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.

High strength strip steels with a yield strength of ~700 MPa are used in the yellow goods industry, with recent trends requiring thicker strips with an improved toughness. Strip steel was produced to thicknesses of 10 – 17 mm, with thinner strips showing improved Charpy impact toughness compared to the thicker strip. It was found that all samples (different thicknesses and test orientations) showed splits on the fracture surfaces in the upper transition region. The hot rolled strip steels showed a predominantly fine-grained (~4 µm ECD) ferrite microstructure, with some coarse grain patches (grains ≥5 µm grouped together). No significant differences were noted for the grain size, yield stress, tensile strength, hardness and texture for the different compositions. Splits were characterised and produced successfully by low blow Charpy testing with acoustic emission (AE) sensors indicating the presence of splits which occurred in the upper transition region predominantly without main crack propagation. Analysis by x-ray tomography and sectioning was carried out showing splits propagate by transgranular cleavage, preferentially following coarse-grained regions, with the 10 mm strip producing deeper splits which was related to elongated grains (high aspect ratio). Therefore the presence of deep splits improved toughness through a reduction of the impact transition temperature, which was achieved by ensuring the presence of a bimodal grain structure, containing coarse elongated grain patches.

Poly(hydroxybutyrate) copolymers are sustainable and biodegradable, but they are known to exhibit secondary crystallisation, which severely reduces the ductility of these materials, thus hindering their current commercial use. Therefore, the main focus of this research was to explore a number of strategies to control the secondary crystallisation behaviour of two Poly(hydroxybutyrate) based copolymers. Blends of P(HB-co-HV)(3 wt % HV) with carbohydrate molecules of varying chain lengths were prepared by melt blending, characterised, and monitored over time to assess their capability to reduce secondary crystallisation. Additives were found to hinder the secondary crystallisation process, demonstrated by a reduction in the percentage change of mechanical properties as the concentration and chain length increased. The effect of storage temperature on the secondary crystallisation behaviour of P(HB-co-HHx)(33 % HHx) was also reported. Samples were stored at a range of storage temperatures and the effects on thermal, chemical and mechanical properties discussed. Increasing storage temperature caused the secondary process to occur to a greater extent, with greater increases in the melting temperature recorded in samples stored at 100 \(^∘\)C (128 \(^∘\)C - 135 \(^∘\)C) compared to samples stored at 7 \(^∘\)C (128 \(^∘\)C – 128 \(^∘\)C). Sub-melting point degradation of the material was also noted.

In this work, the deformation behaviour of Ti-6Al-4V micro-pillars was studied via compression tests at room temperature. Micro-pillars containing either the single α phase region or both α and β phases were prepared and either prismatic < a > slips or < c+a > slips were activated. Thin foils for TEM study were prepared using the FIB from the selected deformed pillars. STEM was used to image dislocations and their interaction with the α/β interfaces. Sample yield strength has been observed in all groups of micro-pillars and its relationship with the β phase, α/β interface, orientation and extrinsic sample size was discussed. The β phase is proved weaker than the α phase and the α/β interfaces behaviour as both the barriers and sources of dislocations. The magnitude of influence depending on the volume fraction, orientation and morphology of the β phase and the α/β interfaces. The work-hardening rate is related to the orientation of micro-pillars and the α/β interfaces in the micro-pillars. The strain bursts can be suppressed by the α/β interface and smaller sample size. The sample with smaller size expressed the larger strain burst in the stress-strain curves. The orientation influences the number and magnitude of strain bursts.

This thesis looks at the effect of micro segregation of Nb, generated from the continuous casting on grain structure development during reheating and deformation. The amount of segregated material present in the microstructure has been quantified in the as-cast condition and that has been used to model grain size distribution after reheating and deformation which is required for subsequent toughness prediction in 0.045 wt % Nb steel. The limits of validity of the original Dutta-Sellars model for predicting the amount of recrystallisation; investigating the influence of the variables such as starting grain size distribution, strain and Nb content has been examined. A modified Dutta-Sellars model considering the starting grain size distribution has been found to be more accurate in quantifying the amount of recrystallisation. This approach has been used to model grain size distribution after deformation both in the homogenised and in presence of Nb segregation. The effect of AlN, to replace Nb(C,N), on pinning the austenite grain boundaries during reheating has been investigated, as the segregation tendency of Al is much less compared with Nb.

The defect structure of single crystal tantalum with orientation 001, 011 and 111 after a 6 GPa shock loading with lateral and back release waves were characterised using scanning electron microscopy (SEM) and transmission electron microscopy. The SEM images were filtered using ImageJ script to determine the type and fraction of deformation twins. A methodology of imaging dislocations in the tantalum single crystals using electron channelling contrast was made with the assistance of the dislocation contrast profile simulation based on the dynamical theory of electron diffraction. The dislocation density distribution was measured using electron channelling contrast imaging (ECCI) technique. The nucleation and growth of the deformation twins are discussed with the aid of finite element simulation of the wave propagation in the material. The defect evolution and response of the single crystals are found to be highly dependent on the loading orientation of the shock wave. The effects of the lateral release wave and back release waves on the deformation mechanisms are also discussed.