The effects of irradiation under a variety of neutron flux and temperature conditions on the transition temperature of a low-carbon, low-alloy steel containing 0.0036 weight per cent boron have been investigated. This steel exhibits greater embrittlement than nominally boron-free steels irradiated under similar conditions ; the differences in behavior have been analyzed in terms of the additional displaced atoms and clusters of defects respectively produced in the iron lattice by the energetic products of the B10 (n, α) Li7 reaction in the boron steel.
Tension specimens prepared from three mild steels have been tested at room temperature after irradiation to approximately the same neutron dose at accurately controlled temperatures in the range of 100 to 350 C; the reactor was operated at three power levels, giving relative neutron dose rates of 1:4:100. The increases in yield stresses are functions of the irradiation temperature and the type of steel but, in agreement with preliminary results reported previously, are not dependent on the neutron dose rate. The absence of a neutron dose rate effect suggests that the nucleation of the damage in steels is not diffusion controlled, and it is probable that the radiation hardening does not depend on the number of displaced atoms but on the number of defect clusters formed in the displacement spikes. It is suggested that the dependency of the radiation hardening on the type of steel is associated with differences in the active nitrogen content.
The effects of irradiation, 2.0 x 1020 neutrons/cm2 (fission) at 60 C, followed by annealing at temperatures up to 500 C on the damage in pure iron have been studied using thin film electron microscope and microhardness techniques. The damage observed in the as-irradiated condition is considered to be interstitial point defect clusters. The clusters disappear and the irradiation-induced increase in microhardness recovers over a relatively narrow annealing temperature range suggesting that interstitials and vacancies are annealing simultaneously by a process of mutual annihilation.