledge of the flow occuring in metal-forming processes is of great industrial importance, and the finite-element technique is the only form of deformation analysis which can predict the flow of the material. The examination of forging operations requires a full elasticplastic treatment to be used. This thesis is concerned with an elastic-plastic, finite-element program which has been developed to investigate three-dimensional examples of this process. The fundamental theory of the finite-element method is first introduced, and then the finite-element program is described in detail. The deformation, and distributions of hardness and die-interface pressure, predicted by this technique for the unlubricated upsetting of a rectangular block are compared with experimental results, and found to be in broad agreement, the differences being attributed to the incorrect imposition of very high friction by the friction-layer technique used in the analysis. With a corrected form of the friction technique, the finiteelement program predicts results for the axisymmetric friction-ring test and a new three-dimensional friction test which are in good agreement with experimental findings up to deformations of approximately 30%; the friction-layer technique used successfully in previous axisymmetric treatments appears to be unsuitable for threedimensional formulations when large deformations are considered. The finite-element program developed here is shown to be capable of modelling an example of a more complicated three-dimensional forging, that of an automobile connnecting rod. The experimental measurements of cteformation and hardness for an aluminium con rod forged using graphite lubrication are found to be in good agreement with the finite-element predictions obtained assuming sticking friction conditions but not with the results of a zero-friction idealisation. Finally, suggestions are made for the future development of this technique.

The enclosed body of work represents a compilation of published papers coveringwork carried out in the Universities of Birmingham, Bath and Southampton, UK, in theSecond University of Rome “Tor Vergata”, Italy, in the Nanoelectronics Research Institute (former Electrotechnical Laboratory) of AIST Tsukuba, Japan, and in the National Institute of Materials Physics (former Institute of Physics and Technology of Materials), Bucharest, Romania, during my research career over more than twenty years. The thesis comprises mainly research work on science and technology of high temperature superconducting cuprates of various compositions (Y-based, Bi-based, Tl-based, Hg-based, (Cu,C)-based) and morphologies (polycrystalline, single crystals, thin films, tapes, artificial superconducting superlattices). The major theme common to most of the research done on the abovementioned materials is the study of vortex matter, dynamics, and pinning, which determine a large number of superconducting properties for various applications. The submitted work presents the results of experimental and fundamental studies of superconducting materials in six main areas. These comprise: (i) synthesis and characterization (current-voltage characteristics, dissipation mechanisms, interaction between inter-and intra-grain vortices) of Y- and Bi-based superconducting ceramics; (ii) experimental and theoretical studies of current-induced unbinding of thermally-created vortex-antivortex pairs; (iii) fabrication and characterization of artificial superconducting superlattices; (iv) fabrication and characterization of anomalous superconductors, two- and multi-component superconductors, and exotic vortex matter; (v) study of vortex matter and dynamics by Scanning Hall Probe Microscopy; and (vi) science and technology of vortex pinning, including through self-assembling nanotechnology of pinning centres. There are a small number of publications that do not fall in the above-mentioned areas, but they are also in the larger field of science and technology of superconducting materials. The papers are presented essentially chronologically, with no attempt to group them into separate research areas, since the common philosophy and approach to the investigation of various superconducting materials is to determine how vortex matter, dynamics and pinning influence their physical properties.

The efficiency of steam reforming depends strongly on the creep resistance of the material used for the reformer tubes. The currently most widely used reformer tube material is HP40 (25 Cr, 35Ni, 40 Fe and 0.4C) austenitic stainless steel. A further improvement in the creep resistance of HP40 is needed for efficiency improvement and for a cost reduction in steam reforming. In order to develop a next generation creep resistant alloy, three HP40-based alloys, namely Alloy A, B and C, with different chemical compositions and/or solidification rate, were studied. Previous tests at 1000 oC and 40 MPa have shown that the creep properties of Alloy C are slightly better than those of Alloy B, both being significantly better than Alloy A. The microstructures of three alloys, under as-cast, crept and heat treated conditions, have been analysed so as to understand their relative creep performance. The small intragranular M23C6 may have contributed significantly to the smaller creep rate, and thus a longer creep life for Alloy B and Alloy C as compared with Alloy A. A microstructure-based climb-glide bypass creep model was described to predict the creep behaviour of HP40. Suggestion on the next generation HP40 alloy has been made.

In recent years, the increasing demand for rare earth magnets for use in motors and machines has led to the need for more efficient use of NdFeB materials. Novel processing techniques have been developed to produce NdFeB materials with excellent magnetic properties without the need for heavy rare earth elements. Two specific novel processes including the Flash Spark Plasma Sintering (FSPS) process and the Hydrogen Ductilisation Process (HyDP) were studied in this project with the focus on the use of microstructural analysis and magnetic measurements to fully understand the mechanisms, hence develop and optimise the processing conditions for production of fully dense anisotropic magnets. The most optimal FSPS sample demonstrates a uniform, fine-grained structure with a high degree of crystallographic alignment, leading to a high coercivity (1438 kA m\(^−\)\(^1\)) and remanence (1.16 T) giving a BH\(_m\)\(_a\)\(_x\) of 230 kJ m\(^−\)\(^3\). While with appropriate s-HDDR and compression conditions, the HyDPed sample exhibits a typical s-HDDR microstructure with a submicron grain size. It is shown in the work that the disproportionated sample shows an enhanced ductility and thus can be shaped at room temperature, and a useful degree of anisotropy is produced within the HyDPed sample during the compression.