Quantitative characteristics of the average size and the area fraction of Mg17Al12 phase were investigated in the center and the edge of the semi-continuously cast AZ61 alloy ingot by image analyzing technique. The differences of the morphology and the distribution of Mg17 Al12 phase in the surface and the center regions were observed by optical microscopy (OM) and scanning electron microscopy (SEM). The result shows that the morphology of dendrite controls the morphology and the distribution of Mg17Al12 phase, the more highly developed dendrite, the finer and the more disperse particles of Mg17AlI2. The Mg17Al12 is finer and more disperse in the center region compared to the surface of the ingot and distributes mainly in the interdendritic area. However, in the surface region most of the Mg17 Al12 particles cluster exhibits on grain boundary and this is contrast with the result of the permanent mould casting. The proportion of Mg17 Al12 phase in the eutectic increases with the cooling rate and it is larger in area fraction in the surface region than that in the center of the ingot. In addition, the back-diffusion in the solid has an effect on the area fraction of Mg17Al12 in the center of the ingot.%通过图像分析技术定量分析Mg17Al12相的大小和面积密度,并采用光学显微镜和扫描电镜观察Mg17Al12相在AZ61镁合金半连续铸锭的边部与心部形貌及分布上的差异.结果表明:枝晶形态可以控制Mg17Al12相的大小和分布,枝晶越发达Mg17Al12相形态越细小分布越弥散.铸锭心部的Mg17Al12比边部的细小弥散并且主要分布于枝晶间而边部的Mg17Al12相主要群聚分布在晶界处,这与普通的永久模铸的结果相反.随着冷却速率增加,Mg17Al12相在共晶中所占比例增加,铸锭边部Mg17Al12相的面积密度比心部的高.另外固相反扩散对心部的Mg17 Al12相的面积密度也有影响.
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