Solidification microstructure selection in aluminium-based alloys.

Iron is the most common impurity found in aluminium. It has a high solubility in molten aluminium and is therefore readily dissolved at all molten stages in production. The solubility of iron in the solid state is, however, very low (0.04%)1. As a result most of the iron present above this level appears as an intermetallic second phase in combination with aluminium and other elements. Numerous AI-Fe and AI-Fe-Si intermetallics have been identified in as-cast commercial purity aluminium and these have been the subject of extensive research because they can influence the material behaviour during subsequent fabrication steps and may have a critical influence on material quality. It is, therefore, of considerable technological interest to be able to exert control over the formation of these phases especially in dilute aluminium alloy compositions. This can be done most readily by controlling cooling rate during solidification or by modification of alloy chemistry to enhance the stability of a desired phase. Several authors have dealt with the effects of further additions on phase stability in the binary AI-Fe alloy system. In particular the effect on phase stability of other transition metals which are known from studies of their equilibrium behaviour to be at least partially soluble in binary AI-Fe phases have shown that additions of these elements can cause significant changes in phase stability with respect to both the cooling rate during solidification and solidification front velocity. It has also been suggested on a number of occasions that the discrepancies often found when comparing the results of published work on both the binary AI-Fe and ternary AI-Fe-Si systems, are attributable even to trace levels of impurity in the compositions investigated. As a result the relative effects of each component present on phase incidence and stability remains uncertain. This is especially true of ternary compositions with low Si:Fe ratios and AI-rich quaternary AI-Fe-Si-Mg alloys where a clear picture of phase incidence versus solidification cooling rate has yet to emerge. To date no systematic investigation has been conducted into the cumulative effect of common alloying additions and impurities on intermetallic phase stability in AI-base alloys. One objective of this thesis work was to determine the effect on phase stability and incidence of cumulative additions of 0.1 %Si, 0.75%Mg and 0.04%Cr to a base composition of AI-0.5wt.%Fe over the range of solidification front velocity between 0.05 and 2.00mmls during steady state growth at an imposed temperature gradient of 8 to 12 Klmm. The effect of minor changes in Fe, Si and Mg content on intermetallic phase selection in AI-Fe-Si-Mg alloys was also investigated under these imposed solidification conditions. Chapter 2 provides a review of the phases reported to have been formed in dilute AI-Fe, AI-Fe-Si and AI-Fe-Si-Mg alloys and the local solidification conditions reported as being necessary for their formation. In addition, the effects reported in the available literature of low levels of impurities on the stability and morphology of the intermetallics will be surveyed. Chapter 3 describes the experimental procedures used including alloy preparation, directional solidification, and the techniques employed in the identification of the intermetallic phases formed. Chapter 4 presents the Results and Chapter 5 is the Discussion which attempts to interpret these results and to draw conclusions concerning their implications which are detailed in Chapter 6