Fig. 5 shows the change of microstructure in specimen B at
slow shot speed of 0.3 m/s during the tensile deformation. It can
be observed that the defect band of positive porosity followed the
contour of die casting (Fig. 5a and g). According to Fig. 5a–c, the
crack initiated at the center of specimen, and propagated to the
surface roughly along the direction perpendicular to tensile stress.
Further observations at the crack initiation site and defect band
revealed that the crack initiated at a gas-shrinkage pore (Fig. 5e),
and the defect band barely changed throughout the tensile deformation
(Fig. 5g–i). This is because the porosity in defect band
was small and individual (Fig. 2d), while the gas-shrinkage pore in
the center was large and complex. On the other hand, the microstructure
around the defect band was dense and fine (Fig. 2d),
while around the gas-shrinkage pore in center was coarse and
comprised large and dendritic ESCs (Fig. 2g). According to Eq. (1),
the actual stress applied on the gas-shrinkage pore was larger than