Friedel Oscillations Induce Hydrogen Accumulation near the sigma 3 (111) Twin Boundaries in gamma-Fe

There is experimental evidence that sigma 3(111) twin boundaries (TBs) in austenitic steels are susceptible to H embrittlement. However, first-principles calculations have demonstrated that these TBs are not trapping sites for H. Density functional theory calculations of the interlayer distances and the solution energy of H in each interlayer space near the sigma 3(111) TB in gamma-iron are performed. It is revealed that they both show oscillating behavior as a result of Friedel oscillations. Although the interlayer space at the TB is smaller and the solution energy is higher than that in the bulk, the second interlayer space near the TB is noticeably larger than the bulk value and hence a lower energy state for H. Friedel oscillations induced by TBs thus provide a thermodynamic driving force for H accumulation. Such enrichment is about 30% at strain-free case and reaches 70% at a 3% tensile strain. It can be partly responsible for the H embrittlement taking place near the TBs.