Bubbles in a champagne glass may add a festive fizz, but the microscopic bubbles that form in a material called metallic glass can signal serious trouble. In this normally high-strength material, bubbles may indicate that a brittle breakdown is in progress.
Johns Hopkins researchers, supervisied by Professor Michael Falk of the Department of Materials Science and Engineering, 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 recently in the journal Physical Review Letters.
"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 Falk. "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."
Most metallic glasses are not transparent or easily breakable, and they will often revert to their original shape after being bent. When enough force is applied, metallic glasses can break. Vacant spaces--microscopic bubbles--are left behind at the site where metallic glasses break. The spontaneous formation of tiny bubbles under high negative pressures is a process known as cavitation, which plays a key role in the failure of metallic glasses.
Falk's team relied on a computer model of a cube of a metallic glass made of copper and zirconium in order to see if they could predict which conditions would cause the bubbles to form. They determined that bubbles form in the presence of high tensile loads, meaning the strong pulling forces more common near the tip of a crack. However, when the pulling forces were at a low level, the metallic glass would reshuffle its atoms to release the stress applied to a particular location.
Falk and his colleagues hope their findings can aid scientists developing new metallic glass alloys for products, such as cell phones and computers, that could benefit from the high strength and elasticity of metallic glass.
The lead author of the Physical Review Letters article was Pengfei Guan, a postdoctoral fellow in Falk's lab. Along with Falk, the co-authors were Shuo Lo, Michael J. B. Spector, and Pavan K. Valavala, who were all part of Falk's lab team at the time the research was conducted. The work was supported by a National Science Foundation grant.