Co-Si金属间化合物深过冷快速凝固及组织演化机理研究

Mechanism of Rapid Solidification and Microstructure Evolution of Co-Si Intermetallic Compounds at High Undercooling

目的探究Co-Si金属间化合物在深过冷条件下产生的再辉现象及对应组织的演化阶段,并分析其形成机理。方法为获得深过冷,结合了两种热力学方法:循环过热法及熔融玻璃净化法,实现了Co2Si金属间化合物(Co-33at%Si)、CoSi金属间化合物(Co-50at%Si)的深过冷快速凝固,并通过光学显微镜、扫描电子显微镜和XRD进行分析。结果发现Co-33at%Si金属间化合物凝固过程中存在两次再辉现象,晶粒尺寸随着过冷度的增加而减小,ΔT=190 K时,二次枝晶间距大约为0.5μm,层片间距为0.289μm,而Co-50at%Si金属间化合物凝固过程仅有一次再辉现象,且随过冷度增加,晶粒尺寸减小。ΔT<50K时,微观组织形貌为树枝晶;ΔT>90 K时,微观组织形貌为细小等轴晶。结论 Co-33at%Si中一次再辉对应于初始枝晶形成及细化过程,二次再辉对应于晶粒间残余液相二次凝固,即晶间细小树枝状共晶的形成。而Co-50at%Si中唯一一次再辉现象则对应枝晶向等轴晶的转变。

The aim of this article is to research the recalescence of Co-Si intermetallic compounds at high undercooling and the evolutionary phase of corresponding organization, and analyze its formation mechanism. By employing the glass fluxing technique in combination with cyclical superheating, the high undercooling rapid solidification of Co2 Si intermetallic compounds(Co-33 at%Si) and CoSi intermetallic compounds(Co-50 at%Si) was accomplished. Two recalescence phenomena were found in the solidification process of Co-33 at% Si intermetallic compounds while a single one was found in Co-50 at%Si. The grain size decreased with the increase of the condenser depression. The secondary dendrite arm space was approximately 0.5 μm and the layer spacing measured was 0.289 μm when ΔT=190 K. Likewise, the microstructure of Co-50 at%Si when ΔT<50 K was dendrite while it transformed into fine equiaxed crystal if ΔT>90 K. In the solidification process of Co-33 at%Si intermetallic compounds, the first recalescence corresponds to the initial dendrite formation and the refining process, secondary recalescence corresponds to intergranular residual liquid’s secondary coagulation, namely the formation of the intergranular fine dendritic eutectic. In contrast, the solidification process of Co-50 at%Si has only one recalescence phenomenon, which corresponds to the transformation of dendrites to isometric crystals.