Nanocrystallization mechanisms in Finemet-type alloys from calorimetric studies

Enthalpy changes during crystallization of Fe73.5Cu1Nb3Si13.5B9 alloy have been obtained by differential scanning calorimetry (DSC) using both isothermal and constant heating rate runs. Residual enthalpy contents after isotherms were measured with a constant heating rate up to 1000 K to determine the crystallized fraction. X-ray diffraction (XRD) on samples annealed in DSC apparatus were carried out to obtain the time dependence of mean grain size and crystallized fraction along isotherms. Taking into account these results, we propose a model where, below 800 K, the crystallization is mainly governed by nucleation and growth process and, above 800 K, by grain growth mechanism. This model gives good agreement with experimental data obtained both along isotherms, by assuming saturation of the nucleation sites, and, also with scan results, by using an activation energy for the growth rate. This formalism gives additional support to the current ideas on the role of copper and niobium atoms in the nanocrystallization of Finemet.