The electrical transport properties of shape memory alloys

The effectiveness of shape memory alloys as active elements in thermal actuators is proved by the huge amount of applications developed in the last few decades. In recent years, interest has kept growing in the sensing capabilities that they show when they are heated by an electrical current. The electrical resistance measured thereby gives an easily available feedback of the actuator displacement, provided that a suitable relation is found between the two quantities. It becomes, therefore, compulsory to investigate several alloys and different testing conditions to best fit applications with a suitable material. In this paper, a recently developed apparatus is presented. It allows driving the shape memory element with a finely controlled current waveform while the electrical resistance and the displacement are precisely measured. Besides simulating real operation conditions, this apparatus does not suffer from the high temperature limitations that affect currently used thermostatic-bath-based equipment, it will allow investigating the martensitic transformation and its fundamental properties in the newly developed high-temperature alloys (e.g. NiTiHf). The results of testing, under several constant stress states, two specific materials are also discussed: equiatomic NiTi wires, which show poor sensing features and melt spun TiNiCu ribbons, with remarkably promising sensing/actuating characteristics.