The polished flat surface of gauge section of failed samples was also examined by SEM
to further study the creep fracture behaviour and typical micrographes are shown in figure 8. It
is seen that, (1) the wedge-like cracks at triple points of grain boundaries are dominant in the
material with clean grain boundaries(Fig.8a). Some small creep voids on regular grain
boundaries are also observed, though they are often associated with intersection site of slip
lines and twins and grain boundaries; (2) the wedge cracks at triple points might initiate the
fracture in the material with low density of precipitates at grain boundaries. The wedge cracks
are seldom observed in materials with high density of precipitates at grain boundaries , instead
creep cracks are often found on normal grain boundaries.