Now, tougher metal materials inspired by nature

Researchers have drawn inspiration from the structure of bones and bamboo to create stronger, tougher materials that can be customised for a range of applications - from body armour to automobile parts.
"If you looked at metal under a microscope you'd see that it is composed of millions of closely-packed grains," said Yuntian Zhu, a professor of materials science and engineering at North Carolina State University, and senior author of two papers on the new work.
"The size and disposition of those grains affect the metal's physical characteristics," Zhu said.
"Having small grains on the surface makes the metal harder, but also makes it less ductile - meaning it can't be stretched very far without breaking," added Xiaolei Wu, a professor of materials science at the Chinese Academy of Sciences' Institute of Mechanics.

"But if we gradually increase the size of the grains lower down in the material, we can make the metal more ductile. You see similar variation in the size and distribution of structures in a cross-section of bone or a bamboo stalk.
"In short, the gradual interface of the large and small grains makes the overall material stronger and more ductile, which is a combination of characteristics that is unattainable in conventional materials.

"We call this a 'gradient structure', and you can use this technique to customise a metal's characteristics," said Wu, lead author of the two papers in Materials Research Letters and Proceedings of the National Academy of Sciences.

Wu and Zhu collaborated on research that tested the gradient structure concept in a variety of metals, including copper, iron, nickel and stainless steel. The technique improved the metal's properties in all of them.
The research team also tested the new approach in interstitial free (IF) steel, which is used in some industrial applications.
If conventional IF steel is made strong enough to withstand 450 megapascals (MPa) of stress, it has very low ductility - the steel can only be stretched to less than 5 per cent of its length without breaking. That makes it unsafe.
Low ductility means a material is susceptible to catastrophic failure, such as suddenly snapping in half. Highly ductile materials can stretch, meaning they're more likely to give people time to respond to a problem before total failure.

By comparison, the researchers created an IF steel with a gradient structure; it was strong enough to handle 500 MPa and ductile enough to stretch to 20 per cent of its length before failing.
The researchers are also interested in using the gradient structure approach to make materials more resistant to corrosion, wear and fatigue.
"We think this is an exciting new area for materials research because it has a host of applications and it can be easily and inexpensively incorporated into industrial processes," Wu said.