Addressing a centuries-old question, researchers have uncovered a key element to how glasses transition into very resilient states. This breakthrough could allow for more reliable ways to use glasses — metallic glasses in particular — in a wide range of applications.
Bulk metallic glasses (BMGs) are a relatively new class of materials made from complex, multicomponent alloys. They have the moldable pliability of plastics at high temperatures but the strengths of metals at room temperature. BMGs owe their properties to their unique atomic structures: When metallic glasses cool from a liquid to a solid, their atoms settle into a random arrangement and do not crystallize the way traditional metals do.
Exactly what happens during this glass transition, for all forms of glass, has puzzled researchers for centuries. Moreover, how the properties of the resulting glasses depend on the transition remains elusive. A study, published Aug. 16 in Nature Communications, provides critical information.
The researchers, led by Jan Schroers, professor of mechanical engineering & materials science, have figured out that the key piece of these puzzles is in a non-standard quantity known as the fictive temperature. Rather than focusing on heat, the fictive temperature focuses on the structure of the material, which is ever-changing in glasses during the transition from liquid to solid and vice-versa.
“Every time you change the temperature a little bit, the entire structure — every atom inside — changes its position,” Schroers said. “That doesn’t happen at all in more conventional materials.”
Focusing on the fictive temperature instead of the ordinary temperature is a dramatic departure from traditional thinking in the field.
“We noticed that the thing the ordinary temperature does — the thermal or kinetic energy — is less relevant,” said co-author Eran Bouchbinder, professor of chemical and biological physics at the Weizmann Institute of Science, in Israel. “Whereas the structural changes — the fictive temperature — that’s very important.”
The findings represent a transformational change in the field and could go a long way to make the processing of BMGs much more reliable. Being able to consistently achieve a BMG with greater resilience — known as “fracture toughness” — is critical to the widespread use of BMGs in load-bearing applications. But due to the many unknowns in the glass transition, much is left to chance in processing BMGs.
Focusing on the fictive temperature instead of conventional temperature, the researchers developed a procedure that allows them to control the structure of the material as it transitions into solid form. This can be carefully tuned in the laboratory.
The ability to change a brittle glass to one that’s ductile and able to easily bend without breaking almost seems “like alchemy,” said Jittisa Ketkaew, a graduate student who led the experiments. Most surprising for the researchers, gradually changing fictive temperatures brought about dramatic changes in fracture toughness. “We repeated and refined experiments over and over until we accepted the outcome, and ultimately developed a theoretical description,” Ketkaew said. “Our findings suggest that you can make any metallic glass ductile or brittle simply by controlling the fictive temperature.”
While this means that all brittle glasses can be made ductile in theory, not all BMGs can easily retain the ductile state when they cool to room temperature. Magnesium and iron-based glasses, for instance, require very high cooling rates that are currently difficult to realize. To expand the range of metallic glasses that can practically be made ductile, Schroers and his team are working on alternative strategies to manipulate fictive temperatures.
Publication: Jittisa Ketkaew, et al., “Mechanical glass transition revealed by the fracture toughness of metallic glasses,” Nature Communications volume 9, Article number: 3271 (2018)