OAK RIDGE, TN—Scientists have measured the highest toughness ever recorded, of any material, while investigating a metallic alloy made of chromium, cobalt and nickel (CrCoNi). Not only is the metal extremely ductile and impressively strong, its strength and ductility improve as it gets colder. This runs counter to most other materials in existence.
The team, led by researchers from Lawrence Berkeley National Laboratory 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,” says project co-lead Easo George, head of 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 (-424 F) is as high as 500 megapascals square root meters. In the same units, the toughness of a piece of silicon is 1, 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,” says research co-leader Robert Ritchie, a senior faculty scientist in Berkeley Lab’s Materials Sciences Division and a 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 (-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 of 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 the atomic level, took the next 10 years.
Many solid substances, including metals, exist in a crystalline form characterized by a repeating 3D atomic pattern, called a unit cell, that makes up a larger structure called a lattice. The material’s strength and toughness, or lack thereof, come from physical properties of the lattice. No crystal is perfect, so the unit cells in a material will inevitably contain “defects,” a prominent example being dislocations—boundaries where undeformed lattice meets up with deformed lattice. When force is applied to the material, the shape change is accomplished by the movement of dislocations through the lattice. The easier it is for the dislocations to move, the softer the material is. But if the movement of the dislocations is blocked by obstacles in the form of lattice irregularities, then more force is required to move the atoms within the dislocation and the material becomes stronger. On the flip side, obstacles usually make the material more brittle, or prone to cracking.
Using neutron diffraction, electron backscatter diffraction, and transmission electron microscopy, researchers examined the lattice structures of CrCoNi samples that had been fractured at room temperature and 20 kelvin.
The images and atomic maps generated from these techniques revealed that the alloy’s toughness is due to a trio of dislocation obstacles that come into effect in a particular order when force is applied to the material. First, moving dislocations cause areas of the crystal to slide away from other areas that are on parallel planes. This movement displaces layers of unit cells so that their pattern no longer matches up in the direction perpendicular to the slipping movement, creating a type of obstacle. Further force on the metal creates a phenomenon called nanotwinning, wherein areas of the lattice form a mirrored symmetry with a boundary in between. Finally, if forces continue to act on the metal, the energy being put into the system changes the arrangement of the unit cells themselves, with the CrCoNi atoms switching from a face-centered cubic crystal to another arrangement known as hexagonal close packing.
This sequence of atomic interactions ensures that the metal keeps flowing but also keeps meeting new resistance from obstacles far past the point that most materials snap from the strain. “So as you are pulling it, the first mechanism starts, and then the second one starts, and then the third one starts, and then the fourth,” explains Ritchie. “Now, a lot of people will say, ‘Well, we’ve seen nanotwinning in regular materials, we’ve seen slip in regular materials.’ That’s true. There’s nothing new about that, but it’s the fact they all occur in this magical sequence that gives us these really tremendous properties.”
The team’s new findings, taken with other recent work on HEAs, may force the materials science community to reconsider long-held notions about how physical characteristics give rise to performance. “It’s amusing because metallurgists say that the structure of a material defines its properties, but the structure of the NiCoCr is the simplest you can imagine. It’s just grains,” says Ritchie.“However, when you deform it, the structure becomes very complicated, and this shift helps explain its exceptional resistance to fracture,” adds co-author Andrew Minor, director of the National Center for Electron Microscopy facility at the Molecular Foundry at Berkeley Lab and professor of materials science and engineering at UC Berkeley. “We were able to visualize this unexpected transformation due to the development of fast electron detectors in our electron microscopes, which allow us to discern between different types of crystals and quantify the defects inside them at the resolution of a single nanometer.”