Most metals are made of crystals which are orderly arrays of molecules forming a perfectly repeating pattern. In many cases the material is made of tiny crystals packed closely together, rather than one large one, and for many purposes making the crystals as small as possible provides significant advantages in properties and performance. However, such materials are often unstable as the crystals tend to merge and grow larger if subjected to heat or stress.
Researchers at the Massachusetts Institute of Technology (MIT) Department of Materials Science and Engineering (DEMSE) in Cambridge, Mass., have been undertaking work funded by the U.S. Army Research Office to find a way to avoid that problem. The result of this research are alloys that form extremely tiny grains – called nanocrystals – that are only a few billionths of a meter across, which retain their nanocrystalline structure and high strength even in the face of high heat. Such materials hold great promise for high-strength structural materials, among other potential uses.
The research was undertaken by graduate student Tongjai Chookajorn, who guided the effort to design and synthesize a new class of tungsten alloys with stable nanocrystalline structures. Her fellow DMSE graduate student, Heather Murdoch, came up with the theoretical method for finding suitable combinations of metals and the proportions of each that would yield stable alloys.
Chookajorn then successfully synthesized and tested the material and demonstrated that it does, in fact, have the stability and properties that Murdoch’s theory predicted. They, along with their advisor Professor Christopher Schuh, department head of DMSE, co-authored a paper outlining their results in a recent issue of Science (Aug.24, 2012).
“For decades, researchers and the metals industry have tried to create alloys with ever-smaller crystalline grains, but nature does not like to do that. Nature tends to find low-energy states, and bigger crystals usually have lower energy,” stated Prof Schuh.
Looking for pairings with the potential to form stable nanocrystals, Murdoch studied many combinations of metals that are not found together naturally and have not been produced in the lab. “The conventional metallurgical approach to designing an alloy doesn’t think about grain boundaries,” Schuh explains, “but rather focuses on whether the different metals can be made to mix together or not. It’s the grain boundaries that are crucial for creating stable nanocrystals. So Murdoch came up with a way of incorporating these grain boundary conditions into the team’s calculations.”
One of the nanocrystalline alloys developed and tested at MIT is a combination of tungsten and titanium. This alloy is exceptionally strong and could find applications in protection from impacts, guarding industrial or military machinery or for use in vehicular or personal armor. Other nanocrystalline materials designed using these methods could have additional important qualities, such as exceptional resistance to corrosion, the team says.
But finding materials that will remain stable with such tiny crystal grains, out of the nearly infinite number of possible combinations and proportions of the dozens of metallic elements, would be nearly impossible through trial and error. “We can calculate, for hundreds of alloys, which ones work, and which don’t,” Murdoch stated.
The key to designing nanocrystalline alloys, they found, is “finding the systems where, when you add an alloying element, it goes to the grain boundaries and stabilizes them,” Prof Schuh says, rather than distributing uniformly through the material. Under classical metallurgical theory, such a selective arrangement of materials is not expected to occur.
The tungsten-titanium material that Chookajorn synthesized, which has grains just 20 nanometers across, remained stable for a full week at a temperature of 1,100 C – a temperature consistent with processing techniques such as sintering, where powdered metal is consolidated in a mold and sintered to produce a solid shape.
Julia Weertman, a materials science professor at Northwestern University, stated “this work represents a significant advancement toward the goal of creating nanocrystalline alloys that are usable at elevated temperatures.” She added that “Schuh and his students, using thermodynamic considerations, derived a method to choose alloys that will remain stable at high temperatures. This research opens up the use of microstructurally stable nanocrystalline alloys in high temperature applications, such as engines for aircraft or power generation.”
Posted by: Paul Whittaker, Editor ipmd.net, [email protected]