Revolutionary Method for Creating Stronger, More Durable Materials

For almost 100 years, researchers believed they understood everything related to how metals bend. This was not the right assumption. Materials science and engineering scientists at the University of Wisconsin–Madison have shown that the rules of metal-bending are not so rigid after all. Their findings have been published in the August 9^th issue of the journal Nature Communications.

Their astonishing discovery not only overturns earlier concepts regarding how metals deform, but could help direct the development of stronger, more long-lasting materials.

“This creates new opportunities for materials design,” says Izabela Szlufarska, a professor of materials science and engineering at UW–Madison. “It adds another parameter we can control to enable strength and ductility.”

Ductility can be defined as the ability of a metal to bend. A majority of methods to boost a metal’s strength do so by compromising flexibility—and as metals become more impervious to bending, they are more likely to snap under pressure.

However, the scientists’ new mechanism for bending might enable engineers to reinforce a material without running the risk of fractures.

It is an advance that is of specific interest to the United States Army, which has a pressing need for robust and durable materials so as to ensure the safety of its troops in war zones.

“Professor Szlufarska has opened up an entirely new area for exploration for structural materials processing and design,” said Michael Bakas, synthesis and processing program manager at Army Research Office in the U.S. Army Combat Capabilities Development Command Army Research Laboratory. “By making such a high-impact discovery, Professor Szlufarska has potentially laid the technical foundation for the development of a new generation of advanced structural materials that could eventually be employed in future Army equipment and vehicles.”

Engineers usually exploit the strength of a metal through methods such as annealing or cold working, which exert their effects via small, yet crucial, structural irregularities known as dislocations.

“Everybody in the metals community knows that dislocations are critical,” says Szlufarska.

It is a truism that has been held since 1934, when three scientists individually understood that dislocation explained an old paradox: Metals are much easier to bend than their molecular structures—which normally take the form of recurrently repeating 3D grids—would indicate.

Dislocations are minute irregularities in the otherwise well-organized crystal lattice of a metal. They form from minor discrepancies—visualize the pages of a book as rows of atoms, and picture how the neat stack of paper becomes mildly distorted at the spot where someone inserts a bookmark.

Regular metals bend because dislocations are able to move, enabling a material to deform without tearing apart every single bond inside its crystal lattice simultaneously.

Strengthening methods usually curb the motion of dislocations. So it was relatively a shock when Szlufarska and colleagues discovered that the material samarium cobalt—called an intermetallic—bent easily, even though its dislocations were sealed in place.

“It was believed that metallic materials would be intrinsically brittle if dislocation slip is rare,” says Hubin Luo, a former staff scientist in Szlufarska’s lab currently working at Ningbo Institute of Industrial Technology in China. “However, our recent study shows that an intermetallic can be deformed plastically by a significant amount even when the dislocation slip is absent.”

Instead, bending samarium cobalt caused narrow bands to develop within the crystal lattice, where molecules took up a free-form “amorphous” configuration rather than the standard, grid-like structure in the rest of the metal.

Those amorphous bands permitted the metal to bend.

It’s almost like lubrication. We predicted this in simulations, and we also saw the amorphous shear bands in our deformation studies and transmission electron microscopy experiments.

Izabela Szlufarska, Professor of Materials Science and Engineering, UW–Madison

A mixture of computational simulations and experimental studies was important to describe the perplexing result, which is why Szlufarska and her group were especially suited to solve this mystery.

It is often easier to carry out theoretical simulations to explain existing experimental results. Here, we first theoretically predicted the existence of shear bands and their role in plasticity in samarium cobalt; these were entirely surprising phenomena. We then confirmed these results experimentally with many different types of experiments to test our theory and to be sure that the predicted phenomenon can be indeed observed in nature.

Hongliang Zhang, Postdoctoral Scholar, UW–Madison

The scientists plan to hunt for other materials that may also bend in this odd manner. Ultimately, they hope to use the occurrence to tweak a material’s properties for flexibility and strength.

This might change the way you look for optimization of material properties. We know it’s different, we know it’s new, and we think we can use it.

Izabela Szlufarska, Professor of Materials Science and Engineering, UW–Madison