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

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.