SELF-DIFFUSION OF CO-58 IN AMORPHOUS CO79NB14B7 DURING ISOTHERMAL CRYSTALLIZATION

The self-diffusion of Co-58 radiotracer atoms in amorphous or isothermally crystallized Co79Nb14B7 was investigated using ion-beam sputtering for serial sectioning. As in other amorphous metallic alloys, the self-diffusivity D(T) in the as-produced melt-spun specimens decreased in the course of time to a final value that is characteristic of the relaxed amorphous state. The influence of isothermal crystallization at 713 K or 750 K on D(T) (663 K) was studied. The state of crystallization was monitored by transmission electron microscopy. Initially D(T) (663 K) decreases by two orders of magnitude as a function of the duration t of crystallization according to a t-1 law, and then shows a t-2/3 dependence. From this, a crystallization enthalpy of 2.8 eV was deduced. The diffusion enthalpy increases from 1.8 eV in the relaxed amorphous state to 2.2 eV after crystallization for 100 h at 750 K. It is proposed that diffusion occurs via a direct diffusion mechanism in both the relaxed amorphous and the nanocrystalline states.