Single crystal Ni-based superalloys are used in the highest temperature components in jet turbine engines owing to their excellent properties under creep conditions. These alloys owe their properties greatly to their chemical composition, and in particular the addition of slow diffusing elements like Re delays the creep deformation significantly. Vacancy diffusion has been suggested to be the rate-controlling process for creep deformation at high temperatures, and elements like Re are expected to interact with the vacancies in slowing them down. This has been investigated in the present work using ab initio calculations. Specifically, dilute as well as non-dilute binary alloys of Re, W and Ta in Ni were considered to study the effect of chemical composition on the rate of vacancy diffusion. Analytical formulations were used to describe the diffusion equations, however their applicability was restricted to the dilute regime. For the calculation of diffusion coefficients particularly in the non-dilute regime, kinetic Monte Carlo simulations were performed. The energies and the diffusion barriers were described using the cluster expansion method. Results suggested appreciable modifications of the vacancy diffusion coefficients, suggesting that the beneficial role of slow-diffusing elements in Ni-based superalloys could be partly explained by their effect on vacancy diffusion.

Chemical composition of structural steels with a ferrite-pearlite microstructure has been developing towards decreasing carbon content, to increase weldability, with increased microalloying element content, to provide grain refinement, solid solution and precipitation strengthening. During the UOE forming of large diameter (more than 400 mm) welded pipes the strength drop from plate to pipe, as a result of reverse deformation (the Bauschinger effect), depends on steel grade, namely microalloying element content, and processing. In this project the microstructure of two Nb- and V-microalloyed steels has been studied with optical, scanning and transmission electron microscopy. The dislocation density and (Ti,Nb,V,Cu)-rich particle diameter, volume fraction and number density were measured for as-rolled and annealed (30 min. at 400 \(^0\)C and 550 \(^0\)C) steels. The Bauschinger effect was measured during compression-tension testing for the same steel conditions. The yield stress drop during reverse deformation has been found to increase with an increase in forward pre-strain, dislocation density and particle number density within the effective particle diameter range of 12-50 nm. On the basis of dislocation-particle interaction analysis, a quantitative model of work-hardening behaviour dependence on particle number density and dislocation density has been derived for the reverse deformation of studied steel grades.

The main focus of the current study was to design, construct and evaluate a common platform to enable the integration of multiple sensing systems for cure monitoring. An Abbe refractometer was selected to house a custom-made cell which accommodated the following optical sensors: (i) Fresnel reflection sensor; (ii) near-infrared transmission sensor; (iii) evanescent wave sensor; and (iv) fibre Bragg grating sensor. The cell was designed such that it was also possible to simultaneously acquire conventional refractive index data during the cross-linking process. Thus, the cross-correlation of data on refractive index, cross-linking kinetics, strain and temperature was facilitated. It was found that the trends observed in the qualitative (intensity-based refractive index monitoring via the Fresnel reflection sensor) and quantitative (optical fibre-based transmission FTIR spectroscopy) approaches were similar during the cure of common thermosetting resin systems. Furthermore, for the first time, S-2 glass® fibres were used to obtain evanescent wave spectra during cross-linking, and excellent correlation was observed with the transmission FTIR spectral data. Fibre Bragg grating sensors were used to infer the magnitude of the residual fabrication strain. Excellent correlation was observed between the refractive index data generated using the Abbe refractometer and Fresnel reflection sensor.

A literature survey on the whole field of pore formation is assembled into the form of a general theory of the causes of porosity in castings. The conventionally accepted modes of feeding are assessed: liquid-, mass-, and interdendritic- feeding; and two further mechanisms are proposed: burst- and solid-feeding. The latter is investigated theoretically using various flow models: elastic-plastic, viscous, creep and Bingham flow. A new theory is proposed for the origin of layer porosity in castings. Experimental work on a wide variety of alloys: Al-Cu, Fe-C, Complex Ni- and Co-base alloys, cast both in air and in vacuum are investigated for the effect of section thickness, taper, and mould and metal temperatures. The formation of porosity appears to change from a non-nucleation to a nucleation mechanism as section thickness increases. A new method of interpreting radiographs based upon a longitudinal line count reveals that solid feeding becomes important in reducing porosity at high mould temperatures. Experiments on the effect of composition of an alloy on porosity cast doubt on the widely accepted theory that the presence of non-equilibrium eutectic liquid reduces porosity, but indicate that the non-equilibrium freezing range of the alloy may be the critical parameter. The effect of pressure on porosity is investigated utilising pressures below atmospheric; the results are inadequately explained by current theories and are discussed in terms of the nucleation and growth of pores; the effect would also appear to have considerable industrial potential for reducing porosity in vacuum cast components.

Aluminium and their alloys readily react with air to form surface oxides and films. If they become entrained in a casting, they form double oxide film defects that reduce the mechanical properties of the component. It is important to determine the nature and behaviour of the films formed, so that defects can be avoided or their internal gas volumes reduced. The reaction of common additions and impurities that may be present in aluminium alloys were melted at 750 °C in air and nitrogen, to determine thickness, composition and structure of possible films that may form in a defect. The effects of molybdenum were also investigated. It has also been suggested that oxides formed on aluminium alloys may act as heterogeneous nucleation points for solidification. Spinel containing copper layers were manufactured by electro-codeposition, and added to commercial purity aluminium to test its potential as a grain refiner. It was compared with non-refined aluminium and aluminium refined with standard Tibor refiner. The lattice parameters of oxides formed were also compared with that of aluminium to ascertain their potential as nucleation centres. The results are discussed in this work.

Pitting corrosion of iron has been studied via artificial pits. Solid corrosion products were observed within the pit, which was characterised as disordered-carbon and Fe\(_3\)C, acted as diffusion barriers for metal ions and slightly increased the solution resistance. Its formation depends upon the interfacial potential. High purity iron was used to calculate an effective diffusion coefficient for metal ions (a combination of self-diffusion and electrical migration) in different MgCl\(_2\) concentrations. The contribution of self-diffusion increases with increasing MgCl\(_2\) concentration if Mg\(^2\)\(^+\) depletion in the pit is considered. The Tafel kinetics of iron dissolution in metal ion saturated solutions was studied in different MgCl\(_2\) concentrations. The Tafel slope (56 to 70 mV/decade) was independent of MgCl\(_2\) and FeCl\(_2\) concentration when FeCl\(_2\) is saturated. The effect of nitrate on the composition/structure of salt layers was characterised. The salt layer is composed of FeCl\(_2\).4H\(_2\)O in chloride-based solutions (isotropic in HCl and anisotropic in HCl with trace nitrate), and Fe(NO\(_3\))\(_2\).6H\(_2\)O in nitrate-based solutions. The dissolution behaviour of iron in chloride/nitrate solutions was studied. Dissolution is uniform under a salt layer, but crevice formation and surface roughening. Potentiodynamic measurements can induce abrupt dissolution/ passivation transitions, resulting in deep and localised attack.

An investigation into the optimisation of magnetic properties of two types of magnetic alloys based on Sm(Co,Cu,Fe,Zr)_z (z = 7.4) and Nd₁₅Fe₇₇B₈ has been carried out. The optimum heat treatment which gave the best magnetic properties for the Sm(Co,Cu,Fe,Zr)_z alloy depended primarily on the sintering and solid solution temperatures. After quenching the alloy from the solid solution temperature, and then ageing isothermally, it was found that the intrinsic coercivity, microhardness and electrical resistance changed with ageing time, and the ageing temperature also affected the rate of these changes. The magnetic properties of specimens produced from the Nd-Fe-B type alloy were also shown to be affected by different heat treatments. The microhardness, microstructure and intrinsic coercivity were affected by an isothermal ageing treatment and the temperature of the treatment. Two routes were used for powder preparation 1n the production of the sintered specimens: i. Mechanical ball milling of crushed ingot material. ii. Hydrogenation followed by ball milling. It was found that higher energy products were achieved in the ball milled material for the two alloys investigated. Two reasons for this were proposed: i. The hydrogenated and ball milled powder had a larger size than the ball milled only powder. ii. The surfaces of the hydrogenated powder were highly reactive in air and some oxidation of the surfaces occurred. Optical and electron microscope studies showed that the variations in the ageing behaviour of both the alloys was concurrent with subtle differences in the microstructure between the samples aged at different isothermal ageing temperatures.

Magnesium hydride is a promising material for hydrogen storage. However, the thermodynamic stability of the hydride coupled with slow reaction kinetics means that it is currently infeasible for application as mobile hydrogen storage media. Thin films of magnesium have been investigated to ascertain the influence of stress on the effects of the sorption thermodynamics and kinetics. It was found that: • The intrinsic stress state can be heavily influenced by deposition conditions. • The influence of the underlying microstructure heavily influences hydrogen sorption mechanics. A model has been proposed which shows regimes dominated by porosity, elastic and plastic contributions and their likely effects on sorption thermodynamics. • Stress analysis using different film thicknesses has been used to model the diffusion behaviour within the films, demonstrating how differential rates and directionality of absorption also affect both kinetic and thermodynamic properties of the films. • A novel approach to tailor thin film stress using flexible substrates was used to investigate and tune thermodynamics. The use of an opto-mechanical hydrogen sensor based on this system design. • Y/Mg multilayer systems were investigated for the first time. The characterisation of these shows that hcp/fcc yttrium occurs at different layer thicknesses, altering the lattice spacings at coherent interfaces with Mg.

Steel properties are controlled by its microstructural parameters, such as grain size, phase balance and precipitates. It is desirable to monitor microstructural changes during processing, allowing in-situ feedback control, or microstructure characterization in a non-contact and non-destructive manner. Electromagnetic (EM) sensors are sensitive to changes in magnetic (relative permeability- dominant effect) and electrical (resistivity­ minor effect) properties, which in steels, vary with microstructure and temperature. EM sensors have been shown to have great potential for assessing steel microstructures (austenite to ferrite transformation or decarburisation). However, the influence of key microstructural parameters is not yet fully understood. This thesis presents a study of the effect of individual microstructural parameters on relative permeability and hence sensor output. In particular the ferrite grain size, pearlite interlamellae spacing, as-quenched martensite carbon content and phase balance were independently studied. The relative permeability of certain steel microstructures was determined using a finite element (FE) model fitted to experimental data. These values agreed with the literature and were used to predict the relative permeability of complex microstructures using an embedded microstructure FE model. Finally a case study on commercial steels was carried out, where the phase balance and tensile strength of dual phase steels were accurately predicted.

Interstitial free (IF) steel is used extensively throughout applications in the automotive, packaging and furniture industries due to its excellent formability and ductility. The manufacturing process ensures excellent material properties for subsequent forming processes are developed through the formation of a fine equi-axed grain structure and crystallographic texture. The annealing process improves the formability of the cold rolled IF sheet, whilst also reducing strength through the recovery and recrystallisation process. After the cold rolling process the grain structure is heavily deformed. During the recovery process the dislocation density is reduced through annihilation and redistribution of dislocations to form sub grains. During the recrystallisation process new grains nucleate and grow into new, strain free, grains. Magnetic properties of ferromagnetic material are known to be affected by microstructural phenomena such as dislocation density, grain boundaries, grain size and texture. It is therefore possible to monitor the recovery and recrystallisation processes using sensors that are responsive to changes in magnetic properties. The purpose of the research completed was to establish whether it would be possible to use electromagnetic (EM) sensors to monitor recovery and recrystallisation processes in-situ during heat treatment, such that EM sensors could then be deployed in a continuous annealing line.