Researchers at The Ohio State University have found that improving the quality of superalloys is instrumental for development of more powerful and environmentally friendly turbine engines.
Results of the study published in the journal Nature Communications detailed how the researchers were able to pinpoint nano twins, microscopic defects that grow inside alloys, and deactivate them in an effort to improve high-temperature properties of superalloys used in jet engines.
“We found that increasing the concentrations of certain elements in super-alloys inhibits the formation of high-temperature deformation twins, thereby significantly improving the alloys’ high temperature capabilities,” said Michael Mills, professor of materials science and engineering and leader of the project.
The defects allow superalloys to deform under heat and pressure, the study found. Tailoring an alloy’s composition and then exposing it to high heat and pressure not only prevents nano twins from forming, it actually makes the alloy stronger.
The technique is called phase transformation strengthening.
Researchers made the discovery when they were studying nano twin formation in two different commercial superalloys, summary of the study outlined. They compressed samples of the alloys with thousands of pounds of pressure at around 1,400 degrees Fahrenheit — a temperature comparable to a running jet engine — and afterward examined the alloys’ crystal structures with electron microscopes and modeled the quantum mechanical behavior of the atoms on a computer.
The temperature and pressure caused nano twin faults to develop within the superalloy crystals in both alloys, in addition to the material composition in and around the faults changing, but in different ways.
Researchers were able to detect these fine-scale movements using the advanced electron microscopes at the university’s Center for Electron Microscopy and Analysis.
“In the first alloy, which was not as strong at high temperature, atoms of cobalt and chromium filled the fault,” said Timothy Smith, a former Ohio State student and lead author of the study. “That weakened the area around the fault and allowed it to thicken and become a nano twin.”
The second ally (titanium, tantalum and niobium), however, tended to diffuse into the faults, resulting in a new and very stable phase of material formed right at the faults.
“We discovered that when the amount of titanium, tantalum, and niobium in the alloy was increased, while decreasing cobalt and chromium, we could actually strengthen the region around the faults and prevent the fault from widening into a nano twin,” Smith said.
The team is continuing to study phase transformation strengthening, to see if tailoring the alloy compositions in different ways might enhance the effect.
The paper’s co-authors included Robert Williams, assistant director of CEMAS; Wolfgang Windl, professor of materials science and engineering; Hamish Fraser, Ohio Eminent Scholar and professor of materials science and engineering; and doctoral students Bryan Esser and Nikolas Antolin, all of Ohio State; Anna Carlsson of FEI/Thermo Fisher Scientific; and Andrew Wessman of GE.
Funding was provided by the GE University Strategic Alliance, National Science Foundation, Center for Emergent Materials, Air Force Office of Scientific Research, and Center for the Accelerated Maturation of Materials.