Environmental Strength Evaluation and Investigation into Degradation Mechanisms of Advanced Metallic Materials based upon Analyses of State of Hydrogen and Nanofractography

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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 hydrogens is applied to analyze the state of hydrogen in the material, or hydrogen trap sites, and correlate the responsible hydrogen in the material to hydrogen embrittlement : the materials investigated are titanium aluminides intermetallic compound and Ti-6Al-4V alloys, and the influence of environment on fatigue crack growth behavior was investigated. In the case of titanium aluminides, the tensile strength of the duplex material decreases in order of a water molecule content in an environment : the strength in vacuum is the highest, and decreases in order of laboratory air and water. In the case of the lamellar material, the fatigue c … More rack growth rate in dry air is higher in the R-C crack plane orientation than that in L-C crack plane orientation : the crack growth rate becomes higher when the crack grows as the lamellae are tearing off. However, in the case of the duplex material, the crack growth rate in the R-C crack plane orientation is smaller in low ΔK (ΔK_<off>) region. When the cathodic charging is applied, the fatigue crack growth rate becomes higher than that in dry air, in particular at higher stress intensity factor range. The hydrogen evolution rate 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 a evolution peak at a 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 fatigue crack growth acceleration. In contrast with these, the crack growth rate of Ti-6Al-4V alloy is sensitive to environment when the crack plane orientation is T-L, whereas the crack growth rate in a solution is almost equal to that conducted in dry air. However, the crack path changes towards the longitudinal direction, when the applied stress intensity factor range exceeds a certain value. The hydrogen evolution kinetics are independent of the testing environment, or the acceleration of crack growth, and the mechanisms of the crack acceleration are discussed. Less