Complex heat treatments and manufacturing processes such as welding involve a wide range of temperatures and temperature rates, affecting the microstructure of the material and its properties. In this work, a diffusion based approach to model growth and shrinkage of precipitates in the alpha + beta field of Ti-6Al-4V alloys is established. Experimental heat treatments were used to validate the numerical predictions of the model for lamellar shrinkage, whilst data from literature have been used to evaluate the numerical model for the growth of equiaxed microstructures. The agreement between measurements and numerical predictions was found to be very good. Experimentally-based approaches were used both to describe the growth of alpha lamellae and martensitic needles while cooling down from temperatures above the beta transus, and beta grain growth for temperatures remaining above the beta transus. Such models were coded in the commercial FE software Visual-Weld for the prediction of microstructure evolution during welding simulations. Experimental welding tests were carried out to validate the predictions. The metallurgical models developed were linked with a mechanical physically based model to predict the flow properties and the initial implementation of the coupled models in Visual-Weld is discussed.

Transparent conducting coatings are used in a wide range of applications, particularly displays and photovoltaic devices, and demand is anticipated to increase with the rise of plastic electronics. Indium tin oxide, deposited by sputter coating, is widely used. Concerns over the cost and long term availability of indium, together with environmental concerns over production, are driving a search for more efficient deposition methods and alternative materials. This work represented the early stage development of industrially applicable coatings, hence the work was performed with a supply chain partnership to facilitate commercial uptake. Sol-gel technology was used to facilitate direct printing of patterned layers. Precursors were selected and a processing route developed. Values for glass transition and crystallization temperature were found to be considerably lower than the accepted bulk value. Dip-coated parts yielded indium tin oxide coatings with transparency of 98% and resistivity of 1.5x10\( -\)\( 2\)\(\Omega\)cm fired in nitrogen and 98% transparency, 3.0x10\( -\)\( 2\)\(\Omega\)cm resistivity fired in air. A printed coating was produced with 93% transparency and 7.0\(\Omega\)cm resistivity. It was established that the limitations to conductivity arose from low film thickness and lack of macro-scale connectivity within the layer. Initial work was also conducted in preparing tin oxide and silicon-doped zinc oxide coatings.

The Bauschinger effects in X70, X80 and X100 high strength low alloy pipeline steels were presented. The microstructure of the as-received alloys was characterized. A variety of microstructures was present across the different alloys, ranging from a refined granular ferrite with small amounts of perlite to a bainitic structure containing martensite/austenite islands, retained austenite and small cementite constituents along with a small amount carbides. Similarly, the dislocation structures varied from homogeneously distributed across the ferrite grains to clusters/walls of dislocations. Mechanical tests on macro and micro sized samples were carried out up to 1% and 2% plastic strains. A micro-device for Bauschinger test was designed and manufactured using micro-electro-mechanical-system (MEMS) technology and was incorporated onto a FIB/SEM in order to prepare the micro sized samples and perform the micro Bauschinger tests. The Bauschinger stress parameter showed that the Bauschinger effect becomes more obvious in samples with higher yield strength and also with increasing pre-strain. In addition, the Bauschinger effect remained similar on the samples of different sizes in the current study. The results indicate that the cause of the early yielding during reverse loading of these alloys is dominated by the dislocation-dislocation interaction.

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.