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Bamboo inspires new process for making metals tougher


July 3, 2014

Researchers have increased the toughness of various metals by giving them a 'gradient stru...

Researchers have increased the toughness of various metals by giving them a 'gradient structure' (Image: Yuntian Zhu)

Steel is a common benchmark against which the strength of materials is measured, with "stronger than steel" a familiar catch cry for those touting the properties of some new space-age material. But now researchers at North Carolina State University have created steel that is stronger than steel using a process that increases the toughness of various metals by altering the microstructures within them.

Inspired by the internal structure of bones and bamboo, which both boast impressive strength-to-weight ratios, the researchers were able to increase the strength and toughness of metals by giving them what the researchers refer to as a "gradient structure." This is a structure where the size of the millions of tightly-packed grains that make up the metal are gradually increased further down into the material.

"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 who collaborated with Yuntian Zhu from NC State on the work.

"But if we gradually increase the size of the grains lower down in the material, we can make the metal more ductile," continues Wu. "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."

In testing the gradient structure approach in a variety of metals, the researchers were able to improve the properties of copper, iron, nickel and stainless steel.

They also tested the technique in interstitial free (IF) steel, which when made to withstand 450 megapascals (MPa) of stress has very low ductility, meaning it can only be stretched to less than 5 percent of its length before breaking. By giving it a gradient structure, the team was able to create IF steel that was strong enough to withstand 500 MPa of stress while being ductile enough to stretch to 20 percent of its length before breaking.

"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," says Wu, with the team also looking to study whether the gradient structure approach could also result in materials that are more resistant to corrosion, wear and fatigue.

The team's work is two papers, the first published in the journal Material Research Letters, and the second appearing in Proceedings of the National Academy of Sciences.

Source: NC State

About the Author
Darren Quick Darren's love of technology started in primary school with a Nintendo Game & Watch Donkey Kong (still functioning) and a Commodore VIC 20 computer (not still functioning). In high school he upgraded to a 286 PC, and he's been following Moore's law ever since. This love of technology continued through a number of university courses and crappy jobs until 2008, when his interests found a home at Gizmag.   All articles by Darren Quick

It would be useful in rockets, but for most people is far away to be useful yet.

Artūrs Pupausis
4th July, 2014 @ 03:05 pm PDT

" being ductile enough to stretch to 20 percent of its length before breaking"

Grammatically this would indicate -ve 80% stretch. Shouldn't the correct sentence read " stretch by 20 percent".

BTW from what I remember from engineering of 50 years ago ductility is property if material being able to withstand reshaping by hammering without breaking or tearing, like hammering a sheet into foil !

4th July, 2014 @ 08:48 pm PDT

Not sure that there is any great discovery here. This just looks like eutectic cell size increasing as you move deeper from the surface. Any metallurgists like to comment?

6th July, 2014 @ 07:53 am PDT

Pretty interesting, but how are they achieving the gradient?

7th July, 2014 @ 10:17 am PDT

"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,”

No. Anyone who knows anything about friction stir welding/processing knows that small grains drastically increase ductility while maintaining strength - it's why some car doors undergo friction stir processing before they are stamped into shape, in order to eliminate cracks and tearing in the locations that undergo the greatest amount of deformation.

Bob, I'm a degreed metallurgist and this structure is an extremely simple version of a standard, well-studied duplex structure like would be found in a duplex stainless steel. The combination of ferrite and austenite phases found in duplex stainless allow it to have excellent toughness, beyond what is normal for a ferritic or austenitic stainless steel. This article suggests that China is perhaps 80 years behind us in metallurgical knowledge.

Justin Chamberlin
7th July, 2014 @ 03:02 pm PDT

"but for most people is far away to be useful yet."?

Really? Not perhaps useful in prosthetics & exoskeleton tech? Where the promise of reducing weight while increasing structural toughness is achievable?

If the projected properties are as accessible in metals as they are in nature's equivalents when resized then one might say where application is the question the sky's the limit, (think aeronautics among many many other things)!

7th July, 2014 @ 05:31 pm PDT
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