The effects of copper and magnesium on the precipitation characteristics of Al-Li-Mg, Al-Li- Cu, and Al-Li-Cu-Mg alloys have been investigated during isochronal and isothermal ageing. In AI-Li-Mg alloys, increasing the magnesium concentration results in stimulation of δ'precipitation by a shift of the α/δ'solvus boundary to higher temperatures. It was shown that for each wt%Mg present in the alloy the α/δ'solvus boundary shifts by 7.0°C. In Al-Li-Cu alloys the concentration of copper has no effect on the position of the α/δ'solvus boundary. The significant stimulation of δ' observed in Al-Li-Cu alloys was shown to be due to the formation of GPCu zones that act as heterogeneous nucleation centres. TEM analysis showed that this heterogeneous nucleation produced composite precipitates consisting of an inner plate of GPI zone and an outer cylindrical shell of δ'. At high copper concentrations (Cu>2.0%) and long ageing times at 150°C, significant retardation of δ' precipitation takes place due to precipitation of the equilibrium T1 and T2 phases. The mechanisms by which copper and magnesium affect the precipitation characteristics of Al- Li-Cu-Mg alloys are different than those operating in the ternary AI-Li-Mg alloys and Al-Li-Cu alloys. In 1.7Lil. 2CuXMg alloys, increasing the magnesium concentration beyond 1.2% causes significant stimulation of δ'precipitation through the formation of Li-Cu-Mg clusters (mechanism referred to as CL δ') that are capable of rapidly developing into δ'. It is proposed that in 1.7Lil. 2CuXMg alloys the initial 1.2%Mg added is consumed in the formation of GPB zones that have very little effect on δ' precipitation. As the magnesium concentration increases to levels higher than 1.2%, the magnesium is free in the matrix to gather both copper and lithium thus forming Li-Cu-Mg clusters which are extremely effective at nucleating δ' In 1.7Li1.2MgXCu alloys the mechanisms by which stimulation of δ' precipitation takes place are again by formation of Li-Cu-Mg clusters (CL δ'), and by nucleation on GPB zones (mechanism referred to as GP δ'). During ageing at 70 and 100°C, and for copper concentrations in the range 0-1.2%, the dominant precipitation mechanism is GP δ'. For higher copper concentrations (1.2<Cu<3.0) the dominant process is CL δ'. Increasing the ageing conditions to 150°C causes precipitation of δ' through classical nucleation and growth for low copper concentrations. For high copper concentrations, the precipitation of δ'comes about through the GP δ'mechanism. Using Kissinger's method, it was found that the activation energy for a' formation in AI-Li-Cu-Mg is equal to 62 kJ/mol, suggesting that the kinetics of the δ'precipitation process are also controlled by the presence of excess vacancies quenched-in from solution heat treatment. It is likely that the Li-Cu-Mg clusters that develop in the alloy also gather excess vacancies thus making the clusters vacancy-rich. For all the alloy systems (Al-Li-Cu, Al-Li-Mg, and Al-Li-Cu-Mg alloys) and independently of the concentrations of copper and magnesium, the largest volume fraction of δ' precipitates form during ageing at 100°C where there is an optimum combination of thermodynamics and kinetics. Ageing the alloys at 150°C (standard heat treatment for lithium containing alloys) and subsequently exposing at 70°C (to simulate service conditions for an aerospace alloy) resulted in embrittlement due to precipitation of additional (fine) δ'. This embrittlement was shown to be closely related to the volume fraction of δ' that precipitates during exposure. In Al-Li-Mg and AI-Li-Cu ternary alloys, increasing the concentration of magnesium and copper respectively, resulted in increased volume fractions of δ' precipitated during exposure and hence increased degrees of embrittlement. For Al-Li-Cu-Mg alloys the maximum volume fraction of δ' precipitated during exposure occurred in the 1.7Li1.2Cu1.2Mg alloy. It was shown that this alloy composition also showed the maximum degree of embrittlement.

The increasing depth, distance from surface connections and falling quality of coal mined are factors that increase the risk of spontaneous combustion in working areas. The trend towards high capital investment, high output faces significantly raises the economic consequences of spontaneous combustion. Much work has been directed towards methods of identifying the liability of a coal to spontaneously combust under given conditions. The cost of prevention and combat of spontaneous combustion underground is high, however this work is carried out with little knowledge of the likely location of a heating in a coalface waste. This thesis investigates the airflow patterns in a coalface waste, with a view towards improving the use of prevention and combat methods. The factors that affect the liability of a coal to spontaneously combust, and the methods of prevention and combat are discussed to provide a background to the subject area. An investigation into the flow of nitrogen that was injected from one hole into a coalface waste as a combat measure was conducted. The results of this showed how the amount of nitrogen entering the waste depended on the rise and fall of the atmospheric pressure. An attempt was made to develop a method of sampling gas from deep within the coalface waste. It proved impossible to sample further than 15m behind the face line. The results from this exercise are presented. The finite element method was used to model the pressure distribution in the waste under differing boundary pressure and waste permeability conditions. A suggested area at risk from spontaneous combustion is presented.

Novel aspects of solid state laser spot melting of aluminium using a pulsed solid state laser were investigated. After a thorough characterisation of the performance of the solid state laser, an initial series of ranging trials were performed to identify parameters which produced cosmetically satisfactory consistent melt spots on the surface of a commercially available aluminium alloy. These melt spots demonstrated a number of features of interest, including symmetrical concentric ring structures on the surface of the spots. A review of published literature on the use of laser beams as an intense radiation source for pulsed laser surface melting was carried out which confirmed that these phenomena have not been researched or reported in any depth. Experimental work identified the conditions under which they could be reliably reproduced, and these conditions are very close to laser parameters used commercially for pulsed laser welding. Further investigations to understand their origin involved using modified aluminium surfaces and temporally shaped laser pulses. Experimental details are included which will allow reliable reproduction of this effect in the future. Specific thresholds were identified for these phenomena and this has led to an improved understanding of solid state laser spot melting on aluminium. It appears that these rings are part of a continuum of irradiance which leads to melt expulsion due to reactive vapour pressure.

As coal mines get deeper, more mechanised and more productive the heat load on the ventilation system increases. In certain cases to the point where serious environmental problems may arise. To continue mining in these demanding conditions the sources of heat must be identified and evaluated so ameliorative measures may be taken. Due to the trend towards mining at greater rates and further from the shaft, mined coal on the conveyors is being recognised as a heat source of growing importance. This thesis describes its investigation. Reviews of heat sources, psychrometry,heat stress indices and heat transfer are included to provide a background framework. The evaluation of the heat released by conveyed coal itself consists of theoretical treatment and laboratory investigations of heat transfer through broken coal. A model conveyor and its instrumentation constructed in a duct are described along with underground measurements at mines. The information obtained from theoretical, laboratory and on site investigations is analysed and summarised to provide a basis for future prediction.

The principal objectives of the research reported in this thesis are: to determine the effect that sinter time has on the metallurgical behaviour of alumina-reinforced aluminium-606lmatrix composites; compare and assess the wear resistance of alumina and silicon carbide reinforced aluminium 6061-matrix composites, together with monolithic aluminium 6061 alloy; determine the effect that reinforcement particle size has on the wear resistance of aluminium 6061-matrix composites; identify the relative merits of two techniques for depositing copper coatings on to alumina reinforcements. Through investigation, a successful method of processing silicon carbide and alumina particulate-reinforced AA6061 composites, fabricated by cold uniaxial pressing with vacuum sintering, has been determined. The processing route is as follows: pressing at 400 MPa; vacuum sinter at 600°C for 30 minutes; solution heat treat for 30 minutes at 530°C then water quench; precipitation (ageing) heat treat for 7 hours at I 75°C, then air cool. Metallurgical examination of composites revealed that magnesium was found to collect at interface regions around alumina particulates, resulting in the depletion of magnesium from the aluminium 6061 matrix. The severe depletion of magnesium from the AA6061 matrix when alumina is used as a reinforcement was found to occur during long (greater than 30 minutes) sintering times using a sintering temperature of 600°C. It is postulated that the formation of spinel (MgA12O4) formed from the reaction of magnesium with alumina is a probable cause for the Mg migration. The composites containing alumina particulates were found to have lower hardness values than the monolithic alloy and composites containing silicon carbide, when sintering took place for longer than 30 minutes. Adding 5 wt% silicon to the AA6061 matrix in composites reinforced with alumina particulates was found to reduce the magnesium depletion for sinter times up to one hour at 600°C and give improved composite bulk hardness. During the research, a need for an improved wear testing machine was identified. Therefore a wear test rig, which allows samples of different materials (under different applied loads if required) to be tested simultaneously without interference between test pieces, was designed and commissioned. Two electroless methods for copper coating alumina particulates were also investigated. One method used formaldehyde as the reducing agent, while the other employed hydrazine-hydrate as the reducing agent. The latter method has proven to be quicker, and with improved results, compared to the traditional method using formaldehyde as the reducing agent.

Diamond-Like Carbon (DLC) coatings are thin protective surface coatings used to reduce friction and minimise wear in a wide range of applications. The focus of this work is the use of DLC coatings within Rolls-Royce’s pressurised water reactors. A strong understanding of material behaviour in this environment is compulsory due to the stringent safety requirements of the nuclear industry. Wear testing of a range of commercial DLC coatings against steel in water, and the dependence of the tribology on normal load, sliding distance, and environmental species, was examined. Wear depth was observed to increase with normal load, and increase non-linearly with sliding distance. Uniquely, it was suggested that the tribology of a DLC coating in water was controlled by the velocity accommodation mode (VAM) of the transfer layer. When interfacial sliding was the dominant VAM, the carbonaceous transfer layer was present at all times, and a low specific wear rate was observed. When shear and recirculation of debris was the dominant VAM, the carbonaceous transfer layer initially present was replaced by iron oxide species, and a high specific wear rate was observed as a result of a three-body mechanism involving hematite. Two individual wear models were developed to predict the wear depth of a DLC coating sliding against steel in water. Each model represents a novel extension to the current literature regarding the modelling of wear. Firstly, an analytical differential equation was derived to predict the wear depth of a ball and a flat surface, in relation to any phenomenological law for wear volume. Secondly, a unique formulation of an incremental wear model for an arbitrary geometry was developed for a DLC coating which included the growth of a transfer layer. An efficient methodology was presented to allow fast integration of the equations whilst damping numerical instabilities. A comparison between the analytic and computational wear models showed a strong agreement in the model predictions, with a comparative error of less than 5%.

Age hardening of Al-Cu-Mg type alloys occurs in two stages separated by a constant hardness plateau when the alloys are aged at 110°C to 240C after solution treatment and quenching. This work aims to develop a physically based two-stage hardening model to predict the yield strength of Al-Cu-Mg alloys with compositions in the (+S) phase region. Experiments by means of hardness and tensile tests, differential scanning calorimetry and transmission electron microscopy (TEM) have been carried out to provide the relevant information for the calibration and validation of the model. The model considers a simplified precipitation sequence which involves a pre-precipitate structure followed by S phase. This pre-precipitate structure is referred to as Cu-Mg co-clusters instead of GPB zones based on atom probe and TEM studies from collaborators and a review of the literature. The competition between the Cu-Mg co-clusters and the S phase is modelled by assuming S phase forms at the expense of Cu-Mg co-clusters. In the model, the solvi of the Cu-Mg co-clusters and the S phase are calculated, the evolution of precipitates in terms of volume fraction, average size and the solute concentration in the matrix are described and the superposition of various contributions from precipitation strengthening, solution strengthening and dislocation strengthening are modelled. Strengthening by Cu-Mg co-clusters and S phase is described by the modulus strengthening mechanism and the Orowan bypassing mechanism, respectively. The predicted contributions to the critical resolved shear stress show that strengthening in the alloys is mainly due to the Cu-Mg co-clusters in the first stage of hardening and due to the S phase in the second stage of hardening. The model takes account of the composition dependency of precipitation rate for Cu-Mg co-clusters formation as well as the amount of Cu and Mg present in undissolved intermetallic phases. With a training root mean square error of 12MPa on an artificially aged 2024 alloy, the modelling accuracy on unseen yield strength data of two other alloys is 16MPa. Using a single set of parameters, the model has been applied to predict the hardness of a 2024-T351 alloy artificially aged at low temperature followed by short term underageing at higher temperature and then room temperature ageing. Good agreement between the predictions and the experiments indicates that the hardness changes during these multi-stage heat treatments can be well interpreted by considering Cu-Mg co-cluster dissolution, S precipitation and Cu-Mg co-cluster re-formation. Application to Al-xCu-1.7Mg alloys (x=0.2, 0.5, 0.8 and 1.1at.%) has shown good predictive capabilities of the model for the first stage of hardening. It is also shown that the model is applicable to Al-Cu-Mg alloys with Si contents at levels of 0.1-0.2wt.%. Modelling results of various Al-Cu-Mg alloys during natural ageing, artificial ageing and multi-stage heat treatments indicate that the model is capable of predicting the evolution of microstructure and the yield strength as a function of composition and heat treatments, and can provide a predictive tool for predicting the strength of Al-Cu-Mg based welds.

Synchrotron radiation computed tomography (SRCT) has been used to analyse the tensile failure mechanisms in carbon fibre/epoxy composites. Two specimen types were analysed – in situ loaded coupons and filament wound samples, taken from incrementally loaded cylinders and scanned “post mortem”. The effects of fibre, matrix and interfacial properties on the initiation and accumulation of fibre breaks have been analysed. Breaks accumulated on a power law curve as a function of fibre stress; however the fibre and matrix moduli had little effect on accumulation. Initial analysis of the fibre Weibull moduli showed little correlation between Weibull modulus and break accumulation. Singlets initiated in low fibre volume fraction areas; however a full investigation into the effects of varying fibre volume fraction has not been possible. Attention was focused on the formation of interacting groups of broken fibres (clusters), as they are believed to be the strength-defining failure event. The in situ coupons had much larger maximum cluster sizes than the filament wound counterparts (14 vs. 9), and a correlation between high break density and low cluster percentage is proposed. No simple correlations were found between fibre/matrix moduli and the clustering parameters. Clusters formed in one load step, and did not grow from singlets or smaller clusters, which suggests a dynamic process. The interface is suggested to be key to damage initiation and propagation. The work provides links between experimental studies and simulation tools by informing and validating a micromechanical tensile failure model. Comparisons between experimental and modelled results found that the model accurately predicted the composite failure strain but not the complex damage accumulation processes. The model under-predicted both cluster size and the proportion of interacting breaks; this is attributed to the inaccurate modelling of the stress transfer process. Both experimentally and analytically the dominant parameter controlling clustering was the overall stress concentration factor. This has been infrequently analysed in work published in the literature, and is the recommended focus of the future work.

No description available.

Two optical full-field strain measurement techniques, Digital Image Correlation and the Grid Method, are applied to characterise the strain-rate dependent constitutive behaviour of composite materials. Optical strain measurement techniques based on full-field images are well established for material characterisation in the quasi-static strain rate region, however in this work they are developed to study the material behaviour at intermediate strain rates, which is relatively unexplored. For this purpose a testing methodology that combines high speed imaging and the use of a high speed test machine is devised. The overall goal is to extract composite materials constitutive parameters to be used in the modelling of strain rate dependent behaviour. Particularly the strain rate dependence of the stiffness of glass and carbon fibre reinforced epoxy materials is investigated. A characterisation procedure based on off-axis specimens with oblique end-tabs is developed and applied to the study of the shear behaviour of a carbon/epoxy composite material. The research in the PhD programme constitutes an essential first step for more profitable applications of full-field measurement techniques to high speed testing. Full-field data acquired with the experimental methodology devised here can be used to investigate non linear material behaviours. Furthermore this experimental methodology, applied to specimens that generate non uniform strain fields, can produce strain maps useful for the application of the Virtual Fields Method. This will lead to a reduction of the experiments needed to characterise materials.