Measurement of heat of transformation in Ti-Ni shape-memory alloy under stress imposition

The heat of transformation of Ti-Ni shape-memory alloy has an important effect on the shape memory characteristics if the alloy is used for actuators. Usually, the heat of transformation is measured by the differential scanning calorimetry (DSC) which is difficult to measure the heat under stress imposition. In this study, a new measuring method of heat of transformation under stress imposition is proposed. A wire of Ti (49.8%)-Ni (50.2%) alloy, 1-min in diameter, is fixed in a chamber of which wall is cooled at 173K, and a d-c electric current is imposed to the wire to increase its temperature up to 373K. Loads are applied to the lower end of the wire and the length of the wire is measured. The heating curve measured by a thin thermocouple welded on the wire is analyzed based on the heat balance composed of Joule heat, heat of transformation, heat transfer to surroundings and heat capacity of the wire. The heat transfer coefficient between the wire and the surrounding is obtained by the heating curve of an 18-8 stainless steel wire. By this method, the heat of transformation and the transformation kinetics are simultaneously obtained from the heating curve of Ti-Ni alloy. The effects of the stress and heat cycle on the above measured results are discussed.