Nucleation and growth mechanisms of nano-scaled Si precipitates in Al-7Si supersaturated solid solution

A dense uniform distribution of nano-scaled Si precipitates has been achieved in Al-7Si alloy by high pressure solution treatment (HPST) and aging treatment. Precipitation behavior of Si phase was investigated using transmission electron microscopy (TEM). The results reveal that Si clusters initially precipitate from the Al(Si) solid solution, providing preferable nucleation sites for the equilibrium Si phase. The Si crystals exhibit spherical shape at first, and then grow rapidly parallel to {111}_Si planes. The shortage of growth ledges on {111} interfaces leads to the inhibition of growth perpendicular to these planes, which results in Si triangles and platelets with high aspect ratio. An equation about the relationship between the relative free energy and the equilibrium ledge densities on Al-Si interfaces was proposed.