Plastic strain localization and fatigue micro-crack formation in Hastelloy X

In polycrystalline metals, local deformation heterogeneities induced by the microstructure influence fatigue crack initiation and micro-crack propagation. The localization in plastic strains associated with heterogeneous deformation has been described as a necessary condition and a precursor for the nucleation of fatigue cracks. However, a clear and quantitative assessment of the correlation between strain localization and fatigue micro-crack lengths requires further investigation. In this work, during interrupted loading experiments, high resolution deformation measurements using digital image correlation are made on polycrystalline Hastelloy X subjected to fatigue loading. The sub-grain level strain measurements are made prior to the formation of micro-cracks. The correlation between the localization of plastic strains very early on during the loading (e.g., less than 1000 cycles) and the micro-cracks which are detected later in the life of the sample (e.g., around 10,000) is discussed in this paper. Particular focus is given to the difference in grain boundary response, either blocking or transmitting slip, and the associated fatigue micro-crack lengths generated in the vicinity of these boundaries. The results show a clear correlation between both the locations and lengths of fatigue micro-cracks, which form later in loading, and the localization of plastic strains very early in the loading process. For the same number of cycles, the transmission of slip across grain boundaries resulted in longer transgranular cracks compared to cracks near grains surrounded by blocking grain boundaries which were shorter cracks and confined within single grains.