Effects of Grain Boundaries’ Nature, Distribution, and Connectivity on the Diffusion of Hydrogen in Fcc Micro and Nano-Structured Nickel

Abstract

Hydrogen embrittlement influences the mechanical properties of the materials as an outcome of the diffusion of hydrogen through the metal, especially in the intergranular defects. Therefore, diffusion through grain boundary networks has been studied using the percolation theory. The correlation between the two types of grain boundaries (Random and Special) has been established using a finite element model where the microstructure of the materials was simulated considering a continuum medium composed of three phases; two for grain boundaries and one for the lattices. It has been shown that the effective diffusivity is dependent on the percolative behavior of random grain boundaries. In this work, we propose to include the distribution of the grain boundary networks, and their connectivity expressed by their junctions, known as triple junctions, due to their high diffusivities and high-volume fractions, especially in nanocrystalline materials. In this aim, we developed different algorithms taking into consideration several distinctive distributions of the four types of triple junctions (J0, J1, J2, J3). 2D and 3D simulations demonstrate the effects of the difference in connectivity of the networks and the impact of the triple junctions on the effective diffusion of hydrogen for nano and micro-size microstructures.

Keywords: Hydrogen diffusion, Polycrystalline structure, Grain-boundary network, Triple junction, Connectivity, Distribution.