3. Electronic Theses and Dissertations (ETDs) - All submissions
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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 Development of titanium alloys for hydrogen storage(2016-10-11) Abdul, Jimoh MohammedThe thesis investigated the effect of partial substitution of Cr or Ti with 2-6 at.% Fe, or 0.05-0.10 at.% Rh/Pd on the structure, hardness, corrosion behaviour and hydrogen storage characteristics of an arc-melted Ti35V40Cr25 at.% alloy. The effects of an annealing and a quenching heat treatment on the properties were also investigated. Melting of the eight alloys was done in a water-cooled, copper-hearth arc melting furnace under an argon atmosphere. Each of the eight ingots was cut into three: one as the as-cast sample and the other two separately quartz-sealed and loaded in two batches in a heat treatment oven and heated to 1000 °C for 1 hour. The first set of quartz tubes were immediately removed and broken in cold water to quench the alloy, hence locking the microstructure. The second batch was loaded into the furnace, heated to 1000 °C for 1 hour and then slowly furnace-cooled. The alloys (as-cast and heat treated) were characterised for phase identification using optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) with Electron Diffraction X-ray Spectroscopy (EDS) using an Oxford system. Thermo-Calc software was used to model the phases using the Solid Solution 4 and Titanium 3 Databases. The hardness values (under a 2 kg load) of all samples were recorded. Potentiodynamic corrosion tests were performed in 6M KOH at 25 °C, and Tafel curves were recorded from -1.4V to -0.2V with a scanning rate of 1mV/sec. A Sievert’s apparatus was used for pressure composition temperature (PCT) measurements at 30, 60 and 90 °C. All the alloys contained a primary bcc (V) phase. The secondary phases were a combination of αTi, Ti(Cr,V)2 Laves phases (C14, C15 or C36) and a minor ωTi phase. The cell volume of the primary (V) phase decreased with addition of Fe and 0.05 Rh but increased with 0.1 Rh and Pd. The hardness of the base alloy increased with additions of Fe and 0.10 at.% Pd, but decreased with additions of Rh and 0.05 at.% Pd. Additions of Rh, Pd and 2 at.% Fe decreased the corrosion rate, while additions of 5 and 6 at.% Fe increased the corrosion rate. The reversible hydrogen storage capacity (RHSC) of the base alloy, otherwise known as useful capacity, was enhanced with addition of Pd and Rh, but decreased with Fe addition. Both annealing and quenching increased the hardness of the 0.05 at.% Rh and all the Fe containing alloys. Heat treatment decreased the hardness of the base alloy, both Pd alloys and v the 0.10 at.% Rh samples. Quenching decreased the hardness of the 0.10 at.% Rh and both Pd-containing alloys. The corrosion rate of the 0, 5 and 6 at.% Fe, 0.05 at.% Rh and the Pd-containing alloys decreased after annealing.at.% FeThe rate increased after annealing the 2 at.% Fe and 0.10 at,% Rh samples. The as-cast sample containing 2 at.% Fe had the lowest corrosion rate (0.0004 mm/y) and the quenched 6 at.% Fe was the least corrosion resistant sample with a corrosion rate of 0.037 mm/y. The quenched 5% Fe alloy had the highest hardness (460 MPa), while the annealed 0.10 at.% Rh sample had the lowest (388 MPa). The quenched 0.05 at.% Pd sample had the highest RHSC (2.28 wt%) while the lowest RHSC of 0.44 wt% was observed in the as-cast 2 at.% Fe sample. Annealing improved the RHSC of all samples except the base Ti35V40Cr25 and 6 at.% Fe alloys, while quenching was detrimental to RHSC of all the samples but the 2 at.% Fe, 0.05 at.% Pd and 0.10 at.% Rh alloys. Increasing the addition of palladium from 0.05 to 0.10 at.% Pd showed no significant improvement on RHSC of the base alloy, thus addition of 0.05 at.% Pd would be sufficient. The RHSC of the annealed 0.05 Rh alloy (2.25 wt% H) was close to the value of the 0.10 at.% Pd, so rhodium could be considered as an alternative to the quenched 0.05 at.% Pd. The RHSC was 1.56, 0.44, 0.75 and 0.68 wt% for 0, 2, 5 and 6 at.% Fe as-cast alloys respectively. Although the 2 at.% Fe alloy had the lowest RHSC, it could find its application as electrode in 6M KOH solution electrolyte because of its low corrosion rate.Item Optimizing the surface integrity of machined Ti-6A-4V using advanced carbide inserts and minimum quantity lubrication(2016) Ofem, Nweoyi LekamThis research is focused on optimizing the surface integrity of Ti-6Al-4V using advanced carbide inserts and minimum quantity lubrication (MQL). Experiments were designed to machine twenty Ti-6Al-4V blocks under dry and MQL lubricating conditions using innovative cemented carbide inserts produced by Liquid Phase Sintering (LPS) and Spark Plasma Sintering (SPS). The cutting speed, feed rate, and depth of cut, were considered as machining parameters, while the cutting force, temperature, tool wear, surface roughness and residual stress were considered as performance characteristics. The results obtained for surface roughness and residual stress measurements were used to analyze the surface integrity of the machined Ti-6Al-4V samples. The influence of sintering techniques and milling parameters on performance characteristics and surface integrity were investigated from experimental results obtained. Discrepancy in behavioral trends with respect to lubricating conditions was also investigated. The analyses of milling results showed that cutting speed played a major role in the optimization of surface integrity of the Ti-6Al-4V work pieces followed by the depth of cut. The results also showed that the LPS inserts performed better than the SPS inserts due to their better mechanical properties (higher fracture toughness (KIC) and transverse rupture strength). The application of MQL was also observed to significantly reduce milling temperatures resulting in better surface integrity. However, an optimized surface integrity of the Ti-6Al-4V samples was achieved during MQL milling with the 10Co-L insert at a low finishing speed of 75m/minute and a reduced depth of cut of 0.5mm.Item Investigation into the microstructure and tensile properties of unalloyed titanium and Ti-6Al-4V alloy produced by powder metallurgy, casting and layered manufacturing(2016) Masikane, Muziwenhlanhla ArnoldABSTRACT Solid titanium (Ti) and Ti-6Al-4V (wt.%) materials were fabricated from powders using spark plasma sintering (SPS), cold isostatic press (CIP) and sinter, layered (rapid) manufacturing, centrifugal and vacuum casing. ASTM Grade 4 Ti, Al and V, 60Al-40V (wt.%) and the pre-alloyed Ti-6Al-4V powders were used as starting materials. The solid Ti and Ti-6Al-4V materials produced by the SPS were compared to the CIP and sinter method on the basis of density, microstructure and chemistry. The materials produced by the CIP and sinter method were also compared to those produced by vacuum casting method on the basis of microstructure, oxygen pick-up, chemistry and room temperature tensile properties. Centrifugal casting was compared to the vacuum casting technique on the basis of microstructural homogeneity. Rapid manufacturing was compared to SPS and CIP and sinter on the basis of microstructural homogeneity, density and tensile properties. The tensile properties of all materials were also compared to their commercial counterparts to investigate the effect of interstitial oxygen. The technology resulting in materials with superior properties was finally identified as most promising for commercial production of Ti-based materials. On the basis of densification, the SPS method appears superior compared to the CIP and sinter and rapid manufacturing method due to the benefit of pressure aided sintering, while the rapid manufacturing method is superior to the CIP and sinter method due to the use of a high power laser resulting in high densification rates. In cases where microstructural homogeneity is the key requirement, the CIP and sinter and rapid manufacturing methods appear superior compared to the SPS method due to longer isothermal holding time and higher sintering temperature and the use of pre-alloyed Ti-6Al-4V powder, respectively. On the basis of oxygen pick-up and additional contamination, the vacuum casting route is inferior due to the tendency of melt-crucible interaction, resulting in the dissociation of ZrO2 and subsequent pick-up of O and Zr. Based on the homogeneity of the microstructure, centrifugal casting is better than vacuum casting. The ductility of vacuum cast Ti was better than that of CIP and sintered Ti, possibly due to limited diffusion of oxygen from the crucible compared to oxygen absorbed from the controlled atmosphere during CIP and sinter. The vacuum casting of the Ti-6Al-4V alloy resulted in dissolution of oxygen and Zr due to melt-crucible interaction. Hence the ductility was worse compared to the alloy produced by CIP and sinter. The rapidly manufactured Ti-6Al-4V specimens exhibited superior ductility and strength compared to all alloys produced by other methods due to the use of high purity starting powder. The tensile properties of these specimens were also comparable to standard requirements. The similarity of the tensile properties of wrought Ti-6Al-4V alloy reported in the literature was an indication of limited oxygen pick-up during rapid manufacturing. Therefore based on low oxygen pick-up, microstructural homogeneity, high density and superior tensile properties, the rapid manufacturing route appears to be the most promising approach for commercial processing of titanium based materials.