Steel is a common benchmark against which the strength of materials is measured, with "stronger than steel" a familiar catch cry for those touting the properties of some new space-age material. But now researchers at North Carolina State University have created steel that is stronger than steel using a process that increases the toughness of various metals by altering the microstructures within them.

Inspired by the internal structure of bones and bamboo, which both boast impressive strength-to-weight ratios, the researchers were able to increase the strength and toughness of metals by giving them what the researchers refer to as a "gradient structure." This is a structure where the size of the millions of tightly-packed grains that make up the metal are gradually increased further down into the material.

"Having small grains on the surface makes the metal harder, but also makes it less ductile – meaning it can’t be stretched very far without breaking,” says Xiaolei Wu, a professor of materials science at the Chinese Academy of Sciences’ Institute of Mechanics who collaborated with Yuntian Zhu from NC State on the work.

"But if we gradually increase the size of the grains lower down in the material, we can make the metal more ductile," continues Wu. "You see similar variation in the size and distribution of structures in a cross-section of bone or a bamboo stalk. In short, the gradual interface of the large and small grains makes the overall material stronger and more ductile, which is a combination of characteristics that is unattainable in conventional materials."

In testing the gradient structure approach in a variety of metals, the researchers were able to improve the properties of copper, iron, nickel and stainless steel.

They also tested the technique in interstitial free (IF) steel, which when made to withstand 450 megapascals (MPa) of stress has very low ductility, meaning it can only be stretched to less than 5 percent of its length before breaking. By giving it a gradient structure, the team was able to create IF steel that was strong enough to withstand 500 MPa of stress while being ductile enough to stretch to 20 percent of its length before breaking.

"We think this is an exciting new area for materials research because it has a host of applications and it can be easily and inexpensively incorporated into industrial processes," says Wu, with the team also looking to study whether the gradient structure approach could also result in materials that are more resistant to corrosion, wear and fatigue.

The team's work is two papers, the first published in the journal Material Research Letters, and the second appearing in Proceedings of the National Academy of Sciences.

Source: NC State