Even if it’s already an old problem, the slip localization induced during a plastic deformation remains a complex subject associated with numerous elementary plasticity processes, the nature and the distribution of dislocations and the state of internal stresses. It is now established that this localization of deformation determines the initiation of surface damage in direct relation to the environment impact. In this sense, hydrogen as oxygen can be seen as one potential determining factor in the processes leading to this damage. Whether it is intrinsic and/or extrinsic, the hydrogen interacts with elasticity and plasticity properties contributing to profound changes in the processes of nucleation and multiplication of defects (vacancies, dislocations…). The distribution of these defects (dislocation structure, slip localization, vacancies clusters, …) and the associated internal stresses are directly affected and modified the condition of crack nucleation.

The goal of this talk is to summarize what is currently known about intra-granular crack initiation under cyclic loading in f.c.c. alloys, and to point out the new challenges that future studies should address. We revisit the implications of elementary plasticity, dislocation structures, and the associated internal stresses, after recalling the fundamental concepts of shielding, defactants, and hydrogen-enhanced vacancy clusters. Then we examine the role of hydrogen in the slip localization process to gain insight into the initiation of intra-granular damage. Crack initiation conditions near cyclic slip bands, associated with local plastic strain irreversibility, are established through atomic force microscopy measurements of surface topography evolution and transmission electron microscopy characterization of deformation localization, for nickel-based alloys exhibiting a wide range of grain sizes, precipitate sizes, and hydrogen contents. The local slip irreversibility, , required for crack initiation has been expressed in terms of surface energy, the elastic energy stored by mobile dislocations, the applied stress amplitude, the dislocation mean free path, the hardening rate, and the elastic properties. It is concluded that the influence of hydrogen on  is multifactorial.