Roles of Crystal Defect and Solute Atom in Hydrogen-induced Embrittlement of Intermetallic Compounds

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Using Ni_3 (Si, Ti), Co_3Ti and Ni_3AI intermetallic compounds with Ll_2-type structure, the influence of the crystal defects such as grain boundaries, dislocations and vacancies, and also solutes such as interstitials, substitutionals and non-stoichiometric atoms has been observed in this study. The following results were obtained.1) It was shown that the grain boundaries in those intermetallic compounds play a role in providing hydrogen diffusion path and also offering hydrogen occupation sites, leading to the reduction of grain boundary cohesive strength and the associated ductility loss. On the other hand, the dislocations and vacancies were suggested to play a role in trapping hydrogen, and as a result, the hydrogen-induced embrittlement can be suppressed. Based on such principal finding, it was shown that the pre-deformation and the shot-peening have the effect of completely suppressing or reducing the environmental embrittlement of those intermetallic compounds.2) It was found that a trace amount of interstitials such as boron and carbon can dramatically suppress the environmental embrittlement of Ni_3 (Si, Ti) and Ni_3Al intermetallic compounds. As the possible mechanisms, it was suggested that such interstilials enrich at the grain boundaries, and block the migration and the condensation of hydrogen, reducing the environmental embrittlement. Also, some substitutional atoms and non-stoichiometric atoms were shown to have the effect of reducing the environmental embrittlement. The mechanisms responsible for such an alloying effect were discussed. Lastly, it was found that surface films having an adequate composition and structure are effective of reducing the environmental embrittlement of Ni_3 (Si, Ti) intermetallic compound.