通过电磁悬浮(EML)熔炼设备对Al-70%Si合金进行深过冷处理, 利用高速摄影仪(HSC)和SEM分别对凝固过程和凝固后的组织进行了观测, 研究了不同过冷度下初生Si的生长规律. 结果表明, 过冷度对初生Si的生长有很大影响. 当过冷度较小时, 初生Si为粗大的长条状, 有特殊的边和面, 且具有明显的孪晶痕迹, 表现出小平面生长的特征; 当过冷度较大时, 初生Si为细小的枝晶和球状晶, 表面光滑, 表现出非小平面生长的特征; 当过冷度居于中间时, 初生Si为粗大的块状和规则排列的枝晶状, 块状有特殊的边和面, 枝晶表面光滑, 表现出小平面和非小平面混合生长的特征. 随着过冷度的增加, 初生Si的生长方式由小平面生长转变为中间方式生长, 再由中间方式生长转变为非小平面生长, 生长方式间发生转变的临界过冷度分别为122 和230 K.

High undercooling processing has long been studied, since crystal growth mode and microstructural evolution are dependent on the undercooling, ΔT. However, in traditional casting, the container wall acts as heterogeneous nucleation site and specimen undercooling is low, which makes it difficult to experimentally reveal the relationship between the crystal growth behavior and undercooling. In order to achieve different undercooling ranging from low to high, many methods have been proposed, such as drop-tube processing, flux processing and electromagnetic levitation (EML). The container wall effects on the purity of the specimen and on the heterogeneous nucleation of undercooled melt can be removed in these methods. Hence, melts can solid in homogeneous nucleation way and achieves high undercooling. Moreover, EML suspends melt droplet stably, and a freely suspended droplet gives the extra benefit to directly observe the solidification process by combining the levitation technique with proper diagnostic means. In this work, Al-70%Si alloy was undercooled by a laser heating EML. The solidification behavior of Al-70%Si alloy melts at different undercooling conditions was investigated during the solidification process by employing a high-speed camera (HSC). After the melts solidification, morphology on the surface of the samples was examined by SEM. The results show that undercooling has great effects on the growth of Si. The primary Si phases are coarse strip with special edges and faces, have obvious traces of twin, and show facet growth characteristic at low undercooling condition. However, the primary Si phases are dendrites and spherulites with smooth surface, and show non-faceted growth characteristic at high undercooling condition. Besides, the primary Si phases are coarse bulks with special edges and faces and dendrites with regular arrangement at moderate undercooling condition, which is the intermediary growth characteristic. As the undercooling increases, the primary Si is refined remarkably and the growth mode changes from facet growth to intermediary growth, and from intermediary growth to non-faceted growth. The critical undercoolings for the transition are 122 and 230 K, respectively. Furthermore, the critical undercoolings were also theoretically calculated using the physical and chemical parameter of Si, which are 108 and 209 K, respectively.