Effects of grain-boundary networks on the macroscopic diffusivity of hydrogen in polycrystalline materials

Abstract

The diffusion heterogeneities of grain boundaries are known to affect the effective diffusivity in polycrystalline materials. However, the correlation between both has not been clearly established. In this work, the influence of the grain boundary topology and connectivity on hydrogen effective diffusivity was investigated, for membranes with different grain sizes. We modeled a polycrystal in two dimensions, as a composite containing three phases, one for the grains and the others for grain boundaries. These phases are characterized by their diffusion coefficient, with a contrast of six orders of magnitude between grain boundaries of different character and three between grains and grain boundaries. The path length and correlation length of high diffusivity boundaries were statistically characterized. We found three different behaviors of the effective diffusivity depending on the grain-boundary networks structure. We observed two linear regimes between the connectivity parameters and the effective diffusivity, and a percolated regime. We quantitatively analyzed the impact of the grain-boundary network structure on the variation of the effective diffusivity, in the two first regimes.