A model for nucleation of nanocrystals in the soft magnetic alloy Fe73.5Nb3Cu1Si13.5B9

Melt-spun ribbons of the alloy Fe73.5Nb3Cu1Si13.5B9 are amorphous, but heat treating them at 550 degrees C for one hour generates a microstructure which consists of DO3 structure Fe-Si crystallites about 10 nm in diameter which are separated from one another by remnant amorphous films that are enriched in B and Nb. Numerous investigators have proposed that nucleation on this exceedingly fine scale occurs at sites which are locally enriched in Fe, this enrichment being caused by partitioning of Cu during the heat treatment. Extended x-ray absorption fine structure (EXAFS) results show that Cu rich clusters with a near fee structure form early in the precipitation process, and that the size of these clusters is influenced by the presence of Nb. These results, anti an extensive transmission electron microscopy study, support an argument that the DO3 precipitates form in intimate contact with the Cu clusters, rather than at sites displaced from them. This model is consistent with results from other experimental studies and with theoretical analyses which show that the surface energy barrier to nucleation of subsequent phases is lowest at the first-formed precipitate. (C) 1997 Acta Metallurgica Inc.