Fatigue crack initiation in nickel-based superalloys studied by microstructure-based FE modeling and scanning electron microscopy

Abstract

In this work stage I crack initiation in polycrystalline nickel-based superalloys is investigated by analyzing anisotropic mechanical properties, local stress concentrations and plastic deformation on the microstructural length scale. The grain structure in the gauge section of fatigue specimens was characterized by EBSD. Based on the measured data, a microstructure-based FE model could be established to simulate the strain and stress distribution in the specimens during the first loading cycle of a fatigue test. The results were in fairly good agreement with experimentally measured local strains. Furthermore, the onset of plastic deformation was predicted by identifying shear stress maxima in the microstructure, presumably leading to activation of slip systems. Measurement of plastic deformation and observation of slip traces in the respective regions of the microstructure confirmed the predicted slip activity. The close relation between micro-plasticity, formation of slip traces and stage I crack initiation was demonstrated by SEM surface analyses of fatigued specimens and an in-situ fatigue test in a large chamber SEM.