PRESSURE AND TEMPERATURE DEPENDENCE OF NICKEL DIFFUSION IN Fe 3 Al: EFFECT OF ORDERING ZS. TO à KEI 1,2 , J. BERNARDINI 1 {, G. ERDE  LYI 2 , I. GO à DE  NY 2 and D. L. BEKE 2 1 Laboratoire de MeÂtallurgie, UMR 6518 CNRS/Universite Aix-Marseille III, case 511, 13397 Marseille cedex 20, France and 2 Department of Solid State Physics, L. Kossuth University, P.O. Box 2, 4010 Debrecen, Hungary (Received 18 February 1998; accepted 28 February 1998) AbstractÐThe pressure and temperature dependence of 63 Ni heterodiusion was investigated in the tem- perature range 888±1282 K, which includes the critical temperature of the A2±B2 order±disorder transition. All the experiments were carried out using the radiotracer method combined with a serial sectioning tech- nique. The high-pressure measurements have shown that the diusion process is controlled by mono- vacancies both in the ordered and disordered structures. The Arrhenius plot presents a downward curva- ture as the result of B2-type atomic ordering and the lattice diusion coecients can be ®tted by a continu- ous curve as a function of temperature and long-range order parameter. Because of the site preference of Ni for a-sites, nickel diusivity is slower and the activation energy of diusion is higher than the corre- sponding values for iron self-diusion. # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. 1. INTRODUCTION Ordered alloys are of interest owing to their unique properties and wide range of applications (high- temperature structural materials, coatings, diusion barriers, contacts and interconnections in the micro- electronics industry), but up to now diusion data reported in the literature are scarce compared to that of pure metals. Since diusion processes at high temperatures are very important for the pro- duction and use of these materials, an understand- ing of diusion phenomena in terms of defect structure and diusion mechanisms is necessary [1]. The Fe 3 Al compound exists in a wide compo- sition range and exhibits a variety of structural transitions between the low-temperature ordered DO 3 structure, the partially ordered B2 structure and the disordered high-temperature A2 structure [2]. Systems exhibiting an A2±B2 tran- sition have already been investigated, pointing to the fact that ordering aects lattice diusion [3±9]. On the basis of a mean®eld theory, neglecting all correlation eects and supposing a vacancy- mediated diusion mechanism, the following Arrhenius expression was derived for volume self- diusion coecients in binary AB and A 3 B systems with an A2±B2 transition [7, 10]: D V  D 0 exp   Q1  bS 2  RT  1 where S is the long-range order parameter, beta is a dimensionless constant, Q the activation energy of diusion in the disordered state, T the temperature, R the universal gas constant and D 0 the so-called pre-exponential factor. The application of this approach seems to be more realistic for diusion in A 3 B systems, where only ``partial'' order is achieved in the B2 structure, than for AB compounds, where the B2 structure is completely ordered [7, 8]. In AB systems the correlation eects are predominant [11], while in A 3 B systems the correlation factor does not vary much with the A2±B2 transition [12, 13]. The use of equation (1) implies that the diusion process is controlled by mono-vacancies. With the help of high-pressure measurements one can check this diusion mechanism experimentally by deter- mining the activation volume DV de®ned through the expression: DV RT @ lnD V @ p  RT @ ln  fa 2  0  @ p 2 where f is the correlation factor, a the lattice par- ameter and n 0 the attempt frequency. The second term on the right-hand side of equation (2) is a small correction term, which is typically not more than a few per cent of the atomic volume [14]. In our calculations this correction term will be omitted. The present paper deals ®rst with the eect of the A2±B2 order±disorder transition on nickel hetero- diusion in the Fe 3 Al compound and, then, with the diusion mechanism in the same compound. Preliminary results on the pressure eect have already been published [8]. Acta mater. Vol. 46, No. 13, pp. 4821±4825, 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)00125-6 {To whom all correspondence should be addressed. 4821