Effects of minor Sr addition on microstructure and mechanical properties of the as-cast Mg-4.5Zn-4.5Sn-2Al-based alloy system

Effects of minor Sr addition (0, 0.2, 0.4, 0.6 and 1.0 mass%) on microstructure and mechanical properties of the as-cast Mg-4.5Zn-4.5Sn-2Al-based alloy system are investigated by SEM, XRD and mechanical testing. The alloy castings are produced by water-cooling steel casting. The Sr-free alloy is composed of four phases i.e. α-Mg, Mg2Sn, Mg51Zn20 and Mg32(Al, Zn)49, while the alloys with the Sr addition of 0.6% or higher consist of α-Mg, Mg2Sn, Mg51Zn20, Mg32(Al, Zn)49 and MgSnSr. Minor Sr addition can effectively refine grains, dendrites and grain-boundary compounds and this effect is more obvious with a higher Sr addition. However, a smaller amount of grain-boundary compounds and a larger amount of the intra-crystalline intermetallics are detected with a higher Sr addition, seemly indicating that the minor Sr addition plays a role in suppressing the eutectic transformation and promoting the formation of the MgSnSr phase. The as-cast alloy with 0.2%Sr addition exhibits the best combined mechanical properties at ambient temperature with the ultimate tensile strength and elongation of 238 MPa and 12.1% respectively. However, excessive Sr addition results in the decline of strength and plasticity. Fractographic analysis demonstrates that quasi-cleavage fracture is the dominant mechanism of these alloys. Furthermore, the Sr addition in the range of 0.2% and 1.0% has no obvious beneficial effect on the ultimate tensile strength of the as-cast alloys and leads to plasticity loss at 175 °C.