Discussion of Effect of Strontium and Phosphorus on Eutectic Al-Si Nucleation and Formation of beta-Al5FeSi in Hypoeutectic Al-Si Foundry Alloys*

The nucleation of Si during the solidification of Al-Si hypoeutectic alloys appears to result from a hierarchy of nucleating substrates operating at progressively lower temperatures: (1) AlP, whose generally particulate morphology can initiate the formation of compact Si particles as seen in hypereutectic alloys; (2) oxide bifilms alone, whose planar form creates platelike Si morphologies; and (3) a currently unknown nucleant that initiates the coral eutectic growth morphology. The consequential growth forms are particulate when initiated on particles and coarse (unmodified) plates when initiated on oxide films. However, when oxide films are deactivated by Sr, eutectic is forced to grow at a lower formation temperature with a consequently fine coral morphology known as modified Si. Increasing Sr additions progressively eliminate each substrate in turn to effect the change from the unmodified to the modified structure.