Equation of state of polycrystalline Ni 50 Al 50
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Equation of state of polycrystalline Ni50 Al50 J. W. Otto Max-Planck-Institut f¨ur Eisenforschung, Max-Planck-Str. 1, 40237 D¨usseldorf, Germany
J. K. Vassiliou Department of Physics, Villanova University, Villanova, Pennsylvania 19085
G. Frommeyer Max-Planck-Institut f¨ur Eisenforschung, Max-Planck-Str. 1, 40237 D¨usseldorf, Germany (Received 10 March 1997; accepted 23 June 1997)
Polycrystalline Ni50 Al50 suitable for high pressure studies was prepared by grinding and subsequent annealing of an inert-gas atomized alloy. The equation of state was determined by energy-dispersive x-ray diffraction in a diamond anvil cell to 25 GPa. The bulk modulus Bo and the pressure derivative of the bulk modulus B0o were found to be Bo 156 6 3 GPa and B0o 4.0 6 0.5.
NiAl is of considerable interest for high temperature applications in the aerospace industry and for stationary gas turbines of electric power plants because of its low specific weight,1 high melting point,2 and good oxidation resistance.3 Its usefulness is limited by low tensile ductility and poor fracture toughness at room temperature4 and inadequate creep resistance at high temperatures.5 (For a review of the properties of NiAl, see Ref. 6.) The reason for the low ductility and the stable B2 superlattice structure rather than the L10 structure of the early transition metal aluminides has been shown to be the extensive Al p –Ni d hybridization with charge transfer from Al to Ni sites.7–12 In order to maximize the charge transfer and to minimize repulsion between Al atoms, the primary Burgers vector in NiAl is a k100l13 which preserves the order and not the ay2 k111l translation typical of many B2 structures. Extensive theoretical and experimental studies of the electronic, elastic, and mechanical properties of pure and alloyed NiAl have been carried out in order to find ways to improve the ductility and the strength at high temperatures.6 Among the information not yet available is the effect of hydrostatic pressure on the crystal structure. An experimental determination of the bulk modulus is of interest both for comparison with theoretical calculations and for predicting how microalloying may improve the ductility through size effects acting like a pressure. The very properties that make intermetallic aluminum-based alloys of great interest for industrial applications make them a challenge for high pressure studies. Their high elastic stiffness requires a large volume ratio of pressure medium to sample in order to avoid shear stresses from direct grain contacts or from bridging across thin films of a pressure medium. Their low scattering power for x-rays (because of the low atomic number of the constituent elements and the relatively large unit cells), on the other hand, requires 3106
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 11, Nov 1997
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a large number of crystallites in the small volume available in high pressure cells. NiAl presents a special problem because of its l
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