Effect of Bi and Zn Additions on the Microstructures and Mechanical Properties of As-Cast Mg–7Sn–2Al-Based Alloys

The microstructure and mechanical properties of as-cast Mg-7Sn-2Al-based alloys containing Bi and Zn were investigated to improve the microstructure and the strength of the alloys. The results revealed that the addition of Bi and Zn can refine the dendritic microstructure and the grain size, especially for the alloy with co-addition of Bi and Zn. The average grain size of Mg-7Sn-2Al, Mg-7Sn-2Al-2Bi and Mg-7Sn-2Al-2Bi-2Zn is 51.9, 43.5 and 28.1 μm, respectively. Besides the Mg 2 Sn phase, additions of Bi and Zn to Mg-7Sn-2Al lead to the formation of Mg 3 Bi 2 phase and promote the formation of Mg 2 Sn phases. Owing to the refinement of microstructure, the strength of as-cast alloys containing Bi and Zn is enhanced significantly, whereas the ductility has no obvious loss. The ultimate tensile strength and ultimate compressive strength of Mg-7Sn-2Al-2Bi-2Zn alloy are enhanced to be 198 and 404 MPa, which is increased by 53.5 and 35.6%, respectively, compared to that of the Mg-7Sn-2Al alloys. The final tensile elongation of the investigated alloys decreases slightly from 9.7% of Mg-7Sn-2Al alloy to 8.7% of Mg-7Sn-2Al-2Bi-2Zn alloy with the increase of the strength. However, the compressive elongation exhibits increase slightly from 18.6% to about 20%. The results indicate that the co-addition of Bi and Zn is beneficial to improve the microstructure and mechanical properties of alloys. The strengthening mechanism of the present alloys was discussed in this manuscript.