Experimental investigation on the role of grain boundary in fatigue crack propagation of Ni-base suepralloys

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Fatigue crack propagation in thin-wall superalloys were experimentally investigated at room temperature, employing miniature compact tension (CT) specimens directly extracted from as-cast Ni-base superalloys. At first, the effect of crystallographic orientation on the fatigue crack propagation was investigated, using a single crystal (SC) Ni-base superalloy with different primary orientations. Based on the test result of the SC specimens, some significant interactions between the crack and grain boundaries were discussed, using a directionally solidified (DS) superalloy. It was found from a series of experiments that the Stage I fatigue crack propagation rate in both SC and DS superalloys were significantly affected by the primary crystallographic orientation and the vicinity of the grain boundaries.