Shape memory alloys (SMAs) have been used as actuators in many different industries since the discovery of the shape
memory effect, but the use of SMAs as actuation devices in aeronautics has been limited due to the temperature
constraints of commercially available materials. Consequently, work is being done at NASA's Glenn Research Center to
develop new SMAs capable of being used in high temperature environments. One of the more promising high-temperature
shape memory alloys (HTSMAs) is Ni19.5Ti50.5Pd25Pt5. Recent work has shown that this material is capable
of being used in operating environments of up to 250°C. This material has been shown to have very useful actuation
capabilities, demonstrating repeatable strain recoveries up to 2.5% in the presence of an externally applied load. Based
on these findings, further work has been initiated to explore potential applications and alternative forms of this alloy,
such as springs. Thus, characterization of Ni19.5Ti50.5Pd25Pt5 springs, including their mechanical response and how
variations in this response correlate to changes in geometric parameters, are discussed. The effects of loading history, or
training, on spring behavior were also investigated. A comparison of the springs with wire actuators is made and the
benefits of using one actuator form as opposed to the other discussed. These findings are used to discuss design
considerations for a surge-control mechanism that could be used in the centrifugal compressor of a T-700 helicopter
engine.
Researchers at NASA Glenn Research Center have been investigating high temperature shape memory alloys as potential damping materials for turbomachinery rotor blades. Analysis shows that a thin layer of SMA with a loss factor of 0.04 or more would be effective at reducing the resonant response of a titanium alloy beam. Two NiTiHf shape memory alloy compositions were tested to determine their loss factors at frequencies from 0.1 to 100 Hz, at temperatures from room temperature to 300°C, and at alternating strain levels of 34-35x10-6. Elevated damping was demonstrated between the Ms and Mf phase transformation temperatures and between the As and Af temperatures. The highest damping occurred at the lowest frequencies, with a loss factor of 0.2-0.26 at 0.1 Hz. However, the peak damping decreased with increasing frequency, and showed significant temperature hysteresis in heating and cooling.
Over the past few decades, binary NiTi shape memory alloys have received attention due to their unique mechanical
characteristics, leading to their potential use in low-temperature, solid-state actuator applications. However, prior to
using these materials for such applications, the physical response of these systems to mechanical and thermal stimuli
must be thoroughly understood and modeled to aid designers in developing SMA-enabled systems. Even though shape
memory alloys have been around for almost five decades, very little effort has been made to standardize testing
procedures. Although some standards for measuring the transformation temperatures of SMA's are available, no real
standards exist for determining the various mechanical and thermomechanical properties that govern the usefulness of
these unique materials. Consequently, this study involved testing a 55NiTi alloy using a variety of different test
methodologies. All samples tested were taken from the same heat and batch to remove the influence of sample pedigree
on the observed results. When the material was tested under constant-stress, thermal-cycle conditions, variations in the
characteristic material responses were observed, depending on test methodology. The transformation strain and
irreversible strain were impacted more than the transformation temperatures, which only showed an affect with regard to
applied external stress. In some cases, test methodology altered the transformation strain by 0.005-0.01mm/mm, which
translates into a difference in work output capability of approximately 2 J/cm3 (290 in•lbf/in3). These results indicate the
need for the development of testing standards so that meaningful data can be generated and successfully incorporated
into viable models and hardware. The use of consistent testing procedures is also important when comparing results
from one research organization to another. To this end, differences in the observed responses will be presented,
contrasted and rationalized, in hopes of eventually developing standardized testing procedures for shape memory alloys.
KEYWORDS: Palladium, Titanium, Nickel, Shape memory alloys, Actuators, Temperature metrology, Particles, Scanning electron microscopy, Chemical analysis, Solid state physics
High-temperature shape memory alloys in the NiTiPd system are being investigated as lower cost alternatives to NiTiPt
alloys for use in compact solid-state actuators for the aerospace, automotive, and power generation industries. A range of
ternary NiTiPd alloys containing 15 to 46 at.% Pd has been processed and actuator mimicking tests (thermal cycling
under load) were used to measure transformation temperatures, work behavior, and dimensional stability. With
increasing Pd content, the work output of the material decreased, while the amount of permanent strain resulting from
each load-biased thermal cycle increased. Monotonic isothermal tension testing of the high-temperature austenite and
low temperature martensite phases was used to partially explain these behaviors, where a mismatch in yield strength
between the austenite and martensite phases was observed at high Pd levels. Moreover, to further understand the source
of the permanent strain at lower Pd levels, strain recovery tests were conducted to determine the onset of plastic
deformation in the martensite phase. Consequently, the work behavior and dimensional stability during thermal cycling
under load of the various NiTiPd alloys is discussed in relation to the deformation behavior of the materials as revealed
by the strain recovery and monotonic tension tests.
Potential applications involving high-temperature shape memory alloys have been growing in recent years. Even in those cases where promising new alloys have been identified, the knowledge base for such materials contains gaps crucial to their maturation and implementation in actuator and other applications. We begin to address this issue by characterizing the mechanical behavior of a Ni19.5Pd30Ti50.5 high-temperature shape memory alloy in both uniaxial tension and compression at various temperatures. Differences in the isothermal uniaxial deformation behavior were most notable at test temperatures below the martensite finish temperature. The elastic modulus of the material was very dependent on strain level; therefore, dynamic Young's Modulus was determined as a function of temperature by an impulse excitation technique. More importantly, the performance of a thermally activated actuator material is dependent on the work output of the alloy. Consequently, the strain-temperature response of the Ni19.5Pd30Ti50.5 alloy under various loads was determined in both tension and compression and the specific work output calculated and compared in both loading conditions. It was found that the transformation strain and thus, the specific work output were similar regardless of the loading condition. Also, in both tension and compression, the strain-temperature loops determined under constant load conditions did not close due to the fact that the transformation strain during cooling was always larger than the transformation strain during heating. This was apparently the result of permanent plastic deformation of the martensite phase with each cycle. Consequently, before this alloy can be used under cyclic actuation conditions, modification of the microstructure or composition would be required to increase the resistance of the alloy to plastic deformation by slip.
The microstructure, transformation temperatures, basic tensile properties, shape memory behavior, and work output for two (Ni,Ti)Pt high-temperature shape memory alloys have been characterized. One was a Ni30Pt20Ti50 alloy (referred to as 20Pt) with transformation temperatures above 230 °C and the other was a Ni20Pt30Ti50 alloy (30Pt) with transformation temperatures above 530 °C. Both materials displayed shape memory behavior and were capable of 100% (no-load) strain recovery for strain levels up to their fracture limit (3-4%) when deformed at room temperature. For the 20Pt alloy, the tensile strength, modulus, and ductility dramatically increased when the material was tested just above the austenite finish (Af) temperature. For the 30Pt alloy, a similar change in yield behavior at temperatures above the Af was not observed. In this case the strength of the austenite phase was at best comparable and generally much weaker than the martensite phase. A ductility minimum was also observed just below the As temperature in this alloy. As a result of these differences in tensile behavior, the two alloys performed completely different when thermally cycled under constant load. The 20Pt alloy behaved similar to conventional binary NiTi alloys with work output due to the martensite-to-austenite transformation initially increasing with applied stress. The maximum work output measured in the 20Pt alloy was nearly 9 J/cm3 and was limited by the tensile ductility of the material. In contrast, the martensite-to-austenite transformation in the 30Pt alloy was not capable of performing work against any bias load. The reason for this behavior was traced back to its basic mechanical properties, where the yield strength of the austenite phase was similar to or lower than that of the martensite phase, depending on temperature. Hence, the recovery or transformation strain for the 30Pt alloy under load was essentially zero, resulting in zero work output.
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