Rapid crystallization of amorphous CoZr and FeB close to eutectic compositions

We present in situ time-resolved X-ray scattering data and transmission electron microscopy studies on ribbons of Co92Zr8 and Fe81.5B18.5. Both systems exhibit a profound change as the crystallization temperature, and hence transformation rate, is raised. At low temperatures (half-time t 1 2 for crystallization of the order of minutes), crystallization occurs by eutectic growth of two crystalline phases; at high rates (t 1 2 of the order of seconds) the mode changes to simultaneous polymorphic crystallization of the two phases, with no eutectic growth. It is suggested that the change of mode occurs when the temperature exceeds the glass transition temperature. © 1994.