Glass that bends? It sounds impossible. But in today's issue of Science, researchers report that they've come up with a new type of metallic glass that flexes and bows like a copper wire. The advance could potentially usher in a new family of wonder materials.
Ultrathin metallic glasses have been around for decades. They became a rage about 10 years ago, when researchers discovered a way to grow them as thick slabs. That opened the door to using these extremely hard and strong materials as everything from novel structural supports in buildings to golf club heads. Unfortunately, bulk metallic glasses also have an Achilles heel: They're as brittle as your average windowpane.
The problem is that the same properties that give metallic glasses their strength also contribute to their propensity to fracture. In conventional metals, atoms line up in a uniform crystalline array, although typically they form a multitude of tiny crystallites packed next to one another. When strain is applied, these grains slip past one another, allowing the metal to bend. In bulk metallic glasses, however, atoms are typically frozen in a random order, like a liquid. Strain in this case becomes localized in a single plane of atoms that slips past its neighbors. Snap! Like an earthquake fault that ruptures, the result is a catastrophic failure of the material. "The challenge has been to introduce graceful failure in these materials," says Reinhold Dauskardt, a materials scientist at Stanford University in California.
In the past, researchers have created metallic glasses that can bend, just a bit, by mixing metal elements and tiny nanoparticles. Fractures in those materials tend to propagate until they run into a nanoparticle, where they are dispersed. Making such composites is difficult and costly. So, Wei Hua Wang, a physicist at the Chinese Academy of Science's Institute of Physics in Beijing, and his colleagues decided to look for a simpler solution. They played around with the composition of a long-known bulk metallic glass made from zirconium, aluminum, copper, and nickel. And they hit upon a simple recipe that yielded a mixture of hard, dense regions of the material surrounded by less dense soft zones. The result was that when the researchers then bent the material, fractures that began in one zone didn't propagate through the neighboring zones. So instead of one major crack fracturing the material, the glass dissipated the force into a multitude of tiny cracks and could bend even more than the previous composites.
"It's a very important result," says Dauskardt. He cautions, however, that the data presented in the paper focuses on what happens when the material is compressed, as opposed to pulled. The latter measures a property known as tensile strength, which is essential for many applications, such as structural supports for buildings. So, if the new materials can withstand those strains, Dauskardt says, they could pave the way for all sorts of amazing materials.