We would like to assert from a sudden
drop of mobile dislocation density that the cavity formation
is a reason for the onset of local necking. Once
cavities are created, mobile dislocations can be absorbed
and annihilated into them to result in a local
strain concentration in their vicinity and their rapid
growth. This idea for the local necking does not contradict
a fact that, in the superplastic deformation of the
present alloy, grain boundary facets can be seen on the
fractured surface and the fracture is essentially intergranular