Shape Memory Alloys
17 July 2007, 13:29
Categories: smt-polymorphic-shape-shifters
Shape memory alloys (SMAs) are smart materials that can change their shape at a specific temperature. Wires that are made of shape memory alloys, such as NiTi, contract when they are converted to the austenitic phase (i.e. heated) and expand when they become martensitic (i.e. cooled). Thus the Joule heating of an electric current will make a SMA wire contract; when the current is switched off, the surrounding air will cool the wire and will cause it to expand.
This phenomenon is being used for a number of applications. The Shape Memory Alloys group from TU-Berlin is using SMAs to adapt the shape of an airfoil for aircraft (model shown above). Their prototype consists of a “slice” of an airfoil which bends so as to minimize the drag of the airfoil under the constraint of a fixed lift force. The group also offers an interactive online simulation, which is useful for examining how SMAs perform under various conditions.
Copper (Cu), nickel (Ni), and titanium (Ti) are common metals in SMAs, however other metals can also be used in the alloys (e.g. platinum, iron, aluminum, gallium, and vanadium) and these elements change the performance of the SMAs. SMAs based on PtFe3 have been shown to be useful for low temperature applications, NiTi doesn’t work very well at very high temperatures, and a report published in Platinum Metals Review states that it appears possible to engineer a shape memory transition in a (Pt, Ni)Ti system anywhere between room temperature and 1000°C, a versatility which is probably unique among all known SMAs. Another report, in the Journal of Magnetism and Magnetic Materials, indicates Ni-Mn-Ga SMAs have been achieved with martensitic transition near room temperature.
Posted by: The Editors
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