4.2.2 | Micro-cracks and fracture
The longitudinal section diagrams of PSP treated blades are shown in Figure 12(e) and (f), when the blades are tested under the vibration stresses below the threshold value. The bulk carbides form near the grain boundaries and on the fracture surface, which are easy to rupture to result in micro-cracks under the CCF load. Therefore, the grain boundaries become weaken during the CCF tests. In Figure 12(f), the micromorphology at the failure location indicates that the blade has failed along the grain boundaries. Actually, under the interaction of the residual stress and CCF load, the maximum stress is in the matrix near the top surface. The micro-cracks initiate from the bulk carbides inside the matrix, propagate along the grain boundaries and finally develop into visible cracks [20,26-27].