Kinetic aspects of nano-crystallization in Finemet-like alloys

Nano-crystalline magnetic alloys, are produced by partial crystallization of precursor amorphous alloy ribbons having specific compositions. The process consists typically of a primary crystallization process. In the present study, we analyze the methods to obtain kinetic parameters of the nano-crystallization process, taking into account the accompanying changes in the compositions of the phases. We have found a non-linear relationship between the evolved enthalpy, used in DSC experiments, and the transformed fraction. That can give false values of the pre-exponential factors in the Johnson-Mehl-Avrami framework, but does not influence the activation energy and the index of the transformation, n. We experimentally observed such a relationship in FINEMET(C)-type alloys. On the other hand, the free enthalpy difference between the amorphous matrix and the segregated crystallites changes during the process. This results in retardation of the kinetics, which can be considered as a change in the activation energy as a function of the transformed fraction, in good agreement with experimental results, which are sometimes assigned to a variable index n. Both isothermal and non-isothermal transformations are analyzed. (C) 2003 Elsevier B.V. All rights reserved.