Drawing inspiration from the structure of bones and bamboo, researchers have found that by gradually changing the internal structure of metals they can make stronger, tougher materials that can be customized for a wide variety of applications -- from body armor to automobile parts.
"If you looked at metal under a microscope you'd see that it is composed of millions of closely-packed grains," says Yuntian Zhu, a professor of materials science and engineering at NC State and senior author of two papers on the new work. "The size and disposition of those grains affect the metal's physical characteristics."
"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," says Xiaolei Wu, a professor of materials science at the Chinese Academy of Sciences' Institute of Mechanics, and lead author of the two papers. "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 customize a metal's characteristics," Wu adds.
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.
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 percent 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.
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