Magnesium alloys are promising candidates for biomedical applications because of their advantageous properties. However, a too high degradation rate can be a problem. It is not desirable for the material to degrade completely before tissue healing has taken place. In this study, the relationship between the microstructure and bio-corrosion behaviour of Mg-3Zn(-0.3Ca) were analysed. It was found that nano-scale precipitates formed during ageing of Mg-Zn decrease the corrosion resistance. The increase in bio-corrosion rate with ageing time appears to be monotonic. The parallel filament-like corrosion trenches mirror the growth direction of the precipitates. The grain size and volume fraction of second phases are both key factors for controlling the bio-corrosion rate of Mg-Zn-Ca. The minimum corrosion rate was observed in the Mg-Zn-Ca solution treated at 420 \(^o\)C for 24 hours which has a balanced second phase volume fraction and grain size. 3D FIB and MicroCT tomography were used to show the internal structure of as-cast Mg-Zn and Mg-Zn-Ca. 3D reconstructions of corroded as-cast MZ3 and MZX30 were also shown. An HA (Ca5(PO4)3\(^.\)(OH)) coating was formed on Mg alloys. It was found that the pitting corrosion on the HA-coated alloys occurs and HA coating cannot provide sufficient protection to the Mg substrate.

LiBH\(_4\) was added to a number of different non-reactive additives in order to investigate their influence on the hydrogen storage properties of LiBH\(_4\). Graphite is a cheap and abundant material that has been used to destabilize hydrogen storage materials such as LiH. Ball milling graphite under Ar was shown to induce a higher amorphous content compared to milling under H\(_2\). The addition of LiBH\(_4\) to graphite milled under Ar resulted in a reduction of 102°C in the decomposition temperature of LiBh\(_4\). The effect of porous additives was investigated through the incorporation of LiBH\(_4\) into zeolite templated carbon (ZTC) and porous silicon. Confinement in these scaffolds resulted in a decrease in the decomposition temperature of LiBH\(_4\) by 125°C. The smaller pore size of ZTC was found to have the greatest effect on the H\(_2\) onset and cyclic stability of LiBh\(_4\). The pre-melting of LiBh\(_4\) into porous scaffolds was shown to eliminate B\(_2\)H\(_6\) release during decomposition. No correlation between the surface area of the additives and decomposition temperature of LiBH\(_4\) was observed. Although none of the materials studied in this work meet the United States DoE targets, confinement of LiBH\(_4\) in porous structures offers a promising approach to unlocking its potential.

This thesis considers the fatigue and fracture behaviour of ultra-high strength steel, AerMet 100. Two variants of AerMet 100 were tested, which had different desulphurisation and deoxidation methods. The first was desulphurised and deoxidised using mischmetal, designated MMAerMet 100. The second was desulphurised using calcium and deoxidised using titanium, designated Ca-AerMet 100. Axial fatigue testing below the 0.2% proof stress was carried out between stress ranges of 900-1700 MPa. Fractography revealed that the fatigue initiation sites could be traced to non-metallic inclusions. Samples tested in the longitudinal orientation had better fatigue lives compared to samples tested in the transverse orientation. A stress intensity factor range at the cracked inclusion at the point of initial fatigue crack growth showed a good correlation with fatigue life. Ca-AerMet 100 consistently had a better fatigue life compared to MM-AerMet 100. Fracture toughness testing showed that both variants of AerMet 100 had similar fracture toughness values and fractured by micro-void coalescence. Fractography revealed that 90% of the fracture surface was made from voids formed from secondary particles with approximate diameters of 90 nm. The work suggests that changing the desulphurisation and deoxidation method for AerMet 100 has not significantly affected the fracture toughness.

Aluminium alloy castings are being used increasingly in safety-critical applications in the automotive and aerospace industries. To produce castings of sufficient quality, it is, therefore, important to understand the mechanisms of the formation of defects in aluminium melts, and important to have a reliable and simple means of detection. During the production of aluminium ingots and castings, the surface oxide on the liquid is folded in to produce crack-like defects (bifilms) that are extremely thin, but can be extremely extensive, and so constitute seriously detrimental defects. However, the presence of bifilms has not been widely accepted, because there has been no single metal quality test that has been able to resolve features that are only nanometres, or sometimes micrometres, in thickness. In the past, porosity has usually been held solely responsible for most failures in aluminium alloys, and hydrogen has been blamed as the actual cause. In this work it has been found that bifilms are the initiator and hydrogen is only a contributor in the porosity formation process. For the first time, evidence is presented for the contribution of air (or perhaps more strictly, residual nitrogen from air) as an additional gas, adding to hydrogen in pores in cast Al alloys. The Reduced Pressure Test (RPT) is used which is a simple and widely known test, is cost effective and involves no complicated equipment or consumables, thus recommending it for implementation on the foundry floor. On this basis, several Al-Si based alloys were studied. A quality index -Bifilm Index- is introduced to quantify the results of the reduced pressure test. In addition, mechanical tests were carried out to correlate bifilm index with mechanical properties.

The aims of this study were to understand and improve the mechanical properties of aluminium Al-Cu-Si-Mg P/M alloy Alumix 123 by application of a warm compaction process. They were achieved by investigating the effect of (a) compaction pressure/temperature, (b) admixed lubricants (e.g. Acrawax C and Kenolube P11) on green/sintered density and mechanical properties of compacts. It was found that compaction at 110ºC led to (1) a reduction in the ejection force up to ~ 40 % and (2) an increase in sintered density up to ~ 98 % of theoretical density. After heat treatment at 200ºC for 5 hours (T6), the tensile strength reached ~ 365 MPa and hardness of ~ 126 HV was achieved. Further improvement of mechanical properties of aluminium alloy Alumix 123 can be achieved by reducing the amount of admixed lubricant to 0.5 wt %, and using Acrawax C as the lubricant rather than Kenolube P11. Taguchi analysis was used to identify which parameter (compaction pressure, temperature and lubricant content) affected the densities and mechanical properties of Alumix 123 specimens with single lubricants the most. It was observed that the most effective parameter in warm compaction of Alumix 123 specimens is the lubricant content.

This project investigated the possibility of tailoring the crystallinity of electrospun fibres (crystallinity studies), and the electrospinning of bi- and multicomponent scaffolds of PLLA. During the crystallinity studies, the effects of various electrospinning process parameters on the crystallinity of electrospun poly(L-lactic acid) (PLLA) fibres were investigated. It was observed that the electrospun fibres had crystallinities between 23 and 46% while that for the as-received granule was 37%, suggesting that the crystallinity of electrospun fibres can be controlled by optimizing the electrospinning process. These results showed that the degree of crystallinity of the electrospun fibres decreased with increasing the polymer solution concentration. Furthermore, an optimum electrospinning voltage at which the maximum degree of crystallinity can be obtained was observed. Finally, bi-component scaffolds based on PLLA and gelatin were electrospun. Multicomponent scaffolds based on PLLA, gelatin and hydroxyapatite (HA) were electrospun followed by electrospraying of the HA phase. Blending gelatin with PLLA resulted in an approximate 50% decrease in fibre diameter. Biocompatibility studies revealed that all scaffolds permitted cell attachment with best results observed on the PLLAGel-HA scaffolds. This was attributed to the exposed HA particles on the surface of the PLLAGel-HA scaffolds which promoted better binding with integrins for osteoconductivity.

Diffusion mechanisms govern a wide range of phenomena in condensed matter including high-temperature deformation. The good influence of slow diffusers such as Si on the creep properties of α-Ti alloys is well documented, as well as the detrimental effect of fast-diffusers such as Fe, Co and Ni. The life-performance of α-Ti alloys at high temperature is also limited by light elements (O, C and N) that promote the fragile α-case phase. The study of diffusion mechanism is experimentally not trivial, since the anisotropy hcp structure of α-Ti requiring single crystal sample. The first-principles approach together with analytical models and Kinetic Monte Carlo simulations can predict the diffusivity values giving additional information on mechanism itself. This work presents the ab initio study of vacancy-mediated diffusion of substitutional atoms as Si, Al, Ga, Ge, In and Sn, interstitial migration of light elements, and anomalous behavior of fast-diffusers. The findings show that the substitutional diffusion is affected by the bonding characteristic. The interstitial sites through which light elements dissolve and diffuse were updated, and they can explain the anisotropy behaviour of these solutes. The results confirm that the anomalous behaviour of fast-diffusers is a results of their ability to dissolve interstitially and substitutionally.

The ageing process of two magnesium alloys with compositions of Mg-1.75Sn-1.93Al and Mg-1.29Sn-2.85Al(at%) have been investigated. Three ageing times, 40, 72 and 160 hrs, were selected to correspond to the early growth, peak hardness and coarsened stage of the alloys and the hardness measured. Subsequently, the precipitates in the alloys have been classified by 4 morphologies, basal plates, 〈112 ̅0〉 laths, pyramidal laths and prismatic rods. and identified as Mg2Sn before the number density and size of the precipitates was measured. This has been achieved through a combination of X-ray Diffraction, Scanning Electron Microscopy and Transmission Electron Microscopy. Moreover, an Orientation Relationship of (0001)α//(111)β,[112 ̅0]α//[110]β was found to be common to three of the four morphologies with the pyramidal laths not conforming to a well-defined orientation relationship. This is then linked through observation to the lack of a common habit plane or growth direction for these pyramidal precipitates. The obtained data has then been used to model the nucleation and growth of the precipitates using a Kampmann-Wagner Numerical framework, where key parameters such as the diffusion rate of Sn and the interfacial energy of the precipitates were fitted. This leads into a simulated strength against the ageing time which has been compared to the initial hardness measurements made. This has good agreement with not only the overall number density and precipitate sizes but also the size distribution of the precipitates The effect of the two microstructures on the differing mechanical properties has been discussed, highlighting the importance of number density over the different precipitate morphologies observed, and future experiments outlined.

Wrought Al-Mn alloys can develop a thin deformed layer on the surface as a result of hot and cold rolling. Subsequent heat-treatment precipitates fine secondary intermetallic particles which effect corrosion susceptibility. This work focuses on the effect of surface preparation and deformation on the electrochemical behaviour of Al-Mn alloys. The first part of the work investigated the effect of surface preparation such as mechanical grinding and polishing, alkaline etching and desmutting, and nitric acid treatment on electrochemical behaviour of an Al-1Mn-0.4Fe-0.3Si model alloy. Different surface preparations of this alloy show different electrochemical behaviour. In the second part of the work, the electrochemical reactivity of the surface layers of commercial rolled AA3005 sheet was investigated by profiling through the surface with GDOES (glow discharge optical emission spectroscopy). The microstructure and electrochemical reactivity was examined at different depths in order to compare the behaviour of the surface layers with that of the bulk alloy. In order to understand the role of deformation on corrosion behaviour of Al-Mn alloy, an Al-1Mn-0.4Fe-0.3Si model alloy was deformed by uniaxial compression and equal channel angular extrusion (ECAE) and followed by annealing. It was found that deformation is likely to have two effects on the surface of Al-Mn alloy. One effect is to cause the precipitation of particles that act as local cathodes and pit initiation sites. The other effect is that formation of precipitates will deplete the adjacent matrix in solute, making it more susceptible to dissolution.

S-phase can be created in austenitic stainless steels by low-temperature thermochemical treatments, which greatly enhanced their hardness, wear resistance and fatigue properties because of the supersaturation by interstitials. One of the technological challenges associated with S-phase surface engineering is that the maximum layer thickness of the S-phase layers is very thin. The thickness of S-phase is restricted by its metastability and precipitation will occur as a result of prolonged treatment. In this project, the effect of in situ tensile stress on the formation of carbon S-phase on 316L austenitic stainless steel was investigated and it was demonstrated that the tensile stress thickened the S-phase layer by promoting the carbon diffusion in austenitic substrate. However, metastable carbides precipitated when applied tensile stress exceeded 40MPa. The thermo-mechanical stability of carbon S-phase was studied by creeping (tensile stress) and HIPping (compressive stress) tests. The results showed that the compressive stress retard the decomposition of S-phase by impeding the carbon diffusion; on the other hand, tensile stress promoted the carbon diffusion. The residual compressive and shear stresses in carbon S-phase was measured be 2.2 GPa and 132 MPa. The wear behaviour of carbon S-phase was studied by dry and oil lubricated reciprocating wear.