M agnesium alloys, as the lightest structural metal, show great potential in the automotive, aerospace and electronics industries due to their perfect combination of properties, including low density, high specific strength and specific stiffness, super damping capacity and good castability [1][2][3][4] . Among all the magnesium alloys, MgAl alloys are most widely used due to their low cost and good mechanical properties at room temperature [5] . However, when the content of Al exceeds 9wt.%, the β-Mg 17 Al 12 phase with low melting point tends to form a network along the grain boundaries, which is detrimental to the mechanical properties. Previous study suggested that the network structure of β-Mg 17 Al 12 phase in high-Al magnesium alloys (Al>9%), such as Mg-15Al and Mg12Al, can be broken by the equal channel angular pressing (ECAP), making the plasticity and strength increase simultaneously [6][7] . In addition, with the increase of Al content, the corrosion resistance of Mg-Al alloys was Abstract:The effects of Sb and Sr on the modification and refinement of Mg 17 Al 12 and Mg 2 Si phases in Mg12Al-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 Mg 17 Al 12 phase was refined and broken into some discontinuous island structures. However, some network Mg 17 Al 12 phases still can be detected in Mg-12Al-0.7Si-0.09Sr alloy. Therefore, Sb performs better in modification and refinement of Mg 17 Al 12 phase than does Sr. Small amounts of fine polygonal shaped Mg 2 Si phases were found in Mg-12Al-0.7Si-0.5Sb alloy, while the morphology of Mg 2 Si 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 Mg 2 Si 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 ºC.
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