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College Park, Maryland      June 6 - 10 , 2004

WP68: Vibrational Entropy of Dissolving Ag in Al

T. Swan-Wood, M. Kresch, J. Lin, M. McKerns, B. Fultz (California Institute of Technology)

There is an unusually strong increase with temperature of the solubility of Ag in fcc Al. This phenomena is not unique to the Al-Ag system and is seen in many binary alloys, particularly those where the impurity has a mass ratio of at least 3 to that of the matrix atom. We hypothesize that low-frequency resonance modes associated with massive solute atoms make a large contribution to the vibrational entropy of mixing. Inelastic neutron scattering spectra were measured on LRMECS at the IPNS from Al, Al-7 at. % Ag, Al-60 at. % Ag, and a 2-phase region (Al-7 at. % Ag and Al-60 at % Ag) at room temperature. Phonon density of state (DOS) curves were obtained by correcting for multi-phonon scattering, Debye-Waller effects, and the thermal factor. The difference in phonon scattering efficiencies of Al and Ag (neutron-weighting) is not large, but we developed methods for correcting this neutron weighting through computer simulations of the lattice dynamics. The phonon DOS from the Al-7%Ag alloy showed a large resonance mode at low energies. This resonance mode made a significant contribution of 0.160 +/- 0.006 kB/atom to the vibrational entropy of the alloy compared to fcc Al, a significant fraction of the total neutron-weight-corrected entropy of mixing of 0.09 +/- 0.01 kB/atom, but there is also a stiffening of some of the vibrational modes. The hcp phase, 60% Ag, showed a large softening of its phonon DOS compared to Al-7% Ag. A full correction for the neutron weighting is not possible, but assuming it is not large we deduce a vibrational entropy for Al-60%Ag that is 1.35 kB/atom larger than for fcc Al. Vibrational entropy makes a large contribution to the thermodynamics of phase stability in the Al-Ag system. The mass and probably the size of Ag atoms may be responsible for some of the trends.

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