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

Abstract

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°C. Under the internal hydrogen conditions, hydrogen-induced loss of ductility was maximized at around 25°C, while it was recovered with increasing/decreasing test temperature and almost fully mitigated particularly at −196°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 microstructural initiation sites of hydrogen-induced micro-cracks in internal hydrogen states were annealing twin boundaries or crystallographic slip planes (i.e., {111} planes) at −40~300°C wherein the loss of ductility was substantial, albeit intergranular fracture prevailed at −196°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 are discussed on the basis of hitherto-known, unique deformation mechanisms driving the plasticity in this alloy in addition to the hydrogen diffusion rate/pathways which are strongly dependent on temperature.