Lee, Seungjun (2019-07). Instabilities on High Temperature Isothermal Creep Performance in Ni-Based Single Crystal Superalloys. Master's Thesis.
Thesis
Nickel-based singe crystal superalloys are a class of metallic materials with a combination of high-temperature strength, toughness, and resistance to degradation in oxidizing environments, which makes them great candidates for aircraft engine turbine blades that operate at temperatures beyond 1000 degrees C. The lifetime of such materials is directly connected to the integrity of the strengthening phase, or the so-called the y' precipitates. Uniaxial thermo-mechanical environments have already been shown to lead to the phase transformations, microstructure gradients, and lattice rotation, which all contribute to the change in mechanical properties. Stress field in turbine blades are, however, multiaxial, and the load bearing section of the turbine blades decreases due to depletion of the y' precipitates during oxidation. In order to accurately predict the lifetime of the turbine blades, it is critical to understand how multiaxiality and oxidation kinetics affect the damage evolution during creep. Samples of the second-generation Ni-based single crystal superalloy, Rene N5, were prepared with specified triaxialities using notches according to Bridgman's equation for creep tests. Through SEM analysis, it was found that the material had too much y/y' eutectics that were not properly eliminated during the heat treatment, which resulted in earlier fractures. Kachanov-Rabotnov creep-damage model with Hayhurst stress criterion was modified to account for the depletion of the strengthening phase due to oxidation. Among many factors that affect the kinetics of oxidation, the effects of surface roughness and the initial plastic strain were studied. According to a number of studies, it was found that the kinetics of oxidation tends to increase with decrease in the surface roughness and increase in initial plastic strain. A dimensionless variable eox was introduced to represent the depleted zone thickness and its evolution was simulated varying the two aforementioned factors. Simulation results of creep curves both in vacuum and in air were analyzed, and the results showed conceptually matching trends found in the literatures.
