A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfillment of the requirements for the degree of Doctor of Philosophy, Johannesburg, August 2017 / Titanium 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. / CK2018

A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy (Metallurgical Engineering) 2017 / The alloy Ti-2.7Al-5.7Fe-6Mo-6V (wt%), commercially known as Timetal 125, is used as a high strength fastener in aerostructure assemblies. Very little information is available on its properties and processing, and this study investigated its consolidation from low cost elemental powders, to achieve the minimum mechanical properties for use as a high strength fastener. Reactions during alloying and its beta transus were investigated by differential thermal analysis. The α+β phase region was established to lie between 590oC and 800oC by thermal analysis, metallography and XRD. The alloy was consolidated to ~99% theoretical density by semi-centrifugal casting, and spark plasma sintering of the blended powders. Various heat treatments were undertaken, and the microstructures were evaluated by optical and scanning electron microscopy. Tensile properties, hardness and density were measured after each heat treatment, to establish the optimal combination of mechanical properties. The experimental Timetal 125 style alloy was found to be a metastable beta titanium alloy, which could be strengthened by ageing. It had a microstructure consisting of alpha grains with fine beta precipitates in the as-cast condition, while the sintered samples had acicular precipitates and larger grains, due to the unusually long period that was required to sinter the samples. The ultimate tensile strength was >1500MPa, and elongation was ~3% in the as-cast condition, thus failing to conform to the Airbus EN6116 standard’s specification for ultimate tensile strength and elongation for a high strength fastener in the as-cast or sintered condition. After annealing the castings at 900oC for 1 hour, the ultimate tensile strength decreased to ~760MPa, while elongation increased to ~15%, which still did not conform to the Airbus standard, due to the low strength. The alloy was solution-annealed at 900oC, followed by water quenching to retain a fully βTi microstructure. The minimum properties for the Airbus standard were achieved after ageing between 500oC and 590oC for 1 hour, with an ultimate tensile strength of ~1285MPa, and elongation of ~6.3%. The strengthening depended on the amount and morphology of αTi precipitates from ageing. The αTi/βTi ratio increased with increasing temperature and holding time (shown by XRD), up to 590oC where the precipitates progressively transformed to βTi. Extending isothermal holding time coarsened the precipitates, which was deleterious to strength. There was generally a positive correlation between mean grain size and temperature or holding time, although competing transformations suppressed grain growth, particularly after heat treatment close to transformation temperatures. Although grain size had an effect on the strength of the Timetal 125 style alloy, the main mechanism was precipitation hardening by the secondary αTi. Extended ageing resulted in the formation of allotriomorphic alpha titanium, and a corresponding decrease in the ultimate tensile strength. It was not possible to subject the sintered samples to tensile testing, due to their shape. However, the sintered samples were less porous and had higher Vickers’ values than the castings, suggesting they had similar, if not higher tensile strengths. The acicular precipitates in the sintered samples were possibly martensite or omega titanium (ωTi, Pearson symbol hP3 and space group P6/mmm) although they were too fine to be detected by X-ray diffraction and too fine analyse separately by energy dispersive X-ray spectrometry. / MT 2017

Chan Fu-wai = 非晶態鐵磁性鐵-磷-碳合金的製作 / 陳輔偉. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2000. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Chan Fu-wai = Fei jing tai tie ci xing tie-lin-tan he jin de zhi zuo / Chen Fuwei. / Acknowledgments --- p.i / Abstract --- p.ii / Table of Content --- p.iv / Chapter Chapter 1: --- Introduction --- p.1 / Chapter 1.1 --- Nucleation --- p.1 / Chapter 1.1.1 --- Homogenous nucleation --- p.1 / Chapter 1.1.2 --- Heterogeneous nucleation --- p.4 / Chapter 1.2 --- The crystal growth --- p.5 / Chapter 1.2.1 --- The solid-liquid interface --- p.5 / Chapter 1.2.2 --- Lateral growth --- p.6 / Chapter 1.2.3 --- Continuous growth --- p.6 / Chapter 1.3 --- Non-equilibrium process --- p.8 / Chapter 1.3.1 --- Reduction of impurities --- p.8 / Chapter 1.3.2 --- Suppression of crystal growth --- p.8 / Chapter 1.4 --- The change in metallurgy --- p.9 / Chapter 1.4.1 --- The metallic glass --- p.9 / Chapter 1.4.2 --- The properties of glass --- p.10 / Chapter 1.4.3 --- The development of amorphous alloy --- p.11 / Chapter 1.5 --- Preparation methods of amorphous alloy --- p.12 / Chapter 1.6 --- The criterion of glass formation --- p.14 / Chapter 1.7 --- The study of this project --- p.15 / Reference --- p.16 / Figure --- p.19 / Chapter Chapter 2: --- Experiment --- p.36 / Chapter 2.1 --- Preparation --- p.36 / Chapter 2.2 --- Experimental procedure --- p.37 / Chapter 2.3 --- Sample analysis --- p.38 / Reference --- p.41 / Figure --- p.42 / Chapter Chapter 3: --- The formation of bulk ferromagnetic Fe-P-C amorphous alloy --- p.45 / Abstract --- p.45 / Introduction --- p.46 / Experiment --- p.47 / Result --- p.48 / Reference --- p.49 / Figure --- p.52 / Chapter Chapter 4: --- Compaction of bulk ferromagnetic Fe-P-C amorphous alloy --- p.57 / Abstract --- p.57 / Introduction --- p.58 / Experiment --- p.58 / Result --- p.60 / Reference --- p.61 / Figure --- p.62 / Chapter Chapter 5: --- Conclusion --- p.65

M. Tech. Metallurgical Engineering. / Aims to study tensile deformation mechanisms of aluminium alloy 5083 using in situ SEM techniques. The objectives to achieve this aim are: to investigate the effect of strain rate on mechanical properties ; investigating anisotropy mechanical properties at varying strain rates ; investigating microstructure anisotropy behaviour at varying strain rates and to investigate the influence of intermetallic particles during tensile deformation.

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Orientador: Maria Helena Robert / Dissertação (mestrado profissional) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-16T19:01:24Z (GMT). No. of bitstreams: 1 Esposte_JulioCesar_M.pdf: 9041212 bytes, checksum: 491e0e0249f5348f5e35f599c930fc71 (MD5) Previous issue date: 2010 / Resumo: Este trabalho teve como objetivo a análise comparativa entre a liga Al-Si5-Cu3-Mg nos estados fundido sob pressão (injetado) e tixoconformado, tomando-se como base um componente comercial suporte de acessórios utilizado em motores a diesel. Caracterizou-se o componente comercial produzido pelo processo de injeção em termos de estrutura e propriedades mecânicas, e por sua vez o produto final obtido pelo processo de tixoconformação é também caracterizado nos mesmos termos a fim de se obter uma base comparativa. Para análise da tixohabilidade, ou seja, capacidade de obtenção de pastas tixotrópicas da liga em estudo, submeteu-se a mesma a análises térmicas utilizando técnicas de DSC, e através de simulação termodinâmica utilizando software THERMOCALC®. São discutidas as temperaturas de transformação e intervalos de solidificação, sugerindo-se janelas de tixoconformação para a liga. A liga foi submetida a tratamentos térmicos de globularização de 120 e 300s, para duas condições de frações sólidas, 45 e 60%, além da avaliação da influência de uma pré-formação a frio por compressão do lingote antes da tixoconformação. A liga mostrou-se interessante para a produção de pastas tixotrópicas, com intervalo de solidificação na ordem de 100°C, além de uma variação da fração líquida com a temperatura favorável para definição da janela de tixoconformação. Este comportamento tixotrópico permitiu o total preenchimento do molde e a obtenção de produtos tixoconformados de boa qualidade, com destaque para a diminuição dos teores de porosidade em comparação aos apresentados pelo componente injetado (cerca de 17 vezes inferiores). O material submetido à prévia deformação a frio apresentou glóbulos de diâmetros reduzidos, provenientes do mecanismo de globularização envolvido que passa pela recristalização do material, e consequente melhora das propriedades mecânicas em relação ao material sem deformação. O produto tixoconformado apresenta propriedades mecânicas que atendem às especificações do material estudado, embora apresentem, para algumas das condições de fabricação empregadas no trabalho, valores de limite de resistência e de dureza inferiores aos do material fundido sob pressão / Abstract: The goal of this work was the comparative study between the Al-Si5-Cu3-Mg alloy obtained by die casting and thixoforging, based on a commercial accessories bracket used in diesel engines. The die casting commercial part was characterized in terms of structure and mechanical properties, and the final sample obtained by thixoforging was characterized in the same terms, in order to have a comparative base. For thixoability analysis, in other words, the capacity to obtain thixotropic slurries of the alloy under study, it was evaluated by thermal analysis through DSC techniques and thermodynamic simulations using THERMOCALC® software. Transformations temperatures and ranges are discussed, proposing thixoforging windows. The alloy was submitted for globularization thermal treatments under 120 and 300s, and solid fractions 45 and 60%, beyond the influence evaluation of a preliminary cold forming before the thixoforging process. The alloy proved to be interesting to produces thixotropic slurries, with solidification range around 100°C, besides a favorable liquid fraction vs temperature to define the thixoforging window. This thixotropic behavior allowed the complete matrix filling and thixoforged samples were obtained with a good quality, highlighting the porosity decreasing in comparison with the die casting component (around 17 times less). Reduced globules sizes were obtained with previous cold forming, due to the involved globularization mechanism of the recrystallization, and consequent mechanical properties improving in face of non cold forming material. Thixoforged product presents mechanical properties according to the studied alloy specifications, although for some manufacturing conditions present lower tensile strength and hardness when compared with die casted component / Mestrado / Materiais / Mestre em Engenharia Automobilistica

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The relatively low oxidation resistance and subsequent surface embrittlement have often limited the use of titanium alloys in elevated temperature structural applications. Although extensive effort is spent to investigate the high temperature oxidation performance of titanium alloys, the studies are often constrained to complex technical titanium alloys and neither the mechanisms associated with evolution of the oxide scale nor the effect of oxygen ingress on the microstructure of the base metal are well-understood. In addition lack of systematic oxidation studies across a wider domain of the alloy composition has complicated the determination of composition-mechanism-property relationships. Clearly, it would be ideal to assess the influence of composition and exposure time on the oxidation resistance, independent of experimental variabilities regarding time, temperature and atmosphere as the potential source of error. Such studies might also provide a series of metrics (e.g., hardness, scale, etc) that could be interpreted together and related to the alloy composition. In this thesis a novel combinatorial approach was adopted whereby a series of compositionally graded specimens, (Ti-xMo, Ti-xCr, Ti-xAl and Ti-xW) were prepared using Laser Engineered Net Shaping (LENS™) technology and exposed to still-air at 650 °C. A suite of the state-of-the-art characterization techniques were employed to assess several aspects of the oxidation reaction as a function of local average composition including: the operating oxidation mechanisms; the structure and composition of the oxides; the oxide adherence and porosity; the thickness of the oxide layers; the depth of oxygen ingress; and microstructural evolution of the base material just below the surface but within the oxygen-enriched region. The results showed that for the Ti-Mo, Ti-Al and Ti-W systems a parabolic oxidation rate law is obeyed in the studied composition-time domain while Ti-Cr system experiences a rapid breakaway oxidation regime at low solute concentrations. The only titanium oxide phase present in the scale for all the binary systems was identified as rutile (TiO2) and formation of multiphase oxide scales TiO2+Al2O3 in Ti-Al system and TiO2+TiCr2 in Ti-Cr system was observed. A thermodynamic framework has been used to rationalize the oxygen-induced subsurface microstructural transformations including: homogeneous precipitation of nano-scaled β particles and discontinuous precipitation of +β phases in Ti-Mo and Ti-W system, evolution of TiCr2 intermetallic phase in Ti-Cr system and ordering phase transformation in Ti-Al system.