Modification and Refinement of Cast Al-Si Alloys

Abstract

In the Al-Si system, minor additions of metallic impurities, Na, Sr and others, modify the micro-structure of eutectic and primary silicon by inducing a high twin density, causing a morphological transition from flake-like to fibrous or spherical shapes. This modification should be distinguished from the inoculation or refining effects of other minor impurities, notably of phosphorous. The modified structural transition can be entirely explained in terms of growth kinetics modified by impurity induced twinning for the silicon phase and numerical modelling of the cooling curves shows that there need be no change in the nucleation behavior. The presence of elements such as phosphorous complicates the picture because in contrast to the former growth controlled process, the latter probably do alter the nucleation kinetics.