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 microstr ucture 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 propo sed that nucleation on this exceedingly fine scale occurs at sites whi ch are locally enriched in Fe, this enrichment being caused by partiti oning of Cu during the heat treatment. Extended x-ray absorption fine structure (EXAFS) results show that Cu rich clusters with a near fee s tructure form early in the precipitation process, and that the size of these clusters is influenced by the presence of Nb. These results, an ti an extensive transmission electron microscopy study, support an arg ument that the DO3 precipitates form in intimate contact with the Cu c lusters, rather than at sites displaced from them. This model is consi stent with results from other experimental studies and with theoretica l 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.