The Electrical Resistance Properties of Shape Memory Alloys
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ABSTRACT Shape memory alloys are known for their ability to build up large deformations under an applied stress, either in martensitic phase or in the pseudoelastic region. The electrical resistance of shape memory alloys, traditionally used to define the transformation temperatures, shows interesting features when considering its modification as a function of strain. The electrical resistance in NiTiCu ribbons, obtained by melt spinning, is here examined, either in martensite or in the pseudoelastic range or under a constant applied stress across the transformation range. In each examined case, the results obtained show that the electrical resistance follows a linear relationship as a function of the imprinted strain. INTRODUCTION Shape memory alloys (SMAs) are known for their functional properties related to their ability to recover a deformation and/or to develop a stress state during a constrained recovery. These properties can actually be exploited in a monolithic sensor-actuator only if a physical property can be adopted to drive the deformation: this appears feasible, at least in well defined cases, when the electrical resistance (ER) shows a linear dependence from deformation, e.g. during the deformation recovery in specimens trained for the Two Way Memory Effect (TWME) [I]. The electrical resistance of SMAs, generally used to define the transformation temperatures, shows interesting features when considering its modification as a fimction of strain. In martensitic phase[2-4], under an applied increasing stress and in absence of a phase transformation, a linear or quasi linear increase with deformation is found for different SMAs. In the transformation range parent phaseAf in the temperature range 70 + 85 'C, where martensite can be stress induced, are given in figure 3a): the pseudoelastic behaviour clearly appears with a hysteresis cycle width which modifies with the test temperature. The transformation strain is completely recovered on unloading. The correspondent ER/ERo vs. F curves, detected at the same time, are given in figure 3b), where, here also, ERo is the ER at F=0. All the curves exhibit in the elastic deformation range a very small dependence upon deformation; when the stress induced transformation sets in, a linear law with a higher slope is found both on loading and unloading, though shifted on the deformation scale of approximately A=0. 1%. The shift seems to be due to a different elastic strain contribution during the direct P->M transformation in comparison to the reverse M->P. Results under constant load As above specified, the specimens were submitted to thermal cycles between 30'C and 90'C across the P
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