Liquid–liquid demixing and microstructure of Co–Cu–Zr alloys with low Zr content

Liquid–liquid phase separation and its effect on the microstructure has been investigated along the quasi-binary line (Co40Cu60)100−xZrx with x = 2, 4, 6, 9 and additionally for (Co50Cu50)94Zr6 and (Co60Cu40)94Zr6. The elemental distributions and the microstructures were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy for samples that were (i) processed by thermal cycling in alumina crucibles at 10, 20 and 30 K/min with simultaneous differential thermal analysis, (ii) rapidly quenched by single-roller melt spinning and (iii) quenched after having been electromagnetically levitated at various temperatures. The metastable miscibility gap of the binary Co–Cu system with phase separation into Co- and Cu-rich liquids transforms into a stable miscibility gap for Zr contents 3 < x < 7.5 with separation into Co/Zr-rich and Cu-rich liquids. In contrast to the binary Co–Cu system where the Cu-rich liquid phase always surrounds the Co-rich phase, the Zr addition modifies the surface tension energies and/or wetting behavior in a peculiar way so that the Co/Zr-rich phase always encloses the Cu-rich liquid phase concerning the ternary Co–Cu–Zr system in that compositional range. The macrosegregation morphologies of the liquid phase separation that built up via Ostwald ripening, gravity-driven convection, collision, coalescence and wetting effects proceed on a very short time scale and even samples that have been prepared by rapid quenching techniques still exhibit phase separated regions in the micrometer regime.