Tracking the onset of plasticity in a Ni-base superalloy using in-situ High-Resolution Digital Image Correlation

The development of discrete strain at the microstructural scale during the onset of plasticity is potentially linked to fatigue crack nucleation in engineering alloys. Hence, quantifying such discrete strain and its evolution is a pathway to understand the microstructural influence on fatigue crack nucleation and to develop crack initiation models, especially those based on crystal plasticity. To date, studying the early stage of plastic deformation in polycrystalline materials has been difficult due to rapid strain evolution and low strain level with very small stress increments. Here, we present work obtained by employing a newly developed in-situ scanning electron microscopy testing station that enables fully automated high-resolution large-area imaging during mechanical loading. Applying High-Resolution Digital Image Correlation (HRDIC) to the captured images enabled the recording of strain pattern evolution at the very early stages of plastic deformation of a nickel-base superalloy. With the assistance of a grain orientation map obtained from Electron Backscatter Diffraction (EBSD), the highly localised deformation characteristics were directly correlated to orientation on a grain-by-grain basis. Using this approach, it was possible to determine the order by which grains shear by slip and observe a change of slip pattern between the onset of plasticity and around 0.2 % proof stress. Differential strain maps were shown to provide a much improved representation of the deformation process at this early stage. The results are discussed in terms of implications when considering fatigue crack nucleation in low cycle and high cycle fatigue regimes.