Scientists in South Korea have invented a new steel alloy that boasts the same strength-to-weight ratio as titanium - the super-strong metal we use to construct jet engines, missiles, spacecraft, and medical implants - but it can be produced for one-tenth of the cost.
In order to develop this new kind of metal, the team from Pohang University of Science and Technology had to overcome a problem that had stumped materials scientists for decades, says William Herkewitz at Popular Mechanics. "In the 1970's, Soviet researchers discovered that adding aluminium to the mix when creating steel can make an incredibly strong and lightweight metal, but this new steel was unavoidably brittle,” he says. "You’d have to exert lots of force to reach the limit of its strength, but once you did, the steel would break rather than bend.”
The issue is that steel on its own is very strong and cheap, but it’s super-heavy. So it’s not that useful in constructing aircraft, and while it’s enjoyed a good run in the car manufacturing industry, the fuel-efficiency people have come to expect is just not possible when you’re trying to support all that hefty steel. According to The Economist, between 1995 and 2011, the weight of steel in an average light vehicle fell from 68.1 percent to 60.1 percent, and it’s only going to get lower.
So you need to mix steel with something to make it lighter, and aluminium is the obvious candidate, because it’s so lightweight and cheap. The problem here is that when you try to mix aluminium and steel - an alloy of iron, aluminium and carbon - sometimes the aluminium and iron atoms would fuse together in weird ways, forming dense, crystalline structures referred to as B2. And while these structures were certainly strong, they were brittle, which means you really didn’t have to do much to them before they’d crumble and break. Scientists tried adding manganese to the mix, which helped reduce the brittleness, but not enough.
And then, lead researcher and materials scientist, Hansoo Kim, had an idea - manipulate the structure of the aluminium-steel alloy on the nanoscale.
"My original idea was that if I could somehow induce the formation of these B2 crystals, I might be able to disperse them in the steel," he told Herkewitz at Popular Mechanics. So his team figured out that if they moved the B2 crystals around and separated them from each other, the rest of the alloy structure could fill in the gaps and protect them from breaking apart.
The key to working this out was adding a tiny bit of nickel, The Economist reports:
"The nickel reacts with some of the aluminium to create B2 crystals a few nanometres across. These crystals form both between and within the steel’s grains when it is annealed (a form of heat treatment). B2 crystals are resistant to shearing, so when a force is applied to the new material, they do not break. This stops tiny cracks propagating through the stuff, which gives it strength. That strength, allied with the lightness brought by the aluminium, is what Kim was after.”
The team has published the results in Nature, and they hope that other materials scientists around the world will use their method to come up with more weird and wonderful new alloys for the market. They’re currently in discussion with POSCO, one of the world’s largest steel manufacturers, to see if they can get their ‘super-steel’ out into the production line.