Glassy Metals May Be Materials of the Future

They're harder, stronger, and basically just better.

By Brad Lemley|Wednesday, April 21, 2004
RELATED TAGS: MATERIALS SCIENCE

The wispy metal strip in my hands is 8 inches long, 1 inch wide, and as thin as aluminum foil.

“Try to tear it,” says William Johnson, a materials science professor at Caltech in Pasadena.

I pull—first gently, but soon with all my might. No go.

“See if you can cut this,” suggests Johnson’s postgraduate assistant Jason Kang, handing me a mirror-bright piece of the same metal. It’s an inch long, a quarter inch wide, and thinner than a dime. I bear down with a heavy-duty pair of wire cutters. The metal will not cut. I try again, squeezing with both hands until my fingers ache. Nothing.

But the most amazing act in this show is yet to come.

“Watch,” says Johnson. From a height of about two feet, he drops a steel ball onto a brick-size chunk of the metal. The ball bounces so high and for so long—1 minute and 17 seconds, with a metronomic tick, tick, tick—that it looks unreal, like some kind of cinematic special effect. “When you try that with regular steel, it goes ‘clunk, clunk, clunk’ and stops,” says Johnson. If the metal were glued to an unyielding surface such as concrete (instead of sitting on Johnson’s oak coffee table, which absorbs a lot of the energy), “the ball would bounce for more than two minutes,” he says. “I’ve done it.”

It’s all astounding, yet oddly familiar. In the typical science fiction film circa 1950, there’s that scene in which scientists return from the just-landed flying saucer and tell the Army brass that no tool known to humankind can cut, burn, bend, or otherwise scar the hull. But the metal in front of me is decidedly terrestrial in origin—it was developed in Pasadena, specifically in the lab down the hall from Johnson’s office.

It is called metallic glass, or amorphous metal, and it appears to be nothing less than an entirely new class of material that can be used to build lighter, stronger versions of anything. “Everything from an Abrams tank to an F-16 jet to a bicycle can be made out of this, and because it is two to three times the strength of conventional alloys, you can halve the weight or more. That’s not evolutionary, it’s revolutionary,” says Johnson. “This is the structural material of the future.”

Strength is not its only virtue. It can also be formed like a plastic. So instead of laboriously making sheet metal and then cutting, machining, and drilling, say, a car fender, all of which weakens the part, a glassy metal fender could be injection-molded in one piece—a breakthrough. “The idea that you can cast something like a plastic part with very high strength is a completely new development,” says materials science professor William Nix of Stanford University, an adviser to Liquidmetal Technologies, which is trying to commercialize the metal.

Better yet, it can be readily made into a foam. “With most metals that’s difficult, because the bubbles want to rise to the surface of the molten metal,” says Johnson. The fact that amorphous metal is thick and like plastic when molten permits the formation of a foam panel that is 99 percent air but roughly 100 times stronger than polystyrene. A sandwich made of two thin sheets of amorphous metal flanking amorphous foam would be strong, light, insulating, fireproof, bug-proof, rustproof, sound dampening, and difficult to penetrate with bombs. Such panels could form buildings, ship hulls, airplanes, and car bodies.

“Glassy metals will be a cut above both metals and plastics,” says Kang, looking up from a plasma arc melter in which he forges new formulations. Asked if his aim is to replace both—which covers a lot of territory—he smiles. “That’s what we’re shooting for,” he says.

But metallic glass has one huge problem—it’s expensive. The first commercialized injection-moldable form costs about $15 a pound to make versus roughly $1 a pound for aluminum and 25 cents a pound for steel. Johnson, Kang, and other researchers are working on variants with cheaper constituents. “I think we can make a viable amorphous steel product. I would call that a very likely development,” says Johnson. Eventually, he says, it could cost the same 25 cents a pound as ordinary steel. “That will change everything,” he says.

Still, fundamental shifts in basic materials don’t happen overnight. Even if bulk metallic glasses become cheap, the world’s metalworking factories will need to be completely retooled to accommodate them. Ted Hartwig, a mechanical engineering professor at Texas A & M University, who has been working on a U.S. Army project to develop large-bore amorphous-alloy ammunition, counsels patience. “Remember when everyone was predicting back in the 1970s that we’d have ceramic engines? Where are they?”

Metallic glass seems an oxymoron. Even the chemistry challenged know that the atomic structures of metal and glass are quite different. Molten metal has an amorphous, or random, atomic structure, but its atoms rapidly clump into a latticework of crystals as the metal cools and solidifies. Molten glass is also amorphous but has exceedingly sluggish crystallization. So even if molten glass cools and hardens slowly, it solidifies in an amorphous state. Glass is truly a frozen liquid.

So metal was metal and glass was glass until the early 1950s, when German scientists made amorphous metals by rapidly cooling vaporized tin and lead at a rate of roughly 1 trillion degrees Fahrenheit per second, a speed that fixed the jumbled atoms in place before they had a chance to form crystals. But these metals were less than one-hundredth the thickness of aluminum foil, “and they crystallized again when warmed to –40°F,” says Johnson. “The aim wasn’t to make anything practical. It was driven by scientific curiosity about how to avoid crystallization.”

The next breakthrough came in 1959 when Pol Duwez, a Belgian-born materials scientist at Caltech, used rapid cooling to make a gold-silicon alloy that remained amorphous at room temperature. It was the first true metallic glass. Again, it was just a gossamer film, because anything thicker could not be cooled quickly enough to “frustrate the crystallization,” as materials scientists say.

By 1990 Japanese researcher Akahisa Inoue and his team at Tohoku University, building on the theories of Harvard University researcher David Turnbull, began casting metallic glasses up to one-quarter of an inch thick. They found they could make “bulk” metallic glass by using three or more elements that differ from one another in atomic size by at least 12 percent. Alloys that meet these conditions solidify according to what’s called the confusion principle: The widely differing atomic radii and the high number of different elements “confuse” the atoms so they don’t know where to go to form crystals as they cool.

Enter William Johnson.

He is 55 years old, soft-spoken and affable but with a stealthy intensity. “Once I get going, I can be kind of overwhelming,” he says, and watching him command grad students with a few quick words, it is clear who is in charge of his lab. As an inquisitive teenager growing up in Bowling Green, Ohio, he wanted to become a particle physicist and unscrew the twisted mysteries of the universe. But at Caltech he crossed paths with quirky physics genius Richard Feynman. “Feynman gave me a long lecture about how if he was a young guy like me, he’d find some other area of physics because the high-energy stuff was overcrowded.” Practical by nature, Johnson had already begun to wonder if he could actually make something with atoms, as opposed to merely puzzling over ghostly traces of their constituents on a linear accelerator’s plates.

“Turns out there was a guy in materials science named Pol Duwez who was making amorphous metals,” says Johnson. Intrigued, he joined Duwez’s lab in 1970 and embarked on a lifelong study of the strange materials.

Even then, it was becoming clear that metallic glasses had useful yet otherworldly properties. Duwez had already created extremely “soft” magnets, meaning their north and south poles could flip easily.

“Somebody realized that if you put them in transformer cores, you could reduce transmission losses” by as much as 50 percent, says Johnson. The Allied Signal Company began doing just that in the 1970s, making thin sheets of rapidly cooled amorphous metal and rolling them into cores. “Today when you drive down a road and see a transformer bucket attached to a power pole, that’s a bucket full of amorphous metal,” says Johnson. “So it’s already out there, quietly, in the world.”

What really excited Johnson was the potential of thick, or bulk, amorphous metal. Analysis of the thin ribbons hinted that a heavy hunk of material, thick enough to be formed into structural shapes, would be like nothing seen on Earth before. Conventional metals dent, tear, and rust because of defects known as grain boundaries and dislocations, in which the crystals are pushed out of alignment and provide entry points for oxidation. Amorphous metals have no crystals that could be affected by such imperfections and hence are springy, extremely strong, and corrosionproof. But a truly thick metal, one with little surface area relative to its mass, would require little or no rapid cooling.

“For years people thought you could only make amorphous metals via rapid cooling,” says Johnson. “Even the Japanese were using it to make their thicker metals. But maybe it wasn’t essential.”

Working with graduate student Atakan Peker, Johnson zeroed in on an alloy with five elements: zirconium, titanium, nickel, copper, and beryllium. Every day, starting in January 1992, Peker patiently mixed these metals in varying amounts. Ten months later, he scored. “Atakan walked in and said, ‘I melted the alloy, and when it cooled back down, I think it was amorphous,’” recalls Johnson.

No cryogenics, no fancy cooling strategies. The stuff cooled to room temperature and solidified as a glass, complete with a characteristic mirror finish. “That was the turning point,” says Johnson, his eyes distant at the happy memory. “The crystallization was orders and orders of magnitude slower than anything we had seen. We discovered later that this stuff was such a good glass former, we could probably make it two inches thick!”

Over the next six months, Johnson and Peker made “several hundred” glassy alloys with varying amounts of those elements, leading to a particularly promising version they dubbed Vitreloy. It proved to be just as amazing as Johnson had predicted. The strongest titanium alloys in common use in the world, when formed into a one-inch-diameter bar, can hoist 175,000 pounds. A same-size bar of Vitreloy can lift 300,000 pounds.

Still, it had a major weakness. Pure Vitreloy had a nasty tendency to shatter like glass. “With a golf driver, a crack would form after a few hits, and eventually you would have club shards flying in all directions,” says Johnson, throwing his hands wide to indicate an exploding clubface. Materials scientists call this catastrophic shear failure. Conventional metals don’t do it because the crystal dislocations bunch up around a crack tip, making the surrounding area stronger. “What this material needed was the ability to fail gracefully, like normal metals do,” says Johnson.

In 2000 Johnson and his team came up with Liquidmetal2, which marries the strength and elasticity of glassy metal to the graceful failure of ordinary metal. It is 80 percent glass and 20 percent crystal. The crystals act like horsehair in old-fashioned plaster, cross-reinforcing the crack-prone amorphous metal. “Now I have matched the toughness and impact resistance of the best alloys out there, with two to three times the strength,” says Johnson. “Now I really have something.”

A bright jumble of shiny parts and gadgets spills across Johnson’s desk: cell phone cases, camera bodies, golf clubheads, folding knives, and nasty-looking three-inch-long cannon shells. All were created by Liquidmetal Technologies. Founded in 1987 and headquartered in Lake Forest, California, with a manufacturing plant in Pyongtaek, South Korea, it is the world’s first commercial manufacturer of bulk metallic glass products and has about a dozen pilot projects under way for the U.S. military and for private companies. “Things are busy,” says Johnson.

Though the material is expensive, Liquidmetal is doing a booming business spraying it over cheaper metals, instantly rendering boilers, oil-well drill heads, and other industrial parts more durable and slippery. “The economizer tubes on our coal-fired boilers were corroding and popping every six months,” says John Berg, maintenance director for Cogentrix Energy, headquartered in Charlotte, North Carolina. But after spray-coating the tube interiors with Liquidmetal in 1992, “it’s still there, and we’ve had virtually no leaks at all.”

Meanwhile, Liquidmetal golf clubs have been on the market since 1998, with about 40,000 drivers sold so far. The clubs exploit the amorphous alloy’s amazing springiness, or what materials scientists call a high elastic-strain limit. Under severe stress, most materials permanently deform. 

Whacking a golf ball with a typical titanium driver head creates a tiny, permanent dimple in the clubface, which steals energy from the ball. But a metallic glass clubface flexes like a trampoline and then springs all the way back, returning all the potential energy to the ball and adding as much as 30 yards to a pro’s drive. 

Liquidmetal tennis racket frames, introduced last year and used by Andre Agassi and other pros, have a similar virtue. “The response you get from the racket is pure . . . there is no energy lost during contact with the ball,” contends Bill Mountford, director of tennis at the United States Tennis Association’s national center in Flushing Meadows, New York.

glassymetals_left
glassymetals_left
Image Courtesy of Professor Michael Widom, Physics Department, Carnegie Mellon University
glassymetals_right
glassymetals_right
Image Courtesy of Professor Michael Widom, Physics Department, Carnegie Mellon University

TOP: A typical two-element alloy consists of iron atoms (red spheres) and boron atoms (blue spheres), which naturally arrange themselves into a crystalline pattern upon cooling. The repeating spaces between the atoms are grain boundaries. Crystals can shift across these boundaries, allowing oxidation and deformation. 



BOTTOM: Introducing a third element with a dramatically different atomic radius, in this case large yttrium atoms (yellow spheres), frustrates the alloy’s tendency to crystallize, so it solidifies in a random, or amorphous, pattern similar to that of glass. Such atomic dynamics can be simulated in computers, sparing researchers endless trial and error in their quest for promising amorphous metals.

Much more is coming. Artificial hips and knees, ultralight cases for phones, PDAs, and laptops, and nondulling surgical blades are all in development. The material is costly, but companies can recoup some expenses because it shrinks far less than conventional metals as it solidifies, which means little or no postforming processing is needed. Amorphous scalpels, for example, pop from their molds already sharp, whereas conventional scalpels require an additional sharpening step. And amorphous blades stay sharp, says Johnson with a smile. “A guy from one of the big razor blade companies keeps calling me, but I haven’t had time to return his calls. Frankly, I don’t think they want amorphous razor blades—they would stay sharp for a year.”

On the military front, the Army is testing an armor-piercing bullet called a kinetic energy penetrator for use by ground-attack jets and armored vehicles, while the Navy evaluates lightweight amorphous-alloy fragmentation bombs. But price remains a barrier. “Right now, it would appear that the cost of these materials will be fairly high and not competitive,” says Robert Dowding, a materials engineer at the Army Research Laboratory at the Army’s Aberdeen Proving Ground in Maryland. He says a chief reason amorphous metals are expensive is that their constituent metals need to be pure: “A small amount of oxygen or carbon in the mix can cause crystallization” and ruin the alloy.

But other researchers are making progress here as well. “Adding certain rare earth elements to the mix attracts impurities,” says John Perepezko, a materials science and engineering professor at the University of Wisconsin at Madison. Such elements allow the use of commercial-grade metals in amorphous alloys, which can dramatically lower costs.

As long as amorphous alloys such as Liquidmetal2 cost $15 a pound, they will remain “boutique metals,” says Johnson, used only for relatively low-volume niche applications. But Joseph Poon, a materials physicist at the University of Virginia, has created laboratory samples of vitreous stainless steel that could propel amorphous metal toward ubiquity. He won’t reveal the exact components but says, “we’ve made it in thicknesses of about the one-centimeter range. The cost commercially would be about the same as the highest form of stainless steel, two or three bucks a pound.” That stuff would be the strongest metal ever made; a rod one inch in diameter would lift 550,000 pounds. The next step, Poon says, is engineering a hybrid version that contains crystals to solve the catastrophic shear failure problem.

“I am shocked at the progress we have made,” says Leo Christodoulou, who heads the Defense Advanced Research Projects Agency’s structural amorphous metals program, which funds Poon’s work and much of Johnson’s. When the program began in 2000, the greatest thickness achieved for amorphous steel was roughly one-thousandth of an inch.

Atomistic modeling will lead to the next breakthrough, Poon says. The traditional method of finding promising alloys was simply to make mix after mix in the lab, a development technique Poon calls “the monkeys-on-a-typewriter thing.” But computer modeling the atomic behavior of various combinations of elements is proceeding rapidly. “Atomistic modeling will help us find the sweet spot where it’s most useful to focus the alloy design work,” Poon says.

Back in Pasadena, down the hall from Johnson’s office, Jason Kang is investigating that sweet spot in the realm of titanium-zirconium–based amorphous alloys similar to Liquidmetal2. Every day he puts pea-size chunks of metallic elements into little crucibles, then zaps them to about 3,000°F with an electrical arc to make an alloy. If upon solidifying the resulting metallic “button” is reflective like a mirror, he knows instantly that he has made a glass. The surface reflects for the same reason the surface of a liquid reflects—the amorphous atoms form a smooth skin that bounces light uniformly. “That’s why you don’t have to machine it after casting,” says Kang, gazing fondly at it. “It just pops out of the mold looking like this.”

Johnson hopes that eventually the world’s engineers and architects will be able to choose from a panoply of amorphous metals and that the catastrophic shear failure problem will be solved for all of them. There will be cheap, abundant vitreous steel, and metalworking factories will be refitted to exploit the material’s boons. The first nation to achieve that happy state will boast greener, safer vehicles, better homes and buildings, more potent arms and sturdier defenses, more efficient industries, and even more successful golfers, batters, and tennis players. “I think we can get there,” Johnson says, admiring the jumble of trinkets before him. “I think all metals will eventually be vitreous. There is a fantastic future.”

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