Theses and Dissertations (Metallurgical Engineering)

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Browsing Theses and Dissertations (Metallurgical Engineering) by Author "Matizamhuka, Wallace, Prof."

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    Studies on the mechanical properties and corrosion behaviour of AI20Be20Fe10Si15Ti35 high entropy alloy produced by mechanical alloying and SPS sintering
    (Vaal University of Technology, 2022-02-22) Onawale, Omoyemi Temitope; Machaka, Ronald, Prof.; Matizamhuka, Wallace, Prof.
    To meet structural, biomedical, and energy-saving application requirements, weight reduction is of utmost concern especially in the transportation sector because it is the feasible means of reducing fuel consumption as well as greenhouse gas emissions. The fabrication of light weight high entropy alloys (LWHEAs) using traditional design methods such as the addition of refractory elements or casting has proven that it is almost impossible to achieve the required properties in LWHEA. The spark plasma sintering (SPS) method was employed to consolidate wet and dry mechanically alloyed (MA) Al20Be20Fe10Si15Ti35. This is owing to the possibility of achieving desired mechanical properties and produce high-performance bulk HEAs with homogeneous and finer grains during the processing route, unlike the conventional casting techniques. The EDS, SEM and XRD examination that was conducted on both wet and dry milled alloy powders reveals a decomposition condition took place at temperatures of approx.. 2000C. We suspect that this is associated with a low melting Fe2Be17 phase, which becomes unstable with increase in temperature. This is in conformity with literature that highlights that the influence of Al and Fe on the performance of Be will depend upon which phases manifest. It has been established that volatility properties of ethanol and Fe must have aided sublimation tendency of wet milled alloy mix especially. This is in addition to evaporation tendency of Be powder. A thermo-analytical process was also conducted on both dry-milled and wet-milled alloy powder. This confirmed a decomposition leading to thermal expansion in both dry and wet milled alloy powder at a rapid rate from room temperature to ~150°C. Hence, Al20Be20Fe10Si15Ti35 milled powder is thermally unstable being the possible cause of premature sublimation that occurred during the sintering process.
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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.