4.1 | Results of untreated blades
The longitudinal section diagrams of untreated blades are shown in Figure 11. In Figure 11(a) and (c), it is found that the cracks initiate near the bulk carbides on the top surface. And the cracks are easy to propagate along the grain boundaries. In fact, the carbides precipitate from the matrix, connect with each other and rupture due to the interaction of the high temperature and CCF load. This may make the grain boundaries become the weaken locations. And the maximum total stress is on the top surface. Therefore, the cracks initiate from the carbides on the top surface and propagate along the grain boundaries (Figure 11 (b) and (d)).