Researchers have made a lighter and potentially cheaper kind of shape-memory alloy: materials that change shape in response to a magnetic field but remember their original shape. The new material, a porous foam made from a nickel-manganese-gallium alloy, stretches slightly when exposed to a magnetic field. It retains its new form when the field is turned off, but it goes back to its original shape when the field is rotated 90 degrees.
Most shape-memory alloys are driven by temperature changes. Magnetically driven alloys, however, respond faster than those that respond to temperature. Another important advantage of materials that change shape under a magnetic field is that they can be activated from a distance, says Robert O’Handley, a materials-science and engineering researcher at MIT. Because magnetic shape-memory materials can be remotely changed, he says that they are particularly promising for biomedical applications. “You could make a stent, where you apply a magnetic field to it from outside the body and gradually open up an artery,” he says.
But magnetic shape-memory alloys have been difficult and expensive to make. The new alloy could be cheaper and easier to synthesize.
And it could be useful in devices that need very precise, repeatable, and rapid positioning, says David Dunand, a materials-science and engineering professor at Northwestern University. Dunand led the work on the new alloy with Peter Mullner, an associate professor at Boise State University. These devices include microscopes, tiny mirrors used in optical communication, and robots used in medicine. Because the foam is light, it could lead to aerospace applications, such as airplane wings that morph to become more aerodynamic.
The alloy that Dunand and his colleagues used is not new. Single crystals of nickel-manganese-gallium are known to stretch by 10 percent when exposed to a magnetic field. But single crystals, in which all the atoms are packed in a regular, repeating pattern, are expensive and time consuming to make.
Normally, the problem is that in polycrystalline metals, the individual crystals have random orientations. In the presence of a magnetic field, they stretch along different directions, pushing against each other and canceling out each other’s motion, Dunand says. “The dream is to make a polycrystal but somehow give space to [the individual crystals] so they can move and not cancel each other’s motions.” This is precisely what happens in the foam because of the pores. The tiny crystals in the alloy get room to stretch, and the foam changes shape. The change is tiny right now–only 0.12 percent–but it’s a start, Dunand says.