(Dr. W. Steven Johnson, advisor)
"Fracture Mechanics Characterization of a Single Crystal Super Nickel Alloy"
An important application of nickel base superalloys is in the industrial gas turbine (IGT) industry, where the high temperature capabilities of these alloys are utilized. Specifically, single crystal nickel alloys are being used to manufacture the first stage turbine blades in the next generation of industrial gas turbines. The elimination of grain boundaries and improved microstructural properties both contribute to the benefits of single crystals. Originally developed for aircraft engines, the use of these alloys for power generation in IGTs requires the development of methods for predicting the life of vital components. One possible avenue for life prediction is the use of fracture mechanics-based crack growth models. The purpose of this research is to gather the necessary data for supporting these models.
The experimental testing includes tension, fracture toughness and fatigue crack growth. The temperatures for the tests ranged from room temperature up to 870°C (1600°F). Among the key trends observed during the testing were an increase in strength up to a peak temperature of 760°C (1400°F), enhanced toughness at elevated temperatures due to plasticity effects, and similar crack growth rates at both room and elevated temperatures (despite differences in crack morphology). Additional observations and conclusions seemed to be consistent with both the unique microstructure of SC nickel and with the work of previous researchers. Along with a thorough understanding of the development, manufacturing and microstructure of these alloys, the material properties obtained during testing should provide the basis for new life prediction models.