Abstract
It was reported that the fatigue crack growth rate of alloy 617 and 625 showed cyclic loading frequency dependence in a high temperature steam environment at 750ºC. In particular, in the alloy 617, the crack path becomes intergranular in the lower loading frequency range, and the growth rate is accelerated. In this study, elemental and crystallographic analysis of the tips of the intergranular cracks propagated under two different loading frequency conditions were conducted by transmission electron microscopy. It was found that Al-enriched and Cr-enriched oxides were formed in the crack tip under both conditions. In addition, Kernel Average Misorientation (KAM) distributions evaluated using transmission electron backscatter diffraction technique indicated that the width of the high KAM value region near the crack was narrow. Therefore, local oxidation morphology and deformation behavior under both conditions showed similar tendency, and it corresponded with the crack growth rate.
