MgO thin films were deposited on Si (100) single crystals, NiW tapes, 310 austenitic stainless steel and Hastelloy C276 by the ultrasonic Spray Pyrolysis technique, using magnesium nitrate and magnesium acetate as precursors at many different conditions. Thermogravimetrical analysis (TGA) of the decomposition of the precursors was used as a guideline temperature for the thin film deposition. Biaxially textured and epitaxial MgO films were deposited on Si substrates using magnesium nitrate precursors. An amorphous MgO thin film was deposited on NiW tapes, NiO buffered NiW, 310-stainless steel and Hastelloy C276 when using low concentration of the magnesium nitrate precursor. Higher concentrations were needed to obtain (200) oriented MgO films on C276. However, NiW tapes and 310-stainless steel were found to be a non suitable substrate for MgO thin film deposition due to surface instability. A (200) oriented MgO thin film was grown on Hastelloy C276 using a magnesium acetate precursor at a much lower concentration compared to the nitrate precursor. The ISD deposition by spray pyrolysis (irrespective of the used precursor and the used substrate) produced MgO films that grew with the (200) normal to substrate surface as distinct to MgO films grown by thermal evaporation which grew with the (200) tilted to the substrate normal. Pulsed laser deposition (PLD) was used to deposit YBCO on MgO buffered substrates. The characterization of the thin films was done using SEM, EBSD, XPS, AFM, X-ray diffraction 2-scans, rocking curve (-scans), phi () scans, X-ray pole-figure measurements, and AC susceptibility. YBCO deposition by PLD on Si substrates buffered with MgO was not successful due to the thermal stresses developed in the MgO buffer. An EDX line scan performed on MgO films deposited on Hastelloy C276 from the acetate precursors confirmed the effectiveness of such layer as a diffusion barrier. It was found that c-axis oriented YBCO films were grown on c axis oriented MgO films deposited on C276 from the nitrate and acetate precursors; however, MgO and YBCO films have a very weak in-plane texture. The AC susceptibility measurements show that the YBCO films have a broad superconducting transition temperature irrespective of the precursor used. To improve the superconducting transition temperatures, different annealing treatments were performed on the MgO buffer deposited on Hastelloy C276, however, it was found that the as deposited MgO buffer films and subsequent deposition of YBCO films gave the best superconducting transition temperatures. In an attempt to improve the in-plane texture of YBCO, CeO2 was deposited by PLD on the spray pyrolysed MgO.

In this study Serratia sp. NCIMB 40259 and R. eutropha H16 were utilised for the accumulation of Polyhydroxybutyrate(PHB). Serratia sp. with high phosphatase activity (HPA)or low phosphatase activity (LPA) were fermented in order to accumulate intracellular PHB and biomineralise hydroxyapatite (HA) on the cell surface. The optimum conditions for the accumulation of PHB, in this study, were found to be after 216 h fermentation, producing a PHB yield of 14.6 %w/w and 3.3 %w/w for LPA and HPA respectively. The molecular and thermal properties of PHB were observed to be largely unaffected by the phosphatase activity of the cells and appeared to be dependent on the fermentation period. R. eutropha cells were fermented with the substrates olive oil and rapeseed oil and compared with glucose. The use of either olive oil or rapeseed oil as carbon sources resulted in the greatest accumulation of PHB. The thermal and molecular properties of the PHB samples were found to be almost identical. The use of oils in the production of PHB would make a viable replacement for the use of sugars as substrates. Electrospun fibres of PHB from glucose, olive oil and rapeseed oil were found to have diameters dependant on their solution concentrations. By adjusting the electrospinning parameters it may be possible to control the crystallinity and the diameters the PHB fibres.

Atmospheric corrosion was investigated using electrochemical and droplet studies. The effects of changes in bulk solution concentration and local pit chemistry on pit propagation and repassivation of 304L and 316L stainless steels were investigated using \(in\) \(situ\) synchrotron X-radiation and electrochemical techniques. Radiography and zig-zag electrochemical sweeps showed that in dilute chloride solutions, partial passivation was observed to initiate locally and propagate across the corroding surface. This caused repassivation gradually rather than as a uniform event. In concentrated chloride solutions, repassivation did not show a sudden drop in current but rather a gradual decrease as potential swept down. Pitting behaviour was also affected by solution concentration. Dilute solutions showed metastable pitting followed by a sharp breakdown (pitting) potential. Concentrated solutions however showed no metastability and a gradual increase in current when pitting. To determine the cause of current oscillations in 304L artificial pits in NaCl:NaNO\(_3\) solutions near the repassivation potential, the salt layers were scanned \(in\) \(situ\) using XRD. The salt layer was confirmed to be FeCl\(_2\).4H\(_2\)O and no nitrate salt was found. A mechanism was suggested to explain the current oscillations in terms of partial passivation being undercut by the advancing corrosion front.

1-3 Piezocomposites show significant improvements over conventional, monolithic transducers. However, a 10 MHz piezocomposite would require a pillar array structure with an aspect ratio of > 9 and a feature size of < 30 μm; which makes fabrication difficult and increasing the operating frequency challenging. Several processes have been developed to improve on the industry standard, dice and fill, but they remain laboratory based. Viscous polymer (VP) embossing has been demonstrated as a route to smaller feature sizes and higher aspect ratios, but the process is limited by the availability of suitable moulds, which must be replicated to make the process cost effective. This thesis reviews the micro replication and fabrication processes necessary to produce moulds for 1-3 piezocomposites, and characterises the replication process from hot embossing to a functioning 1-3 piezocomposite. It demonstrates that the Bosch process and electroforming can be combined to produce a master with a deeper array structure than has been previously demonstrated by these two processes. Piezocomposites have also been made which have pillar widths that are not possible by dice and fill, by filling hot embossed moulds that have aspect ratios of 14 and feature sizes of 30 μm.

Flat rolled Al alloys manufactured using the Direct Chill casting process are used in safety critical applications and therefore need to be of the highest possible quality. Liquid metal is cleaned using a mixture of inert Ar and reactive Cl gases before casting. The first section of this work explores the aetiology of defects in hot rolled Al plate, found using ultrasound. Inclusions extracted from 7xxx plate were examined using scanning electron microscopy. A second type of defect found in 7475 alloy plate was established to be a coarse grain structure. The second section aimed to provide a better understanding of the role of Cl on degassing. The effects of Cl on the degassing performance of a Spinning Nozzle Inert Floatation degassing unit were approached statistically using ALSCAN. Finally, 20 kg melts of Al and Al alloys were doped with H and subsequently degassed, and the reduction in H over time was measured using ALSPEKH. In the third section the LECO H determination device was used to degas Al specimens to very low levels. Degassed specimens were melted under air or N\(_2\) atmosphere, so that when cracks formed in the surface oxide layer either Al\(_2\)O\(_3\) or AlN formed. After subsequent exposure to pure H\(_2\) gas the specimens that formed AlN were found to have absorbed more H than those heated in air.

The directional solidification process is used to produce single crystal turbine blades for modern aero engines. The spacing of primary dendrites (PDAS) within the microstructure are an indication of the thermal gradient attained during solidification; process optimisation is being driven by the desire to refine the single crystal microstructure and maximise the thermal gradient which can reduce the propensity for defects, such as freckles, forming during solidification. Optimisation of the casting process also allows experimental materials to be investigated such as eutectic alloys, which require more stringent process conditions. A CFD-based method has been used in order to predict the thermal field within the ‘single shot’ Bridgman furnace and validated experimentally. The CFD model has been used to design an optimised heating element with the aim of increasing the thermal gradient achievable when casting single crystal alloys. Dendrite arm spacing has been measured from alloys cast using both heating element designs, with the concept experimental heating element showing a reduction in PDAS of up to 30% compared to the standard design for the same furnace conditions. The viability of casting eutectic alloys with such a furnace has also been assessed both experimentally and analytically using a CFD approach.

The present study is concerned with the single crystal growth of functional oxides by the floating zone technique using a four mirror image furnace. This thesis attempts to extend our knowledge of the subject by firstly studying the optical floating zone method as a technique in terms of the experimental growth parameters and then by performing crystal growth and characterization on a number of functional oxides such as rutile (TiO\(_2\)), Co-doped rutile and rare earth orthoferrite crystals (RFeO\(_3\), R=Er and Y). When growing high quality, large and homogeneous crystals using an image furnace, finding the optimum growth parameters (such as growth speed, rotation rate, gas pressure and type of atmosphere) is a crucial first step since it has been found that these parameters can all affect the eventual crystal quality and properties. Comprehensive characterization of an image furnace revealed that the growth behaviour, and consequently, the quality of the as-grown crystal, can be explained by the effects that varying these growth parameters have upon the temperature profiles within growing crystals. Characterization of as-grown crystals has been performed using a combination of different techniques such as scanning electron microscopy, energy dispersive x-ray analysis, x-ray diffractometry, back reflection x-ray Laue, polarised optical microscopy and vibrating sample magnetometry. The best quality TiO\(_2\) crystals were prepared when neither the growing crystal nor the feed rod were rotated during crystal growth. When rotation was employed, this was found to introduce sub-grain boundaries in crystals when the linear growth rate was low, or bubble inclusions when the linear rate was high. The use of lower molten zone temperatures was found to give pale-yellow crystals, while crystals grown using higher molten zone temperatures tended to have the blue colour characteristic of oxygen deficient material. A study of Co-doped TiO\(_2\) single crystals, prepared from the melt using the FZ technique, revealed that Co does not dissolve into the rutile matrix and that the type of atmosphere used during FZ affects the type of cobalt-based second phases formed. Magnetization measurements of Co-doped rutile bulk samples (both single crystals prepared using the FZ technique and polycrystalline powders prepared using a solid state reaction) also indicated that the magnetic properties depend upon the material preparation conditions. An oxygen deficient environment during the preparation of Co-doped TiO\(_2\) powders and single crystals was crucial for the observation of room temperature ferromagnetism, since preparation in oxygen rich conditions led to the formation of paramagnetic material that included the second phase CoTiO\(_3\). Finally, growth of ErFeO\(_3\) and YFeO\(_3\) single crystals by the FZ technique revealed that the best quality crystals were prepared when the growth rate was 6 mm/h or less, since the use of higher growth rates (12, 18 and 24 mm/h) was found to result in the formation of second phase inclusions within the crystals. The magnetic properties of these crystals, as represented by the hysteresis loops, showed that crystals of both materials grown at higher rates have significantly lower coercivities; this was attributed to the effect of the second phase in nucleating reverse magnetic domains and so aiding the demagnetization process.

Production of gas turbines for jet propulsion and power generation requires the manufacture of turbine blades from single crystal nickel-based superalloys, most typically using investment casting. During the necessary subsequent solution heat treatment, the formation of recrystallised grains can occur. The introduction of grain boundaries into a single crystal component is potentially detrimental to performance, and therefore manufacturing processes and/or component geometries should be designed to prevent their occurrence. If the boundaries have very low strength, they can degrade creep and fatigue properties. The root cause for recrystallisation is microscale plasticity caused by differential thermal contraction of metal, mould and core; when the plastic deformation is sufficiently large, recrystallisation takes place. In this thesis, numerical and thermo-mechanical modelling is carried out, with the aim of establishing computational methods by which recrystallisation during the heat treatment of single crystal nickel-based superalloys can be predicted and prevented prior to their occurrence. This work shows that recrystallisation is most likely to occur following 1.5-2.5% plastic strain is applied at temperatures between 1000oC and 1300oC. This emphasises that high temperature deformation is more damaging than low temperature deformation.

Weldable 13%Cr supermartensitic stainless steels are commonly used for subsea pipelines in the oil and gas industry. Although classified as corrosion resistant alloys, these steels can be susceptible to Sulphide Stress Cracking (SSC) when exposed to wet environments containing chlorides, carbon dioxide and low levels of hydrogen sulphide. Standard guidelines stipulate that laboratory SSC tests are performed at 24 °C and at the maximum design temperature, however some studies suggest that the risk of SSC could be greater at temperatures below 24 °C. Seabed temperatures can be as low as 5 °C, so in-service cracking could occur following shut-down conditions even if the material has been qualified at 24 °C. Four-point bend SSC tests performed at 5 °C and 24 °C in simulated seabed environments showed the material was more susceptible to SSC at 5 °C, but only when the as-received pipe surface was compromised. A supporting stress and strain investigation highlighted strain concentrations on the test surface which were coincident with the location of cracking observed in the SSC tests. Finite element simulations were used to demonstrate that tensile stress-strain data should be used over flexural bend data to load four-point bend specimens to the desired loading strain.

In the present study, the effect of hot isostatic pressing (HIPping) variables such as HIPping temperature, HIPping cycle and powder particle size, on the microstructure and mechanical properties of HIPped samples of two Ti alloys have been assessed. Powders of the most commonly used (α + β) alloy, Ti-6Al-4V and one specific beta alloy, Ti-25V-15Cr-2Al-0.2C wt% (burn resistant titanium alloy, BuRTi) were studied. The Ti-6Al-4V powder was made by the plasma rotating electrode process (PREP). BuRTi powders, which were made both by gas atomisation and by PREP were HIPped to investigate the influence of the initial structure of the powder on the microstructure and associated mechanical properties of the HIPped alloy. The PREP Ti-6Al-4V powder was shown to be fully martensitic in the as-atomised condition. The gas-atomised and PREP powders of BuRTi showed very different as-atomised structures, but in both cases the structure was, as expected single phase beta, with the carbon retained insolution. The individual particles of gas-atomised BuRTi powder were always polycrystalline, although there was a significant scatter in grain sizes within different particles. In contrast the individual particles in the PREP powder were either coarse grained polycrystals or single crystals. These differences led to significant differences in the microstructures and properties of HIPped samples. It was found that HIPping of Ti-6Al-4V samples resulted in the formation of equiaxed regions and lath-like microstructure. The small equiaxed regions are formed by recrystallisation which occurs at original particle boundaries where most of the deformation occurs during HIPping; the lath-like microstructure is formed by simply tempering the (less deformed) original alpha prime martensite within the central part of original particles. Among the three HIPping temperatures used, samples machined from powder HIPped at 930°C exhibited a better balance of properties than those HIPped at 880°C or 1020°C. The fatigue properties of samples HIPped at 930°C, made using different HIPping procedures were compared. It was found that samples which contain the as-HIPped surface, which were made using a new HIPping procedure, have better fatigue properties than samples with as-HIPped, machined or electro-polished surfaces which were produced by conventional HIPping . The properties of optimally HIPped Ti-6Al-4V samples are as good as or better than ingot-route samples. In the case of BuRTi the original single crystals or coarse grained polycrystals in the PREP powder are retained after HIPping and limited grain growth occurs in the gas-atomised samples. The tensile strength is comparable for the gas-atomised and PREP samples, but samples tested to failure showed a significant scatter in ductility (a larger scatter in the PREP powder samples) and all fracture surfaces contained large circular fracture initiation sites, with larger sites associated with lower ductility. Initiation occurs in the centre of these circular regions in large grains or in adjacent grains which have similar orientations and the failed region expands symmetrically in powder samples where no texture is expected. The fatigue properties of the PREP samples are much lower whereas the fatigue properties of the gas atomised samples are better than those of samples from ingot route. This behaviour is associated with obvious facetted failure sites in the PREP powder samples where it is suggested that the coarser microstructure has allowed persistent slip to occur leading to localised deformation and to premature failure. These observations are discussed in terms of the potential of net shape HIPping for the production of engineering components and in this context the fact that a new HIPping schedule has been developed during this study, where the fatigue properties of samples containing an as-HIPped surface are excellent, is very significant.