This thesis is aimed at the identification of causes for the instability of Ti-49.8Ni and Ti-40.8Ni-9Cu (at.%)shape memory alloys during thermomechanical cycling. To assess the macroscopical and microstructural behaviour several techniques were employed. Thermo-mechanical tests (TMT) revealed that TiNi did not stabilise under the tested conditions. In contrast, TiNiCu showed a more stable behaviour. Whereas the maximum testing temperatures were found to be detrimental for the strain output in TiNi, none such influence could be found in TiNiCu. In TiNiCu increasing stresses seems to prevent the material stabilisation.During in-situ TMTs under high energy X-ray radiation in a synchrotron, microstructural factors influencing the stability of the material were assessed. No hint on a negative influence of remnant or intermediate phases and texture or variant selection was found. Instead increasingly negative strains build up in austenite possibly causing instabilities. In TiNiCu differences in the processing where found to cause changes in the phase transformation temperatures. These could be linked with differences in the chemical composition and in the level of inhomogeneity of precipitates.

The investigation of the interaction and kinetics of hydride forming materials under high frequency electromagnetic fields was undertaken in a joint research project between the University of Birmingham and C-Tech Innovation ltd. The improvement in the reaction rate of materials with hydrogen is a key step in the development of cost effective hydrogen storage technologies to make an effective fuel cell system for energy storage applications. The use of microwaves and radio frequency fields has been widely reported to improve reaction kinetics in a number of reactions and the evidence for improved diffusion rates suggests that electromagnetic fields could impact on this. C-Tech Innovation Ltd has a long track record in the development of microwave and radio frequency technologies and in the processing of materials using these technologies. The development of specific test and measurement equipment was a key objective of the project and has resulted in the development of a temperature controlled microwave /RF hybrid system to allow measurement of material sorption characteristics at controlled temperatures and pressures. Specifically the equipment allows the exposure of materials to high frequency electromagnetic fields at temperatures up to 800°C, under hydrogen or mixed gas atmospheres of 18 bar and with up to 2.5kW of applied electromagnetic radiation at 2 frequencies.

Pit propagation in stainless steels under electrochemical control was investigated using in situ synchrotron X-ray microtomography, which was used to confirm that the lacy covers commonly found for pits in stainless steels grow through perforation of the metal surface by upward growth of rapidly dissolving lobes from the main pit. In situ synchrotron X-ray radiography has been used to observe the evolution of 2D pits growing at the edge of stainless steel foils under electrochemical control in chloride solutions. Pit growth shape, kinetics and stability under current and potential control at various bulk chloride concentrations have been studied. It was found that the pit depth tends to grow under diffusion control, whereas lateral development is influenced by solution conductivity. The impact of the perforated cover on the pit growth and stability was examined and its formation was found to be similar to the observations from 3D by X-ray microtomography. A method for extracting the key dissolution kinetic parameters from radiographs has been developed. The local anodic current density along the boundary of a pit was directly measured from the rate of advance of the pit into the metal. Then the local metal ion concentration and potential drop inside the pit cavity was back-calculated using transport equations and the requirement to maintain charge neutrality, establishing the relationship between local current density, interfacial potential and metal ion concentration in the solution. The predictive model for pit propagation in stainless steel developed by Laycock and co-workers was examined, its sensitivity to key growth parameters was evaluated, and a modified version of the model was developed based on the kinetic parameters extracted from the radiographic measurements.

This thesis presents a series of broad but systematic and consecutive investigations on the topic of piezoelectric energy harvesting. These include material fabrication and characterisation, harvester fabrication and material parameter selection, electric output and dynamic behaviour tests of energy harvesters, and the feasibility of utilising lead-free piezoelectric materials for energy harvesting. Three lead-based and one lead-free perovskite solid-solutions compositions have been researched individually and compared to each other. In the form of bulk ceramics the lead-free composition is considered capable of replacing the lead-based compositions for vibrational energy harvesting at room temperature. Typical properties of ε\(_r\)≈4700, \(P\)\(_r\)≈9 μC/cm\(^2\), \(d\)\(_3\)\(_3\)≈500 pC/N, \(k\)\(_p\)≈0.51 have been achieved for the lead-free and lead-based compositions respectively. Vibrational energy harvesting based on a novel structure of piezoelectric/silver multi-layer free-standing thick-film unimorph and bimorph cantilevers have been investigated using two of the lead-based compositions. A planar shrinkage difference of 3-6% between the silver and piezoelectric layers is suggested in order to ensure successful fabrication. When tested under harmonic vibration conditions, a comparison of unimorph individual harvesters suggests that higher piezoelectric voltage and electromechanical coupling coefficients may be preferred when selecting materials. Further optimisations involving bimorph devices with tip proof mass have demonstrated maximum harvester outputs (root mean square) of about 9 μW and 2.8 V with approximately 14% bandwidth under resonant vibrations (I 00-150 Hz, 0.5 - I.Og). In addition, the cantilevers have utilised to harvest wind energy with a modified spinning configuration, exhibiting 3.4 V average open-circuit output voltage in optimum wind conditions.

The oxidation resistance of four prototype single crystal nickel-based superalloys for industrial gas turbine applications is studied. All contain greater quantities of Cr than in most existing single crystal superalloys; two are alloyed with Si, one with Re. To explain the results, the factors known to influence the rate of Al\(_2\)O\(_3\) scale formation are considered. Models are developed to predict whether any given alloy composition will form a continuous Al\(_2\)O\(_3\) scale. These are used to rationalise the dependence of Al\(_2\)O\(_3\) scale formation on alloy composition in these systems. The mechanical behaviour of a new single crystal nickel-based superalloy for industrial gas turbine applications is also studied under creep and out-of-phase thermomechanical fatigue (TMF) conditions. Neutron diffraction methods and thermodynamic modelling are used to quantify the variation of the gamma prime (\(\gamma\)’) strengthening phase around the \( \gamma\)’ solvus temperature; these aid the design of primary ageing heat treatments to develop either uniform or bimodal microstructures of the \( \gamma\)’ phase. During TMF, localised shear banding occurs with the \( \gamma\)’ phase penetrated by dislocations; however during creep the dislocation activity is restricted to the matrix phase. The factors controlling TMF resistance are rationalised.

The need for jet engines to burn fuel more effectively at higher temperatures requires the development of nickel-based superalloys containing increasing amounts of the main strengthening, stable, ordered L1\(_2\) intermetallic (Ni\(_3\)(Al, Ti)) \( \gamma\)' phase, with RR1000 being a candidate. Welding of this alloy by conventional methods has been found difficult due to a high susceptibility to heat affected zone (HAZ) liquation cracking. In order to produce welds with good joint integrity, inertia friction welding (IFW), a nominally solid state welding process, has been used to join gas turbine parts made from this alloy, based on the premise that the joining occurs below the melting point of the bulk material. The failure rate, however, is not zero. Detailed microstructural characterisation of the actual weld and of a thermo-mechanically simulated HAZ has revealed for the first time that non-equilibrium constitutional liquation of some strengthening precipitates occurs at the grain boundaries and within the grains of this alloy during IFW, with attendant formation of liquation microvoids within the HAZ. The temperature gradient across the HAZ is predicted to be 1150 \(^o\)C-1286 \(^o\)C. Hafnium-rich oxides were also found to coalesce and become smeared by friction along the weld interface, forming brittle hafnium oxide flakes. However, IFW has been found to be more effective than conventional welding techniques, always producing liquation crack-free welds within 150 \( \mu\)m of the bond line (not common in conventionally welded alloys). Micro tensile testing was used to characterise the local strength within the weld and to rationalise it with the microstructure.

X-ray absorption near edge structure (XANES) experiments on artificial pits of 316L stainless steel were carried out to study the oxidation state and speciation of alloying elements in the pit solution. It was confirmed that the oxidation states of Fe, Cr and Ni are 2+, 3+ and 2+, respectively. Ni(H\(_2\)O)\(_6\)\(^{2+}\) was found to be the main solution species through the pit with no evidence of any Ni-Cl complexes. However, for iron and chromium, hexa-aquo ions (Fe(H\(_2\)O)\(_6\)\(^{2+}\) and Cr(H\(_2\)O)\(_6\)\(^{3+}\)) were found near the pit mouth with chloro complexes close to the dissolving metal surface. The chemistry of molybdenum species in artificial pits of 316L stainless was investigated and the molybdenum oxidation state was found to be 3+. There was no evidence of any Mo(VI) polymolybdates, which have previously been proposed to explain the beneficial effects of Mo on the corrosion resistance of 316L stainless steel. High resolutions measurements did not detect any different molybdenum species adjacent to the salt film. XANES measurements on titanium artificial pits showed a presence of TiCl\(_4\), titanium oxides (rutile and anatse) and metal fragments that were generated during the electrochemical dissolution process. X-ray fluorescence and XANES were also carried out to study the titanium distribution and species in human tissues extracted from the vicinity of failed knee, BAHA (bone-anchored hearing aid) and dental implants. Metal fragments and titanium oxides (rutile and anatase) were found in the tissues. In addition, XANES was carried out on neutrophil cells that had been cultured in the presence of anatase. In one case, a spectrum of rutile was found, suggesting the cells may be able to convert anatase to rutile.

In recent years rare earth metals have hit the headlines due to supply restrictions of neodymium and dysprosium from the main supplier China. The work in this thesis investigates the possibility of recycling sintered NdFeB-type magnets using a combination of hydrogen decrepitation (HD) and the HDDR process (Hydrogenation Disproportionation Desorption and Recombination). In this work the development of microstructure during the HDDR process has been identified and the route taken by hydrogen during absorption and desorption. The microstructure and magnetic properties have been shown to be affected by the process temperature, disproportionation pressure and recombination pressure and time. Sample batches up to 100g exhibit little variation in magnetic properties however 400g batches result in large variations partially due to incomplete recombination. By increasing the disproportionation pressure it was possible to simultaneously process mixed scrap feeds with different Dy and Co levels, however a large variation in magnetic properties was also observed. Optimal processing of sintered Nd\(_{13.4}\)Dy\(_{0.7}\)Fe\(_{78.6}\)Al\(_{0.7}\)Nb\(_{0.4}\)B\(_{6.3}\) was performed by in-situ HD followed by HDDR at 880\(^o\)C with a disproportionation pressure of 1500mbar and recombination under vacuum. The resultant powder exhibited a remanence of 1.08 T, coercivity of 840 kAm\(^{-1}\) and maximum energy product of 178 kJm\(^{-3}\).

This study involved the characterisation of a newly developed 9%CrMoCoVNbNB (FB2) martensitic creep resistant steel for use in large industrial turbine disc and shaft components. A major part of the work involved characterisation of FB2 with incorporation of material, thermal and physical data into a finite element-based model for the simulation of coupled thermo-mechanical working with consideration of grain size evolution based on the implementation of a user sub-routine within the FEM code (QForm). The main focus of the research was to gain a comprehensive understanding of the non-uniform temperature, strain rate, strain and grain size distributions apparent during hot open-die forging.

High Pressure Die Casting (HPDC) is an attractive option for automotive manufacturers, as it has a number of advantages over wrought process routes. An improved understanding of the defects which may result from the process could allow castings to deliver lighter vehicle structures. A novel algorithm was developed to predict the formation of entrainment defects, which may limit the strength of castings. This model was integrated into FLOW-3D, a fluid dynamics solver. Theoretical advances were made, which offer a means of extrapolating a spatial distribution of damage to location specific statistical distributions, an improved way of characterising the contribution of each defect type to strength, and a means of correlating parameters for statistical distributions, allowing the variation in strength may be predicted at arbitrary locations within a casting. Casting and numerical experiments were performed, to evaluate these algorithms and underlying fluid flow solution, and to test the influence of entrainment defects on the strength of HPDC parts. Defects formed by air entrainment were found to significantly limit the strength of the studied castings. The methods and techniques explored in this work showed promise, but further advances would be needed, before ab intio strength prediction for HPDC parts can be realised.