Undercooling of Cu-Co alloys -: Phase morphology in the metastable miscibility gap

The Cu-Co system shows a metastable miscibility gap in the range of the undercooled melt. The binodal separating the (alpha Co + liquid) region and the metastable region have been determined by differential thermal analysis (DTA) with high precision. Electromagnetic levitation (EML) and drop tube experiments have been used to examine the metastable state of Cu-Co alloys. The experiments show that all the three methods allow deep undercooling of the melt into the range of the miscibility gap. Due to the deep undercooling the velocity of the solidification front is very high and the actual microstructure is frozen in during rapid crystallization. The process of demixing can be observed. The microstructure of samples processed in the electromagnetic levitation shows an influence of the electromagnetic stirring due to the induction of electric currents into the melt. Drop tube experiments, which lead to a rapid solidification under reduced gravity conditions, in contrary result in a homogeneous distribution of fine spherical particles of the minority phase. For this reason space experiments under microgravity conditions in the MSL-EML facility are under consideration. In these experiments the stirring effect would be greatly reduced compared to the terrestrial EML.