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).