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.

Hot isostatic pressing (HIP) is of interest to the aerospace industry due to the ability to produce polycrystalline components free from defects, which typically exhibit comparable or superior properties to their cast counterparts. The capability also exists with powder HIP to produce large parts with complex designs to near netshape (NNSHIP) in a single processing step, which can potentially reduce the buy-to-fly ratio ofhigh temperature gas turbine engine components. Such components, however, are manufactured from Ni-superalloys and certain issues exist with the HIP ofNi-superalloys that require addressing. The research presented in this thesis investigates HIP of a Ni-superalloy CM247LC to assess the viability of the process and the alloy for high temperature turbine and combustor casing components of the future. The influence of powder characteristics, post-HIP heat treatment and modification of the HIP procedure, have been investigated to assess the effects on high temperature properties. HIPped CM247LC can significantly outperform cast CM247LC in terms of hot tensile properties (particularly ductility), suggesting NNSHIP is potentially viable. In the current work, however, poor creep resistance was exhibited which is a concern for high temperature components.

High strength, low alloy, hot rolled strip steels with yield stresses in the range 700 to 1300MPa are required for the Lifting and Excavating product sector. Improved combination of strength and toughness in these high strength steels is desired, requiring a detailed understanding of the relationship between microstructure and mechanical properties. In this work 12mm thick 700MPa yield stress strip steels with fully bainitic microstructures, with different compositions and/or processing conditions, have been studied. The microstructures of the steels were investigated with both optical microscopy and SEM. Micro-hardness and Charpy impact tests (at different temperatures) were carried out to investigate the mechanical properties, followed up with fracture surface analysis and unit crack path (UCP) analysis. Three types of bainitic microstructures were identified and quantified, including upper bainite, lower bainite and granular bainite. The fracture surface and UCP analysis indicating that granular bainite is detrimental to toughness, but cannot necessarily be avoided in the steels investigated; while the presence of a small amount of lower bainite (above 5%) was found to improve the impact toughness significantly. Continuous cooling transformation (CCT) diagrams for steels with different compositions were produced, which allowed investigation of the phase transformation behaviour and selection of optimum chemical composition/coiling temperature for improved properties. The effects of alloying elements (B, Mo and V) on the transformation behaviour have been confirmed through the experiments. An optimised combination of alloying and coiling temperature has been proposed and validated via investigating plant trial products with similar alloying and processing parameters.

Two additive processing scenarios were considered covering manufacturing and repair applications. One scenario required evaluation of three processes for the addition of shaped blocks to a casing. The other involved infilling a shallow hemispherical indent with one of three polycrystalline alloys (alloy 718, waspaloy™ and RR1000) using different deposition strategies. Processinduced discontinuities were characterised and controlled using a design of experiments approach. As distinct from welding, process variables of overlap, Toolpath, bead shape,sequencing and incremental height were established as influential variables Toolpath in particular can give markedly different textures and grain alignment which would be expected to influence mechanical anisotropy. Fine pool processes seem less prone to cracking and more likely to yield fine microstructures.