Researchers Develop Magnetic Shape-Memory Metal Foam; Potential for Weight Reduction in Automotive and Aerospace Applications

1 January 2008

The porous nature of nickel-manganese-gallium alloy gives it shape-memory properties. Click to enlarge. Credit: P. Müllner, M. Chmielus and S. Donovan, Boise State University, and D. Dunand and Y. Boonyongmaneerat, Northwestern University.

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), have developed a new class of materials known as magnetic shape-memory foams.

The foam consists of a nickel-manganese-gallium alloy whose structure resembles a piece of 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 strain, up to 10% when a magnetic field is applied.

Strain is the degree to which a material deforms under load. In this instance, the force comes from a magnetic field rather a physical load. Force from magnetic fields can be exerted over long range, making them advantageous for many applications.

The alloy was previously known but not very effective. The research teams increased the strain in polycrystalline Ni-Mn-Ga by nearly fifty times by introducing pores. The alloy material retains its new shape when the field is turned off, but the magnetically sensitive atomic structure returns to its original structure if the field is rotated 90 degrees—a phenomenon called magnetic shape-memory.

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.

—Harsh Deep Chopra, NSF Program Director

Making 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 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 constraint and exhibits a reasonably large degree of strain.

The researchers created the new material by pouring molten alloy into a piece of porous sodium aluminate salt. Once 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 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.

Resources

• Yuttanant Boonyongmaneerat, Markus Chmielus, David C. Dunand, and Peter Müllner, “Increasing Magnetoplasticity in Polycrystalline Ni-Mn-Ga by Reducing Internal Constraints through Porosity”, Phys. Rev. Lett. 99, 247201 (2007) doi:10.1103/PhysRevLett.99.247201

• Stretching More with Pores (Physical Review Focus)

• Magnetic shape memory alloys (University of Helsinki)