'Mind-boggling' alloy is Earth's toughest material, even at extreme temperatures

Researchers have proven that a metallic alloy of chromium, cobalt and nickel is officially the toughest material on Earth — more than 100 times tougher than the wonder material graphene.

In a new study published Dec. 1 in the journal Science, researchers subjected the ultra-tough alloy to extremely cold temperatures, in order to test how fracture-resistant the material is. Scientists have known for years that this alloy is one tough cookie — however, to the team's surprise, the alloy only became tougher and more resistant to cracks as temperatures plummeted. 

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This super-resistance to fracture is in stark contrast to most materials, which only become more brittle in freezing temperatures, according to the study authors.

"People talk about the toughness of graphene, and that is measured at just 4 megapascals per meter," study co-author Robert Ritchie, a professor of engineering at the University of California Berkeley and senior faculty scientist at the Lawrence Berkeley National Laboratory, told Live Science. "The toughness of aluminum alloys used in aircraft is 35 megapascals per meter. This material has a toughness of 450 to 500 megapascals per meter… these are mind-boggling numbers." 

The potential applications of such a tough material range from space infrastructure to fracture-resistant containers for clean energy uses here on Earth. However, Ritchie noted, two of the alloy's three elements (nickel and cobalt) are prohibitively expensive, limiting the alloy's usefulness to the laboratory for the foreseeable future.

Strange alloy

The chromium, cobalt and nickel alloy is an example of a high entropy alloy (HEA). Unlike most alloys, which are made predominantly of one element with lower amounts of additional elements added, HEAs are made of an equal mix of each constituent element. 

This HEA is extremely malleable, or ductile, meaning it can bend under pressure to withstand fracturing, according to the study authors. Several quirks of the alloy's molecular structure make it so extraordinarily malleable. One key mechanism, for example, causes atoms within the alloy to dislocate under pressure, allowing them to shear over one another. This, along with various other mechanisms, allow the material to keep deforming as pressure increases, without fracturing or breaking.

"Each one of these mechanisms kicks in at a later stage when you increase the strain on the material and that's the perfect recipe for high toughness," Ritchie added. "What is remarkable is these mechanisms get more effective in colder temperatures."

The researchers initially tested the alloy's toughness by exposing it to liquid nitrogen at temperatures of around minus 321 degrees Fahrenheit (minus 196 degrees Celsius). When the alloy's toughness only improved, the team wondered how much further they could push the material's limits. 

Dong Liu, a physicist at Bristol University in England, and colleagues designed an experiment to expose the alloy to liquid helium, which can cool to super-frigid temperatures of minus 424 F (minus 253 C). The team then watched neutrons scatter off the material in a process called neutron diffraction to peer into the structure of the alloy and see how cracks formed as pressure increased. 

The experiment showed that when it came to toughness, the alloy blew graphene out of the water.

"Graphene is very high strength, but it doesn't have any damage tolerance," Liu told Live Science. "It's very brittle and shatters just like a mug you throw on the floor that shatters into pieces." 

Another drawback of graphene is that its high strength only holds at exceptionally small, nanometer-level scales, Liu added. Meanwhile, the samples of chromium, cobalt and nickel alloy tested by Liu and her team were cigarette-pack-size, meaning the HEA maintained its toughness at the scale of everyday objects.

Materials of the future

While more testing is needed before this material can be practically applied, Liu is optimistic that the alloy could be used for many projects, both in space and on Earth. For example, the HEA could be used in hydrogen storage containers that could make environmentally friendly hydrogen-powered vehicles more feasible.

"If you drive a car with a hydrogen vessel made from something very brittle you're essentially carrying a bomb around with you,” Liu said. “But not with this material."

Ritchie, meanwhile, is cautious in suggesting potential applications of the alloy, as moving material from the lab to the "real world" requires a lot of knowledge and time, while the costs of nickel and cobalt remain prohibitively high. However, he is interested in developing recipes for new alloys that could be just as tough, using different elements.

"There's 50 usable elements in the periodic table," Ritchie said. "Taking combinations of three, five or seven of them means there are millions of new alloys."