Shape memory alloy
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A shape memory alloy (SMA) (also known as memory metal or smart wire) is a metal that remembers its geometry. After it is deformed, it regains its original geometry by itself during heating (one-way effect) or, at higher ambient temperatures, simply during unloading (pseudo-elasticity). 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.
The two main types of SMA are the copper-zinc-aluminium alloys, and the generally more expensive nickel-titanium (NiTi) alloys. On the other hand, NiTi alloys 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 where the structure starts to change from austenite to martensite upon cooling; Mf is the temperature where the transition is finished. Accordingly, As and Af are the temperatures where the reverse transformation from martensite to austenite start and finish, repectively. 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).
The nickel-titanium alloys were first developed in 1965 by the Naval Ordnance Laboratory and commercialised under the trade name Nitinol (an acronym for NIckel TItanium Naval Ordnance Laboratories).
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. However, these materials are not currently appropriate for applications such as robotics or artificial muscles, due to 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, becoming commercially available in the late 1990's.
There is another type of SMA called ferromagnetic shape memory alloys (FSMA), that change shape under strong magnetic fields.
External links
- Nitinol technology (http://www.nitinol.com/3tech.htm) (from Nitinol Devices & Components)
- Nitinol Technical Data/Application Notes (http://www.jmmedical.com/html/nitinol_technical_information.html#NitinolTechnicalInfo) (from Johnson Matthey, Inc.)
- introductions and comparisons of active materials (http://www.mide.com/active_materials.html) (from Midé Technology Corporation)
- BBC report on medical applications of Nitinol (http://news.bbc.co.uk/1/hi/health/755819.stm)
- SFB 459: A German Research Center for Shape Memory Alloys (http://www.ruhr-uni-bochum.de/sfb459/english/)de:Formgedächtnis-Legierung