The main aim of this work is to study the bonding behaviour of Al alloys as a function of temperature, time and processing conditions from a perspective of oxidation so as to gain a comprehensive knowledge of the oxidation issues during processing and to design better bonding approaches for different alloy systems. Two major parts of work have been carried out during the study: (a) the use of a stacking approach and a double pouring approach to potentially join two Al alloys and their effectiveness are assessed; and (b) a precise investigation of the oxidation mechanisms for Al-Cu and Al-Mg alloys using a combination of theoretical analysis and experimental characterization. The project started with the use of a stacking approach to try to bond two stacked Al alloys. The stacked sample can be viewed as a bi-metal which has an oxide bi-film layer at the bond interface. It was found that the bi-film layer was a physical barrier preventing direct metallic bonding. How this bi-film layer evolves during the bonding process was then investigated. The work then moved on to investigate the oxidation mechanisms of Al-Cu alloys and Al-Mg alloys in greater detail. The thermodynamics, kinetics of oxidation, chemistry and morphology of the oxide scale were particularly studied. Briefly speaking, in the case of the Al-Cu-O₂ system, the oxidation proceeds in the order of amorphous γ-Al₂O₃ - to - crystalline γ-Al₂O₃ - to - α-Al₂O₃; in the case of the Al-Mg-O₂ system, the oxidation proceeds as amorphous γ-Al₂O₃ to MgO to MgAl₂O₄ and the morphology of the oxide scale develops from a protective layer to a porously structured composite layer. A double pouring approach was finally developed to bond Al and Al-5Cu but the method has still not been perfected due to the formation of bi-film defects along the bond interface. Induction melting, squeeze casting, and extrusion bonding were therefore studied as an attempt to reduce the harmful effect of bi-film defects. Although all of the approaches exhibit some limitations, they have potential for future development.

The current thesis takes a step forward into understanding the intergranular stress corrosion cracking (IGSCC) by applying a relatively new micro-mechanical technique to look at the crack growth rate of individual grain boundaries in 304 stainless steel (SS) and to measure fracture toughness for different grain boundaries in Ni Alloy 600. In addition, a model is tested and proposed that could predict crack initiation in 304 SS. Pentagonal cross-section cantilevers 5 μm wide by 25 μm long were milled at individual grain boundaries in both 304 SS and Ni Alloy 600. The cantilevers milled in 304 SS were tested in-situ in a customised stage, using the nanoindenter. Crack growth rate was measured for two different cantilevers to be approximately 40 μm/s (K = 1.1 MPa(m)^(1/2)) and 120 μm/s (K = 1.7 MPa(m)^(1/2)). Cantilevers were milled in Ni Alloy 600 for three different samples: samples that were exposed to simulated pressurized water reactors (PWR) environment for 4500 h, for 1500 h and un-oxidised samples. The fracture toughness calculated for the fractured cantilevers in samples that were exposed for 4500 h was measured to be between 0.73 and 1.82 MPa(m)^(1/2). No intergranular fracture occurred in the samples that were exposed for 1500 h and in the un-oxidised samples. The grain boundary misorientation was measured for the tested cantilevers but no direct correlation was observed between the misorientation angle and the fracture toughness. A Schmid-modified grain boundary stress (SMGBS) model previously used to study the intergranular behaviour of irradiated 316L steel in supercritical water was applied to predict crack initiation in 304 stainless steel. The model was successfully applied and accurately predicted crack initiation. To extend the model, sensitisation was also included. In addition, different areas of the specimen, including the initiation site were analysed using High resolution electron backscatter diffraction (HR-EBSD) technique to measure the geometrically necessary dislocations (GNDs) density. It was observed that the boundary average GNDs is lower for the intact boundaries and higher for the cracked grain boundaries.

This work investigated cathodic disbonding of an unpigmented phenalkamine-cured epoxy coating on mild steel, EC, exposed to 3.5 wt.% NaCl solution. Scanning Acoustic Microscopy (SAM), Scanning Kelvin Probe (SKP), Electrochemical Impedance Spectroscopy (EIS) and optical microscopy have been combined to conduct this study. Several factors affecting the cathodic disbonding process: Film thickness, Cation mobility, Electrolyte concentration, Temperature, Paint composition, Polarisation and Open circuit potential, have been investigated. SAM results show that the disbonding of EC with a linear scribe spreads outwards from the defect with blisters forming at the anodes (as shown in SKP potential maps) within the disbond. The disbonded region does not correspond to complete adhesion loss as verified by peel-testing. Semi-immersion tests show that disbonding under full- and semi-immersion conditions have similar behaviours and both follow parabolic kinetics indicating the disbonding is likely to be controlled by a transport process along the coating/metal interface. An intact epoxy coated mild steel panel coupled with bare mild steel shows that the cathodic reaction beneath the coating obeys Tafel law. A mathematical model simulating cathodic disbonding which produces realistic potential files and shows the oxygen reduction is mostly located near the disbond mouth has been developed.

Adverse tissue reactions to hip prostheses containing CoCr alloys have been widely reported, particularly for implants utilising a metal-on-metal bearing surface or, more recently, a modular taper junction and have been termed Adverse Response to Metal Debris (ARMD). Histological assessments of synovial tissues from patients at revision operation often demonstrate an extensive accumulation of macrophages and abundant tissue necrosis or fibrosis. The inflammatory response starts with the recruitment of immune cells and requires the egress of macrophages from the inflamed site for resolution of the reaction. Metal particles have previously been shown to affect cell migration but the effects of cobalt and chromium on macrophages' motility remain largely unknown. In vitro and in vivo macrophage migration during exposure to cobalt and chromium ions and nanoparticles were examined in this thesis. Cobalt, but not chromium, was found to significantly reduced macrophage motility (>50%). This was found to involve an increase in both cell spreading and the formation of intracellular podosome-type adhesion structures, as well as enhanced cell adhesion to the extracellular matrix (ECM). The formation of podosomes was also associated with the production and activation of matrix metalloproteinase-9 (MMP9) and enhanced ECM degradation. These effects were driven by the down-regulation of RhoA signalling through the generation of reactive oxygen species (ROS). The effect of the Co2+ and Cr3+ metal ions on tissue remodelling and pseudotumour formation which can lead to pain, swelling, limited range of joint movement and extensive tissue lesions, was explored using a multiscale approach. Both 2D and 3D in vitro culture systems were deplored to examine the effects of these ions on human fibroblast activation and mechanobiology. It was observed that Co2+ induced a fibrotic response characterised by cytoskeletal remodelling and enhanced collagen matrix contraction. This was associated with an increase in cell stiffness (~45%) and contractile forces (~80%) measured by atomic force microscopy and traction force microscopy, respectively. These effects were also triggered by the generation of ROS. Moreover, this fibrotic response was enhanced in the presence of macrophages, which increased the prevalence of α-SMA positive fibroblasts and collagen synthesis. These events were verified in vivo by examining the synovial fibroblasts and tissues from hips of patients with metal-on-metal hip implants and patients undergoing primary hip replacement. The findings revealed that fibroblasts isolated from patients undergoing MoM revision THA were more biomechanically active than the control group. Moreover, synovial tissues from patients undergoing MoM revision THA displayed evidence of extensive tissue remodelling and fibrosis. These findings revealed that cobalt leads to adverse tissue reactions via inducing macrophage retention, fibroblast-mediated matrix remodelling and modulating the interplay between macrophage and fibroblast. These distinctive effects can help us understand the pathogenesis of ARMD and the cellular response to cobalt-based alloys, which will inform biocompatibility test protocols and future implant designs.

Tissue engineered heart valve (TEHV) is believed to be a promising candidate for curative heart valve replacements. Collagen, elastin and chondroitin-4-sulfate (C4S) comprise the extra-cellular matrix (ECM) of native heart valves and therefore are suitable materials for TEHV scaffolds. Freeze-drying technique was able to produce scaffolds with relative densities of 0.3%-2.0% and pore sizes of 33.2µm-201.5µm, without having any major effects on the ultra-structures on the scaffold materials. Subsequent dehydrothermal (DHT) treatment and ultra-violet (UV) irradiation introduced inter- or intra-molecular crosslinks in the scaffolds in forms of ester and amide bonds, as well as the accompanying denaturation of the proteins (i.e. ultra-structure transition from helices to random coils). The collagen-based scaffolds had tensile, compressive and effective bending moduli ranging from 39.8kPa to 1082kPa, from 2.4kPa to 213.9kPa, and from 11.0kPa to 415.8kPa, respectively. The different behaviours of the wall stretching and the wall buckling in the individual pores of the scaffolds contributed to the different tensile, compressive and bending moduli. The mechanical properties could be tailored through controlling the freezing temperature, the relative density and the composition of the scaffolds. A lower freezing temperature might lead to lower mechanical properties because different pore structures were introduced. When the the relative density of the scaffold increased, the values of the moduli increased exponentially, with an exponential dependence factor larger for the compressive modulus than for the tensile modulus. Adding elastin or C4S into the collagen scaffolds lowered the mechanical properties due to the decrease in the collagen content. Layered structures that combined collagen-rich layers with elastin-rich and/or C4S -rich layers allowed the scaffolds to make use of the different mechanical properties of different layers, and hence to show anisotropic bending behaviour depending on the loading directions. The lower effective bending modulus (9.6 to 25.0kPa) in the with curvature (WC) direction than that (18.1kPa to 39.3kPa) in the against curvature (AC) direction mimicked the characteristic behaviour of the native heart valves and would be beneficial for a mechanically desirable TEHV. The DHT treatment and UV irradiation were able to increase the mechanical properties of the scaffolds to up to 2.5 times of the original values, by reinforcing the scaffold materials with more crosslinks. In the hydrated status, the hydrophilic C4S improved the water uptake ability of the scaffold and the hydrophobic elastin reduced it. The hydrated layered scaffolds still exhibited bending anisotropy despite much lower effective bending modulus. Finite element models of the scaffolds produced results that were in agreement with the experiments, and enabled us to perform distributed loading and internal stress analysis on the scaffolds. The collagen-based scaffolds were seeded with cardiosphere-derived cells (CDCs), and they attached to the scaffolds and showed visible cell division, proliferation and migration. The CDCs exhibited preferred proliferation behaviours on the collagen-C4S scaffolds to that on the collagen-elastin scaffolds because of the cell affinity to the C4S, as well as the elastin-induced contractile cell phenotype and scaffold volume shrinkage. This difference seemed to be less evident in the layered scaffolds due to the cell communication between the layers. The crosslinking process also had effects on the cell proliferation in the ways that it induced ultra-structure changes or volume shrinkage in the scaffolds. The layered scaffold-cell constructs designed and produced in this study served as a forwarding step towards a mechanically desirable and biologically active TEHV.

Following the discovery of graphene, 2D nanostructures have been noted for their potential in a range of high-impact applications, such as sensing, catalysis, and composite reinforcement. Liquid-phase exfoliation and chemical vapour deposition have been demonstrated and indicate the feasibility of mass-scale production. With the advent of mass-produced 2D nanostructures a key focus of research is to characterise these materials. This thesis is concerned with imaging and structural properties of the 2D nanomaterials, hexagonal boron nitride (h-BN), molybdenum disulfide (MoS₂), tungsten disulfide (WS₂), titanium disulfide (TiS₂) and hexabenzocoronene (HBC), produced via liquid phase exfoliation. HBC strictly speaking is not 2D nanomaterial, however, it can be viewed as transition molecule from benzene to graphene. The data used for characterisation is based primarily on electron diffraction and, in particular, aberration corrected annular dark field (ADF) scanning transmission electron microscopy (STEM). The incoherent nature of ADF STEM provides direct atomic imaging without the contrast reversals upon focus changes seen in conventional high-resolution transmission electron microscopy (HRTEM). The main structural feature investigated in this thesis was the stacking sequences in few-layers h-BN, MoS₂, WS₂ and TiS₂. Simple stacking (AAA) can be distinguished from Bernal (ABA) and rhombohedral (ABC) on the basis of intensity ratio, I_{10̅10}/I_{11̅20} , in diffraction patterns and indirectly in HRTEM images. Nonetheless acquisition of the diffraction patterns suitable for analysis can be challenging due to the sample issues. Non-bulk stacking sequences were reliably confirmed for all above 2D nanomaterials on the basis of atomically resolved ADF STEM. 20 h-BN, 28 MoS₂, 5 WS₂ and 6 TiS₂ nanoflakes were imaged and analysed. Amongst them 2 h-BN, 9 MoS2, 4 WS2 and 1 TiS2 nanoflakes displayed non-bulk stacking. Hence, it appears that 2D WS2 has the greatest affinity for non-bulk stacking. Finally, an interesting structural transformation was observed in HBC molecules. Under the influence of electron beam HBC agglomerates were transformed into crystalline phase with 90^o symmetry.

The main aim of this thesis is to generate a deeper understanding of the mechanical behaviour of cellular materials, specifically for their use in aerospace applications. A closed-cell polymer foam material (Rohacell) of various foam densities was chosen for this investigation, and a comprehensive experimental study was conducted which generated significant findings that hitherto have not been reported in the literature. The research presented in this study revealed the following: The quasistatic response of Rohacell foam displays a compression/tension asymmetry in moduli and strength. In-situ experiments revealed that different macroscopic collapse mechanisms at different foam densities drove this behaviour. Improved experimental methods were developed to characterise the material response at various loading rates. Under compressive loading, as the relative density and loading rate increased, a transition in material behaviour from a ductile to brittle response at very high rates (~5x10^3 s^-1) was found, and tests conducted at different temperatures were used to validate and provide a better understanding of the causes for the observed rate dependency. The compression and tension properties of pre-crushed Rohacell foam loaded in different directions were measured, and with the use of three-point-bend tests it was shown that when the foams’ tension/compression asymmetry, or the changes in stiffness and strength due to pre-crushing (i.e. strain-induced anisotropy), are neglected, this leads to incorrect predictions of the foams’ structural response. Finally, a review of some existing Finite Element foam material models was conducted, and their ability to predict the foam response under complex loading was identified. The new data and understanding generated from this thesis will allow engineers and researchers, who are developing constitutive models for predicting the response of foam materials, specifically in aerospace applications, to account for more aspects of the mechanical behaviours in their Finite Element models.

Anionic chromate (VI) compounds are inhibitive pigments and have been effectively incorporated into organic coatings to protect metal surfaces from aggressive ions, but their risk as a human carcinogen and being harmful to the environment has led to the search of suitable alternatives. Aluminium alloy, AA2024-T3, is the substrate metal alloy used in the experiments and can be found in aircraft fuselage structures due to their high strength-to-weight ratio. However, the presence of intermetallic particles increases susceptibility to localised corrosion. To investigate the protection mechanisms of primers on light alloys, many different factors must be taken into account; from aluminium alloy corrosion processes, the effects of intermetallic additions to coating chemistry, morphology and inhibitive pigments. The chemical environment in which the samples are tested in will also affect the corrosion mechanisms of the alloy as well as the performance of the coatings and release of pigments. It will be important to consider which factors are operating under particular conditions so that experimental results can then be best interpreted. As part of this project, potentiodynamic polarisation, electrochemical impedance spectroscopy and electrochemical noise analysis have been used to investigate the protective mechanisms in which chromate-based paints protect against corrosion and UV-Visible spectroscopy, scanning acoustic microscopy and optical microscopy have been used to investigate pigment release mechanism to identify what characteristics are important when developing new primers.

The nickel-based superalloys have been a key enabler to the development of modern gas turbine engines. Since their introduction the chemical complexity of these alloys has increased significantly, with current generation nickel-based superalloys usually containing over 10 different elements. It is this combination of alloying additions that is responsible for the superior high temperature properties these alloys exhibit. Traditionally, alloy design has invoked considerable use of trial-and-error based approaches involving costly and exhaustive processing backed up by empirical property testing. In this work a computational materials design approach is developed. This method links physically-faithful composition-dependent models with thermodynamic calculations to understand material behaviour. By doing this it is possible to consider large compositional design spaces and isolate alloys expected to have optimal performance for specific applications. The scope of this research has been to apply the computational model to the design of a polycrystalline nickel-based superalloy for turbine disc applications in next generation jet engines. The design trade-offs encountered when developing the new alloy are highlighted. Alloy compositions which are predicted to be optimal for turbine disc applications are isolated. These alloys have been manufactured using a scaled down version of the commercial production method. The newly manufactured alloys have been characterised using microstructural evaluation, mechanical testing and corrosion testing. The experimental results have been compared with modelling predictions in order to determine the capability of the computational approach.

Corrosion at the head-neck junction of total hip replacements is a poorly understood phenomenon with an incidence of 1 - 2 %. Concerns around taper junction corrosion have focused on design factors including changes in taper surface topography and geometry as well as operating conditions such as high bearing surface friction and fluid ingress-egress at the taper junction. Hence, this thesis considered 3 aspects of the head taper junction namely: (1) frictional torque at the bearing surface and below the taper junction for varying head sizes and bearing material combinations, (2) Cobalt and Chromium ion release from CoCr/Ti taper junctions, (3) FE analysis of tapers utilising variables including taper length, material, angle, and clearance under loading conditions representative of walking, hip simulator profiles and stair climb. Bearing friction and the torque about the taper axis beneath the taper junction were positively correlated with the head size (R2 = 0.57 bearing friction, R2 = 0.88 torque) and average surface roughness (Ra) (R2 = 0.66 bearing friction, R2=0.79 torque) of the femoral head. Torque generated on large MoP bearings (0.93 ± 0.2 Nm) was found to be comparable to MoM (0.81 Nm). The median cumulative Cr release rate was at least 2 times greater than that of Co (0.0220 ppb/cycle Cr relative to 0.0109 ppb/cycle Co) due to the acidic environment utilised in the accelerated tests. No statistically significant difference in ion release was found, between the trunnions of different surface finishes. Finite element analyses showed that the largest gaps generated at the mouth of the taper, were associated with smaller taper contact areas. Clearances within ±0.1° enabled the tapers to engage over comparable lengths and therefore did not show differences in taper opening, showing this was influenced by the taper engagement length rather than location (proximal or distal) of contact. Stair climb loading generated the largest taper gaps (80 m) and surface stresses on the head taper (1200 MPa); these were greatest on the shortest trunnion. Although the stair climb loading condition is not currently mandated in testing THR devices, its use could provide a more accurate prediction of taper performance in vivo and may be beneficial to 'beyond compliance' initiatives to improve implant performance.