Microstructural investigation of Sr-modified Al-15 wt%Si alloys in the range from micrometer to atomic scale

Strontium-modified Al-15 wt%Si casting alloys were investigated after 5 and 60 min of melt holding. The eutectic microstructures were studied using complementary methods at different length scales: focused ion beam-energy selective backscattered tomography, transmission electron microscopy and 3D atom probe. Whereas the samples after 5 min of melt holding show that the structure of eutectic Si changes into a fine fibrous morphology, the increase of prolonged melt holding (60 min) leads to the loss of Sr within the alloy with an evolution of an unmodified eutectic microstructure displaying coarse interconnected Si plates. Strontium was found at the Al/Si eutectic interfaces on the side of the eutectic Al region, measured by 3D atom probe. The new results obtained using 3D atom probe shed light on the location of Sr within the Al-Si eutectic microstructure. -- Research highlights: → Microstructural investigation of Sr-modified Al-15 wt%Si alloy after 5 and 60 min of melt holding using 3D microscopy techniques. → FIB-EsB tomography: Fibrous eutectic modification of Si after 5 min of melt holding; unmodified eutectic microstructure after 60 min of melt holding. → Structural and chemical analysis of Fe-rich intermetallic phases using TEM: Fe-rich α-phases are present in the eutectic microstructure after 60 min of melt holding; no Fe rich α-phases are observed within the modified fibrous network after 5 min of melt holding. → 3D-AP analysis of specimens containing A/Si eutectic interfaces: enrichment of Sr is found at the Al/Si interface on the side of the eutectic Al region.