Microstructure of a reactor pressure vessel steel close to the zones of ductile tearing and cleavage

A transmission electron microscopy study of a tempered bainitic nuclear reactor pressure vessel steel was carried out. Cross-section thin foils from a nickel electroplated CT25 specimen fractured in quasistatic loading at 0 °C were prepared in the ductile tearing and cleavage regions of the fracture surface. In the ductile tearing zone, the microstructure was very heterogeneous. Dislocation cells, shear bands, twins and fine heavily deformed subgrains were found. A predominant feature in the zone of cleavage fracture was deformation twinning. The twins occurred in a layer about 200 μm in depth under the fracture surface and not in the deeper parts of the material.