Engineers Edge (January 6, 2008) - In the world of engineering and design materials that are lighter and cheaper are pursued, especially if those characteristics include superior strength and other desirable properties, such as the materials ability to remember its original shape after it's been deformed by an external force.

A new class of materials known as "magnetic shape-memory foams" has been developed by two research teams headed by Peter Müllner at Boise State University and David Dunand at Northwestern University, both funded by the National Science Foundation (NSF).

The foam like structure or shapes consists of a nickel-manganese-gallium alloy whose form resembles Swiss cheese with small voids of space between thin, curvy "struts" of material. The struts have a bamboo-like grain structure that can lengthen or extend, up to 10 percent or the strain when a magnetic field is applied. Strain is the degree to which a material deforms under load. The innovative properties of this material, is that the deforming or shape changing force comes from a magnetic field rather a physical load. Force from magnetic fields can be exerted over large area, making this material structure ideal for many new and innovative applications. The alloy material maintains its new shape when the field is turned off, but the magnetically sensitive atomic structure of the metal foam returns to its original structure or shape if the field is rotated 90 degrees--a phenomenon called "magnetic shape-memory."

Manufacturing large single crystals of the alloy material is too slow and expensive to be commercially viable so, the researchers make polycrystalline alloys, which contain many small crystals or grains. Traditional polycrystalline materials are not porous and exhibit very low or near zero strains due to mechanical constraints at the boundaries between each grain. In contrast, a single crystal exhibits a large strain as there are no internal boundaries. By introducing voids into the polycrystalline alloy, the researchers have made a porous material that has less internal mechanical constraints and exhibits a reasonably large degree of strain or formability.

The researchers created the new material by pouring molten alloy into a piece of porous sodium aluminate salt. When the material cooled, they leached out the salt with acid, leaving behind large voids. The researchers then exposed the porous alloy to a rotating magnetic field. The level of strain achieved after each of the over 10 million rotations is consistent with the best currently used magnetic actuators, and Müllner and Dunand expect to significantly improve the strain when they have further optimized the foam's architecture.
"The base alloy material was previously known, but it wasn't very effective for shape-memory applications," Dunand said. "The porous nature of the material amplifies the shape-change effect, making it a good candidate for tiny motion control devices or biomedical pumps without moving parts."
NSF Program Director Harsh Deep Chopra agrees. "It's the first foam to exhibit magnetic shape memory - it has great potential for uses that require a large strain and light weight such as space applications and automobiles. These materials are able to do more with less material given their foamy structure and provide a sustainable approach to materials development."

The work was funded by NSF through grant DMR-0502551 to expand basic knowledge about the microstructural properties of shape memory alloys influenced by magnetic fields and through grant DMR-0505772 to develop new shape-memory foams.

News adatperd from materials provided by National Science Foundation (NSF).