Effect of hydrostatic pressure on the thermoelastic transformation of Ni-Ti alloy and the entropy of transformation

Abstract

The thermoelastic transformation temperature of a polycrystalline 50·22 at.% Ni-Ti alloy has been determined by measuring its electrical resistance over the temperature range 270K–370K and hydrostatic pressures up to 250 bar. The electrical resistance reaches a peak value at 50% transformation during both cooling and heating of the alloy. The volume change at the parent ↔ martensite phase transformation was measured by dilatometry, and the various energy and entropy changes at the transformation were determined by calor-imetry. The transformation temperature increased with the hydrostatic pressure at the rate of 7 ± 2Kkbar⁻¹, the volume decreased by 0·25 ± 0·05 mlmol⁻¹ and the chemical or structural entropy by 3·6±0·7 J mol⁻¹K⁻¹. Analysis of the calorimetric data gave an entropy change of 3·1 J mol⁻¹ K⁻¹. The contribution from the elastic strain energy is 64Jmor' and from the frictional energy is 62J mol⁻¹. The latent heat of transformation is 995 J mol⁻¹. The change in the entropy at the transformation corresponds to a weighted overall change in the vibrational frequencies by at most 32%, part of which must be due to other effects. Reflection spectroscopy in the far-infrared region or neutron scattering studies can be used to test the validity of our conclusions regarding the change in the phonon spectrum of such alloys at their thermoelastic (martensite) transformations.