Estimation of crack initiation stress and local fracture toughness of Ni-alloys 945X (UNS N09946) and 718 (UNS N07718) under hydrogen environment via fracture surface topography analysis

The variation in mechanical properties of Ni-alloys 945X (UNS N09946) and 718 (UNS N07718) during slow strain rate tests (SSRTs) under two different test protocols, both involving hydrogen (H) charging, was examined through fracture surface analyses. The test protocols used were: H pre-charging of the specimen prior to in-situ H charging during SSRT (P1) and in-situ H charging during SSRT without pre-charging (P2). The ductility (elongation to failure) of Ni-alloy 945X was observed to be inferior to the ductility of Ni-alloy 718 under given SSRT conditions. FRActure Surface Topography Analysis (FRASTA) was applied to evaluate the fracture events during SSRT from crack initiation to final fracture. The fracture events were then correlated with the stress-elongation response to define the stress at which crack initiation occurs. Crack initiation occurs below the material's yield stress under P1 and above its yield stress under P2 for alloy 945X. On the other hand, crack initiation stress for alloy 718 was found to be higher than the material's yield stress under both test protocols. The local fracture toughness was estimated by measuring the Crack Tip Opening Displacment (CTOD) obtained from line profile analysis, based on fracture surface topography data, over the H induced fracture surface area. The average local fracture toughness of alloys 945X and 718 under H decreases significantly when compared with their in-air fracture toughness values and the reduction is higher for alloy 945X than that for alloy 718. Under a H environment, the inferior ductility of Ni-alloy 945X, compared to that of Ni-alloy 718, may be due to significant reduction in the fracture toughness of alloy 945X. The operative mechanism of H assisted cracking in these Ni-alloys is explained based on the effect of microstructural condition on H enhanced - slip localisation and decohesion.