Understanding mechanisms of cyclic plastic strain accumulation under high temperature loading conditions

In-depth analysis of deformation mechanisms, such as slip deformation and slip/twin interaction, on the micro-level can greatly enhance the understanding of the physics behind thermomechanical fatigue damage accumulation in ductile metals. This improved understanding is essential for the development of new fatigue models that are not phenomenological, and have better predictive capability than the current ones. Hastelloy X, a nickel based superalloy has been experimentally studied using multiscale experimentation under different cyclic plasticity and thermomechanical fatigue loading conditions. High resolution ex-situ Digital Image Correlation (DIC) was used to measure plastic strain accumulation as a function of load and temperature. By performing DIC experiments with sub-grain resolution we can relate the measured strain fields to the underlying microstructure through comparison with EBSD scans of the same region of interest. The results reveal a highly heterogeneous material response at the grain level and even at the sub-grain level with high strain concentrations at regions such as twin boundaries and triple points, and with strain variations even within particular grains. By making such measurements at regular intervals of load, the evolution of plastic strain during loading was observed. As loading increased heterogeneity also increased (i.e. regions with high strain became more strained and regions with low strain remained relatively unstrained).