Microstructure of neutron-irradiated iron before and after tensile deformation

Tensile specimens of pure Fe were neutron irradiated at 50–70 °C in three fission reactors to displacement dose levels of 0.0001–0.79 displacements per atom (dpa). Irradiated specimens were characterized using transmission electron microscopy (TEM). Visible defect clusters were not detectable by TEM for doses below ∼0.001 dpa. Both the density and average size of the dislocation loops increased with increasing dose level. Spatially heterogeneous rafts of dislocation loops lying predominantly on {1 1 1} habit planes were visible for doses above ∼0.2 dpa. A high density of small cavities (number density >1024 m−3, ∼1 nm diameter) was detected following irradiation to 0.79 dpa. The neutron irradiation led to an increase in the yield stress and a decrease in the uniform elongation as a function of increasing dose. Examination of specimens after tensile deformation revealed localized deformation in the form of cleared dislocation channels. Relatively few channels were formed in the uniform elongation region of the gage section. Dislocation channels on multiple slip systems were observed in the deformation region near the fracture surface, presumably due to the multiaxial stress state produced during necking and fracture of the tensile specimen.