A model for nucleation of nanocrystals in the soft magnetic alloy Fe 73.5Nb 3Cu 1Si 13.5B 9

Abstract

Melt-spun ribbons of the alloy Fe 73.5Nb 3Cu 1Si 13.5B 9 are amorphous, but heat treating them at 550°C for one hour generates a microstructure which consists of DO 3 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 fcc structure form early in the precipitation process, and that the size of these clusters is influenced by the presence of Nb. These results, and an extensive transmission electron microscopy study, support an argument that the DO 3 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.