Comparison of nanocrystallization processes in Fe73.5Cu1Nb3Si(22.5-x)Bx for x=7 and x=9

We have performed a comparative DSC study of the isothermal enthalpy changes in Finemet Fe73.5Cu1Nb3Si(22.5-x)B-x alloys for x=7 and x=9. The kinetics of nanocrystallization are significantly accelerated when the silicon content is increasing. Similarly, the crystallized fraction reaches its asymptotic value more rapidly and this latter is higher whatever is the isotherm temperature. From XRD measurements we observe that the mean grain radius levels off also more quickly in the alloy richer in silicon. By modelling we obtain relatively good fits with all experimental data. The acceleration of the nanocrystallization kinetic can be interpreted in terms of a lower stability of the amorphous phase which is poorer in boron. The enhancement of nucleation and growth rates in the silicon rich alloy, driven by thermodynamics, seems to overcome more easily the niobium barrier formation which limits the mean grain size.