Theses and Dissertations (Metallurgical Engineering)

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Browsing Theses and Dissertations (Metallurgical Engineering) by Subject "Corrosion resistance"

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    Evaluation of the corrosion behaviour and biocompatibility of Ti-34Nb-25Zr alloy for biomedical applications
    (Vaal University of Technology, 2018-11) Mahundla, Mithavini R.; Shongwe, Mxolisi B., Dr.; Machaka, Ronald, Dr.; Matizamhuka, Wallace R., Dr.
    Pure Ti, Nb, Zr, Al and V powders were used as starting materials. Ti, Ti-6Al-4V and Ti-34Nb-25Zr materials produced by SPS were compared on the basis of density, microstructure, biocompatibility, tensile strength and corrosion resistance. In this study, powder metallurgy (PM) processing route was used to fabricate the alloys. The processing route was mechanical alloying (MA) and spark plasma sintering (SPS). Commercially pure metallic powders (Ti, Nb, Zr, V and Al) of different morphological features and different formulations were prepared. Powder mixing for ternary alloys with Ti as the matrix were conducted in a turbula mixer at a speed of 49 rpm. Followed by mechanical alloying of Ti, Ti-6Al-4V and Ti-34Nb-25Zr in a high energy ball mill for 5h at 500rpm, with a ball to powder ratio of 10:1. Spark plasma sintering of Ti, Ti-6Al-4V and Ti-34Nb-25Zr biomedical alloys was conducted using a hybrid spark plasma sintering furnace at a sintering temperature, heating rate, holding time and pressure of 1200°C, 100°C/min, 10min and 50MPa, respectively. Ti-34Nb-25Zr was fabricated in two ways, fully densified and porous samples. The fully densified sample was fabricated at a sintering temperature, heating rate and holding time and pressure of 1200°C, 100°C/min, 10min and 50MPa, respectively. Whereas, porous Ti-34Nb-25Zr was fabricated using NaCl space holder at a sintering temperature, heating rate, holding time and pressure of 750°C, 50°C/min, 5min and 50MPa, respectively. This was done to compare the solid and porous alloy biocompatibility behaviour. Microstructures, elemental compositions. Phase constitution of the sintered specimens were examined using a field emission scanning electron microscope (FE-SEM) equipped with energy dispersive x-ray spectrometer (EDS) and an x-ray diffractometer (XRD). The microstructure of Ti-34Nb-25Zr had pores and precipitates of niobium. Relative density, micro-hardness, biocompatibility and corrosion test was also conducted on the metallographically polished cross sections of sintered specimens. Ti, Ti-6Al-4V and Ti-34Nb-25Zr alloys made from the irregularly shaped Ti powders and sintered on the hybrid sintering machine yielded higher densifications reaching up to 100 % relative densities. Hardness values ranging from 300-600Hv at a load of 0.5kg. The corrosion resistance of the alloys was higher in the range of 2-4 nA/cm2 exhibiting a passive behaviour in simulated body fluids, such as Hank’s, 0.9wt.% NaCl and eagles minimum essential + 10% fetal bovine serum (E-MEM+ 10% FBS). Biocompatibility tests were conducted (cytotoxicity by WST-1 with SaOS-2 human osteosarcoma cells, protein adsorption and surface wettability). Fibronectin adsorption was less for solid Ti and Ti-34Nb-25Zr (<2ng/mm) compared to Ti-34Nb-25Zr porous and Ti-6Al-4V (4 ng/mm). Albumin adsorption was the highest on Ti substrate (3 ng/mm) than on the fully densified and porous Ti-34Nb-25Zr surfaces followed by less adsorption on Ti-6Al-4V. Surface wettability of Ti and Ti-6Al-4V showed a high contact angle of between 93-98° compared to 86° for the Ti-34Nb-25Zr solid alloy, indicating that Ti-34Nb-Zr alloys exhibited hydrophilic behaviour. The surface wettability results correlated well to less fibronectin adsorption on Ti-34Nb-25Zr solid alloy and excellent adsorption for Ti-6Al-4V. Solid and porous Ti-34Nb-25Zr showed less cell proliferation (0.06 and 0.02% cell viability) which was possibly linked to fibronectin adsorption results. Biocompatibility behaviour of Ti-34Nb-25Zr solid and porous alloys was poorer than Ti (0.20% cell viability) and Ti-6Al-4V (0.23% cell viability). There was poor protein adsorption and cell proliferation on all the alloy substrates.
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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.