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].