Co-Cr alloys are the most widely used metallic biomaterials for metal-on-metal joint prostheses. However, concerns about increased revision rate associated with metal-on-metal replacements have been raised recently. This research was aimed at studying the response of medical grade ASTM Fl537 Co-28Cr-6Mo alloy with different phase constituents to low-temperature plasma carburising (LTPC) to address the tribocorrosion problems of the current metal-on-metal joint prostheses. To this end, n series of heat treatments were designed to produce samples with an a-FCC dominated (>95%), an c-HCP dominated (>97%) and a dual phased (40% c-HCP) microstructures. An optimised low-temperature plasma carburising (LTPC) treatment wns applied to the heat-treated samples. The microstructure of the plasma carburised surface layers were characterised by XRD, GDS, SEMIEDX and TEM, and the properties of the surface carburised layers were evaluated using micro- a•nd nano-indentation, reciprocating wear and electrochemical corrosion tests, and tribocorrosion tests a t different electrochemical potentials. The experimental results have showed that the response of the Co-28Cr-6Mo alloy to the LTPC treatment is highly dependent on the phase constituent. The LTPC treatment can significantly increase the hardness, load bearing capacity and dry wear resistance and tribocorrosion properties of all the Co-28Cr-6Mo samples with different phase constituents.

A study of the processing and characterisation of sintered NdFeB magnets made from recycled feed stock was undertaken. Initially the hydrogen decrepitated (HD) powder was investigated using two different milling techniques. The powders were analysed with optical microscopy, with the aid of a magnetic field. It was found that with light milling the HD powder breaks up to a similar particle size to that of the grain size of the starting material. A data logging system was built to investigate the desorption behaviour of green compacts during sintering. Desorption traces showed desorption from the matrix phase and the intergranular Nd-rich phase. The start of desorption was seen to shift to lower temperatures as the mean particle size of the green compact was reduced. For the processing route used in this work intergranular additions of neodymium hydride were required to increase the density and magnetic properties. To investigate the oxidation behaviour of lightly milled HD powder, powder was exposed to air for varying times. The exposed powder was aligned pressed and sintered. The Nd-rich desorption peak reduced with exposure time, the density and magnetic properties also reduced. Post exposure additions of Intergranular neodymium hydride to the powder recovered density and magnetic properties.

High strength aluminium alloy AA2024 is susceptible to localized corrosion in the form of pitting and intergranular corrosion (IGC). The corrosion behaviour of this alloy is strongly influenced by the presence of different intermetallic particles. In this study, the effect of applied stress and the role of intermetallic particle removal by surface treatment on the initiation and propagation of localized corrosion are investigated. It was found that applying a stress to as polished AA2024 leads to a drop in breakdown potential. The development of micro/nano crevices adjacent to the intermetallic particles is thought to act as initiation sites for localized corrosion, lowering the breakdown potential of stressed AA2024. The electrochemical behaviour of specific intermetallic particles and the particle free matrix of AA2024 have been studied as a function of stress state using the micro-capillary electrochemical cell in combination with an in situ stressing stage. These experiments emphasized the role of intermetallic particles as the possible key contributing factor in determining the corrosion properties of the alloy under stressed conditions. In situ X-ray synchrotron microtomography experiments were used to observe the evolution of corrosion attack as a function of continuous exposure time in both unstressed and stressed AA2024. Quantitative growth rate measurements indicates that application of stresses of 70% yield strength or above has a significant effect on the localized corrosion propagation.

In this study, the inertia welding process was studied by both an FEM model and three analytical models. The thermal analysis shows that there is a steep temperature gradient near the mating surface, which is the cause for the existence of a band of high hydrostatic stress near the weld line. The holding effect of this high static stress is the reason for the presence of the very soft material at the welding interface. The models were used to predict the displacement of the weld line (upset) with a lambda model to describe the constitutive relation of IN718 at high temperature. The results from the different models are in broad agreement. The shear stress induced by friction at the interface is found to enlarge the upset value; its effect must be taken into account if the upset is to be predicted accurately. The extrusion of the burr during the last second of the welding is a direct result of the quick stop of the rotating part due to the balance of the momentum, which is clearly explained by the analytical mechanical model put forward in this work.

Titanium alloys with good combinations of toughness and strength are being designed for aerospace applications: e.g. aeroengine compressors and aircraft undercarriages. It is not an easy process to optimise the balance between toughness and strength in this system, and in practice some aspects of the physical metallurgy of titanium alloys are not very well understood, such as the B2-type ordering in titanium alloys. The aim of this work mainly concerns the development of thermodynamic models, based upon CALPHAD techniques (CALculation of PHAse Diagrams). First-principles calculations were performed in order to provide the thermodynamic modelling with information concerning sublattice occupation. The energies of formation of different point defects in the ordered B2 phase of the Ti-Al binary system have been predicted. The dominant point defects in the sublattice of the B2-TiAl structure were determined to be either substitutional vacancies or anti-site defects, depending on the major element in the alloy. The concentrations of total and thermal point defects were estimated in the present work. The results of first-principles calculations have been adopted in the CALPHAD thermodynamic modelling. Thermodynamic assessment of the Ti-Al-Cr-V quaternary system was carried out, neglecting substitutional vacancies. A two sublattice model for B2 phase (Al,Cr,Ti,V)0.5:(Al,Cr,Ti,V)0.5 was used and a set of self-consistent thermodynamic parameters is presented. The predicted phase equilibria and order/disorder transformation temperature are shown to be in good agreement with experimental information, both in the Ti-Al-Cr-V quaternary system and in the important binary and ternary subsystems. The modelling results were used to predict the B2-stable region, which is important for the design of titanium alloys. Next, substitutional vacancies were taken into account in a thermodynamic model of the B2 phase (Al,Ti,V,Va)0.5:(Al,Ti,V,Va)0.5 in the Ti-Al-V ternary system, taking into account the theoretical results of first-principles calculations. The temperature of the β transus in Ti-6Al-4V alloy has been predicted to aid simulation of microstructure evolution. Experiments were carried out in order to calibrate the thermodynamic models, and to investigate the effect of B2-ordering on the kinetics of α precipitation from the β matrix. By using the obtained thermodynamic parameters, two databases of atomic mobilities were developed – one with substitutional vacancies and the other without. The calculated results from both databases fit the experimental data: concentration profiles, diffusivities and diffusion paths. Finally, microstructural kinetics associated with the β/α phase transformation in the Ti-6Al-4V alloy has been simulated using the phase-field method coupled with the CALPHAD approach. The needle-shape α structure was simulated and the mechanism of formation of Widmanstätten α structure was studied.

In this study, ZnO, MgO, In2O3, SnO2, iron oxide nanowires and nanorods were successfully produced by using Pulsed Laser Deposition. It was observed that the parameters, such as gold nanodots, number of pulses on the targets, temperature, oxygen pressure and substrate, contribute to various results. Different metal catalyst or substrate results in different morphology. Controlling oxygen pressure and substrate temperature was found to be crucial for the ZnO, In2O3 and iron oxide nanowires growth using this technique. The morphology of ZnO nanowires is highly influenced by plasma plume. Increasing the shots on gold target would increase the width of MgO nanorods, but by increasing the number of gold pulses, the distribution of the sizes and dimensions of ZnO nanorods become smaller. However, in the case of In2O3, retaining the total number of gold shots between 5 shots and 25 shots is critical. The variation of the number of laser pulses on oxide target mainly affects the length of nanowires in the case of MgO. However, in regards to In2O3, it does affect both the width and length of the nanowires.

Rare earth metals are at highest supply risk of all metals for clean technologies due to supply restrictions. Rare earth elements are essential for the highest energy permanent magnets which are used in high-tech green applications such as wind turbines and electric motors. This rare earth 'crisis' has been addressed by many authors by reducing or substituting the rare earths for less critical elements. The focus of this thesis, however, is on recycling of rare earth magnetic materials. Previous work has been focused on recycling NdFeB by re-sintering and blending alloying elements to improve the final magnetic properties. However, none of these studies have addressed the systematic addition of alloying elements or the particle size effect on the final magnetic performance of the recycled magnet. The work in this thesis is aimed at maximising the magnetic performance of recycled NdFeB magnets by investigating the effect of the manufacturing route and the influence of adding alloying elements. Uncoated end-of-life sintered NdFeB were exposed to hydrogen at room temperature to break them down into a friable powder; which was then milled, sieved, aligned, pressed and sintered into recycled magnets. The resulting magnets were tested on a permeameter at room temperature, 75 and 150oc to measure the magnetic properties. Scanning electron microscopy was used to assess the microstructures. Mechanical properties such as Vickers hardness and fracture toughness were also studied.

A parametric investigation of the crack susceptibility of laser deposited nickel superalloy CM24 7LC was conducted, using Design of Experiments (DOE) methodology to correlate the laser deposition processing variables (laser power, scanning speed, deposit dilution, laser spot size) against the cracking response. Also investigated was the effect of tool path pattern, powder particle size and substrate preheating. Results showed that cracking occurs during solidification, due to the presence of a low melting point eutectic composition liquid that forms due to elemental segregation of alloying elements, which resides at grain boundaries and other solidification boundaries, at temperatures below the solidus creating a weakened area that separates under an applied tensile stress. Cracking was reduced, but not eliminated, by using a high heat input approach comprising of high laser power, slow scanning speed, large laser spot diameter and low dilution. Different toolpath patterns produced various microstructures, with the cross hatch toolpath producing the lowest cracking response due to the generation of competing stress fields that reduce the net tensile stress available to form cracks. Powder particle size range was also observed to influence the crack density, with finer powders increasing the number of cracks, but decreasing the average crack length.

The microsegregation of alloying elements (Mn, Si and Ni) in as-continuously cast steel slabs were characterised by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). The segregation profiles were obtained by cumulative profiles method using weighted interval rank sort (WJRS) and single element sort (SES) schemes. Analytical approaches (Ciyne-Kurz and Scheil), Thermo-Calc (thermodynamic) and DICTRA (considering the cross-effect of different elements) were used to predict the segregation behaviour of each steel slab. The segregation profiles predicted by DICTRA agree with experimental results better than those predicted by other models for each steel. But for slab I, root mean square deviation (RMSD) value used to determine the difference between DICTRA predicted and experimental segregation profiles was less than the experimental standard deviation. Directional solidification (OS) trials were carried out to verify DICTRA simulations in term of cooling rate. Although, DICTRA prediction using the measured cooling rate fell into the scatter band of WIRS profiles, discrepancy between the simulation and best fit to the profiles was found in the centre of both the dendritic and interdendritic regions. The discrepancy may indicate that the diffusions of Mn, Ni and Si in δ-ferrite used in DICTRA are faster than real situations.

A metal/peptide interface was developed to nucleate hydroxyapatite from aqueous solution. To achieve this, three different modifications were performed on mirror polished Ti-6Al-4V (MPT): 1) thermal oxidation (TO) at 500, 600, 700, 800 °C, 2) chemical treatment with H2O2/HCl and 3) chemical treatment with H2O2/HCl followed by TO at 500, 600, 700, 800 °C. It was found that with the increase of the TO temperature the topography increased and the composition of the surfaces changed, forming TiO2 crystals and increasing the Al2O3 content on the surface. These changes affected locally the surfaces charge behaviour in acidic and alkaline aqueous environments that were clue to the electrostatic interaction between the peptide, which presented positive and negative charged amino acids, and the surface. The peptide adsorption was highest on the surfaces that was chemically treated and thermal oxidised at 500 °C. The peptide coating was confirmed on the surface by the change of the charge behaviour in acidic and alkaline aqueous environments and its chemical composition. In addition, the growth of hydroxyapatite (HA) was obtained on the peptide coated surfaces confirming its bioactivity. Finally, it was shown that the surfaces with HA presented a better cell adhesion than the other modified surfaces.