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.

In this work, the hydrogenation-disproportionation-desorption-recombination (HDDR) processing conditions were optimised for scrap sintered NdFeB magnets from hard disk drives (HDDs). The HDDR process was then performed on scrap sintered NdFeB magnets, which were separated from HDD assemblies using hydrogen, on a large scale in order to produce anisotropic NdFeB alloy powder. The HDDR powder was then used to produce injection moulded magnets on pilot scale equipment (5 kg load) at Kolektor Magnet Technology GmbH, Germany. Sintered NdFeB magnets were separated from cropped hard disk drive corners as a hydrogenated powder, before being purified to remove any contaminants. The material was then HDDR processed using parameters optimised for this material (900 °C and 1200 mbar). The powder was compounded with polyamide 12 at a 90/10 ratio of HDDR powder to binder before injection moulding in a 500 mT field to form bonded magnets. The best bonded magnets produced had a remanence of 0.64 T, a coercivity of 623 kA/m and a BHmax of 61.7kJ/m\(^3\). Despite the mixed composition input feed, there was very little variation in the magnetic properties between 5 randomly selected magnets where a standard deviation for BHmax of 0.34 kJ/m\(^3\) was observed.

This thesis studies dwell fatigue crack growth in an advanced nickel disc alloy RR1000 at elevated temperature. RR1000 with different grain sizes and γ’ precipitate distributions are tested at both 650 and 700°C to investigate the effect of microstructure and testing temperature on dwell fatigue crack growth. Dwell fatigue tests with different dwell times at the peak load are conducted to study the effect of dwell time on crack growth rate and crack propagation mechanism. A transition from cycle-dependent transgranular fatigue crack growth to time-dependent oxide induced intergranular crack growth is found when the dwell time exceeds a critical value. Linear elastic stress intensity factor (K) threshold values for intergranular crack growth under sustained load are also measured to evaluate conditions when intergranular crack growth occurs progressively during the dwell period. Impacts of prior dwell fatigue loading on subsequent fatigue crack growth are also studied.

Martensitic 9-12%Cr-based creep-resistant steels are currently employed in a number of power generation applications as a cheaper substitute for the more expensive Ni-based superalloys. A tempered martensite microstructure with a uniform, fine prior austenite grain size is desired to give the required strength and creep-resistant properties at service entry. Refinement of austenite grain size can be achieved via recrystallisation. During hot opendie forging of high-volume rotor discs utilised in power plants non-uniformities of parameters determining austenite grain size, such as temperature, initial grain size, strain and strain rate across the component occur. The main focus of the research was to gain a comprehensive understanding of the influence of the aforementioned parameters on the development of grain size and kinetics of recrystallisation. Experimental efforts were channelled towards the investigation of static recrystallisation as the process governing the grain size development between the forging sequences. Experimental material for this study was provided by the industrial partner, FOMAS, in a form of a 5-tonne ESR ingot of FB2 steel, stabilised at 760 °C for 24 hours, heat treated at 1150 °C for 10 hours and tempered. Cylindrical axisymmetric test specimens were deformed on the Gleeble 3500TM Thermomechanical Simulator in uniaxial compression mode in the range of temperatures 900-1200 °C, strains 0.15-0.45, strain rates 0.1-5 s-1 and two different initial grain sizes, utilising stress relaxation and double hit methods. Stress relaxation softening behaviour over the range of experimental conditions has been analysed with the final recrystallised fraction being confirmed by optical microscopy. Aspect ratio development, along with the grain size, have been employed as a recrystallisation progress criterion. Considerable scatter and unusual shapes have been shown by the softening curves during the stress relaxation. Deviations in the amount of recrystallisation, when compared with predictions based on the Dutta-Sellars equations have been identified and discussed, based on a modification of the Avrami exponent used. Grain size measurements allowed the necessary parameters for the modelling of recrystallised grain size to be obtained.

The first phase of this thesis characterised the currently commercially available L169 and A201 aluminium alloys in terms of their response to testing at the operating parameters predicted for next generation aero-engine components. The L169 and A201 alloys were initially subjected to ageing trials at 205°C, specimens of both alloys were then fatigue tested at ambient temperature and at 205°C following 1000 hours exposure at 205°C. Detailed micrographic characterisation was undertaken to assess the impact of prolonged elevated temperature exposure on the alloy microstructure. Fractography was undertaken on the failed fatigue specimens to assess the impact of ageing temperature and temperature exposure on fatigue behaviour. The L169 alloy exhibited a significant reduction in properties following 1000 hours exposure at 205°C due to extensive precipitate coarsening. The A201 exhibited comparably better elevated temperature performance due to the increased stability of the Ω- phase precipitate however the extensive shrinkage porosity observed in the alloy had a negative impact on fatigue performance and will limit its use in a pressure tight environment. In addition to the investigation into currently commercially available alloys a detailed investigation was taken into a novel dilute aluminium-copper based castable metal matrix composite with the potential for use at elevated temperatures. This alloy exhibits unique solidification mechanisms which result in an increased resistance to conventional aluminium copper alloy casting defects such as shrinkage porosity, segregation and hot tearing. A detailed investigation was undertaken to assess the impact of chemical composition on the alloys unique solidification behaviour and to assess whether there was any possibility for further optimisation. Following on from this investigation the alloy was characterised in similar terms to the L169 and A201 alloys in terms of its fatigue behaviour at both ambient and elevated temperatures to provide an assessment of the alloys potential to meet the predicted next generation aero-engine component operating conditions.