(A) Vacancy-hydrogen complex in metal
Hydrogen embrittlement, the phenomenon that materials get brittle seriously due to the existence of solute H, in iron-based structural materials is a serious problem from the viewpoint of safety. However, the H solubility in Fe is extremely low and the direct observation of H is quite difficult at the present technique. What the H state is in such Fe-based materials and how such small amount of H causes macroscopic failure of the materials have been still open questions.
Using first-principles calculations, we have first examined the hydrogen-trapping energy of monovacancy (Vac) in Fe, and demonstrated that the VacH2, the monovacancy trapping two H atoms, is energetically dominant at ambient condition. This result indicates that the conventional theory on the number of trapped H would be wrong. Energetics of the Vac and VacH2 bindings was also investigated. In the case of the Vac, binding is found energetically favourable irrespective of the direction and a more compact structure is likely to be formed, suggesting that a vacancy cluster with a spherical structure is energetically more preferable to the separate monovacancies. Regarding the binding of VacH2, on the other hand, the alignment in the <111> direction and keeping the H-Fe bonds are found to be energetically more favourable. This suggests that the formation of plate-shaped clusters on the (100) or (110) plane occurs in the existence of H. Since the (100) planes are the cleavage ones in Fe, the planer vacancy clusters parallel to these planes could enhance the cleavage. The enhancement of the vacancy clusters on the (110) planes, on the other hand, can facilitate the dislocation (linear defects) intersections, because the cutting of the dislocations is usually accompanied by leaving vacancies on the slip planes. This enhancement of the intersections can be closely linked with 'Hydrogen-enhanced local plasticity' (HELP), which is recently regarded as one of the most promising mechanism of the hydrogen embrittlement. Analysis from the atomic level as in this study is expected to play a crucial role in future research on the hydrogen embrittlement mechanism.
(Collaboration with T. Ohno : Y. Tateyama et al., Phys. Rev. B67, 174105 (2003), ISIJ International 43, 573 (2003).)
(B) Microscopic mechanisms of hydrogen embrittlement based on defects
In addition to (3-A), Dr. W. T. Geng and us have published an article on various defects in Fe. We are still keeping this collaboration on grain boundary or other defects in Fe-based materials to elucidate the microscopic degradation mechanism of steel or other Fe-based structural materials.(Collaboration with W. T. Geng and T. Ohno : W.-T Geng et al., Mater. Trans. 46, 756 (2005).)