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, V₉ + W₁ + V₉ and V₁₀ + W₄ + V₁₀ (Vₙ means that a vacancy corresponds to the absence of n W atoms, and Wₘ indicates that there are m W atoms between two vacancies) in various cases of temperature and hydrogen atom concentration. Under the vacancy structure V₉ + W₁ + V₉, 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 V₁₀ + W₄ + V₁₀, hydrogen atoms facilitate vacancy coalescence most strongly when the number is around 45 to 54 in each vacancy.