MO È SSBAUER SPECTROSCOPY EVIDENCE OF A SPINODAL MECHANISM FOR THE THERMAL DECOMPOSITION OF F.C.C. FeCu P. CRESPO 1,2 , N. MENE Â NDEZ 3 , J. D. TORNERO 3 , M. J. BARRO 1 , J. M. BARANDIARA Â N 4 , A. GARCõÂA ESCORIAL 2 { and A. HERNANDO 1 1 IMA, P.O. Box 155, Las Rozas, 28230 Madrid, Spain, 2 CENIM±CSIC, Avda. G. del Amo, 8, 28040 Madrid, Spain, 3 Dpto. QuõÂmica-FõÂsica Aplicada, UAM, Cantoblanco, 28049 Madrid, Spain and 4 Universidad del PaõÂs Vasco, P.O. Box 644, 48080 Bilbao, Spain (Received 20 September 1996; accepted 16 March 1998) AbstractÐMoÈssbauer spectroscopy shows the existence of compositional ¯uctuations, where dierent Fe environments coexist, during decomposition upon heat treatment of metastable f.c.c. FeCu solid solution. The presence of isolated Fe atoms in the Cu matrix, f.c.c. Fe rich Cu, f.c.c. FeCu rich and b.c.c. Fe has been detected in early decomposition stages. At later decomposition stages, low temperature MoÈssbauer spec- troscopy indicates the presence of a broad distribution of Curie temperatures, coexisting with isolated Fe atoms in the Cu matrix, f.c.c. Fe and b.c.c. Fe. # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. 1. INTRODUCTION The Fe±Cu system exhibits a positive heat of mix- ing. However, it is possible to obtain solid solutions in the whole compositional range by dierent tech- niques, in particular by mechanical alloying [1, 2], where mixing at an atomic level occurs [3]. Fe x Cu 1  x alloys obtained by mechanical alloying are f.c.c. for Fe content below 60 at.% and b.c.c. above 70 at.%. During decomposition of the meta- stable ferromagnetic f.c.c. FeCu solid solution, a drastic decrease of the average magnetic moment per Fe atom has been reported [4]. Subsequent magnetic studies have shown that during decompo- sition a broad range of dierent magnetic behavior is achieved and the thermal dependence of the mag- netization shows typical features of multiphase magnetic systems [5]. This behavior cannot be explained by the simple decomposition of the sys- tem into the stable b.c.c. Fe and f.c.c. Cu phases, and hence, it has been proposed that the separation may take place via a spinodal mechanism [5]. The occurrence of a spinodal mechanism is supported also by microstructural studies, transmission elec- tron microscopy [6], neutron diraction and by neu- tron small angle scattering studies [7]. In this work the structural evolution of a meta- stable f.c.c. Fe 50 Cu 50 solid solution after several heat treatments has been studied by MoÈssbauer spectroscopy, due to the high sensitivity of this technique to the local atomic surroundings of the resonant Fe atoms. The decomposition process has been monitored at room temperature and some selected samples have also been studied below room temperature. The thermal dependence of the MoÈssbauer spectra allows dierent processes to be distinguished, such as coherent precipitation, where f.c.c. Fe precipitates in the Cu matrix, and spinodal type decomposition, characterized by strong compo- sitional ¯uctuations. The results point out that dierent Fe environments coexist during decompo- sition, ranging from isolated Fe atoms in the Cu matrix to pure f.c.c. Fe and b.c.c. Fe. 2. EXPERIMENTAL METHODS A metastable f.c.c. Fe 50 Cu 50 (at.%) solid solution has been obtained by mechanical alloying in a Fritsch vibrating mill under a pressure of 1.5 atm of argon. The composition and purity of the sample was checked by X-ray diraction (XRD) and scan- ning electron microscopy (SEM), equipped with energy dispersive X-ray analysis (EDX). The solid solution was obtained after a milling time of 400 h and it consists of powder particles of a size ranging between 200 and 300 mm. The XRD pattern indicates that the solid solution is a face centered cubic structure with a lattice parameter of 3.641 20.002 A Ê . The average crystal size for the f.c.c. Fe 50 Cu 50 phase, determined from the (111) X-ray re¯ection is about 18 nm. The formation of the solid solution has also been con®rmed by EXAFS [3]. The resulting powder was heated up to several temperatures to observe the decomposition of the metastable alloy. The heating was carried out in a dierential scanning calorimeter at 20 K/min Acta mater. Vol. 46, No. 12, pp. 4161±4166, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 PII: S1359-6454(98)00111-6 {To whom all correspondence should be addressed. 4161