The SEM fracture appearances of creep ruptured specimens at 600℃ and 650℃ under different stress levels are shown in Figure 2 and Figure 3, respectively. Typical intergranular brittle fracture and transgranular ductile fracture are observed from the SEM images. As the stress decreases, creep fracture mode converts from intergranular fracture to transgranular fracture. ~800MPa at 600℃ and ~700MPa at 650℃ are proved to be the critical conditions. Above this stress levels, the cavity nucleation, growth and coalescence into macrocracks along grain boundary result in the intergranular fracture mode. On the contrary, below this stress levels, the microstructural degradation leads to the decrease of matrix strength43. The stress concentration formed by the accumulation of dislocations at the particles results in the particle-matrix interface to experience the excess localized plastic deformation. This provides the sites for the nucleation of the cavities and the formation of the macrocracks, leading to transgranular fracture finally44. It can be observed that a mass of ductile dimples are formed, accompanied by prominent localized deformation (i.e. necking) as shown in Figure 2(K) and Figure 3(I, J, K).