The permeability of hydrogen in thin film Pd-Y composite membranes has been investigated for the first time. Thin (5 µm) Pd-Y films were deposited onto macroporous 316L stainless steel substrates by magnetron sputtering. Prior to deposition, a novel laser melting technique was employed in an attempt to reduce the surface roughness and pore size, thus facilitating the deposition of defect-free films. The active hydrogen transport mechanisms of each layer have been determined and the observed hydrogen permeability was analysed using a series resistance type model. The porous substrate was found to contribute significantly to the total transport resistance, and was the predominant source of the non diffusion limited permeation. After correcting for the substrate effects, the permeability of thin film Pd-Y closely matched that in bulk Pd-Y alloys.

Atmospheric localised corrosion of stainless steel has been investigated under salt droplets containing a mixture of MgCl2 and NaCl between the deliquescence relative humidity of the two salts where there was precipitation of NaCl crystals. Dish-shaped pits and crevice-like attack could be observed. Effects of the change of relative humidity (RH) have been studied. A pit that has grown at 33% RH for 1 day will tend to repassivate when the RH is increased to 85% while pits grown at 33% RH for 3 weeks may not repassivate at 85% RH and can continue to grow when the RH is returned to 33%. A pit that has grown at 33% RH can continue to grow after 1 day at 12% RH if the RH is returned to 33%. RH fluctuations, either to low or high RH, lead to greater number of pits. Using the 1D artificial pit method, a potential sweep method was developed to investigate the repassivation process of pits in concentrated solutions, representative of atmospheric conditions. The onset of repassivation is defined as the point where there is no increase in current with an increase in potential. It has been found that repassivation takes place when the concentration of MgCl2 is lower than 3.2 M (equivalent to 64% RH) while there is no repassivation for 3.5 M and 4 M MgCl2 (equivalent to 59% RH and 50% RH respectively). A range of the repassivation potential and the critical metal ion concentration to prevent pits from repassivation was determined.

This work has investigated structural and compositional changes in LiBH\(_4\), Mg(BH\(_4\))\(_2\), Ca(BH\(_4\))\(_2\), LiBH\(_4\)-Ca(BH\(_4\))\(_2\), MgH\(_2\)-B-TiX (TiX = Ti, TiH\(_2\) or TiCl\(_3\)), and hydrided Li-Mg alloy during heating. The crystal and vibrational structures of these borohydrides/composites were characterized using lab-based X-ray diffraction (XRD) and Raman spectroscopy, with particular attention to the frequency/width changes of Raman vibrations of different polymorphs of borohydrides. The thermal stability and decomposition pathway of the borohydrides was studied mainly using differential scanning calorimetry and thermogravimetric analysis, XRD and Raman measurements, whilst the gaseous products during heating were monitored using a mass spectrometry. Hydrogen is the main decomposition gaseous product from all of these compounds, but in some cases a very small amount of diborane release was also detected. These studies suggest that the thermal decomposition of the metal borohydrides occurs via a wide range of reaction pathways, often in several steps, which may involve simultaneous competing reactions. This can include the formation of stable borane intermediates/by-products which largely preclude the possibility of reversibility. Furthermore, the role of diborane in the decomposition and formation of borohydrides, was later studied by heating metal borohydrides (or hydrides) to various temperatures in a gaseous diborane-hydrogen atmosphere; and different types of borane products were observed.

Fast scanning chip-calorimetry was used to explore the crystallisation and melting of three semi-crystalline polymers. The heating and cooling rates required to prevent crystallisation on cooling (from above Tm) and on re-heating (from below T\(_g\)) were determined as: 3,000 and 8,000˚C/s respectively in PCL, 75 and 250˚C/s respectively in PEEK and 10 and 100˚C/s respectively in PLA. The effect of the thermal lag was considered using indium as a standard and corrections of >5˚C were required at rates in excess of 5,000˚C/s. As readily observed in conventional DSC (CDSC), PEEK exhibited a double melting endotherm and this was attributed to a melting-recrystallisation-melting process. The absence of recrystallisation above 250˚C allowed a Hoffman-Weeks analysis to be carried out over a broader temperature range than is general possible in CDSC. The interplay between thermal lag and re-crystallisation was analysed using heating rates covering 5 orders of magnitude. At an optimum heating rate of 1,500 ˚C/s, an equilibrium melting temperature of 359˚C was determined. The consideration of thermal lag led to the measurement of diffusivity using the technique of laser flash apparatus (LFA). The high measurement speed in the LFA allowed a time and temperature resolved study of diffusivity in PLA. LFA, chip-calorimetry and CDSC were used in parallel to explore the cold-crystallisation kinetics of PLA and the development of the relatively unstable α’ crystals. A good correlation between chip-calorimetry and LFA was found, showing an Avrami exponent of 2 and nucleation constant of 6.58 x10\(^5\) and 6.87 x10\(^5\) respectively, corresponding to regime III.

The measurement of properties is very important for both material design and quality control. As materials’ properties are determined by the microstructure of the materials such as grain size or the volume fraction of the present phases, microstructural characterisation is a powerful tool for property prediction. Unfortunately, microstructural characterisation has not been widely applied with all steels such as pearlitic steels or complex multi-phase steels due to their complex microstructures. These microstructures may contain features that cannot be resolved by optical microscopy, and in which important information is contained in their texture rather than simply their grey level. These microstructures were investigated in this study using image texture analysis. Fourier transform-based analysis was applied to pearlitic microstructures to extract the image orientation information. The orientation information as well as the grey value of low pass filtered image was used as predicates in a split-merge algorithm to segment the pearlitic colonies. A supervised classification method based on various statistical measures including a number of 2-point statistics (Grey Level Co-occurrence Matrix measures) was developed to distinguish the bainite (upper bainite and lower bainite), martensite and ferrite phases in steels. The influence of etching on the analysis results was also investigated.

AA5083-H321 is an aluminum alloy commonly used in ship hull superstructures as it has a corrosion resistance which affords an excellent degree of protection in chloride-rich marine environments. Corrosion performance can degrade in a process called sensitisation, due to the precipitation of a β-phase onto grain boundaries with exposure to elevated temperatures over decades of service. Friction Stir Processing (FSP) has been evaluated as a method for locally reversing the degraded microstructure and removing a susceptibility to Intergranular Corrosion (IGC) in immersed and atmospheric conditions. Both the mechanical stirring and heat input to the plate by an FSP tool have been shown to remove the β-phase from grain boundaries which had been precipitated by a sensitisation heat treatment. Sensitisation was shown to cause intergranular corrosion of the alloy; however this susceptibility was removed after microstructural modification by FSP. A re-sensitisation treatment of the previously sensitised and FSP’d region was seen to precipitate coarser and more discrete β-phase particles onto grain boundaries, which corroded at a faster rate than the once-sensitised material under electrochemical testing. This indicates that corrosion resistance degrades more quickly with subsequent sensitisation.

In the pressure infiltration processing of Metal Matrix Composites (MMCs), molten metal is injected into a porous preform. This research investigated ways to optimize the processing and properties of MMCs with Al alloy matrices. A ceramic volume fraction of 0.30 to 0.40 was used to keep a preponderant metallic behaviour and the reactivity of MgO, TiO2, Al2O3-SiO2 and Al2O3-TiO2 preforms was compared to pure Al2O3. Two stages were found during infiltration: first, flow initiation characterised by the dynamic wetting angle ?dyn and, second, the advancing flow in the preform capillaries. Reactions were detected in the MMC but did not significantly influence the ?dyn. Unsaturated fluid flow was evaluated for the subsequent infiltration stage and good agreement was found between a numerical model and experimental data. The MMCs had improved properties compared to the pure alloy. Elastic moduli up to 148 GPa, bending strengths up to 456 MPa as well as reduced wear rate were found. The wear performance of extended ceramic ligaments, found in MMCs with spherical metal ligaments, exceeded all other materials. The improved understanding of the infiltration of preforms and the resulting MMC properties obtained in this research should lead to the development of tailorable composite materials.

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.