The change of microstructure of specimen A at slow shot speed
of 0.3 m/s during tensile deformation was shown in Fig. 4. It was
clear that no defect band was presented in the specimen (see
Fig. 4a). Observations were performed at locations with a gas pore
(see Fig. 4d), gas-shrinkage pore (see Fig. 4g) and shrinkage pore
(see Fig. 4j). The specimen was observed (1) when no tensile stress
was imposed, (2) when a tensile stress of 305 MPa was imposed
and (3) after the specimen fractured. At the stage with a tensile
stress of 305 MPa, there was no significant change at the gas pore
(see Fig. 4b) and gas-shrinkage pore (Fig. 4e), while a clear crack,
roughly perpendicular to the tensile stress exhibited at the
shrinkage pore (Fig. 4h). There were also some secondary cracks
initiated at the gas pore (Fig. 4c) and gas-shrinkage pore (Fig. 4f).
During the tensile deformation, the stress at the regions close