Abstract:
Hydrogen is foreseen as a promising energy carrier that can control global warming by reducing CO2 emissions.
However, hydrogen is associated with an embrittlement phenomenon that imparts substantial damage to the
infrastructure by reducing the ductility, fracture strength, strength bearing capacity, etc., of metallic components. Therefore, understanding hydrogen-induced crack initiation mechanisms in metals are of prime importance. Greater insights into this critical phenomenon are expected if the hydrogen-induced crack initiation can be
correlated with the local microstructure and corresponding stress-strain state towards their propensity for
hydrogen accumulation. With this motivation, in this work, crack initiation is studied for the uncharged and
hydrogen charged nickel specimens during in-situ tensile loading under the scanning electron microscope. By
assuming the material to be elastic at low strains, a novel approach is implemented for generating the microstructural stress maps through strain and stiffness tensor extracted at each point in the region of interest on the
specimen surface using high-resolution digital image correlation (HR-DIC) and Euler angles (given by electron
backscattered diffraction data), respectively. Based on this analysis at low strain, the crack initiation sites for
uncharged and hydrogen charged nickel specimens are correlated with microstructural maps of maximum
Schmid factor, elastic modulus in the loading direction, hydrostatic stress, von Mises stress, and triaxiality factor.
The analysis highlighted two independent factors responsible for hydrogen enhanced decohesion (HEDE) based
intergranular failure observed only at the random grain boundaries, (i) strain localization due to hydrogen
enhanced localized plasticity (HELP) mechanism of hydrogen embrittlement, and (ii) hydrostatic stress-based
hydrogen diffusion to the crack initiation sites. These critical insights thus can help to design hydrogen
embrittlement resistant metals. In addition, the novel experimental approach can be used to calibrate advance
micromechanical models while providing quantitative estimate of the hydrogen distribution in realistic metallic
microstructure responsible for hydrogen-assisted crack initiation with deformation.
