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Low-Density Syntactic Foam Alloy

June 18, 2015

Members of my generation are nostalgic about how simple our lives were in the days before rampant technology. One sure way of assessing the quality of life of people is through their literature, and the predominant literature of my childhood was the Bazooka Joe comics found in Bazooka bubble gum. The jokes in these comics were simple; and, to use an expression from that era, "cornball."

In one memorable strip, Joe and his pal, Mort, are walking past a plumbing supply store, where they see the sign, "Cast Iron Sinks." Commenting on the sign, one of them says, "That's silly. Everyone knows that!" Indeed, one of the first things a child learns about nature is that "heavy" things will sink in water, although this applies when "heavy" refers to density, not mass.

Ivory soap barDensity figured into a popular product of that era, Ivory soap. Ivory soap floats on water since the soap's density has been decreased by incorporating tiny bubbles of air. The density of Ivory soap is about 0.9 g/cc, while the density of most others is slightly above 1.1.[1]

(Left, a modified Wikimedia Commons image.)

Ivory soap was famous for more than just floating. In the late nineteenth century, the company commissioned a chemical analysis of their soap. The resulting slogan, "99 44⁄100% Pure," unintentionally initiated discussions of what the remaining 56/100% might be.

There are many examples of materials having air added to their composition to make them lighter, softer, or more thermally insulating. Two of these, bubble wrap and Styrofoam, are found in nearly every office, and polyurethane foam is used as the padding material for seat cushions. A team of scientists and engineers from the New York University Polytechnic School of Engineering (Brooklyn, New York), Deep Springs Technology (Toledo, Ohio), and the U.S. Army Research Laboratory (Aberdeen Proving Ground, Maryland) decided to apply the same principle to the creation of a lightweight composite alloy.[2-3]

What they created was a matrix of the magnesium alloy, AZ91D, infiltrated with hollow spheres of silicon carbide. This alloy, a syntactic foam, has a density of 0.97 g/cc, so it will float on water (see photograph).[2] A syntactic foam is any matrix material, usually a polymer, that's filled with microballoons of another material. This is the lowest density metal matrix syntactic foam to be created, and it has the desirable property that it will absorb elastic energy at high compressive strain rates.[2]

Micrograph of an epoxy matrix syntactic foam filled with glass microballoons

A Scanning electron micrograph of an epoxy matrix syntactic foam filled with glass microballoons

(Via Wikimedia Commons.)


Syntactic foams using a polymer as a matrix are common, and these foams are being developed to replace metal components in automobiles and watercraft. The magnesium alloy foam is as lightweight as the polymer foams, while offering better material properties.[3] Says Nikhil Gupta, a professor in the Department of Mechanical and Aerospace Engineering of the NYU School of Engineering, and a co-author or the study,
"This new development of very light metal matrix composites can swing the pendulum back in favor of metallic materials... The ability of metals to withstand higher temperatures can be a huge advantage for these composites in engine and exhaust components, quite apart from structural parts."[3]

The magnesium alloy foam shares the lightness of foams, but it it has substantial strength. The enabling agent for the foam is the lightweight silicon carbide hollow spheres developed and manufactured by Deep Springs Technology of Toledo, Ohio. Such spheres withstand a 25,000 pounds/square inch (170 MPa) compressive stress.[2-3] The syntactic foam can be customized through use of different concentrations of the silicon carbide hollow spheres.[3]

The hollow silicon carbide particles also absorb fracture energy, thereby enhancing impact resistance.[3] At high strain rates, the syntactic alloy failed from fracture of the hollow spheres, plastic deformation of the magnesium alloy matrix, and the fracture of intermetallic precipitates at the grain boundaries of the alloy.[2] Fracture of the intermetallics aided in the fracture energy absorption.[2]

AZ91D matrix, SiC, composite

The NYU AZ91D metal matrix syntactic foam, a lightweight magnesium alloy, floating on water.

(NYU Polytechnic School of Engineering image.)


This metal syntactic foam was developed as a replacement for polymer matrix composites when used in applications needing a material with dimensional stability and mechanical property retention at high temperatures.[2-3] Intended applications are for vehicle armor, and automotive and aerospace components.[3] The Ultra Heavy-lift Amphibious Connector (UHAC) vehicle being developed by the U.S. Marine Corps could benefit from such a light weight and high buoyancy material.[3]

Those readers with a chemistry background might be wondering how it's possible to create a useful alloy from magnesium. The chemical element, magnesium, is an early member of the alkaline earths (Group 2 of the periodic table), so it's highly reactive and highly flammable. Just as alloying lead with tin to form the solder alloy reduces the chemical activity of lead to make it safer to use, aluminum does the same for magnesium when alloyed.

Since magnesium-aluminum alloys are extremely lightweight, magnesium is the third most used metal, behind iron (no. 1) and aluminum (no. 2). In 2014, 907,000 metric tons of magnesium were produced worldwide.[4] The following graph illustrates the major uses of this metal.

Historical uses of magnesium, 1975-2003

Historical uses of magnesium in the United States, 1975-2003. (U.S. Geological Survey.)[5)]


References:

  1. According to its Materials Safety Data Sheet, the density of Zest bar soap is 1.14 g/cc, and it contains sodium tallowate, sodium alkyl glycerol sulfonate, magnesium cocoate, sodium cocoate or palm kernelate, water, sodium chloride, coconut or plam kernel fatty acid, fragrance, titanium dioxide, pentasodium pentetate, tetrasodium etidronate, and color agents.
  2. Harish Anantharaman, Vasanth Chakravarthy Shunmugasamy, Oliver M. Strbik III, Nikhil Gupta, and Kyu Cho, "Dynamic properties of silicon carbide hollow particle filled magnesium alloy (AZ91D) matrix syntactic foams," International Journal of Impact Engineering (In Press, April 30, 2015).
  3. A Metal Composite that will (Literally) Float Your Boat, New York University Press Release, May 12, 2015.
  4. Mineral Commodity Summaries 2015, U.S. Department of the Interior, U.S. Geological Survey, January 30, 2015 (2.4 MB PDF File).
  5. Thomas D. Kelly, Grecia R. Matos, et al., "Historical Statistics for Mineral and Material Commodities in the United States," U.S. Geological Survey, Data Series 140, 2014.

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