Design considerations for hydrogen embrittlement resistance: The influence of dislocation density and substructures on hydrogen-assisted cracking in high-Mn TWIP steel
Mohammad Moallemi, Sung-Joon Kim, Hyung-Jun Cho, Sung Ho Kim, Vahid Javaheri
Abstract
This study investigates the hydrogen embrittlement (HE) behavior of high-manganese TWIP-assisted steel by comparing microstructures with high and low dislocation (HD and LD) densities. The HD microstructure exhibited higher hydrogen absorption and a greater density of hydrogen-assisted cracks (HACs) compared to the LD microstructure. Despite lower hydrogen diffusivity, the HD samples experienced significant elongation loss under hydrogen charging, indicating that minimizing pre-existing dislocation structures as a design criterion can enhance HE resistance in high-Mn TWIP steels. This behavior was primarily attributed to the rapid development of HACs. The accelerated failure in HD resulted from two key factors: (1) dislocation multiplication, which increased hydrogen trapping and facilitated crack nucleation, and (2) rapid degradation of the hydrogen-affected region (HAR), which reduced its ability to accommodate deformation, thereby increasing local stress and promoting void formation and coalescence in the hydrogen-unaffected region, ultimately leading to premature fracture. Fractography analysis revealed predominantly intergranular fracture in LD samples. The HE mechanism in LD follows the HELP + HEDE model, where HEDE dominates due to high local hydrogen concentration, and HELP contribution is negligible. In contrast, HD samples exhibited quasi-cleavage features, shallow dimples, and limited intergranular facets, reflecting a combination of HELP-mediated HEDE and localized HELP-driven damage. These observations confirm that dislocation density governs hydrogen distribution and the prevailing HE mechanism, thus controlling the fracture mode in TWIP steels.