Theses and Dissertations (Metallurgical Engineering)

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Browsing Theses and Dissertations (Metallurgical Engineering) by Subject "Alloys."

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    The development of Light-weight High Entropy Alloy (LWHEA) composites Al35Ti35Si(20-x) Be10Bx (x=1,2,3) wt.% prepared by powder metallurgy route
    (Vaal University of Technology, 2022-05-10) Dlamini, Sibongile Mabel; Machaka , Ronald; Matizamhuka, Wallace, Prof.
    High entropy alloys (HEAs) are novel alloys with five or more primary elements in an equiatomic or near-equiatomic proportionate ratio. The configuration entropy in HEAs tends to stabilize the development of solid solutions like body-centred-cubic (BCC), face-centred-cubic (FCC), and hexagonal-closed-pack (HCP). Compared to traditional alloys, the increased number of primary elements present in HEAs causes severe lattice distortion, resulting in higher mechanical properties. HEAs are seen as a radical transformation for the next generation of high-temperature alloys in extreme conditions like aircraft, cutting tools, and bearings. The main objective of this dissertation was to develop new types of Al35Ti35Si(20-x)Be10Bx (x=1,2,3 wt.%) lightweight high entropy alloys using mechanical alloying and Spark plasma sintering to understand better how microstructures evolve during sintering and secondary processing, as well as the mechanical properties that can be derived. The first part of the project involved subjecting the elemental powders (aluminium, titanium, silicon, beryllium and boron) chosen for this work to mechanical alloying for 45 hours. Subsequently, applying plasma sintering to produce all the three fully densified alloy composites: Al35Ti35Si19Be10B, Al35Ti35Si18Be10B2, and Al35Ti35Si17Be10B3 at 1000 ºC with densities 3.48, 3.40 and 3.51 gꞏcm-3, respectively. The sintered alloys showed the formation of BCC and FCC solid solutions as well as ordered solid solution phases such as Ti4Si8/ Ti16Si32, Al4Ti8O2, and B2N2, with a microhardness of 957, 989, and 1093 HV, respectively. The three developed alloys also showed remarkable corrosion resistance in a 3.5 wt.% NaCl solution. Tribological characteristics of the developed Al35Ti35Si(20-x)Be10Bx (x=1,2,3 wt.%) alloys were examined under dry sliding wear conditions with stainless steel as the static friction partner under a 10 N load and a sliding duration of 60 min. The results indicated that the increase of silicon in the alloy has an impact on the friction coefficient and wear rate. High-temperature oxidation test was also conducted for Al35Ti35Si19Be10B, Al35Ti35Si18Be10B2, and Al35Ti35Si17Be10B3 alloys at 700 and 900 ºC for 400 hours and 200 hours, respectively. These alloys showed good resistance to high-temperature oxidation at 900 ºC as compared to oxidation at 700 ºC. The resistance to oxidation was indicated by low weight gain and low rate constant.
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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.