Liquid-liquid phase separation in undercooled Co-Cu alloys

The liquid-liquid phase separation in undercooled Co-Cu alloy melts has been investigated by differential thermal analysis in combination with glass fluxing technique over a composition range of 16.0-87.2 at% Cu. The DTA signals, obtained during isochronous cooling, indicate that this separation process is exothermic and proceeds till the rapid solidification of Co-rich liquid phase occurs. The metastable miscibility gap that was determined directly and reproducibly from the onset temperatures of this process is slightly shifted to the Cu-rich side and roughly symmetrical about a Cu concentration of 53 at.%. The measured critical temperature of phase separation is 1547 ± 1.5 K and is about 108 K below the corresponding liquids temperature. In the present measurements the separated Co-rich liquid always solidified prior to the Cu-rich phase, which crystallized near the peritectic temperature. Lower surface tension and better wetting properties of the Cu-rich liquid phase with glass flux are responsible for the Co-rich phase to be always encased by the Cu-rich phase. In addition, thermodynamic calculations have been accomplished leading to a binodal line, which is in sufficient agreement with the experimental results.