5.2.1 | Vibration stresses below the threshold value
As for the PSP treated blades before CCF tests, the PSP treatment will cause the plastic deformation and a high-level of residual stress. The maximum of the residual stress is located in the matrix near the top surface. Therefore, the bulk carbides near the surface may rupture, which will lead to the initiations of micro-cracks (Figure 14 (d)).
During the CCF tests, due to the interaction of the residual stress and CCF load, the maximum total stress appears in the matrix near the surface. The micro-cracks are prone to initiate at the ruptured carbides and propagate along grain boundaries. As the CCF tests continue, the bulk M23C6 and M6C carbides form around the MC carbides (Figure 14 (e)).
Before the failure, the micro-cracks extend into visible cracks in the matrix near the top surface. However, thanks to the high-level compressive residual stress, the growth rate of cracks can be greatly reduced, which is difficult for micro-cracks to grow into the matrix. Therefore, CCF life has been extended overall compared with the untreated blades under the same CCF load (Figure 14 (f)).