Nanowire Composite Shows Superior Strength
Researchers have created a super strong metallic composite by harnessing the extraordinary mechanical properties of nanowires.
AsianScientist (Mar. 18, 2013) - Researchers from Australia, China, and the United States have created a super strong metallic composite by harnessing the extraordinary mechanical properties of nanowires.
Co-author and Head of the School of Mechanical and Chemical Engineering at The University of Western Australia, Winthrop Professor Yinong Liu, said the work has effectively overcome a challenge that has frustrated the world's top scientists and engineers for more than three decades in nanocomposite design.
"We know that nanowires exhibit extraordinary mechanical properties, in particular ultrahigh strengths in the order of several gigapascal, approaching the theoretical limits. With the fast development of our capability to produce more in variety, more in quantity, and better in shape and size of nanowires, the chance of creating bulk engineering composite materials reinforced by these nanowires has become high," Liu said.
However, all the attempts to date have failed to realize the extraordinary properties of the nanowires in bulk materials. The problem lies with the matrix, said Liu. In a normal metal matrix-nanowire composite, when the composite is pulled to a very high stress, the nanowires experience a large elastic deformation of several percent, but the metals that form the matrix can be stretched elastically to no more than one percent. Beyond that, the matrix deforms plastically, Liu explained.
Plastic deformation damages the crystal structure at the interface between the nanowires and the matrix. In this regard, the properties of the composite are limited by the properties of the ordinary matrix, and not determined by the extraordinary properties of the nanowires.
"The trick is with the NiTi matrix," Liu said. "NiTi is a shape memory alloy, a fancy name but not totally new. It is no stronger than other common metals but it has one special property that is its martensitic transformation. The transformation can produce a deformation compatible to the elastic deformation of the nanowires without plastic damage to the structure of the composite. This effectively gives the nanowires a chance to do their job, that is, to bear the high load and to be super strong," Liu said.
Using this idea, the researchers created composite materials that are twice as strong as high strength steels, that have elastic strain limits up to six percent - which is 5-10 times greater than the elastic strains of the best spring steels currently available - and a Young's modulus of ~30 GPa, which is unmatched by any engineering materials so far.
A number of applications are possible using this nanowire composite, the researchers say. The very low Young's modulus matches that of human bone, making it potentially useful for medical applications as implants.
The ability to produce and maintain extremely large elastic strains also provides an unprecedented opportunity for "elastic strain engineering", which could lead to improvements in many functional properties of solid materials, such as electronic, optoelectronic, piezoelectric, piezomagnetic, photocatalytic, and chemical sensing properties.
The article can be found at: Hao S et al. (2013) A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus and High Strength.
Source: UWA; Photo: pennstatenews/Flickr/CC.
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