Double oxide films (bifilms) are significant defects in light alloy castings which were reported to have detrimental effects on the reliability of the castings. The research reported here was aimed at studying how these defects develop with time. The results suggested that both O and N inside the bifilm would be consumed by reaction with the surrounding melt, and that H might be diffused into the defect. Based on the estimated reactions rates the time required for the consumption of the atmosphere inside a bifilm entrained in pure Al, Al-7wt.%Si-0.3wt.%Mg and Al-5wt.%Mg alloy melts, was determined to be 538, 1509 and 345 seconds respectively. The results also suggested the occurrence of two competing mechanisms during holding of the castings in the liquid state before solidification. The first mechanism was related to the consumption of the bifilm atmosphere, which might reduce the size of bifilms and therefore increase the Weibull moduli the UTS and the % elongation. The other mechanism was the diffusion of H into the bifilms, which would be expected to increase their sizes and reduce the moduli. This research therefore could lead to the development of new techniques by which bifilms might be deactivation in light alloy castings.

Heat resisting centrifugally cast tubes of HP micro alloy (25/35 Cr/Ni, 0.4C), are extensively used in steam reforming, which is currently the dominate technology for hydrogen generation. High pressure of the reacting gases inside the tubes generates high hoop stress in the tube wall and together with the temperature exposure of 900-1050°C causes the tubes to creep along the circumferential direction. The alloy's ability to successfully withstand the severe operating condition is highly dependent on its high temperature creep-rupture behaviour. In recent years a number of manufactures have introduced higher creep-rupture strength versions of the heat resisting HP micro alloy. As a consequence they are able to offer the centrifugally cast tubes in thinner walls. Therefore, there is a need to enhance the creep-rupture strength of the alloy produced by Doncasters Paralloy. The immediate objective of this research project is to establish the mechanisms that govern the creep process in this alloy system. Once these mechanisms are better understood further alloy development within the present alloy composition can be achieved. The advanced stages of the project specifically involve studying the influence of microstructure in relation to creep resistance as a function of compositional modifications.

In the thesis, four DLC coatings were selected: two hydrogen free DLC coatings (GCr and GCrSi) and two hydrogenated DLC coatings (DCr and DCrSi), which are the two main groups in the DLC family. Three annealing conditions (argon annealing, air annealing and HIPping/high pressure argon annealing) were designed to evaluate the stability and performance of the four DLC coatings. The microstructure of the DLC coatings was characterised by Raman spectrum, GIXRD, SEM/Due-beam SEM, EDX and TEM. Scratch tests were performed to assess the adhesion of the coatings. Both the mechanical and tribological properties of the DLC coatings were evaluated including nanohardness, coefficient of friction and wear factor. The influence of high pressure and oxidation was discussed, and the comparison of four different coatings was made.

Inertia weld process variables arc characterised using empirical relations that arc functions of the weld input parameters, allowing the variables to be predicted for any future production welds. The techniques for establishing the empirical relations can be applied to future alloy system for inertia welding by using significantly refined test matrices, reducing the development costs of new welds. Power loss in the bearings for two inertia welding machines is characterised by using hydrostatic bearing theory and several experimental techniques. This allows energy-based empirical relations for new alloy systems to be developed using sub-scale welds only, reducing the cost of implementing future production welds. Average temperature and now stress of the plasticised zone of welds is used to characterise the average shear zone thickness. Two separate models arc developed and arc found to correlate well with each other and with experimental observations. This work helps to improve the mechanistic understanding of inertia weld interfaces and can be used in computational fluid dynamics models to characterise the bond-line cleaning mechanism.

The joining of the titanium alloy Ti-6Al-4V by LFW is analysed by FE and analytic modelling, in both 2D and 3D. For the purposes of model validation and testing, experimental work is carried out using pilot-scale apparatus. Thermocouple wires were inserted in to several welds, to measure thermal values at varying locations. The sensitivity of the measured temperatures and upset rates to the critical process variables; amplitude, frequency and the applied pressure - are shown to be consistent with the predictions of the 2D and 3D modelling. The flash produced is found to be dependent upon the ratio of oscillation amplitude to applied load; when this is large a rippled morphology is produced. The 2D models replicated the flash formation behaviour well. Results for residual stress fields in three principal axes were calculated from the FE model, and compared to experimentally determined residual stress values. The modelled predictions were found to be reasonable. Further, a simple analytical model of the process is proposed, in which mechanical work is balanced against flash formation; at steady-state temperature is predicted to decrease exponentially with distance in the HAZ, and the temperature gradient in the HAZ is predicted to increase as the upset rate increases, as observed.

Two distinct techniques for “optimum” fibre spreading of 2400 Tex E-glass and 12K carbon fibre bundles were designed and developed in this research project. These techniques enabled increases in fibre bundle width of 956% and 1121% for E-glass and carbon fibre bundles respectively. These values surpass anything reported in the literature. Tensile testing was conducted on dry fibre bundles in the as-received and spread state. A reduction in the tensile failure stress and Weibull survival probability was observed with an increase in gauge length. The effect of spreading was determined to be statistically significant at a gauge length of 100 mm; however it was not significant at a gauge length of 50 mm. Composites were manufactured using as-received or spread fibre bundles and their properties analysed and compared. The composites manufactured using spread fibre bundles were significantly thinner than those manufactured using as-received fibre bundles. In hybrid composites the degree of mixing of the two reinforcing fibre types was greater than has been reported in the literature. The overall conclusion from this study is that the utilisation of spread E-glass and carbon fibre bundles can be used as a means for enhancing the apparent failure strain of carbon fibre composites.

The susceptibility of Ti6246 to cold dwell fatigue behaviour has been assessed as part of a wider investigation into the relationship between optical microstructure, crystallographic texture, mechanical behaviour and material processing for various common titanium aerospace alloys. Assessment of the full range of production microstructures for Ti6246 has shown acceptable maximum texture intensities and crystallographic unit size variations. Debit on cold dwell fatigue life has only been observed at very high proportions of the ultimate tensile strength of the material. Heat treatment to eliminate potential ‘good’ factors in the dwell response of conventional microstructures was also noted not to produce a large cold dwell fatigue life debit. The absence of dwell effects may be attributed to one or more factors, including susceptibility of large colony microstructures to other failure modes to the extent that they are activated before the mechanism leading to cold dwell fatigue facet formation. Characterisation of optical microstructures and textures has revealed a range of structures associated with different titanium alloys and product forms. It has been noted that common crystallographic features may be observed in very different microstructures in these alloys. It is concluded from this work that the optical microstructure should not be taken as an accurate indication of the effective structural unit size in titanium alloys.

Despite of the wide application of zirconium and its alloys, wear and corrosion of zirconium components are major issues for demanding applications. To this end, ceramic conversion (CC) treatments at different temperatures for varying time were therefore carried out on both Zr702 and Zircaloy-4 alloys. In addition, first trial of duplex surface treatment was conducted on Zr702 alloy to study the response of the CC formed ceramic layer to oxygen boost diffusion (OBD) and to increase the oxygen diffusion zone (ODZ). Results showed increased anti-corrosion and anti-wear properties due to the generation of dense ceramic layer (mainly m-ZrO\(_2\) and t-ZrO\(_2\)) supported by a hardened diffusion zone on the surface. Pitting corrosion of as received Zr702 and Zircaloy-4 can be eliminated; improvement in wear resistance at best treatment condition has increased 3.7×103 and 8.9×102 times for Zr702 (530°C/10hrs) and Zircaloy-4 (500°C/10hrs) respectively. The main wear mechanism has changed from adhesive wear for the as-received materials to mild abrasive wear of the CC treated samples. OBD treatment in both two-step and three-step duplex surface treatments can further increase the wear performance of Zr702 due to the reduced porosity and cracks in the oxide layer and thickened oxygen diffusion zone.

The dwell fatigue crack growth behaviour of coarse grained RR1000 has been studied using advanced mechanical testing and characterization techniques. Crack growth under sustained loading has also been considered. Environmentally enhanced crack growth at high temperatures in air follows a stress assisted grain boundary oxidation (SAGBO) mechanism, with a 100-300 nm long Al2O3 intrusion ahead of the crack tip. When its thickness of reaches a critical value, this intrusion fails and crack advance ensues. The crack growth process is discontinuous and highly uneven. The dwell fatigue/sustained loading crack growth rates are affected by the stress relaxation ability of the materials, which is related to the size and distribution of tertiary γ' precipitates, testing temperature and stress state. Oxidation damage is deduced to be a dominant factor affecting crack growth over a range of temperature from 600 to 700 °C. At 700 °C oxidation damage begins to have an effect on fatigue crack growth at a dwell time of 30 s. Creep damage is suggested to make the dominant contribution to crack growth at 750 and 775 °C both in air and in vacuum.

Boron addition has been used to refine the microstructures in TiAl castings, such as LPT blades, to improve ductility. However, boride precipitates with undesired morphologies could reduce ductility and even wipe out the entire benefit from grain refinement. Boride morphology in Ti45Al2Mn2Nb1B was found to be closely related to solidification conditions and proper processing windows have been established. In order to improve temperature capability of cast TiAl alloys to cater for the demand in new engines TiAl alloys have to be alloyed with more refractory elements, such as Nb and Ta, which will inevitably affect the boride morphologies. In this study variant alloys based on Ti45Al2Mn2Nb1B have been investigated with emphasis on the relationship between boride morphology, alloying element species and cooling conditions and distinctly different boride formation behaviour in different alloys was observed. In addition, curvy borides were found both on the fracture surface and within the microstructure in tensile test pieces, especially in Hf-containing test pieces, which indicate that curvy borides have the strongest effect in reducing ductility regardless of alloy composition.