Containerless rapid solidification of highly undercooled Co-Si eutectic alloys

Abstract

Droplets of Co–62.1%Si and Co–43.5%Si eutectic alloys with different sizes are rapidly solidified during containerless processing in drop tube. The microstructures of Co–62.1%Si eutectic alloy are mainly characterized by lamellar eutectic plus anomalous eutectic of CoSi2 and Si phases, whereas those of Co–43.5%Si eutectic alloy are mainly characterized by lamellar eutectic plus anomalous eutectic of CoSi and CoSi2 phases. For both alloys, the experimental results show that the microstructural evolution depends mainly on undercooling. In the case of Co–62.1%Si eutectic alloy, the smaller the droplet diameter, the larger the volume fraction of anomalous eutectic. When its diameter is very small, the droplet exhibits anomalous eutectic morphology entirely. As for Co–43.5%Si eutectic alloy, with the decrease of droplet size, the microstructural transition proceeds from lamellar eutectic to anomalous eutectic, even to dendrites. The nucleation rates of each phase have been calculated. The TMK eutectic growth and LKT/BCT dendritic growth theories are applied to analyze the rapid solidification process and investigate the microstructural transition mechanisms. The coupled zone around Co–43.5%Si eutectic alloy has also been calculated on the basis of TMK and LKT/BCT models, which covers a composition ranging from 40.8 to 43.8%Si. And calculated coupled zone of Co–62.1%Si covers a composition ranging from 60.2 to 81.6%Si.