The role of phonons in the thermodynamics of the martensitic transformation in NiTi

Measurements of neutron inelastic scattering, thermal expansion and low-temperature heat capacity were used to study the phonon entropy of the martensite-to-austenite transformation in NiTi, which is found to be (0.5 +/- 0.05) kB atom(-1). The inelastic scattering spectra were corrected for the differing scattering amplitudes of Ni and Ti through a lattice dynamics simulation. The phonon density-of-states (DOS) curves so obtained account for all the transformation entropy measured by calorimetry. From simulations and the measured DOS, the vibrational entropy of austenite was found to be larger than that of martensite because the transverse acoustic (TA(2)) and longitudinal acoustic modes of austenite are softer than those of martensite. Simulations suggest that this originates with a large and negative first-nearest-neighbour transverse force constant. This Phi (1nn)(T) may also be responsible for the soft modes involved in the mechanism of the martensitic transformation.