The development of shape-memory alloys that could operate at temperatures greater than 400 °F would provide an enabling technology for the development of "smart structures" used to control the noise, emissions, or efficiency of gas turbine engines or for the creation of morphing structures permitting more efficient flight over a large envelope of conditions from subsonic to hypersonic. Interest in high-temperature shape-memory alloys (HTSMA) also has been growing in the automotive, process control, and energy industries. However, as is often the case, materials development has seriously lagged component design, with current commercial alloys severely limited in their temperature capability. The two largest challenges to developing a viable HTSMA for demonstration purposes or for commercial use are (1) identification and development of an alloy with an acceptable balance of properties including high transformation temperatures, high work output, stability, and good fatigue life and (2) the availability of the material in an appropriate product form such as fine wire. These challenges are being addressed in a recent collaboration between the NASA Glenn Research Center and Dynalloy, Inc. (Costa Mesa, CA).
Example of a piece of experimental NiTiPt 20-mil-diameter wire developed for shape-memory actuator applications requiring a high-temperature alloy.
Processing procedures have been successfully developed for the wire drawing of a promising new nickel-titanium-platinum (NiTiPt) HTSMA along with postprocessing procedures, such as annealing and training, to prepare the material for actuator applications. The alloy has been processed into 0.020-in.-diameter wire (see the preceding photograph) but could just as easily be processed into finer diameters. After postdrawing heat treatment and training, the behavior of the new NiTiPt wire is extremely stable. The transformation temperatures for the martensite start and austenite finish temperatures are 545 and 570 °F, respectively, and the transformation temperatures are stable with respect to repeated thermal cycling to as high as 840 °F. The hysteresis is narrow, just 25 °F, making the material ideal for actuator applications requiring active control. The transformation strain at 25 ksi is about 1.5 percent, and the corresponding work output is 375 in.-lb/in.3 This is about half the work output that is possible from commercial NiTi wire. However, the NiTiPt wire is capable of repeated actuation in high-temperature environments (up to about 500 °F), whereas commercial "high-temperature" NiTi alloys are limited to about 100 to 200 °F.
Accumulated permanent strain as a function of the number of actuation cycles acting against a 25-ksi (172-MPa) bias stress on a 20-mil wire. The amount of permanent strain accumulated in the trained NiTiPt wire is minimal and less than that of a commercial "high-temperature" NiTi actuator wire even though the NiTiPt wire is operating at temperatures higher than 400 °F.
After appropriate training, HTSMAs also display excellent dimensional stability during repeated actuation against a bias stress of 25 ksi, as shown in the graph. In fact, the trained NiTiPt alloy is dimensionally more stable after repeated actuation than commercial "high-temperature" NiTi alloys, even though there is a 400 °F difference in operating temperature. This dimensional stability or resistance to changes in length during use is critical for applications requiring repeated cycling, to ensure a reasonable fatigue life and to minimize drifting of the zero point in shape-memory alloy actuators (ref. 1). Further improvements in the capabilities of the NiTiPt wire are expected as this new HTSMA is carefully optimized.
Find out more about this research:
Glenn’s materials research:
http://www.grc.nasa.gov/WWW/5000/MaterialsStructures/
Dynalloy, Inc.:
http://www.dynalloy.com/ (external site)
Glenn contacts:
Dr. Ronald D. Noebe, 216-433-2093, Ronald.D.Noebe@nasa.gov; and Dr. Santo A. Padula II, 216-433-9375, Santo.A.Padula@nasa.gov
Authors:
Dr. Ronald D. Noebe, Jeffrey C. Brown, Susan L. Draper, Glen S. Bigelow, and Nicholas Penney
Headquarters program office:
Aeronautics Research
Programs/Projects:
QAT
Last updated: October 16, 2006
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