It has been well understood that if you could decrease the size of the crystals that make up the structures of most metals, you would improve the mechanical properties of those metals, including their strength. However, finding a way to decrease crystal size and maintain that smaller size in the face of heat has proven difficult. Typically, the crystals want to grow larger if exposed to heat or stress.
Now, MIT researchers may have found a way to ensure that the crystals maintain their small size even in the presence of heat and stress, thus achieving the goal of creating stable nanocrystalline materials.
The researchers, who have published their work in the journal Science (“Design of Stable Nanocrystalline Alloys”), came up with a theoretical model for predicting how the mixing of different metals would impact the creation of stable nanostructured alloys.
Heather Murdoch, a graduate student at MIT’s Department of Material Science and Engineering (DMSE), came up with the theoretical model and Tongjai Chookajorn, another graduate student in that department, synthesized the metals to test the stability and properties predicted in Murdoch’s models.
The key to the theoretical model is that it includes considerations of grain boundaries, says Christopher Schuh, head of the material science and engineering department and the two graduate students’ advisor .
“The conventional metallurgical approach to designing an alloy doesn’t think about grain boundaries,” Schuh explains in the MIT press release, adding that typically these models only consider whether two metals can be mixed together.
The first alloy that the researchers came up with was a mixture of tungsten and titanium. It is expected to be unusually strong, which could make it suitable for uses where high-impact considerations are critical, such as industrial equipment shielding or personal armor.
While the alloy the researchers first tested remained stable for a full week at temperatures of 1100 degrees Celsius, it is the possibility of creating entirely new alloys based on the predictive model that has the researchers most excited. “We can calculate, for hundreds of alloys, which ones work, and which don’t,” Murdoch says in the MIT press release.
Julia Weertman, a professor emerita of materials science and engineering at Northwestern University, further notes in the release: “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.”