3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Composition refinement of medium carbon-low alloy steels to improve wearand corroson resistance for rail axle applications(2019) Klenam, Desmond Edem PrimusThe mechanical and corrosion behaviour of three ex-service rail axles were studied and compared with the specifications of standard grades. The axle steels had lower hardnesses, lower yield and ultimate tensile strengths than the recommended standards, although three is a very small data set to arrive at these generalisations. The axle steels had ferrite-pearlite microstructure, with pearlite decreasing from the surface to core, thus decreasing hardness. Similar interlamellar spacings were obtained for the pearlite colonies and there were coarse Widmanstätten and allotriomorphic ferrite. The potentiodynamic corrosion rates of the axle steels in natural rainwater and in Johannesburg Municipal tap water were similar and lower than in synthetic seawater. Similar trends were observed for the immersion tests and there was no discernible effect on the nature of rust, with increasing exposure time. The corrosion rates of the axle steels increased with increasing exposure time. The analysed rust phases were mainly lepidocrocite, iron residue, magnetite and goethite, whereas akaganeite was associated with steels in 3.5% NaCl solution. The corrosion rates of the axle steels in each medium were similar due to small amounts of Cr, Ni, Mo, Cu and Si, as these elements increase corrosion and oxidation resistance in medium carbon steels. Thermo-Calc calculations were used to identify potential steels with lower cementite proportions to improve the corrosion resistance. Three steels from these were selected and produced, then tested, and two steels were identified for further tests, to ascertain whether they could be used as substitute for currently-used axle steels. Thermo-Calc calculations of the standard grades were used as a baseline for the alloy design of the experimental steels, with varying amounts of Cr, Mo, Ni and V, ensuring improved mechanical and corrosion properties. Steels with lower cementite phase proportions and high corrosion indices were identified to improve the corrosion resistance and three selected compositions from the Thermo-Calc calculations were cast, heat treated and microstructures characterised. The air-cooled steels showed laths of bainite and ferrite. Banded ferrite-pearlite was obtained for the furnace-cooled steels, whereas mixed laths of bainite and martensite were observed for the quenched and tempered steels. The corrosion rates of the as-rolled and water-quenched steels, and for the oil quenched and tempered steels, in tap water and in 3.5% NaCl solution were higher than the norii malised, air and furnace-cooled steels. Although the ex-service rail axle steels showed better corrosion resistance than the as-rolled and water-quenched steels, as well as the oil quenched and tempered steels, they were poor compared to the normalised in air and furnace-cooled steels, due to bainite and ferrite of the normalised and air cooled, whereas a ferrite-pearlite and ferrite-bainite microstructure existed for the normalised and furnace-cooled steels. The wear rates of the experimental steels were slightly lower than the ex-service axle steels, showing better wear resistance. The wear rates calculated using the maximum penetration depth and wear track width were slightly higher than those calculated using the Japanese Industrial Standard. The low wear rates of the experimental steels were due to high hardnesses from bainite and martensite microstructures. The main wear mechanisms were material pull-out, smearing and ploughing and the harder microstructure acted as the load-bearing phase. The yield and UTS of the quenched and tempered steels were much higher (668 - 1158MPa, 880 - 1306MPa) than the air-cooled (455 - 587MPa, 767 - 880MPa) and the furnace-cooled (389 - 490MPa, 679 - 706MPa), which was the least, which was greater than that for the ex-service axles. The improved yield and UTS of the quenched and tempered steels were due to increased proportions of martensite and bainite. The normalised, air and furnace-cooled steels showed balanced strength, ductility and corrosion resistance required for rail axles. The heat-treated experimental steels had better mechanical and corrosion properties than the ex-service axle steels, the proprietary and European Standard grades. However, the normalised, air and furnace-cooled SPA_18C and SPA_19C steels were the overall best steels due to their balance of strengths, hardness, ductility, toughness and good corrosion resistance than the other experimental steels, the ex-service axle steels, the proprietary and European Standards. It is also recommended that the rail axle steels with ferrite-bainite are used rather than the ferrite-pearlite steels, due to the balanced mechanical and corrosion properties of the former.Item Quantum transport in low dimensional carbon: from graphene to superconducting diamond(2018) Coleman, ChristopherIn this work investigations into the quantum transport of two different low dimensional carbon systems are presented. Heavily boron-doped nanocrystalline diamond is known to be superconducting and has attracted much research attention as it forms a new class of superconductor; the doped semiconductor superconductor. The system follows a Mott transition with doping concentration and has thus lead to various proposed pairing mechanisms forming the Cooper pair, this includes phonon mediated as well as resonating valence band mechanisms for pairing. Although generally considered a three dimensional system to date very few reports exist on determining the dimensionality of the quantum conduction channels in the system. I investigate the dimensional crossovers in transport using the well-known scaling analysis such as Azlomazov-Larkn and Lerner-Varlomov-Vinokur models to establish the characteristic dimensionality within the fluctuation regime and establish a two dimensional phase that occurs before Josephson coupling between grains becomes significant. The field induced superconductor to insulator transition is then investigated in light of the previously proposed charge-glass state that has been observed in other 2D superconducting systems. It is shown that there the predicted universal scaling analysis can in fact be applied to this system with critical exponents that follow the expected values Further supporting the claim of a 2D system. Subsequently one of the result of the two dimensionality, the Berezinskii-Kosterlitz-Thouless transition is observed through both current-voltage scaling as well as temperature dependent resistance. In addition to these aspects of low dimensional transport the possibility of Andreev bound states is exhibited in the differential resistance, these resonant features are related to the mid gap states observed in layered cuprate systems with anisotropic order parameter (d-wave). Novel magnetoresistance features such as a change from negative to positive magnetoresistance (weak localization to anti-localization) is observed to occur in the fluctuation regime near the established BKT transition point, this is related to normal state electrons that undergo spontaneous time reversal symmetry breaking as the superconducting stage is reach. Such novel transport features are often related to signatures of topological non-trivial phases which can be of use for various quantum computation technologies. This work also investigates the effect of strain deformation on the transport in graphene. Raman annealing is proposed as a non-destructive tool for the formation of nano-scaled deformations, the effect of the deformation on the phonon modes is discussed and related to possible psuedomagnetic field Landau quantization. A novel method utilizing nanomanipulators is employed for fabricating wrinkled multilayer graphene devices allowing for transport features to be compared to flat quasi-2D devices. The magnetoresistance shows a conversion from the 2DEG to a linear positive magnetoresistance with large deformation. These processes can be of interest for carbon based quantum technology. The suitability and possible fabrication methods for creating diamond based superconducting circuit elements are discussed.Item Hard, wear resistant Fe-B-C composites produced using spark plasma sintering(2017) Rokebrand, Patrick PierceFe-B-C composites were produced, from boron carbide and iron powders, using spark plasma sintering. This provided information on the effects of rapid sintering on densification, composition and the microstructure of the materials produced. The composition range included a selection high Fe contents (69.3, 78 and 80.9 vol. % Fe-B4C) and high B4C concentrations (1, 3, 5 vol. % Fe-B4C). The properties of the materials were investigated to determine the potential for using relatively cheap Fe and B4C powders to produce hard, wear resistant materials. High Fe-B4C composites were sintered at 900, 1000 and 1100°C at 60 MPa. Densification increased with increasing temperature and at 1100° each composition achieved ≥ 97 % densification. The materials reacted during sintering with the main phases observed being Fe2B and Fe3(B,C) whilst additional phases formed were FeB, C and Fe23(B,C)6.Comparing the phases that were produced to Fe-B-C phase diagrams showed deviations from expected compositions, indicating the non-equilibrium nature of producing the composites using SPS. Although the composites were not at equilibrium, all the B4C reacted and could not be maintained, even with fast heating and cooling rates. The properties of the materials were dependent on both densification and the phases that were present after sintering. Materials containing higher amounts of the Fe2B phase showed higher hardness and fracture toughness results, up to 13.7 GPa and 3.5 MPa.m0.5 respectively for the 69.3 vol. % Fe-B4C. The materials were sensitive to grain and pore growth which negatively affected properties at 1100°C. The transverse rupture strength of 388.3 MPa for 80.9 vol. % Fe-B4C composite was the greatest, and showed evidence of both intergranular and transgranular fracture. The strength was affected by a fine dispersion of porosity at the grain boundaries, throughout the material, and free carbon in the structure was detrimental to the strength of the 69.3 % Fe-B4C. The wear rates were lower using Si3N4 wear balls compared to stainless steel balls, where 69.3 vol. % Fe-B4C showed the best wear rates, 8.9×10-6 mm3/Nm (stainless steel ball) and 1.77×10-6 mm3/Nm (Si3N4 ball), due to the higher Fe2B composition and free carbon acting as a lubricant during sliding. 1, 3 and 5 vol. % Fe-B4C composites were sintered to densities above 97 % of theoretical at 2000°C and 30 MPa. The formation of a transient FeB liquid phase assisted densification. 1 % Fe-B4C attained hardness and fracture toughness up to 33.1 GPa and 5.3 MPa.m0.5 with a strength of 370.5 MPa. Thermal mismatch between the FeB phase and B4C caused high residual stresses at the interface which led to cracking and pull-out of the FeB phase. Residual carbon at the grain boundary interface exacerbated the pull-out effect. Increasing Fe and the subsequent FeB phase had an embrittling effect. The materials suffered severe wear of up to 36.92×10-6 mm3/Nm as a result of the pull-out with the remaining porosity acting as a stress raiser. 20 vol. % of the Fe in each system was substituted with Ti to reduce the presence of residual carbon. Although in some case the properties of the respective compositions improved, residual carbon was still present in the composites.