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Abstract

This study clarifies the hydrogen embrittlement (HE) behavior in a 1.5 GPa ferrite-martensite dual-phase (DP) steel. Hydrogen pre-charging (3.8 mass ppm diffusible hydrogen), followed by slow strain tensile testing (10-4 s-1), resulted in a brittle fracture at 900 MPa within the elastic regime. Fractographic studies indicated that surface crack initiation consists of intergranular and quasi-cleavage morphology; site-specific transmission electron microscopy (TEM) investigations revealed sub-surface secondary crack blunting by ferrite. A mixed-mode morphology consisting of ductile and brittle features was observed adjacent to crack initiation. It differs from the previous investigation of uncharged DP steel, wherein a predominant brittle fracture was observed. Following significant crack growth, the pre-charged specimen exhibited predominant brittle fracture; site-specific TEM and transmission Kikuchi diffraction studies revealed {100} ferrite cleavage cracking. Electron backscatter diffraction studies were performed on the cross-sectional cracks. We explain the HE via hydrogen-induced fast fracture mechanism. During loading, hydrogen diffuses to the prior austenite grain boundary, resulting in hydrogen-induced decohesion. Subsequent hydrogen diffusion to the crack tip promotes brittle fracture at high crack velocity (>Vcrit). The high crack velocity effectively inhibits crack blunting via dislocation emission, ensuring sustained brittle crack growth even after hydrogen depletion at the crack tip, resulting in {100} ferrite cleavage cracking. Based on TEM observations, we explain the formation of river pattern features on the {100} cleavage surface.