Deformation mechanisms and twin formation observed in Irradiated ferritic/martensitic steels

To study the mechanisms of irradiation-induced embrittlement of ferritic/martensitc steels (FMS), specimens of FMS were irradiated in a target of the Swiss spallation neutron source (SINQ) with high-energy protons and spallation neutrons. The irradiation conditions were in the range of 7.2 – 19.8 dpa (displacement per atom) with 560 - 1750 appm He at 135-300 °C. Transition from ductile fracture of low-dose specimens to brittle fracture of high-dose specimens was observed in tensile tests. After tests, TEM-lamella samples were extracted directly below fracture surfaces using FIB technique. As expected, dislocation channeling phenomenon was observed. Furthermore, for the first time in irradiated FMS, deformation twins were also seen, although just in high-dose and brittle fractured specimens. The twins seem to be initiated at fracture surface, became gradually thinner with distance away from the fracture surface. Features such as twin-precipitates interaction, twin-grain boundary and/or lath-boundary interaction were observed. Twinning bands may be arrested by grain-boundaries or large precipitates, but can penetrate martensitic lath-boundaries. Unlike the case of dislocation channels, small defect-clusters, dislocation loops and dense small helium bubbles were observed inside twin bands. High-resolution TEM observation indicated that the structure of the several atomic layers at twin-boundaries is of fcc structure rather than bcc structure. This implies that the evolution of twining process may be via phase transformation. This hypothesis was confirmed by molecular dynamics (MD) simulation which demonstrated that in iron, at high shear stress levels, transition of bcc structure to fcc structure may take place. Twins are formed when the fcc structure transforms back to bcc. In this contribution, the deformation mechanisms and twin formation in irradiated FMS will be described.