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SYMT: Martensitic Transformations and Shape Memory Effect

SYMT 1: Basics

SYMT 1.2: Invited Talk

Wednesday, March 29, 2000, 14:20–14:50, H16

Lattice Dynamics at Martensitic Phase Transitions — •W. Petry, J. Neuhaus, and K. Nicolaus — Physik Department, E13, Technische Universität München, D-85747 Garching, Germany

The martensitic phase transitions from the open bcc structure to closely packed structures like fcc or hcp are accompanied by particularly low energy phonon dispersions. They are characteristic for particular anisotropic lattice potentials and reveal the trajectories for the displacive transition. Shuffle modes i.e. phonons at the Brillouin zone boundary play a major role in transforming the lattice. Martensitic transitions within close packed structures show considerably different lattice dynamics. Here only specific soft, long wavelength, shear modes are observed. In general the transitions in the pure elements are accompanied by dynamical precursors, and static embryos of the new phase are absent.

For alloys the transition temperature changes drastically for small variations in concentration. The measurements of the lattice dynamics show a strong influence of the degree of order on the martensitic transitions, however only marginally influence of the concentration. It is concluded that the concentration dependence of the transition temperature is dominated by electronic effects.