Browsing by Author "Lang, Candy"

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    Abrasive wear resistance of ruthenium aluminide intermetallic and ferritic steels containing a sigma intermetallic phase
    (1998) Ngakane, M; Lang, Candy
    The abrasive wear resistance of ruthenium aluminide intermetallic and ferritic steel containing an iron-chrome sigma intermetallic phase have been investigated in this study. A medium carbon wear resistant steel (MCV) was used in the study to facilitate comparison between wear resistances in the materials of interest. Specimens of ruthenium aluminide, MCV and ferritic stainless steels containing a sigma phase were produced. The MCV steel was quenched and tempered to match the bulk hardness of the as-received ruthenium aluminide. Five different grades of thermomechanically worked ferritic steels specimens were heat treated to produce different volume fractions of sigma phase. The mechanical properties of the specimens were investigated by compression testing and microhardness measurements. Abrasion testing was carried out on a pin on belt abrasion apparatus. The surface response of the specimens to abrasive wear was characterised by optical and scanning electron microscopy. Microhardness of the specimens were measured with a digital microhardness machine. The wear resistance of ruthenium aluminide was found to be higher than all materials tested in this project. The wear resistance in some of the ferritic steels containing sigma phase was comparable to that of the wear resistant medium carbon steel. The surfaces of the specimens were shown to work-harden during the abrasion process. The corresponding work-hardening results showed that ruthenium aluminide had the highest work-hardening rate. It can be concluded that the work-hardening ability of the test materials correlates with their respective wear resistance properties.
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    Casting platinum jewellery alloys: the effects of casting variables on fill and porosity
    (2005) Miller, Duncan; Keraan, Tauriq; Park-Ross, Penny; Husemeyer, Victoria; Brey, Ali; Khan, Irshad; Lang, Candy
    Comparisons are made between platinum-copper and platinum-ruthenium alloys used for jewellery to evaluate the effects of casting variables. The effects of flask temperatures, investments, and centrifugal speeds on microstructure, percentage fill, and porosity were examined over a range of temperatures. Optimum conditions and materials for successful casting of high quality platinum jewellery alloys, using a Hot Platinum induction melting and casting machine, are described. Suitable choice of investment materials and rotational speeds produced good grid fills with Pt-5%Cu and Pt-5%Ru alloys. Metal porosity was more difficult to control, due to the inherently chaotic nature of the casting process, but casting into a relatively cool mould minimised the probability of bad porosity for both alloys. Pt-5%Ru was found to be successful as a casting alloy when used with induction melting technology. It displayed superior uniformity, hardness and colour, compared with cast Pt-5%Cu alloy.
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    Discovering novel platinum structures
    (2013) Mabunda, Khanyisa Prudence; Lang, Candy
    The Pt[8]Zr ordering transformation in Pt 11 at. % Zr alloys has been studied. The study included determination of the formation of the Pt[8]Zr phase and its order/disorder transformation temperature (Tc). Electron diffraction patterns and dark field images were obtained from transmission electron microscopy to characterize the Pt[8]Zr phase. Microhardness measurements and light microscopy were used to investigate the Pt[8]Zr ordered phase and its effects on hardness and microstructure respectively. The Pt[8]Zr ordered phase was found to be present in the initial as-cast condition and in specimens heat treated up to a temperature of 1070 °C. This is consistent with the DSC measurements of Tc, determined to be 1068 °C. The nucleation of the ordered phase was found to be rapid at any temperature below Tc but the growth of this ordered phase was rather slow, probably due to inadequate vacancies present in the alloy. The domain size of the Pt[8]Zr phase was found to be approximately 10 nm for specimens heat treated at 1000 °C for up to 600 hours.
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    Effect of thermomechanical processing on the surface roughening of AA6061
    (2001) Aphane, Abraham Kalaudia; Knutsen, Robert D; Lang, Candy
    Surface roughening is an undesirable feature in materials. This work investigates the cause of surface roughening in four plates of AA6061 with different thicknesses (Le. 6 mm, 9 mm and two 12 mm). Production of these plates requires thermomechanical processing of the cast slab via a two stage process, namely the initial rolling stage on a hot roughing mill (HRM) and final rolling on the hot finishing mill (HFM). After rolling. the plates are levelled by a tensile stretching process and it is during this process that surface roughening of the plates can arise. Three of these plates exhibited severe roughening and the other 12 mm thick plate exhibited mild surface roughening during the stretch levelling process. Investigations looked closely at how the cause of surface roughening is related to the thermomechanical processing. The four products studied exhibited different degrees of surface roughening during uniaxial tensile deformation. Detailed investigation of the microstructure of these four products was carried out. It was found that the plates that exhibited severe roughening had large flat and elongated grains at the surface. It is the rotation of these surface grains during tensile deformation that was thought to be inducing the roughness in the material. The microstructure evolution was explained relative to the thermomechanical history of the plates. Plane strain compression (PSC) tests were used to simulate the thermomechanical processing cycles on the HFM. Large strain rates used in the industry were compensated with an adjustment in temperature so that an equivalent Zener-Holloman parameter (Z) could be achieved. The microstructure of the PSC specimen was characterised using the electron back scattered technique. It was found that the grain size increases with a decrease in the value of Z.
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    The erosion of titanium aluminide intermetallic alloys
    (1995) Howard, Robert Llewellyn; Ball, Anthony; Lang, Candy
    The erosion behaviour of titanium aluminide intermetallic alloys has not been widely reported in the scientific literature and is part of the current international research effort aimed at exploiting these materials for turbine engine and automotive applications. In the present study titanium aluminides have been subjected to both solid particle erosion and cavitation erosion. The erosion rates have been measured and the damage mechanisms have been identified and discussed in terms of the microstructures and mechanical properties of the titanium aluminide alloys. This has been achieved with a variety of investigative techniques, including electron microscopy, mechanical testing and microstructural examination; and, where necessary, the erosion performance of other materials have been evaluated for comparison. In particle erosion, conducted with air blast rigs at room temperature and at elevated temperature, the titanium aluminide alloys exhibit a ductile mode of material removal, and their limited strain to fracture results in higher particle erosion rates than those for 304 stainless steel. Heat treatment to produce changes in microstructure and hardness does not significantly affect particle erosion performance, and elevated temperature tests reveal an increase in particle erosion rate with increasing temperature. In cavitation erosion, the titanium aluminide alloys exhibit a ductile mode of damage accumulation and material loss, and the rates of material loss are lower than those for other engineering materials such as 304 stainless steel and some hardmetal grades which are currently used in erosive environments. The mechanism of cavitation erosion of the Ti₃Al-based alloy involves the accumulation of strain in phase boundary regions and the preferential removal of the more brittle component of the microstructure. For the Ti₃Al-based alloy, cavitation erosion resistance increases with an increase in hardness produced by heat treatment. The TiAl-based alloys exhibit twinning during the initial stages of cavitation, which is characteristic of the high strain rate deformation of TiAl-based alloys, followed by substantial work hardening and preferential material loss from phase interfaces and twinned regions.
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    Investigating the heat treatment effects on phase transformations and coating morphology in the Cr-Pt coated system
    (2014) Hanief, Nasheeta; Knutsen, Robert D; Topiæ, M; Lang, Candy
    The Cr-Pt system has been extensively studied in recent decades with the aim of providing a reliable thermodynamic description of the phases shown in the equilibrium phase diagram. The Cr-Pt coated system where Pt layers were deposited onto Cr substrates is investigated in this study with particular interest in the formation of Cr-Pt phases and coating morphology as a function of coating thickness and heat treatment parameters (temperature and time). The main aim of this research was to produce the coated system with properties which can be tailored according to the conditions required by their particular application. The system was investigated using several complementary techniques.
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    An investigation of phase transformations in Pt-V coating systems
    (2012) Makhetha, William Motsoko; Lang, Candy; Topic, Mira
    Phase transformations in Pt-V coatings after heat treatment have been investigated. Five Pt- V ordered phases (PtV, PtV 3 , Pt 2 V, Pt 3 V and Pt 8 V) have been previously observed in bulk platinum-vanadium alloys. Phase formation in coatings is expected to be sequential and controlled by the lowest temperature eutectic (liquidus) composition; this allows control of experimental parameters for formation of desired ordered phases. This investigation included fabrication of coatings, heat treatments, morphology characterization and phase analysis. Single and multilayer coatings ranging between 0.07 μm and 0.5 μm were deposited on vanadium and platinum substrates using E-beam deposition. The kinetics of phase transformation were studied by subjecting the coated layers to a variety of heat treatments in the temperature range 600°C to 900°C for 4 and 8 hours. Composition and morphology characterization was carried out using EDS and SEM respectively. XRD was used for phase analysis. Four (PtV, PtV 3 , Pt 2 V, Pt 3 V) out of the five Pt-V ordered phases exhibited in bulk alloys, were successfully formed from the coating system investigated in this project. The first phase formed, and the sequence of phase formation, was found to be different depending on which metal formed the substrate. The vanadium-rich ordered phase (PtV 3 ) was preferentially formed first on vanadium substrates and the sequence of phase formation progressed through ordered phases richer in platinum. The platinum-rich ordered phase (Pt 3 V) was preferentially formed first on platinum substrates and the sequence continued towards formation of ordered phases richer in vanadium. An increase in heat treatment temperature from 600°C to 900°C resulted in rapid kinetics of phase transformation but affected the morphology of the coatings. An increase in coating thickness, number of coating layers, heat treatment time, and temperature resulted in an increase in overall total number of ordered phases and volume of ordered phases.
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    Investigation of the hardening behaviour and ordering transformation in Pt 14 at. % Cu
    (2005) Carelse, Muneeba; Lang, Candy
    The degree of hardening after heat treatment was greater for the initially cold worked specimens than for the initially quenched specimens. Dark field images of initially cold worked specimens after heat treatment showed smaller ordered regions than the initially quenched specimens. This suggests that smaller, more numerous ordered regions impede dislocations to a greater degree than larger ordered regions in this alloy. The hardening mechanism thus appears to arise from dislocation impediment by the ordered regions.
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    Investigation of the hot deformation of sintered titanium compacts produced from direct reduction powder
    (2010) Petersen, Shaheeda; Knutsen, Robert D; Lang, Candy
    The focus of this study was the use of powder metallurgy to produce low cost titanium with comparative mechanical properties to wrought titanium. The objectives of this investigation was to produce sintered titanium compacts that represented metal made by the Direct Powder Rolling method. The critical strain (ɛC) required to induce recrystallization following deformation was determined by hot compressing wrought titanium samples. Finally sintered titanium samples were hot compressed at ɛC and the changes to the microstructure, porosity and mechanical properties was assessed.
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    The milling of tin bronze with a Cu-24.6wt%Sn composition
    (1997) Williams, Garth; Lang, Candy
    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.
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    Order hardening of platinum alloys
    (1999) Towle, Nicholas Richard; Miller, Duncan; Lang, Candy
    The hardening behaviour of three cold-worked platinum alloys, Pt 5 at% Mo, Pt 5 wt% Ru and Pt 5 wt% Cu, has been investigated through a systematic series of heat treatments. All three of the experimental alloys showed a hardness increase during annealing within a specific temperature range. The hardness of the Pt-Mo and Pt-Ru alloys was found to increase rapidly at annealing temperatures above the recrystallisation temperature, with the final hardness similar to the original coldworked hardness. The hardness of Pt-Cu showed an increase of up to 30% at low annealing temperatures of between 200°C and 500°C. In addition, the Pt-Cu alloy also showed the increased hardness found in Pt-Mo and Pt-Ru at high annealing temperatures, but the hardness increase was not to the same extent. Specimens subjected to the annealing treatments were studied by means of optical, scanning electron and transmission electron microscopy, in order to determine the effect of annealing on microstructure and structural order. Resistivity, XRD and OTA techniques were employed in order to study the mechanisms of ordering with temperature, but these techniques did not produce any significant results. It was concluded that the most likely cause for the hardness increase observed in all three experimental alloys was due to a change in structural order upon annealing. The Pt-Mo and Pt-Ru alloys hardened through an increase in short-range order at annealing temperatures above the recrystallisation temperature. The Pt-Cu alloy hardened through the development of long-range order on annealing between 200°C and 500°C. This increase in hardness was in· addition to the high dislocation density in the alloy specimen due to prior cold-work.
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    Precipitation reactions in a Cr-Mn-N austenitic steel with niobium and vanadium additions
    (2000) Basson, Janet Ann; Knutsen, Robert D; Lang, Candy
    Includes bibliographical references.