Acta mater. 48 (2000) 4783–4790 www.elsevier.com/locate/actamat SMALL-ANGLE NEUTRON SCATTERING AND DIFFERENTIAL SCANNING CALORIMETRY STUDIES ON THE COPPER CLUSTERING STAGE OF Fe–Si–B–Nb–Cu NANOCRYSTALLINE ALLOYS M. OHNUMA 1, 2 *, K. HONO 1 , S. LINDEROTH 2 , J. S. PEDERSEN 2 , Y. YOSHIZAWA 3 and H. ONODERA 1 1 National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan, 2 Risø National Laboratory, DK-4000 Roskilde, Denmark and 3 Magnetic and Electronic Materials Research Laboratory, Hitachi Metals Ltd, 5200 Mikajiri, Kumagaya, Saitama 360-0843, Japan ( Received 4 May 2000; received in revised form 14 July 2000; accepted 4 August 2000 ) Abstract—The kinetics of copper clustering and primary crystallization of FINEMET type alloys with the compositions Fe 74.5-x Si 13.5 B 9 Nb 3 Cu x and Fe 77 Si 11 B 9 Nb 3-x Cu x have been studied by small-angle neutron scat- tering (SANS) and high-sensitivity differential scanning calorimetry (DSC) in order to explain the different optimized Cu contents, x, for obtaining the highest permeability in these two alloys. SANS results have shown that the alloys with the optimized Cu contents have the finest nanocrystalline microstructures. Kinetic analyses of Cu clustering prior to primary crystallization have shown that the number density of Cu clusters becomes highest at the crystallization stage of α-Fe primary crystals in the alloy containing an optimized amount of Cu.  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Alloys; Kinetics; SANS and DSC; Magnetic properties 1. INTRODUCTION In 1988, Yoshizawa et al. [1] found that the crys- tallized microstructures of Fe–Si–B amorphous alloys are drastically refined to a nanometer-scale dimension by adding small amounts of Cu and Nb [1]. After annealing Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 amorphous alloy at 823 K for 60 min, the crystallized microstructure con- sists of weakly ordered D0 3 Fe–Si primary particles embedded in a remaining amorphous phase with grain sizes of about 10 nm. These nanocrystalline alloys exhibit excellent permeability while maintaining a high saturation magnetization, and they are commer- cially known as FINEMET. Based on the random ani- sotropy model, Herzer [2] attributed the high per- meability obtained in the nanocrystalline microstructures to the zero magnetocrystalline ani- sotropy resulting from the averaging effect in the ran- domly oriented nanograins. The roles of Cu and Nb additions in nanocrystalliz- ation have been the subject of many investigations. Using atom-probe field ion microscopy (APFIM), Hono et al. [3, 4] reported that the niobium and boron are rejected from the Fe–Si primary particles and par- * To whom all correspondence should be addressed. Fax: +81 298 59 2701. 1359-6454/00/$20.00  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(00)00277-9 titioned in the remaining amorphous phase. Since the enrichment of Nb and B stabilizes the remaining amorphous phase, it suppresses the growth of the Fe– Si primary crystals. It was also confirmed by APFIM [3, 4] and extended X-ray absorption fine structure (EXAFS) measurements [5] that Cu atoms form Cu- enriched clusters prior to the onset of crystallization. This suggests that Cu clustering stimulates the nucleation of the Fe–Si primary particles in the sub- sequent crystallization stage. More recently, the spa- tial distributions of the Cu-enriched clusters were observed directly by the three-dimensional atom probe (3DAP) technique [6], and it was directly shown that the Cu clusters that are formed prior to the nucleation event serve as the heterogeneous nucleation sites for the Fe–Si primary crystals. Hence, the size and distribution of the Fe–Si grains in the final microstructure must be strongly influenced by the kinetics of Cu clustering. In order to increase the saturation magnetic flux density of the FINEMET type alloy, Yoshizawa et al. [7] recently modified the original FINEMET compo- sition to more Fe-rich ones, i.e., Fe 77 Si 11 B 9 Nb 3-x Cu x , by which the saturation mag- netic flux density was improved from 1.24 T to 1.45 T. It is interesting to note that the optimum content of copper, x, for obtaining the highest permeability is