A modified constitutive model considering microstructure degradation of Ni-based superalloys and its application to microstructural damage calculation

Microstructure degradation of Ni-based superalloys operating at high temperatures for long terms inevitably occurs, which has a deteriorating effect on the mechanical properties of the alloy. In this study, the Chaboche elastic-viscoplastic constitutive model of a directionally solidified (DS) superalloy was modified by considering microstructure degradation based on the quantitative description of microstructure morphology. The feasibility and accuracy of the modified model were verified by cyclic tensile tests. The finite element stress-strain simulation results using the modified model showed that compared with the virgin superalloy, the pre-coarsening superalloy exhibit more cyclic softening and mean stress relaxation characteristics, which is illustrated by the variation of dislocation movement behavior. Subsequently, the microstructural damage evolution of the turbine blade was simulated based on the microstructural damage calculation framework, which was established by coarsening and rafting kinetic models, and the capability of the framework was verified by the slice metallographic results of an in-service turbine blade. The simulation results showed that the microstructure evolution at the 60–77% span areas of the leading edge and the trailing edge near blade tip should get attention primarily. Besides, the rafting damage is dominant for long-term service while the coarsening damage occurs more in the single-mission spectrum.