Thermodynamic properties and diffusion thermodynamic factors in B2-NiAl
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INTRODUCTION
ORDERED transition metal aluminides are of interest as high-temperature structural materials due to their very high melting temperatures, good oxidation resistance, and low mass density. A knowledge of their fundamental physical and chemical properties is a necessary prerequisite for the development of technical alloys. The phase diagram for the Ni-Al system[1] is shown in Figure 1. The composition indicated on this and other figures as atomic percent Al refers to the atomic composition of the alloy, i.e., this composition measure excludes vacancies that are present in these alloys. Steiner and Komarek[2] studied the system in the temperature range 1200 to 1400 K and in the composition range 20 to 60 at. pct Al by an isopiestic method and found a very strong composition dependence of the Al activity in the B2 region when close to stoichiometry. The partial molar thermodynamic properties for Ni are not obtained directly by this method. Oforka[3] studied the Ni-Al system over a quite large composition range (10 to 85 at. pct Al) by Knudsen effusion mass spectrometry (KEMS), but only three compositions were in the B2 region and results were given for the 1423 K only. This means that, while activity data were obtained for both components, no information concerning the partial molar enthalpies and entropies were obtained. Wang and Engell[4] determined the thermodynamic properties of the Al component in the temperature range 961 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 (IGCAAR), Kalpakkam, 603102, Tamil Nadu, India. D. KATH, Engineer, is with the Research Center Jülich, 52425 Jülich, Germany. L. SINGHEISER, Director, Research Center Jülich, is also Professor, Technical University Aachen, Aachen, Germany 52062. K. HILPERT, Division Head, Research Center Jülich, is Professor, Technical University Darmstadt, Darmstadt, Germany 64289. Contact e-mail: [email protected] W.A. OATES, Professor, is with the Institute for Materials Research, University of Salford, Salford, M5 4WT, United Kingdom. Manuscript submitted December 11, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS B
to 1278 K and in the composition range 32.2 to 58.7 at. pct Al by an electromotive force method. Jacobson[5] determined the Al activity by the KEMS method at 1371 K for three compositions, but only two compositions were within the B2 region. There have also been some calorimetric investigations. The integral molar enthalpies of mixing for the stoichiometric Ni0.50Al0.50 phase were determined by calorimetry in References 6 through 8, while Henig and Lukas[9] determined this quantity between 42 and 54 at. pct Al. Santandrea et al.[10] has also determined the mixing and reaction enthalpy changes from differential scanning calorimeter (DSC) measurements for various compositions. Meschel and Kleppa[11] and Su et al.[12] have used synthesis calorimetry to m
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