<Original Papers>Specific Heat of the Two Step Transformations of Ti_<50>Ni_<50-x>Fe_x

In order to clarify the mechanisnt of the phase transformation of TiNi, the specific heat of Ti_Ni_Fe_x (x=2.5 and 3.5) was studied in a wide temperature range. (1) Two anomalous peaks were observed in the specific heat vs. temperature curves. These peaks correspond to the respective two transformations from the martensite phase (III) to the intermediate state (II) and from the intermediate state (II) to the parent phase (I). (2) A clear separation of these two transformation temperatures was observed for Ti_Ni_Fe_. The concentration dependence of the peak transformation temperature was -57deg/at% Fe for III→II and -42 deg/at% Fe for II→I. (3) The heats of the III→II and II→I transformations were 197 and 99 cal/mol for Ti_Ni_Fe_ and 102 and 92 cal/mol for Ti_Ni_Fe_, respectively. The heat of the III→II transformation increased with decreasing Fe concentration but the heat of the II→I transformation was almost unchanged with Fe concentration. (4) The specific heat in the intermediate state was 1.5 cal/mol・deg which was larger than that of the martensite phase and the parent phase. The phase transformations of TiNi and Ti_Ni_Fe_x were discussed on the basis of the experimental results of the present and previous work on Ti_Ni_Fe_3