Researchers have combined experimental and phase-field modeling approaches to understand how hierarchical microstructures achieve stability. Larry Aagesen of the Idaho National Laboratory (INL) says, “What we were trying to do is to try to develop ways to stabilize nickel superalloy microstructures for longer. The nickel superalloys are frequently used in high temperature applications such as jet-engine turbine blades.” The research team from INL, Federal-Mogul Powertrain, and the University of California-Santa Barbara published the study in Science Advances.

The researchers took a two-pronged approach, examining nickel superalloys experimentally as well as theoretically with modeling. A main thrust of the study was developing a hierarchical microstructure of ϒ´ phase precipitates in a ϒ matrix, with further ϒ precipitates inside the ϒ´ precipitates. “These ϒ´ precipitates are really effective at blocking dislocation motion,” Aagesen says, “The motion of dislocations is really what degrades the properties of these structures over time and so, because they have these ϒ´ embedded precipitates, they block the motions of dislocations very effectively.”  

Samples of the Ni-superalloy Ni-13.6Al-7.6Co-3.0Ta-1.5W-5.9Ru-1.3Re (by atomic percent) were made and air-cooled, then heat-treated at three different temperatures of 700^oC, 800^oC, and 1000^oC. Initially, ϒ precipitates were not present within the ϒ´ precipitates; they were observed with the annealing at 800°C. The size of the ϒ´ precipitates throughout the heat treatment was recorded, and—surprisingly to the research team—were found initially to not exhibit the expected coarsening behavior, but instead to become smaller. However, by 500 hours they were approximately the size they were initially, and coarsening resumed, though at a slower rate than expected. At this point in time, the ϒ precipitates were also no longer in the ϒ´ precipitates, as they have dissolved out of them. Coarsening was much faster at 1000°C, where the ϒ precipitates within the ϒ´ precipitates were not seen.

Atom probe tomography showed that Co, Ru, and Re were supersaturated in the ϒ´ phase before the heat treatment. These elements cause the formation of the ϒ precipitates within the larger ϒ´ precipitates. The researchers say that other elements could also lead to this kind of behavior.

The researchers used a phase-field modeling approach to explore longer timescales than were possible experimentally, such as 6 × 10⁴ hours. Single ϒ´ precipitates surrounded by ϒ (both of simpler compositions than the experimental part of the study) were examined under conditions of various lattice misfits. From this, it was found that a larger starting size of ϒ´ precipitates may be desirable for alloy stability, as it delays coarsening. This is because these larger particles cause the ϒ precipitates to dissolve into the main structure much more slowly. 

Asked about this study, Alessandro Mottura of the University of Birmingham, an expert in phase stability of these materials, says, “It provides a possible outlook on how to make hierarchical structural alloys that are then more resistant to, essentially, aging of the microstructure, so you can control the microstructure a little better in high-temperature applications.” He says that the most significant parts of the study for the greater materials research community are “the combination of both the experiment and the modeling to give us an idea of the trends that we might expect when we try and seek the use of hierarchical type of microstructures.”

Read the article in Science Advances.