Most metallic things around us—bridges, microchip wires, buildings—are made of arrays of tiny crystals that owe their strength to an orderly, repeating pattern of grains. However, these mixtures, or alloys, of different metals are unstable; under heat or stress they tend to meld together and become larger and weaker. But the right mix can produce a metal that’s stronger, more heat-resistant and capable of creating structures never thought possible.
Finding the ideal alloy crystal for making things from metals has been a frustrating, time-consuming exercise. Typically, finding the right combination of crystals requires tedious, hit-or-miss experiments. But two graduate students at the Massachusetts Institute of Technology (MIT) found a way to predict which metal combinations could create a winning alloy—a discovery that opens the door to quickly creating a whole new world of structures.
These new alloy combinations could produce lighter, but stronger vehicle or body armor for the military, for example. Or, they could make industrial materials that maintain their strength even in extreme heat, underwater craft that could sustain extraordinary pressure, or metals that resist rust on levels unheard of today.
Tonghai Chookajorn, Heather Murdoch, and advisor Christopher Schuh, who led the study that was published the August 24 Science, found that metals engineers had been making alloys the wrong way. Most metallurgists assumed that the metals making up the alloy crystal pattern should be evenly mixed. However, the researchers found that by focusing on metals that instead adhered to the boundaries of crystal grains, they could create metallic combinations that retained their small shape—and new, unique properties—despite heat or other stresses.
These nano-scale combinations had eluded engineers in the past because most tiny metal crystals grow in size if left to their own devices. “Nature does not like (smaller crystal grains),” Schuh said. “Nature tends to find lower-energy states, and bigger crystals have lower energy.”
The research already has yielded one new form of metal. By using their formula of matching metals that stabilized each other’s crystal grain boundaries, Chookajorn and Murdoch found an alloy of tungsten and titanium that remained intact even after enduring a week at 1,100° C.
Just as they did with the new tungsten-titanium alloy, the researchers can now calculate which combinations will or won’t work. “This is one case study,” said Schuh. “But there are hundreds of alloys we could make.”
Tongjai Chookajorn, Heather A. Murdoch, Christopher A. Schuh (2012). Design of Stable Nanocrystalline Alloys Science DOI: 10.1126/science.1224737