Advanced modeling of the ductile–brittle transition in neutron-irradiated steel: A discrete dislocation dynamics approach

Reactor pressure vessel steels are embrittled by neutron irradiation during operation. Evaluating changes in the ductile–brittle transition temperature due to irradiation is crucial for maintaining the structural integrity of nuclear power plants. Therefore, we constructed and validated a dislocation model to describe the ductile–brittle transition caused by neutron irradiation in reactor pressure vessel steels. Macroscopic and microscopic cracks near the tip of the crack were assumed based on the brittle fracture mechanism of the steel material. The behavior of the microscopic cracks that interacted with the dislocations emitted from the cracks was investigated using discrete dislocation dynamics. The proposed model reproduced two fracture behaviors: brittle and non-brittle. In the brittle mode, the microcrack fractured at the side near the macrocrack and could sequentially propagate. Conversely, it fractured at the far side in the non-brittle mode. We clarified the effect of the density, size of the carbide precipitates, and temperature on the fracture mode using our model. A brittle fracture mode could be observed if the density or size of the carbide precipitates in steel increased. We also observed the ductile–brittle transition using our model. Furthermore, the model introduced friction stress applied to the dislocations as an effect of neutron irradiation. We obtained a linear relationship between the friction stress and ductile–brittle transition temperature increment, which was experimentally validated.