Browsing by Subject "Materials Science"

Now showing 1 - 10 of 10
  • Thumbnail Image
    Item
    Open Access
    Adsorption of oxygen molecules on platinum surfaces modified with subsurface atoms of vanadium : a DFT study
    (2014) Matengaifa, R; Lang, C I; Sithole, H
    The aim of this work was to investigate changes in the electronic structure of platinum as a result of alloying with vanadium, and the effects of these changes on O2 adsorption. This is important for the further development of hydrogen fuel cells, because the oxygen reduction reaction (ORR) presently requires O2 adsorption to occur on pure platinum, which is a prohibitively expensive material. A computational study has therefore been undertaken on alloying platinum (which reduces cost) with vanadium (for which there is plentiful experimental data) and the consequences for O2 adsorption. The first moment of the d-band of platinum alloy DOS was used to represent the d-band centre. The d-band centre of Pt-PDOS became lower as a result of hybridisation between platinum and vanadium. The d-band centre of a pure platinum surface with respect to the Fermi level is -1.99eV, but it is shifted to -3.23eV when vanadium atoms are added to the subsurface layer. The adsorption energies of O2 are sensitive to a combination of calculation parameters used. In this work, the calculations were executed using the CASTEP code. This is a plane wave pseudo potential code. The most stabilised geometry of an adsorbed molecule on pure Pt (111) was at the fcc site and had an adsorption energy of -1,91eV. The adsorption energy at the bridge site of Pt (111) is -1.81eV. When subsurface vanadium atoms were introduced, the equilibrium surface-molecule bond lengths increased. The adsorption energy at the fcc site shifted to -1.37eV, -1.43 for the bridge site and -1.45eV for the hcp site. It was concluded that the presence of vanadium atoms in the surface region destabilises an adsorbed oxygen molecule but a more detailed study is needed to show the effect of the solute atoms on the thermodynamics and kinetics of the whole oxygen reduction reaction chain.
  • Thumbnail Image
    Item
    Open Access
    An assessment of polymeric materials and surface treated steels as cavitation erosion resistant materials
    (1983) Barletta, A; Ball, Anthony
    The object of the research described in this thesis was to optimise the choice of materials used for vital components of hydraulic machinery. Frequently these components are damaged by a process known as cavitation erosion and the operation and efficiency of machines are seriously impaired. Nineteen different polymers which have potential for use in hydraulic components have been eroded by liquid cavitation, employing the stationary specimen system. An attempt has been made to correlate the extent of erosion with the mechanical and chemical properties of the polymers. Modes of erosion of different materials were studied by scanning electron microscopy and a strong correlation was found between these modes and the resistance to erosion. Heterogenous polymers (mixture of two homogenous components), together with the poly amides and polyethylenes, showed the highest erosion resistances. The effect of prior immersion (3 weeks at 70°C) in either a dilute or concentrated form of hydraulic fluid has been investigated for both polyacetal and ultra high molecular weight polyethylene samples in order to simulate service conditions. The polyacetal samples showed improved erosion resistance relative to the samples stored in air or water (3 weeks at 70°C). In contrast, the ultra high molecular weight polyethylene samples failed in a catastrophic manner by solvent stress cracking.
  • Thumbnail Image
    Item
    Open Access
    Cavitation erosion of WC-Co
    (1987) Hankey, S E; Ball, Anthony
    An investigation involving the vibratory cavitation erosion of WC-Co alloys was undertaken in order to determine the mechanisms of material removal. Nineteen grades of WC-Co alloys were studied. These alloys had been previously characterised according to microstructural and mechanical properties. Further characterisation by way of Young's modulus and density of the materials was undertaken. An investigation of the i nfluence of various parameters on cavitation erosion established a binder content dependence on erosion. For two grain sizes, erosion was found to increase to a maximum at 12 vo1-% binder content (1.8 μm grain size) and 23 vol-% binder (2.8 μm grain size). The main mode of material removal was found to be cobalt removal followed by WC grain pull-out. In high binder content alloys, cobalt removal was predominant with little loss of WC grains. X-ray diffraction showed that the allotropic phase transformation of the binder under cavitational attack was beneficial to the erosion resistance of these alloys. The erosion of low binder content alloys was controlled by the contiguity of the WC skeleton. Maximum erosion occurred at binder contents which corresponded to the combination of a fragile WC skeleton and a small volume of available cobalt for strain induced transformation.
  • Thumbnail Image
    Item
    Open Access
    A constitutional study of a dual phase steel containing 12% chromium
    (1983) Schaffer, G B; Ball, Anthony
    This thesis involved a study of the phase transfonnations in a chromium containing corrosion resistant dual phase steel, designated 3CR12. The objectives included the detennination of time-temperature-transformation (TTT) diagrams for the transformations between austenite and ferrite and an investigation into the factors controlling these reactions. The austenite decomposition reaction for a high nickel alloy, 3CR12Ni, and the effect of varying titanium concentrations on the equilibrium phases present in 3CR12, were also examined. Dilatometry was used to determine the transformation temperatures between austenite and ferrite and the Ms temperatures for the alloys investigated. The kinetics of the reactions were investigated by optical microscopy using two different etching techniques while the volume fractions of the various constituents were determined by a point counting method. Transmission electron microscopy was used to study the carbide morphologies and the nucleation and growth modes of the phases during the transformations. The distribution of the alloy elements were determined by microhardness measurements, an electronprobe microanalysis and a Kevex spectrometer attached to a scanning electron microscope. The 3CR12 alloy used in this study did not become fully austenitic above the Ae₃; it lies in the nose of the gamma loop of the Fe-Cr phase diagram. Two temperature regimes were identified on the decomposition of austenite. At 750°c the existing ferrite grains grew into the austenite matrix, while at 650° C and 700°C new ferrite was sympathetically nucleated i.e. it was heterogeneously nucleated on existing ferrite/austenite grain boundaries. Two types of carbide morphologies were formed. These were random precipitation within the ferrite grains and interphase precipitation. The TTI diagram showed conventional "C" curve kinetics. The austenitisation reaction occurred by a para-equilibrium mechanism. The rate controlling process was the structural change from ferrite to austenite; the reaction was not long range diffusion controlled. The speed of the reaction increased continuously with increasjng transformation temperature. No growth of ferrite occurred on isothermal transformation of 3CR12Ni at temperatures below the Ae₁. Increasing the bulk titanium content increased the Ms, Ae₁ and Ae₃ temperatures of 3CR12 due to the removal of carbon from, and the addition of titanium to, solution.
  • Thumbnail Image
    Item
    Open Access
    Fatigue and fracture behaviour of PVC at elevated temperatures
    (2008) Von Zwiklitz, L; Tait, Robert B
    A series of failures in underground water-carrying uPVC piping in a luxury resort in Dubai gave rise to a research opportunity to determine the effect of temperature on fatigue life performance of uPVC piping. Two different modes of testing were used to determine this temperature effect, namely SN and Fracture Mechanics Paris testing. The temperatures tested were 20%C and 45%C. In addition to temperature tests, a potential ageing effect was also investigated by comparing pipes which had been in service in the resort, and previously unused piping. The SN tests consisted of externally and symmetrically stressing, across the diameter, sections of pipe from the luxury Madinat Jumeirah resort in Dubai where the failures had occurred. The Paris equation generating FM tests used Compact Tension specimens and produced an equation relating the crack growth rate to the cyclic stress intensity amplitude. In addition, material properties were measured which could then be used for fatigue lifetime predictions. In addition to the lifetime tests, fracture toughness tests were also completed. These were done with a view to determining the fracture toughness of the material, and also to ascertain if there was an orientation effect for crack growth. The potential ageing effect was also investigated. This was achieved by means of using differently orientated specimens. SENB specimens were used to determine circumferential fracture toughness and C-Shaped specimens for longitudinal cracks (the direction of on-site crack growth). Fracture surfaces were inspected and calculations performed to indicate critical flaw sizes were broadly consistent with linear elastic fracture mechanics (LEFM). Using the material properties garnered from the Paris fatigue and fracture toughness tests, lifetime predictions were made and compared to the SN data for cycles to failure The research studies revealed that there was a distinct difference in fatigue performance as a result of a temperature increase from 20%C to 45%C, as characterised by both SN and Paris fatigue tests. There was also, but to a lesser degree, an ageing effect. The temperature performance factor for the SN curves was between 1.6 and 4.6, while for the Paris characterisation the temperature performance factor was between 3 and 3.5.
  • Thumbnail Image
    Item
    Open Access
    Feasibility study into the use of digital image correlation for creep strain monitoring of fossil power plant welds
    (2018) Cardenas, Nicolas; Knutsen, Robert D; Becker, Thorsten
    The life span of high temperature power plant pipework is principally a function of material creep damage - an irreversible plastic deformation of the material when subjected to temperatures and loads beyond a certain threshold. Within Eskom, the South African parastatal power utility, creep damage is primarily quantified by way of Metallographic Replication (replicas). This is a quasi NonDestructive Examination (NDE) technique that looks at the microstructure of the sample in question. Although well-known and used extensively, replicas, as with any technology, have their shortcomings. Extracting of replicas and their subsequent analysis are manual processes that inherently suffer from subjectivity. Furthermore, storage and archiving of vast quantities of physical replica slides for future reference is cumbersome - a challenge that digitisation can address. The aforementioned vulnerability to analysis subjectivity and benefits of digitisation are areas which a technology known as Digital Image Correlation (DIC) - a non-contact, full field, deformation measurement technique - can potentially address. Some research has been done on using DIC for power plant creep measurement; however literature quantifying its performance in this specific application is scarce. This study thus looks into setting up a DIC system optimised for measuring strain in an area of the pipework welds known as the Heat Affected Zone (HAZ) - the weakest part of the weldment. The achievable accuracy is established and the major parameters that affect DIC accuracy are investigated, elucidating the trade-offs between optimising each. In addition, two scenarios exist for the acquisition of DIC measurement data from a plant: when the plant is operating (online); or when the plant is shut down for maintenance (offline). The encumbrance of imaging a hot surface makes the former scenario the more demanding, and was thus investigated. This data was subsequently used to substantiate whether DIC has the potential to be used online (i.e. at elevated temperatures) or is limited to use during shut downs.
  • Thumbnail Image
    Item
    Open Access
    The influence of nitrogen on the deformation behaviour of a modified AISI type 200 series alloy
    (1993) Biggs, Taryn; Knutsen, Robert D; Shaw, M P
    This thesis investigates the influence of nitrogen concentration on the deformation behaviour of a modified AISI 200 type alloy. The Fe-18Cr-4Ni-7Mn base alloys contained a range of nitrogen contents from 0 to 0.27 wt%. The tensile behaviour was characterised by uniaxial tensile testing. The room temperature yield strength was shown to decrease with increasing nitrogen content for nitrogen contents less than 0.1 wt%. This decrease was attributed to the presence of secondary phases in the annealed state. An increase in yield strength was observed with further additions of nitrogen and this is probably due to solid solution strengthening and a Cottrell interaction. The variation of room temperature ultimate tensile strength showed no dependence on nitrogen content. Elevated temperature tensile tests (120DC) showed an increase in ultimate tensile strength and yield strength with nitrogen content suggesting that solid solution strengthening and a Cottrell interaction are occurring in this alloy range. Room temperature deformation was shown to induce a transformation from austenite to martensite throughout this alloy range, the degree of transformation decreasing with increasing nitrogen content. This transformation was shown to provide considerable strengthening and work hardening to the alloy. The low stacking fault energy (SFE) of this alloy range ensured that cross-slip was significantly inhibited during room temperature deformation and the deformation mode was observed to be planar glide. Nitrogen was shown to increase the SFE of this alloy range thus causing a decrease in the inhibition of cross-slip with increasing nitrogen content. Increasing the nitrogen content thus causes a decrease in strength if the contributions of a deformation-induced transformation, SFE variation and secondary phases are considered but causes an increased strengthening contribution due to solid solution strengthening and a Cottrell interaction. Hence as the nitrogen content increases different strengthening mechanisms are activated and no overall trend of room temperature ultimate tensile strength with nitrogen content is observed. The ductility and formability of the steels did not appear to show any distinct trends with changes in nitrogen content.
  • Thumbnail Image
    Item
    Open Access
    The influence of water composition on the pitting behaviour of stainless steel
    (1985) Capendale, A E; Noel, R E J
    The new concept of hydropower has been found to be technically feasible in South African gold mines. Chilled mine service water is piped from the surface to deep level stope; where the hydrostatic pressure provides power for stoping machinery. This water varies widely in composition and acidity. High concentrations of sulphate, chloride and nitrate are present. These ions are derived from the leaching of oxidised sulphides from the broken rock, the fissure water and the dissolution of blasting fumes. In order to minimise the deterioration of stoping machinery by corrosion and synergistic corrosive abrasive effects, a compromise between selecting a suitable corrosion resistant material and treating the mine service water to an acceptable level of corrosiveness is being sought.
  • Thumbnail Image
    Item
    Open Access
    The microstructure and some phase transformation characteristics of the dual phase steel 3CR12
    (1983) Protopappas, Eugenie; Ball, Anthony
    Oilatometry and metallography have been used to establish the phase transformation diagram of a steel containing 12% chromium and 0,03% carbon. A dual phase material with attractive mechanical properties and adequate corrosion resistance can be achieved by careful control of alloy content and heat treatment. The work has demonstrated the influence of the addition of alloying elements (in particular nickel), and thermal history. The dual phase (ferrite plus martensite) nature of the steel has been identified and characterised by both optical and transmission electron microscopy. The islands of lath martensite and the attendant high dislocation densities present good mechanical behaviour. In particular, the continuous yielding, work hardening, formability and toughness can be attributed to this microstructure. The thesis provides a base for the discussion of methods of "designing" the steel for particular applications.
  • Thumbnail Image
    Item
    Open Access
    Wear of aluminium MMCs against automobile friction materials
    (1994) Howell, Gavin John; Ball, Anthony
    Two magnesium/silicon aluminium alloys each reinforced with 20 vol. % SiC particulates have been worn against three different automobile friction linings (brake pads). Two of the friction linings are commonly used against cast iron brake rotors while the third has been formulated for use against aluminium MMC brake rotors. Wear processes at the interfaces of the specific rotor - pad combinations have been characterised through the analysis of friction traces and the use of optical and electron microscopy. Models on the interdependence of friction and wear, and models of wear mechanisms for aluminium MMCs and cast iron sliding against friction materials have been proposed and discussed. For an aluminium MMC sliding against an organic pad formulated for use against cast iron, wear rates are low and friction is constant due to the formation of a solid lubricant layer at the wear interface. When this MMC is worn against a semi-metallic pad formulated for use against cast iron, wear rates are extremely high due to two and three body abrasion which lead to subsurface delamination and early melt wear in the MMC. For an aluminium MMC developed for its use in automobile brake rotors sliding against a semi-metallic pad specifically formulated for its use against MMC brake rotors, wear rates at low loads are low although friction traces are irregular and fracture of the SiC particulates occurs at the lowest load and sliding velocity. This fracture of SiC is caused by the abrasive action of hard alumina particles within the pad. At high loads and sliding velocities cohesiveness of materials within the pad is poor and the wear rate of the MMC is extremely high. At the highest load/sliding velocity combination, the wear resistance of the MMC is inferior to that of its unreinforced matrix. If the structure and composition of friction linings are arranged correctly, the wear resistance and frictional performance of aluminium MMC brake rotors are superior to those of cast iron brake rotors. In addition, the lower density of aluminium MMCs provides for an economic advantage over cast iron with respect to efficient use of fuel, and fabrication expenses.