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概要

State of microstructure in metallic materials can be expressed as numerical values based on a concept of system free energy, and the energy value is used as a measure of damage analysis of the materials. In this study, change in the system free energy of the FG-HAZ in 9Cr heat resistant ferritic steels during creep is evaluated as summation of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. Changing rate of system free energy depends on applied stress and is expressed quantitatively as a numerical formula using rate constants. Furthermore, it is suggested that a steel ruptures when the applied stress exceeds a limited stress which depends on the microstructural state expressed by the system free energy of the state. Thus, the relationship between the limited stress and system free energy makes it possible to predict the rupture life of the steel.

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