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Discovered the most resistant material in the world

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Scientists have measured the highest toughness of a material ever recorded by studying a metal alloy of chromium, cobalt and nickel ( CrCoNi ). Not only is the metal extremely ductile (which in materials science means highly malleable) and incredibly strong (that is, it resists permanent deformation ), but its strength and ductility improve as it gets colder . This is in contrast to most other existing materials.

 

The team, led by researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) and Oak Ridge National Laboratory, published a study describing the record-breaking results in Science. “When designing structural materials, you want them to be strong but also ductile and resistant to fracture,” said project co-leader Easo George, chair of advanced alloy development and theory at the University of Tennessee. “Typically, it’s a trade-off between these properties. But this material is both, and instead of becoming brittle at low temperatures, it becomes harder.” CrCoNi is a subset of a class of metals called high entropy alloys (HEA).equal mixture of each constituent element. These balanced atomic recipes appear to give some of these materials an extraordinarily high combination of strength and ductility when stressed , which together constitute the so-called “toughness”. . HEAs were first developed about 20 years ago, but the technology needed to push materials to their limits in testing wasn’t available until recently.

“The toughness of this material near the temperatures of liquid helium (20 kelvin, -424 Fahrenheit) reaches 500 megapascals square root. In the same units of measure, the toughness of a piece of silicon is equal to one, that of the aluminum airframe of passenger aircraft is about 35, and that of some of the best steels is about 100. . So, 500 is a huge amount. So, 500, that’s a staggering number,” said research co-leader Robert Ritchie, senior scientist in Berkeley Lab’s division of materials science and Chua professor of engineering at UC Berkeley.

Ritchie and George started experimenting with CrCoNi and another alloy that also contains manganese and iron (CrMnFeCoNi) a long time ago. They created samples of these alloys, then lowered the materials to liquid nitrogen temperatures (about 77 kelvins, or -321 F) and discovered impressive strength and toughness. The researchers immediately wanted to continue work with tests at liquid helium temperatures, but it took 10 years to find facilities that would allow samples to be stress tested in such a cold environment and to recruit team members with the analytical tools and experience needed to analyze what is happening in the material at the atomic level. Fortunately, the results have been worth the wait.

 

The making of super products but in due time

Now that the CrCoNi alloy has been discovered, this and other HEAs are one step closer to adoption for special applications. While these materials are expensive to create, George envisions their use in situations where environmental conditions could destroy standard metal alloys, such as in the freezing temperatures of deep space . He and his Oak Ridge team 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 made to have similar properties. .

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While the progress is exciting, Ritchie cautions that real-world use may still be a ways off — for good reason. “When you fly an airplane, would you like to know that saving us from falling to 40,000 feet is an airframe alloy developed just a few months ago? Or would you like the materials to be mature and well understood? That’s why structural materials can take many years, even decades, to really get used to.”

  • Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys at 20 kelvin (science.org)
 

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