What's stronger than steel, impossible to see through and was created just about 50 years ago? The answer: glass. Metallic glass, that is.
"What the word 'glass' really means is a solid material that is amorphous, one in which the atoms are irregularly arranged," explained Bucknell Professor Wendelin Wright, jointly appointed in mechanical engineering and chemical engineering. "The metals found in nature are crystalline, which means their atoms are arranged in an ordered pattern. Metallic glasses look the same as other metals, but their disordered structure is quite different."
Created in the 1960s, metallic glasses are formed by mixing several molten metals, and then cooling them at up to a million degrees per second to avoid crystallization — so quickly that the atoms don't have time to form an ordered pattern. The result is an extremely strong metal. Fascinated by some of its properties, Wright has been studying the material throughout her career.
"The weakest metallic glass is at least as strong as the strongest steel, so you can imagine the possible applications," Wright said. It's currently used to manufacture items such as sports equipment, transformers, and electronics cases. Larger applications are unlikely at this point, because of one significant problem with the material: "It is very brittle. It resists high stresses, but then breaks without warning." Wright and her team are working to understand why metallic glasses behave that way.
Models have predicted that the material's brittle nature is due to what are basically avalanches of rearrangements of groups of atoms, known as shear transformation zones (STZ), that form and propagate when pressure is applied. But experimental support for that theory has been difficult to come by. One of the challenges — these slip avalanches happen within a couple of milliseconds or less (it takes hundreds of milliseconds to blink your eyes).
Wright led the experimental effort involving high time-resolution stress measurements of the avalanche behavior and worked with several researchers, including an undergraduate and post doctoral scholar at Bucknell and collaborators at the University of Illinois, Urbana Champaign and Johns Hopkins University. The team believes they have overcome the experimental challenges to provide evidence showing slip avalanches of STZs as the underlying cause of deformation in metallic glasses.
"We were excited when we realized what we had," Wright said. "These findings could provide valuable insight into not just informing research on how other materials deform, but also into how we can harness the engineering potential of these extraordinary materials."
The results were recently published in the physics journal, Physical Review Letters, where it was selected as an Editors' Suggestion and featured on the Physical Review Letters homepage, a designation only given to about one in eight accepted submissions.
"It still feels amazing to me that I had the opportunity to contribute as a co-author of a paper in a respected journal as an undergraduate," said Rachel Byer '13.
Byer worked with Professor Wright throughout her time as a physics major at Bucknell. She added that this undergraduate research experience is the key to her success as she pursues a Ph.D. in materials science and engineering at Virginia Tech.
"Having the opportunity to work on this project was rewarding because everyday I learned something new," Byer said. "My time with Professor Wright helped me grow so much as a researcher and materials scientist, and I use those research and problem solving skills I learned at Bucknell every day."
Metallic glass is a strong but brittle material which can resist high stresses but, as seen in this video, fracture without warning.
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