Aluminium alloys are the most promising lightweight materials used in the automotive industry to achieve weight reduction for improving fuel efficiency and reducing CO2 emissions. High pressure diecasting (HPDC) is a fast and economical near-net shape manufacturing method to produce engineering components. About 80% of cast aluminium alloys are currently manufactured by HPDC. The increased demands of manufacturing structural components by HPDC process require high strength Al-alloys for the automotive industry. However, the currently available die cast Al-alloys are unable to fulfil this requirement. Al-Mg2Si alloy is known as an alloy capable of providing superior high strength by Mg2Si particles. However, Al-Mg2Si alloy is not applicable in the HPDC process because of the severe die soldering problem. This has limited its applications throughout industries. Moreover, the existing studies on the Al-Mg2Si alloy are mainly focused on the hyper-eutectic alloys and limited information is available for hypo-eutectic alloys. Generally, the mechanical properties of Al-alloys are determined by the alloy composition, the defect levels in the components, the microstructure which is mainly controlled by the casting process and heat treatment process. Due to the high cooling rate provided by the die block in the HPDC process, the refined microstructure in the die cast Al-Mg2Si alloys can be obtained to improve the mechanical properties. Therefore, the development of high strength Al-Mg2Si based alloys for the HPDC process is significant for manufacturing quality automotive components. The present study mainly focuses on the alloy development for the HPDC process. In order to make die castable Al-Mg2Si based alloys, the effect of excess Mg has been investigated to modify the hypo-eutectic Al-Mg2Si system for improving the mechanical properties. The effect of excess Mg on the solidification and microstructural evolution, and the mechanical properties of Al-Mg2Si alloys, has also been investigated by the combination of thermodynamic calculation and the experimental validation. The excess Mg in the hypo-eutectic Al-Mg2Si alloys has been found to be able to shift the eutectic composition to a lower Mg2Si content, which means that the hypo-eutectic composition of Al-Mg2Si alloy can be at eutectic or hyper-eutectic compositions after adding different levels of excess Mg. The experimental trials have also found that Al-8Mg2Si-6Mg alloy provides the best combination of strength and ductility in the as-cast castings made by the HPDC process. This can be further enhanced by adding 0.6wt.% Mn, which exhibits yield strength of 189MPa, UTS of 350MPa, and elongation of 6.5%. Investigations have also revealed that the Al-8Mg2Si-6Mg alloy exhibits a relatively high tolerance to the Fe impurity because of the insignificant reduction of ductility of the alloy. The elongation is still at a level of 5% when Fe is at 1.6wt.% in the alloy. Furthermore, Cu and Zn can further enhance the mechanical properties of the Al-8Mg2Si-6Mg-0.6Mn alloy. Cu contents between 0.31wt.% and 0.92wt.% in the Al-8Mg2Si-6Mg-0.6Mn alloy can increase the yield strength from 193MPa to 207MPa, but decrease the UTS from 343MPa to 311MPa, and the elongation from 4.8% to 3.8% under as-cast condition. This can be attributed to the formation of hot tearing defects in castings. Therefore, the Cu content in the alloy should be limited to a low level. On the other hand, zinc can be controlled to a level of 4.3wt.%, which will dramatically increase the tensile strength of the alloy. More importantly, Zn can significantly increase the mechanical properties of the alloy after a quick T6 heat treatment under a condition of solution at 490oC for 15 mins and ageing at 180oC for 90 mins, at which the yield strength is 345MPa, UTS is 425MPa, and elongation is 3.2 %. In the present study, the solidification and microstructural evolution, the relationship between the microstructure and mechanical properties, and the strengthening mechanisms in the developed alloy are discussed on the basis of the experimental results. A two stage solidification has been recognised to be responsible for the microstructure formation in the HPDC process. The primary α-Al phase is formed as prior phase for the hypo-eutectic alloy and the primary Mg2Si phase is formed as prior phase for the hyper-eutectic alloy. The solute elements including Mg, Mn, Fe, Cu, and Zn can enhance the solution strengthening and/or the precipitation strengthening in the alloys, but alter the solidification ranges, which will affect the formation of defects in the castings. In the quick T6 heat treatment, the AlMgZn phase is dissolved into the Al phase during solution treatment and precipitated during ageing treatment. The quick heat treatment is also found to be able to spheroidise the Mg2Si phase. Only η′ MgZn phase is precipitated during aging in Zn containing alloys. The alloy with 4.3wt.% Zn provides the best combination of the mechanical properties because of the high density of MgZn precipitates in the α-Al phase.

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The development of aluminium alloys that can be processed by Rapid Solidification (RS) techniques for use in high temperature applications has recently been an area of intense study. One of the alloy systems of interest is Al-Cr-Zr-Mn. This work comprises a study of the microstructure and tensile properties of alloys of this system processed by melt spinning, high pressure gas atomisation (HPGA) and chill casting. The RS microstructures of Al-Cr and Al-Zr binary alloys were also compared with those of the quaternary alloys. The variety of microstructures observed in the powders of the quaternary alloys was consistent with the different cooling rates and nucleation temperatures experienced by droplets of different sizes, A cubic phase not previously reported was observed in the finer powder. The transition from a partitionless to a cellular microstructure occurred at estimated solidification front velocities similar to those predicted by morphological stability theory. The distribution of discrete Al[13]Cr[2] intermetallic particles within Al-Cr gas atomised powders of different sizes was found to be consistent with a probabilistic model of nucleants distributed in the volume of the alloy melt. Based on these results the original Al-5.2Cr-1.4Zr-1.3Mn (wt%) alloy was diluted to give an Al-3.3Cr-0.7Zr-0.7Mn (wt%) alloy so that the bulk of the powder (the sub-45mum size fraction) did not contain coarse intermetallic particles but exhibited a mainly cellular microstructure. A relationship has been determined between the thickness of wedge shaped chill castings and powder diameters for. similar microstructures. Prediction of alloy compositions designed to give a particular microstructure in a specified powder size can therefore be tested by a simple casting technique. The mechanical properties of the original and optimised quaternary alloy powders consolidated by Conform and extrusion have been determined and related to the as-consolidated and aged microstructures. The extruded powders of both alloys exhibited better properties than the Conformed powder. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The optimised alloy had a consistently better ductility. Neither of the alloys retained its strength after prolonged treatment at 400°C, but the results suggest that a service temperature of 300°C may be possible.

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An AA5083 aluminium alloy sensitized in service after 40 years exposure to ground atmosphere temperature is studied. Nitric acid mass loss test (NAMLT) is used to determine the susceptibility to intergranular corrosion (IGC). The degree of sensitization in various areas through the alloy thickness was found different, which can be associated with non-homogeneous Mg distribution through the alloy plate thickness. Structure characterisation confirmed that the in-service sensitization is associated with the formation of the ' phase and a cubic Al-Mg transition phase with magnesium content between the GP zones and the '' phase at the grain boundaries. In order to simulate the in-service sensitization process and to gain insight into the sensitization mechanism, the sensitization of AA5083 alloy at relatively low temperatures, namely 70 and 100°C, is studied. For the AA5083 alloy sensitized at 70°C, although the mass loss value is below 15 mg/cm2, ' phase is observed as individual precipitates at grain boundaries. The AA5083 alloy after exposure to 100°C for 240 hours is susceptible to IGC since the ' precipitates have grown continuously at the grain boundaries. Additionally, the effect of sensitization in AA5083 alloy on stress corrosion cracking (SCC) is also investigated using constant displacement double cantilever beam (DCB) testing. It is found that the cracking length increases with the degree of sensitization. The population density of the crack branches also increases with the degree of sensitization. The metal between different small branches is known as ligament. And with high degree of sensitization, the ligaments between crack branches have become brittle. Therefore, small branches became connected to form a continuous crack with the crack propagating.

Aluminium alloys are essential to a variety of industry sectors, particularly transport, where they are used in the production of cars and aeroplanes. However, aluminium alloys are susceptible to degradation through corrosion which can compromise the integrity of components manufactured from this material. Therefore research into the means by which these alloys degrade is important. This thesis aims to understand how one of the more potentially damaging types of corrosion, known as pitting corrosion, occurs in the important aluminium alloy 2024-T3 (AA2024-T3). In order to study this phenomenon, this thesis first characterises the alloy microstructure in detail, particularly the type and distribution of intermetallic particles since these play an important role in corrosion processes. The microstructure was studied using an electron microprobe analysis of a 5 mm x 5 mm area of AA2024-T3 and some 80,000 particles were characterised. This investigation was one of the most comprehensive studies to date of any aluminium alloy. Of the particles studied, it was found that the major types included the S and θ phases and a number of compositions based around AlCuFeMn and AlCuFeMnSi. Depletion zones were an integral feature of the alloy microstructure. Pair correlation functions were used to determine the degree of clustering and it was found that there was both inter particle as well as intra particle clustering. Inter particle clustering was observed at length scales well beyond 50 µm. A detailed study of corrosion on AA2024-T3 was undertaken by examining the surface after corrosion over a time period spanning 2.5 minutes to 120 minutes. From this investigation, a hierarchy of the localised corrosion was observed as it was very apparent that particles of particular elemental compositions were more susceptible to attack much sooner than other compositions. Larger corrosion attack sites on the surface, which were called co-operative corrosion, were attributed to intermetallic clustering affects and changes in chemical composition such as Cu-enrichment. These results were used to develop a detailed model of the initiation of stable pitting corrosion in AA2024-T3, which will lead to a better understanding on how to prevent pitting attack on commercially important aluminium alloys. AA2024-T3 is rarely used in the polished state, for real world applications is it generally finished by mechanical or chemical processing. In the final part of this thesis, the influence of clusters on metal finishing was examined using a standard aluminium chemical deoxidiser. It was found that the etch rate of this deoxidiser increased dramatically with the increase in temperature. Under certain processing conditions only the intermetallic particles are etched out and these retain the history of the spatial distribution of the clustering of the intermetallic particles. This leaves a cluster of 'holes' which could trap metal finishing solution and lead to severe subsurface attack

The use of aluminium alloy films, and in particular Al-1%-Si, is common in many microelectronic interconnection schemes. Specular reflectivity and sheet resistance measurements are often used to characterise the deposited film. These and other properties, such as electromigration resistance, are dependent on the film's structure. An understanding of the relationship between the film structure and its properties is therefore important. Al-1%-Si films were deposited using dc magnetron sputtering onto (100) silicon substrates with or without thermal or deposited oxide layers. The film grain size and orientation were determined using scanning electron microscopy and X-ray diffraction measurements respectively. The orientation was quantified by the ratio of the X-ray diffraction intensities from the (111) and (200) planes. The structure of Al-1%-Si films depended strongly on film thickness using continuous mode deposition, and on substrate position using batch mode. For both modes of deposition the X-ray diffraction ratio was dependent on the substrate used. By careful choice of monitor substrate, and control of deposition conditions, the film structure could be successfully predicted and controlled. The specular reflectivity of Al-1%-Si films has been shown to depend on their roughness, optical properties and grain size, and the measurement wavelength. Of particular importance in determining the specular reflectivity is the substrate used as well as its position for batch mode deposition. The critical dimension size and automatic alignment accuracy obtained during the photolithographic patterning of the deposited film depended on its specular reflectivity. This shows that specular reflectivity control, especially for a batch deposition process, is important. Finally, room temperature sheet resistance has been shown to vary with the substrate used and substrate position during batch mode deposition.