Precipitation-strengthened, Ni-based superalloy 718 has an excellent high-temperature strength as well as corrosion/oxidation resistance, thereby used in various industrial applications including aerospace equipment. However, the alloy is known to be susceptible to hydrogen embrittlement (HE), requiring proper understanding on its hydrogen embrittlement processes and mechanisms for ensuring the safety use in hydrogen infiltrating environments with wide range of operating temperatures. 
　In a previous study, the authors performed a series of room temperature tensile tests under internal- (tests with the specimens with uniformly pre-charged with hydrogen) and external- (tests with non-charged specimens in hydrogen gas) hydrogen conditions in order to clarify the characteristic HE behaviors in those two distinct types of the hydrogen supply [1]. Thus, the present investigation deals with the similar internal/external-hydrogen tests at the temperature range of 233~573 K as a continuation of such a prior work with incorporating the influence of temperature variation.　In the case of internal hydrogen tests, the maximum loss of ductility was observed near 300 K and rather recovered at lower/higher temperatures, whereas in the external hydrogen tests, it became more remarkable as a monotonic function of the increasing temperature. Observations via scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) techniques showed that, when the ductility loss was particularly pronounced, faceted fracture due to twin boundary or slip plane {111} separations were common outcome under both the internal- and external-hydrogen. However, when the ductility loss was relatively moderate under external-hydrogen below 300 K, intergranular cracking was the predominant failure mode. It will be shown that this pronounced transition of the hydrogen-assisted fracture characteristics can be ascribed to the temperature-dependent change of the microstructural hydrogen penetration and diffusion pathways, in combination with the unique deformation mechanism of this alloy accompanying precipitate-shearing-induced planer dislocation glide.

Temperature-Effects on the Internal/external-Hydrogen-Driven Tensile Ductility Loss and Relevant Failure Mechanisms in a Ni-Based Superalloy 718

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

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