Nucleation path of helium bubbles in metals during irradiation

Abstract

A thermodynamic formalization is developed for description of the nucleation and growth of helium bubbles in metals during irradiation. The proposed formalization is available for evaluating both microstructural changes in fusion first wall materials where helium is produced by (n, α) nuclear transmutation reactions, and those in fusion diverter materials where helium particles with low energy are directly implanted. The calculated nucleation barrier is significantly reduced by the presence of helium, showing that a helium bubble with an appropriate number of helium atoms depending on bubble size can nucleate without any large nucleation barriers, even at a condition where an empty void has very large nucleation barriers without helium. With the proposed thermodynamic formalization, the nucleation and growth process of helium bubbles in iron during irradiation is simulated by the kinetic Monte Carlo (KMC) technique. It shows the nucleation path of a helium bubble on the (N He, N V) space as functions of temperatures and the concentration of helium in the matrix, where N He and N V are the numbers of helium atoms and vacancies contained in the helium bubble, respectively. Bubble growth rates depend on the nucleation path and suggest that two different mechanisms operate for bubble growth: one is controlled by vacancy diffusion and the other is controlled by interstitial helium diffusion.