Glass has many useful properties: strength, rigidity, and transparency.
But, it is also fragile; when it cracks it reveals its brittleness and shatters almost instantaneously. Contrast that behavior with steel and other alloys, which are tough, but deform and fail easily. Combine the pros of glass and metal and discard the cons and you could have a strong and tough material.
"Strength and toughness are actually very different, almost mutually exclusive," explains Marios Demetriou of the California Institute of Technology, "Generally, materials that are tough are also weak; those that are strong, are brittle". Demetriou and colleagues have now developed a metallic glass that combines the advantages of each class. [Demetriou et al., Nature Mater (2011) doi:10.1038/NMAT2930].
By combining the noble metal palladium with a small proportion of silver and a mixture of the metalloids phosphorous, silicon, and germanium, the Caltech team has developed an alloy that is at once strong and tough.
The glass-forming ability of palladium- metalloid systems was first recognized by Caltech's Pol Duwez and colleagues in the mid-1960s.
Although other bulk versions of palladium- metalloid glasses have been developed since, this is the first to demonstrate such a unique combination of properties, Demetriou told us.
"Our study demonstrates for the first time that this class of material, the metallic glasses, has the capacity to become the toughest and strongest ever known," Demetriou says. These materials will, the researchers say, "push the envelope of damage tolerance accessible to a structural metal."
Superficially, the materials are metallic, entirely opaque and possess metallic conductivity. It is their internal glassy structure that endows them with the strength and hardness of glass. The disordered structure avoids the possibility of extended crystal defects that make the lattice structure of metals susceptible to damage when deformed. A glass has much smaller defects activated only by much higher stresses. "When defects in the amorphous structure become active under stress, they coalesce into shear bands, that rapidly extend and propagate through the material," explains Demetriou. "And when these shear bands evolve into cracks, the material shatters."
The team has now found that worsening the problem could be the solution. The shear bands in the new palladium alloy work together under stress forming networks that block cracks from propagating. This prevents the material from shattering but does not compromise its glassy strength.
Unfortunately, mass production of the new metallic glass may be limited because of the prohibitively high cost of palladium. Nevertheless, because of the high biocompatibility of the alloy, the researchers anticipate applications in biomedical and dental implants where the material cost is usually offset by the high fabrication cost. Future versions based on a cheaper metal like iron, copper, or nickel could find widespread use in structural applications, like automotive and aerospace components.
David Bradley