High entropy alloys (HEAs) are a group of metals containing a subset of CrCoNi. HEAs are made from an equal mixture of each constituent element, unlike all current alloys, which all have a high proportion of one element and smaller additions of other elements. Some of these materials appear to possess an incredibly high mix of strength and ductility under tension, which together form what is known as “toughness” due to these balanced atomic compositions.
Since they were created about 20 years ago, HEAs have attracted a great deal of scientific attention. However, until recently, the equipment needed to test the materials to their limits was not readily available.
In a new study, scientists reported on the heaviest material on Earth. Scientists from Lawrence Berkeley National Laboratory (Berkeley Lab) and Oak Ridge National Laboratory measured the highest toughness ever measured of any material while investigating a metal alloy made of chromium, cobalt and nickel (CrCoNi). Not only is the metal incredibly strong (meaning it resists permanent deformation) and extremely ductile (meaning very malleable in materials science), but it also becomes more ductile and stronger as it cools. Unlike most other materials in use, this.
Co-project leader Easo George, the governor’s chair for advanced alloy theory and development at ORNL and the University of Tennessee, said: “When you design structural materials, you want them to be strong, but also tough and resistant to breakage. Usually it is a compromise between these properties. But this material is both, and instead of becoming brittle at low temperatures, it becomes tougher.”
Research co-lead Robert Ritchie, a senior faculty scientist in Berkeley Lab’s Materials Sciences Division and the Chua Professor of Engineering at UC Berkeley, said: “The toughness of this material at temperatures of liquid helium (20 Kelvin, -424 Fahrenheit) is as high as 500 megapascal square root meters. In the same units, the toughness of a piece of silicon is one, the aluminum airframe in passenger aircraft is about 35, and the toughness of some of the best steels is about 100. So 500, it’s a staggering number.
Nearly a decade ago, scientists began experimenting with CrCoNi and another alloy containing manganese and iron (CrMnFeCoNi). They created samples of the alloys, reduced the materials to liquid nitrogen temperatures (about 77 Kelvin, or -321 F), and discovered impressive strength and toughness.
They wanted to run tests at liquid helium temperatures to immediately confirm their findings. Still, it took the team the next decade to find facilities that would allow them to do this, as well as team members who had the necessary analytical skills and training to study the material’s behavior at the atomic level.
Scientists used neutron diffraction, electron backscatter diffraction and transmission electron microscopy to observe the lattice structures of CrCoNi samples that were fractured at room temperature and 20 K. operate in a specific sequence when force is applied to the material.
Regions of the crystal lying on parallel planes first slip away from each other due to sliding dislocations. This motion displaces layers of unit cells, creating a form of obstruction in the direction perpendicular to the sliding motion. When the metal is subjected to further pressure, a process known as nanotwinned takes place, where parts of the lattice form a mirrored symmetry with a boundary in the middle. The CrCoNi atoms move from a face-centered cubic crystal to another arrangement known as tight hexagonal packing as forces continue to act on the metal. This transformation takes place as forces continue to act on the metal.
Ritchie said, “This sequence of atomic interactions keeps the metal flowing, but also encounters new resistance from obstacles far beyond the point where most materials break from the stress. So as you pull it, the first mechanism starts, and then the second starts, and then the third starts, and then the fourth.
“Now a lot of people will say, well, we’ve seen nanotwinning in mainstream materials, we’ve seen a shift in mainstream materials. That’s true. There is nothing new about that, but the fact is that they all occur in this magical order that gives us these amazing properties.”
“The findings, along with other recent work on HEAs, may force the materials science community to rethink long-held beliefs about how physical characteristics drive performance. It’s funny because metallurgists say that a material’s structure determines its properties, but the structure of the NiCoCr is the simplest you can imagine – it’s just granules.”
Co-author Andrew Minor, director of the National Center of Electron Microscopy facility at the Molecular Foundry at Berkeley Lab and professor of Materials Science and Engineering at UC Berkeley, said: “However, if you deform it, the structure becomes very complicated and this shift helps to explain the exceptional resistance to breakage. We were able to visualize this unexpected transformation thanks to the development of fast electron detectors in our electron microscopes, which allow us to distinguish between different types of crystals and being able to quantify the defects in it with a resolution of a single nanometer – the width of only a nanometer few atoms – which, it turns out, is about the size of the defects in the deformed NiCoCr structure.”
- Dong Liu et al. Exceptional fracture toughness of medium- and high-entropy CrCoNi-based alloys at 20 Kelvin. Science. DOI: 10.1126/science.abp8070