Study of mechanism of RQ embrittlement of Fe73.5Cu1-Nb3Si13.5B9 nanocrystalline soft magnetic alloy

Rapidly quenching embrittlement (RQE) sometimes appears in Fe73.5Cu1-Nb3Si13.5B9 nanocrystalline soft magnetic alloy (FINEMET) during production process. As a result of RQE the ductility of the as-quenched ribbon drops seriously. The mechanism of RQE was elucidated in the recent work of current authors. It was believed that RQE is due to the structural relaxation but not related to α-Fe(Si) crystallization. In this paper, the high resolution TEM (HRTEM) method and image digital processing were applied to analyze the HRTEM images of two FINEMET rapidly quenched ribbons with different thicknesses. In the thinner ductile sample, the ordering domains with the size of about 3 nm are observed. In the thicker RQE sample, the metastable nanocrystalline domains with the size of 18 nm are observed along with the structural relaxation. These domains seem to have Fe3B-like metastable phase structure on nanometer scale. The result indicates that the local atomic ordering regions extend when RQE induced by structural relaxation occurs.