Abstract
One of the fundamental aspects of hydrogen embrittlement is based on the consequences of the solute on the elementary mechanisms of plasticity [1]. Recently we have highlighted that the hydrogen-induced microplasticity antagonistic processes is a key feature to improve the resistance to hydrogen embrittlement [1-4]. In this work, the impact of hydrogen on elastic modulus, elementary processes (emission, mobility etc.) associated with dislocation, global behavior and internal stresses are investigated in nickel single crystals. We focused on the impacts of strain rate and hydrogen concentration on the hardening rate. The impact of hydrogen on the monotonic and cyclic plasticity of <001> oriented nickel single crystal was investigated using loading and unloading tests and nanoindentation. Static and dynamic nanoindentations were performed on undeformed and pre-strained samples with and without hydrogen. The indented surfaces were analyzed by SEM-FIB, EBSD and TEM to characterize the development of dislocation structures and any other defects and hence to establish the hydrogen-plasticity correlation near surface. Hydrogen induced impact on maximum shear stress to activate dislocations, hardness and elastic modulus was observed in static nanoindentation experiment. The long-range internal stresses developed in the hydrogen charged samples during the dynamic nanoindentation were compared to the results of TEM (dislocation density) and cyclic micro-tensile test (effective and back stresses). A competition between cyclic hardening/softening was observed with and without hydrogen, attributed to the hydrogen induced differences in the development of dislocation structures and subsequent internal stresses. Both can be correlated with hydride and/or vacancies formation.