Molecular dynamics simulation for coalescence of vacancies in tungsten crystal

We performed molecular dynamics simulations of coalescence of two vacancies in a tungsten (W) crystal to elucidate the effect of temperature and hydrogen atoms. Simulations were performed for two types of vacancy structures, ${\mathrm{V}}_9+{\mathrm{W}}_1+{\mathrm{V}}_9$ and ${\mathrm{V}}_{10}+{\mathrm{W}}_4+{\mathrm{V}}_{10}$ (${\mathrm{V}}_n$ means that a vacancy corresponds to the absence of $n$ W atoms, and ${\mathrm{W}}_m$ indicates that there are $m$ W atoms between two vacancies) in various cases of temperature and hydrogen atom concentration. Under the vacancy structure ${\mathrm{V}}_9+{\mathrm{W}}_1+{\mathrm{V}}_9$, we observed vacancy coalescence for all the cases of the temperature and the number of hydrogen atoms. Evaluating the potential energy required for removing one of the W atoms between two vacancies, we found that high temperature and existing hydrogen atoms in the vacancies facilitate vacancy coalescence, and that under the structure ${\mathrm{V}}_{10}+{\mathrm{W}}_4+{\mathrm{V}}_{10}$, hydrogen atoms facilitate vacancy coalescence most strongly when the number is around 45 to 54 in each vacancy.