By Diana Lutz

Asked what they're up to, Ames Lab scientists Scott Chumbley and Alan Russell reply that they're working on new composites. But, they hasten to add, they're not working on what you think they're working on.

The word composite carries with it some negative baggage, explains Chumbley, a metallurgist at the Lab and associate professor of materials science at Iowa State University (ISU). "If we go to a conference," adds Russell, an associate scientist at the Lab and assistant professor of materials science at ISU, "and say we're going to talk about composites, we have to quickly explain we're talking about a different sort of composite and that our composites have advantages the others don't. It's kind of an evangelical thing."

A composite is a mixture of two different, usually quite dissimilar, materials. The strengths of one material are complementary to those of the other, so the properties of the composite are superior to those of its constituents. "In theory," Chumbley adds darkly.

The idea of composite materials is as old as adding straw to clay to make bricks. Most modern, high-tech composites, however, consist of a ceramic in a metal, a ceramic in a ceramic or a ceramic in a polymer. Aluminum reinforced with fibers of the ceramic silicon carbide is a classic example. The composite material combines the strength and stiffness of silicon carbide with the ductility of aluminum.

The Ames Lab composites, on the other hand, consist of two immiscible metals (metals that do not form alloys), such as magnesium and titanium. Whereas ceramic- and polymer-containing composites can be cantankerous materials, difficult to make and difficult to work, the metal-metal composites have the properties that have made metals utilitarian materials since the Bronze Age.

Like much science, the research had accidental beginnings. It grew out of an effort to make wire of a niobium-tin alloy (Nb3 Sn), which is one of the best low-temperature superconductors. "It's very difficult to make wire out of this compound," Chumbley explains," because it tends to be brittle. So the people who were trying to make wire took copper and niobium and melted them together. Because copper and niobium are immiscible, the niobium solidified within the copper into little treelike structures called dendrites. They were able to draw that material down into fine wire because copper and niobium are ductile. As the copper rod got smaller, so did the niobium, forming tiny filaments within the copper. Then they took the drawn wire and passed it through molten tin. The tin atoms went right through the copper and combined with the niobium to form niobium-three-tin. And that's how they made windable wire out of a brittle material.

"But while they were doing this," he concludes, "they noticed that the copper wire with niobium in it -- forget the tin -- had very high strength, much higher than you'd expect. You'd expect the strength of the composite to be intermediate between that of copper and niobium, but it was higher than that of niobium. In other words, adding a weaker element to niobium made it stronger instead of weaker." Russell adds, "Joze Bevk, who was then at Harvard University, first noticed this strengthening, but very quickly thereafter, John Verhoeven, Bill Spitzig and Ed Gibson at Ames Lab took Bevk's work and expanded it substantially."

Many of the niobium ribbons in a transverse section of the copper-niobium composite are only 10 nanometers across, which is about the width of 30 atoms. In the as-cast composite, however, they might be five microns across, which is equivalent to many thousands of atoms. It is the fineness of the microstructure that makes the composites strong.

Metals deform and ultimately fail because they contain defects, called dislocations, that can move with relative ease through the metal's crystalline structure. The ribbons strengthen the metal by making it hard for dislocations to form and, once they form, to move. Thirty atoms across "is so small that it is very expensive energetically to produce a dislocation inside it, much more so than in a bulk crystal," says Russell, "and then if the dislocation moves, it can move only a short distance before it hits a boundary between dissimilar metals and stalls."

The search for a champion metal-metal composite is guided by both scientific and practical criteria. But the bottom line is that the scientists must beat the best structural alloys now in use. For the aircraft industry, which values lightness, that means beating aluminum alloys. And for the automobile industry, which values both lightness and cheapness, that means beating the strength-to-weight ratios of steel at a moderate cost.

"That kind of limits the area of your focus," Chumbley says, "because they're not going to be interested in anything heavier or more expensive than that."

"For the most part," Russell says, "we're looking at the big three light metals: aluminum, titanium and magnesium. The other light elements are either very expensive or reactive."

Russell qualifies this statement by remarking that Timothy Ellis, an associate metallurgist at the Laboratory "who owns half the world's supply of the rare-earth element scandium over in the Materials Preparation Center," talked them into investigating the properties of a scandium-titanium composite. "We just know this is going to be the one with the fabulous properties," he says, laughing. "Scandium costs about ten times more than gold."

The team is having the best luck with magnesium and aluminum. "If someone were to come to me and say, 'Pick two systems; we'll give you money to investigate thoroughly two of the 15 systems you've tried,' I'd pick magnesium-titanium and aluminum-niobium," says Russell.

The magnesium composite is interesting because of its surprising high-temperature performance. In general, heat coarsens the microstructure of metal-metal composites, decreasing their strength. "However, we have one famous exception to this: magnesium," says Russell. The two scientists have combined magnesium with titanium, niobium and iron. These composites deformed well, and their strength increased, but they were no better than the best commercial alloys.

Then an engineer at a company they were working with suggested they look at how these composites performed if they were held at high temperatures for a long time. "So we tried it," Russell says. "We went in thinking they were probably going to coarsen and wouldn't do too well. Everybody was startled when they were unchanged. These composites have the best high-temperature stability of any magnesium metal ever seen on the planet Earth, which seems ironic, given magnesium's reputation for flammability."

Metastable aluminum binaries also look promising. "The two big ones are aluminum-niobium and aluminum-titanium," Russell says. "Last summer we got up to a tensile strength of 1,200 megapascal. That's about 170,000 pounds per square inch, which is equivalent to hoisting an adult blue whale out of the ocean on an aluminum cable 1.8 inches thick. A typical commercial aluminum alloy is going to fracture at 300 or 400 megapascal. A really extreme, not-very-ductile, pushing-the-envelope kind of precipitation-hardened aluminum alloy might make it up to 700. Well, we're almost double that, and the composite is really well behaved. So we think this is pretty neat."

A conversation with Dick Lederich, senior technical specialist at McDonnell Douglas, provides some idea of the competitive environment for metal-metal composites. To interest his company, Lederich says, a composite must beat the monolithic titanium alloys, particularly the titanium-aluminum-vanadium alloy (Ti6Al4V) used in roughly 90 percent of their applications.

Lederich feels the "aluminum-titanium and titanium-yttrium metal-metal composites have potential for airframe skins." Compared to the monolithic alloys if not to other composites, however, they require "extensive processing," which is "a major disadvantage." But, Lederich says, the "innovative group" at the Ames Lab is already considering several promising solutions to this problem, such as 90-degree extrusion, which puts more "work" into a piece than the normal extrusion process, or, in the case of the yttrium composite, deliberate oxidation.

Chumbley and Russell remain confident that they're on to something; they're as worried about the PR problem as the technical ones. "We're still battling a misconception when people hear 'composites'," laments Chumbley. "We need better marketing and a name that won't remind people of expensive, difficult-to-machine materials."

When the writer suggests "immisch metals," a conflation of "immiscible" and "misch" (German for mixed), the scientists reply by e-mail, "We are of the opinion that perhaps the perfect name for these new materials (ahem, how shall we put this diplomatically?) still remains to be found."

For more information:
Scott Chumbley, (515) 294-7903, chumbley@iastate.edu
Alan Russell, (515) 294-3204, russell@ameslab.gov

Current research funded by:
Basic Energy Sciences Office, DOE
L.W. Huncke Foundation
Arloe Paul
Samuel Hamilton and William Oppold
Engineering Research Institute of the ISU College of Engineering
NASA

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Last revision: 4/17/98 sd

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