G. Airoldi

The electrical transport properties of shape memory alloys

Abstract 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.

The two-way memory effect by the pre-strain training method in a 50Ti40Ni10Cu (at.%) alloy

The two-way memory effect (TWME) can be settled after a training of the shape memory alloy apt to stabilize the preferential growth (shrinkage) of martensite domains by decreasing (increasing) temperature. That appears as a consequence of a pattern of line defects which act as nucleation sites of selected variants growth. Though several training procedures have been suggested, a clear cut view of the most efficient one has not yet been reached. Recently a pre-strain method has been adopted to settle the TWME in TiNi alloys, with the highest reversible strain obtained for a plastic pre-strain introduced at temperatures (T) M{sub s}TA{sub f}. The TWME on 50Ti40Ni10Cu (at%) alloy has already been investigated with a training procedure based on a sequence of thermal cycles under applied stress involving the B2(parent) {leftrightarrow} B19(orthorhombic) transformation. Interest is here given to the pre-strain training TWME in a 50Ti40Ni10Cu (at%) alloy introducing the plastic prestrain either in monoclinic (B19{prime}), or in orthorhombic (B19), or in parent (B2) phase in the range of stress induced martensite.

DSC calibration in the study of shape memory alloys

The unusual mechanical properties (i.e. shape memory effect and superelasticity) of shape memory alloys (SMA) rely on the thermoelastic martensitic transformation (TMT) which is a first-order solid-solid, non-diffusive phase transition, athermal in character.Differential scanning calorimetry (DSC) is often used as a convenient method of investigating the thermal properties ofSMAs. The common practice of standard temperature calibration, required for a correct instrument performance, is here critically discussed in relation to the study of both the direct exothermic transformation on cooling, and the reverse endothermic transformation on heating in a NiTiSMA. The DSC results show that, with the standard temperature calibration, the instrument is calibrated on heating but un-calibrated on cooling. A general method is advanced to overcome this problem, intrinsically related to the dynamic character of DSC.ZusammenfassungAußergewöhnliche mechanische Eigenschaften (z. B. Form-Memoryeffekte und Superelastizität) von Formmemory-Legierungen (SMA) beruhen auf einer thermoelastischen martensitischen Umwandlung (TMT), bei der es sich um eine athermische, nichtdiffusive Feststoff-Feststoff-Phasenumwandlung erster Ordnung handelt.DSC wird oft als praktische Methode zur Untersuchung der thermischen Eigenschaften von SMAs benutzt. Die übliche Anwendung der für eine präzise Gerätefunktion erforderlichen Standardtemperaturkalibration wird hier in Bezugnahme sowohl auf die direkte exotherme Umwandlung beim Abkühlen als auch auf die umgekehrte endotherme Umwandlung beim Erhitzen von NiTi SMA diskutiert. Die DSC-Ergebnisse zeigen, daß das Gerät durch diese Standardtemperaturkalibration zwar für das Erhitzen, nicht aber für das Abkühlen kalibriert ist. Es wird eine allgemeine Methode zur Überwindung dieses Problemes entwickelt und in Bezug zum dynamischen Charakter der DSC gesetzt.

The two-way memory effect in a 50 at.% Ti-40 at.% Ni-10 at.% Cu alloy

The two-way memory training across the B2 to or from B19 transformation of a 50 at.% Ti-4O at.% Ni-10 at.% Cu alloy has been investigated here following both the strain and the electrical resistance change. Training loops of the form (P to M transformation to strain to shape recovery on reverse transformation) under applied stress levels of 51, 70.1, 89.2 and 108.3 MPa have been examined. The chief aim of the paper has been both to detect the electrical resistance change across the B2 to or from B19 transformation, which is linearly related to the strain, and to discuss the different contributions related to the electric transport properties. The results found here provide sound support for selecting the electrical resistance as a state control variable in a two-way memory effect device.

The Electrical Transport Properties of Shape Memory Alloys Under a Stress State

The electrical resistance of shape memory alloys under an increasing stress state is here examined: the relevance of the transformation strain connected with the stress induced transformation and the concomitant electronic resistivity change at the transformation are examined for the best alloys now in use.

Electric Transport Properties of a NiTi Shape Memory Alloy Under Applied Stress.

It is well known that mechanical properties of Shape Memory Alloys are strongly dependent upon the test temperature (T) respect to transformation temperatures: the stress-strain curves however hinder the true deformation processes acting. Electrical resistance(ER), a physical property sensibly affected by electronic structure modifications, traditionally used to follow the growth of thermal martensite, is here investigated to follow the modifications of a NiTi alloy, in an initial single phase structure, under applied stress. ER measurements are here detected with the aim to distinguish different deformation processes at four test temperatures T i (i=1,..,4): in martensitic phase,either at T 1 f or at T 2 s within the hysteresis cycle; in parent phase, either at A f 3 or at M s 4 within the hysteresis cycle, where T 2 = T 4 . Results are examined in comparison with previous obtained data and discussed at the light of imprinted deformation.