PHASE TRANSFORMATIONS IN Ti50Ni50-xFex ALLOYS

Abstract

The phase transformations in Ti50Ni50-xFex alloys have been studied by means of electrical resistivity measurement, differential scanning calorimetry (DSC), dilatometry, observation of shape changes, optical microscopy, transmission electron microscopy (TEM) and electron diffraction. During cooling, the electrical resistivity rapidly increased at M's (starting temperature for the parent to intermediate phase transformation) and then reached maximum at Ms (starting temperature for the intermediate to martensite phase transformation). M's and Ms corresponded with temperatures of abrupt rising of two exothermic peaks and two breaks in DSC and thermal expansion curve on cooling, respectively. The shape changes during heating and cooling for Ti50Ni47.5Fe2.5 were devided into two steps. Correspondingly, two kinds of surface relief were distinctly observed. In the parent to intermediate phase transformation, its metallographic feature exhibited the thermoelastic nature. In electron diffraction, many extra reflections occured close to one-third and one-haef positions of the B2 reciprocal lattice on cooling. As an intermediate state, the needle shaped domains with 1/3B2 extra spots were observed in electron microscopy. Judging from morphology and temperature range of the structural change, these domains coincided with the surface relief of the parent to intermediate phase transformation in optical microscopy. The crystal structure, internal defects and morphology of martensites were also described. As an applied investigation for development of practical use of TiNi alloys, the drastic reversible shape memory (RSM) effect associated with the two-step transformation was found to be obtained by a constrained aging in Ni rich TiNi alloys.