Hydrogen embrittlement of single-phase strain-hardened nickel-based UNS N08830 alloy

Abstract

This work evaluated the hydrogen embrittlement resistance of the nickel-based UNS N08830 alloy through a hydrogen charging study, slow strain rate (SSR), and fracture toughness tests. The microstructure evaluation showed large-size austenitic grains with no evidence of second phase precipitation, a high fraction of low-angle grain boundaries, and texture of {100} and {111} planes in the rolling direction. From SSR tests results, strain to fracture and reduction of area embrittlement indexes of 25.3 and 42.1% were found, respectively. A modest drop in fracture toughness of approximately 20% was observed. From fractography, it was observed a prevailing mixed micromechanism of fracture comprised of microvoids coalescence and quasi-cleavage flat facets in secondary cracks aligned with the rolling direction. The quasi-cleavage flat facets showed nanovoids at the slip line intersections, which in turn confirmed Hydrogen-Enhanced Localized Plasticity (HELP) as the prominent embrittlement mechanism. Because of the material's crystallographic texture, a good part of hydrogen was not transported to a direction normal to the applied stress as it would happen for pure diffusion but instead followed the dislocations in the rolling direction. That effect caused that less hydrogen was concentrated in the main crack tip, which inevitably increased the overall energy for fracture.