Metastable liquid phase separation in Co - Cu alloys.

Two Co – Cu alloys were studied by drop tube processing technique in a view of investigating the effects of rapid solidification on the phase transformations and microstructural evolution in the metastable alloys. The as – solidified samples had diameters ranging from 53 – 850+ μm and these were analysed using various characterization techniques such as optical (OM) and scanning electron (SEM) microscopy, x- ray diffraction (XRD) and differential thermal analysis (DTA). The Cu – 50 at. % Co alloy was observed to experience liquid phase separation at lower undercooling than the Cu – 68.5 at. % Co alloy. This is found to be in accordance to the asymmetrical metastable miscibility gap determined for the alloy system. Significant number of liquid phase separated structures were observed at cooling rates in excess of 15000 Ks-1, evidenced by a range of microstructural morphologies including stable core shell structures, evolving core shell structures and structures in which the demixed liquid phases were randomly distributed. A large number of these structures experienced multiple liquid phase separation processes. The configuration of the core shell structures were found to be independent of the composition of phases and their relative abundance, with the core always formed by the higher melting point phase. The optimum production of the core shell structures were found to be a function of cooling rate.