One of the most effective ways to increase the performance, reduce the noise and emissions, and improve the overall efficiency of subsonic to hypersonic aircraft is to replace various static structures with adaptive or reconfigurable components. Designs for everything from adaptive inlets, nozzles, flaps, and other control surfaces, to variable-geometry chevrons, reconfigurable blades, and active hinges for the operation of various doors and panels have been developed. However, enabling such dramatic design changes requires the use of high-energy-density functional materials such as shape-memory alloys (SMAs).
The most viable SMAs have been based on binary nickel titanium (NiTi), but this class of SMA has a very low temperature capability, generally in the range of -100 to 80 °C. Many of the envisioned applications in aeronautics require SMAs that have temperature capability far in excess of this level but that can still generate significant work output. Alloying additions including palladium (Pd), platinum (Pt), gold (Au), halfnium (Hf), and zirconium (Zr) have been shown to increase the transformation temperatures for NiTiX alloys, but to date, almost all research on high-temperature SMA (HTSMA) behavior has been performed under stress-free conditions. The mere exhibition of shape-memory behavior at elevated temperature is not sufficient for evaluating the potential of these materials, especially when the primary application calls for integration of the HTSMA within adaptive structures or as a component in a solid-state actuator (ref. 1). Instead, work output (or the ability of the material to recover strain against some biasing force) is the most important property for screening the viability of different alloys. Because such data do not exist in the open literature, load-bias testing was performed at the NASA Glenn Research Center to quantify the work output of several HTSMA systems (e.g., NiTiPd, NiTiPt, and NiTiAu) over a wide range of temperatures. The load-bias testing consisted of constant-load, strain-temperature tests, where the load was applied to the sample at room temperature and then held constant as the sample was heated and cooled through the transformation regime (for details, see ref. 1).
The graph summarizes the data generated during this survey. Each data point represents a unique alloy composition with the maximum work output plotted (for details on many individual alloys in the figure, see ref. 2). The bars surrounding each data point indicate the full temperature range of the transformation from the martensite finish temperature Mf to the austenite finish temperature Af for each alloy. For active control of a component using SMAs, the environmental temperature must be below this transformation temperature range, whereas passive control of an actuator would require the environmental temperature to pass through this temperature range. Consistent work output of between 8 and 11 J/cm3 was achieved for alloys with potential operating capability between 100 and approximately 300 °C. To put this level of work output into perspective, 10 J/cm3 is roughly equivalent to the work performed by a piece of wire 0.04 in. in diameter by 25 in. long lifting an attached 110-lb weight a distance of 0.5 in.
Maximum work output and transformation temperature range (Mf to Af) for various NiTiPt, NiTiPd, and NiTiAu alloys. Each data point represents a unique alloy. In general, work output drops off appreciably for alloys with transformation temperatures above 300 °C, although a second generation of alloys is being developed for use at higher temperatures.
Long description of figure.
For alloys with transformation temperatures above 300 °C, the work output drops off appreciably. To address this issue, Glenn researchers are currently developing a new generation of advanced SMAs with significantly improved properties for use at higher temperatures.
Last updated: December 14, 2007
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