A new method of deactivating nano twins to improve the high-temperature properties of superalloys may lead to a hotter and ultimately cleaner jet engine.
Researchers at The Ohio State University have tailored an alloy’s composition and exposed it to high heat and pressure to not only prevent nano twins—microscopic defects that grow inside alloys and weaken them, allowing them to deform under heat and pressure—but to also make the alloys stronger.
The engineers tested the technique, which is being called “phase transformation strengthening,” by eliminating the formation of nano twins and decreasing alloy deformation by half.
Michael Mills, Ph.D., a professor of materials science and engineering and a leader of the study, said when an engine can run at very high temperatures they consume more fuel thoroughly and produces lower emissions.
The advancement could be a cost-saver for the industry, which has already begun implementing the alloy modifications, said Mills in an exclusive interview with R&D Magazine.
“Our industry colleagues tell us that increasing the operating temperature of turbine engines, for example for aircraft, by even a few degrees can lead to significant reduction in operating cost over the life a fleet of aircraft,” Mills said. “At higher operating temperature, turbine engines are more efficient and have lower greenhouse gas emissions.”
According to Mills, alloys are currently designed on computers—practically atom by atom—and the aim of the experiment is address a deficit in the “quantitative, comprehensive understanding” of how the exotic metal-based materials deform under high stress.
The discovery was made when the science team was studying nano twin formation in two different commercial superalloys. They compressed samples of the alloys with thousands of pounds of pressure at around 1,400 degrees Fahrenheit, which is comparable to running a jet engine.
They also examined the alloys’ crystal structures with electron microscopes and modeled the quantum mechanical behavior of the atoms on a computer.
In both alloys, testing the temperature and pressure caused nano twin faults to develop within the superalloy crystals and the material composition in and around the faults changed.
Some elements, including atoms of nickel and aluminum through a sequence of atomic-scale jumps, diffused away from the faults, while others diffused into the faults.
“In the first alloy, which was not as strong at high temperature, atoms of cobalt and chromium filled the fault,” Timothy Smith, former student at Ohio State and lead author of the study, said in a statement. “That weakened the area around the fault and allowed it to thicken and become a nano twin.”
However, in the second alloy, where nano twins didn’t form, the elements titanium, tantalum and niobium tended to diffuse into the faults instead, which resulted in a new and very stable phase of material formed right at the faults.
The new phase resisted the formation of nano twins.
According to the researchers, the tendency for particular atoms to diffuse into the nano twin faults depends on the overall composition of the alloy.
Mills explained how the technology can further be advanced.
“The ability to understand these alloys down to the atomic scale using advanced microscopy and modeling techniques has opened up new insights into the deformation behavior of these alloys,” Mills said. “This ability has only recently become possible and we expect continued, improved insights as we continue these studies.”
The study appeared in Nature Communications.