Crack Growth Process of Ni-Single Crystal with Cathodic Hydrogen Charging

Notched tensile tests {orientation: tensile axis [001], direction of notching [010]or[110] } were performed to investigate the crack growth process in Ni-single crystal with hydrogen cathodic charging. Y-shaped hillocks and striation-like patterns were observed on the fracture surfaces of hydrogen embrittled specimens using the scanning electron microscope (SEM). The Y-shaped hillocks are exactly matched on both fracture surfaces of a specimen. In addition, cleavage fracture is occurred between a Y-shaped hillock and next one. It means that the cleavage fracture is occurred at first, and the Y-shaped hillock is secondary formed. It is considered that the striation-like pattern is formed by the localized slips and cleavage fracture between them. Peach-Koehler’s law leads that the dislocations on the equivalent slip systems with (111　)　[101] are subjected to the highest force of all dislocations at the crack tip. Furthermore, Ni-hydride was not observed on the surface of specimen just after charging hydrogen. Thus, it was concluded that Y-shaped hillock and striation-like pattern were occurred in Ni-single crystal with charging hydrogen under condition that Ni-hydride is not formed.