The impact of Mn and Al on the trapping and diffusion of hydrogen in γ-Fe: An atomistic insight

• Published in: International journal of hydrogen energy 2024-09, Vol.83, p.731-744
• Main Authors: Das, Bikram Kumar, Chakraborty, Poulami, Lu, Mingyuan, Bonilla, Mauricio Rincón, Akhmatskaya, Elena
• Format: Article
• Language: English
• Subjects: Density functional theory; Hydrogen diffusion; Hydrogen embrittlement; Hydrogen trapping; Steel
• ISSN: 0360-3199
• DOI: 10.1016/j.ijhydene.2024.06.322

Common alloying elements such as Mn and Al can significantly influence the local dynamics of Hydrogen in steel, promoting or attenuating the mechanisms associated with Hydrogen induced Embrittlement (HIE). Here, we propose a first principles-based framework to systematically unlock the physical underpinnings of such influence in Mn/Al-alloyed γ-Fe. Our framework can be readily adapted to analyse H behaviour in the bulk phase of any face-centred cubic (FCC) Fe-X-Y alloy, provided that solutes X and Y substitute the Fe sites. In our scheme, all thermodynamically stable substitutional solute sites were identified (≤5.4 wt% Mn; ≤4 wt% Al) up to the third nearest neighbour (NN) shell of a single H atom. The impact of Mn/Al on H-binding was quantitatively evaluated, indicating a surprisingly strong correlation with the local Al distribution regardless Mn content, and indirect stabilization by Al when present in the 2nd NN shell. Nonetheless, Al strongly repels H bonding. The contradictory role of Al was explained in terms of bonding/anti-bonding orbitals occupancy in H-M interactions (M = Al, Mn, Fe). The barriers to H hopping between adjacent local environments and the corresponding jump frequencies were subsequently calculated, providing insights into the limits imposed by the presence of Al and Mn on H mobility in Mn/Al-alloyed γ-Fe. Most notably, presence of Al in the 2nd NN shell of H severely reduces the H jump frequency, leading to potential irreversible trapping at high Al contents. Such behaviour may critically contribute to mitigate H-induced delayed fracture in Al-rich austenite steel. [Display omitted] •H-binding strength is strongly correlated with local distribution of Al, regardless Mn content.•Al repels H but indirectly strengthen the H-binding when present in the 2nd NN shell.•The unique dual role of Al on H-binding is explained via COHP analysis.•Reversible and irreversible trapping sites are isolated for Fe-Mn-Al alloys.•Irreversible H-trapping may contribute to mitigate H-induced delayed fracture.