Internal and External Hydrogen-related Loss of Ductility in a Ni-based Superalloy 718 and Its Temperature Dependence

Toward a better understanding of the hydrogen embrittlement characteristics in nickel-based superalloy 718, tensile tests were performed under hydrogen pre-charged states (internal hydrogen) as well as in hydrogen gas environment (external hydrogen) at various temperatures ranging from -196 to 300 degrees C. Under the internal hydrogen conditions, hydrogen-induced loss of ductility was maximized at around 25 degrees C, while it was recovered with increasing/decreasing test temperature and almost fully mitigated particularly at -196 degrees C. On the other hand, under the external hydrogen conditions, deleterious impact of hydrogen on the ductility monotonically increased with temperature elevation. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) analyses on post-mortem samples revealed that the micro structural initiation sites of hydrogen-induced micro-cracks in internal hydrogen states were annealing twin boundaries or crystallographic slip planes (i.e., {111} planes) at -40 similar to 300 degrees C wherein the loss of ductility was substantial, albeit intergranular fracture prevailed at -196 degrees C, accompanying minimum embrittlement effect. Meanwhile, in the case of external hydrogen states, the fracture modes were transitioned from intergranular to slip plane cracking with increasing temperature in response to the augmentation of embrittlement magnitude. The rationales of these multiple hydrogen-related failure modes and their roles on macroscale material performance mechanisms driving the plasticity in this alloy in addition to the hydrogen diffusion rate/pathways which are strongly dependent on temperature.

Automated summary

The study investigated the hydrogen embrittlement characteristics in nickel-based superalloy 718 through tensile tests under different hydrogen environments. It was observed that hydrogen-induced loss of ductility was maximized at 25 degrees C under internal hydrogen conditions, while the deleterious impact of hydrogen on ductility increased with temperature elevation under external hydrogen conditions. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) analyses revealed the micro structural initiation sites of hydrogen-induced micro-cracks at different temperatures in internal hydrogen states.