Fatigue crack propagation in thin-wall superalloys components were experimentally investigated at room temperature, employing miniature compact tension (CT) specimens directly extracted from an actual post-service gas turbine vane as well as as-cast single crystal and poly-crystal Ni-base superalloys. At first, the effect of crystallographic orientation on the fatigue crack propagation rate was investigated, using a single crystal Ni-base superalloy, CMSX-4, with different primary orientations. Based on the test results of the CMSX-4, some significant interactions between the crack and grain boundaries were discussed, using a conventional casting polycrystalline superalloy, IN939. Finally, the fatigue crack propagation resistance of the post-service gas turbine superalloy was examined, extracting the miniature CT specimens from the leading and trailing edge of the post-service vane. It was found from a series of investigations that the crack propagation rates in the miniature specimens were generally higher than those in the standard size specimen. It was also shown that the fatigue crack propagated with significantly higher rate in the trailing than in the leading edge. These findings were reasonably interpreted in terms of the crack propagation mode and grain boundary role, which were affected by the specimen thickness and the in-service damage in the actual gas turbine.
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