Twinning phenomenon found in nanocrystalline aluminum

Using a powerful electron microscope to view atomic-level details, Johns Hopkins researchers have discovered a “twinning” phenomenon in a nanocrystalline form of aluminum that was plastically deformed during lab experiments. The finding will help scientists better predict the mechanical behavior and reliability of new types of specially fabricated metals. The research results, an important advance in the understanding of metallic nanomaterials, were published in a recent issue of the journal Science.

At the microscopic level, most metals are made up of tiny crystallites, or grains. Through careful lab processing, however, scientists in recent years have begun to produced nanocrystalline forms of metals in which the individual grains are much smaller. These nanocrystalline forms are prized because they are much stronger and harder than their commercial-grade counterparts. Although they are costly to produce in large quantities, these nanomaterials can be used to make critical components for tiny machines called microelectromechanical systems, often referred to as MEMS, or even smaller nanoelectromechanical systems, NEMS.

But before they build devices with nanomaterials, engineers need a better idea of how the metals will behave. For example, under what conditions will they bend or break? To find out what happens to these new metals under stress at the atomic level, Johns Hopkins researchers, led by Mingwei Chen, conducted experiments on a thin film of nanocrystalline aluminum. Grains in this form of aluminum are 1,000th the size of the grains in commercial aluminum.

Chen and his colleagues employed two methods to deform the nanomaterial or cause it to change shape. The researchers used a diamond-tipped indenter to punch a tiny hole in one piece of film and subjected another piece to grinding in a mortar. The ultra-thin edge of the punched hole and tiny fragments from the grinding were then examined under a transmission electron microscope, which allowed the researchers to study what had happened to the material at the atomic level. The researchers saw that some rows of atoms had shifted into a zig-zag pattern, resembling the bellows of an accordion. This type of realignment, called deformation twinning, helps explain how the nanomaterial, which is stronger and harder than conventional materials, deforms when subjected to high loads.

“This was an important finding because deformation twinning does not occur in traditional coarse-grain forms of aluminum,” said Chen, an associate research scientist in the Department of Mechanical Engineering in the university’s Whiting School of Engineering. “Using computer simulations, other researchers had predicted that deformation twinning would be seen in nanocrystalline aluminum. We were the first to confirm this through laboratory experiments.”

By seeing how the nanomaterial deforms at the atomic level, researchers are gaining a better understanding of why these metals do not bend or break as easily as commercial metals do. “This discovery will help us build new models to predict how reliably new nanoscale materials will perform when subjected to mechanical forces in real-world devices,” said Kevin J. Hemker, a professor of mechanical engineering and a co-author of the Science paper. “Before we can construct these models, we need to improve our fundamental understanding of what happens to nanomaterials at the atomic level. This is a key piece of the puzzle.”

The nanocrystalline aluminum used in the experiments was fabricated in the laboratory of En Ma, a professor in the Department of Materials Science and Engineering and another co-author of the research paper. “This discovery nails down one deformation process that occurs in nanocrystalline metals,” Ma said. “This is the first time a new mechanism, which is unique to nanostructures and improbable in normal aluminum, has been conclusively demonstrated.”

Other co-authors of the paper were Hongwei Sheng, an associate research scientist in the Department of Materials Science and Engineering; Yinmin Wang, a graduate student in the Department of Materials Science and Engineering; and Xuemei Cheng, a graduate student in the Department of Physics and Astronomy.