Antagonist softening and hardening effects of hydrogen investigated using nanoindentation on cyclically pre-strained nickel single crystal

Abstract

Since hydrogen-plasticity interaction is a key feature in the study of hydrogen embrittlement, a series of nanoindentation tests were carried out to evaluate the possible impact of hydrogen on the dislocation structures formed during cyclic pre-straining of nickel single crystal. Ex-situ nanoindentation and in-situ electrochemical nanoindentation tests were performed on cyclically pre-strained nickel single crystal and nickel-hydrogen systems to characterise the change in the reduced modulus and hardness. Although the reduced modulus was less affected by the conditions explored, the change in hardness highlights the competition between the hardening and softening processes associated with hydrogen. When hydrogen was pre-charged before the cyclic tests, the dislocation microstructures were softer than those induced by cyclic tests in hydrogen-free metals. In contrast, when in-situ electrochemical nanoindentation tests were performed, the hydrogen ingress on cyclically strained nickel single crystal induced a hardening of the microstructure. This effect was due to a pinning effect by the solute on the dislocations. The competition between these processes is discussed in terms of hydrogen and vacancy concentrations.