The alloy Fe65Ni20Nb6B9 was obtained from the elemental constituents in a high-energy planetary ball mill and was studied by 57 Fe Mössbauer effect spectroscopy, Mössbauer thermal scans and X-ray diffraction. The as prepared nanocrystalline alloy consisted primarily of metastable bcc α-Fe(Ni) nanocrystals (57 nm average size) and small amounts of γ-(Fe,Ni) with Ni concentration of about 58%. Up to about 693 K only defect recovery is inferred. Between 693 and 873 K the α-phase transformed gradually into the fcc γ-phase, whose starting Ni concentration decreased continuously with increasing temperature, reaching a final value which was below 32 at. %. Introduction Magnetic nanocrystals embedded in magnetic and non-magnetic amorphous matrices are of great interest nowadays because of different reasons. On the one hand the behavior of an ensemble of magnetic nanoparticles with interactions of increasing intensity (from free particles to collective systems) is a subject of great interest, which presents basic questions to be answered [1]. On the other hand, one of the most promising routes for developing softer and better magnetic materials for technological use is the manufacturing of dense dispersions of nanocrystals with high saturation magnetization and high permeability embeded in magnetic amorphous matrices [2]. One criterion used to attain this goal is the achievement of systems in which the exchange length lK ≈ (A/K) 1/2 >δ, where A is the exchange constant, K the anisotropy energy density and δ the characteristic size of magnetic entities which have a defined easy direction (as nanocrystals in the above referred dispersions). Such criterion is based on the anisotropy random model, which predicts that under the stated condition the effective anisotropy will vary as δ [3,4] and the system will therefore have softer magnetic properties as δ becomes smaller. In this work we study the alloy Fe65Ni20Nb6B9 obtained from the elemental powders in a highenergy planetary ball mill. Fe and Ni bear magnetic moments and their high global concentration (85 at.%) should impart a high saturation magnetization to the system, whereas Ni, especially within an fcc (γ) phase, would contribute to the softening of the magnetic response. Nb was added to reduce atomic diffusion and hence to prevent grain growth above certain critical size within the nanometric scale. B was used to promote amorphization or disorder in the space left between nanocrystals. Ball milling provides a non-equilibrium route which is proven suitable for the preparation of this sort of nanocomposites. The aim of this article is to present a characterization of the system and of its evolution with temperature from the point of view of composition, structure and microstructure. The system’s, magnetic properties will be published elsewhere. To this end, we applied local and non-local experimental techniques such as Mössbauer effect spectroscopy (MS), Mössbauer thermal scans (MTS)[5] and X-ray diffraction (XRD). Journal of Metastable and Nanocrystalline Materials Online: 2004-07-07 ISSN: 2297-6620, Vols. 20-21, pp 571-575 doi:10.4028/www.scientific.net/JMNM.20-21.571 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-19/03/20,17:06:55) These preliminary results indicate that the as-milled system is stable up to about 753 K, and that combinations of α/γ phases at different ratios and different individual compositions can be obtained in a controlled manner by performing thermal treatments of specific lengths at specific temperatures. Experimental details Mechanical alloying was carried out in a high-energy planetary ball mill (Fritsch Pulverisette P7) starting from pure element powders: Fe of 99.7% purity, with a particle size under 8 μm; Ni of 99.8% purity, with a particle size of 3 -7 μm; Nb of 99.85% purity, with a particle size under 74 μm and B of 99.6% purity, with a particle size of 50 μm. The alloy was milled at the mill operation velocity of 600 r.p.m., in CrNi steel vials, for 10, 20, 40 and 80h. The ball diameter was 12 mm, the powder/balls mass ratio was fixed at 1:5, about 10 g of alloy were produced. The containers were sealed in a glove box with a stationary argon atmosphere. Mössbauer effect experiments were carried out with the nuclear 14.4 keV radiation of 57 Fe (∼ 30 mCi 57 CoRh source) in two modes: conventional spectra MS taken at fixed temperatures beween RT and 900 K, and thermal scans MTS [5] recorded at fixed Doppler velocities at a temperature rate of 2 K/min. Also XRD studies were performed with the Cu Kα radiation in two modes, in order to allow a direct comparison between them and the Mössbauer results: one set of diffractograms with extended angular range taken at fixed temperatures during long times (isothermal mode), and another one with reduced angular range where temperature was varied as a succession of ramps (periods between diffractograms) and plateaus (periods of data recording) with an average rate of 2 K/min (scan mode). Results Fig. 1a shows the XRD patterns of samples measured at different temperatures from RT to 873 K. The as prepared sample diffractogram contains only a few broad reflections which were ascribed to a bcc (α)-Fe(Ni) phase (although in principle the presence of Nb and/or B cannot be excluded). A Rietveld analysis yields an average grain size of 57 nm. The main peak (located at about 44.5 degrees) is slightly asymmetric and a faint reflection is present at about 51 degrees. Rietveld and peak shape analysis of diffractograms indicate that these features originate from a minority fcc (γ) Fe1-xNix phase (approximately 5 to 15 % in weight). From the lattice parameter (0.3575 nm) obtained from the Rietveld analysis a Ni concentration of 56-60 at. % was deduced [6]. Only defect removal is observed for thermal treatments of up to 693 K. Well developed peaks of the γ-phase are already visible in the diffractogram taken at 773 K, they grow with temperature and become dominant at 873 K. The pattern labeled RT2 was obtained at room temperature after measuring the sample at 773 K and indicates that within our experimental conditions the α→γ transition is not reversible. Fig. 1b contains MS spectra taken at the same temperatures. From RT to 673 K they reflect a broad distribution of hyperfine fields. This is in agreement with the composition determined from XRD, since γ-Fe1-xNix (0.56 ≤ x ≤ 0.60) has a magnetic ordering temperature between 850 and 870 K[7]. The inclusion of a small paramagnetic contribution (no more than 3% of Fe probes) slightly improves the spectra theoretical fits. This contribution increases at 773 K, consistently with the development of fcc reflections, and becomes dominant at 873 K. In the RT2 spectra, however, the fraction of probes in paramagnetic environments returns to the small previous values, indicating that the γ-phase formed at 773 K is ferromagnetic at RT. This is consistent with its Curie temperature (about 700 K) as estimated from the average composition (42 % Ni) determined by XRD. 572 Metastable, Mechanically Alloyed and Nanocrystalline Materials 2003