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Uniaxial cyclic visco-plastic behaviour of a nickel-based superalloy, RRIOOO, has been successfully described under strain-controlled conditions by Tong and associates using Chaboche type of models with two kinematic hardening components. In this work, the ability of the original model was evaluated to consider unbalanced stress-controlled conditions at selected stress ranges and ratios. A third kinematic hardening component was introduced in the constitutive model to enabled an improved solution with which both ratchetting and cyclic deformation can be modelled adequately. The role of each kinematic hardening parameter in the description of ratchetting was examined and discussed with respect to the modified model response.

Quantum Tunnelling Composite (QTC) is a metal polymer composite, commercialised and produced using a patented manufacture process. This process ensures that the metal particles within an elastomerie polymer matrix maintain a highly fractal surface morphology where nano-scale point features are retained on the particles and are coated in polymer. This structure provides unique electronic behaviour including very high resistivity, of order 10 ΜΩ, above the expected percolation threshold and an exponential increase in conductivity under all types of mechanical deformation. Increase in sample compression leads to a lower electrical resistance through the material. Current-voltage characteristics show a hysteresis effect due to current storage in the QTC material as a current is passed. The hysteresis is shown to be reduced as the applied compression on the material is increased until an Ohmic regime is reached at very high compressions, above 70% linear compression. At these high compressions Joule heating is also proven to occur as a result of the power dissipated in the sample by I-V cycling. The Joule heating is sufficient to influence the physical characteristics of the sample and expand it, creating a current limiting device. QTC samples loaded with acicular electro-conductive particles in small fractions, less than 10% by weight, showed less sensitivity to applied compression in terms of electrical response. These samples appear to exhibit less white noise characteristics and indicate a combination of field assisted quantum tunnelling and percolation mechanism, Intrinsically conductive QTC samples were developed. These were made using QTC granules mixed into a polymer solvent solution. Upon depositing onto electrodes the solvent was allowed to evaporate leaving the constituent polymer binding the QTC granules together, compressing them into a conductive state. Samples were exposed to volatile organic compound (VOC) vapours in the concentration range 10 ppm to 100,000 ppm, causing swelling and void filling in the binding polymer. Combinations of these processes caused an increase in sample resistance, from ~50 Ω to excess of 10 ΜΩ. Sample composition and physical parameters have significant effect upon the response characteristics of the sensors. A system of experiments was undertaken and optimum sample composition was determined. Response to environmental changes were investigated^ namely temperature response and response to varying concentration of exposed solvent. It was found that samples produced using Polyphenylene Oxide ( PPO) and Polyvinyl Chloride (PVC) based binding polymer were more resistant to temperature change from 30 С to 80 С due to their molecular structures. Sensor response to different vapour concentrations was found to exhibit two distinct response regimes. High concentration exposures were found to exhibit a swelling mechanism with a CASE-II diffusion model fitting the data well. Whereas at low concentrations a void-filling based change in sample dielectric constant was attributed to the electronic response to vapour exposure. These predictions were also confirmed using a Quartz Crystal Microbalance (QCM) to measure mass uptake of vapour molecules and polymer density under similar test conditions.

An additive layer manufacturing (ALM) technique, electron beam melting, has been used for the production of simple geometries, from pre-alloyed Ti-6Al-4V powder. Microstructure, texture and mechanical properties achieved under standard conditions have been investigated, alongside numerical modelling of the electron beam and attempts to modify solidification through the addition of boron. Experimentation reveals an asymmetric electron beam which can be manipulated to produce different material responses. The electron beam has been used in this work, at its least powerful, as a means of preheating powder particles and, at its most powerful; to produce what is effectively a macro-scale electron beam weld. Numerical modelling and extraction of solidification parameters reveal that solidification occurs in the columnar region - columnar grains are observed experimentally and are a feature of the process. I Observed microstructures indicate a complicated thermal history that is capable of producing diffusion-less and diffusional transformation products. Electron backscatter diffraction (EBSD) and prior ~ grain reconstruction reveal a strong texture perpendicular to the build axis. Mechanical properties, tested over a range of build temperatures, are sensitive to temperature over the tested range of 625 - 700 °e. Attempts to disrupt columnar solidification via the addition of boron to Ti-6Al-4V, before subsequent EBM processing, were unsuccessful. Solidification remained in the columnar regime with no refinement in grain morphology observed.

The dimensional instability of metallic materials, following machining (or any method of material removal), is a common problem in the aerospace industry. This is because large parts have to be manufactured to very close tolerances and as much as 95% of the initial material may need to be removed. For machined, relatively complex, aerospace parts it would not be uncommon to see around 50% difference in the simulated results from the actual results, with even simple geometries showing around 19%-40% error. It is believed that a significant portion of these errors are due to a lack of: understanding of how the residual stresses redistribute following material removal; residual stress measuring capabilities; finite element (FE) modelling capabilities; knowledge of the additional residual stresses induced from the machining process (from varying cutting speeds, feed rates, tools, temperatures etc) and finally how the multiaxial residual stresses alter the shape of the part. This project improves the knowledge base of many of these topics. This work involved the development of an automated layer removal method for residual stress measurements in cold rolled sheet metal. This was needed due to the number of residual stress measurements that were required for modelling purposes. The method can determine the residual stresses in the rolling direction for cold rolled sheet material using a single strain gauge. Moreover, it is simple to set up and the total cost of the test rig and subsequent measurements is relatively low. Furthermore, in this project, a number of sheet metal aluminium parts were formed to different radii. The parts were then subsequently chemically milled in stages; the dimensional changes following each material removal being measured. This process was simulated using a commercially available FE package called PAM-STAMP. The FE results for distortion did not show good agreement when compared to the measured results; however, the residual stress measurements provided an unexpected understanding into the cause of the error. The results of the project improve the understanding of how residual stress change the shape of components and give an insight into the modelling parameters needed.

Metal matrix nanocomposites (MMNCs) can significantly improve mechanical properties of light alloys such as aluminium alloys beyond the properties of conventional metal matrix composites (where the reinforcement particles are micronsized). Therefore, MMNCs are potentially strong candidates for use in the automotive industry, where the mechanical performance and energy conservation are highly demanded. However, the challenge is to incorporate ceramic nanoparticles into liquid metals due to their large surface – to – volume ratio and poor wettability. In the present study, several nanoparticle feeding mechanisms (the most critical factor in the fabrication of nanocomposites by the ultrasonic method) were explored. SiC and TiB2 nanoparticles with an average diameter between 20nm and 30nm were dispersed through liquid A356 alloy with a green compact nanoparticle incorporation method under ultrasonic cavitation and streaming. The green compact method which has been developed during this project was found to be a promising mechanism achieving the engulfment and relatively effective distribution of the nanoparticles into the melt. Advanced FEGSEM and TEM techniques were used for the microstructural characterisation of the nanocomposites. The microstructural studies reveal that the nanoparticles were embedded into A356 alloy without any observed intermediate phase between the particles and matrix. It has been shown that with only 0.8 wt.% addition of the nanoparticles, the hardness was considerably improved. The nanocomposite billets were reheated into the semi-solid state to be thixoformed at a solid fraction between 0.65 and 0.70 for near net shape components with reduced porosity. The feasibility of thixoforming for aluminium nanocomposites was demonstrated. The microstructures, hardness and tensile mechanical properties of the thixoformed nanocomposites were investigated and compared with those of the asreceived A356 and thixoformed A356 alloys. The tensile properties of the thixoformed nanocomposites were enhanced compared to thixoformed A356 alloy without reinforcement, indicating the strengthening effects of the nanoparticles.

It is well known that the finishing temperature of 6xxx alloys determines the resultant texture, which clearly has a strong effect on formability. Specifically, the texture is determined by whether the finishing temperature is above or below the ß transus. This study was initiated in order to directly determine the mechanisms that take place during the hot deformation of AA6111 to directly explain these observations. Accordingly, the effect of inter-pass time during thermomechanical processing of AA61111 on flow behaviour and microstructure evolution has been investigated. This was achieved using plane strain compression testing undertaken on the Sheffield Thermomechanical Compression (TMC) facility, using the hit-hold-hit-quench approach at temperatures of 320°C, a strain rate of 85 s'1 to an initial strain of 0.5, unloaded and held for delay times of 0,6,60,600 and 6000 seconds, followed by the second deformation. Hardening of the alloy was observed, the extent of which was dependent on the hold time. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. TEM identified precipitation, predominantly ß and Q phases, on dislocation lines, the size of which was a function of the hold time. In contrast to conventional ageing studies, the Q phase was found to be the majority precipitate phase. The coarsening rate of the Q during the hold period of the precipitates was considerably faster than for coarsening following a conventional precipitation treatment. The size of the microband structure at the end of the double deformation was a function of the hold time, suggesting that coarsening of the precipitates during the hold had altered the Zener pinning potential. Texture analysis through EBSD revealed that the texture was mainly composed of the α and β fibres, indicating a classic hot deformation texture with no evidence of a recrystallisation texture. This, the EBSD maps themselves, optical and transmission electron microscopy indicated that no recrystallisation had occurred for any inter-pass delay time. Small changes in deformation texture were observed with changes in inter-pass delay time, however, such differences were believed to be small. Deformation substructure increased in size (e.g. microband width) with increased inter-pass time, as expected, which with the increase in precipitate size led to the softening observed at 600 and 6000 seconds. The implications of these observations are discussed.