Damage evolution in neutron-irradiated Cu during neutron irradiation

Abstract

We fabricate Cu of residual-gas-free by melting them in vacuum of 10 −5 Pa. Both residual-gas-free specimen and as-received specimens which were estimated to contain 4 ppm hydrogen atoms were neutron-irradiated at 200°C and 300°C with temperature-controlled rig in Japan Material Testing Reactor (JMTR). Neutron fluence ranges from 5.3 × 10 18 to 1.0 × 10 20 n/cm 2. Irradiated specimens were observed by electron microscopy. In copper, both stacking fault tetrahedron (SFT) and voids were observed. The number density of voids decreased with increasing the fluence. The size of voids increased with the fluence. The voids formed uniformly in specimens at the low fluence, while some of voids were observed near dislocations at the high fluence. The number density of SFT increased with the fluence at 200°C. The number of vacancies which are accumulated in a void is 350 times larger than that in a SFT in a specimen irradiated to 5.3 × 10 18 n/cm 2 at 200°C. At low fluence the number density of voids is same for as-received specimens and residual-gas-free specimens. The difference of the number density of voids between these two specimens was observed at high fluence in which the density is low in the residual-gas-free copper. Results are modeled as follows. Small vacancy clusters move during an irradiation. Voids nucleate when the coalescence of small vacancy clusters occurs. The mobility of voids with gas atoms is lower than that without gas-atom. This causes the number density of voids in as-received copper to be larger than that in residual-gas-free copper.