This Al 12.7wt%Si alloy has been modified with 0.02wt%Sr. This has the effect of refining the microstructure in a similar manner to increasing the cooling rate. Modified alloys have higher toughness and improved tensile strength and ductility in comparison to unmodified alloys.
A similar effect can be achieved using sodium. However, strontium is less easily lost by evaporation or oxidation during processing.
The addition of strontium has also shifted the eutectic composition slightly from 12.7wt% Si. This alloy now contains a small fraction of primary aluminium dendrites. Primary silicon should be avoided in these alloys since the brittle silicon reduces the tensile strength, ductility and fatigue resistance.
Compare the microstructure with the unmodified 12.7wt% Si eutectic alloy.