The U.S. National Science Foundation (NSF) has recently reported
that two research teams have developed a new porous foam of an alloy
that changes shape when exposed to a magnetic field. The NSF states
that this new material is able to remember its original shape after it’s been deformed by a physical or magnetic force.

Voids of space between thin, curvy struts of metal alloy give the alloy magnetic shape memory.

In the world of commercial materials, lighter and cheaper is usually
better, especially when those attributes are coupled with superior
strength and special properties, such as a material’s ability to
remember its original shape after it’s been deformed by a physical or
magnetic 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 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
percent when a magnetic field is applied. Strain is the degree to which
a material deforms under load. In this instance, the force came 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 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."

Making large single
crystals of the alloy material is too slow and expensive to be
commercially viable — one of the reasons why gems are so costly — 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 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.

Via NSF