Precipitation-Strengthened, High-Temperature, High-Force Shape Memory Alloys
- Created: Friday, 01 August 2008
Shape memory alloys capable of performing up to 400 °C have been developed for use in solidstate actuator systems.
Shape memory alloys (SMAs) are an enabling component in the development of compact, lightweight, durable, high-force actuation systems particularly for use where hydraulics or electrical motors are not practical. However, commercial shape memory alloys based on NiTi are only suitable for applications near room temperature, due to their relatively low transformation temperatures, while many potential applications require higher temperature capability. Consequently, a family of (Ni,Pt)1–xTix shape memory alloys with Ti concentrations x ≤ 50 atomic percent and Pt contents ranging from about 15 to 25 at.% have been developed for applications in which there are requirements for SMA actuators to exert high forces at operating temperatures higher than those of conventional binary NiTi SMAs. These alloys can be heat treated in the range of 500 °C to produce a series of fine precipitate phases that increase the strength of alloy while maintaining a high transformation temperature, even in Ti-lean compositions.
The absolute minimum requirement for determining the operating temperature of an SMA is the temperature of the martensite-to-austensite, solid-state phase transformation, which is the source of the shape memory behavior. However, the present high-temperature, high-force SMAs can be used at temperatures up to 400 °C not only because they exhibit a range of high transformation temperatures, but because these materials also exhibit high strength (resistance to dislocation mediated deformation processes) in both the austenite and martensite phases, have a relatively high recovery temperature, and also exhibit excellent dimensional stability (little or no irrecoverable strain component during the transformation process). Consequently, these alloys are attractive for use in performing actuator and control functions particularly in the aggressive environments often encountered in aerospace, automotive, and down-hole energy exploration applications.
The composition of an alloy of this type is given generally by the empirical formula Nix–y–zPtyMzTi100–x, where M can be Au, Pd, or Cu; x, y, and z are atomic percentages; 50 ≤ x ≤ 55; 10 ≤ y ≤ 30; 0 ≤ z ≤ 10. These alloys are precipitation-hardenable, but unlike in prior Ti-rich NiTi alloys, the slightly Ti-lean composition prevents the formation of the Ti2Ni phase, which is a coarse globular phase that cannot be thermally tailored in as much as it appears during solidification. Instead, one can rely on precipitation of fine (Ni,Pt)3Ti2 structures and other intermetallic phases for enhanced performance. These precipitates are often lath-like in structure and, in many cases, submicron in size. The precipitate volume fraction can also be tailored through heat treatment and alloy composition. These precipitates result in additional strengthening of the austenite (high-temperature) matrix phase and increase the resistance of the martensite phase against slip, without exerting a significant effect on the detwinning stress. Thus, the over-all effects are greater specific work output with better dimensional stability, and superior mechanical properties, especially at high temperatures.
The desirable properties of these alloys include the following:
- Their transformation temperatures variously remain stable or increase with aging time.
- They exhibit specific-work-output levels >9 J/cm3 and good work performance, comparable to those of conventional binary NiTi alloys.
- Unlike NiTi and other NiTi-based ternary SMAs, these alloys do not exhibit transformation temperatures lower than those of corresponding stoichiometric alloys; indeed, these alloys exhibit transformation temperatures higher than those of similar alloys that have Ti-rich compositions.
- Unlike in the prior NiTi-based ternary SMA alloys, which exhibit decreases in transformation temperatures with increased aging time or thermal cycling, these alloys exhibit stabilization of, or increases in, transformation temperatures with aging.
- These alloys can be processed into such bulk forms as bar, rod, sheet, plate, and wire through conventional thermomechanical processes. Because these alloys have high recrystallization temperatures (700 to 800 °C), they are amenable to heat treatment and aging after thermomechanical processing, without adversely affecting grain sizes.
- The high recrystallization temperatures also make these alloys suitable for use in applications in which they could be subjected to significant heating above their rated temperatures.
This work was done by Ronald D. Noebe, Susan L. Draper, and Michael V. Nathal of Glenn Research Center and Edwin A. Crombie of Johnson Matthey, Noble Metal Products N.A.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17993-1.