US researchers develop crack-resistant metallic glass

The new metallic glass is a microalloy featuring palladium, a metal with a high bulk-to-shear stiffness ratio that counteracts the intrinsic brittleness of glassy materials.

‘Because of the high bulk-to-shear modulus ratio of the palladium-containing material, the energy needed to form shear bands (or narrow zones of intense shearing strain) is much lower than the energy required to turn these shear bands into cracks,’ said Robert Ritchie, a materials scientist who led the Berkeley contribution to the research.

The result is that glass undergoes extensive plasticity in response to stress, allowing it to bend rather than crack.

Glassy materials have a non-crystalline, amorphous structure that make them inherently strong but invariably brittle. While the crystalline structure of metals provides microstructural obstacles such as inclusions and grain boundaries that inhibit cracks from propagating, there is 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.

In earlier work, the Berkeley-Cal Tech collaborators fabricated a metallic glass, dubbed DH3, in which the propagation of cracks was blocked by the introduction of a second, crystalline phase of the metal. This crystalline phase, which took the form of dendritic patterns permeating the amorphous structure of the glass, erected microstructural barriers to prevent an opened crack from spreading.

Now, the collaboration has produced a pure glass material whose chemical composition acts to promote extensive plasticity through the formation of multiple shear bands before the bands can turn into cracks.

‘The addition of the palladium provides our amorphous material with an unusual capacity for extensive plastic shielding ahead of an opening crack. This promotes a fracture toughness comparable to those of the toughest materials known. The rare combination of toughness and strength, or damage tolerance, extends beyond the benchmark ranges established by the toughest and strongest materials known,’ said Ritchie.

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

The new metallic glass was fabricated by Marios Demetriou at Cal Tech in the laboratory of William Johnson. Characterisation and testing was done at Berkeley Lab by Ritchie’s group.

‘Traditionally strength and toughness have been mutually exclusive properties in materials, which makes these new metallic glasses so intellectually exciting,’ Ritchie said.

The characterisation and testing research at Berkeley Lab was funded by DOE’s Office of Science. The fabrication work at Cal Tech was funded by the National Science Foundation.