Atomistic processes of damage evolution in neutron-irradiated Cu and Ni at high temperature

Abstract

This paper consists of two parts. In part 1, the experimental results of damage evolution of neutron-irradiated Cu and Ni are described. In part 2, results of computer simulations are described with linkage of experimental data to explore the atomistic process of damage evolution. To study experimentally the atomistic processes of damage evolution in neutron-irradiated Cu and Ni in part 1, we prepare two types of specimens for both metals. One is as-received specimen from manufacturer. Another is a residual-gas-free specimen which is prepared by melting as-received metals in highly evacuated vacuum at 10−5 Pa. Specimens are irradiated with fission neutrons in the temperature-controlled-irradiation capsule at JMTR (Japan Materials Testing Reactor). TEM (Transmission Electron Microscope) observation shows that the dislocation structure is developed by the aggregation of interstitial clusters in irradiated metals. It is found that the number density of void which are observed in specimens, both as-received and residual-gas-free, that are irradiated to a low fluence such as 5.3 × 1018 n/cm2 at high temperature of 200°C is the same. This suggests that gas atoms are not responsible for the nucleation of voids at high temperature above 200°C in neutron-irradiated Cu and Ni. There are two characteristic temperatures of T sft and T void for the formation of stacking fault tetrahedra (sfts) and voids at high temperature, below T sft only sft forms and above T void only voids are observed. T sft is 180°C and 250°C for Cu and Ni, respectively. T void is 250°C and 270°C for Cu and Ni, respectively. In situ annealing experiments of neutron-irradiated specimen are carried out to examine the behavior of voids and sfts at high temperature. It is found that voids move as a cluster and that sfts coalesce and disappear spontaneously without shrinkage of their size. In part 2, Computer simulations of molecular dynamics and molecular statics are carried out to study the atomistic process of damage evolution in neutron-irradiated Cu and Ni at high temperatures. Interstitial clusters relax to a bundle of ⟨110⟩ crowdions and move one-dimensionally with a small activation energy such as 0.001eV. The migration of interstitial bundles reacts sensitively to strain fields. Interstitial clusters then form their grouping. The activation energy of an interstitial bundle to change their crowdion direction to another one is about 1 eV. This is an important factor for the evolution of dislocation structure. At high temperatures, a vacancy cluster of sfts and voids relaxes to a movable structure of string shape. Vacancy clusters move and coalesce with other clusters. The activation energy is as small as those that vacancy clusters move as a cluster without an evaporation as a single vacancy. Voids can nucleate at high temperature without trapping of any gas atoms in small vacancy clusters. Voids nucleate uniformly in specimens irradiated to a low fluence. Micro-voids migrate under the influence of strain fields and segregate near dislocation lies. At high temperature, vacancy clusters relax to the movable structure of string shape. This may explain the results of recent varying temperature irradiation at high temperature. At high temperature, vacancies are stored in a supersaturated state in a crystal as small vacancy clusters and clustering of vacancies proceed by cluster migration.