New steel alloy is both strong and ductile

For the steel industry, there may now be a way out of a dilemma that has existed ever since people first began processing metal. In a paper in Nature, scientists at the Max-Planck-Institut für Eisenforschung in Düsseldorf, Germany, report a new type of metallic material that is both extremely strong and highly ductile. Up to now, one of these material properties could only be improved at the expense of the other, but this new advance could alter that trade-off, leading to the creation of lighter metallic components with thinner walls.

Ideally, steels and steel-related alloys should be both strong and ductile. They need to be ductile so that they bend rather than break when subjected to strong forces, but also need to be strong so that they do not warp when subjected to weak forces. A team led by Dierk Raabe, director at the Max-Planck-Institut für Eisenforschung, and Cemal Cem Tasan, formerly head of a research group at the institute but now a professor at the Massachusetts Institute of Technology, has now succeeded in combining both properties in one material.

"We pursued a new strategy in the development of this material, which generally opens up new possibilities for the design of metallic materials," says Raabe. The new material is based on high-entropy alloys, which contain similar amounts of five or more different metals distributed without any identifiable order. High-entropy alloys have been the subject of extensive testing by materials scientists over recent years, but have proved too brittle for many applications.

The great strength of high-entropy alloys comes from the disorder of their component atoms. This disorder makes it difficult for defects in the alloy, known as dislocations, to move through its crystal structure when the alloy is deformed. But this also makes the alloy very brittle under high enough pressures.

Steels that mainly comprise iron, along with small quantities of other elements like carbon, vanadium or chrome, are, on the other hand, often ductile, but are not strong enough for many applications. These steels are ductile because they can switch from one crystal structure to another, which uses up energy that would otherwise cause damage. Many steel components such as car body parts are made up of lots of tiny areas that alternate between two different crystal structures.

This coexistence of different crystal structures had always been considered detrimental for high-entropy alloys. "We have now turned this conception on its head, as recent studies have shown that this is not the important factor," explains co-author Zhiming Li.

Together with his colleagues, Li searched for a material that is as strong as a high-entropy alloy, but, like particularly ductile steels, possesses two coexisting crystal structures. They eventually came up with an alloy made from 50% iron, 30% manganese and 10% each of cobalt and chrome.

"With this alloy, we have shown that our concept works," says Raabe. "If we further improve the microstructure and the composition, we can even further enhance the strength and ductility." This is what the researchers are now working on.

This new alloy can be processed just as easily and cost-efficiently as a particularly ductile steel and absorb as much impact energy when incorporated in the body of a car. At the same time, the alloy is strong enough that thin sheets made from it do not give way when subjected to a weak force.

This story is adapted from material from Max-Planck-Gesellschaft, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.