Volume 6, Issue 1 (January 2009)

    Effect of Inhomogeneity in Aligned Grains on Creep-Fatigue Crack Opening and Propagation Behavior of Directionally Solidified Superalloy

    (Received 8 November 2007; accepted 19 September 2008)

    Published Online: 2009

    CODEN: JAIOAD

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    Abstract

    Directionally solidified superalloy, which has elongated large grains, is used for gas-turbine blade application because of its high creep strength. Since the grain size is not small enough in comparison with the component size or the crack size, the inhomogeneous microstructure strongly affects the crack propagation behavior in such a structure. The authors investigate the effect of microstructural inhomogeneity on crack propagation morphology and crack propagation rate (da/dN) under a creep-fatigue condition. The macroscopic direction of the main crack is perpendicular to the loading axis, though the path is microscopically bumpy due to the effect of inhomogeneous multi-grains. Several subcracks are found around the main crack, and they occasionally coalesce with each other. In order to understand the local fluctuation of (da/dN) due to the inhomogeneous multi-grains, the grain shape and its crystallographic orientation are identified by means of a scanning electron microscope (SEM) and an electron back scattering diffraction (EBSD). The subcracks tend to initiate at the high angle grain boundaries oriented nearly perpendicular to the loading axis. The local fluctuation of da/dN is influenced by two factors. One is the cracking path; the intergranular crack shows higher da/dN. The other factor is the magnitude of Young’s modulus of the grain (in loading direction) at the crack tip. The higher Young’s modulus leads to a lower da/dN in transgranular cracking. The detailed observation of local displacement near the crack tip clarified that the magnitude of cyclic displacement inversely related to the Young’s modulus of the grain in the loading axis, and results in the high strain concentration at the crack tip in the grain with low Young’s modulus. The experiment result suggests that the strain around the crack tip governs the crack propagation rate though it is affected by the inhomogeneous microstructure.


    Author Information:

    Yamamoto, Masato
    Central Research Institute of Electric Power Industry, Materials Science Research Laboratory, Komae, Tokyo

    Kitamura, Takayuki
    Kyoto University, Department of Mechanical Engineering and Science, Kyoto, Kyoto

    Ogata, Takashi
    Central Research Institute of Electric Power Industry, Materials Science Research Laboratory, Komae, Tokyo


    Stock #: JAI101539

    ISSN: 1546-962X

    DOI: 10.1520/JAI101539

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    Author
    Title Effect of Inhomogeneity in Aligned Grains on Creep-Fatigue Crack Opening and Propagation Behavior of Directionally Solidified Superalloy
    Symposium Seventh International ASTM/ESIS Symposium on Fatigue and Fracture Mechanics (36th ASTM National Symposium on Fatigue and Fracture Mechanics), 2007-11-16
    Committee E08