Shape memory alloy
Description
A metal alloy that can be deformed then return to its original shape with heat. Shape-memory alloys (SMA) take advantage of phase transformations; liquid to solid or liquid to gas transitions are widely known, but changes can also occur with solid phases when the atoms or molecules that make up a material adopt a different pattern. An SMA has two phases: austenite (symmetrical) and martensite (asymmetrical). The switch between the two phases can be induced by temperature or pressure. The first shape metal alloy discovered in the 1950s by William Buehler at the U.S. Naval Ordinance Laboratory (composed of Nickel (55%) and Titanium (45%)) was called Nitinol. In Nitinol, the phase change is just above room temperature. One example shows a crumpled spoon spring back to its original shape when placed in a cup of warm water. Less expensive shape memory metals are composed of copper-zinc-aluminum or copper-aluminum-nickel alloys, as well as many others.
Synonyms and Related Terms
shape memory alloy; SMA; memory metal memory alloy; smart metal; muscle wire; Nitionol (Deut.)
Applications
- Construction in earthquake areas to allow yield but no permanent deformations
- Used in reinforced concrete to sense cracks then contract to close them
- Used medicinally for clogged arteries, bone repair, valve actuation and dental braces
Physical and Chemical Properties
- High strength along with elasticity
- Bio-compatible
- Lightweight
- Expensive
Resources and Citations
- Wikipedia: Shape-memory alloy accessed October 2024
- Nitinol: Website
- Confluent Medical: Nitinol Facts
- Cosmos Magazine: Metal with a memory
- Desroches, Reginald & Smith, B. (2004). Shape memory alloys in seismic resistant design and retrofit: A critical review of their potential and limitations. JOURNAL OF EARTHQUAKE ENGINEERING. 8. 415-429. 2004.