Crack growth process in Ni-Single crystal with hydrogen cathodic charging

Notched tensile tests {orientation tensile axis [001] and [(1) over bar(1) over bar1], direction of notching: [010], [011], [112]} were performed to investigate the crack growth process in Ni-single crystal with hydrogen cathodic charging. It was investigated that the crack growth direction tends to be < 011 > on {001} with any tensile direction. Y-shaped hillocks and striation-like pattern were observed on the fracture surfaces of hydrogen embrittled specimens using a scanning electron microscope (SEM) and an atomic force microscope (AFM). The striation-like patterns were not matched on both fracture surfaces of specimens, though the Y-shaped hillocks were exactly matched. Moreover, it was indicated that the striation-like pattern is perpendicularly formed to the crack growth direction on the fracture surface behind the crack tip and its shape results in an obtuse angle with respect to the fracture surface. The Y-shaped hillock was formed between micro-cracks located at crack tip by shear fracture. Furthermore, we proposed a crack growth model in Ni-single crystal at hydrogen atmosphere from the observation of the fracture surfaces.