Electronic Theses and Dissertations (Masters)
Permanent URI for this collectionhttps://hdl.handle.net/10539/38876
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Item Bio-tribocorrosion behavior of low- cost titanium alloys in simulated body fluids(University of the Witwatersrand, Johannesburg, 2023) Rundora, Nicola Ruvimbo; Bodunrin, M.O.; Merwe, Van der; Klenam, D.E.PTitanium and its alloys are considered the gold-standard material in the manufacturing of biomedical implants. This is due to their superior mechanical properties and resistance to corrosion in comparison to the other biomaterials on the market. Titanium and its alloys are however very expensive in comparison and their application and accessibility is thus limited. Issues of toxicity as in the case of the commercial alloy (Ti-6Al-4V) have also been reported. One approach to combat these issues has been the development of new low-cost and less expensive α+β type titanium alloys through the modification of the commercial alloy composition. These efforts led to the development of the following alloys: Ti-3Fe, Ti-4.5Al-1V-3Fe, and Ti-6Al-1V-3Fe. To determine whether these alloys can be utilised for the manufacturing of biomedical implants, they have to undergo a series of screening tests. Some of those tests include wear, corrosion, and tribocorrosion tests. This is the basis of this study. Dry sliding ball-on-disk wear tests were conducted on these alloys. Ti-4.5Al-1V-3Fe and Ti-6Al-1V-3Fe had superior wear resistance to Ti-3Fe, the alloy where the full substitution of V with Fe was employed. Ti-4.5Al-1V-3Fe and Ti- 6Al-1V-3Fe alloys wear resistance was closely comparable to that of the commercial alloy, Ti- 6Al-4V. The corrosion behavior of the experimental alloys was investigated in 0.9 wt.% NaCl solution, Hanks Balanced Salt Solution, prediabetic Hanks Balanced Salt Solution, and diabetic Hanks Balanced Salt Solution. It was important to study the behavior of these alloys in a simulated diabetic environment because of the increase in the number of people living with diabetes and their increased risk of developing different bone and joint disorders. The corrosion behavior of Ti-3Fe was superior to that of the commercial alloy and the other experimental alloys in 0.9 wt.% NaCl solution and Hanks Balanced Salt Solution. In the prediabetic and diabetic Hanks Balanced Salt Solution, the experimental alloys resistance to corrosion was comparable to that of the commercial alloy with Ti-4.5Al-1V-3Fe having the best resistance to corrosion in the prediabetic solution and Ti-6Al-1V-3Fe having the best resistance to corrosion in the diabetic solution. Ti-4.5Al-1V-3Fe and Ti-6Al-1V-3Fe showed tribocorrosion resistance that was comparable to the commercial alloy in both the normal HBSS and diabetic HBSS under both open circuit potential and potentiostatic conditions. These alloys showed better tribocorrosion behavior in the normal HBSS under potentiostatic conditions compared to Ti-3Fe and the commercial alloy. In the diabetic HBSS under potentiostatic conditions, Ti-4.5Al-1V-3Fe had the best resistance to tribocorrosion. The iv | P a g e glucose addition had a slightly negative effect on the tribocorrosion behavior as the wear rates and total worn volume values in such solutions were slightly higher than in the normal solutions. Ultimately, under corrosion and tribocorrosion testing conditions, the partial substitution of V with Fe as beta stabilisers in low-cost titanium alloy designs offered superior properties over the total replacement of V with Fe.Item Investigation of corrosion behaviour of aluminium alloy 7075 processed by laser shock peening without coating(University of the Witwatersrand, Johannesburg, 2024) Shonhai, Natsai; Cornish, L.A.Laser shock peening (LSP) is a surface treatment to induce beneficial compressive residual stresses in metallic structures, thus improving their fatigue resistance. This technology has the potential to improve aeronautical component performance during application and maintenance. Aluminium alloys are used in the aviation industry due to their high strength-to-weight ratios and ease of design and manufacturing. However, they are susceptible to localised corrosion in some aggressive environments. This study investigated the corrosion behaviour of AA7075-T651 after LSP without a protective coating (LSPwC). Variations in power intensity (PI) of 1-6 GW/cm2, coverage (Np) of 2.5-67 spots/mm2 and spot size (SS) of 0.5-1.5 mm were explored. Surface modifications were evaluated using stereo microscopy, scanning electron microscopy (SEM), contact profilometry and Vickers microhardness tests. Three types of corrosion tests were conducted in 3.5 wt% NaCl solution on the peened and unpeened samples: potentiodynamic polarisation tests, 30-day immersion tests and stress corrosion cracking by three-point bending for 40 days. Microscopic examination revealed rough surfaces with areas of melting and solidification in LSPwC samples. Surface roughness increased in all samples post-LSPwC due to induced plastic deformation and surface ablation. Increasing PI and Np led to increased surface roughness. All peened samples had increased microhardness with a positive correlation with Np. Potentiodynamic polarisation revealed higher corrosion rates for most LSPwC samples, likely due to increased surface roughness which reduced corrosion resistance. Corrosion rate had a positive linear correlation with PI and Np with aluminium oxide formation and pitting as the dominant corrosion mechanisms. Improved corrosion resistance after LSPwC was observed in some samples with specific parameters. Notably, one sample had a corrosion rate four times lower than the unpeened one. The three-point bending induced tensile stresses on the peened surfaces, which led to the formation of multiple tangled cracks on the top surface of all specimens. The LSPwC samples were less susceptible to SCC and the best resistance was observed for Sample P3-SS1-Np50.38 which had no visible cracks in the sample cross-section.