Modification and refinement effects of Sb and Sr on Mg17Al12 and Mg2Si phases in Mg-12Al-0.7Si alloy

The effects of Sb and Sr on the modification and refinement of Mg17Al12 and Mg2Si phases in Mg-12Al-0.7Si alloy were investigated and compared. The microstructure and mechanical properties of Mg-12Al-0.7Si alloy and its modification mechanism by Sb and Sr were investigated using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and differential thermal analysis (DTA). The results indicate that by adding 0.5wt.% Sb to the Mg-12Al-0.7Si alloy, the Mg17Al12 phase was refined and broken into some discontinuous island structures. However, some network Mg17Al12 phases still can be detected in Mg-12Al-0.7Si-0.09Sr alloy. Therefore, Sb performs better in modification and refinement of Mg17Al12 phase than does Sr. Small amounts of fine polygonal shaped Mg2Si phases were found in Mg-12Al-0.7Si-0.5Sb alloy, while the morphology of Mg2Si phases in Mg-12Al-0.7Si-0.09Sr alloy changed from the coarse Chinese script shapes to fine granule and irregular polygonal shapes, indicating that the effects of modification and refinement on Mg2Si phase are more significant by adding 0.09wt.% Sr than 0.5wt.% Sb. The ultimate tensile strengths of the Sb and Sr modified Mg-12Al-0.7Si alloys were considerably increased both at room temperature and at 200 degrees C.