Theses and Dissertations (Metallurgical Engineering)

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

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    Biocompatibility evaluation of sintered biomedical Ti-24Nb-4Zr-8Sn (Ti2448) alloy produced using spark plasma sintering (SPS)
    (Vaal University of Technology, 2018) Madonsela, Jerman S.; Yamamoto, Akiko, Dr.; Machaka, Ronald, Dr.; Shongwe, Brendon, Dr.; Matizamhuka, Wallace R., Dr.
    Solid titanium (Ti), Ti-6Al-4V (wt.%), and Ti-24Nb-4Zr-8Sn (wt.%) materials were fabricated from powders using spark plasma sintering (SPS). The starting materials comprised of elemental powders of ASTM Grade 4 titanium (Ti), aluminium (Al), vanadium (V), niobium (Nb), zirconium (Zr), and tin (Sn). The powders were initially characterised and milled prior to sintering. The micronpowders were milled in an attempt to produce materials with nanostructured grains and as a result improved hardness and wear resistance. The produced solid Ti-24Nb-4Zr-8Sn alloy was compared to solid titanium (Ti) and Ti-6Al-4V (Ti64) on the basis of density, microstructure, hardness, corrosion, and biocompatibility. Relative densities above 99.0% were achieved for all three systems. CP-Ti and Ti64 had both 100% relative density, and Ti2448 showed a slightly lower density of 99.8%. Corrosion results showed that all three materials exhibited good corrosion resistance due to the formation of a protective passive film. In 0.9% NaCl Ti2448 had the highest current density (9.05 nA/cm2), implying that its corrosion resistance is relatively poor in comparison to Ti (6.41 nA/cm2) and Ti64 (5.43 nA/cm2), respectively. The same behavior was observed in Hank's solution. In cell culture medium, Ti2448 showed better corrosion resistance with the lowest current density of 2.96 nA/cm2 compared to 4.86 nA/cm2 and 5.62 nA/cm2 of Ti and Ti64 respectively. However, the current densities observed are quite low and insignificant that they lie within acceptable ranges for Ti2448 to be qualified as a biomaterial. Cell proliferation test was performed using murine osteoblastic cells, MC3T3-E1 at two cell densities; 400 and 4000 cells/mL for 7 days incubation. Pure titanium showed better cell attachment and proliferation under both conditions suggesting that the presence of other oxide layers influence cell proliferation. No significant difference in cell proliferation was observed between Ti64 and Ti2448.
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    Development of a potentially low young's modulus (Ti-34Nb-25Zr-XFe) base alloy for orthopaedic device application
    (Vaal University of Technology, 2019-03) Nemavhola, Mavis Khathutshelo; Machaka, Ronald, Dr.; Shongwe, Mxolisi Brendon, Dr.; Matizamhuka, Wallace R., Dr.
    Elemental titanium (Ti), niobium (Nb), zirconium (Zr), and iron (Fe) powders were used to fabricate four near-β alloys with non-toxic of composition Ti-34Nb-25Zr, Ti-34Nb-25Zr-0.4Fe, Ti-34Nb-25Zr-1.2Fe, and Ti-34Nb-25Zr-2Fe (wt. %) (TNZ and TNZF) using spark plasma sintering (SPS) of nano-crystalline powders attained by high energy ball milling. The fabricated alloys were compared to Ti-34Nb-25Zr (used as a benchmark alloy in this study) and comparison was made with the commercially used Ti base alloys produced either by conventional methods or powder metallurgy. The powder mixtures were milled for 5 hours using a Simoloyer high energy ball mill with a ball to powder ratio of 10:1 and a rotational speed of 1000 rpm. This was followed by sintering the mechanically alloyed powders at 1100 ºC for 10 minutes with a pressure of 50 MPa and a heating rate of 100 ºC/min using an H-HP D25 spark plasma sintering furnace (FCT System, Germany). The powders were characterised for particle size and crystal structure using SEM and XRD. The consolidated components were characterised with regards to density, microstructure, mechanical properties. The electrochemical behaviour of the alloys was investigated using a Digi Ivy DY2300 series potentiostat. Three corrosion medium, Sodium chloride (NaCl), phosphate buffered saline solution (PBS) and Dulbecco’s modified eagle’s medium that mimic the conditions in the human body were used. Mouse myoblast cell line (C2C12) was used to investigate the biocompatibility of the sintered alloys in 1010x5 mm specimens using standard colorimetric assay MTT. Both electrochemical and biocompatibility test were conducted in triplicates and the results compared with that of the benchmark. Results of mechanical alloying of powder mixtures demonstrated an inhomogeneous structure. Milling for 5 hours resulted in agglomeration of small Fe and Zr particles. Milling for 3 hours resulted in a better distribution of elements compared to longer milling times. Therefore, sintering powders milled for 3 hours would have yielded better results. The densification results were acceptable and ranged between 97-99% of theoretical densities. Although some porosity was observed, especially on the un-etched microstructure. An insignificant decrease in density was observed when 1.2 (wt. %) Fe was added. The sintered samples had microstructures which were not homogenous. However, the addition of Fe yielded a more homogeneous microstructure compared to the one with less Fe. Therefore, TNZF with 2 (wt. %) Fe had a more homogenous microstructure. Sintering at 1100 ºC resulted in undissolved niobium and titanium which were observed in the microstructure as dark and white areas. The hardness of the TNZF alloys were comparable and lied between 373 and 432 Hv. These hardness values are higher than other similar titanium-based alloys fabricated using conventional methods. The addition of Fe to TNZ showed an insignificant decrease in hardness. The addition of Fe was found to decrease the Young’s Modulus of TNZ from 119.1 to 80 GPa with an addition of 2 wt.% Fe. However, an unacceptable reduction (230.91 to 158.2 MPa) in strength was also noticed. Pseudo passivation was observed when the alloys were immersed in 0.9 % Sodium Chloride (NaCl) which could be attributed to the inhomogeneity in the microstructure. The possibility of pitting corrosion was also observed. The alloy containing 2 Fe (wt.%) was found to be more corrosion resistant than the other alloys. The TNZF alloys exhibited better corrosion resistance in 0. 9% NaCl compared to phosphate buffered solution (PBS) and DMEM. The corrosion behaviour in PBS and DMEM cannot clearly be explained from the graphs. The morphology of the corroded samples was almost the same for all the alloys in different corrosion media. The microstructures showed pits which could have been from the pores that acted as initiation sites for pitting. In cell culture for 1 and 7 days, the cell viability for TNZF alloys was greater than that of the control group (TNZ). A significant decrease in cell viability for TNZF was observed in cell culture for 4 days. The addition of Fe on TNZ do not cause toxic effects and show good cell adhesion, indicating in-vitro cytocompatibility. The greatest cell viability of 102±3.0 % for Ti-34Nb-25Zr-2Fe. The analysis of cell morphology indicated good cell-substrate interaction. The TNZF alloys developed in this study can be suitable candidates for orthopaedic implant application due to their low Young’s modulus, corrosion resistance and superior biocompatibility. However, the strength needs significant improvement. The advantage of this biomaterial, when compared to commercial alloys, is the absence of cytotoxicity elements such as Al and V.
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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 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.