Thermodynamic activities and partial enthalpies of mixing in the solid solution of Fe in Ni 3 Al
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I.
INTRODUCTION
THE g' phase of the type Ni3Al is important for the strengthening of the face-centered cubic (fcc) g matrix in Ni-base superalloys[1] and for the development of alloys on the basis of ordered intermetallic phases.[2] The Ni3Al phase has a fcc L12 structure. The Al atoms at the cube corner sites are surrounded by 12 Ni atoms as nearest neighbors. The Ni atoms at the face center sites have the coordination number 12, too, and are surrounded by 4Al and 8Ni atoms. The properties of g'-hardened alloys and of the g' phase can be improved by the addition of further elements, such as boron or transition metals. Boron is dissolved interstitially. Transition metals occupy different lattice sites in the g' phase. Mo¨ssbauer effect measurements on Ni3Al,[3,4] axial electron-channeling microanalysis,[5] and atom probe fieldion microscope measurements[6] revealed a preference of the Fe atoms for the Al sites. Analysis of the g'/g equilibrium in the Ni-Al-Fe system by the cluster variation method with the Lennard–Jones potential[7] showed that the preferential substitution of Fe for Ni or Al in the g' phase (Ni3Al) depends strongly upon the stoichiometry of this phase. Other theoretical publications on the site preference of a third component in the Ni3Al lattice are by Ochiai et al.[8] as well as Guard and Westbrook.[9] Three different types of substitutional behavior are distinguished in the two publications: (a) substitution of the third component for Ni, (b) for Al, and (c) for Ni and Al. A substitutional behavior of type (c) was inferred in the two publications from the direction of the g' solubility lobe in the Fe-Ni-Al phase diagram. Different theoretical approaches were made to explain the solubility behavior.[8,9] Wu et al.[10] used the tetrahedron approximation of the M. ALBERS, Ph.D., a former Graduate Student with Institute of Physical Chemistry, Technische Hochschule Darmstadt, Germany, is now with Leybold Vakuum GmbH, 50986 Ko¨ln, Germany. D. KATH, Engineer, is with the Institute of Materials in Energy Systems, Research Centre Ju¨lich. K. HILPERT, Professor, is with the Institute of Materials in Energy Systems, Research Centre Ju¨lich, 52425 Ju¨lich, Germany, and with the Institute of Physical Chemistry, Technische Hochschule Darmstadt. Manuscript submitted November 27, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
cluster variation method to model generally the ternary site occupation in L12 ordered intermetallics. The phase diagram of the Ni-Al-Fe system is given by Bradley,[11] as well as Budberg and Prince.[12] Kek et al.[13] determined calorimetrically the enthalpy of formation of Ni3Al with different additions of iron. The determination of a complete set of partial thermodynamic properties for the ordered intermetallic phase Ni3Al with different additions of iron is reported in this work for the first time. The data obtained are discussed with respect to the solubility behavior of iron. The investigations were carried out by vaporization measurements using Knudsen effusion mass spectromet
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