The Effect of Crystal Structure and Microstructure on the Hydrogen-Induced Embrittlement of Intermetallic Compounds

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Using Ni_3(Si, Ti), and Co_3Ti intermetallic compounds with L1_2-type structure, the influence of atomic composition at grain boundaries, grain size and the second-phase dispersions has been observed in this study. The following results were obtained.1) The atomic compositions at the grain boundaries of Ni_3(Si, Ti) alloy were analyzed by means of Auger spectroscopy, and then correlated with the tensile ductility in air. Consequently, it was found that grain boundary compositions such as boron as well as constituent atoms strongly affected the tensile ductility of Ni_3(Si, Ti) alloy deformed in air. It was suggested that boron atoms enriched at the grain boundaries affect decomposition process of moisture in air, or block the migration and the condensation of hydrogen, reducing the environmental embrittlement.2) It was found that the Nb containing second-phase dispersions suppress the moisture-induced embrittlement of Ni_3(Si, Ti) alloys though those enhance the strength of Ni_3(Si, Ti) alloys. This result was interpreted so that the absorbed hydrogen can be trapped within the second-phase dispersions or at interface between the second-phase dispersions and the matrix, or in the plastically deformed regions adjoining the second-phase dispersions. Based on these results, high-temperature structural material with high fracture toughness at low temperature and also high flow strength at high temperature was developed.3) It was found that fine-grained microstructure was effective in reducing the moisture-induced embrittlement of Co_3Ti alloy. Also, the moisture-induced embrittlement of Co_3Ti alloy was reduced in the low deformation rate region. As a corresponding mechanism, oxidation and related oxide product formed at free surface of Co_3Ti alloy reduce the decomposition of moisture in air, resulting in the moisture-induced embrittlement.