Hydrogen embrittlement and trapping at crack tips in Ni-single crystals

Abstract

Hydrogen assisted crack growth was examined in cyclically hardened and notched Ni-single crystals. Tests were performed in different aqueous electrolytes at cathodic potentials as well as in hydrogen gas at different pressures and temperatures. Comparisons between the crack growth rates in alkaline and acid solutions and in hydrogen gas yielded estimates of the effective hydrogen activity at the crack tip for various overpotentials. A new sectioning technique involving so-called micro-milling was used for detailed fractographic examinations. These examinations show that the cracks in Ni-single crystals {orientation: specimen axis [100], surfaces: (110)} grow in hydrogen by a mixture of slip along (111) and local brittle fracture along (100). A simple trapping model is presented which assumes that a high density of deep traps with an apparent binding energy of 32 kJ/mol H is produced at the crack tip within a zone of less than 10 nm during crack growth. This model describes the pressure, temperature and frequency dependence of the crack growth rate, da dN , without any additional fitting parameters.