A radial metallographic cross-sections were prepared from the leading-edge and midchord regions of the failed blade #1 (Fig. 14).
A magnification micrograph examination of the leading edge cross-section 1L revealed a dark region with a depth of approximately 6 lm from the surface with the anomaly. The dark region presents a large amount of precipitates along the grain boundaries with irregular shapes and forming continuous networks at some areas (Fig. 15a). Similar examination of the area away from the dark region revealed a limited amount of precipitates along the grain boundaries (Fig. 15b). A forging lap was also identified at the transition radius of the cross-section 1L under the blade platform with signs of microstructural changes and shear bands (Fig. 15c).
Micrograph of a cross-section 1M through the midchord fracture surface of blade #1 revealed limited plastic deformation and a transgranular fracture path consistent with an HCF mechanism. The microstructure was typical of a heat-treated alloy forging, consisted of smaller recrystallized grains with a size of ASTM #9, and larger deformed grains with a size of ASTM #5 (Fig. 16).