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10/7/03

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Thermodynamic Investigation of the A2/B2 Region of the Fe-Al System by Knudsen Effusion Mass Spectrometry L. BENCZE, D.D. RAJ, D. KATH, W.A. OATES, J. HERRMANN, L. SINGHEISER, and K. HILPERT The vaporization of Fe-Al alloys has been investigated in the temperature range of 1140 to 1600 K by Knudsen effusion mass spectrometry. Eleven different compositions were examined in the composition range between 30 and 51 at. pct Al, mostly in the B2 region of the phase diagram. The partial pressures and thermodynamic activities of both Fe and Al were evaluated: directly from the

o /IM , and also measured ion intensities for a component in both the alloy and the pure element, IM

from the ion-intensity ratios of the alloy components, IAl/IFe, by means of a Gibbs–Duhem integration. Reliable partial and integral molar enthalpies and entropies of mixing have been obtained by mass spectrometry for this system for the first time. Nearly temperature-independent integral enthalpies and entropies of mixing over the wide temperature range investigated were found, with the mixing entropies being large and negative.

I. INTRODUCTION

THE ordered intermetallic phases FeAl and NiAl and other aluminides are of interest as high-temperature structural materials due to their very high melting temperatures, good oxidation resistance, and low mass density. Knowledge of their fundamental physical and chemical properties is the necessary prerequisite for the development of technical alloys. Systematic vaporization studies were, therefore, carried out to determine for the first time the partial thermodynamic properties of both components by direct measurements over the wide homogeneity range of FeAl B2 phase. The results obtained allow, for example, the evaluation of thermodynamic factors that are important for the correlation between the chemical and self-diffusion coefficients. The Fe-rich portion of the Fe-Al phase diagram, as assessed by Kubaschewski,[1] is shown in Figure 1. It can be seen that the B2 phase has been subdivided into the B2, B2(1), and B2(h) subregions, as suggested originally by Köster and Gödecke[2,3,4] on the basis of the temperature dependence of the elastic constant and thermal-expansion coefficients. There is, however, considerable doubt about these subregions. The neutron-diffraction measurements of Inden and Pepperhoff[5] showed that they cannot be attributed to different ordered structures. Broska et al.[6] have suggested that the regions may differ in their defect structures, viz., single vacancies, divacancies, and triple defects. It has also been suggested that they might be related L. BENCZE, Professor, is with the Department of Physical Chemistry, Roland Eötvös University, 1117 Budapest, Hungary. D.D. RAJ, Scientific Officer, is with the Materials Chemistry Division, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, 603102, Tamil Nadu, India. D. KATH, Engineer, and L. SINGHEISER, Director, and K. HILPERT, Division Head, are with the Research Center Jülich, 52425 Jülic