Data rich imaging approaches assessing fatigue crack initiation and early propagation in a DS superalloy at room temperature

Crack initiation and early propagation behavior of the directionally solidified (DS) superalloy CM247LC has been assessed by data rich imaging approaches. These include conventional characterization methods such as replica record analysis, 3D optical surface imaging, optical and scanning electron microscopy (SEM) as well as more recent techniques like digital image correlation (DIC) and synchrotron radiation computed tomography (SRCT). Three modes of secondary crack behaviors were found during evaluation of the fatigue process. The early stages of fatigue damage were controlled by microstructure-induced cracking, mainly consisting of carbide cracking. Fatigue damage was then promoted via slip band cracking and opening mode controlled carbide-cracking. The mechanisms of these different cracking behaviors are associated with the plastic zone of the main crack tip. Even though the early localized strain levels were of the same intensity within slip bands and at the intersection sites with carbides, carbide-induced cracking occurred prior to slip band cracking, characterized by SEM-DIC. This indicated that carbide-induced cracking was more likely to occur in the early stages of the fatigue process. Early crack growth behaviors were further investigated in situ at the microstructural scale via SRCT. The effect of carbides on crack initiation and propagation processes were evaluated in 3D. This revealed the phenomenon around pores, that cracks simultaneously grew on different slip planes in 3D, contrary to previous theories that such cracks tend to grow on a single favourable slip plane (in polycrystalline alloys). The SRCT result demonstrates the importance and necessity of 3D characterization of the crack propagation behavior at sub-surface, which is not fully analyzed by 2D characterization.