Microstructure of slow-cooled wedge-cast Cu58Co42 alloy with a metastable liquid miscibility gap

Abstract

The Cu-Co equilibrium phase diagram (figure 1) contains a metastable liquid phase miscibility gap where the liquid phase will separate into two, provided it can be undercooled sufficiently before solidification sets in [1]. This has been achieved by casting a Cu58Co42 alloy in a wedge-shaped mould where the as-cast microstructure showed evidence for liquid phase separation prior to solidification, e.g. formation of Co-rich droplets in a Cu- rich matrix [2,3]. Using backscattering SEM the dark Co-rich phase can easily be distinguished from the bright Cu-rich matrix (figure 2). The big dark particles are the Co-rich droplets that solidified after phase separation in the undercooled liquid. As cooling progressed in the liquid droplets the composition of the Co-rich phase would change and small droplets of Cu-rich liquid would form and be pushed towards the interior leaving the rim unaffected.