Microstructure characterization of as-cast Al-Mg-Si alloys with high content Li element addition

The effects of adding a large quantity of Li on the microstructure and solidified precipitation behavior of an as-cast Al-Mg-Si alloy was investigated by thermodynamic calculation and microstructural characterization. The addition of Li substantially changed microstructural features and solidified precipitate phases of the Al-Mg-Si alloy. According to thermodynamic calculations, in addition to the Mg2Si phase, the second phases of Li-containing alloy also included delta-AlLi and Al2LiMg precipitates, in contrast to the base Al-Mg-Si alloy. Unusually, the Al2LiMg phase was found even when the content of Mg was below 2 wt%. The content, quantity, and distribution of the second phases in different alloys were revealed by optical microscopy, scanning electron microscopy and x-ray diffraction analysis synthetically. AlLiSi ternary compound was only found in the alloy with excess Si content. In addition, transmission electron microscopy observations were used to characterize crystal structures of the Li-containing phases. The results showed that a delta'-Al3Li metastable phase, instead of a delta-AlLi phase, was precipitated because of the occurrence of non-equilibrium solidification.