The objective of this research was to study the difference in microstructure and high temperature fatigue behaviour between three RR1000 to IN718 inertia friction welds and to provide further understanding of the inertia friction welding process. Between the three welds no significant differences in the weld microstructures were found. Also high temperature fatigue crack growth tests within 0.3 mm of the weld interface, showed no difference in crack growth rate due to the three different sets of welding parameters. The cracks were found to propagate from RR1000 through the weld interface into IN718 passing a 10-30 mm wide zone, allowing higher crack growth rates. Fractographic studies have shown that these higher crack growth rates are caused by a higher tendency to intergranular cracking. In the present welds a semi-solid weld contact layer of 10 - 30 mm thickness developed, being an intermediate alloy of varying composition of the two base materials. In the surrounding material constitutional liquation of NbC particles in IN718 and of primary γ′ in RR1000 occurred. Similar welding process characteristics during the final second of the three welding cycles resulted in the observed similar weld microstructure and high temperature fatigue properties. It was further found that the local fatigue crack growth rate increase occurs in the weld contact layer.

In this work an investigation into the effect of alloying additions (Mn, Cu, Sb and Zr) on microstructure and corrosion of Zn alloys and hot dip galvanised coatings was undertaken. The first part of this thesis focuses on the effect of alloying additions on the corrosion of Zn alloys. The result shows that Mn is the most beneficial addition, which can significantly improve the resistance of Zn. The effect of Cu depends on its concentration. A high level of Cu addition has a deleterious effect on the corrosion resistance as the Cu-rich particles are catalytic cathodic sites for oxygen reduction. Additions of Zr and Sb were found to have minor effect on the corrosion behaviour of Zn alloys. The effect of these additions on the microstructure of hot dip galvanised coatings was investigated in the second part. Both Zr and Mn can inhibit the layer growth of active steels with high Si content. Thus, Mn and Zr might be an alternative addition to Ni which can control the excessive reaction of the active steels. Addition of 0.8 wt % Cu significantly increases the coating thickness of the galvanised steel containing 0.02 wt % Si. The growth kinetics of the alloy layers follows a linear law. The final part of this thesis focuses on the effect of these additions on the atmospheric corrosion resistance and electrochemistry of hot dip galvanised coatings. Among the alloying additions investigated in this study, Mn is the most beneficial addition to the Zn bath and can significantly improve the resistance of the hot dip galvanised coating to atmospheric corrosion. The effect of other additions on corrosion resistance is minor. The beneficial effect of Mn addition is mainly due to the formation of a Mn-rich oxide layer on the top surface during the galvanising process, which can greatly inhibit the cathodic reactivity of the hot dip galvanised coating. Coupled with the relatively low cost and ease of alloying of this element, these various factors suggest that Mn might have broader applications in general galvanising.

Atmospheric pitting corrosion of stainless steel was studied to determine pit growth mechanisms and kinetics. Inkjet printing of chloride was used to investigate the growth of atmospheric corrosion pits. It has been shown that the pit size increases with increasing chloride deposition density, as well as increases with increasing deposit diameter. Atmospheric pit growth was characterised in situ and in real time with synchrotron X-ray microtomography. Most pits were found to have open mouths and shallow depths. Growth of a deep pit was also observed in a pre-existing defect. Pit growth in depth does not appear to be under diffusion control. Electrochemical kinetics of the metal dissolution reaction including the Tafel slope as well as the critical metal ion concentration for pit propagation were studied with lD artificial pits in high chloride concentration solutions relevant to atmospheric conditions. The diffusion-limited current density and ratio of the critical metal ion concentration for pit propagation to the saturation concentration were found to decrease with increasing chloride concentration. However, there is no significant effect of the chloride concentration on the Tafel slope. The pitting potential and repassivation potential were measured on abraded wires and were found to decrease with increasing chloride concentration. Salt layer formation has been observed on ID artificial pits in 1 M and 6 M solutions with synchrotron XRD. The dominant phase of the salt layer was found to be FeCl\(_2\)•4H\(_2\)0. The formation of FeCl\(_2\)•2H\(_2\)0 was also observed, predominantly at higher applied potentials. This work can be used to provide a basis for developing a model to predict pitting corrosion of stainless steel under atmospheric corrosion conditions, for example in the case of storage of stainless steel intermediate nuclear waste containers.

The aim of this work is to establish the influence of the many process variables on the microstructure and the nature of internal stress in IN718 samples produced directly from powder using direct laser fabrication, which enables production of solid samples directly from a CAD file. The process variables that have been studied include, specimen geometry, laser power, laser traverse speed, the detailed laser path and powder feed rate. It has been found that the detailed microstructure is strongly influenced by all of these variables with the propensity for the production of equiaxed or columnar grains being strongly influenced by laser power. The texture is correspondingly strongly influenced by changes in processing conditions. The extent of precipitation of the various phases expected in IN718 was also found to be influenced by the process conditions. The level and nature of the residual stress in the sample and in the substrate have been determined for a wide range of experimental conditions and using neutron diffraction. It has been found that the level of these stresses could be reduced to a minimum value of about 300 MPa, but could not be eliminated. A simple 3D thermo-mechanical model was developed to understand the residual stress distribution, which agreed closely with the experimental measurements.

A complete and comprehensive understanding of dwell crack growth behaviour is required for two compressor and turbine disc alloys, Udimet 720Li and RR1000, both having a fine grain microstructure. The effect of temperature, dwell time and dwell load has been studied in air and vacuum along with detailed fractographic and microstructural analysis to understand the relevant contributions of oxidation, creep and microstructure. The study has been extended to determine the effect of an overload segment in the load waveform, the rationale being that this type of waveform better models real loading cycles on engines in service. In support with extensive modelling carried out within Rolls-Royce plc in order to understand the stress state ahead of the crack tip and its relaxation over dwell time, the effect of overload factor, test temperature and dwell time has also been examined empirically. In good agreement with the findings of others, crack growth rates according to overload cycling are shown to propagate significantly slower than crack growth rates according to (otherwise similar) dwell-only loading, or static loads; even at high temperature and in an oxidising environment. A good agreement between modelled predictions and experimental results has been achieved, indicating that the retardation of crack growth rates is mainly affected by mechanical factors.

Experiments have been carried out to develop process-routes for bonding Ti6Al4V couples and Ti45Al2Mn2Nb1B couples using a combination of solid state diffusion bonding, powder sintering and brazing. The aim was to firstly produce air-sealed semi-bonds at moderate diffusion bonding conditions (i.e. low bonding temperature or low pressure) that did not downgrade the properties of the bonded components, and then complete the bonding by Hot-Isostatic-Pressing (HIPping) without the use of encapsulation. In the first method, Ti6Al4V powder was used as interlayer when two Ti6Al4V blocks were diffusion bonded; and two TiAl powders, Ti48Al2Mn2Nb and Ti45Al2Mn2Nb1B, were used as bonding interlayer respectively when the Ti45Al2Mn2Nb1B couples were diffusion bonded. It was found that air-sealed bonds could be achieved only with the Ti45Al2Mn2Nb1B couples after the first-step bonding. HIPping and post-bond heat treatments were carried out to fulfill the bonding. Microstructural assessment and measurements of tensile strength and fatigue properties of the bonded samples were carried out and it was found that the bonded samples had properties comparable to those of the parent material. In the second method, the brazing alloy TiCuNi-60 was used to seal the periphery of specially designed Ti6Al4V samples. Brazing was carried out in vacuum and the brazing time was optimised so that an air tight seal was produced, but the microstructural changes associated with liquid phase diffusion between the braze alloy and the Ti6Al4V were minimised. After brazing, the vacuum-sealed bonds were HIPped to produce fully bonded samples. The tensile properties of the bonds were shown to be comparable with those of the bulk material. A slight decrease in fatigue properties was found in the bonded samples, which was associated with inclusions on the interface caused by contamination before bonding. Analysis of the factors controlling the bonding of nominally flat surfaces and of surfaces with powder interlayer has been carried out in order to explain the observations and the conditions required for successful low temperature, low pressure bonding defined.

A programme of research was undertaken to identify the mechanisms of formation of voids and defects in aluminium alloy C354 and C355 castings. The following aspects of the material processing were studied as independent and linked effects: casting technique and associated variables, hot isostatic pressing cycle parameters, and heat treatment cycle. Microstructure related driving forces for defect formation were quantified using differential scanning calorimetry and quantitative metallography and surface effects investigated using x-ray photoelectron spectroscopy. It was shown that by controlling key variables within casting, the component can become less sensitive to subsequent defect evolution from further thermal processing. Optimised parameters were defined and the mechanism of defect formation elucidated. A Non Destructive Testing (NDT) method for the detection of defects within C354 components was developed.

This study investigated the feasibility of manufacturing bimetallic fuel pump bearings by Hot Isostatic Pressing (HIPping). The aim of the project was to reduce processing costs whilst maintaining or improving product quality. The process involved compacting, sintering and diffusion bonding Al 7wt%Si powder to 30 % leaded bronze. The pre-processing HIP conditions including surface preparation of the leaded bronze liner, powder size, degassing temperature and degassing dwell time, had a significant effect on the bond strength. By optimising the HIP conditions, it was then possible to achieve a highly compacted Al-Si casing that had bonded to the leaded bronze by forming a thin, uniform and continuous diffusion bond interface. The interface consisted of three intermetallic layers; Al2Cu, AlCu and Al2Cu3. Due to the brittle nature of the intermetallic layers, an increase in width resulted in a reduction in strength. An optimum bond width has been established and resulted in bond strengths up to four times greater than the current production bearings manufactured by flame spraying. The project developed a manufacturing process for powder HIP bearings that offered a potential cost saving of 18%.

Thermal barrier coatings (TBCs) are used to protect high-pressure stage 1 turbine components in aero engines. At present the full potential high-temperature capabilities of TBCs cannot be utilised due to the difficulties in estimating the remaining useful life of in-service TBCs. State of the art non-destructive techniques, such as photo-luminescent piezospectroscopy (PLPS) have aided in furthering the understanding of damage evolution mechanism techniques, but are limited in applicability at temperature. In this work, a new force-balance model is presented for calculating the growth stress in a thermally grown oxide (TGO) layer at oxidation temperatures. Furthermore, a new experimental technique is explored for observation of the full-field strain distribution using synchrotron X-ray microtomography (SX μCT) coupled with digital volume correlation (DVC). The forcebalance method relates the creep in bondcoats of precision-machined cylindrical micro-specimens to the stress acted on the bondcoat by the TGO. These precisionmachined specimens were volumetrically imaged at the I12 JEEP beamline of Diamond Light Source (DLS) to reveal the three-dimensional evolution of TBC microstructure with time at temperature. The time-dependent volumetric image data acquired at DLS were processed using commercial digital volume correlation code to compute full-field displacement and strain distribution.

The primary focus of this study was to investigate the effect of removing the binder, by specified method, on the tensile strength of E-glass fibre bundle and composites. The methods investigated for removing the binder from E-glass fibres were: (i) fibre spreading; (ii) acetone-based treatment; and (iii) heat treatment in air and in a vacuum. In the first phase of the research, the effect of the above mentioned binder removal methods were investigated using the single-bundle tensile test. Binder removal via fibre spreading did not cause any reduction in the properties of E-glass fibre bundles. However, binder removal by acetone extraction led to a decrease of 37% in the tensile strength. The most detrimental effect on the tensile strength was found to be when E-glass was exposed to temperatures in excess of 450 °C. The percentage reduction in tensile strength for E-glass fibre bundle for 450 °C, 550 °C and 650 °C were 60%, 66% and 90% respectively. In the second phase of the research, E-glass bundles that were subjected to the above-mentioned treatments were used to fabricate single bundle composites. The procedure for manufacturing these composites was developed. It was established that the reduction in the strengths of the E-glass composites after specified treatment could be correlated to the reduction in properties experience by the fibre bundles. Attempts were made to analyse the treated fibres using a range of analytical techniques such as X-ray diffraction, thermographic analysis, differential scanning calorimetry and infrared spectroscopy analysis. Heat treating E-glass fibres in the absence of air was shown to bring about a reduction in the tensile strength by 58% as compared to 78% when the fibres were previously heated in air at 650 °C.