Multi-scale approach of the consequences of hydrogen on cyclic behaviour of nickel single crystal

A multi-scale study on the influence of hydrogen on cyclically strained oriented nickel single crystal is conducted in order to understand the impact of the solute on the microstructure of strained nickel single crystal. At macroscale, uniaxial cyclic tests are performed to evaluate the impact of pre-charged hydrogen on the work hardening of the material. We noted that hydrogen induces a competition between softening and hardening of the metal. Then, by separating the stress induced by short-range (represented by the effective stress) and long-range (represented by the back stress) interactions between dislocations, we noted hydrogen reduces the effective but has a more complex behaviour with back stress.

Therefore, observations with transmission electron microscope and nano-indentation tests have been performed on cyclically pre-strained nickel single crystal with and without hydrogen. The dislocation organisation induced by the cyclic tests is similar for nickel with and without hydrogen. In both case, the dislocation arrangement can be affiliated to a composite structure with a wall phase containing mainly edge dislocation dipoles and a channel phase where cross-slip event of screw dislocations occurred. We noted that hydrogen impacts the density and the distribution of dislocations and consequently, modifies the internal states of the different phases at microscale. At this length scale, we observed that hydrogen hardens the wall phase while it softens the channel phase. By correlating the results from microscale with the observed competition between hardening and softening effect of the solute at macroscale, we manage to quantify and explain mechanical behaviour of cyclically strained nickel single crystal with hydrogen.