4.2.1 | Phase precipitations and transitions
Figure 12 shows the SEM morphology and EDS analyze results of the PSP treated blades while the vibration stress is below the threshold value. The major contents of different phases based on the EDS spot scanning are listed in Figure 12(d). According to the elemental analysis results, the phases labelled (a) and (b) are the γ’ phase and γ phase. The phase labelled (c), which is rich in C, Ti, W and Co elements, is considered to be the bulk MC carbides. Furthermore, the phase labelled (d) that contains the insoluble W element and is short of Ti and Cr elements is supposed to be the M6C carbides. Besides, the phase labelled (e) should be the M23C6carbides which mainly consist of C and Cr elements [24].
In fact, the MC carbides may degenerate into the M23C6 or M6C carbides during the CCF tests in the form of the following equations:
MC+γ→M23C6+γ′ (1)
or
MC+γ→M6C+γ′ (2)
Especially, the formation of the M23C6carbides results in the depletion of the Cr element in the γ phase, which further leads to the generation of cationic vacancies. These cationic vacancies may enlarge, gather, connect with each other and develop into the micro-holes [25].