Aerospace applications typically require electronic products with not only higher levels of reliability than the consumer electronics industry but also longer service lives within demanding working environments. The transition will inevitably mean changes to design and manufacturing procedures, which is likely to incur a significant cost to the business. For example, the best candidate Pb-free solder alloys have been shown to require higher soldering temperatures and have higher surface tensions. Moreover, a reduction in product safety and reliability is not acceptable to the industry. This present work is divided into three sections. Firstly, the effect of increased component soldering temperatures on the integrity of the epoxy laminate material used for manufacture of printed circuit boards (PCB) has been assessed. Secondly, the required changes in soldering process parameters have been investigated for a range of solders and PCB finishes, largely due to the different wetting characteristics brought about by the increased surface tension of the Pb-free solders. Thirdly, the reliability of SnAgCu solder is assessed in comparison to the currently utilised SnPbAg solder alloy. This has been achieved firstly by accelerated thermal cycling, as the dominant mode of failure in a solder joint is typically thermo-mechanical fatigue and as such is already well researched. In addition, the mechanical fatigue properties have been assessed using a novel accelerated vibration test method and then finally, the two individual accelerated environmental tests of thermal cycling and vibration have been combined in a novel way to assess whether the combination is especially dangerous for SnAgCu solder reliability. A secondary objective of the combined environment test was to see if the well established thermal cycling test method for demonstration of product reliability can be further accelerated while still producing solder joint failure representative of those in-service. The present work shows that SnAgCu solder has inferior thermo-mechanical and mechanical fatigue life to SnPbAg solder. A combined environment test has been developed which effectively combines the single environments of thermal and vibration. The combination of thermal cycling with superimposed vibration is especially dangerous for SnAgCu solder, where an 89% reduction in the characteristic life is observed when compared to the equivalent thermal cycling characteristic life. It is suspected that a large reduction in life will be observed in SnPbAg solder, but not as pronounced as SnAgCu due to SnPbAg solders ability to better withstand plastic deformation that is induced by thermal cycling.

To improve the efficiency of turbine aero engines, higher operating temperatures and weight savings are being investigated. Alloys such as RR1000 are being trialled as they perform better at higher temperatures than current nickel-based superalloys. To achieve weight savings, inertia welding is being trialled for turbine discs but current post weld heat treatments reduce fatigue life. In this investigation, a number of novel post weld heat treatments were trialled aimed at improving post weld microstructure and fatigue properties. Extensive microstructural characterisation and mechanical testing were used to assess the effect of these treatments on both parent and weld materials. Post weld heat treatment (PWHT) was found to have a great effect on the size and distribution of γ' and carbides, particularly when a PWHT temperature of 980ºC or above was used. The effect of this microstructural change on the hardness of the weld and parent material has also been characterised. Extensive total life fatigue testing was carried out at 650ºC. It was found that failure can occur in both the parent and weld material, although it is deduced that the yield stress of the weld needs to be surpassed to see weld failure (plasticity in the weld). Increasing dwell time at peak load reduced the life of these components. Two mechanisms for crack growth were observed with initiation either at the surface or at a large Hf rich particle subsurface. Oxidation was found to have a large effect on both initiation and growth of fatigue cracks. By introducing a sharp pre-fatigue crack into samples, static load testing was used to determine a threshold value of K (stress intensity factor) for growth and growth rates were plotted at different K values. It was seen in these tests that PWHT had a large effect on growth rates and threshold values of K.

This work covers two projects, with the common theme being fine metallic powders and the use of polymer binder systems to aid processing.

Linear friction welding (LFW) has been selected as a screening method to investigate the solid state joining of the Ni-based superalloys: IN718, RR1000, Alloy10, IN100, IN713LC and CMSX4, with the volume fraction (Vf) of γʹ range from 20-70%. Similar welds of each alloy and one dissimilar weld, IN718-IN713, are studied in this project. The extreme thermomechanical history during LFW results in dynamic recrystallisation in the weld zone, dissolution and reprecipitation of the strengthening phase in the HAZ. Sustained load crack growth (SLCG) threshold testing was carried out at 650°C in air for all the similar welds. Weld IN718 which had a low Vf of γʹ+γʹʹ, had the highest threshold (25MPa√m) and highest SLCG resistance. However, there was no such clear effect of γʹ size and volume fraction on threshold value (9-19MPa√m) in welds with a high γʹ content. The crack growth rates of all these welds accelerated sharply from 10-5 to 10-2mm/s over a limited increase in K. A comprehensive mechanical assessment of dissimilar weld IN718-IN713 was carried out, including tensile tests at room temperature and 630ºC, low-cycle and high-cycle fatigue tests at 630ºC on notched specimens, fatigue crack propagation testing at 630ºC, and stress rupture testing at 650ºC.

The steel industry is facing significant competition on a global scale due to the drive for light-weighting and cheaper more sustainable construction. Not aided by oversupply in geographic sectors of the industry, there is significant competition within the slowly shrinking sector. The recent growth in developing countries through installation of modern plant technology has led to the reduction in unique selling points for mature steelmaking locations. As such, to compete with the equalling product capability and innate cheaper production costs within developing areas the industries in Europe and North America require significant improvements in productivity and agile resource management. To date the basic oxygen furnace has been somewhat treated as a black box within industry, where only control parameters are monitored, not the fundamental mechanisms within the converter. Studies over the past 30 years have shown the basic oxygen furnace is unable to attain the thermodynamic minimum phosphorus content within the output liquid steel. Coupled with the need to drive down resource cost, with a potential for high content phosphorus ores the internal dynamic system of the basic oxygen furnace requires more rigorous understanding. With the aid of in-situ sampling of a pilot scale basic oxygen furnace, and laboratory studies of individual metal droplets suspended in a slag medium (known to be a key driving environment for impurity removal) the present project aims to provide insight into the transient interfacial area between slag and liquid metal through basic oxygen steelmaking processing. Initially the macroscopic dynamics including the amount of metal suspended in the gas/slag/metal emulsion, the period of time it is suspended for, and the speed at which it moves, is investigated. It was found that these parameters vary greatly through the blow, with a normal peak in residence times near the beginning of the blow and a dramatic increase in metal circulation rates at the end of the blow, when foaming is reduced or collapsed. Further to this, a method of interrogating the size of metal droplets within the slag layer using X-ray computed tomography is introduced. The study then progresses into the microscopic environments that individual droplets are subjected to during steel processing. Initially the cause of spontaneous emulsification in basic oxygen furnace type slags is investigated through high temperature-confocal scanning laser microscopy/X-ray computed tomography led experimentation, with the addition of null experiments conducted to rationalize the experimental technique. It was found that the flux of oxygen across the interface was the cause and thus the confirmation of material transfer across the interface being the driving force. Furthermore the physical pathway of emulsification is interrogated and quantified, with in-situ observation of spontaneous emulsification in the high temperature-confocal scanning laser microscope enabled through use of optically transparent slags. The life cycle of perturbation growth, necking and budding is observed and quantified through high-resolution X-ray computed tomography. In addition a phase-field model is developed to interrogate slag/metal systems in 2D and 3D variations, giving rise to the ability to track the cause of emulsification and to predict its occurrence. Finally the project progresses with the in-situ investigation of spontaneous emulsification as a function of initial metal composition. The behaviour of droplet spontaneous emulsification is seen to reduce in severity and subsequently to decline into a non-emulsifying regime below a critical level. Free energy calculations coupled with a measure of the global interfacial tension increase give quantifiable reasoning as to the behaviour seen.

The aim of this research project is to provide the academic and industrial community with a numerical tool that can be used for describing extreme flow cavitation scenarios and the atomisation process of these multiphase jets in a low-pressure environment. The research lies in the intersection of Numerical Analysis, Applied Physics and programming. From the physical point of view, the project has two different strands: The first is developing a methodology for channel flows due to a rapid pressure drop which is possible to result into various flow regimes inside the channel. The second step is to track the liquid fragmentation of the liquid jet downstream the channel exit and describing the atomisation process to liquid ligaments and blobs to droplets. Using a fully Eulerian approach, this research aims towards a holistic approach that addresses some of the major challenges that govern superheated jets atomisation. The finite volumes method in a compressible framework is used utilising various models for modelling the underpinning physics of flashing jets. Flashing occurs either if a liquid follows an isothermal depressurisation or isobaric heating. In both cases, the fluid fails to adjust to the local changes in pressure and temperature admitting a metastable state which makes the process more challenging to understand. The Homogeneous-Relaxation-Model (HRM) is used for modelling the heat transfer under sudden depressurisation conditions accounting for the non-equilibrium vapour generation. A new pressure equation is proposed which employs the continuity equation indirectly. The pressure responds to compressibility and density changes due to the rapid phase change and includes the surface tension contribution in the pressure-velocity coupling algorithm. The coupling of the continuity and momentum equation with the HRM and the interface tracking method is thoroughly described. The result of this coupling is a conserved numerical method that is capable of characterising the flow regimes and the impact of bubble nucleation on the mass flow rate. The present study presents a numerical approach for simulating the atomisation of flashing liquids accounting for the distinct stages, from primary atomisation to secondary break-up to small droplets Following the Eulerian-Lagrangian-Spray-Atomisation approach, the concept of the surface density Σ is introduced into the methodology for the spray dynamics. The proposed approach has the advantage of avoiding the unrealistic common assumption of pure liquid at the nozzle exit. It models the change in the regime inside the nozzle treating flashing in a unified approach simulating the metastable jet both inside and outside the nozzle. Important mechanisms such as thermal non-equilibrium, aerodynamic break-up, droplet collisions and evaporation are modelled in a novel atomisation model. The modified Σ- equation employed a new source term proposed for cryogenic jets. A wide range of numerical tests is presented for validation and obtaining insights for the underlying physics. Short and long nozzle geometries are tested for both low and high-pressure releases for flashing water, R134A, liquid nitrogen and LNG. Results for turbulent flows for both sub-cooled and superheated liquids are presented showing that the proposed approach can accurately simulate the primary atomisation.

<p> Unstable failure in underground mining occurs when a volume of material is loaded beyond its strength and displaces suddenly. It is recognized on various scales, from small rock bursts to the collapse of pillars or entire sections of a mine. The energy that is released during smaller scale events is manifested through the ejection of material, which can pose a hazard to the safety of miners. Larger scale events generate seismic waves as mine workings are damaged and may entrap miners or terminate production. </p><p> This dissertation focuses on the analysis of unstable failure in an underground room and pillar mining environment. The potential for violent pillar failure is assessed using numerical modeling techniques and a parametric approach to loading conditions and material strength properties. The magnitude of instability is quantified by calculating the release of kinetic energy that occurs as failure progresses in each simulation. </p><p> Fundamental mechanisms associated with the release of kinetic energy are analyzed in a series of finite difference models, and the results are compared with analytical solutions to illustrate the applicability of the energy calculations to increasingly complex modes of failure. Back analyses are performed on two room and pillar mine collapse events from the western United States by constructing large-scale models and reproducing widespread failure. The values of energy released in two-dimensional models are extrapolated by assuming a depth of failure in the third direction, and the total energy values are compared to the documented seismic magnitudes from each collapse through empirical equations. With further development of this numerical modeling approach, energy consideration may be used to study the potential for instability in a wide variety of mining excavations and identify the associated range of hazards.</p>

Vermiculite is a clay mineral that is generally used for a wide range of applications such as in agricultural, horticultural and construction industries. This is due to its various properties which include high porosity, lightweight, thermo-insulating, non-toxic and good absorption capacity when exfoliated. The objective of this research was to critically evaluate the fundamental interaction of electromagnetic waves with vermiculite from different source locations and to understand the mechanism of exfoliation in an applied microwave field. When vermiculite minerals are placed under the influence of high electric fields, they expand due to the rapid heating of their interlayer water, which subsequently builds up pressure that pushes apart the silicate structure. The degree of exfoliation is directly related to the intensity of the applied electric field. The principal areas covered in this thesis include: a detailed review of the fundamentals of microwave processing and issues surrounding scale up; a critical literature review of vermiculite mineralogy, and previous methods of vermiculite processing and their limitations; understanding the interaction of microwave energy with vermiculite by carrying out mineralogical and dielectric characterisation; microwave exfoliation tests of vermiculite minerals from different source locations and a comparative energy and life cycle analysis of microwave and conventional exfoliation of vermiculite. A detailed review of the literature revealed that conventional exfoliation of vermiculite by gas or oil fuelled furnaces has significant limitations such as emissions of greenhouse gases, high-energy requirements (greater than 1 GJ/t), health and safety issues and poor process control. All work reported so far on microwave exfoliation of vermiculite has been limited to laboratory scale using domestic microwave ovens (2.45 GHz, power below 1200 W) and the route to scale up the process to industrial capacity has not given due consideration. Mineralogical characterisation of vermiculite from different geographical locations (Australia, Brazil, China and South Africa) revealed that only the sample from Brazil is a pure form of vermiculite while the other samples are predominantly hydrobiotite. All the samples have varying degrees of hydration with the Brazilian sample having the highest total water content. The presence of water in any form in a material influences its dielectric response and ultimately the microwave absorbing properties. The dielectric characterisation carried out on the different vermiculite samples shows that the vermiculite mineral structure is effectively transparent to microwave energy, but it is possible to selectively heat microwave absorber, which is the interlayer water in the vermiculite structure. The continuous microwave exfoliation tests carried out at both pilot scale at 53-126 kg/h and the scaled up system at 300-860 kg/h demonstrated that microwave energy can be used for the industrial exfoliation of vermiculite at high throughputs and is able to produce products below the specified product bulk densities standard required by The Vermiculite Association (TVA). The degree of vermiculite exfoliation depends on factors such as power density, feedstock throughput, energy input, interlayer water content, particle size of the feedstock, and vermiculite mineralogy. The highest degree of exfoliation was recorded for the Brazilian sample, which also had the highest water content. Life cycle analysis (LCA) frameworks by the International Organisation for Standardisation (The ISO 14040: principles and framework and ISO 14044: Requirements and guidelines) and British standards institution (PAS2050) were used to carry out comparative life cycle analysis of vermiculite exfoliation using microwave heating and conventional (industrial and Torbed) heating systems. The results showed that the microwave system potentially can give an energy saving of about 80 % and 75 % over industrial and Torbed Exfoliators respectively, and a carbon footprint saving potential of about 66 % and 65 %. It can be concluded that the reduced dust emission and noise from the microwave system would improve the working conditions, health and safety. Furthermore, the methodology discussed in this project can be used to understand the fundamental of microwave interaction with perlite and expanded clay, which are minerals with similar physical and chemical compositions as vermiculite.

Single crystal Ni-based superalloys are used in the highest temperature components in jet turbine engines owing to their excellent properties under creep conditions. These alloys owe their properties greatly to their chemical composition, and in particular the addition of slow diffusing elements like Re delays the creep deformation significantly. Vacancy diffusion has been suggested to be the rate-controlling process for creep deformation at high temperatures, and elements like Re are expected to interact with the vacancies in slowing them down. This has been investigated in the present work using ab initio calculations. Specifically, dilute as well as non-dilute binary alloys of Re, W and Ta in Ni were considered to study the effect of chemical composition on the rate of vacancy diffusion. Analytical formulations were used to describe the diffusion equations, however their applicability was restricted to the dilute regime. For the calculation of diffusion coefficients particularly in the non-dilute regime, kinetic Monte Carlo simulations were performed. The energies and the diffusion barriers were described using the cluster expansion method. Results suggested appreciable modifications of the vacancy diffusion coefficients, suggesting that the beneficial role of slow-diffusing elements in Ni-based superalloys could be partly explained by their effect on vacancy diffusion.

Chemical composition of structural steels with a ferrite-pearlite microstructure has been developing towards decreasing carbon content, to increase weldability, with increased microalloying element content, to provide grain refinement, solid solution and precipitation strengthening. During the UOE forming of large diameter (more than 400 mm) welded pipes the strength drop from plate to pipe, as a result of reverse deformation (the Bauschinger effect), depends on steel grade, namely microalloying element content, and processing. In this project the microstructure of two Nb- and V-microalloyed steels has been studied with optical, scanning and transmission electron microscopy. The dislocation density and (Ti,Nb,V,Cu)-rich particle diameter, volume fraction and number density were measured for as-rolled and annealed (30 min. at 400 \(^0\)C and 550 \(^0\)C) steels. The Bauschinger effect was measured during compression-tension testing for the same steel conditions. The yield stress drop during reverse deformation has been found to increase with an increase in forward pre-strain, dislocation density and particle number density within the effective particle diameter range of 12-50 nm. On the basis of dislocation-particle interaction analysis, a quantitative model of work-hardening behaviour dependence on particle number density and dislocation density has been derived for the reverse deformation of studied steel grades.