Evidence of a Coupling between Magnetic and Chemical Interactions in Iron-Based Alloys
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EVIDENCE OF A COUPLING BETWEEN MAGNETIC AND CHE'MICAL INTERACTIONS IN BASED ALLOYS
IRON-
V. PIERRON-BOHNES, M.C. CADEVILLE AND F. GAUTIER L.M.S.E.S. (LA CNRS no 306) Inst. Le Bel 4, rue B.Pascal 67070 Strasbourg F ABSTRACT The simultaneous occurrence of chemical and magnetic interactions in transition metal alloys greatly influences their thermodynamic properties. However the exnerimental evidence of the Counling between both interactions has not been demonstrated so far. The present study shows that such a coupling exists in iron-based alloys (FeCo, FeMn, FeV) and that it modifies qualitatively the chemical short range order. This is evidenced from its temperature dependence in both ferromagnetic and paramagnetic phases which displays a marked change of behaviour at the Curie temperature (T cm). Theoretical models which account qualitatively for the whole of the observed behaviours are briefly discussed.
INTRODUCTION The influence of magnetism on the stability of phase diagrams is a wellestabiished fact from both experimental and theoretical points of view. Numerous experimental examples are given in a recent review article by Mliodownik i] whereas a brief summary of theoretical approaches will be found in paper hereafter referred to as [2]. In the present article we wish to show the influence of magnetism on the chemical short range order (SRO) from an investigation of its temperature denendence over a T range which encompasses T in dilute FeCo, FeV and Fe'fn alloys. This paper summarizes (1) the metfodology used to carry out such a study from low temperature N'4R measurements ; (2) previous and new data obtained in FeCo [2, FeV [3,4] and FeMn alloys and (3) a theoretical description of SRQ based on anlsing-Heisenberg Hamiltonian t-eated in both point and pair (Bethe) mean field approximations (MFA). METHODOLOGY A study of SRO in alloys is carried out in two steps. The first step consists of determining the metallurgical process to obtain alloys in thermodynamical equilibrium state, and the second one is the measurement of the corresponding local order. Obtention of the Equilibrium State In the case of local order in a solid solution, the equilibrium state is attained by a mechanism of vacancy jumps. As previously shown [5] the ordering rate can be accurately described by a single exponential process with a relaxation time T(T) which obeys an Arrhenius law T(T)
= T
eE
(1)
/kT
The self-diffusion energy S E SD is the sum of the formation and migration energies of vacancies, whereas T contains their formation and migration entropies. In transition metal alloys, ESD ranges between 2 and 3 eV and T 16 14 between i0and 10sec. Such values are deduced from a kinetics stugy which can be carried out through measurements of resistivity [4,5,6], NMR r27 and so on, by means of isochronal or isothermal anneal sequencies. Then the sample is annealed during almost 15 T(T) to ensure that the equilibrium Mat. Res. Soc. symp. Proc. Vol.
21 (1984) @Elsevier Science Publishing Co.,
Inc.
402
state is attained. The diffusi
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