Nanotwinned materials display unique properties

15 March 2012

Professor Jian Lu, Chair Professor of Mechanical Engineering at the City University of Hong Kong and Professor Ke Lu of the Chinese Academy of Sciences (CAS) Institute of Metal Research in Shenyang have developed a novel processing technique for transforming metallic surfaces into nanocrystalline structures.

Unlike conventional, coarse grain materials, nanocrystalline structures are made up of ultra-fine grains in the nanometer scale and a large number of grain boundaries. This unique structure gives nanocrystalline materials increased strength and other enhanced physical properties compared to convention materials. The technique, known as surface mechanical attrition treatment, utilises plastic deformation to refine the surfaces of metals and alloys, turning them into nano-sized crystallites. This method efficiently creates nano-structured surfaces without changing the chemical composition of materials. It also eliminates the bonding problems that occur when coating methods are used to deposit nanocrystallite layers onto metals or alloys. This flexible, low-cost approach has generated considerable interests in academia and industry, leading to four patents and numerous publications in international journals.

The two research teams are also studying nanotwinned materials, so called because the atomic structure on one side of a grain boundary is a mirror reflection of the other. Grain refinement confers strength to metals and alloys. However, this process reduces the ductility of the material, causing them to be broken easily when deformed. The unique boundary structure of nanotwinned materials is able to block dislocations from propagating throughout the metal or alloy, thus preventing the material from breaking easily when stressed. These metals and alloys thus have high strength as well as high ductility, two of the most desirable properties of materials used in modern technologies.

J. Lu and K. Lu were awarded a grant under the Chinese Academy of Sciences (CAS) – Croucher Funding Scheme for Joint Laboratories. The highly-competitive CAS-Croucher Funding Scheme enables researchers in Hong Kong universities and the institutes of the Chinese Academy of Sciences to work together on highly specific scientific topics with the intention, in the longer term, to establish a Joint Laboratory. Since the first selection exercise in 2004, a total of thirteen teams of researchers have been selected for support under the scheme.

For more information on the scheme, and a list of projects supported since 2004 click here.

1. Tong, W.T., Tao, N.R., Wang, J. Lu, J. & Lu, K. (2003). Nitriding Iron at Lower Temperatures. Science 299, 686-688.

2. Zhu, L., Ruan, H., Li, X., Dao, M., Gao, H. & Lu, J. (2011). Modeling grain size dependent optimal twin spacing for achieving ultimate high strength and related high ductility in nanotwinned metals. Acta Materialia 59, 5544-5557.

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