PHONONS AT MARTENSITIC PHASE-TRANSITIONS OF BCC-TI, BCC-ZR AND BCC-HF

The phonon dispersion of the bcc phase of the pure group 4 metals Ti, Zr and Hf is dominated by a valley of anomalous low frequency and strongly damped phonons along [2ξξξ] propagation, which are related to the β to ω and β to α martensitic phase transitions. The low lying L2/3(111) phonon does not change with temperature and is due to a bcc inherent weakness towards ω Displacements. The T11/2(110) phonon decreases drastically upon approaching T0and is a precursor fluctuation of the β to α transition. The entropy increase due to these low frequency and strongly damped anharmonic phonons stabilizes the bcc structure at high temperature with respect to the hcp or ω structure. Different to alloys the overdamped phonons, which achieve the displacements for the martensitic transition do not condensate to (quasi)static displacements. © 1991, Taylor & Francis Group, LLC. All rights reserved.