Researchers observe fundamental mechanism causing material 'work hardening'

Researchers have directly observed the microscopic mechanism involved in "work hardening," a process by which materials like metals become stronger when deformed by hammering or stretching. The research provides a deeper understanding of how this process strengthens materials and could have wide-ranging impacts on material design and manufacturing, where work hardening techniques are used to strengthen everything from car frames to overhead power wires. 

Much like the blacksmiths of the Iron Age understood, hammering metal can improve its strength. Researchers at the U.S. National Science Foundation Materials Research Science and Engineering Center at Harvard University used a confocal optical microscope to observe the detailed hardening mechanism in colloidal crystals — particles 10,000 times larger than atoms that spontaneously form a crystal structure at high concentrations — which they used to mimic smaller atomic systems that are more difficult to observe. Their findings, published in Nature, showed that colloidal crystals exhibited far stronger work hardening potential than other materials while allowing scientists to gain better understanding of the particle "defects" that enable the hardening processes.

Previous research has shown that imperfections in the structure, known as dislocations, form a network of defects which cause the work hardening. 

"What hasn't been clear is the full complexity of the interactions between the defects in these atomic crystals that lead to hardening," said Ilya Svetlizky, a postdoctoral fellow and co-first author of the paper.

The results reveal that the process is governed primarily by the geometry of the particles and the defects. The crystals became stronger because of how their dislocation defects interact and entangle with one another. These observations reveal the universal mechanisms of work hardening that also apply more generally to all materials. Soft colloidal crystals can contain a very high density of these defects, giving them a high capacity for hardening.

"This research tells us something fundamental and universal about how materials get stronger," said researcher and co-author David A. Weitz. "These are amazing materials: Even though they are very soft, work hardening makes them into the strongest materials known."