Aug 24, 2012
Nanocrystalline alloys can take the heat
Using a combination of experiments and analytical thermodynamic modelling, researchers at the Massachusetts Institute of Technology have produced a new tungsten-based nanocrystalline alloy that is stable above 1000 °C. The breakthrough result – nanocrystals are rarely stable at such high temperatures – will be a great help when it comes to developing new materials in which high strength and impact resistance are all important.
Nanocrystalline metals are much stronger than their bulk counterparts but, unfortunately, they are also more unstable. One particular problem is that grains of the nanocrystals can grow and merge together at high temperatures – something that softens the metal. This is not exactly a welcome phenomenon, given the elevated temperatures routinely employed in metals processing. Although researchers have tried to devise methods to prevent such unwanted grain growth, no really satisfactory solution has yet been found.
A team led by Christopher Schuh may now come up with a way. The researchers have designed and fabricated alloys containing nanocrystals that do not coalesce at high temperatures. “Our design method is based on calculating the energies of all the atoms/bonds in a given alloy, including those in crystal environments and those that are located at the grain boundaries, between crystals,” Schuh told nanotechweb.org. “We specifically calculate the effects that different alloying elements have on the structure’s energy, and seek to identify which elements can stabilize the grain boundaries and so lead to and maintain the nanocrystalline state.”
Industry has been trying to create alloys with ever-smaller crystalline grains for years now, he says, but nature does not like to do that because she tends towards low-energy states, which inevitably means bigger crystals.
Following on from the calculations, the MIT team then successfully synthesized the alloys that they had designed using suitable combinations of different metals in pre-defined proportions. The researchers also looked at metal blends that are not usually found together and that have never been produced before – even in the lab. For example, in this preliminary work on tungsten alloys, they considered combinations of 12 different elements, including titanium, chromium, copper, silver, and eight others. “Conventional approaches to designing alloys do not generally take grain boundaries into account, but instead focus on how compatible the different metals are,” explains Schuh. “However, grain boundaries are crucial for creating stable nanocrystals, so we decided to include these in our calculations.”
One particular alloy based on tungsten and titanium the team created contains about 20 atomic % of titanium and has grains that are just 20 nm in size. It was found to remain stable for a whole week at annealing temperatures of 1100 °C and retained its exceptional strength during this time. It could thus find use in applications where high-impact resistance is needed, such as in industrial machinery or in armour, say the researchers.
“The methodology we developed in our work could easily be exploited to make, other, new nanostructured materials with equally good or even better strength and stability, and additional desirable properties like corrosion resistance,” says Schuh. “Finding such alloys would be almost impossible by conventional trial and error methods, but we are able to calculate which metal combinations work and which do not,” add team members Heather Murdoch and Tongjai Chookajorn.
Indeed, the team is now busy detailing how its theoretical approach can be extended to many other alloys and structures – work that will be detailed in a future publication.
The present results are reported in Science.
About the author
Belle Dumé is contributing editor at nanotechweb.org.