Steel and plastic. These two materials are essential to so much of our infrastructure and technology, and they have a complementary set of strengths and weaknesses. Steel is strong and hard, but difficult to shape intricately. While plastic can take on just about any form, it’s weak and soft. So wouldn’t it be nice if there were one material as strong as the strongest steel and as shapeable as plastic?
Well, a lot of scientists and technologists are getting excited about a relatively recent invention called metallic glass with both of those properties, and more. Metallic glasses look shiny and opaque, like metals, and also like metals, they conduct heat and electricity. But they’re way stronger than most metals, which means they can withstand a lot of force without getting bent or dented, making ultrasharp scalpels, and ultrastrong electronics cases, hinges, screws; the list goes on. Metallic glasses also have an incredible ability to store and release elastic energy, which makes them perfect for sports equipment, like tennis racquets, golf clubs, and skis.
They’re resistant to corrosion, and can be cast into complex shapes with mirror-like surfaces in a single molding step. Despite their strength at room temperature, if you go up a few hundred degrees Celsius, they soften significantly, and can be deformed into any shape you like. Cool them back down, and they regain the strength. So where do all of these wondrous attributes come from? In essence, they have to do with metallic glass’ unique atomic structure. Most metals are crystalline as solids.
That means that if you zoomed in close enough to see the individual atoms, they’d be neatly lined up in an orderly, repeating pattern that extends throughout the whole material. Ice is crystalline, and so are diamonds, and salt. If you heat these materials up enough and melt them, the atoms can jiggle freely and move randomly, but when you cool them back down, the atoms reorganize themselves, reestablishing the crystal.
But what if you could cool a molten metal so fast that the atoms couldn’t find their places again, so that the material was solid, but with the chaotic, amorphous internal structure of a liquid? That’s metallic glass. This structure has the added benefit of lacking the grain boundaries that most metals have. Those are weak spots where the material is more susceptible to scratches or corrosion. The first metallic glass was made in 1960 from gold and silicon. It wasn’t easy to make. Because metal atoms crystallize so rapidly, scientists had to cool the alloy down incredibly fast, a million degrees Kelvin per second, by shooting tiny droplets at cold copper plates, or spinning ultrathin ribbons.
At that time, metallic glasses could only be tens or hundreds of microns thick, which was too thin for most practical applications. But since then, scientists have figured out that if you blend several metals that mix with each other freely, but can’t easily crystallize together, usually because they have very different atomic sizes, the mixture crystallizes much more slowly. That means you don’t have to cool it down as fast, so the material can be thicker, centimeters instead of micrometers.
These materials are called bulk metallic glasses, or BMGs. Now there are hundreds of different BMGs, so why aren’t all of our bridges and cars made out of them? Many of the BMGs currently available are made from expensive metals, like palladium and zirconium, and they have to be really pure because any impurities can cause crystallization. So a BMG skyscraper or space shuttle would be astronomically expensive. And despite their strength, they’re not yet tough enough for load-bearing applications.
When the stresses get high, they can fracture without warning, which isn’t ideal for, say, a bridge. But when engineers figure out how to make BMGs from cheaper metals, and how to make them even tougher, for these super materials, the sky’s the limit.