Non-isothermal nanocrystallization of Fe83.3Si4B8P4Cu0.7 (NANOMET®) alloy: modeling and the heating rate effect on magnetic properties

The kinetics of the nanocrystallization of a Fe83.3Si4B8P4Cu0.7 amorphous alloy by using differential scanning calorimetry has been investigated by non-isothermal annealing in a wide heating rate range from 10 degrees C min(-1) to 200 degrees C min(-1). The amorphous alloy was prepared by melt-spinning and showed a two-stage crystallization. In the first crystallization stage, alpha-Fe nanocrystals are formed. This phase is responsible for the good soft magnetic properties. The activation energy during the crystallization of the alpha-Fe nanocrystalline phase was determined from an isoconversional approach, and was found to be distinctively varying during the crystallization. An attempt was made to model the crystallization in an interval where the activation energy is relatively constant. The Malek criterion indicated that the Kolmogorov-Johnson-Mehl-Avrami model is not appropriate, whereas the Sestak-Berggren model meets the criterion. The acquired model shows good agreement with the experimental results. The magnetic properties of annealed ribbons at different heating rates show an initially steep decrease of the coercivity (H-c) from 82 A m(-1) at a heating rate of 10 degrees C min(-1) to 20 A m(-1) above 100 degrees C min(-1), whereas saturation magnetization (M-s) is practically invariant at ca 175 Am-2 kg(-1). This dependence is explained as due to diminishing grain size with higher heating rates, which is further supported by x-ray diffraction measurements.