The need for increased thermal efficiency, which provides a reduction in energy consumption, is driving the use of materials with higher temperature capabilities for a variety of applications such as land-based power systems and aircraft engines. These materials are subjected to environmental and thermomechanical extremes for sustained periods during normal operation. The operating conditions are in a regime where several time-dependent damage mechanisms, such as creep deformation, oxygen diffusion, and crack tip oxidation, can operate during cyclic loading and during the dwell period. Therefore, the fundamental mechanisms governing time-dependent damage and degradation must be well understood. During elevated temperature creep-fatigue loading, polycrystalline Ni-base superalloys have exhibited intergranular failure due to oxygen embrittlement. Controlling the grain boundary character distribution (GBCD) has been shown to influence the crack growth rate, but the actual effect on the crack tip kinetics is not well understood. The primary goal of this research endeavor is to assess the influence of the grain boundary character distribution on the crack tip kinetics (i.e. the rates of creep deformation, crack extension, and oxygen diffusion along grain boundaries) of nickel and nickel alloys during elevated temperature creep-fatigue cycling. The crack tip kinetics govern the crack growth rate, so if the kinetics can be quantitatively described as a function of temperature, hold time, and GBCD, then those aspects can be incorporated into a life prediction model.
NON-TECHNICAL SUMMARY: The objective of this project is to determine the effect of different amounts of cold working on the elevated temperature fatigue crack growth behavior of nickel and nickel-base superalloys and provide an outreach program that will reach multiple schools and grade levels through an interactive inquiry-based approach. Nickel is the major alloying element in some of the most advanced alloys, which are used in demanding applications, such as land-based power systems, aircraft engines, and chemical processing equipment, and these components require long-term mechanical property retention when exposed to elevated temperature and cyclic loads. Understanding the fatigue behavior during elevated temperature operating conditions is critical to predicting a component?s long-term performance. The research proposed in this project will be integrated with educational activities in a number of ways and for a large range of educational levels, from K-12 to graduate student training. The central component of the K-12 educational outreach will be a program the PI is developing called Metal Madness. The Metal Madness program will be presented in middle school classrooms and will have age-appropriate basic curriculum in what metals are and how they behave. The initial target audience for this program will be the middle schools in the north Alabama region whose student population is predominantly composed of underrepresented groups. The outcomes of this project will increase the understanding of elevated-temperature fatigue while educating graduate and undergraduate students, teachers, high school and middle school students. The outcomes are also expected to improve public safety and well-being through better predictive capabilities for energy producing components, such as aircraft engines and land-based power generation systems.
