Microscopically Structuring Steel Like Bamboo Makes It Stronger Yet More Flexible

Making a steel tool? You want both qualities.

The experimental steel is made of grains that go from small to large

Yuntian Zhu

People's teeth and bamboo stalks may not seem very durable compared to bars of steel. But, a new series of experiments finds, making metals mimic those materials could improve metals' endurance and strength.

A team of chemists from China and the U.S. manufactured steel with a particular microstructure, inspired by teeth and bamboo. The resulting material was both more flexible and able to handle higher amounts of stress than conventionally made steel. In factories, you want both qualities. Structural steel should be able to handle a lot of stress, but it should also bend a little when it comes near its stress limit. That way, it will give engineers more time to fix it before it fails, instead of shattering suddenly.

At the surface, the newly developed steel is composed of grains that are 96 nanometers wide, or about 1,000 times thinner than a sheet of paper. Deeper down into the metal, however, the grains become gradually larger. At its core, the steel has grains about 35 micrometers in size, or more than 300 times wider than the grains at the surface. Of course, all this change in grain size happens over a very short distance. The entire sheet of steel is only one millimeter thick. And remember, the small-to-big transition has to happen at both the top and bottom surfaces of the steel sheet.

Deeper down in the metal, the grains become gradually larger.

The small grains on the surface of the steel help make the metal harder. Meanwhile, the larger grains deeper inside allow the steel to bend. Many things in the natural world also have microstructures that have a gradient of grain sizes from surface to center. The gradient helps them deal with stresses such as weather and wearing.

The China-U.S. team published their steel gradient findings in two papers in the journals Materials Research Letters and the Proceedings of the National Academy of Sciences. The team isn't the only one to have tried creating metals with gradients. In the past few years, other chemists and materials scientists have made copper and stainless steel like this. (The latest gradient-grained steel is not stainless steel. Instead, it's of a type of steel called interstitial-free.) Those groups, too, found improved flexibility with strength in their metals.