Bubbles in champagne in a glass may add a festive fizz to the drink, but microscopic bubbles that form in metallic glass can signal serious trouble. In this normally high-strength material, bubbles may indicate that a brittle breakdown is in progress.
That’s why researchers at Johns Hopkins University used computer simulations to study how these bubbles form and expand when a piece of metallic glass is pulled outward by negative pressure, such as the suction produced by a vacuum. Their findings were published in the May 3 issue of Physical Review Letters (DOI: 10.1103/PhysRevLett.110.185502).
“A lot of people are interested in metallic glasses because of their strength and their potential use to make better cell phone cases, computer housings and other products,” said Michael L. Falk, who supervised the research. “But what precisely causes these materials to break apart or ‘fail’ has remained a mystery. By studying the behavior of the bubbles that appear when these glasses crack, we were able to learn more about how that process occurs.”
The nearly random arrangement of atoms gives metallic glasses distinctive mechanical and magnetic properties. Most metallic glasses are reasonably elastic and often spring back to their original shape after being bent. Still, when a powerful enough force is applied, they can break.
“Our lab team is interested in learning just how susceptible metallic glasses are to fracturing and how much energy it takes to create a crack,” said Falk, a professor in the Whiting School of Engineering's Department of Materials Science and Engineering. “We wanted to study the material under conditions that prevail at the tip of the crack, the point at which the crack pulls open the glass. We wanted to see the steps that develop as the material splits at that location. That's where dramatic things happen: atoms are pulled apart, bonds are broken.”
At the site where this breakup begins, a vacant space—a bubble—is left behind. Falk’s research group discovered that cavitation—spontaneous formation of tiny bubbles under high negative pressures—plays a key role in the failure, or breakdown, of metallic glasses.
Falk said, “Once it appears, it releases energy as it grows bigger, and it may eventually become big enough for us to see it under a microscope. But by the time we could see them, the process through which they had formed would be long over.”
Therefore, to study the bubble’s nucleation, Falk’s team used a computer model of a cube of a metallic glass made of copper and zirconium, measuring only about 30 atoms on each side. By definition, a bubble appears as a cavity in the digital block of metallic glass, with no atoms present within that open space.
The simulations revealed that these bubbles emerge in a way that is well-predicted by classical theories, but that the bubble formation also competes with attempts by the glass to reshuffle its atoms to release the stress applied to a particular location. That second process is known as shear transformation. As the glass responds to pressure, the researchers found which of the two processes has the upper hand—bubble formation or shear transformation—varies. They determined that bubbles dominate in the presence of high tensile loads. But when the pulling forces were small, the atom reshuffling process prevailed.
Falk and his colleagues hope their findings can help scientists developing new metallic glass alloys for products that can take advantage of the material’s high strength and elasticity, along with its tendency not to shrink when it is molded to a particular shape. These characteristics are important for applications, for example, in cell phones and computers.
“Our aim is to incorporate our findings into predictive models of failure for these materials,” Falk said, “so that they can be optimized and used in applications that require materials that are both strong and fracture-resistant.”
