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)).