It seems hard to believe that glass could be stronger than steel, but a team of researchers has developed a super-strong metallic glass that has incredible plasticity when placed under stress, making it as strong and tough as metal. Typically, the structure of glass is strong but brittle which can cause cracks to develop and spread. The new metallic glass features palladium which has a high “bulk-to-shear” stiffness ratio. This allows the metallic glass to bend rather than crack – giving it a fracture toughness that goes beyond the limits of some of the strongest and toughest materials known.

The team from the Berkeley Lab and the California Institute of Technology (Caltech) created a pure glass material with a unique chemical composition that when placed under pressure, makes the glass form multiple shear bands rather than developing a crack. This property makes it much more damage-tolerant than other metallic glass.

Robert Ritchie, a materials scientist who led the Berkeley contribution to the research, said, “These results mark the first use of a new strategy for metallic glass fabrication and we believe we can use it to make glass that will be even stronger and more tough,”

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

Initially, the Berkeley-Caltech collaboration made a metallic glass where the propagation of cracks was blocked by micro-structural barriers. This new work produced a glass that increased plasticity ahead of an opening crack, through the addition of palladium. The initial samples of the glass were microalloys of palladium with phosphorous, silicon and germanium, this yielded glass rods that were about one millimeter in diameter. By adding silver the researchers were able to expand the thickness of the rods to six millimeters.

“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 said.

Ritchie co-authored the paper, “A Damage-Tolerant Glass”, published in the journal, Nature Materials, which described the research. The other co-authors were Marios Demetriou (who actually made the new glass), Maximilien Launey, Glenn Garrett, Joseph Schramm, Douglas Hofmann and William Johnson of Caltech.

The new metallic glass was fabricated by co-author Demetriou at Caltech in the laboratory of co-author Johnson. Characterization and testing was done at Berkeley Lab by Ritchie’s group.