New glass world’s strongest material?

Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory and the California Institute of Technology have discovered a new type of metallic glass with a strength and toughness beyond that of any known material. The glass is a microalloy featuring palladium, a metal with a high “bulk-to-shear” stiffness ratio that counteracts the intrinsic brittleness of glassy materials.

“Traditionally strength and toughness have been mutually exclusive properties in materials, which makes these new metallic glasses so intellectually exciting,” said Robert Ritchie, a materials scientist who led the Berkeley contribution to the research. “We’re bucking the trend here and pushing the envelope of the damage tolerance that’s accessible to a structural metal.”

According to Ritchie, the new material possesses the inherent plusses of both materials. Glassy materials, for example, have a non-crystalline, amorphous structure that make them inherently strong but invariably brittle. The crystalline structure of metals, on the other hand, provide microstructural obstacles that inhibit cracks from propagating; however, there’s nothing in the amorphous structure of a glass to stop crack propagation. The problem is especially acute in metallic glasses, where single shear bands can form and extend throughout the material leading to catastrophic failures at very small strains.

“Because of the high bulk-to-shear modulus ratio of palladium-containing material, the energy needed to form shear bands is much lower than the energy required to turn these shear bands into cracks,” explains Ritchie. “The result is that glass undergoes extensive plasticity in response to stress, allowing it to bend rather than crack.”

The initial samples of the new metallic glass were microalloys of palladium with phosphorous, silicon and germanium that yielded glass rods approximately one millimeter in diameter. Adding silver to the mix enabled the Cal Tech researchers to expand the thickness of the glass rods to six millimeters. The size of the metallic glass is limited by the need to rapidly cool or “quench” the liquid metals for the final amorphous structure.

“The rule of thumb is that to make a metallic glass we need to have at least five elements so that when we quench the material, it doesn’t know what crystal structure to form and defaults to amorphous,” Ritchie says.

Described in the journal Nature Materials, the new metallic glass was fabricated by co-author Marios Demetriou at Caltech in the laboratory of co-author William Johnson of Caltech, one of the pioneers in the field of metallic glass fabrication. Characterization and testing was done at Berkeley Lab by Ritchie’s group.
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