5.1 | Damage evolution mechanisms of the untreated blades
Figure 14 shows the damage evolution mechanisms of untreated blades during CCF tests under different vibration stresses. The initial state of the blades is shown in Figure 14 (a). The surfaces of turbine blades are directly exposed to air and oxidized during the CCF tests. The bulk carbides precipitate near grain boundaries.
During the CCF tests, the maximum stress appears on the surface and causes the elastic or plastic strain. The high-level stress may cause the dislocations and migration of grain boundaries. The carbides that precipitate from the matrix and aggregate along the grain boundaries can hinder the increase of dislocation. Due to the high-level stress, the large bulk carbides may rupture accompanied by the initiation of micro-cracks (Figure 14 (b)).
Before the failure, with the cyclic loading and damage accumulation, micro-cracks initiate on the top surface, gradually gather and extend into visible cracks. The visible cracks propagate along grain boundaries that are full of carbides. (Figure 14 (c)).