Ashby map showing specific strength versus specific modulus for nanotwinned (nt) Al–Fe and various alloys. Li et al. Click to enlarge.

A metal’s crystal lattice is made up of a repeating sequence of atomic layers. If one layer is missing, there is said to be a stacking fault. Meanwhile, “twin boundaries” consisting of two layers of stacking faults can form. One type of stacking fault, called a 9R phase, is particularly promising, said Xinghang Zhang, a professor in Purdue University’s School of Materials Engineering and the corresponding author of the Advanced Materials paper.

It has been shown that twin boundaries are difficult to be introduced into aluminum. The formation of the 9R phase in aluminum is even more difficult because of its high stacking fault energy. You want to introduce both nanotwins and 9R phase in nanograined aluminum to increase strength and ductility and improve thermal stability.

—Xinghang Zhang

Now, researchers have learned how to readily achieve this 9R phase and nanotwins in aluminum.

Purdue postdoc Sichuang Xue is lead author of the Nature Communications paper, which is the first to report a “shock-induced” 9R phase in aluminum. Researchers bombarded ultrathin aluminum films with tiny micro-projectiles of silicon dioxide, yielding 9R phase.

Here, by using a laser-induced projectile impact testing technique, we discover a deformation-induced 9R phase with tens of nanometers in width.

—Sichuang Xue

The microprojectile tests were performed by a research group at Rice University, led by professor Edwin L. Thomas, a co-author of the open-access Nature Communications paper. A laser beam causes the particles to be ejected at a velocity of 600 meters per second. The procedure significantly accelerates screening tests of various alloys for impact-resistance applications.

The Advanced Materials paper describes how to induce a 9R phase in aluminum not by shock but by introducing iron atoms into aluminum’s crystal structure via a procedure called magnetron sputtering. Iron also can be introduced into aluminum using other techniques, such as casting, and the new finding could potentially be scaled up for industrial applications.

The resulting “nanotwinned” aluminum-iron alloy coatings proved to be one of the strongest aluminum alloys ever created, comparable to high-strength steels.

Molecular-dynamics simulations, performed by professor Jian Wang’s group at the University of Nebraska, Lincoln, showed the 9R phase and nanograins result in high strength and work-hardening ability and revealed the formation mechanisms of the 9R phase in aluminum. Understand new deformation mechanisms will help us design new high strength, ductile metallic materials, such as aluminum alloys.

—Xinghang Zhang

The research was mainly funded by US Department of Energy’s Office of Basic Energy Sciences, Materials Science and Engineering Division. The researchers have filed a patent application through the Purdue Research Foundation’s Office of Technology Commercialization.

The transmission electron microscopy work for the research was supported by a new FEI Talos 200X microscope facility directed by Haiyan Wang, Purdue’s Basil S. Turner Professor of Engineering; and the “in situ micropillar compression” work in scanning electron microscopes was supported by Purdue’s Life Science Microscopy Facility, led by Christopher J. Gilpin, director of the facility. These advanced microscopy facilities were made possible with support from Purdue’s Office of the Executive Vice President for Research and Partnerships.

The team included researchers from Purdue’s School of Materials Engineering, Department of Materials Science and NanoEngineering at Rice University, the Department of Engineering Physics at the University of Wisconsin-Madison, State Key Lab of Metal Matrix Composites, the School of Materials Science and Engineering at Shanghai Jiao Tong University, Department of Materials Science and Engineering at China University of Petroleum, California Institute of Technology, Louisiana State University and the University of Nebraska-Lincoln.

Resources

• Sichuang Xue, Zhe Fan, Olawale B. Lawal, Ramathasan Thevamaran, Qiang Li, Yue Liu, K. Y. Yu, Jian Wang, Edwin L. Thomas, Haiyan Wang & Xinghang Zhang (2017) “High-velocity projectile impact induced 9R phase in ultrafine-grained aluminium” Nature Communications 8, Article number: 1653 doi: 10.1038/s41467-017-01729-4

• Q. Li, S. Xue, J. Wang, S. Shao, A. H. Kwong, A. Giwa, Z. Fan, Y. Liu, Z. Qi, J. Ding, H. Wang, J. R. Greer, H. Wang, X. Zhang (2018) “High-Strength Nanotwinned Al Alloys with 9R Phase” Adv. Mater. 2018, 1704629 doi: 10.1002/adma.201704629