Enhanced ductility in Al-Si-Cu-Mg casting alloys with high Si content

It has recently been suggested that the beta-Al5FeSi and theta-Al2Cu phase intermetallic particles are refined and dispersed in the presence of high silicon, hence improving the ductility of Al-Si-Cu-Mg alloys. However, no metallographic evidence was presented to support these claims. Therefore, a study into the effect of Si content in the range 4.5 - 9%, on the morphology and distribution of Fe-rich and Cu-rich intermetallic phases has now been conducted. It is shown that Si indeed exerts a refining effect on the beta-Al5FeSi phase platelets while it disperses the Cu-rich intermetallic particles. In alloys with low Si content, the Fe- and Cu-rich particles form long and closely intertwined clusters. Microcracks originating in cracked intermetallics extend and propagate along the clusters with little plasticity, hence the low ductility of the alloys. At high Si contents, the intermetallic phases appear more dispersed and the clusters of particles are small and isolated from each other. Microcracks resulting from the cracking of the intermetallics are short and isolated as well, hence the increased ductility of the alloys. The mechanisms by which the refining and dispersion of intermetallic phases occur are discussed.