4.3.2 | Micro-cracks and fracture
The longitudinal section of the PSP treated blades is observed based on SEM while vibration stresses of CCF tests are above the threshold value (Figure 13(e) and (f)). It can be observed that there are long needle-like MC carbides and σ phases. Besides, the micro-holes appear near the carbides, σ phases and grain boundaries. And the MC carbides rupture to result in the initiation of micro-cracks owing to brittleness. The micro-cracks and micro-holes may interact, grow and further propagate into the visible cracks. According to the previous researches, the long needle-like MC carbides are the weaken locations and harmful to the strength and plasticity of alloys. Thus, the cracks easily propagate along long needle-like MC phases so that there are wedge-shaped notches on the fracture surface (Figure 13 (f)) [30-31].