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].