KINETICS OF THE AMORPHOUS-TO-NANOCRYSTALLINE TRANSFORMATION IN FE73.5CU1NB3SI13.5B9

Isothermal measurements of the electrical resistance R were performed in samples of the alloy Fe73.5Cu1Nb3Si13.5B9 at temperatures where the nanocrystalline phase develops. At each temperature, nanocrystallizat ion is associated to a very slow decrease of R(t) compared to conventi onal crystallization processes. X-ray-diffraction data were obtained o n similar samples annealed at the same temperatures for selected times . The evolving nanocrystalline fraction turns out to be essentially pr oportional to the variation in the electrical resistance. The form of the kinetic law describing the R(t) decays is obtained by making use o f a novel procedure, developed to analyze time-dependent properties ch aracterized by a complex behavior. Nanocrystallization in the consider ed alloy appears as initially described by a Johnson-Mehl-Avrami kinet ics with exponent n congruent-to 4, suggesting that the early stages o f the process involve homogeneous nucleation and three-dimensional gra in growth. For longer times, corresponding to the hearth of the nanocr ystallization process, and at all temperatures, the functional form of the kinetic law drastically changes to a much slower power law of the type R is similar to At(-alpha). The exponent alpha turns out to be e ssentially independent of temperature. Such a change presumably reflec ts a reduction in the grain growth velocity and/or in the nucleation r ate.