Modelling hydrogen permeation in gradient-structured steel fabricated by severe shot peening

Gradient-structured materials induced by severe surface plastic deformation (SPD) exhibit exceptional strengthductility synergy, fatigue resistance, and significant potential for hydrogen embrittlement resistance. However, the physical relationship between microstructural gradients and hydrogen transport needs further elucidation. We develop a two-step method to simulate hydrogen permeation in the gradient structure of dislocation cell size formed by severe shot peening. The simulated microstructure is compared to the reported experiments. Our results show that the hydrogen diffusivity in the cell size gradient layer is significantly lower than in the bulk material, up to 4.2 times reduction. However, its effectiveness in reducing apparent diffusivity decreases as the proportion of the gradient layer diminishes, and extensive coverage does not further reduce the apparent diffusivity when coverage is large than 400%. A predictive model for the effective hydrogen diffusivity in gradient structures is also proposed, which aligns well with the simulation results. This work establishes a physical correlation between SPD processes, microstructural gradient structures, and hydrogen permeation performance, paving the way for designing microstructural gradients resistant to hydrogen embrittlement.