Precursors of amorphization in supersaturated Nb-Pd solid solutions
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C. Ettl and K. Samwer Institutfiir Physik, Universitdt Augsburg, D-86135 Augsburg, Germany
W.B. Yelon University of Missouri Research Reactor, Columbia, Missouri 65211
W. L. Johnson W.M. Keck Laboratory of Engineering Materials 138-78, California Institute of Technology, Pasadena, California 91125 (Received 7 February 1994; accepted 12 September 1994)
The possibility that crystal-to-amorphous phase transformations can be induced by one or more underlying instabilities of the crystalline phase has been investigated in highly supersaturated solid solutions of Nb-Pd. Several unusual properties were discovered that may be identified as precursor effects of the collapse of the bcc cr-Nb terminal solution to the amorphous phase. Elastic neutron diffraction measurements of a - N b solutions found, with increasing Pd concentration, an anomalously large increase of the average atomic root-mean-square displacement to about half of the value at which the Lindemann criterion predicts the lattice should melt. Low-temperature heat capacity measurements yielded a concomitant decrease in the Debye temperature, suggesting that supersaturation causes an elastic modulus to soften. Single crystals of a - N b solutions at high supersaturations have a highly anisotropic structure that is visible in transmission electron microscopy images; it is consistent with the development of a soft phonon mode leading to a bcc-to- ( 1
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FIG. 3. Linear thermal expansion coefficient aL of Nb-Pd alloys determined by neutron diffraction (filled circles) and dilatometry (open circles) plotted against Pd concentration. The apparent divergence of aL at 42 at. % Pd in the neutron data is an artifact of statistical fluctuation due to the large uncertainty for that data point. The roughly linear dependence of ai (dashed line) spans the entire concentration range.
the value of aL at 150 K determined by dilatometry to the average aL calculated from the neutron diffraction data between 12 and 300 K; evidently, the divergence in thermal expansion at 42 at. % Pd suggested by the latter data is an artifact of statistical fluctuation. Plotting aL against Pd concentration demonstrates that a roughly linear relationship exists across the entire composition range from pure Nb to pure Pd, even though the 52 and 100 at. % Pd samples have the fee rather than the bcc structure. Thus, the gradual increase in a^ with Pd content in the bcc sample probably results from the concentration-weighted average of the anharmonicity in interatomic potentials that gives rise to thermal expansion, rather than from a developing instability of the crystalline structure. C. Atomic mean-square displacement The temperature dependence of the scattering intensity in elastic neutron diffraction is contained in the Debye-Waller factor e~2M where M is a monotonically increasing function of temperature T.24 This factor accounts for the reduction in intensity associated with thermally induced oscillations of the diffracting atoms in a crystal about their idea
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