Nanocrystallization in FINEMETs

X-ray diffraction analysis (including small-angle X-ray scattering (SAXS)) was used to study nanocrystallization in the amorphous Fe73.5Si13.5B9CU1Nb3 alloys upon isochronous and isothermal annealings at 430 and 550 degreesC. The beginning stages of the transformation are characterized by transient temperature-time ranges in which the structural parameters of the newly for med alpha -Fe(Si)-phase grains gradually reach values typical of a truly cr ystalline state only at a grain size of similar to4 nm. The kinetics of iso thermal transformations is described by Avrami exponents n = 0.4-0.9. These magnitudes are substantially lower than that (n = 2.5) predicted theoretic ally for phase transformations that occur via the mechanism of nucleation a nd growth. The peculiarities of the transformation kinetics and the experim entally observed nonmonotonic behavior of the temperature and time dependen ces of the SAXS intensity suggest that, upon the nanocrystallization in FIN EMETs, it is the diffusional growth of nanocrystals (of more than 3 nm in s ize) that is the determining factor rather than nucleation and growth. The relation between the microheterogeneous (nanocluster) structure of melts an d of amorphous alloys produced by melt quenching is discussed; the specific features (caused by the microheterogeneous structure) of the transformatio n of FINEMETs to the nanocrystalline state are also considered.