Atomic Short Range Order in a Fe-Ni Invar Alloy
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ATOMIC SHORT RANGE ORDER IN A Fe-Ni INVAR ALLOY
P. CENEDESE , F. BLEY, S. LEFEBVRE~i *Institut Laue Langevin, 156 X, 38042 Grenoble Cedex, France 0 Institut National Polytechnique de Grenoble - Laboratoire de Thermodynamique et Physico-Chimie M6tallurgiques associ6 au C.N.R.S. (L.A.29) - E.N.S.E.E.G. Domaine Universitaire, B.P.75 38402 Saint Martin d'H~res, France ; *C.E.C.M.C.N.R.S., 15, rue Georges Urbain, 94400 Vitry sur Seine, France.
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The structural state of Fe-Ni Invar has been extensively studied. In particular, the possibility of some clustering together with short range ordering has been reported [1 , 2]. In order to determine precisely short range order parameters in this alloy, neutron diffuse scattering measurements have been performed on a f.c.c. single crystal Fe 0 6 9 8 Nio0 30 2, using the four circles, three axes 6 diffractometer D1OatI.L.L.Ni 2 has been used in order to increase the contrast between the two species [3 , 4]. The sample has been annealed 109 hours 0 at 470 C. The diffuse intensity appears as a very weak modulation of the Laue monotonic diffuse scattering, showing maxima at 100 and 110 positions of the reciprocal lattice, and also weak maxima under Bragg peaks (Fig. 1).
LoueA200 unit
j220
i
;
FIG. 1. Diffuse intensity, after corrections, along superstructure
fundamental lines.
100
210
320
The procedure of Borie and Sparks has been applied to separate the different components of the diffuse scattering. Modifications of this procedure [5] allow averaging of equivalent points in reciprocal space in the course of the separation. The displacement terms are weak but separable. The results presented here have been obtained after neglecting the displacements terms, so that the experimental diffuse intensity is considered as short range order intensity. The averaged short range order intensity is isointensity map in the 001 plane.
presented on Fig.
2 as an
The short range order parameters aimn are quite the same as those obtained after separation of displacement terms. They are weak, and the correlation length is short only the two first otmn have noticeable values (010= -0.020 ; t20= 0.045). The small absolute value of a 1 0 compared to a200 is compatible with the possibility of some clustering (Table 1).
Mat.
Res.
Soc. Symp. Proc. Vol.
21 (1984) OElsevier Science Publishing Co.,
Inc.
352
a 0 0 0 is quite equal to 1, that means that the measurements and the corrections are good. The short range order intensity is parameters (compare Fig. 2 and Fig. 3).
FIG. 2. Isointensity map of experimental short range order intensity
rebuilt with 5 short range order
FIG. 3. Isointensity map of short range order intensity, rebuilt with 5 a mn"
TABLE I. Short range order parameters, and pair interaction potentials. Standard deviation of the a mn is given in units of the last digit. Zmn
91mn
V mn(meV)
000 110 200 211 220 310 222 321 400
1.0214 (30) -0.0201 (8) 0.0451 (10) 0.0046 (5) 0.0042 (8) (5) 0.0003 0.0042 (9) 0.0008 (4) 0.0037 (10)
0.038 2.81 -6.43 -0.77 -0.01 -0.32 -0
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