What do you do if you want a material that's as hard as glass, but that can bend without shattering, like steel? Well, if you're a researcher at the California Institute of Technology (Caltech), you invent metallic glass. There are several types of metallic glass – which is said to be stronger than steel or titanium – all of which consist of a metal with the disordered atomic structure of glass. Although it's been possible to produce the material in bulk since the early 90s, the production process has limitations, that have kept metallic glass from coming into common use. Now, however, a Caltech team has come up with a new process, in which the material can be shaped as easily as plastic.
In the conventional method of shaping metallic glass, the solid alloy must first be heated to over 1000C (1832F). This liquefies it, and puts it comfortably above its transition phase temperature of 500-600C (932-1112F), below which it begins to crystallize. Once liquefied, it is then poured into metal molds, where it cools before crystallizing and becoming a solid again.
Unfortunately, however, such molds are typically only designed to withstand temperatures up to 600C. Being repeatedly exposed to the 1000-degree liquid is hard on them, and as a result, they need to be frequently replaced. The liquid metallic glass is also fluid enough that it has a tendency to splash and break up when being poured, which results in finished cooled parts with structural defects.
In order to address these problems, the Caltech researchers have utilized a process known as ohmic heating. Using it, they fired an electrical pulse of over 1,000 joules in about 1 millisecond (which is around one megawatt of power), into a 20 x 4 mm metallic glass rod. This heated it to around 550C (1022F) in approximately half a millisecond, at which point it became liquefied, but was still very close to its transition phase. This was not a problem, however, as it was then immediately injected into a mold and cooled. The entire shaping process took less than 40 milliseconds, and resulted in a structurally-sound metallic glass toroid – a donut-shaped object.
The technology is called rapid discharge forming. It has been patented, and is now being developed for commercialization by the Caltech spin-off company Glassimetal Technology.
The research was published last week in the journal Science.
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