Abstract The Co-12%Si hypoeutectic, Co-12.52%Si eutectic and Co-13%Si hypereutectic alloys are rapidly solidified in a containerless environment in a drop tube. Undercoolings up to 207K (0.14T_E) are obtained, which play a dominant role in dendritic and eutectic growth. The coupled zone around Co-12.52%Si eutectic alloy has been calculated, which covers a composition range from 11.6 to 12.7%Si. A microstructural transition from lamellar eutectic to divorced eutectic occurs to Co-12.52%Si eutectic droplets with increasing undercooling. The lamellar eutectic structure of the Co-12.52%Si alloy consists of Co and Co3Si phases at small undercooling. The Co3Si phase cannot decompose completely into Co and Co2Si phases. As undercooling becomes larger, the Co3Si phase grows very rapidly from the highly undercooled alloy melt to form a divorced eutectic. The structural morphology of the Co-12%Si alloy droplets transforms from Co primary phase plus lamellar eutectic to anomalous eutectic, whereas the microstructure of Co-13%Si alloy droplets experiences a `dendritic to equiaxed' structural transition. No matter how large the undercooling is, the εCo solid solution is the primary nucleation phase. In the highly undercooled alloy melts, the growth of Co and Co3Si phases is controlled by solutal diffusion.
Fund: Project supported by the National Natural Science Foundation of China (Grant Nos 50221101, 50291015 and 50271058), Fok Ying-Tong Education Foundation, China (Grant No 71044) and Doctorate Foundation of Northwestern Polytechnical University, China (Grant N
Cite this article:
Yao Wen-Jing (姚文静), Wei Bing-Bo (魏炳波) Microstructural evolution during containerless rapid solidification of Co-Si alloys 2003 Chinese Physics 12 1272
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