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 in Science magazine. Graduate student Tongjai Chookajorn, of MIT’s Department of Materials Science and Engineering (DMSE), and fellow DMSE graduate student, Heather Murdoch, devised the theoretical method for finding suitable combinations of metals and the proportions of each that would yield stable alloys. They then successfully synthesized the material and demonstrated that it does, in fact, have the stability and properties that Murdoch’s theory predicted. 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 consider grain boundaries, rather focuses on whether the different metals can be made to mix together or not. However, it is often 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. Making the crystals as small as possible often provides significant advantages in performance, the paper suggests. 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 armour. The researchers stress that this fundamental research could lead to a wide range of potential uses.
Other nanocrystalline materials designed using these methods could have additional important qualities, such as exceptional resistance to corrosion, the team says.
However, 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. So the researchers devised a way of finding the systems where, when you add an alloying element, it goes to the grain boundaries and stabilizes them, rather than distributing uniformly through the material. Under classical metallurgical theory, such a selective arrangement of materials is not expected to occur.
One tungsten-titanium material that was synthesized has grains just 20 nanometers across, and remained stable for a full week at a temperature of 1,100°C – a temperature consistent with processing techniques such as sintering. 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 suggest.
