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.

Hospital acquired infections is a modern day reality which plagues health care system around the world. Infections are the cause of 37,000 deaths and a 20 billion euro expense in EU annually. Despite the attention that such infections receive in scientific literature, very few viable solutions have been put forward and implemented. This study systematically developed, characterised and optimized novel long-lasting antibacterial surface coatings for use in medical devices, surgical instruments and hospital equipment. The reactive magnetron-sputtering deposition technique was successfully employed to combine the high mechanical performance of the nitrogen AISI 316 S-phase coating and the antibacterial efficacy of the elements silver and copper. Various configurations including homogeneous monolayers and layered multilayers were investigated for the first time in literature. It was found that substitutional silver and copper atoms can reside within the S-phase lattice while the good corrosion and wear resistance of S-phase are maintained by carefully controlling deposition parameters. Silver was observed to significantly increase the nitrogen uptake up to a previously unreported level of 50at% which has been termed 'ultra-saturation'. Copper based multilayers were found to have a 100% bacteria elimination rate, while silver monolayers also exhibited a good antibacterial efficacy against Staphylococcus aureus.

NiTi-based shape memory alloys (SMAs) have been developed as high power density micro-actuators for small scale and/or light weight actuation systems; this provides opportunities for actuators to be installed in regions where conventional actuators are unattractive due to their size, weight or power consumption. SMA based actuators could be applied across a greater range of applications if the SMA that provides the force could be joined to other light weight engineering materials, such as Ti-6Al-4V. The scope of this work is to describe the reasons why conventional fusion based welding and brazing procedures fail to provide strong joints between Ti-6Al-4V and NiTi-based SMAs, then detail a novel brazing method that can join these materials. This novel joining method involves using a localised heating method to braze the parent metals together. This localised brazing method prevents the shape memory properties from being compromised. The strength of the joints produced in this work have been related to their microstructure, which in turn have been related to the processing steps used to produce the joint. A study of the processing parameters was conducted to investigate the potential of this method as a large scale production joining method.

The motivation for this work was provided by the results of an interdisciplinary, multi-university research programme funded by the Ministry of Defence. The significant finding in question was that the stress corrosion cracking (SCC) resistance of AISI type 304 stainless steel (SS) alloys can be significantly improved by the addition of the platinum group metals (PGM), ruthenium and palladium. The increased SCC resistance could be attributed to the enhancement of the cathodic hydrogen reduction reaction. Thus, the primary objective of this work was to confirm that the increase in hydrogen production at the surface would not counteract the improvement in SCC resistance by increasing the susceptibility to hydrogen embrittlement (HE). Electrochemical hydrogen charging was employed and melt extraction was used to measure the bulk concentration of absorbed hydrogen as a function of alloy chemistry. Both Ru and Pd doped experimental 304 SSs showed a decrease in the concentration of absorbed hydrogen compared to an experimental standard 304 SS (i.e., a controlled, non PGM-doped reference 304). This result is thought to be due to the PGMs enhancing the recombination kinetics of adsorbed hydrogen. Ru proved more effective than Pd at hindering absorption and this is likely associated to the more homogenous distribution of Ru in solid solution. Slow strain rate tensile (SSRT) testing was performed in air at sub-ambient temperatures on pre-hydrogen charged specimens. PGM addition was found not to increase the susceptibility of 304 to HE. Conversely, an approximately equal enhancement of the HE resistance was observed for the addition of both Ru and Pd. The modest improvements are attributed to the reduced concentration of absorbed hydrogen, with a further possible beneficial trapping effect of Pd concentrated bands, contributing to the HE resistance of Pd doped 304.