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

The crystallization behaviour of Fe74Dy6B20 and Fe70Dy10B20 amorphous alloy s was carefully investigated by differential scanning calorimetry, Mossbaue r spectrometry and x-ray diffraction up to 800 degrees C. Calorimetric stud ies were performed in Limited temperature ranges that were progressively ex tended. For Fe74Dy6B20, after partial crystallization into the tetragonal F e3B compound, the remaining amorphous part segregates into two amorphous 'p hases', respectively enriched and impoverished in dysprosium. Tetragonal Fe 3B further transforms into orthorhombic Fe3B. Metastable Dy3Fe62B14 compoun d then forms from the Dy-impoverished amorphous fraction, and subsequent cr ystallization of the Dy1+epsilonFe4B4 phase occurs in the Dy-enriched fract ion. Finally, Dy3Fe62B14 decomposes into bcc iron, Dy1+epsilonFe4B4 and iro n borides. The nature of the first crystallization product suggests the exi stence of local environments of t-Fe3B type for this Dy concentration. The crystallization process of Fe70Dy10B20 strongly differs from that of Fe74Dy 6B20 Segregation phenomena occur in the amorphous state prior to any crysta lization. If the nature of the first crystallization product is assumed to be correlated with short-range order in the amorphous state, our results su ggest that the local environments differ from those of Fe74Dy6B20, as they probably involve dysprosium atoms. This behaviour would agree with a previo us Mossbauer study performed on the as-quenched amorphous alloys, providing evidence for a structural modification of the iron environments in the rar e earth concentration range 8-9 at.%.