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.

In centrifugal casting, molten metal is introduced into a mould which is rotated at high speed. The centrifugal force helps to fill thin sections but this benefit may be offset by the effect of the turbulent flow on the casting quality. In this research, the effect of direct and indirect gated mould designs on the quality and reliability of aluminium alloy investment castings made by centrifugal casting was investigated. The scatter in the ultimate bend strength and the modulus of elasticity was analyzed using the Weibull statistical technique, which showed that the Weibull modulus of both properties was significantly improved for the indirect gated cast test bars compared to the direct gated bars. A detailed microstructural characterization was carried out on the cast test bars, which included grain size, dendrite cell size and porosity. Scanning electron microscopy was used to examine and analyze the presence of defects on the fracture surfaces such as shrinkage pores, entrapped bubbles and oxide films resulting from surface turbulence during mould filling. The results indicated a clear correlation between the mechanical properties and the presence of casting defects. Water modelling experiments were carried out using purpose-built experimental centrifugal casting equipment and filling sequences recorded using a high speed video camera. The water modelling results showed that the general tendency for the direct and indirect gated mould designs was that the higher the rotational velocity, the lower the filling length and consequently the lower the filling rate. Subsequently, this information was used to validate the computer software ANSYS CFX. An excellent correlation was obtained between the experimental water modelling and simulation results for both direct and indirect gated moulds.

The microstructure evolution of a sand cast AZ91 magnesium alloy during heat treatment (solution treatment and subsequent ageing) were characterized quantitatively using a combination of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The area fraction of discontinuous precipitates and number density of continuous precipitates (N\(_V\)) in the AZ91 alloys with and without pre-deformation were measured using OM and TEM, respectively. Based on these metallurgical evidences, the existing precipitation strengthening mode for AZ91 was modified and the effect of pre-deformation on the precipitation strengthening of AZ91 was investigated. Al-Mn-(Mg) particles in the size range of 20-200 nm have been found in the as-cast AZ91. Their morphologies, chemical composition and structures were investigated using TEM. It was found that these particles have a chemical composition of (Al \(_6\)\(_.\)\(_5\)\(_2\)Mn) \(_1\)\(_-\)\(_x\)Mg\(_x\) (x < 0.13) and a decagonal quasi-crystalline structure. These particles were stable during a solution treatment and acted as preferential nucleation sites for continuous Mg\(_1\)\(_7\)Al\(_1\)\(_2\) precipitates during the subsequent ageing. The results obtained using two electron tomography (ET) approaches were also summarized in this thesis. One uses HAADF-STEM for Mg\(_1\)\(_7\)Al\(_1\)\(_2\) precipitates on Al-Mn-(Mg) particles. The other technique is BF-STEM applied to study Mg\(_1\)\(_7\)Al\(_1\)\(_2\) precipitates on the dislocations.

The focus of this research was to design, optimise and deploy a practical multi-measurand sensor (MMS) for process monitoring of glass and carbon fibre preforms in an autoclave. The MMS design was based on the optical fibre-based extrinsic Fabry-Perot interferometric (EFPI) strain sensor. The EFPI sensor consisted of a pair of cleaved optical fibres, with a defined gap that were housed in a precision bore capillary. Interferometric interrogation was used to measure the cavity length hence, permitting the strain to be determined. The basic EFPI sensor design was modified in the following manner to enable multi-measurand monitoring. Firstly, a fibre Bragg grating (FBG) was inscribed on one of the optical fibres that were destined to be housed in the capillary; the end-face of this cleaved optical fibre was sputter coated with Au/Pd to produce a reflective surface. This FBG is in a relatively strain-free condition and only responds to temperature. Secondly, a technique was developed to manufacture optical-quality end-faces of the capillary and this too was sputter coated with Au/Pd. Thirdly, secondary cleaved optical fibres were packed around the lead-in optical fibre of the EFPI sensor with a defined (secondary) cavity thus, permitting transmission/reflection FTNIR spectroscopy. Finally, cleaved secondary optical fibres were also secured to the lead-in fibre and these served as Fresnel reflection sensors. The interrogation of the MMS was carried out using a conventional fibre-coupled multi-channel FTNIR spectrometer. The feasibility of monitoring strain, temperature, cross-linking kinetics and refractive index simultaneously during the processing of glass and carbon fibre preforms was demonstrated.