College Park, Maryland      June 6 - 10 , 2004

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MP44: Coherent Inelastic Neutron Scattering Study of Lattice Dynamics and Vibrational Entropy in Polycrystalline Fe₇₁Ni₂₉ Undergoing a Low Temperature Martensitic Transformation

O. Delaire, M. G. Kresch, T. M Kelley, B. T. Fultz (California Institute of Technology)

Using the Pharos time-of-flight spectrometer at the Lujan center, we measured the neutron scattering function S(|Q|,E) for the alloy Fe₇₁Ni₂₉ as function of temperature from 300K to 80K. We observed in situ the structural and dynamical effects of the fcc-bcc martensitic transformation taking place at low temperatures, with an emphasis on the entropy of the transformation. To gain insight into the phonon entropy of this martensitic transformation, we studied the lattice dynamics across the phase boundary. We tried to extract information about the inter-atomic force-constants from the Q- and E- dependence of the coherent scattering signal recorded with the pixelated Pharos detector array. The coherence in the data can be directly related to the polycrystalline averaged phonon dispersions for the two phases of the material. The inelastic scattering function S_inel(|Q|,E) for the high temperature phase was compared to a computer simulation based on the Born-von Karman model and inter-atomic force-constants determined previously. The good agreement between the experimental data and the simulation suggests new approaches for investigating the lattice dynamics of polycrystals. The vibrational entropies of the low- and high-temperature phases were calculated from the phonon DOS curves. We found a significant entropy change upon transformation of -0.13 kB per atom, typical of martensitic transformations.

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