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 α-phase morphology in the unmodified alloy proceeded during eutectic growth according to a model proposed. In the unmodified alloy, only Fe-rich α-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 α-phase and a Fe-rich δ-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 α-phase could be observed.