5.2.2 | Vibration stresses above the threshold value
Before CCF tests, the states of the blades are the same as those of the blades mentioned in Section 5.2.1 (Figure 14 (g)).
During the CCF tests, the microstructure of blades changes differently. According to the threshold vibration stress (194.0MPa), the relationship between the threshold vibration stress and yield stress of K403 at 540℃ is compared (Table 2). It reveals that the total stress (the sum of the half of threshold vibration stress and static stress) is above the yield stress (σb0.2). According to the researches of Taira.S et al. [32]and Wick.A et al.[33], the significant relaxation of the compressive residual stress occurs when the total stress is above the yield stress. Thus, the compressive residual stress is gradually released as the CCF tests are conducted [34]. This indicates that the PSP treatment has no positive effect when the total stress is close to or above the yield stress (Figure 14 (h)).
Furthermore, the material plastic flow stress is achieved so that the dislocations, stacking faults and micro-holes generate. This provides favorable conditions for the carbon atom diffusion and nucleation of carbides [35]. Apart from the formation of bulk M23C6 and M6C carbides, a kind of long needle-like MC carbides appears in the grains accompanied by the precipitation of needle-like σ phases. The damage evolution has the following features:
Owing to the above three reasons, the initiation and propagation process of cracks are accelerated by the precipitation of lamellar MC carbides and σ phases [37].­
Before the failure, the residual stress is fully released and the maximum stress appears on the top surface. Besides, the precipitation of the lamellar MC carbides and σ phases weakens the mechanical property of the superalloy. Therefore, the cracks initiate on the top surface and rapidly propagate along MC carbides. Therefore, the CCF life of the PSP treated blades decreases compared with that of the untreated blades ( Figure 14 (i)).