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.

Entrainment defects such as double oxide films and entrapped bubbles occur frequently in aluminium alloy castings during the mould-filling process, and are very detrimental to both mechanical properties and reproducibility of casting properties. However, the behaviour of entrainment defects in the liquid metal and their evolution during the casting process are still unclear, and the distribution of these defects in casting remains difficult to predict. An algorithm, named the oxide film entrainment model (OFEM), that has the potential to predict the formation and distribution of entrainment defects in castings was studied and tried to validate using A356 alloy. The initial validation of OFEM used previous reported experimental data on mechanical properties of castings. Then modelling of three common entrainment mechanisms in fluid flow were conducted, and the predicted defects quantities in the samples were compared with the bending strengths of the castings. A further validation compared the tensile strength of the cast test bars with either the predicted number of defects, or the defect concentration within the bars obtained from the simulation. A general relationship between the mechanical strength of the cast test bars and the quantity of estimated defects was apparent. This research also assessed the behaviour of entrainment defects in the liquid state and during solidification using ultra-fast synchrotron X-ray radiography. The reconstructed images showed the 3D morphologies of entrainment defects and revealed the morphological evolution of the defects. Currently, the OFEM algorithm has not yet been fully validated. However, modelling work highlighted the potential of the method as an indicator for the entrainment severity in different mould designs. The effects of different modelling conditions on the modelling results were discussed, and some useful courses were suggested to achieve higher accuracy of model.

in vivo degradation of Ti implants has remained as a concern despite its perceived excellent corrosion resistance. Elevated levels of metal have been detected both locally and systemically, often leading to unfavourable biological responses. Corrosion of Ti, Zr and their alloys (Ti6Al4V and TiZr) were studied in simulated physiological solutions. It was found that albumin, the most prevalent tissue fluid protein, induced a time dependent dissolution of Ti6Al4V in the presence of H2O2, an inflammatory biomolecule commonly found in peri implant sites, in 0.9% NaCl. However, the corrosion of Zr was observed to be unaffected by the presence of H2O2 and/or albumin in 0.9% NaCl. Furthermore, TiZr alloys have been shown to possess enhanced passivity in comparison to CP Ti in the various exposure conditions including highly acidic (HCl), oxidative environments (H2O2 in 0.9% NaCl) and cell cultures (macrophage). Corrosion products of Zr were characterised in situ by synchrotron X ray methods, which were found to be ZrOCl2 ∙ 8H2O, tetragonal ZrO2, and Zr metal fragments in 0.9% NaCl regardless of the presence or absence of H2O2 and/or albumin. The presence of Zr metal fragments as a result of a corrosion process indicates the generation of metal species in the absence of wear.

In the present study, the effect of lamellar spacing, volume fraction of equiaxed gamma grains and lamellar orientation on fatigue crack propagation threshold have been assessed for three as-cast γ-TiAl alloys, Ti45Al2Mn2Nb1B (4522XD), Ti45Al2Mn2Nb (4522) and Ti46Al8Nb (at %). The influence of alloying elements, Nb and Ta on fatigue threshold was also studied with five forged alloys, Ti45Al2Mn2Nb1B, Ti45Al2Nb2Ta1B, Ti45Al2Mn2Ta1B, Ti45Al2Mn4Ta1B and Ti45Al2Mn4Nb1B. The fatigue crack propagation threshold tests were carried out at 650°C in air at a stress ratio R=0.1 and frequency of 10HZ. In addition, the microstructural characterisation of fatigue crack growth fracture surface was also examined by SEM. The samples assessed for fatigue crack propagation threshold failed mainly by translamellar fracture and interlamellar fracture. Interlamellar cracking occurs in lamellar colonies at any lamellar orientation at high stress intensity factor, ΔK, while at low ΔK cracking occurs in lamellar colonies only when the lamellar interface is parallel to the fracture plane. Therefore, the fracture behaviour is determined by both lamellar orientation and ΔK. The fatigue crack propagation threshold shows little sensitivity to lamellar spacing and volume fraction of equiaxed gamma grains with 4522XD variants in the studied range.

This thesis presents a new approach to fabricate high precision micro machine components from stainless steel and stainless steel ceramic composite materials, using Softlithography and powder metallurgy processes. Three different 316-L stainless steel powders, including 5, 10 and 16 μm in size, and two different ceramics powders, including 400 nm alumina and 320 nm titania, were tested. The PhD research process can be divided into three main stages. In the first stage, high quality SU-8 master moulds and their negative replicas soft moulds are produced using Softlithography technique. The second stage includes preparing the stainless steel slurries, filling the soft micro moulds, obtaining the green micro components, de-binding and sintering. In the third stage, the fabrication process has been developed further to produce stainless steel-ceramic composite micro components. Fabrication process in each stage was investigated in detail and the optimum properties were produced. Dispersant acrylic-based binder is adopted in this research successfully in producing damage-free green micro components. A cold isostatic pressing technique is also adopted to improve the densities and linear shrinkages of the stainless steel green and sintered micro components. A new mixing method is used to improve the homogeneity of the ceramic inclusions in the stainless steel matrix of the composite micro components. Characterization of the sintered stainless steel and composite micro components in terms of shape retention, density, linear shrinkage, internal structure, hardness and surface roughness were investigated in detail. The resultant stainless steel and composite micro components retain the same high geometric quality as the SU-8 master moulds.

Nickel Manganite (NiMn2O4) thin films (~3µm thickness) have been deposited on polycrystalline alumina substrates using pulsed laser deposition (PLD). The influence of deposition conditions, including substrate temperature and oxygen partial pressure, on the structure and properties of the films has been investigated. The effect of post-deposition annealing at 800°C in oxygen has also been studied. The structure and microstructure of the films were characterised by X-ray diffraction (XRD), and scanning electron microscopy (SEM). A four-wire resistance measurement technique was also carried out to measure the resistance and to calculate the films electrical parameters. Optimisation of the deposition parameters enabled films to be produced which exhibit characteristic negative temperature coefficient of resistance (NTC) thermistor behaviour. The dependence of the structure and properties of the films on deposition conditions will be presented and discussed. This study has demonstrated that PLD is a promising technique for fabricating high quality NiMn2O4 thin films suitable for use as NTC thermistors.

Friction Stir Welding (FSW) is known to result in a complex microstructural development, with features that remain unexplained, such as: the formation of the onion rings structure. Moreover, various microstructural factors have been suggested to control the strength in Al-Mg AA5xxx welds. The influence of the basemetal microstructural parameters (e.g. grains, intermetallic particles, stored energy) on the microstructure-property development has not been previously investigated, and is the subject of the present work. To rationalise the microstructural and local strength (hardness) development, especially within the heat affected zone (HAZ), a simple and rapid 3-D heat transfer model was established to predict the thermal fields associated with FSW. This numerical model utilises the alternating direction implicit method to simulate the transient thermal cycle based on the process parameters, thermo-physical and thermo-mechanical properties of the material. The model was fitted for the friction coefficient and contact conductance between the sheet and the backing plate using experimental torque and force data, as well as in-situ thermocouple measurements for AA2xxx and AA5xxx welds. The model predictions were consistent with the microstructural and microhardness development in the welds. Gleeble thermal simulations showed that the heating rate during welding affects the recrystallisation start temperature, which could delay or speed up recrystallisation. In the thermo-mechanically affected zone (TMAZ), the onion rings structure was studied in several AA5xxx and AA2xxx welds. This follows a thorough microstructural investigation of the basemetals sheets prepared by direct chill and continuous casting, to establish the influence of the microstructural heterogeneity in the basemetal on the onion rings formation and the microstructural development. Stereological studies of the intermetallic particle distributions in the basemetal and the welds revealed that there is a direct relation between the banding of constituent particles (Al(Fe,Mn)Si or Al6(Fe,Mn) in AA5xxx) or equilibrium phases (Al2CuMg or Al2Cu in AA2xxx) along the rolling direction, and the formation of the onion rings. A clear onion rings structure was defined by three microstructural features, which are: 1) the existence of fine and coarse grain bands, 2) grain boundary precipitates coinciding with the fine grain bands, and 3) coarse particle segregation in the coarse grain bands. Upon etching, these microstructural heterogeneities form the unique onion rings etching profile. The formation of the onion rings was rather independent of the process parameters and alloy type, as long as the intermetallic particles are banded regardless of their types. However, alloys with high area fraction of intermetallic particles (~> 0.02) were found to produce more pronounced microstructural heterogeneities, which resulted in a stronger etching intensity. The microstructural heterogeneities within the AA5xxx welds, especially the interaction between the dislocations and the fine Al6(Fe,Mn) dispersoids, indicated that establishing a structure-property model requires the incorporation of the various strengthening factors. Stereological studies of the grain size and intermetallic particle distributions in the TMAZ indicated that the hardness is a combination of various microstructural factors, with grain-boundary strengthening as the main factor, with additional contributions by Orowan strengthening by the Al6(Fe,Mn) particles in specific locations, as well as a minor contribution by solid solution strengthening which resulted from the dissolution of Mg2Si during welding. The high dislocation stored energy in the TMAZ, as measured by differential scanning calorimetry, was associated with the geometrically-necessary dislocations which resulted from the interaction with the intermetallic particles and grains, but do not contribute to the hardness.