In order to study the atomistic processes of damage evolution experimentally, 2 types of specimen were prepared for both metals. One was an as-received specimen and the other was a residual-gas free specimen which was prepared by melting the as-received metal in a vacuum of 10-5Pa. The specimens were irradiated with fission neutrons in a temperature-controlled capsule. Transmission electron microscopic observations showed that a dislocation structure developed via the aggregation of interstitial clusters in the irradiated metals. It was found that the number-densities of the voids which were observed, when irradiated to a low fluence (5.3 x 1018/cm2) at a high temperature (200C), were the same in both as-received and gas-free specimens. This suggested that gas atoms were not responsible for the nucleation of voids above 200C in neutron-irradiated Cu and Ni. There were found to be 2 characteristic temperatures for the formation of stacking-fault tetrahedra and voids at high temperatures. Only stacking-fault tetrahedra formed below the stacking-fault tetrahedra temperature and only voids were observed above the void temperature. The stacking-fault tetrahedra temperature was equal to 180 and 250C for Cu and Ni, respectively. The void temperature was equal to 250 and 270C for Cu and Ni, respectively. Annealing experiments were carried out in situ on neutron-irradiated specimens in order to examine the behavior of voids and stacking-fault tetrahedra at high temperatures. It was found that voids moved as a cluster, and that stacking-fault tetrahedra coalesced and disappeared spontaneously without decreasing in size. Computer simulations involving molecular dynamics and molecular statics were carried out in order to study the atomistic processes of damage evolution in neutron-irradiated Cu and Ni at high temperatures. It was found that interstitial clusters relaxed into a bundle of <110> crowdions, and moved 1-dimensionally with an activation energy of the order of 0.001eV. The migration of such interstitial bundles reacted sensitively to strain fields. The interstitial clusters then formed groups. The activation energy which was required for an interstitial bundle to change its crowdion direction to another one was of the order of 1eV. This was an important factor in the evolution of the dislocation structure. At high temperatures, a vacancy cluster of stacking-fault tetrahedra and voids relaxed into a movable string-like structure. The vacancy clusters moved and coalesced with other clusters. The activation energy was so small that the vacancy clusters moved as a group without evaporation as a single vacancy. Voids could nucleate at high temperatures without trapping any gas atoms within small vacancy clusters. The voids nucleated uniformly in specimens which had been irradiated to low fluences. Micro-voids migrated under the influence of strain fields, and segregated near to dislocation lines. At high temperatures, the vacancy clusters relaxed into the mobile string-like structures. It was concluded that, at high temperatures, vacancies were stored in a supersaturated state - as small clusters - and that clustering of the vacancies proceeded via cluster migration.

Atomistic Processes of Damage Evolution in Neutron-Irradiated Cu and Ni at High Temperature. Y.Shimomura, I.Mukouda, K.Sugio, P.Zhao: Radiation Effects and Defects in Solids, 1999, 148[1-4], 127-59