Simulation of Atomic Configurations in Short Range Ordered Ni 3 Fe
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SIMULATION OF ATOMIC CONFIGURATIONS IN SHORT RANGE ORDERED Ni 3Fe M
S. LEFEVBRE 1 P. CENEDESE, F. BLEY, Ilnstitut National Polytechnique de Grenoble - Laboratoire de Thermodynamique et Physico-Chimie Mgtallurgioues (L.A. E.N.S.E.E.G. Domaine Universitaire, B.P.75 38402 Saint Martin d'Hlres ; 1Institut Laue Langevin, 156 X 38042 Grenoble Cedex ; 111C.E.C.M.-C.N.R.S., 15, rue Georges Urbain 94400 Vitry-sur-Seine, France
29),
ABSTRACT A procedure similar to that of Gehlen and Cohen and Williams is used to simulate atomic configurations of short range ordered Ni 3 Fe. The short range order parameters were previously measured after different heat treatments. Probabilities of different clusters are computed from those models : triangles, tetrahedrons, clusters as defined by Clapp. These probabilities vary with the enhancement of short range order. It is shown that most of the clusters the probabilities of which increase with short range order, are directly related with the LI 2 ordered structure: either they have the L1 2 structure, or the Li 2 structure modified by one or two antiphase boundaries. Such clusters are identified in the model in order to display perfectly ordered domains.
INTRODUCTION Short range order parameters in Ni 3Fe have been determined previously, by thermal neutron diffuse scattering measurements, after different heat treatments [1, 2 , 3]. Annealings above the critical temperature of ordering 0 0 Tc (% 500 C), at 507 C, 535°C and 685°C, provide equilibrium short range 0 order, and short annealings below Tc (1/2 hour at 472°C and I hour at 385 C) provide transient short range order, before long range order is established. Lately, additional measurements, after annealing at 503°C, allow us to have a better idea of the variation of short range order parameters near Tc. So, we thought it might be interesting to seek the evolution of atomic configurations with annealing temperature, using geometrical simulations. Gehlen-Cohen-Williams type procedures have been used by many authors to simulate atomic configurations of short range ordered alloys [4, 5 , 6]. In these geometrical simulations, the exchange of atoms of different kinds, located in lattice sites, is done only if the exchange provides values of the short range order parameters a tending towards the requested ones.
SIMULATION PROCEDURE The principle of the procedure used here is quite similar to those described in [4, 5, 6] some details are different, and will be given. A f.c.c. 4 lattice containing N = Nx&N * Nz sites (here N = 13500), is built from a simple cubic lattice, by exclusion of vacant sites. Each site of the f.c.c. lattice is registred by a number (one dimensional area), and by cartesian coordinates. The initial occupancy of the sites by 0 or I-atoms is either random, or segregated, the concentration of atoms being fixed. The exchanges Mat. Res. soc. Symp. Proc. Vol. 21 (1984)
sElsevier Science Publishing Co.- Inc.
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of 0 and I- atoms, taken at random, are done in order to fit the eight first short range order parameters (
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