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Microscopic Metallic Bubbles

July 9, 2013

Bubbles may add a festive fizz to champagne, but the microscopic froth that forms 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.

That’s why Johns Hopkins researchers, supervised by materials scientist Michael Falk, studied 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 journal Physical Review Letters.

The team’s computer simulations revealed that bubbles emerge in a way that is well predicted by classical theories, but that bubble formation also competes with attempts by the glass to reshuffle its atoms to release the stress applied to a particular location in a process called “shear transformation.” As the glass responds to pressure, which of the two processes – bubble formation or shear transformation – has the upper hand varies, the team found. For instance, they determined that bubbles dominate in the presence of high tensile loads, meaning the strong pulling forces that are more common at the tip of a crack. But when pulling forces were at a low level, the atom reshuffling process prevailed.

Falk, a professor in the Department of Materials Science and Engineering, said people are interested in metallic glasses because of their strength and potential for use in products such as cell phone cases and computer housings. The team is  hopeful its findings can help scientists who are developing new metallic glass alloys for products that can take advantage of the material’s high strength and elasticity, along with its tendency to resist shrinking when it is molded into a particular shape.

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The lead author of the Physical Review Papers article was Pengfei Guan, a postdoctoral fellow in Falk’s lab. Along with Falk, the co-authors were Shuo Lu, 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 NSF supported the work.

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