The effects of dislocation density and configuration, and grain boundary properties on hydrogen induced stress cracking of conventionally processed and additively manufactured Inconel 625

This study examined the hydrogen embrittlement properties of both conventional and additively manufactured (AM) Nickel Alloy 625. The conventional Nickel Alloy 625 samples were prepared to contain altered grain boundary properties and dislocation densities and were evaluated in relation to AM 625 specimens which displayed distinct dislocation configurations and columnar grain structures. Step loading and slow strain-rate tensile tests with hydrogen charging, fractography, crack analysis and hydrogen desorption tests were performed to determine the hydrogen effects in the wrought material in “as received”, “annealed” and “annealed & pre-strained” conditions and AM 625. The hydrogen embrittlement resistance of as-received wrought 625 (W625) and annealed (ANN) wrought 625 (ANN W625) showed no comparative difference between their hydrogen charged condition and their baseline uncharged Fast Fracture Strength (FFS) tests, both failing at 12 incremental steps despite the removal of grain boundary precipitates and the larger grain size in the annealed condition. Annealed & pre-strained wrought (ANN W625_S), with similar flow stress and dislocation density as AM 625, showed an increased HE resistance compared to W625 and ANN W625 which was attributed to a reduction in the mobile hydrogen due to trapping at the random dislocation arrangement. W625, ANN W625, and AM 625 showed higher mobile hydrogen concentrations through hydrogen desorption tests, however the AM 625 material still maintained a higher threshold stress and higher ductility compared to W625 and ANN W625. This work suggests that the dislocation density and arrangement are important contributors to the increase in hydrogen embrittlement resistance of AM 625 compared to W625 and ANN W625, which is supported by the fractography and crack analysis. • AM 625 shows higher HE resistance than wrought forms under hydrogen charging. • Pre-strained wrought alloy mimics AM 625 dislocation density and strength. • Dislocation density and arrangement enhance HE resistance in AM 625. • Increased dislocation density in Annealed W625 provided increased hydrogen traps. • Fractography and desorption confirm role of dislocations in HE resistance.