Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

Abstract

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.