An investigation of micro-mechanisms in hydrogen induced cracking in nickel-based superalloy 718

Abstract

Hydrogen embrittlement of the nickel-iron based superalloy 718 has been investigated using slow strain rate tests for pre-charged material and also in-situ hydrogen charging during testing. Fractography analyses have been carried using scanning electron microscopy, electron back-scattering diffraction and orientation image microscopy concentrating on the influence of microstructural features and associated micro-mechanisms leading to hydrogen induced cracking and embrittlement. It was observed that hydrogen induced transgranular cracking initiates at micro-voids in the crystal lattice. Similar behaviour has been observed in multi-scale finite element chemo-mechanical numerical simulations. In contrast, hydrogen induced localized slip intergranular cracking was associated with the formation of micro-voids in intergranular regions. The effects of grain boundary and triple junction character on intergranular hydrogen embrittlement were also investigated. It was observed that low end high angle misorientations (LHAM), 15°<θ ≤ 35°, and critical high angle misorientations (CHAM), 35° < θ ≤ 50°, are preferential sites for hydrogen induced cracking. In contrast, few or no hydrogen induced cracks were observed at low angle misorientations (LAM), 0°≤ θ ≤ 15°, high end high angle misorientations (HHAM), 50°<θ ≤ 55°, or special GB misorientations (SGB), θ > 55°. Finally, the use of grain boundary engineering techniques to increase the resistance of super alloy 718 to hydrogen induced cracking and embrittlement is discussed.