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The relationship between alloy composition, microstructure, mechanical and thermal processing and mechanical properties (strength and toughness) in 2xxx (Al-Cu-Mg) and 7xxx (Al-Mg-ZnCu) series aluminium alloys has been investigated for use in the production of high-strength lightweight components.

In commercial thermomechanical processing, metals and alloys are often subjected to complex deformation histories, including changes in the strain rate, temperature and strain path. Due to the complicated nature of the sequences of deformations that thermomechanical processes need to create a final product, the strain path experienced by the majority of volume elements in the material is complicated. However, the relationship between the strain path and the microstructure/texture evolution is not well predicted by current models used in industrial processes. The important properties in metals are sensitive to the anisotropic microstructure of the metal produced via thermomechanical processing. This anisotropy includes crystallographic texture, grain shape, the shape and spatial distribution of particles and deformation induced dislocation boundaries. An understanding of the origin of these microstructural features is, therefore, critical for future improvements in properties. To some extent, under linear deformation conditions the evolutions of parameters are well known. This can not be said, however, for non-linear deformation conditions. In this work the effect of strain path on microstructure and texture evolution of the aluminium alloy AA5052 under hot working conditions has been investigated. Specimens were produced with a wide range of strain paths with many also statically annealed to investigate different stages of recrystallization. The microstructures and textures created in the specimens were then studied using electron backscattered diffraction (EBSD) and the subsequent develop of a program that analyses the recrystallization fraction of the material in function of the crystallographic orientatio. The deformed strain path specimens were created using the Arbitrary Strain Path machine at the University of Sheffield, which is capable of torsion and tension/compression testing. Tests from a single step of torsion through to multiple steps with combined forms of strain, using one, two or more steps of deformation with a value of equivalent strain of 0.25 and small reverse deformation for selected tests of equivalent strain of 0.05. I , , The microstructures formed by the applied deformation and annealing processes were analyzed using EBSD to create profiles that show the behaviour of the microstructures and textures under the different strain paths. More detailed analysis was then carried out using in-house software. The program used quaternion geometry to calculate the mean orientation of each grain and then used the deviation from the mean to analyse the extent of substructure within the grains which enabled the recrystallization fraction for each strain path condition to be calculated. The results show a clear difference between the recrystallization behaviour and the respond of microstructure result of strain path with an equivalent strain of 0.05 of deformation applied on the reverse deformation. The small reverse conditions are quite different showing that applying the small reverse deformation after two steps of forward deformation presents material with significant substructure than just two steps of forward deformation, meanwhile when the small reverse deformation is applied after one step of forward deformation the amount of material with significant substructure decreases. 1I

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The components of a modem internal combustion engine are required to give extreme reliability over extended periods of operation and none is exposed to more arduous conditions than the piston, especially in the pin boss and crown regions of pistons for diesel engines. The increasing emissions requirements and performance targets demanded of direct injection diesel engines has resulted in steep increases in both specific powers and maximum cylinder pressures. This has in turn lead to greater temperatures and pressures being felt by the piston. The adaptation of the piston design to these increasingly demanding load and temperature conditions has required a continuous improvement and innovation in the field of materials and process technologies. The vast majority of the internal combustion engine pistons produced globally are made by a gravity die casting process using Al-Si based alloys. Although Al-Si alloys have been the subject of a great deal of research over the last 30 years, the majority of work has been based on fairly rudimentary characterisation of the microstructures as a function of alloy chemistry and cooling rate. Most of the attention has been paid to the silicon morphology and distribution rather than on a fundamental knowledge of the development of the complex microstructures and intermetallic phases that arise in commercial alloys. However, the properties of cast near-eutectica: luminium-silicon alloys are very strongly influenced by the microstructure, i.e. the primary aluminium, and the interdendritic microconstituents, such as secondary phases, intermetallics, inclusions and porosity. A fine and uniform grain size is often desired as it improves mechanical properties of castings such as tensile strength, ductility and fatigue resistance, and at the same time aids castability, improves porosity distributions and reduces hot tearing susceptibility. A thorough phase characterisation has been carried out using a number of techniques including optical and electron microscopy with electron backscatter diffraction (EBSD), and image analysis. Use was also made of thermodynamic modelling to predict the volume fraction and distribution of phases within the microstructure as a function of chemical composition and process parameters. From this analysis a detailed understanding of the phases occuffing in multicomponent Al-Si alloys was established. Furthermore, additions associated with grain refining, i.e. Ti, Zr and V, have been investigated systematically using commercial and model alloy systems. All three additions were observed to refine the structure of the castings through the formation of the phase A13Ti, although combined additions with Zr were found to be less efficient due to a 'poisoning' effect on the A13Ti. It was also established that there is a strong competition between the effects of grain refiners and P, with the formation of AbTi reducing the nucleating efficiency of AIP to silicon. The nucleation and growth of the primary silicon phase were thus examined by EBSD. AIP was confirmed as nucleating the silicon epitaxially, after which growth continues by surface nucleation, although the presence of twins were seen to influence the shape of the crystal. Finally, suggestions have been made as a consequence of this work for the future development of piston alloys.