Thermodynamic competition and hydrogen redistribution by coherent nanoprecipitates in high-strength steels

Designing nanoprecipitates as static hydrogen traps is a widely used strategy to mitigate hydrogen embrittlement (HE) in high-strength steels, yet increasing trapping capacity alone may not ensure resistance under dynamic loading. Here, the effect of Cu-rich precipitate evolution (B2 → BCC/9 R → FCC) on HE in low-alloy steels containing 0.2–1.7 wt.% Cu was systematically investigated. The 1.0Cu steel exhibited the lowest hydrogen embrittlement index ( I HE = 1.4%) and the best ductility retention. Density functional theory calculations combined with transmission electron microscopy showed that coherent/semi-coherent BCC/9R Cu-rich precipitate interfaces have a low hydrogen solution energy (−0.202 eV), making them thermodynamically favorable hydrogen-trapping sites. These interfaces may competitively trap hydrogen relative to dislocation-associated sites during deformation, thereby promoting hydrogen redistribution and reducing dislocation-mediated hydrogen transport and HELP-related localized plasticity. In contrast, the 1.7Cu steel, despite its higher apparent hydrogen concentration (1.4 × 10 −6 mol/cm 3 ), contains coarser FCC-related precipitates that introduce local stress concentrations and promote detrimental hydrogen accumulation. These results suggest that hydrogen-resistant alloy design should focus not simply on maximizing static trapping capacity, but on regulating precipitate structure and interfacial state to enable more effective dynamic hydrogen management.