Engineers achieve longstanding goal of stable nanocrystalline metals

Aug 23, 2012 by David Chandler, MIT News Office
A transition electron microscope image of the structure of the new tungsten-titanium alloy, after being exposed to a high temperature of 1,100 degrees Celsius for a week. The image shows that the alloy retains its nanocrystalline structure even after this heat treatment. Image courtesy of Chookajorn et al, from Science

Most metals—from the steel used to build bridges and skyscrapers to the copper and gold used to form wires in microchips—are made of crystals: orderly arrays of molecules forming a perfectly repeating pattern. In many cases, including the examples above, the material is made of tiny crystals packed closely together, rather than one large crystal. Indeed, for many purposes, making the crystals as small as possible provides significant advantages in performance, but such materials are often unstable: The crystals tend to merge and grow larger if subjected to heat or stress.

Now, MIT researchers have found a way to avoid that problem. They've designed and made alloys that form extremely tiny grains—called nanocrystals—that are only a few billionths of a meter across. These alloys retain their nanocrystalline structure even in the face of high heat. Such materials hold great promise for high-strength , among other potential uses.

The new findings, including both a theoretical basis for identifying specific alloys that can form nanocrystalline structures and details on the actual fabrication and testing of one such material, are described in a paper published Aug. 24 in Science.

Graduate student Tongjai Chookajorn, of MIT's Department of Materials Science and Engineering (DMSE), 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 the material and demonstrated that it does, in fact, have the stability and properties that Murdoch's theory predicted. They, along with their advisor Christopher Schuh, the Danae and Vasilis Salapatas Professor of and department head of DMSE, are co-authors of the paper.

For decades, researchers and the metals industry have tried to create alloys with ever-smaller crystalline grains, Schuh says. But, he adds, "nature does not like to do that. Nature tends to find low-energy states, and bigger crystals usually have lower energy."

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 ," Schuh explains, but rather focuses on whether the different metals can be made to mix together or not. But, he adds, it's the grain boundaries that are crucial for creating stable . So Murdoch came up with a way of incorporating these grain boundary conditions into the team's calculations.

Why go to the trouble of designing such materials? Because they can have properties that other, more conventional metals and alloys do not, the researchers say. For example, the alloy of tungsten and titanium that the MIT researchers developed and tested in this study is likely exceptionally strong, and could find applications in protection from impacts, guarding industrial or military machinery or for use in vehicular or personal armor. But the researchers stress that this fundamental research could lead to a wide range of potential uses. "This is one case study, but there are potentially hundreds of alloys we could make," Schuh says.

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 says.

The key to designing nanocrystalline , they found, is "finding the systems where, when you add an alloying element, it goes to the grain boundaries and stabilizes them," 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 degrees Celsius—a temperature consistent with processing techniques such as sintering, where powdered material is packed into a mold and heated to produce a solid shape. This means this alloy could easily become a practical material for a variety of applications where its high strength and impact resistance would be important, the researchers say.

Explore further: Faster switching helps ferroelectrics become viable replacement for transistors

Related Stories

MIT thinks small to find safer metals

Feb 21, 2006

MIT researchers have devised a new method for shrinking the size of crystals to make safer metal alloys. The new materials could replace metal coatings such as chromium, which is dangerous for factory workers ...

New super strong alloy discovered

Sep 08, 2010

(PhysOrg.com) -- International team of researchers has discovered a new super-strength light alloy and had their key findings published in Nature Communications.

Model simulates atomic processes in nanomaterials

Mar 01, 2007

Researchers from MIT, Georgia Institute of Technology and Ohio State University have developed a new computer modeling approach to study how materials behave under stress at the atomic level, offering insights that could ...

Japanese material scientists develop new superelastic alloy

Jul 01, 2011

(PhysOrg.com) -- Working out of Tokyo University, scientists in the Department of Materials Science, have developed a new metal alloy that unlike other “superelastic” alloys can resume its original shape in temperatures ...

Discovery might improve titanium alloys

Oct 20, 2005

Two University of Maryland scientists say they've developed a modification of titanium alloys that will expand their uses and make them safer.

Recommended for you

New world record for a neutron scattering magnet

3 minutes ago

A unique magnet developed by the Florida State University-headquartered National High Magnetic Field Laboratory (MagLab) and Germany's Helmholtz Centre Berlin (HZB) has reached a new world record for a neutron ...

The science of charismatic voices

17 hours ago

When a right-wing Italian politician named Umberto Bossi suffered a severe stroke in 2004, his speech became permanently impaired. Strangely, this change impacted Bossi's perception among his party's followers—from appearing ...

Urban seismic network detects human sounds

17 hours ago

When listening to the Earth, what clues can seismic data reveal about the impact of urban life? Although naturally occurring vibrations have proven extremely useful to seismologists, until now the vibrations ...

NIST 'combs' the atmosphere to measure greenhouse gases

18 hours ago

By remotely "combing" the atmosphere with a custom laser-based instrument, researchers from the National Institute of Standards and Technology (NIST), in collaboration with researchers from the National Oceanic ...

Supersonic laser-propelled rockets

19 hours ago

Scientists and science fiction writers alike have dreamt of aircrafts that are propelled by beams of light rather than conventional fuels. Now, a new method for improving the thrust generated by such laser-propulsion ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

Moebius
not rated yet Aug 24, 2012
Sounds like a good candidate for swords and knives.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.