High-Temperature SMAs for Actuator Applications

Work output is comparable to conventional SMA alloys but with transition temperatures significantly exceeding those of conventional materials.

Compositions and production processes have been developed for making NiTi-based shape-memory alloys (SMAs) that can be tailored for use as actuator materials at temperatures exceeding those of conventional alloys. Whereas conventional shape-memory alloys are limited to use at temperatures well below 100 °C due to low transformation temperatures, these high-temperature shape-memory alloys (HTSMAs) have transformation temperatures exceeding 300 °C while maintaining many of the other attributes associated with NiTi alloys, most importantly high work output (see Figure 1). Other attractive properties of this family of NiTiPt HTSMAs include usefully high values of tensile ductility, relatively narrow hysteresis, good oxidation resistance up to 600 °C, and excellent thermal and dimensional stability. Just as important, these alloys can be readily processed into various structural forms such as thin rod and fine-diameter wire by conventional processes (see Figure 2). These materials hold promise for expanding the variety of applications in which SMAbased actuators could be used.

The compositions of the present alloys can be summarized generally as

xTi + yPt + zM + (bal.)Ni,

where
M is one or more of Au, Pd, and Cu;
x, y, and z are atomic percentages;
x lies between 50 and 52;
y lies between 10 and 25;
z lies between 0 and 5; and
optionally, as explained below, the general composition described thus far can be modified by microalloying with C.

Ti-rich Ti-Ni-Pt alloys, including ones of the present type, are multi-constituent materials consisting of a Ti₂(Ni,Pt) phase within a Ti(Ni,Pt) matrix that is capable of undergoing a thermoelastic martensitic transformation (which is the desired shape-memory transformation). Ti₂(Ni,Pt) forms as a coarse interdendritic structure during melting and is undesired because it affords no benefit; is not amenable to modification or suppression through heat treatment; tends to embrittle the alloy, thereby making thermomechanical processing more difficult; and reduces the fatigue life, the fracture strength, and the martensite volume fraction of the alloy. However, a small amount of Ti₂(Ni,Pt) phase is tolerable by keeping x fairly close to 50 at.%.

Furthermore, the Ti₂(Ni,Pt) phase can be significantly reduced in volume fraction or even eliminated by deoxidation of the melt through the introduction of controlled amounts of C, CO, or simply by using a graphite crucible during melting. In addition to suppressing the Ti₂(Ni,Pt) phase, this type of treatment will result in micron-size TiC particles and submicron particles of a new Tirich NiTiPt intermetallic phase that is amenable to heat treatment by typical solution and precipitation processes.

In general, alloys of this type can be produced by essentially any melting process, such as arc melting, induction melting, and vacuum arc remelting. The cast ingots are homogenized, typically for 72 hours at 1,050 °C in vacuum, and then subjected to thermomechanical processing. These alloys are amenable to such conventional processes as rolling, extrusion, swaging, forging, and drawing. A preferred process for making thin rod or wire includes multiple hot extrusion at temperatures above 800 °C, which can be followed by cold drawing to produce fine diameter wire. Final actuator properties are optimized in such materials through a combination of cold work, heat treatment, and training. (As used here, “training” is a term of art signifying a subprocess in which an SMA is subjected to additional thermomechanical processing.) In principle, it should also be possible to make thin films of these alloys by sputtering.

This work was done by Ronald D. Noebe, Susan L. Draper, and Michael V. Nathal of Glenn Research Center, and Anita Garg of Amber Research Corp.

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-18054-1.