Fig. 6 shows the change of microstructure in specimen B at
slow shot speed of 0.15 m/s during the tensile deformation. According
to Fig. 6a–c, the crack initiated at both the defect band and
the center of specimen, and both cracks propagated to each other
roughly along the direction perpendicular to tensile stress. Further
observation at the crack initiation sites revealed that the crack
initiated at a gas-shrinkage pore in the center and the aggregated
porosity in defect band (Fig. 6e and h). This is because with a lower
slow shot speed, more large and dendritic ESCs would be created
in the slow shot stage, which resulted in large and connected
(Fig. 2e) porosity in defect band.