Hydrogen assisted crack initiation and propagation in a nickel-based superalloy

Abstract

To understand the mechanism for hydrogen-induced embrittlement in a nickel-based superalloy, detailed electronmicroscopy characterisation has been employed on the UNS N07718 (Alloy 718) after hydrogen charging and slow strain rate testing to investigate the strain localisation and damage accumulation caused by hydrogen. Transmission Electron Microscopy analysis demonstrates that the microstructure of the material after tension is characterised by planar dislocation slip bands (DSBs) along {111}γ planes. Consistent results from Electron Channeling Contrast Imaging (ECCI) reveals that cracks always propagate along planar DSBs in the presence of hydrogen. This phenomenon is rationalised by the evident nucleation of nanoscale voids along the DSBs, especially at the intersections between nonparallel DSBs. The proposed mechanism, confirmed by both the ECCI analysis and fractographic study by Scanning Electron Microscopy, indicates that the interaction between the hydrogen and dislocations along the DSBs leads to void nucleation. Furthermore, the results suggest that coalescence and widening of voids via the dislocation process promote the crack propagation along the DSBs in hydrogen charged Alloy 718.

Mechanism of heat affected zone cracking in Ni-based superalloy DZ125L fabricated by laser 3D printing technique

“…However, the fracture surface of the artificial crack is typically flat and some traces and shear bands can be clearly seen from the inset in Fig. 7c [36]. By comparing Fig.…”

Effect of Hydrogen Precharging on Mechanical and Electrochemical Properties of Pure Titanium

“…Figure shows the microhardness distribution of the sample after hydrogen precharged for 75 h, and the results indicate that the thickness of hydrides layer was about 450 μm. Meanwhile, as these samples were precharged with a constant electric current density, the thickness of titanium hydrides layers in other samples can be estimated based on the equation , where t is the charging time (h) and x is the thickness of titanium hydrides layer. As the thickness of hydrides layer was 450 μm when the hydrogen precharging time reaches 75 h, the equation can be transformed into .…”

Hydrogen embrittlement controlled by reaction of dislocation with grain boundary in alpha-iron

“…Nevertheless, one still needs to be reminded that, it is less likely that any single mechanism for hydrogen embrittlement is exclusively responsible for all hydrogen induced embrittlement process. Firstly, there are experimental observations that the hydrogen assisted cracking sometimes initiates or propagates at the intersection of localized slip bands in the grain interior in some metals (Nibur et al, 2009;Tarzimoghadam et al, 2017;Zhang et al, 2016). For such cases, it is likely that the dislocation-dislocation reaction can result in an 'activated structure' in the highly tangled dislocation junctions.…”