A new quenching and ageing treatment to refine microstructures of a number of cast .TiAI-based alloys has been assessed. This has been done by using Jominy end quenching, to better understand the continuous cooling transformation behaviour of the different alloys, followed by an investigation of the response of the transformed samples to ageing in optimally quenched samples. Ti-46AI-8Ta, Ti-46Al-8Nb, Ti-46AI-5Nb-W, Ti-47AI-2Nb-lMn-lW-O.2Si and Ti-48AI-2Nb-2Cr, have been investigated and the importance of boron content, grain size and oxygen content have been investigated for Ti-46AI-8Nb. It has been found that heavy elements such as Nb and Ta broaden the range of cooling rates over which a fully massively structure can be obtained. The addition of boron leads to the formation of fully lamellar structures over a wide range of cooling rates by suppressing the massive transformation, the feathery and the Widmanstatten transformation. The prior a. grain size also suppresses the massive transformation independent of whether the grain size was achieved by heat treatment or by addition of boron. It has been found that the responses of both high and low oxygen-containing samples are similar at high cooling rates, but are very different at intermediate and low cooling rates. The amount of massive gamma which appears to be nucleated away from the original alpha boundaries is also strongly influenced by cooling rate and oxygen content. It is proposed that the low diffusivity of such heavy elements retards diffusion-controlled phase transformations (feathery and lamellar) so that the partitionless massive transformation can take place at lower cooling rates. In fine-grained samples the dominance of the lamellar microstructure is interpreted in terms of the role of grain boundaries in nucleating lamellae at temperatures above the massive start temperature. The complex role of oxygen is interpreted in terms of the ·stabilisation of a. by oxygen and by the extent of the segregation of oxygen during quenching. The observations made using the Jominy end quenching technique led to the selection of salt bath quenching as the optimum technique to develop crack-free fully massively transformed samples and these have been used to investigate the influence of different ageing schedules (carried out using the HIP (hot isostatic pressing) cycle used for • structural castings) on the microstructure and properties of quenched and aged samples. It has been found that the formation of subgrains during the massive transformation, precipitation of a on the sub grain boundaries and precipitation of a on the four {Ill} planes of the gamma within these sub grains during ageing lead to \:onsiderable refinement of microstructures. The influence of the ageing treatment has been studied by ageing at different temperatures within the two phase field and by two step ageing at low and high temperatures. It has been found that the finest microstructures are obtained by low temperature ageing followed by short, higher temperature ageing. The influence of the ageing sequence on the microstructures is explained in terms of the factors influencing the nucleation and growth, of a precipitates at different ageing temperatures. Room temperature tensile tests have been carried out on salt bath quenched samples aged using either one step or two step ageing. It has been found that one-step ageing brings a more significant improvement in room temperature tensile properties than two-step ageing, where the best properties are found in samples aged at low temperatures, followed by a short excursion into the single a phase field. Two separate appendixes, studying mechanism of Widrnanstiitten transformation and embrittlement of TiAl-based alloys after exposure at service temperatures are given at the end of the thesis.

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Titanium Metal Matrix Composites (TiMMC) have been gaining momentum in the aerospace applications in the past two decades. The advantages of TiMMC include superior stiffness-ta-weight and strength-ta-weight ratios, compared to conventional aerospace steels. The low pressure shaft in the aeroengine is one of the components that could benefit from the superior material properties of TiMMC. Potentially, the use of TiMMC could provide a combination of higher torque density (higher torque for a given diameter) and lower weight for these shafts. However, little has been done to investigate the torsional behaviour of TiMMC shafts. This thesis investigates the torsional behaviour of TiMMC shafts with different fibre orientations, through experimental tests. A material database was established for the TiMMC system used for each fibre orientation. Further to that, a comprehensive failure investigation was carried out on the tested specimens in order to understand the failure mechanisms. Samples from the fractured specimens were polished and etched to study their microstructure and internal features. Numerical models were developed to predict the global orthotropic elastic material properties of TiMMC using the unit cell concept. The material properties were then used in the tube models which predict the shear moduli for different fibre orientations. The results were validated with the experimental test results. To minnimise weight, aeroengine shafts aim to have thin walls, which could result in buckling instability. Numerical models were therefore developed, to investigate the buckling behaviour of TiMMC shafts. Hill's potential fun ction in ABAQUS was used, which define the anisotropic yield behaviour, using user-defined stress ratios. As a result of this research, the understanding of the torsional behaviour of TiMMC has been strengthened.

Titanium nitride coatings, because of their high hardness, resistance to corrosion and wear and gold-like appearance, are widely used in industrial applications ranging from coating of decorative items to protection of cutting and forming tools against wear and corrosion. Because of their excellent tribological properties especially their suitability in cutting, abrasive and erosive wear application, titanium nitride coatings have attracted considerable research. The suitability of TiN coatings as excellent tribological coatings is explained by their high hardness, good adhesion to steel substrate and chemical stability. However, there is one critical shortcoming which affects the use of TiN coatings in commercial products. It is that TiN coatings show high friction coefficients against some counterfaces.

Rapid manufacture refers to those processes in which a component can be manufactured without the need for specific tooling. This reduces the time between the design stage and production of finished parts compared with traditional methods such as forging or casting. There are a number of laser based rapid manufacture processes currently available such as selective laser sintering and direct laser deposition which are both based on adding powder form material to a substrate in a layer-by-Iayer fashion. Subtractive methods such as laser milling have been developed to a lesser extent but are lirriited by low removal rates and problems caused by heat affected zones. Here, three new subtractive methods of laser rapid manufacture of near net shape titanium components were developed. Each was investigated and the most promising was chosen for further research. The chosen method is based on water assisted laser processing of partially sintered titanium powder. A block is made by partial sintering of titanium powder in a furnace such that it has the properties of being strong enough to be handled but still porous, with the powder particles retaining their spherical shape. The block is then soaked in liquid so that the interconnected pores become filled. A laser is used to irradiate the surface of the block which causes material removal from the irradiation site in a predictable way. The process was characterised by investigating the influence of process parameters: sintering temperature, laser energy density, powder particle size, laser repetition rate and type of liquid used to soak the blocks. The process was also analysed using high- . speed photography, emission spectroscopy and the probe-beam extinction technique to gain an understanding of the material removal mechanism. A computational model of the process was proposed based on the Smoothed Particle Hydrodynamics method. It was found that the mechanism of material removal is based on the explosive boiling of the liquid in the pores of the partially sintered block caused by heating from the laser. The rapid expansion of the exploding water creates a pressure which breaks the weak bonds between particles and ejects them from the surface. The new method can be faster than the current additive laser based rapid manufacture techniques and uses lower laser powers than the current laser milling processes. This is achieved through the choice of experimental conditions and laser parameters.

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A research programme has been undertaken to investigate the subsurface deformation microstructures in aerostructural titanium alloys that result from industrial surface finishing processes. Microstructural analysis of the region immediately below the treated surface has been performed for high speed machined and shot peened material, with emphasis placed on characterising the mode of plastic deformation through high resolution scanning electron microscopy and electron backscatter diffraction. Both shot peening and high-speed machining result in a plastically deformed subsurface layer, with experimental evidence collected through electron backscatter diffraction suggesting that the mode of plastic deformation is influenced by local textural heterogeneities (microtexture) in addition to alloy chemistry and substrate temperature. The stability of the deformed subsurface microstructure following exposure to elevated temperature is investigated for the near-alpha titanium alloy Ti-834. Here, shot peening leads to an increase in oxygen uptake kinetics during exposure to laboratory air in the temperature range 600°C to 700°C. The increased levels of subsurface oxygen versus the non-shot peened condition were measured by secondary ion mass spectrometry and fatigue testing has suggested a lowering of the high cycle endurance limit in shot peened Ti-834 following prolonged thermal exposure in air. The outcomes of this research suggest that the mode (and magnitude) of the residual plastic strain introduced by shot peening and high speed machining may be predicted; should an improved understanding of microtexture evolution during the primary processing (such as hot-working) of titanium alloys be achieved.

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