“Metallic glasses are strong, smooth, hard, and corrosion resistant; they do not scratch or bend easily and are aesthetically very appealing,” said Robert Maass, professor of materials science and engineering. “Metallic glasses belong to the toughest structural materials we know.”
In spite of metallic glasses’ positive attributes, their use is currently limited. “Right now, you can only buy really expensive watches, cell phones, and golf clubs that use metallic glasses,” Maass said.
However, same properties of the material that make metallic glasses attractive are also associated with their shorter lifespan. “To make a metallic glass alloy, you have to essentially trick nature and capture the atoms in a disorganized state, by melting the metal and cooling it down very fast, in order to achieve all of its attractive qualities.” Normally, the atoms comprising the metallic elements in the alloy would crystalize into an organized structure when cooled slowly to a solid.
“In this disorganized state the atomic structure is continuously evolving and trying to move into a more stable state,” said Amlan Das, a graduate student in materials science and lead author of the paper. “As the material ages and the atoms find more relaxed bond configuration, the movement of the atoms slows down.” This relaxation is associated with the gradual deterioration of the material as it ages, much like how plastic starts to crumble after lying around for a few years.
Previous research has demonstrated that various disordered materials, including metallic glasses, experience this gradual slowing as a monotonous trend. “For example, if you make a sample of metallic glass, you’ll see a rapid movement of atoms at first, and leave the sample in the same conditions, you’ll see the movement of atoms slow down at a steady rate as the sample ages,” said Das.
In this study, Das experimented with the application of a range of stresses at different points in the aging process to see its effect on the movement of atoms. “We applied gentle bending stresses that did not break or permanently deform the sample,” he said. “What we found was that these small stresses disrupted the monotonous, steadily slowing movement. Since the data interpretation was very involved, our collaborator Peter M. Derlet, who is a Prof. at ETH/Paul Scherrer Institute in Zurich, ran unique atomistic simulations. These simulations helped to explain what we see, showing that the overall change is due to some small localized clusters that sometimes moved faster within a sea of slow-moving atoms.”
“This is an incredibly difficult experiment to perform, and our lab is uniquely situated to capitalize on expertise in mechanical deformation and synchrotron-based research,” Maass said. “A unique feature of this research was using x-ray photon correlation spectroscopy at Argonne National Laboratory to measure how fast the atoms were moving under different conditions. This technique has not been applied to this particular question.”
To reach Robert Maass, call 217-300-0665; email rmaass@illinois.edu
The paper “Stress breaks universal aging behavior in a metallic glass” is available here: https://doi.org/10.1038/s41467-019-12892-1