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 $\varepsilon$Co and Co3Si phases at small undercooling. The Co3Si phase cannot decompose completely into $\varepsilon$Co and $\alpha$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 $\varepsilon$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 $\varepsilon$Co and Co3Si phases is controlled by solutal diffusion.
Received: 11 April 2003
Revised: 27 May 2003
Accepted manuscript online:
PACS:
81.20.Ev
(Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation)
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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