A wide range of publications over the past 15 years have documented the rapid strides have been made in developing alternatives to conventional actuators in realistic applications through the use of smart materials. Even with recent advances, though, an early review of smart material candidates by Crawley (1994) reflects the relative merits of different actuation technologies. Most of the materials considered in Table 1 have rather limited strain capability. Shape Memory Alloys (SMAs), on the other hand, offer a very high strain capability (up to 5-8%), which leads to the potential for large amplitude motion with greatly reduced volume, weight and moving parts relative to competing concepts. Furthermore, their high elastic modulus suggests SMA actuators can provide a high ratio of force to volume that make them well suited to the challenges of providing high force actuation in confined spaces. The relatively low bandwidth of SMAs is not a constraint for many important problems and can be circumvented by use of feedback control of wire temperature, effective use of convective cooling, or appropriate mechanical design implementation.
The most practical shape memory materials are alloys of nickel and titanium, though in some cases alternate alloys or optimized doping can enhance selected material properties such as transition temperature. SMAs exist in one of two stable crystalline phases: a high temperature 'austenite' and a low temperature 'martensite'. The austenitic phase has a conventional stress-strain curve with a higher linear modulus of elasticity while the martensitic phase exhibits a stress plateau between two yield points (Figure 1). Transition between the austenite and martensite phases can be triggered by applied stress, but it is more typically accomplished through temperature change. NiTi alloys are good conductors, so direct ohmic heating is a popular and robust way to control the shape memory effect. Phase transformation temperatures over a range of temperatures are available, depending on the particular alloy used; the martensite phase for most SMA materials typically occurs at room temperature, with the transition temperature occurring at approximately 70 °C for the most common 50/50 NiTi alloy. Given that the bulk of most commonly used NiTi actuation materials have transition temperatures under 100C, this limitation can pose a strong challenge to the use of SMAs for high temperature applications.
Figure 1: Schematic of the major material properties of SMAs: phase change with variations in temperature and stress (left); and hysteresis of the stress/temperature behavior of SMAs in their transition between martensite and austenite phases (right).
There has thus been an increasing level of interest in High Temperature SMAs (HTSMAs) in recent years. The list of alloys that exhibit shape memory behavior and which have higher transition temperatures than conventional NiTi include NiTiPd, NiTiPt, NiTiHf, NiTiZ, NiAl, and Fe-based compositions. Limited phenomenological and transformation structure data for these alloys, however, has prevented widespread practical application. Also, a basic challenge is that mechanical properties can be degraded for alloys operating at elevated temperatures (e.g., reduced work output and higher creep levels). Ternary alloys have recently been studied in considerable detail by Noebe et al. 2004; of these, the Ni30Pt20Ti50 alloy tested at NASA/Glenn was found to have particularly promising properties. Measured transformation temperatures for this "20-Pt" alloy were in the 250 °C range, as shown in Figure 2. Very importantly, the maximum work capability appeared to be limited only by the tensile ductility of the material and to be comparable to typical binary NiTi alloys while exhibiting little hysteresis. This combination of high transition temperature, low hysteresis, and high work output makes the 20-Pt alloy of NiTi a very promising HTSMA for actuator applications, though higher transition temperatures may well be achievable with further modification of alloys.
Figure 2: Strain temperature plot showing work output of 20-Pt NiTi alloy (left); low hysteresis exhibited by this material (right) (figure from Noebe 2004).
Figure 3: Stress/strain curve for 20-Pt NiTi alloy (data from R. Noebe, NASA/GRC, March 2005)
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