The influences of internal and external hydrogen on the tensile ductility loss and fracture behaviors of a precipitation-hardened Ni-based superalloy 718 were investigated via slow strain rate tensile (SSRT) testing under hydrogen pre-charged conditions (internal hydrogen) or in gaseous hydrogen environments (external hydrogen) . Severe degradation of tensile ductility was confirmed both in internal and external hydrogen conditions, and the degree of such degradation became more significant with increasing hydrogen content or hydrogen gas pressures. Moreover, the loss of tensile ductility was more pronounced in internal hydrogen conditions than external hydrogen environments. In association with such degradation of macroscopic tensile ductility, hydrogen also altered fracture mode from ductile microvoid coalescence to some brittle appearances. Whereas typical intergranular fracture combined with a decent fraction of quasi-cleavage fracture appeared on the fracture surface formed in external hydrogen environments, several types of unique faceted characteristics were found on the fracture surfaces in internal hydrogen conditions. The detailed observation of the mid-sectioned lateral surfaces of post-mortem samples successfully revealed that the observed distinctions consisted of the fracture along grain boundaries and {111} crystallographic planes including annealing twin boundaries, besides the frequency of the cracking along twin boundaries evidently increased at higher hydrogen concentration. On the basis of the series of experimental results, the initiation and propagation mechanisms of those hydrogen-induced cracks are discussed in terms of hydrogen distribution, intrinsic deformation character of the material itself as well as the alteration of plastic deformation mode caused by dissolved hydrogen.

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