Amorphous to crystalline transformation ofFe80B20

Abstract

The transformation from the as-quenched amorphous to the crystalline state of ${\mathrm{Fe}}_{80}$ ${\mathrm{B}}_{20}$ was followed isothermally at relatively low temperatures (580 to 630 K) with Mössbauer-effect spectroscopy (MES). Both the Allied Chemical Metglas 2605 ${\mathrm{Fe}}_{80}$ ${\mathrm{B}}_{20}$ and twin-roller quenched (RQ) ${\mathrm{Fe}}_{80}$ ${\mathrm{B}}_{20}$ were studied. Before the onset of crystallization the Mössbauer recoil-free fraction of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclei was constant but it increased by about 15% during crystallization. This implies that on the average the Fe atoms are more firmly bound in the crystalline than in the amorphous state. From a combination of MES, x-ray diffraction, and optical microscopy it was found that (a) the MG 2605 ribbons are rather inhomogeneous in the sense that 10% to 15% of the ribbon cross section is considerably more resistant to crystallization than the remaining part, (b) no such inhomogeneity is present in the RQ ribbons, (c) the crystallization takes place by the eutectic type of reaction, the crystallization products are $\alpha$-Fe and the (metastable) tetragonal ${\mathrm{Fe}}_3$B compound, and (d) a quantitative analysis of the kinetics of the transformation suggests that crystallization occurs only by the growth of (crystalline) nuclei which are already present in the as-quenched amorphous material.