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.

As a result of the increasing energy demand but a heavy dependence on easy-to-produce conventional oil, vast reserves of recoverable heavy oil have been left untapped. According to the International Energy Agency, IEA, fossil fuels – oil, coal, natural gas – will still predominate, despite a decline in their overall share, towards meeting the increasing world energy demand. While heavy oil has been predicted to account for an increasing share, contributions from conventional light oil have been predicted to drop from 80% to 53% in the next two decades (IEA, 2013b). Therefore, the large reserves of the under-utilised heavy oil, if extracted cost-effectively and in an environmentally friendly manner, will facilitate the meeting of both the short and long term energy demands. In this work, different thermal heavy oil recovery processes were reviewed with particular attention given to the air injection processes. In-situ combustion, ISC, has been identified as the most efficient and environmentally friendly technique used to recover heavy oil. Until the last decade, there was only a small interest in the conventional ISC. This is due to the complex nature of the processes taking place during ISC and the lack of success recorded over the years. The successful pilot scale testing of the Toe-to-Heel Air Injection, THAI, by Petrobank has revived interest both industrially and in the academic environment. Experimentally, THAI has been consistently proven to exhibit robust and stable combustion front propagation. Among the advantages of THAI is the ability to incorporate the in-situ catalytic upgrading process, THAI-CAPRI, such that further catalytic upgrading is achieved inside the reservoir. To realise the theoretical promise offered by THAI-CAPRI, there is a need to develop a reliable numerical simulation model that can be used to scale laboratory experiments to full field scale. Even for 3D combustion cell experiments, only one such model exists and it is incapable of predicting the most critical parameters affecting the THAI process. Therefore, the subject of this work was the development and identification of an accurate and reliable laboratory scale model that can then be used to develop field scale studies and investigate the effect of reservoir geology on the THAI process. However, because of the significant uncertainty introduced by the kind of kinetics scheme used and the fact that the main mechanism through which fuel deposition takes place is still a contentious issue, three different kinetics schemes, based on Athabasca bitumen, have been tested for the model of the 3D combustion cell experiment. All the models offered an insight into the mechanism through which oxygen production begins. They revealed that oxygen production was as a result of the combustion front propagating along the horizontal producer (HP). They also showed that the presence of coke inside the horizontal producer is an essential requirement for stable combustion front propagation. It was also observed that LTO is not the main mechanism through which fuel is deposited as oxygen does not bypass the combustion front. The models also showed that the temperature around the mobile oil zone (MOZ), where catalytic reaction in the CAPRITM is envisaged to be located, will not be sufficient to make the hydro-treating catalysts effective. Therefore, it is concluded that some form of external heating must be used in order to raise the temperature of the catalyst bed. Two out of the three different Arrhenius kinetics schemes that were successfully used to history-match the 3D combustion cell experiment were adjusted and implemented in field scale simulations. This is because the kinetics parameters obtained from the laboratory scale model cannot be used directly for the field scale simulation as they led to excessive coke deposition. A comparative study, between the two kinetics schemes, showed that the adjusted direct conversion kinetics predicts higher oil rate, and higher air rate can be injected right from the initiation of the combustion compared to in the case of the split conversion kinetics. The direct conversion kinetics was then used to study the field performance because it provided a more realistic representation of the physicochemical processes than the split conversion kinetics. The study revealed that even if the combustion front swept the whole reservoir length, it has to propagate along the horizontal producer for oxygen production to take place. It was observed that the combustion zone does not only have to cover the whole reservoir length but also has to expand laterally in order to produce the whole reservoir. For heterogeneous reservoirs, the THAI process was found to have larger air-oil ratio (AOR) in reservoir containing a discontinuous distribution of shale lenses compared to the homogeneous model. However, overall, the THAI process is only marginally affected in terms of cumulative oil recovery. The combustion front was found to propagate in a stable manner just like in the homogeneous model. However, further study is needed to investigate the effect of different permeability distributions would have on the THAI process. This should allow the optimum location of the wells to be determined. Studies of the effect of bottom water (BW) on the THAI process have shown that the oil recovery is heavily affected depending on the thickness of BW zone. It was found that the location of the HP well relative to the oil-water interface significantly affects the oil production rate and hence the cumulative oil produced. More oil is recovered when the HP well is located inside the BW zone. It was found that a ‘basal gas layer’, just below the oil-water interface, is formed when the HP well is located in the BW zone. The study has shown that there is a limit to BW thickness above which the THAI process cannot be applied to a BW reservoir. However, future work is needed to determine this BW thickness. The reservoir cap rock, depending on it is permeability and porosity, only marginally affects the oil recovery in the THAI process. It was found that the cap rock aids in heat distribution to the extent that most of the upper oil layer is mobilised. However, the effect is observed to be less pronounced with increased permeability and porosity. Future work should look into whether longer operation period has an adverse effect on the stability of the combustion front, and thus on the overall performance of the THAI process.

Economic markets continue to become more complex, creating challenges for organisations with the scope, complexity, and interdependencies of emerging risks necessitating a more robust and integrated approach to risk management and internal control. To this effect, enterprise risk management (ERM) has been the topic of increased attention with regulatory, business and academic arenas alluding to the need for ERM and improved internal control systems. The traditional practices of managing risks in silos has had implications on the internal control systems of organisations as the organisational focus has been on those directly related to financial operations and reporting. Therefore, the aim of this study was to determine the extent to which organisations have implemented ERM, particularly those in the mining industry, and then understand the impact this ERM has on the internal control system. To this end, qualitative research with an exploratory design was conducted. Twelve executives and senior managers across eight organisations, who are responsible for risk management and assurance of internal controls, were interviewed to uncover their distinctive insights regarding this phenomena. The rich data that was unearthed was analysed using thematic analysis techniques. The evidence from the study showed that while the ERM practices of organisations are between intermediate and mature levels, more work still needs to be done in order to institutionalise ERM. Furthermore, ERM improves the internal control system of the organisation, however the maturity of the ERM process and the leadership in the organisation are big determinants of the extent to which the improvements can be realised. Other factors were identified which necessitate enhancement and sustainability of the ERM capability and formed the basis of a model for enterprise risk and control integration that was developed. The results of this research provided additional insights that will bolster the advancement of internal control management through ERM. / Dissertation (MBA)--University of Pretoria, 2013. / lmgibs2014 / Gordon Institute of Business Science (GIBS) / MBA / Unrestricted

<p> Thirty cross-bred steers (initial BW 375 &plusmn;37.2, replicate 1; and 535.0 &plusmn; 39.4 kg, replicate 2) were utilized to investigate the effects of Mo water concentration on performance, carcass characteristics, and mineral status of feedlot steers fed a growing and finishing diet for 151 and 112 d for replicate 1 and replicate 2, respectively. The experimental design was a randomized complete block design. Steers were blocked by weight and then divided into 2 weight block replicates each consisting of 15 steers. Steers were randomly assigned within block to one of 5 treatments (3 pens/treatment; 1 steer/ pen; 2 replicates/treatment). Water treatments consisted of: 1) 0.0 &mu;g, 2) 160 &mu;g, 3) 320 &mu;g 4) 480 &mu;g Mo/L, and 5) 960 &mu;g of supplemental Mo/L added as Na₂MoO₄ to the drinking water. Steers were housed in individual pens that contained individual 265 L water tanks for monitoring water intake. Daily water intake was recorded for each steer. Steers were individually weighed on 2 consecutive days at the beginning and end of the experiment and interim weights and jugular blood samples were obtained every 28 d. Liver biopsies were obtained on d0 and 84 from each steers. Steers were transported to a commercial abattoir, slaughtered, and individual carcass data and liver samples were collected. Initial BW was used as a covariate for statistical analysis of the data and significance was determined at <i>P</i> &le; 0.05. No differences were observed for final BW (<i>P</i> &le; 0.98). Overall ADG, DMI, feed efficiency and water intake were similar across treatments. Hot carcass weight, dressing percentage, yield grade, LMA, adjusted fat thickness, KPH, and marbling scores were similar across treatments. Liver and plasma Cu, Mo, and Zn concentrations were similar across treatments. These data indicate that water Mo concentration had no impact on performance, mineral status, water intake, and carcass characteristics in feedlot steers fed a high concentrate diet.</p>

Additive manufacturing of aluminium alloys using selective laser melting (SLM) is of research interest nowadays because of its potential benefits in industry sectors such as aerospace and automotive. However, in order to demonstrate the credibility of aluminium SLM for industrial needs, a comprehensive understanding of the interrelation between the process parameters, produced microstructure, and mechanical behaviour is still needed. This thesis aims at contributing to developing this comprehensive understanding through studying the various aspects of the process, with investigation of the powder raw material to the near fully dense samples, focussing on the alloy AlSi10Mg. The primary building blocks in the SLM process are the single tracks. Their formation is affected by the physical properties of the material that control the laser-material interactions. Keyhole mode melting was found to be dominant when processing AlSi10Mg, producing conical-shaped melt pools. Porosity was not evident in single tracks and individual layers. Satellites and balling defects, however, were observed on top of the tracks and layers at higher scan speeds, which contribute to porosity formation with layer progression. The combination of process parameters controls the amount of porosity formed, with the scan speed controlling the type of pore; metallurgical or keyhole pore. A pre-melt scan strategy significantly reduced porosity and successfully produced 99.8% dense samples. Furthermore, the pre-melt scan strategy was seen to effectively reduce the number of pores developed when using powder that does not fully comply with the process standards. The gas flow rate within the process chamber controlled laser spatter and condensate removal during processing, which in its turn affected the degree of porosity in the samples. The SLM process resulted in an AlSi10Mg alloy with a characteristically fine microstructure, with fine equiaxed grains at the melt pool core and coarser elongated grains at the boundary. The material showed a strong texture, owing to directional solidification. Cellular dendritic Al with inter-dendritic Si was observed. The material was subjected to a T6 heat treatment that transformed the microstructure into spheroids of Si in the Al matrix. This study investigated, for the first time, the local mechanical properties within the SLM material using nanoindentation. This showed a uniform nano-hardness profile that was attributed to the fine microstructure and good dispersion of the alloying elements. Spatial variation within the material was recorded after the T6 heat treatment due to phase transformation. This study is also the first to report on the compressive behaviour of solid SLM material, which is important for developing prediction and simulation models. The heat treatment softened the material and provided it with an increased ductility under indentation, tensile, and compressive types of loading. In addition, the material showed good fatigue performance, which was further improved by heat treatment and machining to obtain a smoother surface roughness. This investigation has, therefore, developed an understanding of the various aspects of the SLM process yielding near fully dense parts and defined the microstructure-mechanical property interrelation promoting the process for Al alloys in a number of industrial sectors.