Undercooling, Liquid Separation and Solidification of Cu-Co Alloys

Large undercooling can induce not only various solidification pathways, but also a precursor reaction, or liquid separation. This paper deals with the latter effect of undercooling using examples of the Cu-Co system which has a flattened liquidus. Bulk Cu-Co alloys (about 7mm diameter) at compositions ranging from 10 to 90 wt pct Co were highly undercooled using a fluxing technique. Except for Cu-90 wt pct Co, liquid separation was directly observed as undercooling exceeded a critical value depending on the composition. It was also confirmed by a microstructural transition from dendrites to droplets above the critical undercooling. Finally, theoretical calculations regarding the metastable miscibility boundary and maximum droplet radius were made to analyze the experimental results.