Study on the Process of the Structural Relaxation of Vacancy Clusters by the Visualizing Method with an MD Simulation

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Computer simulations of molecular dynamics (MD) were used to study the clustering of vacancies in nickel. This is to investigate the atomistic processes of vacancy agglomeration in irradiated metals. First, PKA cross section was calculated to know how many Frenkel pairs form by an irradiation experiment. Next, simulations were carried out with DYNAMO ver. 8.5 code by implementing the potential of the embedded atom method (EAM). First, the perfect crystal was thermalized at 1300K, 1500K, 1700K and 1800K.Then, four triangular vacancy clusters whose sizes were 10 were introduced on (111) planes at randomly selected positions. MD simulations were performed in a very small time step of outputs to observe the atomistic processes of the movement of a vacancy. It was found that vacancy clusters moved and agglomerated with keeping the cluster. Thus a vacancy did not evaporate from a cluster to be absorbed in a larger cluster. There were three different processes in an agglomeration of vacancy clusters. One was an atom caged in a tetrahedral structure of vacancies moved to another lattice site. Another was an atom caged in a hexahedral structure of vacancies moved to another lattice site. The other was an atom moved along a <110> row. The migration energy was 1.13eV(=E_m) for a single vacancy and 0.4eV for a small vacancy cluster (3v). The binding energy of 17v cluster and a single vacancy was 0.8eV=E_b. So the energy for a single vacancy to evaporate from a cluster was E_m+E_b=1.93eV.Therefore it means that a vacancy does not evaporate from a cluster.