Shape memory alloy

A shape memory alloy (SMA) (also known as memory metal or smart wire) is a metal that "remembers" its geometry. After a sample of SMA has been deformed from its "original" conformation, it regains its original geometry by itself during heating (one-way effect) or, at higher ambient temperatures, simply during unloading (pseudo-elasticity or superelasticity). These extraordinary properties are due to a temperature-dependent martensitic phase transformation from a low-symmetry to a highly symmetric crystallographic structure. Those crystal structures are known as martensite and austenite.

Overview

The three main types of SMA are the copper-zinc-aluminium, copper-aluminium-nickel, and nickel-titanium (NiTi) alloys. NiTi alloys are generally more expensive and possess superior mechanical properties when compared to copper-based SMAs. The temperatures at which the SMA changes its crystallographic structure are characteristic of the alloy, and can be tuned by varying the elemental ratios. Typically, M_s denotes the temperature at which the structure starts to change from austenite to martensite upon cooling; M_f is the temperature at which the transition is finished. Accordingly, A_s and A_f are the temperatures at which the reverse transformation from martensite to austenite start and finish, respectively. It is important to note that repeated use of the shape memory effect may lead to a shift of the characteristic transformation temperatures (this effect is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material).

In this figure, ξ (xi) represents the martensite fraction.

One-way vs. two-way Shape Memory

Shape memory alloys may have different kinds of shape memory effect. The two most common memory effects are the one-way shape memory and the two-way shape memory. A schematic view of the two effects is given in the figure below.

SMAtwoway.jpg

In the figure above, the procedures are very similar: starting from martensite (a), adding a reversible deformation for the one-way effect or severe deformation with an irreversible amount for the two-way (b), heating the sample (c) and cooling it again (d). With the one-way effect, cooling from high temperatures does not cause a macroscopic shape change. A deformation is necessary to create the low-temperature shape. On heating, tranformation starts at A_s and is completed at A_f (typically 2 to 20 °C or hotter, depending on the alloy or the loading conditions). A_s is determined by the alloy type and composition. It can be varied between −150 °C and maximum 200 °C.

The two-way shape memory effect is the effect that the material remembers two different shapes: one at low temperatures, and one at the high temperature shape. This can be obtained also without the application of an external force (intrinsic two-way effect). The reason the material behaves so differently in these situations lies in training. Training implies that a shape memory can "learn" to behave in a certain way. Under normal circumstances, a shape memory alloy "remembers" its high-temperature shape, but upon heating to recover the high-temperature shape, immediately "forgets" the low-temperature shape. However, it can be "trained" to "remember" to leave some reminders of the deformed low-temperature condition in the high-temperature phase. There are several ways of doing this.

History

The nickel-titanium alloys were first developed in 1962–1963 by the Naval Ordnance Laboratory and commercialized under the trade name Nitinol (an acronym for Nickel Titanium Naval Ordnance Laboratories). Their remarkable properties were discovered by accident: anecdotally, samples of the alloy were being subjected to strength tests by being pounded with hammers to see how much force was necessary to deform them. After several dents had been created, the researchers left the samples on a windowsill and went to lunch; upon their return, they discovered that the dents had "repaired" themselves.

The range of applications for SMAs has been increasing in recent years, with one major area of expansion being medicine: for example, the development of dental braces that exert a constant pressure on the teeth. There have also been limited studies on using these materials in robotics (such as "Roboterfrau Lara"), as they make it possible to create very light robots. Weak points of the technology are energy inefficiency, slow response times, and large hysteresis.

Metal alloys are not the only thermally-responsive materials, as shape memory polymers have also been developed, having become commercially available in the late 1990s.

There is another type of SMA called ferromagnetic shape memory alloys (FSMA), that change shape under strong magnetic fields. These materials are of particular interest as the magnetic response tends to be quicker and more efficient than temperature-induced responses.

Materials

Materials having the memory effect:

Ag-Cd 44/49 at.% Cd
Au-Cd 46.5/50 at.% Cd
Cu-Al-Ni 14/14.5 wt.% Al and 3/4.5 wt.% Ni
Cu-Sn approx. 15 at.% Sn
Cu-Zn 38.5/41.5 wt.% Zn
Cu-Zn-X (X = Si,Sn,Al) a few wt.% of X
In-Ti 18/23 at.% Ti
Ni-Al 36/38 at.% Al
Ni-Ti 49/51 at.% Ni
Fe-Pt approx. 25 at.% Pt
Mn-Cu 5/35 at.% Cu
Fe-Mn-Si
Pt alloys
Co-Ni-AL
Co-Ni-Ga

External links

Nitinol technology (from Nitinol Devices & Components)
Shape Memory Alloys and Their Applications - Introductory information on Shape Memory Alloys
Nitinol Technical Data/Application Notes (from Johnson Matthey, Inc.)
introductions and comparisons of smart active materials (from Midé Technology Corporation)
BBC report on medical applications of Nitinol
SFB 459: A German Research Center for Shape Memory Alloys
SMAterial.com - phenomena, crystallography, model, simulation and applications of SMA - Has .gif animations demonstrating the effect.
Texas A&M University's Shape Memory Alloy Research Team - SMA overview, publications, etc.
Institute of Physics, ASCR - Research Projects, publications, events & conferences, functional materials

References

Duerig, TW, KN Melton, D Stöckel amd CM Wayman. "Engineering Aspects of Shape Memory alloys". ISBN 0-750-61009-3. London: Butterworth Heinemann, 1990.
K. Shimizu and T. Tadaki, Shape Memory Alloys, H. Funakubo, Ed., Gordon and Breach Science Publishers, 1987