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

This paper consists of two parts. In part 1, the experimental results of da mage evolution of neutron-irradiated Cu and Ni are described. In part 2, re sults of computer simulations are described with linkage of experimental da ta to explore the atomistic process of damage evolution. To study experimen tally 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 spe cimen which is prepared by melting as-received metals in highly evacuated v acuum at 10(-5) Pa. Specimens are irradiated with fission neutrons in the t emperature-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 clus ters in irradiated metals. It is found that the number density of void whic h are observed in specimens, both as-received and residual-gas-free, that a re irradiated to a low fluence such as 5.3 x 10(18) n/cm(2) at high tempera ture of 200 degrees C is the same. This suggests that gas atoms are not res ponsible for the nucleation of voids at high temperature above 200 degrees C in neutron-irradiated Cu and Ni. There are two characteristic temperature s 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 degrees C and 250 degrees C for Cu a nd Ni, respectively. T-void is 250 degrees C and 270 degrees C for Cu and N i, respectively. In situ annealing experiments of neutron-irradiated specim en are carried out to examine the behavior of voids and sfts at high temper ature. It is found that voids move as a cluster and that sfts coalesce and disappear spontaneously without shrinkage of their size. In part 2, Compute r simulations of molecular dynamics and molecular statics are carried out t o study the atomistic process of damage evolution in neutron-irradiated Cu and Ni at high temperatures. Interstitial clusters relax to a bundle of [11 0] crowdions and move one-dimensionally with a small activation energy such as 0.001 eV. The migration of interstitial bundles reacts sensitively to s train fields. Interstitial clusters then form their grouping. The activatio n energy of an interstitial bundle to change their crowdion direction to an other one is about 1 eV. This is an important factor for the evolution of d islocation structure. At high temperatures, a vacancy cluster of sfts and v oids relaxes to a movable structure of string shape. Vacancy clusters move and coalesce with other clusters. The activation energy is as small as thos e that vacancy clusters move as a cluster without an evaporation as a singl e vacancy. Voids can nucleate at high temperature without trapping of any g as atoms in small vacancy clusters. Voids nucleate uniformly in specimens i rradiated to a low fluence. Microvoids migrate under the influence of strai n fields and segregate near dislocation lies. At high temperature, vacancy clusters relax to the movable structure of string shape. This may explain t he results of recent varying temperature irradiation at high temperature. A t high temperature, vacancies are stored in a supersaturated state in a cry stal as small vacancy clusters and clustering of vacancies proceed by clust er migration.