3.2 | Crack initiation and propagation
Figure 8 shows the partially broken blades. And the crack sources on the fracture surface of the broken turbine blades are observed by the optical microscope (OM) (Figure 9(a)). The number of crack sources of the PSP treated blades (C2, C5) is less than that of the untreated (B2, B4) blades. Therefore, it is determined that the PSP treatment can significantly reduce the number of crack sources while the vibration stresses are below the threshold vibration stress. In contrast, the number of crack sources of PSP treated blade (C9) is more than that of untreated blade (B9) while the vibration stress is above the threshold value. This indicates that the number of crack sources cannot be reduced by the PSP treatment while the vibration stresses are above the threshold vibration stress.
The crack sources of untreated (B2, B9) and PSP treated (C2, C9) blades are shown in Figure 9 (b)-(e). The cracks of B2, B9 and C9 blades initiate from the top surface. In contrast, the crack initiation position of the C2 blade is the matrix near the top surface. Especially for the C9 blade, the lamellate structure can be observed near the crack source on the fracture surface.
Figure 10 shows the fatigue striations of the crack propagation section on the fracture surface of untreated (B2,B9) and PSP treated (C2,C9) blades. All the selected observation areas are 100um away from the crack source at the critical position. According to the measurement results of fatigue striations marked in Figure 10, the width of fatigue striations for the PSP treated blade (C2) is smaller than that of the untreated blade (B2). This indicates that the PSP treatment is able to reduce the growth rate of the cracks due to the compressive residual stress while the vibration stresses are below the threshold vibration stress, for which it is beneficial to improve the fatigue property of superalloy. However, the crack growth rate of B9 blade is greatly accelerated by the large vibration stress compared with that of B2 blade, which demonstrates that CCF damage significantly increases. Especially for the C9 blade, the fracture surface near the crack source is characterized by the small dimples accompanied with the lamellar structure.