Abstract
In order to apply hydrogen as the next-generation energy, it is necessary to understand the hydrogen embrittlement (HE) mechanism. In the past studies investigating the HE mechanism, pure Ni has been widely used as a model material owing to its simple microstructure and high HE sensitivity. In pure Ni, it has been pointed that hydrogen accumulated at grain boundaries promotes intergranular fracture, resulting in degraded strength and ductility. There are two proposed processes of hydrogen accumulation at grain boundaries: hydrogen transportation by dislocation motion and hydrogen trapping by local equilibrium. The authors have visualized the accumulation of hydrogen at grain boundaries of pure Ni by local equilibrium via secondary-ion mass spectrometry (SIMS), and confirmed that the hydrogen contributed to the intergranular fracture. Additionally, the SIMS analysis detected the strong signal of hydrogen
at the same location as the signal of sulfur segregated at grain boundaries, suggesting that the sulfur has a hydrogen trapping effect. Previous literatures have reported that intergranular sulfur increased hydrogen at grain boundaries and promoted hydrogen-induced intergranular fracture. The author's previous study has also confirmed accumulations of hydrogen and sulfur at grain boundaries were confirmed; however, the change in the degree of ductility loss depending on the amount of sulfur at grain boundaries was not investigated. Therefore, the objective of this study was to clarify the correlation between the amount of sulfur segregation at grain boundaries and hydrogen-induced ductility loss. Auger electron spectroscopy (AES), thermal desorption analysis (TDA), and slow strain rate tensile (SSRT) test were conducted using pure nickel with different amounts of sulfur segregation at grain boundaries. The amount of intergranular sulfur, hydrogen at grain boundaries, and ductility loss in each pure nickel were obtained from AES, TDA, and SSRT tests, respectively. As a result, it was found that the amount of hydrogen at grain boundaries increased with more sulfur segregation, leading to increased loss of hydrogen-induced ductility.