Metal Alloy Identified as the Toughest Material on Earth
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A team of researchers from the Lawrence Berkeley National Laboratory and Oak Ridge National Laboratory has identified the toughest material on Earth – a metal alloy of chromium, cobalt and nickel. Contrary to other materials, its strength increases as it gets colder.
The perfect combination of metals
The alloy, known as CrCoNi, is an example of a high entropy alloy (HEA), which – unlike alloys in use today – contains equal proportions of its constituent elements. HEAs were first developed ~20 years ago, but technologies capable of testing the extreme limits of these materials have only recently become available.
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The research co-leaders, Professor Robert Ritchie from the University of California, Berkeley and Professor Easo George from the University of Tennessee, first started work on CrCoNi 10 years ago, initially finding the alloy to be exceptionally tough at liquid nitrogen temperatures (~ -200 degrees Celsius). They wanted to take it a step further and see how tough CrCoNi is at liquid helium temperatures (~ -250 °C).
What is “toughness” and how do we measure it?
Toughness is the combination of strength and ductility (resistance to fracture) a material possesses. To measure toughness, a pristine specimen is pulled until it fractures, and the force needed to do so is measured. For fracture toughness tests, the force required to grow a crack intentionally made in the sample is measured.
In their most recent research paper, published in Science, Ritchie and George have quantified the toughness of CrCoNi at liquid helium temperatures and used neutron diffraction, electron backscatter diffraction and transmission electron microscopy to reveal how the atomic structure of CrCoNi contributes to its remarkable strength.
Just how tough is CrCoNi?
“The toughness of this material near liquid helium temperatures (20 °K, -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,” said Ritchie.
The toughness of CrCoNi comes from the atomic structure of its crystalline lattice, which changes when force is applied to the alloy. A trio of atomic interactions occurs in sequence, followed by a rearrangement of the unit cells within the lattice, which enables CrCoNi to resist stress past the point where other materials would have fractured.
“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,” explained Ritchie. “The fact they all occur in this magical sequence gives us these really tremendous properties.”
“The structure of the CrCoNi is the simplest you can imagine – it’s just grains,” Richie said. Co-author Professor Andrew Minor continues: “However, when you deform it, the structure becomes very complicated, and this shift helps explain its exceptional resistance to fracture.”
A strong resistance to fracture at frigid temperatures make CrCoNi ideal for application at environmental extremes, such as in deep space. Understanding how CrCoNi is so tough brings such uses of the alloy, and HEAs in general, a bit closer.
The researchers caution that although their progress in this field is exciting, new structural materials can take a long time to be used in real-world applications, as they need to be well-understood before they can replace well-tested and trusted existing materials.
Reference: Liu D, Yu Q, Kabra S, et al. Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys at 20 Kelvin. Science. 2022;378(6623):978-983. doi: 10.1126/science.abp8070
This article is a rework of a press release issued by the Berkeley Lab. Material has been edited for length and content.