The microstructure evolution in the course of crystallization of a splat-quenched Fe73,5Si13.5B9Nb3Cu1 amorphous alloy was investigated by atom probe field ion microscopy (APFIM) and high resolution transmission electron microscopy (HRTEM). All the alloying elements were found to be distributed homogeneously as an amorphous solid solution in the as-quenched state. At an initial stage of annealing, a concentration fluctuation of Cu was found to occur. Cu formed clusters of a few nanometer diameter and their composition was found to be approximately 30 at.% Cu at the beginning. In the later stage, a b.c.c. FeSi solid solution and the B and Nb enriched amorphous phase with the smaller Si content were found to coexist. In addition to these two phases, Cu enriched particles containing approximately 60 at.% Cu were found to be present in the intergranular regions, although we were not successful yet to determine whether this was a crystalline or amorphous phase. Based on these observations, we discuss the crystallization process of this alloy at 550°C which leads to the emergence of excellent soft magnetic properties.
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