In the present work, phase transformations in TIMETAL LCB tita- nium alloy and their influence on mechanical properties were studied. Different initial conditions were prepared by solution treating above β-transus immedi- ately followed by heat treatment in α/β temperature regime. These resulted in different grain boundary α thicknesses and contiguities at a fixed α vol- ume fraction. The subsequent ageing response of this material was studied by low temperature ageing at 400 ◦ C, 450 ◦ C and 500 ◦ C. Phase transformations were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and resistivity measurements. Mechan- ical properties were investigated using microhardness measurements and tensile tests. It has been proved that metastable ω phase is formed during annealing at 400 ◦ C and 450 ◦ C. ω particles further transform to very fine precipitates of α phase when exposed to annealing for longer time periods. These fine precip- itates significantly contribute to increase of microhardness and achieving high value of yield stress. Keywords: Metastable beta Ti alloys, phase transformations, microstructure changes, ω phase 1

Commercially available titanium (Ti) alloys, such as Ti–6Al–4V and c.p. Ti, even though established in clinical use as load-bearing bone implant materials in orthopedics and dentistry, possess significant drawbacks that may lead to implant failure: i) presence of alloying elements with harmful health effects, ii) high Young’s modulus (E > 100 GPa) compared to human cortical bone (Ebone = 10 – 30 GPa), and iii) lack of antibacterial activity against multidrug-resistant bacteria, which may lead to implant-associated infections. To overcome the first two drawbacks, a new generation of biocompatible metastable β-type Ti alloys has been developed, in particular β-type Ti–Nb alloys, which are versatile candidates due to their low Young’s modulus, high strength-to-weight ratio and improved corrosion resistance. The present work aims to tackle all three aforementioned issues by developing novel β-type Ti–45Nb-based alloys with potential intrinsic antibacterial activity by adding antibacterial gallium (Ga) and copper (Cu) in minor amounts (up to 8 wt.%) via metallurgical route. Nine alloys with the following chemical compositions: (100-x)(Ti–45Nb)–xGa, (100-x)(Ti–45Nb)–xCu (where x = 2, 4, 6, 8 wt.%), and 96(Ti–45Nb)–2Ga–2Cu, based on alloy design approaches, were produced by controlled casting and homogenization treatment. The effect of antibacterial alloying additions on phase constitution, mechanical characteristics, corrosion, and tribocorrosion response in a simulated physiological environment has been investigated. All nine alloys in the homogenized state display a single-phase β (BCC) phase microstructure, whose lattice parameter is proved to be sensitive to Ga and Cu content, with an almost linear contraction. The mechanical characteristics are strongly influenced by Ga and Cu addition, with a general strengthening effect mainly attributed to substitutional solid solution strengthening, and to grain boundary strengthening in case of Ga. Deformation behavior indicates high mechanical stability of the β phase, suggesting dislocation slip as dominant deformation mechanism. The results demonstrate that strategic alloy design is an effective method to significantly increase strength without adversely affecting the Young’s modulus, which remains in the range of good biomechanical compatibility (E = 64 – 104 GPa). Evaluation of the corrosion response and metal ion release in simulated physiological environment demonstrates the high corrosion resistance of the nine alloys, whereas tribocorrosion wear resistance increases upon Ga addition. Further thermal (aging) treatments, carried out on a specific Cu-containing alloy, proved the feasibility of tailoring enhanced mechanical, chemical and potentially antibacterial properties by thermally-induced precipitation of Ti₂Cu intermetallic compound. These novel developed alloys are considered to be promising candidates for biomedical bone implant applications.

Additive Manufacturing (AM) processes are gaining prominence in industry as they can build parts to near-net-shape with minimal postprocessing. Metal laser AM techniques, such as Selective Laser Melting (SLM), offer rapid cooling rates on the order of 10^5-10^6 K/s. This is due to a highly-focused laser heating a microscopic volume in an otherwise lower-temperature environment. Hence, metal laser AM can manufacture novel, out-of-equilibrium microstructures that cannot be produced in near-net-shapes with other processes. It is desirable to optimize feedstocks for metal AM processes to leverage their advantages. One option of optimizing feedstocks is through in-situ alloying, or by using elemental powders. Elemental powders homogenize over the course of multiple laser passes, or intrinsic heat treatments. However, rapid cooling rates prevent the homogenization of a layer when first printed. To investigate the homogenization process, this thesis used synchrotron X-ray Diffraction (sXRD) to track the phase transformations during the SLM of a 14-layer single wall (single-hatch, multilayered) of Ti-1Al-8V-5Fe (Ti-185) from elemental Ti, Fe and an alloyed AlV powders, capturing frames at 250 Hz. Infrared imaging was performed simultaneously on the surface at 1603.5 Hz to observe the temperature changes at the surface. Post-mortem electron microscopy was performed on cross-sections of the wall perpendicular to the scanning direction to observe the changes in the microstructure with respect to the build direction. Specifically, Electron Dispersive X-Ray Spectroscopy and Electron Backscatter Diffraction were performed to observe the alloying elemental distribution and microstructure of the wall with respect to the build direction. The research performed found that in the melted zone, phase transformation times below 50 ms yielded a partially-alloyed microstructure, with regions concentrated and dilute in alloying elements. Partial mixing was diffusion-induced by laser beam heat and the exothermic heat of mixing of Ti-185 from its constituent elements. Further diffusion during reheating cycles yielded an alloyed microstructure. / Thesis / Master of Applied Science (MASc)