Combined in-situ SAXS/WAXS and HRTEM study on crystallization of (Cu60Co40)1 − xZrx metallic glasses

CuZr as well as CoZr are well known metallic glass-formers in a wide compositional range. Since the binary Cu–Co system exhibits a metastable liquid–liquid miscibility gap, i.e. Cu and Co tend to separate from each other, the ternary Cu–Co–Zr system is a promising candidate to form phase separated glass–glass composites. In this work (Cu60Co40)1 − xZrx metallic glasses with relatively low Zr contents of x = 37 and x = 32 were prepared by melt spinning and investigated by in-situ small-angle and wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC). Certain heat treated samples were additionally investigated by high-resolution transmission electron microscopy (HRTEM). Even for x = 32 there are no indications for any kind of phase separation in the as-quenched state within experimental resolution, i.e. the critical temperature Tc for a liquid–liquid phase separation has already decreased from 1556 K for binary Cu60Co40 to a temperature below the glass transition temperature Tg = 762(5)K found for (Cu60Co40)68Zr32. Combined in-situ SAXS/WAXS and HRTEM investigations reveal that thermal annealing also does not induce an amorphous–amorphous phase separation. Instead the formation of nano crystallites of a so far unknown Cu-rich/Zr-poor phase with relatively low activation energy for crystallization Ea = 116(7) kJ/mol at temperatures far below the crystallization temperature deduced from DSC measurements is observed.