Richard Musgrove
Dr Richard Musgrove is a zoologist and science journalist. He writes for Cosmos and Double Helix.
A new nickel-based super-metal alloy that maintains strength and flexibility over an 800oC temperature range has been developed in Korea.
A team at researchers at the Republic of Korea’s Pohang University of Science and Technology (POSTECH) called its high entropy allow (HEA) the first of the ‘hyperadaptors’.
“Our HEA breaks through the limitations of existing alloys and establishes a new class of temperature-insensitive materials,” says coauthor, Professor Hyoung Seop Kim. “The Hyperadaptor concept represents a breakthrough in developing next-generation materials with consistent mechanical behaviour even under extreme conditions.”
Metal alloys typically contain more than one element. The most familiar, steel is at least 98% iron with around 0.1 to 0.2% carbon added, with variations including stainless steel, which contains chromium to make it corrosion-resistant. Bronze is 88-95% copper, and 5-12% tin, and humans have been making that for at least 5300 years.
HEAs are a little more complex, typically containing five or more elements in about equal proportions. Like metal smoothies, no ‘flavour’ dominates. Originating in the early 2000’s, scientists are looking for stronger, tougher or more corrosion-resistant alloys.
And ‘entropy’? Remembering the Second Law of Thermodynamics from high school physics, natural process invariably become more disordered over time. Their entropy increases, like mess in a teenager’s bedroom. HEAs make use of this principle. The more randomly-mixed different elements you have, the greater the entropy.
But strangely, high entropy helps to stabilise the solid phase for these alloys. So if mixed thoroughly in the molten phase, the resulting hardened alloy tends to be mechanically tougher and more thermally stable than those with fewer elements.
The most famous HEA is probably the ‘Cantor alloy’ named after Professor Brian Cantor of the University of Oxford. An alloy of cobalt, chromium, iron, manganese and nickel (Co, Cr, Fe, Mn and Ni), this material is still flexible under “cryogenic temperatures,” typically liquid nitrogen at -196oC.
Everyday metals are sensitive to changing temperatures, says lead author Dr Hyojin Park, and are normally optimised for a particular thermal range. Outside that range they don’t do so well, which limits their effectiveness in environments “with dramatic temperature fluctuations,” says Park.
The POSTECH HEA handled the team’s 800oC temperature change without missing a mechanical beat. Temperatures ranged from a cryogenic -196°C to a blistering 600°C, typically found in gas turbines.
Made from 35% nickel, with the combination of iron, cobalt and chromium at 53% , plus aluminium (7%) and titanium (5%) their HEA was a ‘face-centred cube (FCC), says Park. That’s one atom at each corner of the cube (8 corners in total), plus one atom at the centre of each face of the cube (6 faces).
FCC super metals can be bent and stretched without breaking and are great for wiring and foil. The cube’s corners would be one element, and the face centres, another, which locks in the structure, giving it extra strength, meaning it won’t deform over time. And the aluminium and titanium are like reinforcing rods, giving extra thermal stability and meaning that their HEA could still flex over a range of temperatures, says Park.
Park concludes that their hyperadaptor HEA would be particularly useful for demanding applications like rockets, jet engines, car exhausts, power plant turbines, wherever metals had to cope with extreme and rapid temperature changes. The paper was published in Material Research Letters.
Super metals and batteries
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