3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Phase equilibrium diagram of Co-Fe-Pd(2019) Ukabhai, Kiyaasha DyalThere is potential to use Co-Fe-Pd rare earth-doped alloys for magnets. The system offers possible alternatives for the heavy and expensive rare-earth magnets. The phase diagram of Co-Fe-Pd was derived nearly sixty years ago and is inconsistent. Therefore, the system needs to be studied again to provide an accurate and updated phase diagram to facilitate the development of the potential magnetic alloys. Samples of different compositions of Co-Fe-Pd were made in a button arc furnace, under an argon atmosphere on a water-cooled copper hearth. They were sectioned and part was kept in the as-cast state and the other part annealed. The samples were analysed in a scanning electron microscope with energy dispersive X-ray spectroscopy to observe the microstructures and to determine the overall and phase compositions. Samples were also analysed in an X-ray diffractometer to confirm the phases. The same analysis procedure was followed for the as-cast and the annealed samples. Coring occurred in many samples, which was expected due to the wide solid solutions in the system. The microstructures were not similar to any of the previous work. There was better agreement of the as-cast samples with the assessed 25°C isothermal section of Raghavan [1992Rag], than with the experimental one of Kuprina and Grigorev [1961Kup]. The deduced liquidus slope agreed with both of the liquidus surfaces published. Some of the phases were too fine to be analysed in the as-cast samples. It was expected that annealing the samples would have coarsened the grains, but the phases were still to fine to analyse. The 1000°C alloys were not homogenous and apart from three alloys which had two phases, the others comprised only one phase. The FePd and FePd3 regions of the Fe-Pd binary diagram did not extend perceptibly into the ternary system. The 1000°C isothermal section was comprised mainly of the A1 (γFe, αCo, Pd) phase. The samples were also annealed at 650°C. The ordered α’ peaks were difficult to identify by XRD, and FePd and FePd3 were difficult to distinguish from A1 (γFe, αCo, Pd). There was a A1 (γFe, αCo, Pd) miscibility gap in the system, around Co71:Fe16:Pd13 – Co31:Fe22:Pd47. The hardness of the alloys increased with increased Fe and decreased Pd contents. The three conditions followed the same trends for hardnesses.Item Influence of temperature on the metal dusting of alloy 800(2018) Morudu, Kholo VeronicaMetal dusting (MD) is a severe form of corrosion in which iron, steels, and nickel (Ni) and cobalt (Co) based alloys disintegrate into a metal or carbide powder with a coke deposit when exposed to strongly carburising gases (carbon activity, ac>1) at elevated temperatures (400800°C). Temperature affects both the driving force and rate of the reaction, represented by gas phase supersaturation with carbon, and the rates of the various processes involved in converting that energy difference into the dusting process. Therefore, process streams such as reformer gas can be benign when hot, but becomes aggressive below critical temperatures. There are different views in literature about the effect of temperature on metal dusting of different materials and alloys. Alloy 800 experiences metal dusting (MD) at 525°C, which is the temperature of the tube sheet of reformers in petrochemical industry. This alloy is specifically used for tube ferrules in the reformers. The reformer trains can reach a critical (highest) internal temperature of 650°C. Therefore, these two temperatures were compared. The effect of temperature and exposure time on the metal dusting of Alloy 800 were investigated in terms of the form of attack and the degradation mechanism. From the results obtained, it was observed that the longer exposure periods result in more carbon deposition and the carbon filaments in the coke become finer as compared to the nanotubes obtained after shorter exposure periods. The alloy suffered metal dusting attack after a relatively short exposure period of three days (72 hours) at both temperatures of 525°C and 650°C, with very little coking.Item Investigation of the C-Ni-V ternary phase diagram and development of an optimum wear resistant alloy(2018) Fabuyide, Abosede AdefunkeThe C-Ni-V ternary phase diagram was investigated. The eutectics of ten as-cast samples produced by Apata [2014Apa1] were re-analysed. The results obtained were compared with previous analyses by Apata [2014Apa1] and Muobeleni [2013Muo]. The differences in eutectic analyses were identified. Six new samples were produced and analysed in the as-cast condition, as well as after annealing at 1000°C for 1000hours and water quenched. The phases in the alloys were identified using SEM-EDX, EPMA and XRD. The solidification projection of the new samples was constructed and the extensions of the binary phases were determined. A new liquidus projection was derived using results of the new samples and Apata’s [2014Apa1] samples and four ternary invariant reactions were identified, L + ~VC + βV2C → σ′, L + βV2C → σ′ + (V), L + ~VC + σ′ → (Ni) and L + ~VC → (Ni) + C, of which were slightly different from Apata [2014Apa1] and Muobeleni [2013Muo]. The isothermal section of the new sample at 1000°C was plotted Six VC-Ni alloys (from VC and Ni powders) and seven V-C-Ni alloys (from VC, V and Ni powders) were produced by spark plasma sintering. The sintered density, hardness, fracture toughness, wear performance and corrosion performance in 1M H2SO4 and 3.5wt% NaCl environments of the alloys were evaluated. Alloys with 10 at.% Ni had the preferred combination of properties, and Alloy V48:C42:Ni10 (at.%) was the optimum alloy with the following properties: 97.6% sintered density, wear rate of 3.03 ± 0.00(1) 10-6 mm3/Nm against a Si3N4 ball, corrosion rates of 0.0176mm/y in 1M H2SO4 environment, and 0.00295mm/y in 3.5wt% NaCl environment.Item Corrosion characteristics of steels and metallic alloys used as construction materials in plants exposed to fluorine containing acids(2018) Van der Merwe, RynoThe two hydrofluoric acid (HF) storage tanks used for holding 70% technical grade HF product at the HF plant at Necsa started leaking in March 2012. An evaluation of the failure was conducted in the form of a corrosion failure analysis. It was confirmed that a higher than usual nitric acid (HNO3) content in the technical grade HF stream changed the corrosion mechanisms typically experienced within the HF storage vessels, which then caused the tanks to fail. Immersion type corrosion experiments were done to safely simulate the corrosive environment experienced by the mild steel, stainless steel and nickel alloys used on site, and to predict the change in corrosion rates and characteristics associated with the HNO3 contamination in the HF production plant circuit. Since the corrosion resistance of mild steel in HF is heavily dependent on the thickness of the protective scale on the steel, a series of planned interval corrosion tests (PICTs) was done to reproduce and then examine the oxidefluoride barrier on mild steel coupons in pure 70% HF prior to corrosion tests. These shorter PICTs were also done on the stainless steel and nickel alloys and showed that the prepassivation step had a surface cleaning effect when exposed for only 24 h. Eleven day corrosion tests were conducted to establish the effect of HNO3 concentration and temperature on mild steel corrosion in 70% HF, and to determine the change in corrosion rates and mechanisms associated with HNO3 contamination (0.1-1% HNO3) of the downstream HF products. The corrosion was characterized by analysing the corroded coupons for mass loss, apparent corrosion rates, acid consumptions, visual observations of scale formation and pits, as well as depth profiles from scanning electron microscopy and energy dispersive spectroscopy analyses. Linear relationships were frequently observed when analysing mass losses for the coupons over time, making it possible to define corrosion rates in terms of first order reaction kinetics. The harshest corrosive condition for mild steel in HF was determined to be 1% HNO3 in 70% HF at a constant temperature of 25ºC. The corrosion characteristics of alloys used in the HF plant, as affected by HNO3 impurities (in the range 50–10000 ppm) in the final HF acid product (70% Technical grade) were successfully established. Normalized SA516 Grade 70 mild steel and Monel 400 were found not adequate for use as construction materials in a plant where HNO3 contamination was >100 ppm. However, the corrosion resistance of SS 904 L was suitable under these conditions and was recommended for applications in HF solutions where the presence of an oxygen-containing acid (e.g. HNO3) is consistent. It was recommended that Alloy 31, Alloy 33 or Nirosta 4565S, with higher chromium content (>20 wt% Cr), should be considered for construction material of the HF plant when HNO3 contamination becomes unavoidable. However, if the continued use of mild steel at the plant cannot be avoided, other inhibition strategies tailored to the selective consumption of HNO3 in the HF product stream need to be investigated.Item Development of PGMs-modified TiAl-based alloys and their properties(2017) Mwamba, Ilunga AlainTitanium aluminides Ti3Al (α2), γ-TiAl and TiAl3 have received much attention for potential applications where light weight for energy saving, room temperature corrosion resistance in aqueous solutions, high-temperature oxidation resistance, or where combinations of the above are needed. Gamma-TiAl of composition Ti-47.5 at.% Al with additions of platinum group metals (PGMs: Pt, Pd, Ru and Ir) was investigated for microstructure, hardness, room temperature aqueous corrosion, high-temperature oxidation resistance, mechanical alloying and consolidation by spark plasma sintering, and coating on titanium Grade 2 and Ti-6Al-4V substrates. Gamma-TiAl of Ti-47.5 at.% Al produced by melting and casting gave a microstructure consisting of γ grains and lamellar grains with alternating of α2 and γ phase lamellae. Additions of 0.2, 1.0, 1.5, and 2.0 at.% PGMs introduced new phases of high PGM contents. The γ and lamellar phases were still present. The additions of PGMs significantly improved the aqueous corrosion properties at room temperature, by improving the pitting corrosion resistance of the γ-TiAl alloy by modifying its hydrogen evolution of the cathodic reaction. The presence of PGMs also influenced the oxidation behaviour of γ-TiAl at 950°by forming the Z-phase which stabilized a continuous protective Al2O3 phase. However, Ti-47.5 at.% Al, being a two-phase alloy (α2+γ), PGMs could not sustain a stable Z-phase, as it transformed into an oxygen supersaturated Ti3Al, which subsequently led to the formation of TiO2+Al2O3, a non-protective oxide mixture. The optimal PGM addition to γ-TiAl was 0.5 at.%, with iridium giving the best room temperature corrosion and high-temperature oxidation resistance. Mechanical alloying of Ti and Al pure powders with PGM additions gave powders where α2 and γ were only identified after heat treatment. Consolidation of the mechanically alloyed powders by spark plasma sintering gave different microstructures from the cast alloys, with continuous α2 and γ phases and evenly distributed nanometer-sized alumina, and much higher hardnesses. Cold spraying the mechanically alloyed powders on to titanium Grade 2 and Ti-6Al-4V substrates gave coatings of irregular thickness, dense near the substrates with porosity at the top, giving poor oxidation protection.Item An investigation of the C-Ni-V ternary phase diagram and development of abrasion-resistant alloys(2016) Apata, Ayodeji OluwatoyinThis study investigated the C-Ni-V ternary phase diagram and identification of possible abrasive-resistance alloys. Twenty-four alloys were made from the elemental components and were arc-melted under an argon atmosphere, using Ti as an oxygen-getter. These alloys were analysed in both the as-cast condition, and after annealing for 1000oC and water quenching. Microstructural characterization was carried out in a SEM with EDX, and was done to confirm the phases. The results were used to plot a solidification projection and all binary phases extended into ternary, except for ~Ni8V, ~NiV3 and ~V2C which were not found, due to the sample compositions chosen. The extensions of the binary phases were: ~Ni3V: ~15 at.% C; ~Ni2V: ~20 at.% C; σ′: ~18 at.% C; (Ni): ~27 at.% C; ~V8C7: ~2 at.% Ni; ~V6C5: ~2 at.% Ni and ~VC: ~2 at.% Ni. The liquidus surface was derived, and three ternary invariant reactions were identified. The isothermal section at 1000oC was also constructed. Hardness of the alloys was studied in both conditions. Alloys with (Ni) (188-402HV5) were found to be ductile with low hardness. Alloys containing (V), ~Ni3V and ~Ni2V were identified as hard phases (533-1052 HV5). Alloys with σ′ phase were very hard (1065-1266 HV5) extremely brittle with cracks. Fracture toughness of the C-Ni-V alloys 0.9-5.2 (MPa.m-1/2) were comparable with ceramics 0.5-5.3(MPa.m-1/2). The wear behaviour of the alloys was characterized by sliding the carbide alloys against a Cr-steel ball in a pin-on-disc configuration. There were several co-existing wear mechanisms: abrasion, adhesion and the formation of a thin tribolayers. The wear coefficients for a 10N contact load after a sliding wear path of 300m varied between 0.1 x 10-6and 7.6 x 10-6(mm3/Nm), which was not as good as WC-Co hard metals, but close.Item Corrosion and other properties comparison of AISI 316L stainless steel surface alloyed with Ru/Ni mixtures with the parent metal and with Hastelloy© C-276(2016) Lekala, Makgale BarclaysThe surfaces of AISI 316L stainless steel plate were laser alloyed with ruthenium powder as well as a mixture of ruthenium and nickel powders using a Nd:YAG laser set at fixed operating parameters. The microstructure, elemental composition, and corrosion characteristics of the alloyed zone were analysed using optical and scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and corrosion potential measurements. EDS analysis of the alloyed specimen showed that through the laser surface alloying, 2 mm surface layers with 12.5wt % Ru and 5.2wt% Ru were produced on an AISI 316L stainless steel. Similar microstructures which were dendritic and columnar grains, typical of weld beads under non-equilibrium cooling conditions were observed for all samples. Hardness profile measurements showed a significant increase from 160 HV for the substrate to a maximum of 247 HV for the alloyed layer. Using an Autolab potentiostat, the corrosion behaviour and resistance of the laser alloyed layers, substrate AISI 316L, and Hastelloy© C-276 were evaluated and compared in sulphuric acid solution of different concentration and temperatures. The Hastelloy© C-276, followed by the 12.5wt% Ru presented the most noble corrosion potential (Ecorr) and the lowest corrosion current density (icorr). However, in 60wt% H2SO4 and 40oC, the 5.22 wt% Ru alloys exhibited slightly better anticorrosive properties than 12.5wt% Ru. The observed corrosion potential, Ecorr, for untreated AISI 316L stainless steel sample in 40wt% sulphuric acid solution at 40oC was -277 mV. The 5.22 wt% Ru and 12.5wt% Ru alloyed stainless steel samples presented -240 mV, and 61 mV respectively in the same solution. Besides showing comparable performance to 5.2wt%Ru sample within specific short potential ranges, Hastelloy© C-276 was generally superior in all solutions. In addition it was found that the stability of the passive layer was improved with additions of Ru. Based on the developed costing equation the cost of 5 mm AISI 316L stainless steel plate with surface area (A = 1 m2) surface alloyed with 5.2wt% Ru to a depth of 2 mm using Nd: YAG laser is estimated at R15 989, and it is less than the cost of a Hastelloy© C-276 plate of similar size which is estimated at R19 900. As the material thickness increases, the cost benefit of laser surface treatment increases and vice versa. Reduction of the Ru additions to levels below 5.2wt% would improve cost competition without detracting from performance.