Fractal morphologies from decomposition of Fe-Ni-Invar alloys
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Fractal morphologies from decomposition of Fe–Ni–Invar alloys Q. Li, A. Wiedenmann, and H. Wollenberger Hahn-Meitner-Institut Berlin, Glienicker Str. 100, D-14109 Berlin, Germany (Received 3 June 1994; accepted 17 September 1996)
Small angle neutron scattering investigations performed on Fe 12x –Nix alloys with 0.26 < x < 0.45 revealed a thermodynamically driven decomposition below 800 ±C. The miscibility gap extends at least from 30 at. % Ni to 45 at. % Ni. The diffusioncontrolled decomposition produces fractal morphologies during the early stages of the process. During annealing the structure of the precipitated phase densifies continuously from mass fractals with small dimensionality s1 < dm < 3d to surface fractals with 2 < ds < 3. The results are compared with recent simulations of aggregation and growth as well as with a new analysis of spinodal decomposition, both predicting fractal morphologies.
II. EXPERIMENTAL
I. INTRODUCTION 1
The classical phase diagram of Fe –Ni alloys shows a single phase austenite (fcc) for the Invar composition around Fe–35 at. % Ni at temperatures above 450 ±C. However, many observations of anomalous properties as compiled in recent reviews2,3 suggest the presence of heterogeneities in these alloys, which are not in accordance with the picture of a stable phase. Meteorites of these compositions have been found decomposed into a disordered Fe3 Ni phase with fcc structure and an ordered phase of FeNi with L10 structure. Decomposition was also observed in Fe –Ni and Fe –Ni –Cr–Invar alloys after irradiation with electrons, neutrons, and ions4 ; however, it was not clear whether this process is radiation-induced or thermodynamically driven and accelerated by radiation-enhanced diffusion. The Small Angle Neutron Scattering (SANS) investigation performed on Fe –34 at. % 62 Ni after annealing at 625 ±C over 230 days gave clear evidence for a thermodynamical driven phase separation, giving rise to concentration and magnetization fluctuations.5 Proton irradiation of this alloy at the same temperature produced a considerable acceleration of the process.6 This phase separation was confirmed by means of conversion electron M¨ossbauer spectroscopy in thin layers of Fe12x –Nix with 0.32 < x < 0.39 annealed at 625 ±C.7 The present investigation aims to verify the postulated miscibility gap and to characterize the morphology and kinetics of the decomposition process. Due to the sluggish diffusivity at the applied annealing temperatures below 800 ±C, very small amplitudes of the concentration fluctuations are expected. By means of the SANS technique, small modifications of the microstructural parameters can be studied and have indeed been observed during the early stages of the decomposition.8 J. Mater. Res., Vol. 12, No. 1, Jan 1997
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A. Preparation
High purity Fe (99.999 at. %) and Ni (99.9999 at. %) of natural isotopic composition were used for the preparation of Fe 12x –Nix alloys in the com
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