Crystallization of cobalt amorphous alloys under field annealing

Crystallization of Co-rich amorphous ribbons annealed under a 10 Oe external magnetic field at the early 30 minutes from their glassy status to supercooled liquid status is investigated by high-resolution transmission microscope (HR-TEM), Selected Area Fourier Transform (SA-FT), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results indicate that the short-rang ordering feature can be refined very well in specimen annealed under temperatures about 87.4 degrees C below their glass transition (Tg), showing refined salt-pepper morphologies with a mean length changing from 1.2 +/- 0.8 nm to 0.8 +/- 0.2 nm and a mean width shifting from 0.5 +/- 0.2 nm to 0.3 +/- 0.1 nm. When the amorphous ribbons are field-annealed at temperature near to Tg (i.e., 450 degrees C), ultra-fine nanocrystalline structures can be formed on the top surface of ribbons with size of 3.5 +/- 0.5 nm and inter-grain spacing of about 0.4 +/- 0.2 nm even though the inner parts of the ribbons are still in amorphous phases. The nanocrystalline areas are featured by the formation of doped hcp cobalt phase orientated along the c-axis, with the inter-plane spacing ranging from 4 A to 6 A. When the annealing temperature is above Tg, the grain sizes are increased dramatically with multi-phased nanocrystals precipitating from the amorphous substrate, and finally reaching almost complete crystallization at 600 degrees C, causing greatly coarsening of the nanocrystal structures.