In-situ study of the effect of hydrogen on fatigue crack initiation in polycrystalline nickel

Abstract

Correlating hydrogen embrittlement phenomenon with the metallic microstructural features holds the key for developing metals resistant to hydrogen-based failures. In case of fatigue failure of hydrogen charged metals, in addition to the hydrogen-based failure mechanisms associated with monotonic loading such as HELP, HEDE etc., microstructural features such as grain size, type of grain boundary (special/random), fraction of special grain boundaries; their network and triple junctions can play a complex role. The probable sites for fatigue crack initiation in such metals can be identified as the sites of highest hydrogen concentration or accumulated plastic strain. To this end, we have developed an experimental framework based on in-situ fatigue crack initiation and propagation studies under scanning electron microscope (SEM) to identify the weakest link in the metallic microstructure leading to failure. In-situ fatigue experiments are performed on carefully designed polycrystalline nickel (99.95% pure) specimens (miniaturised, shallow-notched & electro-polished) using a 10 kN fatigue stage inside the SEM. Electron Back Scattering Diffraction (EBSD) map of the notched region surface helps identify the distribution of special/random grain boundaries, triple junctions and grain orientation. The specimen surface in the shallow notched region for both the hydrogen charged and un-charged specimens are then carefully studied to correlate the microstructural feature associated with fatigue crack initiation sites. Such correlation of the fatigue crack initiation site and microstructural feature is further corroborated with the knowledge of hydrogen trapping and grain’s elastic anisotropicity to be either the site of high hydrogen concentration, accumulated plastic slip or both.