We perform an in situ investigation of hydrogen-assisted crack initiation in initially flaw-free samples of polycrystalline Inconel 725. We design specialized tensile specimens, introduce hydrogen into them using electrochemical charging, and perform in situ tensile tests in a scanning electron microscope (SEM). We use electron backscattered diffraction (EBSD) to correlate crack initiation sites with the underlying microstructure; then we use digital image correlation (DIC) to measure the strain and elucidate the role of plastic deformation in crack initiation. We charged the specimens to different surface H concentrations, capture the formation of cracks from beginning to failure by SEM, analyzed the crystal structure and strain evolution at each cracking location, and compared their behaviors with each other. Based on analysis of numerous initiation events, we identify several recurring characteristics of crack initiation, namely: a) crack initiation is intergranular at low H concentrations and low plastic strains and becomes increasingly transgranular as H concentrations or plastic strains increase, b) intergranular cracks initiate preferentially at locations where a slipping grain boundary intersects a thin twin lamella (&lt;4 mm thick). Several cracks are seen to initiate at coherent twin boundaries. Potential mechanisms for hydrogen-assisted crack initiation will be discussed. The outcomes of this research will be beneficial for future microstructural design and better life time prediction of Ni based superalloys.

In Situ Investigation of Crack Initiation in Hydrogen Embrittled Inconel 725

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

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