An alloy of chromium, cobalt and nickel (CrCoNi) is the toughest material ever tested, and its characteristics could make it useful in the construction of aircraft or spacecraft.
Toughness is a measure of a material鈥檚 resilience against fracturing, and it isn鈥檛 the same thing as hardness, which is a measure of a material鈥檚 ability to withstand deformation. For instance, you can鈥檛 put a dent in a diamond, famously known to be the hardest material yet discovered, but you might be able to crack one.
鈥淛ust the other day, I went to Google and looked up 鈥榳hat鈥檚 the toughest material in the world?鈥� and what came back was diamond, which is 100 per cent wrong,鈥� says聽 at Lawrence Berkeley National Laboratory. 鈥淒iamond is very brittle, it doesn鈥檛 have any toughness.鈥�
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Often, when you increase the strength of a material, you lower its toughness: hard materials are brittle. What is remarkable about CrCoNi is that there is no trade off, says Ritchie, who has been studying the alloy with his colleagues. As it gets stronger, it gets tougher.
Materials often become less tough and more brittle at low temperatures. But in tests at just 20 degrees above absolute zero, CrCoNi displayed a toughness of 500 megapascals square root metres 鈥� essentially a measure of the energy required to break it. This is far higher than the toughness of aluminium used in aircraft and even the best steels.
Analysis of CrCoNi after fracturing showed that its structure creates a series of obstacles to fracturing, one at a time. First, areas of metallic crystals slide across each other until the patterns no longer line up, creating more resistance.
After that, the alloy displays a phenomenon called nanotwinning, when impacts break crystals within the alloy into smaller crystals, and some of them flip orientation, increasing the overall strength of the structure. The crystalline structure can also morph from a cubic to hexagonal arrangement.
These slips and rearrangements are common in isolation, but the CrCoNi alloy displays them all, and in a repeating order, which means that the metal is extraordinarily resistant to fracturing, says Ritchie.
Ritchie says research into alloys like this could uncover even tougher substances as well as other materials that surpass currently available ones in all sorts of ways.
鈥淭here鈥檚 millions, probably trillions, of new compositions out there that might have interesting properties,鈥� he says. 鈥淧eople are looking at it for all sorts of different situations, for making materials for batteries, for catalysts, looking at certain high-entropy ceramics, all sorts of things.鈥�
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