Smart metal foam
Dr. Neil Canter, Contributing Editor | TLT Tech Beat March 2010
A new class of materials lighter than conventional metal alloys still retain excellent mechanical properties.
KEY CONCEPTS
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Metal foam is a relatively new class of materials that is lighter in weight than conventional metal alloys yet retains excellent mechanical properties.
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A smart metal foam has been developed based on a combination of nickel, manganese and gallium that changes shape in the presence of a magnetic field and retains the shape when the field is removed.
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One potential application for this smart metal foam is in micropumps.
Foam is usually considered to be a negative factor if not a curse when present in lubricant applications. This phenomenon occurs due to the entrainment of air in either an aqueous or oil medium. In the case of metalworking fluids, the presence of foam deprives the fluid from actually contacting the surface of a cutting tool and a workpiece during a metal-removal operation. This factor leads to inferior performance.
But foam can impart beneficial properties to a metal alloy. Metal foams are a relatively new class of materials that have the potential to be used in structural applications because they are lighter in weight than conventional metal alloys but can retain excellent mechanical properties. Now a collaborative research program between Northwestern University in Evanston, Ill., and Boise State University in Idaho has led to some very unusual metallic foams.
David Dunand, the James N. and Margie M. Krebs professor of material science and engineering at Northwestern, describes how metal foams are prepared. “Instead of just casting by pouring molten metal into an empty mold,” he says, “the metallic melt is cast onto an aggregate of inorganic oxide particles (such as sodium aluminate) under vacuum. After cooling and cutting the alloy/oxide composite with a saw, the oxide is removed by dissolution in a mineral acid such as hydrochloric acid. The result is a metal alloy sponge or foam with voids replicating the shape, size and connectivity of the original oxide particles.”
Dunand indicates that the trick is finding a suitable mineral acid that will not dissolve the metal and an oxide that is heat-resistant but still dissolves in acid.
Certain metal alloys have been known to change states when placed in a magnetic field. In a previous TLT article, the concept of magnetic refrigeration was examined as an alternative to gas compression (
1). An exotic metal alloy based on manganese, iron, phosphorus and germanium is described that moves reversibly from a disordered paramagnetic state to an ordered ferromagnetic state in a magnetic field. During this process, heat is absorbed from a refrigerator and then expelled.
Metal alloys based on nickel, manganese and gallium also exist that change their shape in the presence of a magnetic field and retains the shape when the magnetic field is removed. These materials then revert back to their original shapes when the magnetic field is rotated by 90 degrees.
Peter Mullner, professor of materials science and engineering at Boise State, says, “This shape memory effect occurs when the direction of the magnetic field is changed. Alternatively, the magnetic-shape memory effect occurs if the metal works against an external load such as a spring. The metal alloy expands when the magnetic field is applied and then contracts when the magnetic field is removed. Metal alloys receptive to this process can expand and contract hundreds of times per second.”
One of the problems in working with these magnetic-shape memory alloys is that the effect is only seen with single crystals, which are difficult to work with and too expensive to be commercially viable. An example of a single crystal is a gem such as a diamond. It is more desirable to prepare magnetic-shape memory alloys out of polycrystalline alloys that are much easier and cheaper to prepare. Such an approach has not been successful until now.
POLYCRYSTALLINE METAL FOAMS
Dunand and Mullner have developed polycrystalline metal foams that function as a magnetic-shape memory alloy. The metal alloy used is also a combination of nickel, manganese and gallium.
The magnetic-shape memory effect is related to a temperature-dependent phenomenon in which the metal alloy transitions during cooling from the austenite phase at elevated temperature to the martensite phase at lower temperature. Mullner explains, “This phase transition is similar to a substance moving from a liquid to a solid. In our case, movement of the single crystal version of the alloy to the martensite phase leads to the formation of crystallographic twins, which shear back and forth in the presence of a magnetic field. This shearing effect gives the alloy its memory capability.”
Magnetic-shape-changing alloys generate stretching and shrinking, which is expressed as a parameter known as magnetic field induced strain (MFIS). Single crystals exhibit high MFIS values up to 10%. Dunand says, “Single crystals are not constrained by neighboring grains, which makes it easy to shear twins and, as a result, readily change shape.”
Polycrystals are much easier to prepare from a molten state than single crystals. But twinning in polycrystals is canceled because neighboring grains deform in incompatible directions and hinder each other.
The researchers overcame this problem by introducing porosity into the structure, making it foam-like. This reduced the number of neighbors for each grain, which could behave much more like a single crystal. Initial efforts led to the creation of a bamboo grain structure in the struts, which contain grains that span their whole width, 200 to 300 microns, and are connected at nodes. This arrangement does allow the struts to display a small MFIS due to the added freedom introduced by the pores.
The researchers decided then to mix two sizes of oxide particles to prepare a foam with two different pore sizes. Dunand says, “We processed the molten metal alloy into a metal foam, leading to a material that has a porosity of 62%. The presence of the two different pore sizes enabled the formation of struts and nodes on two size levels. Besides the big struts and nodes seen previously, a network of finer struts and nodes is developed among pores that are 50 micron in length.”
This finer structure enables twinning to occur in the presence of a magnetic field to change the shape of the metal foam. MFIS for this polycrystalline, magnetic-shape memory metal foam can reach up to almost 10%.
An image showing the two-tier structure of the metal foam is shown in Figure 2. Mullner says, “Now that we have developed this polycrystalline material, the porosity will enable it to be used in applications such as a micropump. The metal foam will be able to move liquids in the presence of a magnetic field without any moving parts. One advantage of a magnetic-shape memory foam as compared to other smart actuator technologies is a large work output, making these materials suitable for micropumps.”
Figure 2. The two-tier structure of the smart metal foam enables the material to change shape in the presence of a magnetic field and retain that shape when the field is removed. (Courtesy of Boise State University and Northwestern University)
Other applications include sonar devices, precision actuators and magneto-mechanical sensors. Future work will involve optimizing the architecture of the foam to achieve better performance. Dunand adds, “We also need to understand what is going on in the nodes and then apply this idea to other alloys.”
The researchers have filed a patent application, and further information can be found in a recent article (
2).
REFERENCES
1.
Canter, N. (2009), “Magnetic Refrigeration: Another Way to Cool,” TLT,
65 (5), pp. 12–13.
2.
Chmielus, M., Zhang, X., Witherspoon, C., Dunand, D. and Mullner, P. (2009), “Giant Magnetic-Field-Induced Strains in Polycrystalline Ni-Mn-Ga Foams,”
Nature Materials,
8 (11), pp. 863–866.
Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.