The effects of Mn and Sn on the microstructure of Al-7Si-Mg alloy modified by Sr and Al-5Ti-B were studied. The results show that the well-modified eutectic Si structures can be obtained by adding different amounts of Sr; however, columnar dendrites structure is observed with high content of Sr, indicating a poisoning effect of the Al-5Ti-B grain refinement. In addition, Sr intermetallic compounds distribute near the TiB2 particles, which agglomerate inside the eutectic Si. The mechanism responsible for such poisoning is discussed. The addition of Mn changes the morphology of iron intermetallic compounds from β-Al5FeSi to α-Al(Mn,Fe)Si. Increasing the amount of Mn changes the morphology of α-Al(Mn,Fe)Si from branched shape to rod-like shape with branched distribution, and finally converts α-Al(Mn,Fe)Si to Chinese scripts. The microstructure observed by transmission electron microscopy (TEM) shows that Mg is more likely to interact with Sn in contrast with Si under the effect of tin. Mg2Sn compound preferentially precipitates among the Si-Si interfaces and Al-Si interfaces.
