Influence of the rare earth concentration on the crystallization process of Fe-Dy-B amorphous alloys. Study of Fe74Dy6B20 and Fe70Dy10B20 alloys

Abstract

The crystallization behaviour of Fe74Dy6B20 and Fe70Dy10B20 amorphous alloys was carefully investigated by differential scanning calorimetry, Mössbauer spectrometry and x-ray diffraction up to 800 °C. Calorimetric studies were performed in limited temperature ranges that were progressively extended. For Fe74Dy6B20, after partial crystallization into the tetragonal Fe3B compound, the remaining amorphous part segregates into two amorphous `phases', respectively enriched and impoverished in dysprosium. Tetragonal Fe3B further transforms into orthorhombic Fe3B. Metastable Dy3Fe62B14 compound then forms from the Dy-impoverished amorphous fraction, and subsequent crystallization of the Dy1 + Fe4B4 phase occurs in the Dy-enriched fraction. Finally, Dy3Fe62B14 decomposes into bcc iron, Dy1 + Fe4B4 and iron borides. The nature of the first crystallization product suggests the existence of local environments of t-Fe3B type for this Dy concentration. The crystallization process of Fe70Dy10B20 strongly differs from that of Fe74Dy6B20. Segregation phenomena occur in the amorphous state prior to any crystallization. If the nature of the first crystallization product is assumed to be correlated with short-range order in the amorphous state, our results suggest that the local environments differ from those of Fe74Dy6B20, as they probably involve dysprosium atoms. This behaviour would agree with a previous Mössbauer study performed on the as-quenched amorphous alloys, providing evidence for a structural modification of the iron environments in the rare earth concentration range 8-9 at.%.