The effects of complex inoculants on the as-cast and solid-solution state microstructure of AZ91D magnesium alloy and the grain refinement mechanism were investigated by OM, SEM, EDS and XRD. The results show that the grain size of AZ91D magnesium alloy decreases effectively after the complex carbon inoculants are added. The best refinement result can be achieved when the mass ratio of MgCO3, C2Cl6 and SiC is set as 3:1:1. On one hand, the amount of Al4C3 particles induced by carbon-containing compound including MgCO3 and C2Cl6 can directly serve as the heterogeneous nucleus of primaryα-Mg phase. On the other hand, some of the fine Al4C3 and SiC particles are pushed by solid−liquid interface betweenα-Mg phases and the melt, and distribute along the crystal boundary, so as to impede the growth ofα-Mg grains, and theα-Mg grains are greatly refined finally.%采用OM、SEM、EDS、XRD等研究复合变质剂组成对AZ91D镁合金铸态和固溶态组织的影响，并分析晶粒细化作用机理。结果表明，适当加入三元复合碳变质剂可使 AZ91D 镁合金的晶粒尺寸显著减小，在相同的实验条件下，当3种碳化物组分MgCO3、SiC、C2Cl6的质量比为3：1：1时，能够获得最好的细化效果。一方面， MgCO3、C2Cl6在熔体中分解出的C与Al发生原位反应，生成的大量Al4C3颗粒均匀分散到熔体中，直接成为α-Mg的形核核心；另一方面，在α-Mg晶体生长过程中部分细小的Al4C3颗粒与SiC颗粒一起被固液界面推移至晶界，起到阻碍晶体长大作用，使α-Mg晶粒得到有效细化。
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