Soft Phonon Modes and the Propagation Velocity of Martensite

Abstract

THE interpretation of martensitic phase transformations remains a central problem in physical metallurgy. Studies of lattice properties such as elastic and anelastic behaviour, thermal expansion and thermal conductivity have resulted in knowledge of the way in which such transformations proceed at the atomic level1–8. In particular, the martensitic phase changes in Au–Cd (ref. 1), In–T1 (refs. 2 and 7) and TiNi (refs. 5 and 6) are preceded by “softening” of certain elastic coefficients and the transitions result from the development of lattice instability evidenced as a soft acoustic mode; as the transition temperature is approached and the incipient mechanical instability rises, certain lattice vibration modes undergo a considerable energy decrease and as their frequency falls the wavelength increases and the interatomic binding forces are decreased. Ultimately, the vibration amplitude and anharmonicity are so large that the atoms adopt new sites. On the basis of this model properties which depend on the lattice vibration spectrum are expected to exhibit marked anomalies in the vicinity of the transformation. One property directly manifesting anharmonicity of lattice vibrations is the thermal expansion; a pronounced peak in the thermal expansion coefficient occurs centred around the martensitic transformation in TiNi (ref. 6), the attendant anharmonicity being reflected in the Gruneisen parameter which rises from the normal background value of about 2 to more than 40 in the transition region.