The complex characteristics of crystallization of the Fe75Si15B10 glassy ribbon

The Fe75Si15B10 glassy ribbon was examined in the course of various isothermal and dynamic heat treatments. The number, type and mechanism of formation of the crystallization products formed during two crystallization stages in this alloy were determined by differential scanning calorimetry (DSC), thermomagnetometry, transmission electron microscopy and X-ray diffraction measurements. It is concluded that the first DSC peak is due to the crystallization of alpha-Fe(Si) or Fe3Si and the composite microcrystals containing Fe3B cores with alpha-Fe(Si) envelopes. The metastable Fe3B subsequently transforms into the stable Fe2B in the second DSC peak. The remaining amorphous matrix crystallizes by the eutectic reaction also in the second DSC peak forming the Fe3Si and Fe2B eutectic structures. The apparent activation energy E-1* decreases during the first crystallization stage from 480 kJ (g atom(-1)), characteristic of the Johnson-Mehl-Avrami (JMA) nucleation-and-growth kinetics of the as-quenched sample, to 350 kJ (g atom)(-1) implying a dominance of the diffusion-controlled growth of iron in the already pre-nucleated sample. Simultaneously, the JMA exponent n(1) decreases from 2.5 to 1.5. The apparent activation energy E-2* depends on the temperature of the first crystallization stage. It decreases with increasing T-a from 384 to 282 kJ (g atom)(-1). This phenomenon was attributed to modification of the chemical composition of the remaining eutectic after the temperature dependent primary crystallization of Fe3B within the composites. The value of n(2) is 3.