EFFECT OF HYDROSTATIC-PRESSURE ON THE THERMOELASTIC TRANSFORMATION OFNI-TI ALLOY AND THE ENTROPY OF TRANSFORMATION

The thermoelastic transformation temperature of a polycrystalline 50.2 2 at.% Ni-Ti alloy has been determined by measuring its electrical res istance over the temperature range 270 K-370 K and hydrostatic pressur es up to 250 bar. The electrical resistance reaches a peak value at 50 % transformation during both cooling and heating of the alloy. The vol ume change at the parent reversible arrow martensite phase transformat ion was measured by dilatometry, and the various energy and entropy ch anges at the transformation were determined by calorimetry. The transf ormation temperature increased with the hydrostatic pressure at the ra te of 7 +/- 2 K kbar(-1), the volume decreased by 0.25 +/- 0.05 ml mol (-1) and the chemical or structural entropy by 3.6 +/- 0.7 J mol(-1) K -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 64 J m ol(-1) and from the frictional energy is 62 J 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 vibrati onal frequencies by at most 32%, part of which must be due to other ef fects. Reflection spectroscopy in the far-infrared region or neutron s cattering studies can be used to test the validity of our conclusions regarding the change in the phonon spectrum of such alloys at their th ermoelastic (martensite) transformations.