Includes bibliographies. / The effects of high energy milling on tin bronze with the composition Cu-24.6wtSn have been examined using hardness testing, optical microscopy, scanning electron microscopy, transmission electron microscopy and x-ray diffraction. High energy milling has caused mechanical alloying of an elemental copper and tin powder blend, and mechanical milling of a cast powder and a melt quenched powder. Nanocrystalline grains with a size between 5 nm and 50 nm have been directly observed in the final milled powder. The powder consist of the a phase and 8 phase and is partially amorphous. An extension of the solid solution solubility has also been detected due to milling. The formation of the metastable tin-rich 11 phase has been observed in the intermediate stage of mechanical alloying of the elemental powder blend due to the higher diffusivity of tin in copper over copper in tin. The formation of the 11 phase during mechanical alloying of tin bronze with the composition Cu-24.6wtSn has not been reported before. The morphological development of the three initial powders has proceeded by different mechanisms during milling due to the different hardness and toughness of the starting powders. Milling of the elemental powder blend and the cast powder proceeds via classic mechanisms for milling of ductile powders and brittle powders respectively, while milling of the tougher melt quenched powder proceeds via a combination of the two mechanisms. An attempt to process the milled powder into a bulk state using various thermomechanical techniques while still retaining a nanocrystalline grain size has not succeeded. The high diffusivity of the material at elevated temperatures has led to grain growth into the micrometer range even at relatively low thermo-mechanical processing temperatures. The milled powders have poor compaction properties due to the highly deformed structure and therefore the processed material has poor properties compared to a cast material.

Bibliography: p.103-116. / Comminution is the process which aims at increasing the surface area and the resultant liberation of a particular constituent from the mass of a solid. The autogenous mill uses tumbling to effect comminution, but instead of special milling bodies being added, pebbles of the material to be comminuted are used. The autogenous comminution process utilises less than 0,1 per cent of the energy input. The principal objective of the present work was to analyse autogenous milling behaviour in terms of the individual comminuting mechanisms and to establish the inter-relationships between the main process variables, namely rock petrography, size distribution of the feed, applied load, relative velocity and environment. In this manner the optimisation of the process and an improvement of its efficiency was sought. In addition the establishment of testing procedures to predict the autogenous milling behaviour of a given type of rock was aimed. In the present work the gold bearing Witwatersrand quartzite was used, although the findings are applicable to other types of rocks. Since fracture phenomena are involved in all comminuting mechanisms of impact-compression, chipping and abrasion, slow compression and Brazilian tests were performed. The grain size and the mineral composition of the rock has been found to have a large influence on the local stresses required for these processes. Indeed the results show that the fragility and therefore ease of comminution increases with increasing grain size of the quartzite. Brazilian tests on drill cores of varying diameters may allow the prediction of the critical size of rock of the mill feed which can survive in a mill of given characteristics.

A range of carbon-graphites with differing properties has been evaluated for wear resistance. These include carbons with a high degree of graphitic order (natural and synthetic graphite), those with little or no such order (pitch bonded cokes, glassy carbons) and impregnated grades. Testing has been carried out using abrasive wear, dry sliding wear, particle erosion, slurry erosion, cavitation erosion and the corresponding wear rates have been related to the bulk properties of the different materials. In all tests; hardness, elastic modulus, porosity and the presence of fillers were found to influence the wear rates of the various grades. Maximum wear rates were consistently observed with the softer, more porous unfilled carbons.

Bibliography: pages 129-139. / Silicon nitride, Si₃N₄, ceramics were produced using either silicon or silicon nitride powder. The silicon was reaction bonded in nitrogen atmosphere to form reaction bonded Si₃N₄,which was then sintered between 1700°C and 1800°C to form a dense Si₃N₄ ceramic. The silicon nitride powder compacts were also sintered between 1700°C and 1800°C. In order to achieve densification Y₂O₃-A1₂O₃ additive combination was used in both processing routes. The physical and mechanical properties of the Si₃N₄ materials was found to be dependent on the processing conditions. The post sintered reaction bonded Si₃N₄ materials had the highest densities and hardness values, while the sintered Si3N4 materials had the highest strength and toughness values. The microstructure was also influenced to a great extent by the processing conditions, and this in tum influenced the mechanical properties of the ceramics.

A study has been made of the effects of combinations of pressure and temperature ori six polycrystalline, synthetic graphites, in the high pressure domain (> 1 GPa).The graphites were investigated in three different conditions: (1) the "as received" condition (AR condition),(2) after exposure to pressures of about 3 GPa at room temperature (in a piston-cylinder device – PC condition) and (3) after exposure to temperatures of about 1500°C at pressures of about 5.5 GP a (high temperature- high pressure, or HTHP, condition). Their microstructures have been compared on the basis of X-ray diffraction measurements to determine their crystallite sizes (L˳ and L˳), interplanar spacings (c and a) and textures. Optical and scanning electron microscopy were used to examine their fracture surfaces and macro porosity. Mercury porosimetry provided a means of establishing the pore size distribution of pores of less than 20 1-1m diameter. Bulk and skeletal densities were determined from mercury porosimetry and helium pycnometry respectively. The effects of PC and HTHP conditioning on their mechanical properties, were measured by both uniaxial compression fracture tests, and by electrical resistivity measurements. In addition, the triaxial behavioursof the six graphites in the AR condition were evaluated from piston-cylinder compression tests. All the isopressed graphites were found to have very similar crystallite sizes, interplanar spacings and textures in the AR condition. The extruded graphite had larger crystallite dimensions, and was slightly less isotropic, than the other grades. Fracture occurred due to cleavage of the basal planes of crystallites in the filler particles or binder. The size, shape and orientation of filler particles and porosity with respect to the applied stress field determined whether fracture was intergranular, or trans granular, in nature. Limited basal plane slip and sub-critical microcracking caused uniaxial compressive stress-strain curves typical of those of polycrystalline graphites,i.e. convex with respect to the strain axis. Fracture strengths and strains were related to the proportion of amorphous, intercrystallite bonding and, to a lesser extent, to porosity.

Bibliography: pages 76-82. / An investigation involving the erosion of WC-Co alloys by solid particle impact erosion was undertaken to determine the mechanism by which material is removed. For this purpose a simple particle-gas stream erosion apparatus was employed. The nineteen different WC-Co alloys studied were initially characterised according to mi crostructural and mechanical properties. An investigation of the influence of various parameters on erosion was conducted to establish the manner by which the WC-Co alloys were eroded. A limit in erosion rate occurred with increasing particle size for all samples, which is associated with ductile erosion. The variation of erosion rate with the angle of impact was found to be dependent on the binder content. A maximum in erosion occurred at a 90u angle of incidence for the low cobalt content alloys and in the region of a SOU angle of incidence for high cobalt content alloys. Thus suggesting a predominantly brittle mode of erosion, with a ductile mode becoming more important with increasing binder content. The erosion rate was found to increase with decreasing hardness. For impact angles of 45u and greater, the hardness effect was masked by microstructural influences. Examination of the steady state eroded surface and the single particle impact event, using the scanning electron microscope revealed three modes of material removal. These may occur simultaneously, the predominant mode, however, changes with binder content. For WC-Co alloys containing less than 10 wt-% cobalt, cobalt extrusion was observed as being the controlling mode of material removal. Maximum carbide grain cracking was associated with a cobalt content of 10 wt-%. Above this binder level ductile cutting of the matrix became an increasingly important mode of material removal.

Many of Eskom's coal fired power plants have an average age of 170 000 hours and a few operating close to 300 000 hours. Main steam temperatures experienced in a power plant vary between 535-555°C. These operating conditions place main steam pipe components to operate within the creep regime. It is of utmost importance for safety and plant health that these critical components are managed to determine the remaining life and risks associated with high temperature exposure for prolonged periods of time. Non-destructive testing (NDT) methods are utilised extensively on Eskom power plants to determine the remaining life and replacement strategies for critical components. Surface replication is used as a life assessment tool for creep damage quantification of main steam pipe work. A large part of maintaining plant is repair welding on creep aged and sometimes creep aged material as entire system replacements are impractical and time consuming. By repair welding new material onto creep aged material, mechanical and microstructural properties of the creep aged material deteriorates. The study of this work is focused on characterising the as-received materials from Eskom power plants and using these creep aged materials to create cross-weld samples with virgin material. The cross-weld samples were creep-rupture tested at high temperature and low stress conditions to determine the fracture location of repair welded cross-weld samples. Once ruptured, the zone of rupture, was identified and created in a larger volume by simulation using Gleeble® thermo-mechanical equipment. The as-received base materials were subjected to different operating conditions hence contain different degrees of creep damage. The microstructural evaluation of the creep damaged material was conducted using optical microscopy, scanning electron microscopy (SEM), coupled with more advanced electron backscattered diffraction (EBSD). Microhardness and hot tensile testing were included to characterise the mechanical degradation of the as-received material. The fracture location of the creep-ruptured cross-weld samples were investigated using optical microscopy, SEM and EBSD and occurred on the outer region of the heat affected zone (HAZ) of the creep aged material. The fine grained microstructure with coarse precipitation of this region is characteristic of the fine grain heat affected zone (FGHAZ). The occurrences of voids predominantly occur in this narrow region with very few voids in the adjacent base/weld material. As this zone is of particular interest due to it being the weakest region in repair welded joints, the need to investigate it further is important. A larger testing volume of the FGHAZ was created by applying a weld thermal cycle simulation to the as-received base materials. The impact of this simulation was determined microstructurally by optical microscopy and mechanically by hardness and tensile testing. The FGHAZ has low creep resistance and is most susceptible to failure due to the small grained microstructure. Due to the numerous small grains, there is a high effective diffusion coefficient (HEDC). The multi axial stresses induced during in service/ creep testing conditions together with the HEDC causes voids to form at an accelerated rate. Significant void coalescence promotes the formation of micro cracks which in turn lead to macro crack formation and eventually failure.

Includes bibliographical references. / Includes bibliographical references (leaves 96-103). / The high usage of Al and its alloys in both the automotive and aerospace industries is attributed to its excellent specific strength and corrosion resistance. High demand of Al usage has led to the improvement of both the casting techniques and joining processes, in order to improve on the quality of the final product. The selection of the manufacturing process for Al and its alloys is based on the capabilities of the specified requirements for components and the alloy used. High pressure die casting (HPDC) is the most widely used casting process in the automotive industry due to its high production rate, and ability to produce complex shaped components. However, HPDC is prone to porosity making it difficult to heat treat and weld. Semi solid metal (SSM) forming has the potential to produce near-net-shape components with high integrity. Due to laminar filling characteristics of SSM HPDC, low porosity or porosity free castings can be produced. This offers the opportunity to apply heat treatment as well as weld the SSM HPDC components. A high continuous wave Nd: YAG laser has been used to investigate the weldability of SSM cast A356 alloy. The CSIR rheo-process was used to prepare the aluminium A356 SSM slurries and thereafter plates (4X80X100 mm3) were cast using a 50 Ton Edgewick HPDC machine. Plates in the as cast, T4 and T6 heat treatment conditions which had passed radiography inspection were then laser welded. Some of the initial as-cast plates that were welded were subjected to pre or post weld T4 or T6 heat treatment and are referred to as pre-weld T4 or pre-weld T6 and post-weld T4 or post-weld T6 specimens.

Includes bibliographical references (leaves 145-149). / Corrugated board is a composite sandwich type material used in the packaging industry worldwide. In the design of corrugated boxes, the stacking strength is an important design parameter. Current research shows that box failure is influenced by the flexural rigidities of the panel and its transverse shear rigidities. McKinlay proposed a new method to measure the MD transverse shear stiffness of corrugated board. This research was aimed at designing a fixture to perform the MD shear test and to evaluate its performance. In addition, the properties that influence box strength were to be investigated. These properties were then to be used in improved box strength predictions. It was found that the designed MD shear fixture was able to measure the transverse shear stiffness of corrugated board in the MD direction with a high degree of accuracy and reproducibility. This method was much easier to perform than the standard block shear test method and also much quicker. This was a very important factor considering the application of this testing method in a research and development environment. In addition, the stiffness test exhibited good possibilities for use as a quality control tool. Extensive testing showed that the material used in the manufacture of corrugated board had a strong influence on board and box strength. In addition, it was found that the separation of the faces in a corrugated board structure had an influence on the strength and stability of the box. Factors such as the manufacturing process and board structure were also found to have an effect on box strength. Box strength predictions were performed using the methods available in the literature. These predictions had good correlation with the experimental box compression values. It was shown that box strength can be accurately predicted from liner and fluting properties and this capability is an important tool in box strength design.

Includes bibliographical references. / A study has been made of the fatigue crack initiation and fatigue crack growth behaviour of three different steels in wire form, namely, an austenitic AISI 304 stainless steel, a corrosion resistant ferritic steel, 3CR12, and pearlitic high carbon steel. The stainless steel wires were produced in the laboratory at a drawing speed of 50 mm min-1, without intermediate annealing, whilst the high carbon pearlitic steel was manufactured commercially. Studies were made on stainless steel wires as a function of drawing strain between 0.09 and 0.585. Fatigue testing was carried out on an ESH servo hydraulic testing machine on both notched and unnotched samples and the S-N curves were used to evaluate the fatigue properties of the steels. Tests were performed with sinusoidal loading and load ratios of R= 0.048 and R=0.22 at a frequency of 2Hz. The microstructural evolution during drawing was characterised by optical and transmission optical microscopy, and x-ray diffraction. Fatigue crack growth and fracture surfaces were studied using scanning electron microscopy. In general, the fatigue limit was enhanced by increased drawing strain, but such strain also increased the subsequent crack propagation rates. The highest value of fatigue limit of 630 MPa was exhibited by the commercial pearlitic steel despite of its high notch sensitivity. Both shot peening of the steel wire surface and reducing the surface roughness by manual polishing increased the fatigue limit between 40 and 25 % respectively. The fatigue limit of AISI 304 stainless steel wire was improved from 215 MPa to 650 MPa after drawing to 0.585 strain. This improvement is attributed to the deformation-induced phase transformation of (ϒ) austenite to α'-martensite. X-ray diffractometer traces show that the amount of strain-induced martensite varied from 8% in the wires drawn at low strain (0.09) to 36% in the wire samples drawn to 0.585 strain. This study has established that approximately 20% of deformation-induced martensite, through drawing strain, is a critical amount which determines the subsequent fatigue response of this steel. If the amount of previously developed martensite is less than the critical amount of 20%, the martensite formed during the fatigue process will act beneficially by retarding fatigue cracking, raising the fatigue limit and resulting in a ductile fatigue fracture surface. However, in the presence of more than 20% of martensite, any martensite induced by cyclic strain will encourage more rapid crack initiation compared to a material containing less than 20% martensite which leads to more brittle fracture surface characteristics. The fatigue limit of 3CR12 steel wire was also improved from 130 MPa to 310 MPa (maximum stress) after drawing to 0.68 strain. The experimental results indicate that the use of drawn 3CR12 ferritic steel for wire application under cyclic conditions is restricted to low stress levels. However, the application of heat treatment and the resultant development of a dual-phase microstructure, improved the fatigue limit to 470 MPa. Based on the findings in this study, recommendations regarding material selection and drawing process optimisation for wire production to improve the fatigue performance of AISI 304 stainless steel is given.