Analysis of eutectic silicon modification during solidification of Al-6Si using in-situ neutron diffraction

Aluminum-silicon alloys contain coarse, acicular Si plates that are detrimental to material properties. However, trace additions of Sr have been found to modify the eutectic Si to have a fine, fibrous structure, which improves elongation, strength, and thermal conductivity. In this study, in-situ neutron diffraction was conducted during the solidification of unmodified and Sr-modified binary Al-6 wt.% Si to obtain a novel view of the modification mechanism. Neutron diffraction intensity data was collected at temperatures ranging from 660 °C to 200 °C and was integrated to create fraction solid curves for the individual Al and Si phases in each alloy. Scanning electron microscopy revealed that the Sr-modified alloy microstructure contained many micro-sized Al-Si-Sr intermetallics scattered about the eutectic regions at the Al-Si interfaces, and a few Si flakes close to some β-(Al,Si,Fe) intermetallics, suggesting important interactions between both the Fe-bearing phases and Sr in the solidification of eutectic Si. Comparing the isolated phase evolution temperatures for each alloy revealed in-situ for the first time that Sr suppresses the nucleation temperatures of both the eutectic Al and eutectic Si phases by several degrees. Accordingly, the eutectic reaction was observed to evolve at a higher primary Al solid fraction with Sr modification.