Professor Dunand discovers shape-memory metal foam
Some metal alloys change their shape when exposed to magnetic field. For some metal crystals, such as off-stoichiometric Ni2MnGa Heusler alloys, the deformation can be as high as 10%. Magnetic shape-memory describes a phenomenon when the alloy retains the new shape upon removal of the magnetic field, and the original shape is restored by applying an opposite field. In general, there is little shape-memory effect in a polycrystalline sample because of random orientation of the grains; any change in one grain is cancelled by an opposing change in another grain oriented in the opposite direction. In an article published with coworkers in Physical Review Letters (99, 247201 (2007), December 14, 2007), Professor Dunand reported that Ni-Mn-Ga foams with 55% and 76% open porosity display a fully reversible magnetic field-induced strain of up to 0.115% without bias stress, which is about 50 times larger than nonporous, fine-grained Ni-Mn-Ga. He believes that the material can have applications like in sonar, actuators and magnetomechanical sensors, very small motors and other applications that require small, rapid displacements. It may have applications in aircrafts, changing the shape of airplane wings slightly to reduce aerodynamic resistance based on the speed of the aircraft, making the flight more fuel efficient at all speeds. For more information, see McCormick News article by Emily Ayshford.
H. Kung, Feb 19, 2008.