Interfacial Relay Trapping of Hydrogen in Metastable Austenitic Steel: A Molecular Dynamics Study

Hydrogen (H) embrittlement in metastable austenitic steels is strongly influenced by the migration and trapping of interstitial H at multiple crystal interfaces. We built an atomistic tri-interface Fe-H supercell model containing an incoherent BCC/FCC (α'/γ) phase boundary which is a non Kurdjumov-Sachs (non-KS) interface, a semi-coherent α'/γ KS interface, and a high angle FCC/FCC (γ/γ) grain boundary (HAGB). Then, we tracked 1.0 at% H diffusion using molecular dynamics simulations in the tri-interface model. At 300 K, H first rapidly enriches the incoherent non-KS phase boundary, while the semi-coherent KS interface acts merely as a permeable pass-through. Subsequently, H drains to the deeper HAGB. The BCC region is rapidly depleted to a very low level of H concentration. Raising the temperature to 473 K accelerates but does not reorder this sequence. The results provide an atomistic basis for relay-type trapping that links transformation interfaces to late-stage enrichment of random γ/γ GBs, and offer a transport-based framework for interpreting H embrittlement within a fixed-lattice Fe-H model that intentionally omits plastic deformation, alloying/segregation chemistry, and externally applied stress effects. • Tri-interface MD isolates hierarchical H trapping in metastable austenitic steel. • Ultrafast BCC transport drives rapid H enrichment at incoherent phase boundaries. • Incoherent non-KS phase boundaries act as high-capacity transient relay stations. • Semi-coherent KS interfaces act merely as permeable pass-throughs for hydrogen. • H eventually drains to deep γ/γ grain boundaries, explaining late-stage cracking.