Atmospheric corrosion of aluminium alloy AA2024 was investigated using in situ synchrotron micro-tomography, which allows visualisation in a non-destructive manner in real time. The effect of atmospheric variables such as salt type, humidity, exposure time and salt deposition density on the corrosion rate was investigated. It was found that corrosion fissures grow along grain boundaries parallel to the rolling direction of the alloy, reaching a limiting depth, and then spread laterally. The volume of corrosion increases with salt density and relative humidity. Salt type has a limited effect on the volume of corrosion in microtomography measurements where the droplet is constrained at the top of a pin, but in parallel lab-based experiments on plate surfaces, it was found that NaCl and simulated ocean water droplets spread laterally, leading to increased corrosion owing to an increase cathodic area, whereas pure MgCh and CaCh droplets do not spread. Preliminary microtomography work on cycling the relative humidity showed transient increases in localised corrosion during wetting and drying phases, often associated with rapid growth of pmt of a localised cmTosion site, or initiation of new sites.

This document is a compilation of my selected research publications in micro and nano fabrications. The papers are largely arranged in chronological order to show the development of research interests. The research works are grouped into three sections. Section one consists of 34 research papers on micro fabrication in various materials. The research was motivated by the development of a finger nail sized micro engine as explained in Papers 1 and 2. Section two of the document includes some research activities and achievements on nanocomposite materials embedded in metallic and ceramic matrices. Section 3 includes the papers to reflect the research in developing nanostructure fabrication processes. The research contained in this DSc submission shows a continuous exploration and development of novel micro/nano fabrication processes. Although the submission covers research activities spanning 15 years, from 2000 to 2015, many of the research results represent the top technology of the time. They have contributed to the ever progressing manufacturing capability of the world. The research has encompassed both theoretical and experimental studies, contributing to the understanding of the processes and materials involved.

This thesis describes computational studies, syntheses and characterization of Cu-Au, Pd-Au, and Pt-Au bimetallic nanoparticles. The computational methodology is a combination of a genetic algorithm coupled with an empirical potential and density functional theory, which is used to study theoretically the geometrical structure, chemical configuration, and electronic properties of 38 atom Cu-Au and 40-atom Pt-Au nanoparticles. Experimental Cu Au and Pt-Au nanoparticles are synthesized by wet-chemical methods: the two-phase method combined with a galvanic exchange procedure (dodecanethiol-stabilized Cu-Au nanoparticles), the polyol method (Polyvinylpyrrolidone stabilized Pt-Au nanoparticles), and the Turkevich-Frens method combined with a successive reduction procedure (citrate-stabilized Pt-Au nanoparticles). The Pd-Au nanoparticles which are characterized in our work were synthesized by a microbial method. The nanoparticles are characterized using transmission electron microscopy (TEM) and scanning TEM (STEM) combined with high angle annular dark field (HAADF) imaging, energy dispersive X-ray elemental mapping using a silicon drift detector (SDD), tomography, and electron energy loss spectroscopy (EELS). Sizes and shapes of the Cu-Au, Pd-Au, and Pt-Au nanoparticles are studied by TEM. Morphological evolution and aggregation of the Cu-Au nanoparticles are also observed under the TEM electron beam. SDD-EDX elemental mapping combined with HAADF contrast is used to study the chemical configuration of all the three systems. HAADF-STEM tomography is performed for the Pd-Au nanoparticles. Surface plasmon resonances of the Cu-Au and Pd-Au nanoparticles are studied using EELS. The structures and configurations of the theoretical bimetallic clusters and the experimental bimetallic nanoparticles are found to be composition-dependent.

The development of the alternating current field measurement (ACFM) technique for high-speed inspection of railway wheels/track is important to ensure the integrity of railway assets. The stress and conditions at the wheel/rail contact patch, severity of rolling contact fatigue (RCF) damage and changes in microstructure and hardness with tread depth for railway wheels are investigated. This study employs experimental measurement of RCF defects in railway wheels to understand ACFM sensor signal - RCF defect relationships. The influence of sensor frequency and speed, sensor angle relative to crack angle, lift-off distance, crack propagation angle and inter-crack spacing distance are investigated. Low rather than high frequency sensors are better suited at sizing cracks in railway wheels and track. The optimum signal is obtained when the sensor is oriented parallel to the crack angle. The signal is reduced at higher speeds. The maximum change in the normalised Bx signal component of the sensor’s magnetic field is greater for closely spaced cracks (< 5 mm) and thus overestimates the defect. Inter-crack spacing distances of 15-20 mm yield estimates for crack pocket depths that are oversized by 20-36%. The inner cracks in a cluster with four defects yields higher Bₓ values than expected and thus significantly over sizes the defects at 15-20 mm inter-crack spacing distances, whereas, the sensor provides reasonable depth estimates for the outer cracks. Crack propagation angle affects the signal. Scans conducted with the probe oriented at 90° to the cluster of cracks consisting of a deep central crack surrounded by shallow cracks results in a distinct central sensor Bₓ signal that consists of a peak due to a flux leakage effect, thus, identifying the critical defect.

Microporous materials (with pores of less than 2 nm in diameter) have attracted considerable attention due to the variety of applications such as gas storage. Physisorption of molecular hydrogen offers several advantages over chemical absorption, namely, fast kinetics and complete reversibility. The overall aim of this work was to investigate the potential of microporous materials for hydrogen storage, with particular attention given to a relatively new class of material: Polymers of Intrinsic Microporosity (PIMs). Generally the PIMs were seen to adhere to Chahine’s rule, which predicts a linear correlation of hydrogen adsorption capacity, at 77 K, with surface area (1 wt.% per 500 m\(^2\) g\(^{-1}\)). IRMOF 1 and Cu-BTC exhibited the largest gravimetric storage capacities of 4.86 and 4.50 wt.% at 77 K and 15 bar, respectively. The largest for a microporous polymer was 3.26 wt.% at 77 K and 15 bar, for the methyl triptycene-based PIM. Two empirical equations, Sips and Tóth, were used in addition to a multi-parameter Virial type thermal equation to fit hydrogen adsorption curves over a range of temperatures (77 to 137 K), in order to calculate the enthalpy of adsorption for all materials as a function of hydrogen adsorption. The findings in this investigation suggest that there is a trade-off between gas sorption capacity and enthalpy of adsorption where dispersive van der Waals interactions dominate adsorption. It is unlikely that the optimal enthalpy of adsorption (of ca. 20 kJ mol\(^{-1}\)) will be achieved by simply reducing the pore size of the material.

Implanted titanium (Ti) devices such as dental implants have been shown to produce metallic species within adjacent tissues. The effect of the presence of these species within oral epithelial tissues is currently not well characterised or known. This thesis investigates the effects of TiO\(_2\) nanoparticles (TiO\(_2\) NPs) at a range of concentrations on oral epithelial cells in the context of cell viability, cellular functions and interactions via a variety of methods. A co-culture model was established, and the difficulties of using a nano-scale insoluble stimulus were explored, and high content screening techniques were shown to be potentially more appropriate methods than conventional assays in this context. Interactions between TiO2 NPs and oral epithelial cells were imaged and investigated using a variety of imaging techniques. Oral epithelial cells were shown to take up TiO\(_2\) NPs within vacuole type structures. Cell viability appeared to not be affected at lower concentrations. Gene expression changes of oral epithelial cells in response to TiO\(_2\) NPs in the presence and absence of pathogenic bacteria were investigated. Cytokines important in cell-cell signalling were shown to bind TiO\(_2\) NPs, therefore creating potential for TiO\(_2\) NPs within tissues to modify immune responses within tissues adjacent to implanted Ti devices.

The objective of this research was to study the difference in microstructure and high temperature fatigue behaviour between three RR1000 to IN718 inertia friction welds and to provide further understanding of the inertia friction welding process. Between the three welds no significant differences in the weld microstructures were found. Also high temperature fatigue crack growth tests within 0.3 mm of the weld interface, showed no difference in crack growth rate due to the three different sets of welding parameters. The cracks were found to propagate from RR1000 through the weld interface into IN718 passing a 10-30 mm wide zone, allowing higher crack growth rates. Fractographic studies have shown that these higher crack growth rates are caused by a higher tendency to intergranular cracking. In the present welds a semi-solid weld contact layer of 10 - 30 mm thickness developed, being an intermediate alloy of varying composition of the two base materials. In the surrounding material constitutional liquation of NbC particles in IN718 and of primary γ′ in RR1000 occurred. Similar welding process characteristics during the final second of the three welding cycles resulted in the observed similar weld microstructure and high temperature fatigue properties. It was further found that the local fatigue crack growth rate increase occurs in the weld contact layer.

In this work an investigation into the effect of alloying additions (Mn, Cu, Sb and Zr) on microstructure and corrosion of Zn alloys and hot dip galvanised coatings was undertaken. The first part of this thesis focuses on the effect of alloying additions on the corrosion of Zn alloys. The result shows that Mn is the most beneficial addition, which can significantly improve the resistance of Zn. The effect of Cu depends on its concentration. A high level of Cu addition has a deleterious effect on the corrosion resistance as the Cu-rich particles are catalytic cathodic sites for oxygen reduction. Additions of Zr and Sb were found to have minor effect on the corrosion behaviour of Zn alloys. The effect of these additions on the microstructure of hot dip galvanised coatings was investigated in the second part. Both Zr and Mn can inhibit the layer growth of active steels with high Si content. Thus, Mn and Zr might be an alternative addition to Ni which can control the excessive reaction of the active steels. Addition of 0.8 wt % Cu significantly increases the coating thickness of the galvanised steel containing 0.02 wt % Si. The growth kinetics of the alloy layers follows a linear law. The final part of this thesis focuses on the effect of these additions on the atmospheric corrosion resistance and electrochemistry of hot dip galvanised coatings. Among the alloying additions investigated in this study, Mn is the most beneficial addition to the Zn bath and can significantly improve the resistance of the hot dip galvanised coating to atmospheric corrosion. The effect of other additions on corrosion resistance is minor. The beneficial effect of Mn addition is mainly due to the formation of a Mn-rich oxide layer on the top surface during the galvanising process, which can greatly inhibit the cathodic reactivity of the hot dip galvanised coating. Coupled with the relatively low cost and ease of alloying of this element, these various factors suggest that Mn might have broader applications in general galvanising.

Atmospheric pitting corrosion of stainless steel was studied to determine pit growth mechanisms and kinetics. Inkjet printing of chloride was used to investigate the growth of atmospheric corrosion pits. It has been shown that the pit size increases with increasing chloride deposition density, as well as increases with increasing deposit diameter. Atmospheric pit growth was characterised in situ and in real time with synchrotron X-ray microtomography. Most pits were found to have open mouths and shallow depths. Growth of a deep pit was also observed in a pre-existing defect. Pit growth in depth does not appear to be under diffusion control. Electrochemical kinetics of the metal dissolution reaction including the Tafel slope as well as the critical metal ion concentration for pit propagation were studied with lD artificial pits in high chloride concentration solutions relevant to atmospheric conditions. The diffusion-limited current density and ratio of the critical metal ion concentration for pit propagation to the saturation concentration were found to decrease with increasing chloride concentration. However, there is no significant effect of the chloride concentration on the Tafel slope. The pitting potential and repassivation potential were measured on abraded wires and were found to decrease with increasing chloride concentration. Salt layer formation has been observed on ID artificial pits in 1 M and 6 M solutions with synchrotron XRD. The dominant phase of the salt layer was found to be FeCl\(_2\)•4H\(_2\)0. The formation of FeCl\(_2\)•2H\(_2\)0 was also observed, predominantly at higher applied potentials. This work can be used to provide a basis for developing a model to predict pitting corrosion of stainless steel under atmospheric corrosion conditions, for example in the case of storage of stainless steel intermediate nuclear waste containers.

The aim of this work is to establish the influence of the many process variables on the microstructure and the nature of internal stress in IN718 samples produced directly from powder using direct laser fabrication, which enables production of solid samples directly from a CAD file. The process variables that have been studied include, specimen geometry, laser power, laser traverse speed, the detailed laser path and powder feed rate. It has been found that the detailed microstructure is strongly influenced by all of these variables with the propensity for the production of equiaxed or columnar grains being strongly influenced by laser power. The texture is correspondingly strongly influenced by changes in processing conditions. The extent of precipitation of the various phases expected in IN718 was also found to be influenced by the process conditions. The level and nature of the residual stress in the sample and in the substrate have been determined for a wide range of experimental conditions and using neutron diffraction. It has been found that the level of these stresses could be reduced to a minimum value of about 300 MPa, but could not be eliminated. A simple 3D thermo-mechanical model was developed to understand the residual stress distribution, which agreed closely with the experimental measurements.