Metallic Materials by means of Localized Hydrogen Distribution and Nanoscopic Damage Analyses

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Advanced metallic materials are sensitive to an environment and in order to clarify the degradation mechanisms and to improve the toughness of these metallic materials in a service environment, we must clarify the process of hydrogen entry as well as the hydrogen trap sites in the material. In this research project, thermal desorption spectroscopy of hydrogen and nanoscopic in situ observation of the process of embrittlement by means of atomic force microscopy are applied; the thermal desorption spectroscopy analyze the state of hydrogen in the material, or hydrogen trap sites, and the responsible hydrogen in the material is correlated to hydrogen embrittlement. The hydrogen evolution rate of TiAl is increased by cathodic charging, with lower temperature peaks and higher ones. The peaks at lower temperatures are correlated with hydrides decomposition and detrapping of hydrogen from microstructural imperfections such as microvoids. As received materials also shows an evolution peak at a … More higher temperature, and the evolution rate is almost independent of cathodic charging. In addition, the evolution rate at a higher temperature above 800 ℃ is increased by cathodic charging. These hydrogens are considered to have an important role on crack growth acceleration. The crack tip displacement (CTOD) of the embrittled crack is smaller than that of a fatigue crack in dry air, indicating that the embrittled crack has sharper crack than that of fatigue crack that is determined by the stress intensity factor. However the CTOD of the embrittled crack has large scatter band because the CTOD is influenced by the structure of the material and the localized hydrogen content in the material. The static SC crack grows in a steady manner in the order of urn without any retardation, when it grows along the grain boundary. However, the crack is reaching the triple grain boundary, the crack growth rate is decreasing with an increase in the CTOD. However, the crack passes over the triple grain boundary, the CTOD is decreased with an increase in the crack growth rate. When the cathodic current is applied, the crack path near the sample surface is transgranular, and the crack grows with a zigzag manner, and the crack grows with acceleration and retardation in the order of urn due to the amount of the localized hydrogen content in the material. Less