Nucleation of the nanocrystalline phase in Fe73.5Cu1Nb3Si13.5B9 (abstract)

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Abstract
A new class of soft magnetic alloys derive their attractive properties from the fact that they contain a high volume fraction of iron rich, nanometer scale, DO3 structure precipitates. These precipitates are imbedded within, and separated from one another by, a portion of the amorphous phase from which they formed. In an effort to determine the mechanism by which the addition of modest amounts of Cu and Nb to Fe‐Si‐B amorphous alloys induce the formation of the metastable nanocrystalline structure, melt‐spun samples of Fe73.5Cu1Nb3Si13.5B9, Fe76.5Cu1Si13.5B9, and Fe74.5Nb3Si13.5B9 were heat treated at 550 °C for times ranging from 2 to 60 min and studied by means of transmission electron microscopy (TEM), x‐ray diffraction, and extended x‐ray absorption fine structure (EXAFS) analysis. EXAFS spectra from the Cu absorption edge in the Fe73.5Cu1Nb3Si13.5B9 alloy show that tiny clusters of a Cu rich phase are present even in the amorphous, as‐melt‐spun ribbon, and that these clusters have close atomic packing. These clusters are found to coarsen and increase in number during the first few minutes of heat treatment. TEM observations show that a high proportion of the precipitates are nucleated within this same short time, supporting the belief that the clusters catalyze nucleation of the DO3 precipitates. Examination of the alloys without Cu or Nb indicate that the Nb is important both for stabilizing the amorphous phase component and for inducing formation of the Cu clusters.

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