Influence of dislocation cells in the hydrogen diffusivity, trapping and embrittlement of wrought and additively manufactured Inconel 718

Abstract

Hydrogen embrittlement (HE) raises a major concern for the mechanical integrity of high-strength alloys, such as Ni-based superalloys exposed to hydrogen-rich environments. The diffusion and trapping of hydrogen atoms are critical factors governing HE. In this study, the role of microstructure, particularly dislocation cells, a characteristic microstructure of alloys made by Laser Powder Bed Fusion (LPBF), on hydrogen diffusion, trapping and embrittlement of additive manufactured (AM) and wrought Inconel 718 was investigated. For the first time, trapping behaviour in hydrogen-saturated AM Inconel 718 was analysed by thermal desorption spectroscopy (TDS) coupled with numerical simulations. A high density of hydrogen traps in the cell walls attributed to dense dislocations and Laves was responsible for the local accumulation of hydrogen, causing significant loss in the cohesive strength and triggering cracking along the dislocation cell walls. This influential role of dislocation cells alters the fracture behaviour from intergranular (seen in the wrought condition) to intragranular for the AM condition. In addition, the cellular network of dislocations acts as a short circuit accelerating hydrogen diffusion, enabling faster and deeper penetration of hydrogen in the AM condition. The results obtained in this study indicate that the higher HE susceptibility of AM Inconel 718 alloys is intrinsically associated with the interaction of hydrogen with dislocation walls.