Abstract:
Hydrogen deteriorates the fatigue behaviour of metals by attacking their microstructure in different ways. For
better clarity about hydrogen-based degradation of metals, it is vital to identify the microstructural configurations that promote fatigue crack initiation (FCI) in the presence of hydrogen. Intergranular regions in polycrystalline metals are more prone to hydrogen attack due to the prevailing atomic structure, elastic anisotropy,
and plastic inhomogeneities causing increased hydrogen accumulation in this region. In this work, FCI is studied
in a model material nickel in the uncharged and hydrogen charged state during in-situ strain controlled low cycle
fatigue (LCF) testing under a scanning electron microscope (SEM). Crack initiation sites are characterized by
investigating the elastic modulus in the loading direction as well as the maximum Schmid factor of the crack
neighbouring grains extracted through the electron backscattered diffraction (EBSD) data. The crack frequency
for the uncharged and hydrogen charged specimens is then analyzed using the difference in the elastic modulus
(ΔE), the difference in the maximum Schmid factor (Δm), and ΔE=Δm ratio between the crack neighbouring
grains. The comparison shows that for the hydrogen charged specimens, intergranular FCI sites show high values
of ΔE=Δm compared to the uncharged specimens. These findings provide a predictive model for hydrogen linked
FCI in metals. In addition, the synergistic role of the Hydrogen Enhanced Local Plasticity (HELP) mediated
Hydrogen Enhanced Decohesion (HEDE) mechanism responsible for FCI is also demonstrated.
