Glassy palladium rods, with diameters ranging from 3 to 6 mm. (Source: Caltech/Marios D. Demetriou)
A transmission electron micrograph shows the amorphous structure of glassy palladium. The area shown is 10 nm x 10 nm. (Source: Caltech/Carol Garland)
Their new alloy, which is a combination of the palladium, a small fraction of silver, and a mixture of other metalloids has shown itself in tests to have a combination of strength and toughness at a level that has not previously been seen in any other material, reports Marios D. Demetriou, who led the research. The tests were published in a recent issue of the journal Nature Material.
The problem with trying to increase strength in ordinary metals is that their atoms are organized in a crystal lattice, he adds. "And whenever you try to make something as perfect as a crystal, inevitably you will create defects," he says. Those defects, under stress, become mobile, and other atoms move easily around them, producing permanent deformations. While this rearrangement around defects results in an ability to block or cap off an advancing crack, producing toughness, it also limits the strength of the material.
On the other hand, glass has an amorphous structure, its atoms scattered about without a specific discernible pattern. In metallic glasses like the new alloy this results in an absence of the extended defects found in crystalline metals. The actual defects in glasses are generally much smaller in size and only become active when exposed to much higher stresses, resulting in higher strengths. However, this also means that the strategy used in ordinary metals to stop a crack from growing ever longer—the easy and rapid rearrangement of the atoms around defects into a sort of cap at the leading edge of a crack—is not available.
"When defects in the amorphous structure become active under stress, they coalesce into slim bands, called shear bands, that rapidly extend and propagate through the material," says Demetriou. "And when these shear bands evolve into cracks, the material shatters."
In the new palladium alloy, so many shear bands form when the material is put under stress that it "actually leads to higher toughness, because the bands interact and form networks that block crack propagation," Demetriou says. In other words, the number of shear bands that form, intersect, and multiply at the tip of an evolved crack is so high that the crack is blocked and cannot travel very far. In essence, then, the shear bands act as a shield, preventing shattering. Thus, the palladium glass acts very much like the toughest of steels, using an analogous blocking mechanism of arresting cracks.
The researchers suggest that the palladium alloy described in the paper could be of use in biomedical implants, such as dental implants. Many noble-metal alloys, including palladium, are currently used in dentistry due to their chemical inertness and resistance to oxidation, tarnish, and corrosion. It is also suitable for other structural applications like automotive and aerospace components, although cost will prohibit its being used for any large-scale manufacturing process.