PARTITIONING BEHAVIOUR OF Al IN A NANOCRYSTALLINE Fe 71.5 Si 13.5 B 9 Nb 3 Cu 1 Al 2 ALLOY P.J. Warren 1 , I. Todd 2 , H.A. Davies 2 , A. Cerezo 1 , M.R.J. Gibbs 3 , D. Kendall 4 and R.V. Major 4 1 Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH UK 2 Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, UK 3 Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK 4 Telcon Ltd., Crawley, UK (Received July 9, 1999) (Accepted July 19, 1999) Keywords: Atom probe; Soft magnetic; Amorphous; Nanocrystalline Introduction The nanocrystalline Fe-Si-B-Nb-Cu alloy, known as FINEMET, is now a well established commercial soft magnetic material exhibiting excellent permeability while maintaining a high saturation magneti- sation (1,2). It is produced by melt-spinning to form a ribbon with an amorphous structure. This ribbon is then partially devitrified by annealing in the temperature range 520 –580°C in order to develop the desired magnetic properties. In the optimum magnetic state the partially devitrified alloy has a nanocomposite microstructure consisting of randomly oriented Fe-Si nanocrystallites of diameter 10 –20nm, Cu-rich particles of diameter 5–10nm and an amorphous matrix phase, enriched in niobium and boron. A number of attempts have been made to improve the magnetic properties of the Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 alloy by partial substitution of additional alloying elements for Fe (e.g. Co (3,4) Al (5,6)) as well as full or partial substitution of V, Cr, Mo, Ta etc. for Nb (e.g. (7)). In the case of partial substitution of Fe by Al, Lim et al. (5) found that additions up to 1%Al improved the d.c. coercivity (H c ) but caused a deterioration in both the saturation magnetisation ( I ) and saturation magnetostriction ( s ). Recently a systematic study has been undertaken for substitutions of up to 10 at.% of Fe by Al (6). It was observed that the d.c. coercivity H c showed a significant decrease for a substitution of 2 at.% Al and then remained approximately constant up to 8 at.% Al. It was proposed that this reduction in H c was due to a decrease in the magneto-crystalline anisotropy caused by incorporation of Al into the nanocrystallites. Subsequent investigation of the magnetostriction behaviour and a.c. magnetic prop- erties of Fe (73.5-X) Si 13.5 B 9 Nb 3 Cu 1 Al X in the range 0  X  6 (8) showed that substitution of 2% Al for Fe produced an alloy which after annealing exhibits a lower coercivity and magnetostriction and a higher  0.4 than that of the base alloy. The purpose of this investigation was to use 3-dimensional atom probe microanalysis to experimen- tally quantify the local phase chemistries of this nanocomposite microstructure, with particular refer- ence to the partitioning behaviour of the Al. Pergamon Scripta Materialia, Vol. 41, No. 11, pp. 1223–1227, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter PII S1359-6462(99)00279-1 1223