New Principle in Material Science

Photo: Model of highly ordered, necklace-like patterns

Materials scientists have known that a metal's strength (or weakness) is governed by dislocation interactions, a messy exchange of intersecting fault lines that move or ripple within metallic crystals. But what happens when metals are engineered at the nanoscale? Is there a way to make metals stronger by manipulating their nanostructures?

By performing 3-D atomic simulations of divided grains of nanostructured metals, lead researcher Huajian Gao and his team observed that dislocations organize themselves in highly ordered, necklace-like patterns throughout the material. The nucleation of this dislocation pattern is what determines the peak strength of materials, the researchers report. The finding could open the door to producing stronger, more ductile metals, said Gao. "This is a new theory governing strength in materials science," he added.

Divide a grain of metal using a specialized technique, and the pieces may reveal boundaries within the grain that scientists refer to as twin boundaries. These are generally flat, crystal surfaces that mirror the crystal orientations across them. The Chinese authors created nanotwinned boundaries in copper and were analyzing the space between the boundaries when they made an interesting observation: The copper got stronger as the space between the boundaries decreased from 100 nanometres, ultimately reaching a peak of strength at 15 nanometres. However, as the spacing decreased from 15 nanometres, the metal got weaker.

So the researchers dug a little further. The scientists reproduced their experiment in computer simulations involving 140 million atoms. They used a supercomputer, which allowed them to analyze the twin boundaries at the atomic scale. To their surprise, they saw an entirely new phenomenon: A highly ordered dislocation pattern controlled by nucleation had taken hold and dictated the copper's strength. The pattern was characterized by groups of atoms near the dislocation core and assembled in highly ordered, necklace-like patterns. "They're not getting in each other's way. They're very organized," Gao said.

From the experiments and the computer modelling, the researchers theorize that at the nanoscale, dislocation nucleation can become the governing principle to determining a metal's strength or weakness.; Source: Brown University