Measured and calculated thermoelastic properties of supersaturated fcc Ni(Al) and Ni(Zr) solid solutions
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Measured and calculated thermoelastic properties of supersaturated fcc Ni(Al) and Ni(Zr) solid solutions J. Bøttiger Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark
N. Karpe Department of Solid State Physics, The Royal Institute of Technology, S-10044 Stockholm, Sweden
J. P. Krog Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark
A. V. Ruban Center for Atomic-scale Materials Physics and Physics Department, Technical University of Denmark, DK-2800, Lyngby, Denmark (Received 29 October 1996; accepted 30 December 1997)
Metastable face-centered cubic (fcc) solid solutions of Ni12x Alx and Ni12x Zrx have been prepared in thin-film form using dc planar magnetron sputtering in a UHV system. In both these alloy systems, extended solubilities in the fcc phase and a pronounced (111) texture are observed after sputter deposition. An amorphous phase is found to form in Ni12x Alx for x > 0.30 and in Ni12x Zrx for x > 0.05. Lattice constants, thermal expansion coefficients, and Debye temperatures were derived from x-ray diffraction measurements. These parameters were also calculated by using ab initio methods in the framework of the local-spin density and coherent potential approximations for the electronic subsystem and the Debye–Gr¨uneisen model for the vibrational properties of the nuclei subsystem. Experiment and theory are compared and discussed.
I. INTRODUCTION
During recent years, considerable progress in ab initio calculations of the electronic structure of solids has been achieved, enabling accurate first-principle calculations of the thermodynamic properties of pure elements and completely ordered alloys. The situation is, however, dramatically different for random alloys where the broken translational symmetry makes ordinary band structure methods inappropriate. Direct calculations of such alloys are too cumbersome.1 Therefore, at present only approximate schemes can be used in the calculations of the electronic structure and ground state properties of real alloys. The simplest and in many cases sufficient approach is a mean-field treatment of the electronic structure of random alloys in the coherent potential approximation (CPA).2 It has recently been shown3–9 that the CPA can be sufficiently accurate to calculate various thermodynamic properties of metallic alloys. This is the reason why methods based on this approximation are promising for solving various problems in materials science. Nevertheless, the use of the CPA implies some restrictions, as this approximation is not able to take into account short-range order (SRO) and local relaxation (LR) effects (i.e., atom movements away from the symmetric lattice positions). An alloy in this approximation is considered as completely random with atoms in perfect J. Mater. Res., Vol. 13, No. 6, Jun 1998
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lattice positions. Therefore, the CPA may give very poor results for some thermodynamic properti
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