4.3.1 | Phase precipitations and transitions
The SEM morphology and EDS analyze results of the PSP treated blades at
the fracture location are shown in Figure 13. The element compositions
of different phases observed from the EDS spot scanning are listed in
Figure 13(d). According to the previous investigations, the phases
labelled (a) and (b) are the γ phase and γ’ phase, respectively.
Besides, the long needle-like phases labelled (c), which are rich in the
Ti, Mo,W and C elements, are observed on the fracture. They should be
the proeutectoid MC carbides precipitating from the matrix. The long
needle-like MC carbides are prone to rupture owing to the brittleness
under the CCF load. Based on the results mentioned in section 4.2, the
phases labelled (d) and (e) are the
M23C6 and M6C carbides,
respectively. Furthermore, the needle-like phases labelled (f), which
abound in the Ni, Cr, Mo and W elements, are confirmed to be the σ
phases. Actually, the precipitation of carbides resulting in the
depletion of C elements and enrichment of insoluble W and Mo elements
causes the formation of the σ phases [28-29].