系统研究了Al含量对铸态和挤压态Mg-2.5Sn-2Ca-xAl (x=2、4、9，质量分数，%)合金微观组织和力学性能的影响。结果表明，随着Al含量的升高，合金的强度有所降低，延伸率增加。Mg-2.5Sn-2Ca-2Al、Mg-2.5Sn-2Ca-4Al和Mg-2.5Sn-2Ca-9Al合金的屈服强度分别约为370、325和290 MPa，延伸率分别约为6.2%、11.0%和12.0%。第四组元Al元素的加入改变了Mg-Sn-Ca合金中纳米尺寸第二相的类型和含量。Mg-2.5Sn-2Ca-2Al和Mg-2.5Sn-2Ca-9Al合金中分别形成高密度的G.P.区和Mg₁₇Al₁₂第二相，Mg-2.5Sn-2Ca-4Al合金中未见明显的纳米相析出。高密度的G.P.区阻碍再结晶晶粒长大的效率较Mg₁₇Al₁₂纳米相更为显著，因此Mg-2.5Sn-2Ca-2Al合金的再结晶晶粒更为细小(约0.5 μm)。同时TEM观察表明，Mg-2.5Sn-2Ca-2Al合金的晶粒内部还存在较高密度的位错，且这些位错一般与G.P.区伴生，因而合金内部还保留有较高密度的亚晶片层组织(片层厚度0.2~1.0 μm)。大量G.P.区以及残余位错的存在会成为新产生位错运动的障碍，这些均会对Mg-2.5Sn-2Ca-2Al合金高的屈服强度做出贡献，但同时也会损伤合金材料的塑性。在Mg-2.5Sn-2Ca-9Al合金中，由于高含量Al元素的存在以及Mg₁₇Al₁₂纳米相阻碍位错运动能力相对较弱，导致残余位错密度更低，所以Mg-2.5Sn-2Ca-9Al合金表现出了更大的晶粒尺寸、较低的屈服强度以及更高的塑性。

There is considerable demand for high-performance, low-cost, and rare-earth-free magnesium alloys in several industrial applications because of their energy conservation potential. However, the mechanical properties of the currently available rare-earth-free magnesium alloys cannot satisfy the industrial requirements. Therefore, a novel rare-earth-free magnesium alloy with high strength, excellent ductility, and good formability must be urgently developed. In this study, the microstructure and mechanical properties of the Mg-2.5Sn-2Ca-xAl (x=2, 4, and 9, mass fraction, %) alloys in the as-cast and extruded states when different amounts of Al content are added are systematically studied. As indicated by the results, the strength and elongation of the alloy decrease and increase, respectively, with the increasing Al content. The yield strengths of the Mg-2.5Sn-2Ca-2Al, Mg-2.5Sn-2Ca-4Al, and Mg-2.5Sn-2Ca-9Al alloys are approximately 370, 325, and 290 MPa, respectively, and their elongations are approximately 6.2%, 11.0%, and 12.0%, respectively. The type and content of the nanosecond phase of the Mg-Sn-Ca-based alloy changed because of the addition of the fourth type of Al element. High-density G.P. zones and a second phase of Mg₁₇Al₁₂ can be observed in the extruded Mg-2.5Sn-2Ca-2Al and Mg-2.5Sn-2Ca-9Al alloys, respectively; however, nanophase precipitation cannot be observed in case of the extruded Mg-2.5Sn-2Ca-4Al alloy. The high-density G.P. zones hinder the growth of the recrystallized grains more efficiently than the Mg₁₇Al₁₂ nanophase; thus, the recrystallized grains of the extruded Mg-2.5Sn-2Ca-2Al alloys are finer (approximately 0.5 μm) than the extruded Mg-2.5Sn-2Ca-9Al alloy. Based on TEM images, high-density dislocations can be observed inside the extruded Mg-2.5Sn-2Ca-2Al alloy grains and G.P. zones can be observed toward the side of the dislocations; thus, the high density subgrain lamella structure is retained in the alloy (lamella thickness: 0.2~1.0 μm). The movement of the newly generated dislocations is inhibited by the large number of G.P. zones and residual dislocations, increasing the yield strength and decreasing the plasticity of the Mg-2.5Sn-2Ca-2Al alloy. The Mg₁₇Al₁₂ nanophase that was formed in the Mg-2.5Sn-2Ca-9Al alloy because of the addition of high Al content exhibits a weak ability to hinder the movement of the dislocations, resulting in low-density residual dislocation. Therefore, the Mg-2.5Sn-2Ca-9Al alloy, exhibits a large grain size, low yield strength and high plasticity.