Three-dimensional visualization of the microstructure development of Sr-modified Al-15Si casting alloy using FIB-EsB tomography

Unmodified and Sr-modified (after 5 and 120 min of melt holding) Al-15Si alloys were investigated by transmission electron microscopy and focused ion beam tomography using energy-selective backscattered electrons for imaging. The three-dimensional visualization of the microstructure provided not only the true morphology of the Al-Si eutectic and Fe-rich intermetallics, but also allowed us to estimate their volume fractions. The evolution of Fe-rich alpha-phase morphology in the unmodified alloy proceeded during eutectic growth according to a model proposed. In the unmodified alloy, only Fe-rich alpha-phase was found, whereas in the Sr-modified alloy after 5 min of melt holding, two morphologies of Fe-rich phases were observed, namely a Fe-rich alpha-phase and a Fe-rich delta-phase. Both phases segregated mainly at modified eutectic grain boundaries. After 120 min of melt holding, the eutectic microstructure is similar to the unmodified structure again and only the Fe-rich alpha-phase could be observed. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.