Thermodynamic and kinetic factors controlling the formation of nanocrystalline FeCuNbSiB materials

A detailed study of nanocrystallization by thermal annealing of amorphous Fe73.5Cu1Nb3Si17.5B5 is presented. Experimental technique s include X-ray diffraction, transmission electron microscopy, Mossbauer spectroscopy and differential scanning calorimetry. Isothermal annealing in the temperature range 450-515 degrees C results in a sharp increase of the transformation rate and a subsequent slowing down of the process at long times (typically up to 4h). The evolution of the nanocrystalline phase has bee followed as a function of annealing time at 490 degrees C. The results show that a quasi DO3 structure emerges directly from the amorphous matrix and the faster changes correspond to an increase of the amount crystalline phase. the first stages of the crystallization process (up to 10% of crystalline phase achievement) under both isothermal and continuous heating regimes are controlled by homogeneous nucleation and three dimensional growth whose rates, and the viscosity, are calculated as a function of temperature. The further nanocrystallization is controlled by diffusion limited growth.