5.2.2 | Vibration stresses above the threshold value
Before CCF tests, the states of the blades are the same as those of the
blades mentioned in Section 5.2.1 (Figure 14 (g)).
During the CCF tests, the microstructure of blades changes differently.
According to the threshold vibration stress (194.0MPa), the relationship
between the threshold vibration stress and yield stress of K403 at 540℃
is compared (Table 2). It reveals that the total stress (the sum of the
half of threshold vibration stress and static stress) is above the yield
stress (σb0.2). According to the
researches of Taira.S et al. [32]and Wick.A et al.[33], the
significant relaxation of the compressive residual stress occurs when
the total stress is above the yield stress. Thus, the compressive
residual stress is gradually released as the CCF tests are conducted
[34]. This indicates that the PSP treatment has no positive effect
when the total stress is close to or above the yield stress (Figure 14
(h)).
Furthermore, the material plastic flow stress is achieved so that the
dislocations, stacking faults and micro-holes generate. This provides
favorable conditions for the carbon atom diffusion and nucleation of
carbides [35]. Apart from the formation of bulk
M23C6 and M6C carbides,
a kind of long needle-like MC carbides appears in the grains accompanied
by the precipitation of needle-like σ phases. The damage evolution has
the following features:
- Firstly, in the initial several CCF cycles, the residual stress has
not been fully released. The total stress is very close to or above
the yield stress, which facilitates the precipitation of MC carbides
and σ phases. The long needle-like MC carbides and σ phases are
sensitive to the high-level stress. Therefore, the MC carbides rich in
insoluble W and Mo elements can be easily ruptured to produce the
micro-cracks due to the brittleness.
- Secondly, the precipitation of σ phases rich in Cr, Ni, W and Mo
elements leads to the depletion of these elements in the matrix, which
weakens the solid solution strengthening effect of γ’ phases.
Besides,the cationic vacancies resulting from the depletion of Cr and
Ni elements may increase, grow and gather with each other to develop
into holes. These holes cause stress concentration so as to further
promote the micro-cracks to propagate [36].
- Lastly, the long needle-like MC carbides, which pass through the γ
phases, are actually lamellar. The micro-cracks are prone to initiate
and propagate along with the interfaces between γ phases and MC
phases, which greatly reduces the strength and plasticity of
superalloy.
Owing to the above three reasons, the initiation and propagation process
of cracks are accelerated by the precipitation of lamellar MC carbides
and σ phases [37].
Before the failure, the residual stress is fully released and the
maximum stress appears on the top surface. Besides, the precipitation of
the lamellar MC carbides and σ phases weakens the mechanical property of
the superalloy. Therefore, the cracks initiate on the top surface and
rapidly propagate along MC carbides. Therefore, the CCF life of the PSP
treated blades decreases compared with that of the untreated blades (
Figure 14 (i)).