ATOMISTICS OF VOID FORMATION IN IRRADIATED COPPER

Computer simulations of molecular dynamics and molecular statics are carried out to study the atomistics of void formation in irradiated copper. The isotropic many-body potential of the embedded atom method due to Daw and Baskes [5] is implemented. It is found that a relaxation of tri-vacancy to the Damask-Dienes type of stacking fault tetrahedron (3v-sft) is an important step for the formation of sft. In a 3v-sft, an atom inside a tetrahedral 4v moves with large amplitude at low frequency. To form a void in irradiated copper, gaseous atoms such as hydrogen and helium have to be included in a vacancy cluster. Hydrogen atoms are introduced in the present work. Gaseous atoms suppress the 3v-sft type of relaxation. The structure of hexa-vacancy fluctuates between a sft and a void. A hydrogen atom trapped in a hexa-vacancy preserves the structure of a void. In order to grow such a microvoid to a large void, several hydrogen atoms have to be included in a microvoid. A 8v-void which traps only singly hydrogen atom grows to sft by further absorption of vacancies, while it grows to a void when two hydrogen atoms are trapped. A sft of nine-vacancy which is composed by the mixture of a 6v-sft and 3v-sfts converts to a void when four hydrogen atoms come to the cluster. For the case of the formation of a tetrahedral 10v-void in the core of a displacement damage cascade, the vacancy cluster itself is stable. By absorbing vacancies, it grows to a large sft in which the 10v-void is still included. Even when only one hydrogen atom is trapped in vacancy clusters which grow from the 10v-void and whose size is below the 17-vacancy, it relaxes to avoid. Trapped hydrogen atoms move on the surface of a void and suppress the relaxation of sft. When hydrogen atoms come to a sft which grows from the 10v-void and whose size is larger than the 18-vacancy, it does not relax to a void. It is concluded that the critical step of void nucleation is in the range of six to fifteen vacancies in the presence of gaseous atoms. The preferential formation of voids along dislocation lines, which is observed in irradiated metals, is due to the existence of a high concentration of trapped gaseous atoms along dislocation lines.