Hydrogen-Enhanced Fracture Behavior of AM (Additive Manufacturing) Inconel 718 Alloy Using Slow Strain Rate Test Method in Hydrogen Environment

Abstract

Hydrogen-enhanced fracture (HEF) behavior of AM (additive manufacturing) Inconel 718 (IN718) specimens, either pre-charged or pre-charged/in-situ charged in aqueous NaCl solution under a cathodically applied potential, was exmined by using a slow strain rate test (SSRT) method at a strain rate of 1 x 10-6 /sec. Under certain hydrogen charging condition, the susceptibility to HEF of AM IN718 specimen was substantial with starter cracks formed on the specimen' surface. Detailed fractographic and EBSD analyses showed that the mechanisms for starter crack included slip band cracking and cracking following delta precipitates, while it was intergranular cracking for the CM (conventional manufacturing) IN718 counterpart. AM specimens exhibited relatively higher reduction in tensile elongation (RTE) under mild hydrogen exposure, but showed lower RTE than CM specimens when subjected to in-situ charging, suggesting improved resistance in more aggressive environments. The dominancy of crack mechanism also varied with the severity of hydrogen environment. These findings implied that the distribution and morphology of delta precipitates, acting as hydrogen trapping sites, play a critical role in determining the fracture mode and HEF sensitivity of AM IN718 alloy.