Dose dependence of defect accumulation in neutron irradiated copper and iron

In order to investigate the difference in defect accumulation between fcc Cu and bcc Fe, tensile specimens were neutron irradiated at ≃70 °C in the HFIR reactor at Oak Ridge National Laboratory to fluences in the range of 4.5×1020–4.7×1024 n/m2 (E>1 MeV) corresponding to displacement dose levels in the range of about 0.0001–0.8 dpa. Irradiated specimens were characterized using positron annihilation spectroscopy, transmission electron microscopy and electrical conductivity measurements. A limited number of iron specimens were also tensile tested. At 0.0001 dpa, a low density of very small vacancy clusters (1–3 vacancies) were detected in iron, while bigger three-dimensional cavities were observed at higher doses. Both their density and average size increased with increasing dose level. In contrast, no such cavities were observed in copper. Irradiation led to an increase in yield stress and a decrease in the uniform elongation for iron.