Peering into the crystal
Many solids, including metals, exist in a crystalline form characterized by a repeating three-dimensional atomic pattern, called a unit cell, which forms a larger structure called a lattice. The strength and stiffness of a material, or lack thereof, comes from the physical properties of the mesh. No crystal is perfect, so unit cells in a material will inevitably contain ‘defects’, a notable example being dislocations – the boundaries where the undistorted lattice meets the deformed lattice. When force is applied to the material—think, for example, of bending a metal spoon—shape is changed by the movement of turbulence across the lattice. The easier it is to move the turbulence, the softer the material. But if the movement of dislocations is blocked by obstacles in the form of lattice irregularities, then more force is required to move the atoms within the dislocations, and the material becomes stronger. On the flip side, obstacles usually make the material more brittle and prone to cracking.
Using neutron diffraction, electron backscattering and transmission electron microscopy, Ritchie, George and colleagues at the Berkeley Laboratory, University of Bristol, Rutherford Appleton Laboratory and UNSW examined the lattice structures of the examined CrCoNi samples. Fractured at room temperature and 20 K (For strength and ductility measurements, a pure metal sample is drawn until it breaks, whereas for fracture toughness tests, a sharp crack is intentionally inserted into the sample before it is drawn and the pressure needed to grow the crack is then measured.)
The images and atomic maps generated from these techniques revealed that the alloy’s hardness is due to three dislocation hindrances that come into play in a specific order when force is applied to the material. First, moving dislocations cause regions of the crystal to slip away from other regions on parallel planes. This movement displaces layers of unit cells so that their pattern in the perpendicular direction does not match the sliding movement, which creates a kind of obstacle. The extra force on the metal creates a phenomenon called nanotwinning, in which regions of the lattice form reverse symmetry with a boundary between them. 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 shifting from a face-centered cubic crystal to another arrangement known as closed hexagonal packing.
This sequence of atomic interactions ensures that the metal continues to flow, but also keeps new resistance from obstructions beyond the point at which most materials break from strain. “When you pull it on, the first mechanism starts and then it starts the second, and then it starts the third, and then the fourth,” Ritchie explained. “Now, a lot of people will say, well, we’ve seen nanobreeding in ordinary materials, we’ve seen slippage in ordinary materials. That’s right. There’s nothing new about that, but it’s the fact that they all happen in this magical sequence that gives us these really awesome properties.” .
The team’s new findings, taken together with other recent work on HEAs, may force the materials science community to revisit longstanding notions about how physical properties lead to performance. “It’s interesting because metallurgists say a material’s structure determines its properties, but NiCoCr’s structure is the simplest you can imagine — it’s just a grain,” Ritchie said. “However, when you deform it, the structure becomes very complex, and this transformation helps explain its exceptional resistance to fracture,” added co-author Andrew Minor, director of the Molecular Foundry’s National Center for Electron Microscopy at Berkeley Lab and professor of materials science and engineering at UC Berkeley. . “We were able to visualize this unexpected shift due to the development of fast electron detectors in our electron microscopes, which allow us to distinguish between different types of crystals and identify defects within them with a resolution of one nanometer — the width of only a few atoms — which, as it turns out, are the size of the defects.” in the deformed NiCoCr structure.”
The CrMnFeCoNi alloy was also tested at 20 K and performed impressively, but it did not achieve the same toughness as the simpler CrCoNi alloy.
forging new products
Now that the inner workings of CrCoNi alloys are better understood, they are getting closer to being certified for special applications. Although these materials are expensive, George expects them to be used in situations where extreme environmental conditions could destroy standard metal ingots, such as in the freezing 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 are global shortages of cobalt and nickel because of their demand in the battery industry—could be persuaded to have similar properties.
Although the progress is exciting, Ritchie cautions that real-world use may still be a long way off, with good reason. “When you fly on an airplane, do you want to know that what keeps you from falling 40,000 feet is an airframe alloy that was developed just a few months ago? Or do you want the materials to be mature and well understood? That is why structural materials can take Many years, or even decades, until it comes into real use.
This research was supported by the Department of Energy’s Office of Science. The low-temperature mechanical testing and neutron deflection were 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 user facility of the Department of Energy’s Office of Science at Berkeley Laboratory. Other authors on this project are Chen Yu, Saurabh Capra, Ming Jiang, Joachim Paul Forna-Kreutzer, Rubing Zhang, Madeline Payne, Flynn Walsh, Bernd Glodowatz, and Mark Asta.
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