School of Chemical and Metallurgical Engineering (ETDs)

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Browsing School of Chemical and Metallurgical Engineering (ETDs) by Author "Cornish, Lesley"

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    Influence of copper on the corrosion and mechanical properties of Grade 4 titanium for biomedical applications
    (University of the Witwatersrand, Johannesburg, 2022-12) Hadebe, Nomsombuluko Dayanda Elizabeth; Cornish, Lesley; Chown, Lesley H.; Smit, Melanie; Mwamba, Alain
    This study assessed the effect of Ti2Cu and its proportions on the corrosion resistance, and compared the results to Grade 4 commercially pure titanium. The Thermo-Calc program with the TTTI3 (Ti-alloy) database was used to predict the phases. Materials Studio software was used to model the crystal structures and XRD patterns of the phases of Ti-Cu alloys. Ti-Cu samples with 0, 5, 15, 25, 33, 40, 47 and 50 wt % Cu were produced. Composition, microstructures, phases, hardness and corrosion resistance were studied in the as-cast and annealed conditions (750° and 900°C water quenched). The CP Ti samples comprised basket-weave acicular microstructures. The Ti-5Cu samples comprised lamellar (αTi) and Ti2Cu phases. The Ti-15Cu, Ti-25Cu and Ti-33Cu alloys comprised (αTi) dendrites and sparse eutectic of Ti2Cu and (αTi). The ((βTi) dendrites decomposed to (αTi) and Ti2Cu, and could not be retained due to insufficient fast quenching. The Ti-40Cu and Ti-47Cu samples had minor titanium oxide dendrites which solidified first and then Ti2Cu nucleated on them and grew as dendrites, surrounded by the Ti2Cu + TiCu eutectic. In the Ti-50Cu sample, TiCu was the true primary phase and grew as needles, and was subsequently surrounded by a coarse TiCu + Ti2Cu eutectic. No Ti3Cu phase was observed. The microstructures of the as-cast alloys agreed with the Cu-Ti phase diagram of Ansara et al. (2021) and Dyal Ukabhai et al. (2022) with the congruent formation of Ti2Cu, as well as no Ti3Cu. The addition of copper to titanium increased the hardness, while annealing decreased the hardness of the Ti-Cu alloys. Addition of copper above 5 wt % Cu and annealing decreased the corrosion resistance of the samples, but since copper ions in liquid solutions promote the antimicrobial activity, some corrosion is necessary to allow the copper ions to be available. The corrosion tests showed that the corrosion rates obtained were very low, below 0.13 mm/y, which is an acceptable corrosion rate for biomaterial applications. Ti-5Cu showed the best corrosion resistance.
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    Thermo-mechanical processing and testing of titanium alloys for potential dental applications
    (University of the Witwatersrand, Johannesburg, 2022-12) Nape, Kgetjepe Tlhologelo; Chown, Lesley H.; Cornish, Lesley
    New titanium alloy compositions were identified for potential dental implants on the basis of having two-phase microstructures for good mechanical properties and by avoiding problematic elements to increase biocompatibility. The Thermo-Calc program with the TTTI3 (TT Ti-alloy) database was used to calculate new Ti compositions, without toxic Al and V as alloying elements. The aim was to mimic the α+β phase proportions in Ti-6Al-4V and Ti-10.1Ta-1.7Nb-1.6Zr (TTNZ) (an analogue for Ti-6Al-4V). Copper (Cu = 1, 3, 5 and 10 wt%) was varied to give the Ti2Cu phase, which gives good hardness and antibacterial properties. A cost analysis was done and the less expensive Ti-6Nb-4Zr-xCu and Ti-8Nb-4Zr-xCu (x = 0 and 5 wt%) compositions were selected for experimental work. The samples were made by arc-melting and prepared for microstructural studies to understand the influence of alloying elements, and to compare with the commercial Ti-6Al-4V and reported Ti-10.1Ta-1.7Nb-1.6Zr (TTNZ) alloys. Hot deformation of the as-received Ti-6Al-4V and TTNZ alloys was investigated, using a Gleeble 3500® Thermo-mechanical Simulation Facility, at 850°C and 950°C and strain rates of 0.1 s-1 and 10 s-1. The as-cast Ti-6Nb-4Zr-xCu and Ti-8Nb-4Zr-xCu (x = 0 and 5 wt%) alloys comprised αTi and βTi, with Ti2Cu once Cu was added, although EDX indicated some inhomogeneity. The XRD analyses identified αTi and small amounts of βTi with solid solution (shifted peaks), with some Ti2Cu. The Ti-8Nb-4Zr alloy (285 ± 7 HV) had similar hardness to Ti-6Nb-4Zr (280 ± 13 HV), and was considered the better alloy. Adding 5 wt% Cu increased the hardness due to Ti2Cu. With the Gleeble, deformation at 950°C and 10 s-1 led to a finer Ti-6Al-4V microstructure, whereas finer Ti-10.1Ta-1.7Nb-1.6Zr (TTNZ) microstructures occurred at 850°C and 10 s-1. The XRD of all deformed Ti-6Al-4V and Ti-10.1Ta-1.7Nb-1.6Zr samples indicated αTi and βTi, with shifted βTi peaks. The Ti-6Al-4V (324 ± 9 HV) deformed at 850°C and 0.1 s-1 had higher hardness than both deformed TTNZ samples. Higher flow stress were obtained at higher strain rate (10 s-1) and lower temperature (850°C). The Ti-6Al-4V alloy had higher flow stress than the TTNZ alloy. Therefore, the TTNZ alloy was considered better, due to its lower flow stress, which indicated better formability. The new alloys had similar hardnesses to Ti-6Al-4V, and were higher than for TTNZ, suggesting that they might have similar properties to Ti-6Al-4V.