High frequency ultrasound is receiving increased attention for medical imaging in areas such as ophthalmology and dermatology. Recent advances in the manufacture of fine scale piezocomposite materials mean there is great potential for commercial transducers far superior to conventional devices currently in clinical use. This Thesis reports the fabrication and characterisation of passive materials suitable for use in high frequency piezocomposite transducer devices. Epoxy composites have been fabricated using tungsten and alumina as filler material with volume fractions up to 0.4. Acoustic impedance and attenuation has been determined for different filler volume fractions to provide data for modelling to aid transducer design and also determine the filler volume fraction required to provide optimal results. Acoustic impedance values of 3-15MRayl were measured for the materials made in this work and the influence of filler particle size and shape is also discussed. Piezocomposite transducers have been constructed using material developed in this work and compared to devices made with more conventional passive materials. In addition to the fabrication of the composite samples for characterisation a process for incorporating material into transducers is described showing how the fabrication can be a part of the transducer construction resulting in an efficient and neat package.

The transition toughness of low alloy steels used in pressure vessels is of key importance to establishing the safe operation of a number of structures; of these, the integrity of a nuclear reactor vessel is of greatest concern. The through life toughness of such vessels is a combination of the start of life properties and irradiation damage response of the material. Modelling of the inherent scatter of toughness measurements has received much academic interest since the mid-twentieth century and is found to be dependent on a number of metallurgical factors and failure modes; therefore, the micro-mechanisms of the ductile and brittle failure are explored and an assessment of the current best thinking on the modelling of crack arrest toughness is also considered. It has been established in this work that a highly accurate representation of a large toughness database can be achieved by the inclusion of constraint loss effects and the interaction between initiation and arrest toughness distributions.

This present study is concerned with developing a new alloy system which is capable of forming a metallic glass on rapid solidification of the melt, rather than modifying a known glass forming composition, and assessing its glass forming ability. Iron (Fe) was chosen as the solvent element because it is significantly cheaper than the base elements found in some other metallic glasses and does not require the addition of large quantities of expensive alloying elements to enable vitrification. A ternary system using carbon (C) and boron (B) was studied initially as these metalloids are known to aid glass formation in other systems. Manganese and molybdenum were selected as secondary alloying additions in order to determine if they would have an effect on the Fe-C-B alloy with the best glass forming ability. A combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffractometry and secondary ion mass spectroscopy was used to investigate the microstructure of as-cast and rapidly solidified alloys. Differential scanning calorimetry (DSC) was used to investigate the thermal behaviour of the alloys. The ability of the iron-based alloys to form a glass on rapid solidification from the melt could not be predicted by observation of the as-cast microstructure or through computational methods. It was found that vitrification of the ternary system was only possible for compositions which were close to a eutectic point and that stabilisation of the supercooled liquid was caused by competition for nucleation between austenite and metastable phases, rather than between primary equilibrium solidification products. Of the ternary compositions where an amorphous phase was produced it was concluded that Fe₈₀.₉C₅B₁₄.₁ had the best glass forming ability (GFA). It was determined that the addition of manganese and/or molybdenum to the base composition generally had the effect of improving the GFA through the increased complexity of the system making it more difficult for recrystallisation to occur. Of the multi-component alloys it was concluded that Fe₆₀.₉Mn₁₀Mo₁₀C₅B₁₄.₁ had the best GFA as it had the highest values for each of the parameters used to describe GFA. It is believed that this is due to competition between the austenite and alpha stabilisers (manganese and molybdenum respectively) causing enhanced stability of the supercooled liquid.

Laboratory and synchrotron X-ray fluorescence (XRF) have been used to investigate the microscopic and macroscopic distribution of metallic contaminants in membrane electrode assemblies (MEAs) which were used in proton exchange membrane fuel cell (PEM FC) stacks. The laboratory XRF results were consistent with the synchrotron XRF results. Higher levels of contaminants observed for the areas near to the coolant outlet than the areas near to the coolant inlet. The cathode side of MEAs showed higher levels of contamination than the anode side of the MEAs. Synchrotron XRF maps of MEAs generally showed higher levels of contaminants on the cathode side compared with the anode side. Fe was mainly observed in the cathode side microporous layers, whereas Ni, Cr and Cu were mostly accumulated in the cathode side or in the membrane. Synchrotron XRF maps of MEA plan views showed a crack-like distribution for Fe and Pt which were similar to cracks in the microporous layer of the MEAs. A novel electrochemical cell that simulated galvanic and crevice corrosion, temperature cycles for a PEM fuel cell, and pressure across the stacks was designed and used to discriminate between the corrosion behaviour of candidate coatings for bipolar plates.

The solidification behaviour of Ti45Al2Mn2Nb1B (at.%) has been studied together with its response to HIPing (Hot Isostatic Pressing) in order to understand the mechanism of grain refinement in castings and to understand the influence of surface changes occurring during HIPping on the properties of HIPped net shape cast turbine blades. Samples which had been rapidly cooled from near the melting point from a Bridgman furnace, where a thermal gradient was imposed, have been used to understand the grain refinement mechanism and the details of the solidification sequence. In addition the structure of powder samples, which have been gas-atomised and hence very rapidly cooled have also been used to further the understanding of solidification and of the role of borides. It has been shown that borides themselves play an important role in grain refinement. It has been shown that HIPping results in the formation of a surface which is caused by oxidation from the oxygen present in the argon used in the HIP. The details of the chemistry and microstructure of the surface layers have been shown to be influenced by oxygen partial pressure, by HIPping time and HIPping temperature. Conventional HIPping conditions lead to a surface which contains a γ-layer which does not appear to downgrade either the tensile properties or fatigue properties of the samples and may slightly improve the corrosion resistance. Further work is required to produce net shape castings, which have properties comparable with conventionally cast Ti4522XD, but the present work shows that these could then be HIPped without the γ-layer contained surface causing any downgrading in properties.

Type 347L and Type 309L stainless steels used as weld cladding materials of nuclear reactor pressure vessels were investigated under realistic conditions of fabrication and service. Electron backscatter diffraction and metallographic observations were used in combination with double loop electrochemical potentiokinetic reactivation (DLEPR) test and ASTM-262-A test to determine the precipitation behaviour and its influence on intergranular corrosion (IGC) resistance as a function of post welding heat treatment (PWHT) time at 600 oC and after simulated service ageing at 425 °C. It was shown that during the first stage of PWHT, M23C6 carbides precipitate in both alloys as a result of δ ferrite decomposition. This increased sensitization of the materials is due to the creation of a chromium-depleted zone, which was found to be replenished after 40 h of treatment, recovering the resistance of the materials to IGC. After combined treatment of PWHT + simulated service, 309L was shown to be re-sensitized whereas 347L remained resistant. Sigma phase was found to precipitate during the last stages of PWHT with a higher tendency on 309L. Two characteristic reactivation potentials were found for both materials. A specific split-cycle DLEPR was applied to show that this technique can be further developed to identify deleterious phases in austenitic stainless steel welds. Micro hardness and Charpy impact tests showed an increase in hardness and a decrease in absorbed energy of materials without PWHT after ageing at 425 oC.

This thesis is concerned with the porosity formation mechanism during electron beam welding of titanium-based alloys. During the welding of titanium alloys for aerospace engine applications, porosity is occasionally found in the solidified welds. Hence the key factors responsible for porosity formation need to be identified, and guidance to minimise porosity occurrence needs to be provided. The research conducted in this work is twofold. First, porosity formed in electron beam welded titanium samples is characterised to rationalise the porosity formation mechanism. Second, models based on sound physical principles are built to aid understanding of porosity formation, and to provide predictive capability. Porosity formed in electron beam welds is characterised using metallographic sectioning, high resolution X-ray tomography, residual gas analysis (RGA), scanning electron microscopy (SEM) and energy and wavelength dispersive spectroscopy (EDS/WDS) analysis. The results confirm porosity formed in electron beam welded titanium-based alloys is associated with gas dynamics; hydrogen is very likely to be responsible for porosity formation. A coupled thermodynamic/kinetic model is proposed to study the hydrogen migration behaviour during electron beam welding process, and then the effect of hydrogen on bubble formation is investigated via quantitative modelling, backed up by targetted experimentation

Low temperature plasma surface engineering has been a useful method for increasing the hardness and wear resistance of austenitic stainless steel without reducing the corrosion resistance of this alloy. Plasma carburising is of particular interest as it produces thicker hardened layers than plasma nitriding, and an equivalent improvement in the tribological and corrosion performance of the base material. In this project, the active screen (AS) plasma technique was used to carburise austenitic stainless steel AISI 316 and the obtained layer of carbon expanded austenite was compared with the one produced by conventional DC plasma treatments. The hardening and wear resistance produced by AS and DC plasma carburising were equivalent. With regard to corrosion, the AS treated material performed better than its DC counterpart as a consequence of the improved surface quality of the former. The mechanism of AS carburising was comparatively studied with its AS nitriding counterpart. Different experimental arrangements and two plasma diagnostic techniques were used for this purpose: optical emission spectroscopy and electrostatic probes. The evidence shows that AS nitriding relies on the deposition of iron nitrides and the active species in the plasma to produce hardening, whilst AS carburising requires the plasma activation and moderate ion bombardment.

The compaction, sintering and heat treatment processing conditions for the commercial powder, Ecka Alumix 231 with the nominal composition of Al-15Si-2.5Cu-0.5Mg(wt%) have been optimized in this study. It has been found that densification of Ecka Alumix 231 depended largely on the sintering atmosphere and the amount of liquid phase presented. Sintering in nitrogen atmosphere was found to be beneficial due to the formation of AIN which later induced pore filling effect. Presence of hydrogen in the sintering atmosphere, however, was not desirable. Decomposition of MgH2 at higher temperature was suggested to increase the pore pressure and caused detrimental effect on compact densification. The effects of alloy additions (i.e. Cu and/or Ni) on sintering and heat treatment response were also investigated in this study. Addition of copper was found to enhance the sintering response by a significant increase in the sintered density,reduced the peak ageing temperatures and time. Nickel addition, however, was detrimental to both sintering and heat treatment response since it reduced the amount of copper content dissolved in the α-aluminum due to the formation of Al\(_3\)(Ni,Cu)\(_2\) phase. The precipitation sequence for Al-Si-Cu-Mg-(Ni) was suggested to be: Supersaturated solid solution (SSS)→GP zones → θ” → θ’ → θ.

An investigation into the fundamentals of the deposition of silicon carbide within porous silicon carbide fibre preforms using microwave-enhanced chemical vapour infiltration has been carried out. The study of the kinetics of deposition revealed an Arrhenius behaviour of the matrix growth rate against the temperature in the range 800-1000°C and a linear dependence on the pressure in the range 20 - 70 kPa. This is typical of a surface-reaction limited regime. The morphology of the SiC deposited changed with both temperature and pressure. Increases in both lead to a transition from a smooth, globular deposit morphology to something that was rougher and more angular; this corresponded to the transition from a nucleation to a growth regime. Stoichiometric SiC was predominantly found in the central region of the samples infiltrated at 1000°C, but the deposit became more silicon-rich (up to 2.6 at %) the farther from the initial deposit. Dielectric properties showed that ZMI Tyranno silicon carbide fibres readily absorbed microwave energy. In specific conditions of temperatures and pressures, 900-950°C and 50 kPa, an inside-out deposition pattern was observed indicating a temperature gradient across the preform. Deposition of silicon carbide and silicon caused the gradual flattening of the temperature gradient.