Atomistic Mechanism of Nucleation and Growth of Voids in Cu Studied by Computer Simulation
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Atomistic Mechanism of Nucleation and Growth of Voids in Cu Studied by Computer Simulation Y. Shimomura and I. Mukouda Applied Physics and Chemistry, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527 JAPAN ABSTRACT A TEM observation of fission neutron-irradiated copper at 300oC shows that the maximum size of stacking fault tetrahedra (sft) observed is 6 nm of edge length which corresponds to a cluster of 280 vacancies and the minimum size of voids is 2.2 nm in diameter which corresponds to a cluster of 470 vacancies. The result suggests that a vacancy cluster whose size is smaller than 300 vacancies grows to sft while a cluster whose size is larger than 500 vacancies relaxes to a void in 300oC-irradiated copper. A computer simulation of molecular dynamics (MD) with an isotropic EAM potential examined this model. It is found that a vacancy cluster that is smaller than 300 vacancy segregates to a (111) platelet and relaxes to an sft. Small vacancy clusters which are generated at damage cascade cores aggregate to spherically distributed vacancies for the size of more than 500 vacancies, and relax to several (111) platelets, which finally form a vacancy (111) polyhedron. Inside a polyhedral vacancy platelet, vacancies are confined and grow to a void at high temperature. INTRODUCTION Shimomura and Mukouda [1] reported that a void could be formed by clustering of only vacancy. In fission-neutron-irradiated copper at 300oC to 7 x 1017n/cm2 at Kyoto University Reactor (KUR), the number density of voids exceeds the number density of hydrogen and helium atoms that are generated by the transmutation reaction [2]. A copper foil irradiated in this experiment was prepared to be a very low content of residual gas atoms by melting in highly evacuated vacuum. It is reported by the present authors that vacancy clusters move as a cluster at high temperature and coalesce to a larger cluster [3, 4]. The objective of the present work is to show how a vacancy cluster can grow to a void by clustering of only vacancies at high temperature. This means that a void can be formed without an inclusion of gas atoms in vacancy clusters in neutron-irradiated copper at high temperature. Of course if gas atoms are included in a small vacancy cluster, a void formation is promoted significantly as reported on experiments of multi-ion beam irradiated copper [5]. MODELING Suggested Modeling on Void Formation from TEM Observations of Neutron-irradiated Copper at High Temperature Shimomura and Mukouda [1,6] reported that both stacking fault tetrahedra (sft) and voids were observed in neutron-irradiated copper at 300oC to 7 x 1017n/cm2 in a temperature-controlled rig at KUR. The maximum edge length of observed sft was 4.5 nm (a cluster of 171 vacancies) and the minimum diameter of void observed was 2.2 nm (a cluster of 470 vacancies). We examined the maximum edge length of sft in pure copper irradiated to high fluence. In pure copper irradiated at 300oC to the fluence of 1 x 1020n/cm2 at Japan Materials Testing React
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