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High impact amorphization. (Technology Spotlight).

Ballistics experts have puzzled over the loss of impact resistance in an extremely hard and lightweight ceramic material called boron carbide, sometimes used in protective armor. The material does an excellent job of blocking low-energy projectiles such as handgun bullets, but shatters too easily when struck by more powerful ammunition. Writing in the March 7 issue of the journal Science, researchers from The Johns Hopkins University and the U.S. Army Research Laboratory say they have discovered that higher-energy impacts cause tiny bands of boron carbide to change into a more fragile glassy form, which was determined by observing the atomic structure of boron carbide fragments retrieved from a military ballistic test facility. This high-impact pressure amorphization has previously been seen in minerals and semiconductors, but the researchers say they are the first to report such behavior in a ceramic as hard as boron carbide. The extremely high velocities and pressures associated w ith impact of a high-powered projectile appear to cause microscopic portions of the material's crystalline lattice structure to collapse. "It'S like having a sturdy table and suddenly kicking the legs out from underneath it' said Mingwei Chen, associate research scientist in the Department of Mechanical Engineering at Johns Hopkins and lead author of the Science article. Chen came up with a way to position ultra-thin edges of the fragments so that their atomic structure could be viewed through a high-resolution transmission electron microscope at Johns Hopkins. Localized areas that initially appeared to be cracks in the material were found to consist of the new glassy form of boron carbide. Under normal conditions, atoms in boron carbide form a crystal lattice. In the 2-nanometer glassy bands, however, the atoms were in a jumbled or disordered arrangement. "This discovery was very enlightening, because it tells us that under extremely high pressures the crystal structure collapses and forms these nano-scale amorphous bands;' said Kevin J. Hemker, a professor in the Department of Mechanical Engineering, co-director of the electron microscope lab, and senior author of the Science article. "Then the material fractures along these bands because t he glassy material appears to be weaker than the crystalline boron carbide." Having found why boron carbide abruptly loses its protective capabilities, the researchers hope they have opened a door toward development of a new form of the material that will do a better job of keeping soldiers and police officers safe. The observations also provide experimental evidence that extreme conditions in pressure, temperature and/or loading and quenching rates can lead to the creation of entirely new materials or structures with substantially altered physical and mechanical properties.

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Date:Apr 1, 2003
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