Say Hello to the Toughest Material on Earth

The team, led by researchers from Lawrence Berkeley National Laboratory (Berkeley Lab) and Oak Ridge National Laboratory, published a study describing their record-breaking findings in Science on Dec. 2, 2022. “When you design structural materials, you want them to be strong but also ductile and resistant to fracture,” said project co-lead Easo George, the Governor’s Chair for Advanced Alloy Theory and Development at ORNL and the University of Tennessee. “Typically, it’s a compromise between these properties. But this material is both, and instead of becoming brittle at low temperatures, it gets tougher.”

CrCoNi is a subset of a class of metals called high entropy alloys (HEAs). All the alloys in use today contain a high proportion of one element with lower amounts of additional elements added, but HEAs are made of an equal mix of each constituent element. These balanced atomic recipes appear to bestow some of these materials with an extraordinarily high combination of strength and ductility when stressed, which together make up what is termed “toughness.” HEAs have been a hot area of research since they were first developed about 20 years ago, but the technology required to push the materials to their limits in extreme tests was not available until recently.

“The toughness of this material near liquid helium temperatures (20 kelvin, -424 Fahrenheit) is as high as 500 megapascals square root meters. In the same units, the toughness of a piece of silicon is one, the aluminum airframe in passenger airplanes is about 35 , and the toughness of some of the best steels is around 100. So, 500, it’s a staggering number,” said research co-leader Robert Ritchie, a senior faculty scientist in Berkeley Lab’s Materials Sciences Division and the Chua Professor of Engineering at UC Berkeley.

Ritchie and George began experimenting with CrCoNi and another alloy that also contains manganese and iron (CrMnFeCoNi) nearly a decade ago. They created samples of the alloys then lowered the materials to liquid nitrogen temperatures (around 77 kelvin, or -321 F) and discovered impressive strength and toughness. They immediately wanted to follow up their work with tests at liquid helium temperature ranges, but finding facilities that would enable stress testing samples in such a cold environment, and recruiting team members with the analytical tools and experience needed to analyze what happens in the material at an atomic level took the next 10 years. Thankfully, the results were worth the wait.

Now that the inner workings of the CrCoNi alloy are better understood, it and other HEAs are one step closer to adoption for special applications. Though these materials are expensive to create, George foresees uses in situations where environmental extremes could destroy standard metallic alloys, such as in in the frigid temperatures of deep space. He and his team at Oak Ridge are also investigating how alloys made of more abundant and less expensive elements – there is a global shortage of cobalt and nickel due to their demand in the battery industry – could be coaxed into having similar properties.

Though the progress is exciting, Ritchie warns that real-world use could still be a ways off, for good reason. “When you are flying on an airplane, would you like to know that what saves you from falling 40,000 feet is an airframe alloy that was only developed a few months ago? Or would you want the materials to be mature and well understood? That’s why structural materials can take many years, even decades, to get into real use.”

This research was supported by the Department of Energy’s Office of Science. The low-temperature mechanical testing and neutron diffraction was performed at the ENGIN-X ISIS Facility at the Rutherford Appleton Laboratory, led by first author Dong Liu. Microscopy was performed at the National Center for Electron Microscopy at the Molecular Foundry, a DOE Office of Science user facility at Berkeley Lab. The other authors on this project were Qin Yu, Saurabh Kabra, Ming Jiang, Joachim-Paul Forna-Kreutzer, Ruopeng Zhang, Madelyn Payne, Flynn Walsh, Bernd Gludovatz, and Mark Asta.

Founded in 1931 on the belief that the biggest scientific challenges are best addressed by teams, Lawrence Berkeley National Laboratory and its scientists have been recognized with 16 Nobel Prizes. Today, Berkeley Lab researchers develop sustainable energy and environmental solutions, create useful new materials, advance the frontiers of computing, and probe the mysteries of life, matter, and the universe. Scientists from around the world rely on the Lab’s facilities for their own discovery science. Berkeley Lab is a multiprogram national laboratory, managed by the University of California for the U.S. Department of Energy’s Office of Science.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.

Text: Aliyah Kovner
Source: https://newscenter.lbl.gov/2022/12/08/say-hello-to-the-toughest-material-on-earth/