: Photo: Dave Bullock/Wired.comPASADENA, California -- Researchers at Caltech are pioneering new ways to make superstrong metals that are twice as tough as titanium, and twice as elastic. These "metallic glass" composites are so strong a 3mm rod can support a 2-ton truck and they bend instead of snapping like most other metals of their kind, which are called "glass metals."
The new metals can potentially be used in industries from aerospace to automotive, as well as in consumer electronics. Because the alloy is so strong, less metal is needed, so spacecraft and cars would be lighter.
Glass metals have been around since the '50s. They get their exceptional strength from their disordered atomic structure (hence the "glass" name), whereas most metals have a weaker, crystalline atomic structure that follows a pattern. The downside of the glass structure is that it makes the metal brittle when it's put under too much pressure. The new composites have dendrites of normal crystalline metal structures running through the glass component, which greatly increases the pressure threshold of the alloys.
Making metal composites starts with a special arc welder that completely melts a sample, breaking its crystalline structure and uniformly mixing its atoms. Here, an arc of plasma springs from an electrode to a sample of titanium alloy, melting it instantly. The sample now has the structure of a regular glass metal. Forming the crystalline dendrites comes later in the process.
: Photo: Dave Bullock/Wired.comThe plasma arc melter can be used to melt nearly any metal except beryllium. When beryllium is melted, it produces vapor that mixes with air and oxidizes forming beryllium-oxide, a dangerous carcinogen. The samples that contain beryllium (even a tiny amount) must be melted inside a similar plasma arc melter inside a room that has negative pressure to prevent the beryllium-oxide from escaping.
: Photo: Dave Bullock/Wired.comA piece of extremely dark welding glass prevents the brilliant white light from blinding the experimenter while the sample melts. When the shield is removed, an incredibly bright beam of light shines on the wall, lighting up the room in the process.
: Photo: Dave Bullock/Wired.comAn ingot of metallic glass glows bright orange after it's heated to more than 3,000 Kelvin with an arc of plasma. The copper base is flooded internally with cold water to prevent the copper from vaporizing when the sample is melted.
: Photo: Dave Bullock/Wired.comNow that the sample alloy has been melted into a homogenous glass, it's time to form the dendrites inside. Ph.D. candidate Douglas Hofmann must first make sure that water is flowing through the copper tray where the sample rests or the tray will rupture from the heat.
Next, the glass vacuum tube that holds the sample and the tray must be emptied of air and replaced with a noble gas such as Argon (held in the blue tanks). This prevents the sample from oxidizing. Finally, Hofmann cranks the dial on the radio frequency inductor to heat the metal sample on the tray to 800-1,000 degrees Celsius.
: Photo: Dave Bullock/Wired.comThe radio frequency inductor coils heat an alloy sample to between 800 and 1,000 degrees Celsius in a matter of seconds. The goal here is to heat the sample below its melting point to allow only a specified portion of the atoms to form in a crystalline structure. This is the groundbreaking technique that creates the fortifying dendrites within the glass structure.
About 200 volts at 50 amps of radio-frequency energy is pumped through the coil, which heats the sample using induction. The coil itself doesn't get hot, but the sample obviously does. The radio frequency induction provides more control during heating than the arc melter -- control that allows scientists to tweak the composition of the alloy to their specifications.
: Photo: Dave Bullock/Wired.comA sample of metallic glass composite cools on the melting trough.
: Photo: Dave Bullock/Wired.comThis copper tray failed instantly and ruptured when a student forgot to turn on the cooling pump during the experiment. The copper has a much lower melting point than the various metals that melt atop it, but thanks to its high level of thermal conductivity, it transfers the heat into the water -- as long as the water is moving.
: Photo: Dave Bullock/Wired.comSeveral ingots of metallic glass composite are ready to be machined and mechanically tested.
: Photo: Dave Bullock/Wired.com This scanning electron microscope takes detailed photos of the surface structure of materials, including the metallic glass composite that Hofmann is creating.
: Image courtesy Douglas C. Hofmann/Nature 451 A microscope image shows how the crystalline dendrites affect the way the metals handle pressure. On the left is a composite with a smaller percentage of dendrites, in the middle is a sample with a higher percentage, and on the right is a pure glass metal with no dendrites.
: Image courtesy Douglas C. Hofmann/Nature 451This electron micrograph shows a sample with both crystalline dendrites (labeled "bcc" for body-centered cubic) and glass structures. Compare the ordered geometric matrix of the atoms on the left to the random placement of the molecules on the right (glass).
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