Radiation softening of high purity iron by 14 MeV neutrons

In order to obtain fundamental knowledge of the damage structure after 14 MeV neutron irradiation and to study the nature of the interaction of the defects with dislocations, experiments of mechanical properties of high purity iron have been conducted after irradiation in RTNS-II at 20 K. Remarkable softening is observed in as-irradiated specimens in a temperature range 50-77 K, whereas hardening is observed below 50 K down to 4.2 K. The cross-over temperature from hardening to softening increases by annealing; softening is still observed just below the annealing temperature. Remarkable difference with fission neutron irradiation is that softening is observed even after room temperature annealing. The fluence dependence of the softening has also been studied. The difference in the annealing behavior and the fluence dependence of the softening after fission- and fusion-neutron irradiations is discussed in terms of the difference in the cascade structure. (orig.)