Shape memory alloy

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A shape memory alloy (SMA, also known as memory metal) 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.

Contents

[edit] 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, Ms denotes the temperature at which the structure starts to change from austenite to martensite upon cooling; Mf is the temperature at which the transition is finished. Accordingly, As and Af 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).

Image:Sma_wire.jpg

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

[edit] 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.

one way shape memorytwo way shape intrinsic memory

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 As and is completed at Af (typically 2 to 20 °C or hotter, depending on the alloy or the loading conditions). As 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.

[edit] 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" [1]), 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.

[edit] Crystal structures

Why does a metal or an alloy observe these qualities of memory? Dr. Frederick E. Wang[1], an expert in crystalline structures, was the one who came with the early answers to the phenomenon. According to him, Nitinol undergoes phase changes while remaining a solid. Normally these phase changes occur in an alloy when heated to its melting point. Different phase changes occur at different temperatures. [2] These phase changes, known as martensite and austenite, "involve the rearrangement of the position of particles within the crystal structure of the solid." In shape memory alloys, these phase transformations occur below its melting point. Thus, the alloys can retain their shape without melting. Some alloys change shape within a small difference in temperature. Under the transition temperature, Nitinol is in the martensite phase and can be bent into various shapes. To set the "parent shape" the metal must be held in position and heated to about 500 ° C. (varies for different SMAs). The high temperature "causes the atoms to arrange themselves into the most compact and regular pattern possible" resulting in a rigid cubic arrangement known as the austenite phase. This shape is registered in the crystalline structure when the alloy is cooled and any changes in the original shape is resisted, hence the name "muscle wire."

[edit] Manufacture

Researching now.

[edit] Properties

Very strong and contains many different metals.

[edit] Applications

[3]

The first consumer commecial application for the material was as a shape memory coupling for piping, eg; oil line pipes for industrial applications, water pipes and similar types of piping for consumer/commercial applications. The late 1980's saw the commercial introduction of Nitinol as an enabling technology in a number of mininally invasive endovascular medical applications. While more costly than stainless steel, the self expanding properties of Nitinol alloys manufactured to BTR (Body Temperature Response), have provided an attractive alternative to balloon expandable devices. On average, 50% of all peripheral vascular stents currently available on the worldwide market are manufactured with Nitinol.

[edit] Materials

Materials having the memory effect at different temperatures and at different percentages of its solid solution contents.

  • 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 XXX
  • 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

[edit] External links

[edit] References

  • Duerig, TW, KN Melton, D Stöckel and CM Wayman. "Engineering Aspects of Shape Memory alloys". ISBN 0-7506-1009-3. London: Butterworth Heinemann, 1990.
  • K. Shimizu and T. Tadaki, Shape Memory Alloys, H. Funakubo, Ed., Gordon and Breach Science Publishers, 1987
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