Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al-Si-based alloys

The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al-5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al-5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the alpha-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}(alpha-Al) planes from supersaturated solid solution in the alpha-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the alpha-Al matrix still remained cube to cube. The beta-Al5FeSi intermetallic was also observed, depending on subsequent controlled cooling.