The randomly ordered atoms of glasses can at times arrange themselves into crystals, weakening them. Now researchers have discovered this crystallization is apparently due to avalanches of particles within glasses, report findings detailed in the Proceedings of the National Academy of Sciences.
Glasses are more like motionless liquids than solids. They are unstable, capable of transforming into crystals possessing orderly rows of atoms, a process known as devitrification. Glasses can crystallize even after long times of apparent stability.
“Many glassy materials like ceramics or metallic glasses are subject to a loss of their properties because of devitrification,” says physicist Chantal Valeriani at the Complutense University of Madrid. “Understanding the mechanism of this process is crucial to trying to extend the life of glassy materials.”
The mechanisms underlying devitrification were elusive. To discover more about this enigma, Valeriani and her colleagues conducted molecular dynamics computer simulations of glasses, exploring the crystallization of the most basic glass-forming system, one made just of simple hard spheres all of equal size that cannot overlap in space.
The models revealed the crystallization is caused by clusters of particles within the glass collectively undergoing avalanches — large, abrupt rearrangements of structure happening sporadically in random locations throughout the glass.
“We expected to find crystallites developing gradually as a consequence of the restless rattling motion that particles have in a glass,” Valeriani says. “Instead, we found that crystallites grow intermittently, in a series of discontinuous steps.”
The particles in these avalanches mostly are not themselves ones that become crystalline. Rather, the small random disturbances caused by the avalanches trigger crystallization in regions near the avalanches that are already semicrystalline, or partially ordered.
The scientists now plan to investigate how devitrification proceeds in more complex systems, “such as mixtures of spheres of different sizes that bear a greater similarity to real glassy materials,” Valeriani says. “We are convinced that the experience and insight we have gained from our previous study with hard spheres will be highly valuable for this new research.”
The researchers would also like to find out what triggers avalanches in these glasses in the first place, as well as how they proceed and why they stop. “By understanding how exactly avalanches are triggered, one could come up with ideas on how to prepare glasses where avalanches and therefore crystallization is avoided,” Valeriani says.