Note the unusual distribution of phases in the eutectic microconstituent.
The photomicrograph at left was taken at 50X magnification. Below, the magnification is 500X.
The etchant used here was a one percent solution of hydrogen fluoride (HF) in water. Don't try making or using this etchant without first preparing yourself for the extreme hazard to your person from the HF acid.
There is "too much" primary alpha for the given composition, so there has been eutectic divorcement, wherein the later alpha that formed was deposited on the primary alpha. Eventually, the eutectic formed in the classical coupled manner.
This alloy has, in addition to the 4.5% to 6.0% silicon, 0.6% copper and 1.0% iron. Silicon platelets are all that is left of the eutectic. Some precipitation has occured in the alpha.
Note that this alloy is intended to have both primary alpha and eutectic alpha plus silicon in the two-phase microstructure.
Specimen 2 is a higher silicon aluminum alloy.