Characterisation of intermetallic phases in multicomponent Al-Si alloys for piston applications

Al-Si based casting alloys are widely used for piston applications. This is due to their combination of properties, which include good castability, high strength, light weight, good wear resistance and low thermal expansion. In order for such alloys to meet increasingly demanding operational requirements, such as higher service temperatures and pressures, multicomponent Al-Si alloys, which contain several other alloying additions (Cu, Ni, Mg, Mn and Fe), have been used to further enhance the high temperature strength and fatigue resistance. Improved material properties are strongly dependent upon the morphologies, the thermal and mechanical properties, and the distribution of the intermetallic phases present in these alloys, which are in turn a function of alloy composition and cooling rate. Therefore, the main aim of this work was to characterise the many complex intermetallic phases in multicomponent Al-Si alloys. Five main areas of interest were investigated in this research. Firstly, thermodynamic modelling has been used to predict phase formation in complex alloys, which has been compared with measurements from differential scanning calorimetry (DSC). Secondly, the presence of additional elements in multicomponent Al-Si alloy systems allow many complex intermetallic phases to form, which make microstructural characterisationn on-trivial, as some of the phases have either similar crystal structures or exhibit subtle changes in their chemistries. A combination of electron backscatter diffraction (EBSD) and energy dispersive X-ray analysis (EDX) have therefore been used for discrimination between the various phases. It is shown that this is a powerful technique for microstructure characterisation and provides detailed information which can be related to the microstructuree volution during initial casting and subsequent heat treatment. Additionally, the complex morphologies of intermetallics have also been observed using 3D X-ray tomography. In this present work, a number of different experimental techniques were used to provide a rapid means of phase discrimination in order to validate microstructural evolution models. Thirdly, the mechanical properties of individual intermetallics have been investigated as a function of temperature using high -temperature nanoindentation. In particular, the hardness and modulus of a number of phases have been measured for a range of alloy compositions. The creep behaviour of intermetallic phases was also investigated, since this is important in the determination of the high temperature mechanical properties of alloys. Fourthly, the coefficients of thermal expansion of intermetallic phases were measured by high temperature X-ray diffraction, and thermal expansion anisotropy was also explored to investigate the formation of microcracking. Finally, in order to investigate the effect of both applied mechanical and thermal loads on the formation of cracks, Eshelby modelling has been used to predict the internal stresses of the different intermetallic phases and alloys, with the aid of the experimental data obtained in this work. The phase identity, composition, and the corresponding physical and mechanical properties can be used to inform alloy design strategies which, may facilitate the development of new alloys with improved properties.