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, nanocrystallization is associated to a very slow decrease of R(t) compared to conventional crystallization processes. X-ray-diffraction data were obtained on similar samples annealed at the same temperatures for selected times. The evolving nanocrystalline fraction turns out to be essentially proportional to the variation in the electrical resistance. The form of the kinetic law describing the R(t) decays is obtained by making use of a novel procedure, developed to analyze time-dependent properties characterized by a complex behavior. Nanocrystallization in the considered alloy appears as initially described by a Johnson-Mehl-Avrami kinetics with exponent n congruent-to 4, suggesting that the early stages of the process involve homogeneous nucleation and three-dimensional grain growth. For longer times, corresponding to the hearth of the nanocrystallization 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 essentially independent of temperature. Such a change presumably reflects a reduction in the grain growth velocity and/or in the nucleation rate.