Roles of hydrogen and helium atoms on the formation of voids and bubbles in irradiated copper and nickel

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Computer simulations of molecular dynamics are carried out to study atomistics of the formation of voids and bubbles in irradiated copper and nickel. An isotropic EAM (embedded atom method) many body potential of Daw and Baskes formulation is implemented. In copper, a relaxation of tri-vacancy to the Damask-Dienes type is a fundamental step for the relaxation of vacancy clusters which are larger than tetra-vacancy. Vacancy clusters relax generally to the stacking fault tetrahedra (sft). In the tri-vacancy of the Damask-Dienes type (3v-sft), an atom at the apex of triangular tri-vacancy moves towards the center of the tetrahedron and shows the resonance vibration with large amplitude of low frequency. This vibration can occur even at low temperature such as 10 K. Due to the resonance vibration, the structure of hexa-vacancy fluctuates between a stacking fault tetrahedron (6v-sft) and a void. When a hydrogen atom is trapped at the hexa-vacancy of 6v-sft, it forces the relaxed central atoms of 3v-sft to push back to original vacancy positions to form a void. Micro-voids such as 4v to 10v is possible to be formed during clustering of vacancies. Such micro-voids could be included in grown large sfts as the results of the relaxation of arrival vacancies at void surface to 3v-sft. A trapped hydrogen atom moves fast along surface of 6v-voids, prevent hexa-vacancy to relax to sft and lead to grow a 6v-void to a large void. With structural fluctuations of sft portion in small vacancy clusters in copper, it is possible for a vacancy cluster whose size is smaller than 14 vacancies to relax to a void when hydrogen atoms are trapped in it. Relaxations of small vacancy clusters in nickel are studies by the MD simulation. In nickel a triangular tri-vacancy does not relax to a 3v-sft but stay as a triangular 3v on (111) plane. Vacancy clusters grow as a compound of triangular 3v. In several vacancy clusters of 6v and 7v, a 4v-void was observed to be included in them. When a helium atom is introduced at small vacancy clusters in nickel, a He atom moves to outside of vacancy cluster. Therefore a He atom in nickel does not contribute to relax vacancy clusters to voids. A hydrogen atom trapped in vacancy clusters in nickel moves as the same way as in copper. In nickel, there is no easy moving atoms such as atoms in 3v-sft in copper. several hydrogen atoms have to be trapped to form voids. In copper, bubbles form with trapping a large number of gas atoms to vacancy clusters whose size is below 14v.