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Constitutive modeling of cyclic plasticity at elevated temperatures for a nickel-based superalloy
During the operation of turbines in jet engines or in power plants, high thermal and intermittent mechanical loads appear, which can lead to high-temperature fatigue failure if thermal and mechanical loads vary at the same time. Since fatigue testing is a time-consuming process, it is important to develop realistic material models with predictive capabilities that are able to extrapolate the limited experimental results for cyclic plasticity within a wide range of temperatures. To accomplish this, an approach based on a representative volume element (RVE), mimicking the typical γ/γ′ microstructure of a Ni-based single crystal superalloy, is adopted for cyclic loading conditions. With the help of this RVE, the temperature- and deformation-dependent internal stresses in the microstructure can be taken into account in a realistic manner, which proves to be essential in understanding the fatigue behavior of this material. The material behavior in the elastic regime is described by temperature-dependent anisotropic elastic constants. The flow rule for plastic deformation is governed by the thermal activation of various slip systems in the γ matrix, the γ′ precipitate and also by cube slip along the γ/γ′ microstructure. This phenomenological crystal plasticity/creep model takes different mechanisms into account, including thermally activated dislocation slip, the internal stresses due to inhomogeneous strains in different regions of γ matrix channels and in γ′ precipitates, the softening effect due to dislocation climb, the formation of 〈112〉 dislocation ribbons for precipitate shearing, and Kear-Wilsdorf locks. This constitutive law is parameterized based on experimental data for the CMSX-4 single-crystal superalloy by applying an inverse analysis to identify the material parameters based on many low cycle fatigue tests in the intermediate temperature and high stress regime. The identified material parameters could predict cyclic plasticity and low cycle fatigue behavior at different temperatures. The model does not only reliably reproduce the experimental results along different crystallographic loading directions, but it also reveals the relative importance of the different deformation mechanisms for the fatigue behavior under various conditions.