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Abstract

Hydrogen transport behavior in metals is greatly influenced by their microstructure. Different types of trapping sites like dislocations and grain boundaries can result in significant trapping effect which not only increases total hydrogen concentration but also has a significant effect on diffusivity. The hydrogen transport behavior in metals is also influenced by the application of mechanical loading. Therefore, for the proper modeling of hydrogen diffusion, these influences must be considered. In order to realize this, a micromechanical model based on coupled crystal plasticity and hydrogen diffusion is developed and applied to a polycrystalline microstructure to study the trapping effect of dislocations through a series of simulation tests.

First, a study of a precharged material is carried out where hydrogen is allowed to redistribute under the influence of mechanical loading applied as 20% biaxial strain. It is observed that two locations emerge in the material at which locally, total hydrogen concentration is higher. It is shown that hydrogen tends to move from compressive regions to tensile regions. In the next step, the influence of pre-straining (20% biaxial strain) on the hydrogen diffusion is analyzed. This is analogous to the residual stresses inside material due to previous manufacturing steps. Lastly, a series of permeation tests is performed to observe the trapping effect of dislocations on effective diffusivity. It is shown that effective diffusivity keeps on decreasing with stronger traps and the effect is more dominant at a higher mechanical loading where due to large plastic deformation, trap hydrogen density increases considerably. It can be concluded that considering only the dislocation traps in ferrite, a heavily deformed ferritic material has a very low effective diffusivity. With this work, it is demonstrated how the micromechanical modeling can support the understanding of hydrogen diffusion in combination with the mechanical loading at the microstructural level.