Twisted steel stays strong and pliable

Instead of deforming the steel by hammering it or bending it, the researchers took small cylinders of steel and twisted them. The result is a steel cylinder with the best of both worlds: strength and ductility. (Credit: Steve Snodgrass/Flickr)

In steel making, two desirable qualities—strength and ductility—tend to be at odds. Stronger steel is less ductile, and more ductile steel is not as strong.

But new research shows that when cylinders of steel are twisted, their strength is improved without sacrificing their ability to stretch.

Strength and ductility are both crucial material properties, especially in materials used in structural applications. Strength is a measure of how much force is required to cause a material to bend or deform. Ductility is a measure of how much a material can stretch without breaking. A material that lacks strength will tend to fatigue, failing slowly over time. A material that lacks ductility can shatter, causing a sudden and catastrophic failure.

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Steel is one of the rare materials that is both strong and ductile, which is why it’s ubiquitous as a structural material. As good as steel is, however, engineers are constantly working to make it better. The problem is that methods of making steel stronger tend to sacrifice ductility and vice versa.

“We call it the strength-ductility tradeoff,” says senior author Huajian Gao, professor of engineering at Brown University. He and his colleagues have found a way around that tradeoff in cylinders made with a particular kind of steel called twinning-induced plasticity (TWIP) steel.

The technique is described in the journal Nature Communications.

TWIP steel can be made stronger through what’s called work hardening. Work hardening is the process of strengthening steel by deforming it—bending it, flattening it, or hammering it on a forge. When TWIP steel is deformed, nanoscale structures called deformation twins form in its atomic lattice.

A new twist

Deformation twins are linear boundaries with identical crystalline structures on either side, forming a mirror image across the boundary. Twin structures are known to make TWIP steel much stronger, but just like other ways of hardening steel, there’s a ductility tradeoff.

To evade that tradeoff, Gao and colleagues introduced a new twist—literally—on the deformation process. Instead of deforming the steel by hammering it or bending it, the researchers took small cylinders of TWIP steel and twisted them.

The twisting motion causes molecules in the outer parts of the cylinder to deform to a much greater degree than molecules toward the core. The idea is a little like runners on a track. Those running in the outside lanes have more ground to cover than runners on the inside.

Because the twisting motion deforms the outside more than the inside, deformation twins form only toward the surface of the cylinder. The core remains essentially untouched.

‘Best of both worlds’ steel

The result is a steel cylinder with the best of both worlds—the surface of the cylinder becomes stronger and more resistant to cracking, while the inside retains its original ductility.

“Essentially we partitioned the material into a hardened part near the surface and a softer part near the core,” Gao says. “This allowed us to double the strength without sacrificing ductility.”

The work in the lab was done with very small cylinders—on the order of centimeters long. However, nothing indicates that the process can’t be scaled up to larger cylinders, Gao says.

Eventually, he and his colleague hope their technique could be used to pre-treat steel that requires a cylindrical shape—axles or drive shafts on cars for example. In particular, Gao sees torsioned steel as a good option for axles on high-speed trains.

“It’s critical to have high strength and high ductility for such an axle component,” he says. “So it’s critical in this kind of system to push this strength-ductility limit as far as possible.”

Other co-authors are from the Chinese Academy of Sciences, Shanghai University, University of Science and Technology, Beijing, and Zhejiang University.

Source: Brown University