Stronger steel with a twist
Mike McLeod
Materials Automotive Metal Fabrication material science slideshowMaterial scientists double TWIP steel's strength without loss of ductility.
A TWIP steel cylinder deformed by twisting preserves ductility at the core (a). Parallel lines indicate more “deformation twins” closer to the surface (b, then c), a measure of greater strength. (Photo credit: Gao lab/Brown University)
However, engineers at Brown University, in partnership with universities in China, say they’ve developed a simple procedure that strengthens steel without losing any of the material’s plasticity. So simple in fact, that it’s just a matter of twisting it.
The new technique, described in Nature Communications, relies on the special properties of twinning-induced plasticity (TWIP) steel, which gets stronger when it’s deformed. According to the researchers, this is due to nanoscale structures in the metal, called deformation twins, that form in TWIP’s atomic lattice when it’s deformed. While these twin structures make TWIP steel much stronger, they also make it more brittle.
To get around that, the researchers twisted small cylinders of TWIP steel down their long axes, which created more of the strength-inducing deformation twins near the surface but left the core’s atomic structure relatively untouched.
“Essentially, we partitioned the material into a hardened part near the surface and a softer part near the core,” said said Huajian Gao, professor of engineering at Brown and research lead. “This allowed us to double the strength without sacrificing ductility.”
So far, the process has only been used on small steel cylinders, but Gao and his colleagues don’t see any reason why the technique wouldn’t scale. They envision using the process to pre-treat steel for any application requiring a cylindrical shape such as axles or drive shafts on cars or for axles on high-speed trains.
“It’s critical to have high strength and high ductility for such an axle component,” Gao said. “So it’s critical in this kind of system to push this strength-ductility limit as far as possible.”
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