"New strategy for optimizing the nanostructured metals"
In a report published in the April 14 issue of Science , a team of scientists observes that a nanostructured metal can be hardened by annealing and softened when subsequently deformed, which is in contrast to the typical behavior of a metal. Dr. Xiaoxu Huang, co-author of the report, answers Scitizen's questions.
Dr. Huang, what is a nanostructured metal compared to an ordinary metal?
A metal is considered as nanostructured when the size of the structure unit (what we call the grain size) within the metal is below a few hundreds of nanometers. In an ordinary metal the grain size is typically in the range of a few to hundreds micrometers.
Could you briefly explain what do you mean by annealing and by deformation?
Annealing means to heat a metal sample to and keep it at certain high temperatures for some time. Annealing is a conventional technique used to obtain a compromise between strength and ductility of deformed metals. To deform a metal means to change its dimension or shape. For example, a metal rod can be deformed to be longer by pulling it, and a metal ingot can be deformed to be thin sheet by rolling.
So what did you observed?
Our work is focused on the strength and ductility of nanostructured metals. We observed a very new and unusual phenomenon, opposite to our today’s knowledge about metals. Our nowadays common knowledge is that a metal gets harder when it is deformed and on the other hand, it becomes softer by being heated up. But what we found is that the nanostructured metals, in our case nanostructured aluminum, are softened by being deformed and hardened by annealing.
What technique did you use?
First, let me tell you that I'm working in the Center for Fundamental Research: Metal Structures in Four Dimensions, Risø National Laboratory, Denmark, and our center is supported by the Danish National Research Foundation. For this work we had collaboration with Osaka University, Japan. The techniques involved are those for sample preparation, mechanical test and microstructural characterization. The technique we used for the sample preparation is a rolling process called accumulative roll bonding, which has been developed in Osaka University. The mechanical test is done by tensile testing. The technique we used to have a close look at the microstructure is mainly electron microscopy, including transmission electron microscopy and high-resolution electron microscopy. As we described in the report, the microstructure is characterized by a very fine layered structure with the boundary spacing within two hundreds nanometers. Line defects, known as "dislocations" are present in the layer structure. The amount of dislocations is changed by annealing or deformation. We think this change modifies the mechanical properties, but in an opposite way in the nanostructured metals, as we observed.
You suggest that for materials such as nanostructured aluminum, deformation should be used as an optimizing procedure instead of annealing...
That’s right. All the nanostructured metals have very high strength, much stronger than ordinary metals, but they normally have very limited ductility. To make the nanostructured metals ductile we now suggest to use deformation instead of annealing.
What are the industrial implications of your discovery?
Our studies have direct implications for industry. Firstly, the material we used in our study (aluminum) is widely used in the industry. Secondly, the process we used to prepare the nanostructured aluminum (rolling) is a routine industrial process. Thirdly a combination of cold deformation and annealing is involved in many of routine industrial processing. Therefore our finding and the suggested new strategy for optimizing the strength and ductility of nanostructured metals may be an inspiration in the application and development of industrial processes.
Finally, what are the next steps now for your research?
To study this phenomenon in more detail and also to study similar phenomena in other metals.
Dr. Huang, thank you.
By Francesca Gilibert and Gilles Prigent
Dr. Xiaoxu Huang works at the Center for Fundamental Research: Metal Structures in Four Dimensions, Materials Research Department, Risø National Laboratory, Denmark X. Huang et al., Science, Vol 312, April 14 2006, pp 249-251