Ohio State scientists find a way to remove defects from from alloys to enable jet engines to run hotter and cleaner

Researchers at Ohio State University have found a way to deactivate ‘nano twins’ to improve the high-temperature properties of superalloys that are used in jet engines. The discovery could speed the development of powerful and environmentally friendly turbine engines of all sorts, including those used for transportation and power generation.

Nano twins are microscopic defects that grow inside alloys and weaken them, allowing them to deform under heat and pressure. In their work, the engineers describe how tailoring an alloy’s composition and then exposing it to high heat and pressure can not only prevent nano twins from forming, it can also make the alloy stronger.

“Strong, heat-resistant alloys enable turbine engines to run cleanly and efficiently,” explained Michael Mills, professor of materials science and engineering and leader of the project at Ohio State. “When an engine can run at very high temperatures, it consumes its fuel more thoroughly and produces lower emissions.”

The researchers made the discovery when they were studying nano twin formation in two different commercial superalloys. They compressed samples of the alloys with thousands of pounds of pressure at around 1400°F and afterward examined the alloys’ crystal structures with electron microscopes and modelled the quantum mechanical behaviour of the atoms on a computer.

In both alloys, the temperature and pressure caused nano twin faults to develop within the superalloy crystals. And, in both alloys, the material composition in and around the faults changed, but in different ways.

Through a sequence of atomic-scale jumps, some elements, such as nickel and aluminium, diffused away from the faults, while others diffused into the faults. The researchers were able to detect these fine-scale movements using 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, lead author of the study. “That weakened the area around the fault and allowed it to thicken and become a nano twin.”

But in the second alloy—the one that didn’t form nano twins—the elements titanium, tantalum and niobium tended to diffuse into the faults instead. As a result, a new and very stable phase of material formed at the faults. The new phase was so stable that it resisted the formation of nano twins.

The tendency for particular atoms to diffuse into the nano twin faults depends on the overall composition of the alloy, the researchers found. Smith added: “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.”

The team is continuing to study phase transformation strengthening, to see if tailoring the alloy compositions in different ways might enhance the effect.

Author
Tom Austin-Morgan

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