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Titanium is well known for its excellent properties, such as high strength-to-weight ratio and outstanding corrosion resistance. However the high cost of this metal has confined its applications to those mostly within the aerospace and military industries. The high purchase price of titanium is primarily driven by the need for intricate metal extraction processes, as well as the sensitivity towards conventional metal working operations. Among the potential solutions, powder metallurgy (P/M) technology provides an economical approach to bring down the price of finished titanium products. However, there are still many problems, such as the residual porosity in the sintered body, that need to be overcome. In this thesis, a fundamental study was carried out focusing on the P/M press-and-sinter technique, using commercially pure titanium (CP Ti) as well as two binary titanium alloys, namely Ti-Ni and Ti-Sn. The influence of several processing parameters including compaction pressure, lubricant type/concentration, sintering time/temperature were performed on both the CP and binary systems. The principal tools utilized for mechanical characterization were hardness and tensile testing, whereas optical microscopy, x-ray diffraction (XRD), and scanning electron microscopy were employed to identify the microstructural features present. Press-and-sinter P/M strategies were successfully developed for all of the blends studied. For CP-Ti, a maximum tensile strength >750MPa and near full theoretical density (~99%) were achieved. Transitions in the size and the size distribution of pores and ?-Ti grains were also observed and quantified. It was found these transitions, as well as the powder impurities present (i.e. oxygen and carbon), greatly influenced the final mechanical properties. In the case of the binary alloys, it was shown that liquid phase sintering (LPS) significantly improved the sintered density for the Ti-10%Ni composition, when sintered at l100°C. A eutectic microstructure (CP-Ti + Ti2Ni), coupled with grains of CP-Ti, were identified as the principal phases present. On the other hand, the Ti-Sn alloys only showed a modest increase in sintered density compared to the CP-Ti, owing to the high solubility of Sn in Ti. In terms of crystal structure, XRD highlighted that the Sn containing samples were fully CP-Ti.

M. Tech. (Department of Metallurgical Engineering, Faculty of Engineering Technology), Vaal University of Technology. / This work is an investigation in the development and characterisation of porous Ti-Sn-SiC and Ti-Nb-SiC composites. Pure Titanium (Ti), Tin (Sn), Niobium (Nb) and Silicon carbide (SiC) powders were used as starting materials. The Ti-Sn-SiC and Ti-Nb-SiC composites were produced by powder metallurgy (PM) press-and-sinter route. The Sn is an α-phase stabilizer while Nb is a β-phase stabilizer in Ti alloys. A systematic study of binary Ti-Sn and Ti-Nb alloys was conducted with the addition of SiC particles. The addition of Sn influences the microstructure of the titanium alloy. With increasing the percentage of Sn content, the density of the samples decreases on the Ti-Sn alloys. An increase in the Sn content from 10 to 25 wt. % content resulted in decreased hardness. The Ti-Sn binary revealed stability of the HCP phase with increasing composition of the Sn content. The porous structures of the Ti-Sn-SiC composites were evenly distributed throughout the materials. The sintered densities increase from 94.69% to 96.38%. XRD analysis detected the HCP crystal lattice structure for the Ti5.4Sn3.8SiC and Ti5.6-Sn3.8-SiC composites. XRD pattern of the Ti5.8-Sn3.8-SiC reveals both the HCP and FCC crystal structures. The HCP phase has lattice parameters a= 2.920 Å; c=4.620 Å with smaller c/a ratio of 1.589. Additionally, FCC lattice parameter a=5.620 Å Fm-3m # 225 was obtained both for Ti5.8Sn3.8SiC and Ti6.0Sn3.8SiC XRD patterns. On the other hand, Optical microscopy analysis of the Ti-Nb alloys revealed the equiaxed grains composed of the light β-phase segregating on the grain boundaries. The Ti9Nb1 has low Vickers hardness of all alloys while Ti8Nb2 and Ti7.5Nb2.5 alloys are harder due to high amount of Nb content. Generally, the densities of the Ti–Nb alloys increased with increasing Nb content. HCP and BCC phases have the lattice parameters a = 2.951 Å, c = 4.683 Å and 3.268 Å, respectively. An HCP (α′) phase was detected in the Ti8.5Nb1.5 alloy with lattice parameters a = 5.130 Å, c = 9.840 Å while a BCC phase had a = 3.287 Å. The sintered Ti8Nb2 alloy also had the α′-phase with a = 5.141 Å, c = 9.533 Å and BCC phase with a = 3.280 Å lattice parameters. On the contrary, the Ti7.5Nb2.5 alloy formed the α′-phase of a = 5.141 Å, c = 9.533 Å and BCC with a = 3.280 Å lattice parameters. For the 10 and 15 wt.% Nb alloys, very porous structures were observed. The pores appear spherical and widely distributed. As the Nb content is increased to 20 wt.% (Ti7Nb2SiC) and 25 wt.% (Ti7Nb2.5SiC), porosity was minimized. The sintered densities of the Ti-Sn alloys are decreasing from 95.90% to 92.80% with increased amount of Sn in the Ti, while the sintered densities of Ti-Sn-SiC are increasing from 94.69% to 96.38%. The high porosity, which developed in Ti7Nb1SiC and Ti7Nb2.5SiC, affected the densities of these composites. The sintered densities of Ti-Nb alloys are increasing from 92.08% to 97.65% with increased amount of Nb in the Ti. In terms of hardness Ti7Nb1SiC and Ti7Nb2.5SiC resulted in the lowest while Ti7Nb1.5SiC and Ti7Nb2SiC composites were 511.74 HV and 527.678 HV. The porosity levels were increased by the addition of SiC in the Ti-Sn-SiC and Ti-Nb-SiC composites. The XRD analysis revealed phase transformation on the Ti-Nb alloys and Ti-Nb-SiC composites.