Ordering and disordering in anisotropic L1 0 -FePd
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BB5.37.1
Ordering and disordering in anisotropic L10-FePd A. Kulovits1, W.A. Soffa2, W. Püschl1 and W. Pfeiler1 Institut für Materialphysik, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria 2 Dept. of Materials Science and Engineering, University of Pittsburgh, 842 Benedum Hall, Pittsburgh, PA 15261, U.S.A. 1
ABSTRACT Nearly equiatomic FePd (Fe-52at.%Pd) alloys have been deformed by cold-rolling to 60% thickness reduction. Ordering and disordering was studied during isochronal and isothermal annealing by residual resistometry (REST) in the deformed as well as in the recrystallized state. In both cases a first order phase transition with a broad thermal hysteresis is observed. Resistivity values corresponding to thermal equilibrium of LRO-states, however, result lower in the deformed case. This is interpreted as a consequence of internal stresses leading to a preference of one variant of ordered domains and consequently to a higher degree of LRO. A detailed study by an isothermal small-step annealing treatment yields two counteracting exponential processes during order-order relaxation with an activation energy of 2.7eV and 2.4eV, respectively.
INTRODUCTION Intermetallic compounds offer very promising technological properties such as a high corrosion resistance and an unusual high-temperature strength. Therefore materials of the class ‘intermetallics’ occupy a prominent position among the high-temperature structural materials of the future. Recently, a subgroup of the intermetallics moved into the focus of interest which are ferromagnetic at ambient temperature and due to their L10-type of atomic ordering display a very marked mechanical and magnetic anisotropy. Their ‘easy axis’ of magnetisation corresponds to the c-axis of the tetragonally distorted ordered domains and it is hoped to achieve a very high density of magnetic and magneto-optic recording by a preferential domain orientation with the caxis perpendicular to the surface, especially if these materials can be stabilised in the form of magnetic nanostructures. Thermodynamic stability is a basic precondition for technical application and performance of materials. Therefore, there is a growing interest in the kinetic parameters which are involved in the establishment of long-range order (LRO) and thereby determine the stability of these materials. In the present paper we report on a study of the evolution of LRO after different thermomechanical pre-treatment by measurement of residual electrical resistivity (REST). This method is very well suited to monitor the fine variation of LRO because of its ultra high resolution for the corresponding structural changes [1-3]. We present a detailed study during isochronal annealing and preliminary results obtained during an isothermal small-step annealing treatment.
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EXPERIMENTAL The sample material was alloyed by the Department of Materials Science and Engineering, University of Pittsburgh. Its composition was determined as 52%±0.05% Fe and 48%±0.05% Pd by wet-chemical analysis (Institut für
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