Amorphous-to-crystalline transformation of ${\mathrm{Fe}}_{82}$${\mathrm{B}}_{12}$${\mathrm{Si}}_6$

The transformation from the as-quenched amorphous to the crystalline state of ${\mathrm{Fe}}_{82}$ ${\mathrm{B}}_{12}$ ${\mathrm{Si}}_6$ (METGLAS® 2605S) has been investigated by Mössbauer spectroscopy, x-ray diffraction, and density measurements. The first step of the transformation is found to be structural relaxation during which atomic rearrangements towards a more stable amorphous state take place. In the second step, crystalline Fe-9 at.% Si alloy precipitates out until the metastable amorphous phase ${\mathrm{Fe}}_{77}$ ${\mathrm{B}}_{18}$ ${\mathrm{Si}}_5$ is formed. The final products of crystallization are found to be Fe-9 at.% Si alloy and ${\mathrm{Fe}}_2$B. The distribution of the Si atoms amongst the bcc lattice points of the precipitated Fe-Si alloy is random at low annealing temperatures $T_A$. However, at higher $T_A$ short-range ordering of Si atoms appears and increases with increasing $T_A$. During crystallization the Mössbauer recoil-free fraction of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclei increased by 39%. The density also increased from 7.28 to 7.49 g/${\mathrm{cm}}^3$. These two observations imply that the Fe atoms are more firmly bound in the crystalline than in the amorphous state. During structural relaxation an increase of atomic ordering was observed; this is proposed as the origin of the small increase in the magnetic hyperfine field with increasing $T_A$ before the onset of crystallization.