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Researchers from Brown University and the Institute of Metals Research of the Chinese Academy of Sciences have found a new way to use nanotwins – tiny linear boundaries in the atomic network of the world. a metal with identical crystal structures on both sides – to create stronger metals.
In a newspaper in the newspaper Science, the researchers showed that the variation in the spacing between the twin boundaries, as opposed to maintaining a constant spacing, produced considerable improvements in the strength and rate of hardening of the metal – the extent to which a metal became stronger when he was deformed.
Huajian Gao, a professor at the Brown School of Engineering who co-directed the work, said the research could point to new manufacturing techniques for high-performance materials.
"This work deals with what is called a degraded material, that is, a material in which the internal composition varies somewhat gradually," Gao said. "Gradient materials are a hot area of research because they often have desirable properties over homogeneous materials, in which case we wanted to see if a nanotin spacing gradient produced new properties."
Gao and his colleagues have already shown that nanotwins can themselves improve the performance of materials. Nano-spun copper, for example, has been shown to be significantly stronger than standard copper, with exceptionally high fatigue resistance. But this is the first study to test the effects of variable nanotwin spacing.
Gao and his colleagues created copper samples using four separate components, each with a different spacing of nanotwins. Spacing from 29 nanometers between limits to 72 nanometers. Copper samples were composed of different combinations of the four components arranged in different orders over the entire thickness of the sample. The researchers then tested the strength of each composite sample, as well as the strength of each of the four components.
The tests showed that all the composites were stronger than the average strength of the four components from which they were made. Remarkably, one of the composites was actually stronger than the most powerful of its components.
"To give an analogy, we think that a chain is as strong as its weakest link," Gao said. "But here we have a situation where our channel is actually stronger than its strongest link, which is really amazing."
Other tests have shown that the composites also had above-average levels of work hardening of their components.
To understand the mechanism behind these increases in performance, the researchers used computer simulations of the atomic structure of their samples under stress. At the atomic level, metals respond to stress by the movement of dislocations – line defects in the crystal structure where the atoms are displaced. The way these dislocations develop and interact is what determines the strength of a metal.
Simulations revealed that the density of dislocations is much higher in gradient copper than in a normal metal.
"We have found a unique type of dislocation, called bundles of concentrated dislocations, which leads to dislocations of an order of magnitude denser than normal," Gao said. "This type of dislocation does not occur in other materials and that is why this copper gradient is so strong."
Gao said that although the research team is using copper for this study, nanotwins can also be produced in other metals. It is therefore possible that the nanotin gradients improve the properties of other metals.
"We hope these results will inspire people to experiment with twin gradients in other types of materials," Gao said.
Other authors of the study were Zhao Cheng, Haofei Zhou, Lu Qiuhong and Lei Lu. The research was funded by the US National Science Foundation and the National Natural Science Foundation of China.
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Material provided by Brown University. Note: Content can be changed for style and length.
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