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

Measurements of neutron inelastic scattering, thermal expansion and low-tem perature heat capacity were used to study the phonon entropy of the martens ite-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 diff ering scattering amplitudes of Ni and Ti through a lattice dynamics simulat ion. The phonon density-of-states (DOS) curves so obtained account for all the transformation entropy measured by calorimetry. From simulations and th e measured DOS, the vibrational entropy of austenite was found to be larger than that of martensite because the transverse acoustic (TA(2)) and longit udinal acoustic modes of austenite are softer than those of martensite. Sim ulations suggest that this originates with a large and negative first-neare st-neighbour transverse force constant. This Phi (1nn)(T) may also be respo nsible for the soft modes involved in the mechanism of the martensitic tran sformation.