Most metals from the steel used to build bridges and skyscrapersto the copper and gold used to form wires in microchips are madeof crystals: orderly arrays of molecules forming a perfectlyrepeating pattern. In many cases, including the examples above, thematerial is made of tiny crystals packed closely together, ratherthan one large crystal. Indeed, for many purposes, making thecrystals as small as possible provides significant advantages inperformance, but such materials are often unstable: the crystalstend to merge and grow larger if subjected to heat or stress. Now, MIT researchers have found a way to avoid that problem.
They vedesigned and made alloys that form extremely tiny grains callednanocrystals that are only a few billionths of a meter across.These alloys retain their nanocrystalline structure even in theface of high heat. Such materials hold great promise forhigh-strength structural materials, among other potential uses. The new findings, including both a theoretical basis foridentifying specific alloys that can form nanocrystallinestructures and details on the actual fabrication and testing of onesuch material, are described in a paper published in Science . Graduate student Tongjai Chookajorn, of MIT s Department ofMaterials Science and Engineering (DMSE), guided the effort todesign and synthesize a new class of tungsten alloys with stablenanocrystalline structures. Her fellow DMSE graduate student,Heather Murdoch, came up with the theoretical method for findingsuitable combinations of metals and the proportions of each thatwould yield stable alloys.
Chookajorn then successfully synthesizedthe material and demonstrated that it does, in fact, have thestability and properties that Murdoch s theory predicted. They,along with their advisor Christopher Schuh, a professor ofmetallurgy, are co-authors of the paper. For decades, researchers and the metals industry have tried tocreate alloys with ever-smaller crystalline grains, Schuh says.But, he adds, Nature does not like to do that. Nature tends tofind low-energy states, and bigger crystals usually have lowerenergy. Looking for pairings with the potential to form stablenanocrystals, Murdoch studied many combinations of metals that arenot found together naturally and have not been produced in the lab.
The conventional metallurgical approach to designing an alloydoesn t think about grain boundaries, Schuh explains, but ratherfocuses on whether the different metals can be made to mix togetheror not. But, he adds, it s the grain boundaries that are crucialfor creating stable nanocrystals. So Murdoch came up with a way ofincorporating these grain boundary conditions into the team scalculations. Why go to the trouble of designing such materials? Because they canhave properties that other, more conventional metals and alloys donot, the researchers say.
For example, the alloy of tungsten andtitanium that the MIT researchers developed and tested in thisstudy is likely exceptionally strong, and could find applicationsin protection from impacts, guarding industrial or militarymachinery or for use in vehicular or personal armor. But theresearchers stress that this fundamental research could lead to awide range of potential uses. This is one case study, but thereare potentially hundreds of alloys we could make, Schuh says. Other nanocrystalline materials designed using these methods couldhave additional important qualities, such as exceptional resistanceto corrosion, the team says. But finding materials that will remainstable with such tiny crystal grains, out of the nearly infinitenumber of possible combinations and proportions of the dozens ofmetallic elements, would be nearly impossible through trial anderror.
We can calculate, for hundreds of alloys, which ones work,and which don t, Murdoch says. The key to designing nanocrystalline alloys, they found, is finding the systems where, when you add an alloying element, itgoes to the grain boundaries and stabilizes them, Schuh says,rather than distributing uniformly through the material. Underclassical metallurgical theory, such a selective arrangement ofmaterials is not expected to occur. The tungsten-titanium material that Chookajorn synthesized, whichhas grains just 20 nanometers across, remained stable for a fullweek at a temperature of 1,100 C a temperature consistent withprocessing techniques such as sintering, where powdered material ispacked into a mold and heated to produce a solid shape.
This meansthis alloy could easily become a practical material for a varietyof applications where its high strength and impact resistance wouldbe important, the researchers say. Julia Weertman, a professor of materials science and engineering atNorthwestern Univ., says this work represents a significantadvancement toward the goal of creating nanocrystalline alloys thatare usable at elevated temperatures. She adds that Schuh andhis students, using thermodynamic considerations, derived a methodto choose alloys that will remain stable at high temperatures This research opens up the use of microstructurally stablenanocrystalline alloys in high temperature applications, such asengines for aircraft or power generation.
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