Stronger, more ductile steel the promise of new technique

Researchers have found a way to improve the strength of steel without sacrificing ductility.

The team, from Brown University in the US, focused on a particular type of steel called twinning-induced plasticity, or TWIP.

TWIP steel can be made stronger through a process called work hardening, which involves deforming, bending, flattening and hammering it on a forge.

When TWIP steel is deformed, nanoscale structures called deformation twins form in its atomic lattice. Deformation twins are linear boundaries with identical crystalline structures on either side, forming a mirror image across the boundary.

These structures are known to make TWIP steel much stronger but, just like other ways of hardening steel, there's a ductility trade-off.

To overcome this, the team from Brown 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 caused the molecules in the outer parts of the cylinder to deform to a much greater degree than molecules toward the core.

Because the twisting motion deformed the outside more than the inside, deformation twins formed only toward the surface of the cylinder. The core remained essentially untouched.

This meant that the surface of the cylinder became stronger and more resistant to cracking, while the inside retained its original ductility.

"Essentially, we partitioned the material into a hardened part near the surface and a softer part near the core," explained lead researcher Huajian Gao. "This allowed us to double the strength without sacrificing ductility."

The work in the lab was done with very small cylinders just a few centimetres long. However, Gao believes the process could be scaled up.

Eventually, the researchers hope their technique could be used to pre-treat steel that requires a cylindrical shape. 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 concluded. "So it's critical in this kind of system to push this strength-ductility limit as far as possible."

Laura Hopperton

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