Nanocrystal structure at the stages prior to crystallization of amorphous Fe73.5Si13.5B9Cu1Nb3

The atomic structure of Fe73.5Si13.5B9Cu1Nb3 (FINEMET) alloy was investigated in accordance with the temperature and time of annealing by means of X-ray diffraction, small-angle X-ray scattering, and transmission electron microscopy (TEM). The structure of as-quenched amorphous state is microheterogeneous with the sire of heterogeneities of about 2 nm. Transformation from amorphous to nanocrystalline state includes the intermediate stage, when precursor-type nanocrystals are being formed. The sire of alpha-Fe(Si) precursor-type nanocrystals increases up to 3.5-4 nm in this intermediate region. At the same time, the intensity of small-angle scattering is raised, micro-microdiffraction pattern becomes more distinct, and average neighbouring interatomic distance is decreased by about 2%. The structure state of precursor-type nanocrystals is characterized by definite series of predominant interatomic distances. Such series appears to be intermediate between observed for amorphous and crystalline states remaining close to the b.c.c. type topology. The intermediate stage is ended by appearance of the first crystalline sign on the diffuse X-ray diffraction pattern. Structural characteristics get out on saturation with the time of annealing. Partly the size of alpha-Fe(Si) nanocrystals reaches the maximum value of about 4 nm at 430 degrees C and 9 nm at 550 degrees C. Structure of nanocrystals is ordered as D0(3) type not only at 550 degrees C, but weak signs of the order are observed at 430 degrees C too. At the initial stage (to similar to 4 nm), the growth of nanocrystals is not described by the relationship as d(t) similar to t(1/2), so it is not diffusion-controlled. Both an ousting of Nb by the interface and ifs accumulation on the front of crystallization depress the process of alpha-Fe(Si) nanocrystals' growth and promote formation of Nb-rich regions. The size of heterogeneities is about 3.5 nm in such Nb-rich regions at 550 degrees C in the range of saturation.