Metallurgical Engineering

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Browsing Metallurgical Engineering by Subject "Dissertations, Academics -- South Africa."

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    The evaluation of Ta contents on strengthening effect and oxidation resistance of gamma titanium aluminide (𝑦-TiAl-%xTa) alloys.
    (Vaal University of Technology, 2020-04) Cobbinah, Prince Valentine; Machaka, Ronald, Dr.; Shongwe, Brendon, Dr.; Matizamhuka, Wallace R., Dr.
    Intermetallics are described as an ordered alloy phase formed between two metallic elements. An alloy is said to be ordered provided two or more sublattices are needed to describe its atomic structure. An example is titanium aluminides (TiAl). Over the years, TiAls have been some of the most interesting and highly researched high-temperature structural materials. The benefits TiAls offer include low density, high strength-to-weight ratio, excellent corrosion and oxidation resistance, and are ductile at their operating temperatures while also maintaining good structural stability. The substitution of nickel-based superalloys (NBSAs) with the lightweight TiAls for certain stress and temperature applications stand to improve the efficiency of systems and reduce emissions of CO2 and NOx. However, at ambient temperatures, the low workability of TiAls makes their fabrication challenging. In this study, 𝛾-TiAl based alloys of composition Ti-46.5Al-%xTa (x = 0.8, 4 and 8 at.%) were fabricated through the powder metallurgy (PM) route which included mechanical alloy and consolidation by spark plasma sintering. For comparisons, a reference alloy following the renowned and commercially available Ti-48Al-2Cr-2Nb (at.%) composition was included. The main objective of the study was to evaluate the influence tantalum (Ta) contents had on strengthening effect and oxidation resistance of the 𝛾-TiAl based alloys. Particle size distribution, X-ray diffractometry (XRD) at room and high temperatures and scanning electron microscopy (SEM) analyses were used to characterise the as-received elemental powders and sintered alloys. The predominant phases observed from the XRD and SEM analyses of the sintered alloys were the 𝛼2-Ti3Al, 𝛾-TiAl and 𝛼-Al2O3. Electron backscatter diffraction (EBSD) analysis showed an additional phase of TaAl present in the alloys. Furthermore, mechanical properties were measured by compression tests carried out at room temperature and elevated temperatures up to 1000 °C. The results show that the addition of 0.8 at.% Ta increased the compressive strength of the 𝛾-TiAl alloy from 980 N.mm-2 at room temperature to 1280 N.mm-2 at 850 °C. At 1000 °C, the strength of the alloys increased with increasing Ta additions. Steady flow stresses were observed for the reference alloy and 𝛾-TiAl alloy containing 0.8 at.% Ta at their ultimate stresses. Lastly, isothermal oxidation tests in air conducted were at 850 °C for 360 hours and at 1000 °C for 100 hours. The 𝛾-TiAl alloys with 4 and 8 at.% Ta obeyed the parabolic growth law with nearly constant oxidation rates at both tested temperatures. The increasing Ta presence by doping effect suppressed pronouncedly TiO2 growth by decreasing oxygen vacancy concentrations and favoured the formation and growth of a more adherent, denser and protective Al2O3 oxide.