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Glassy Metals

Two seemingly unrelated events in early-1990s materials research have evolved into one of DARPA’s most intriguing new areas of focus. One was the air force’s ongoing quest for stronger, lighter materials to build better planes. The second exploded into view after the Persian Gulf War. During the conflict, United States-led forces used shells made of radioactive uranium-238 to attack Iraqi tanks. Instead of flattening on impact, uranium-238 peels away in layers and actually sharpens, making it more destructive than conventional shells. But some veterans’ groups soon claimed the radioactive residue caused health problems. Plus there was an expensive environmental cleanup required. All this led the army to seek a nonradioactive replacement for its uranium projectiles.

In the mid-1990s, these parallel needs led DARPA to the Caltech lab of William L. Johnson, a pioneer in a field known as “glassy metals.” Such materials look like ordinary metals, but they have a key difference: they’ve been fabricated so their atomic structures aren’t orderly, or “crystalline,” but rather random or “amorphous” in nature-like the atomic structure of glass. Scientists have known for at least a decade that a random atomic structure in a metal alloy can confer greater strength and more resistance to fracture and corrosion than are provided by crystalline structures, which contain more defects that make for weakness than amorphous structures. The problem is, glassy metals are extremely difficult and expensive to produce. In most cases, therefore, they have only existed as laboratory curiosities (an exception is Johnson’s glassy zirconium-beryllium alloy, now used in high-end golf clubs). But working under army sponsorship, Johnson’s lab in 1997 came up with a glassy tungsten that not only self-sharpened-making it a potential replacement for uranium shells-but pointed the way to techniques for mass-producing glassy metals with broader applications.

Looking to direct more firepower into this potential breakthrough area, DARPA this spring began a four-year, $30 million thrust to fund efforts to model the atomic interactions that take place as metals are mixed and cooled. The hope is that this insight will lead to glassy versions of widely used metals such as aluminum, titanium and iron that can be fabricated by the ton in existing factories. The first glassy metals were discovered “by trial and error, by happenstance, some might even say alchemy,” says Leo Christodoulou, DARPA’s manager of the new program, called Structural Amorphous Metals. “What we are trying to do is put [more] science behind this program, try to understand the fundamental physics.”

DARPA’s effort attacks the problem from several different angles. For starters, a team led by Johnson that includes seven university labs and three military research groups will do the underlying scientific studies and computational work and create new samples. The prototype materials will then be passed to industrial partners for small-scale fabrication and testing. (As of mid-August, the partner companies had not been announced.)

Whether any fundamentally new, factory-ready-metals recipes will emerge from this collaboration is an open question. But the potential payoff is clear: Johnson’s group, for instance, is working on glassy aluminum and magnesium alloys that would possess twice the strength of their crystalline counterparts. That means less material would be needed to, say, build a fighter jet or a 747, enabling it to save fuel or carry heavier payloads. “If we can successfully do this, then this is the material aircraft will be built out of in 15 years,” Johnson says. “It will become a major paradigm shift in the way we use metals.”

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