Migration of rhenium and osmium interstitials in tungsten

Abstract

Tungsten is expected to be a promising plasma-facing material for future fusion devices, but radiation-induced precipitation (RIP), which leads the material to hardening, is a concern at their practical use. One of the keys to accurate prediction of the emergence of RIP is migration of solute atoms, rhenium and osmium, that are produced by nuclear transmutation through irradiation. We conduct a series of numerical simulations using an atomic kinetic Monte Carlo method and investigate the migration of these solute atoms in the form of tungsten-rhenium and tungsten-osmium mixed dumbbells, considered to be the most efficient "carriers" of the solute atoms. We find that the low rotation energy barrier of these mixed dumbbells leading to three-dimensional migration greatly influences their diffusivities. The result also suggests that, although these dumbbells have three-dimensional motion, one cannot simply reduce their migration behavior to that of vacancy-like spherical objects.

Suppression of radiation-induced point defects by rhenium and osmium interstitials in tungsten

“…Notice that N visit values for SIAs in W crystals are, meaningfully, smaller than the others, indicating that this trajectory is sparser than the others. The difference is caused by a specific feature of SIAs in W crystals, that is, they have microscopically 1D motion and intermittently change their direction by rotation events10. Consequently, they have less chance of recombining with a vacancy compared with W-Re and W-Os mixed dumbbells and have greater chance of escaping the cascade-displacement influenced region.…”

“…We analyze the kinetics of the pure/mixed dumbbells by using event-driven atomic kinetic Monte Carlo (AKMC) method10. Our purpose is to analyze the trajectory of these dumbbells and to verify that the mixed dumbbells enhance vacancy-interstitial recombination compared with the pure dumbbells.…”

“…To conduct the AKMC and its post analyses shown in Fig. 3, we use a programming code specially developed for this purpose using a KMC development tool called PAKSS10; this is open source software developed by the first author. We set the attempt frequency of 10 13   s −1 for any events.…”

“…Another hypothesis is that vacancy or interstitial clustering might cause the aggregation, but the RIP is not correlated to the growth of voids and dislocation loops8. The remaining possibility seems to be that it is caused by the recombination of a vacancy and an interstitial, both of which are associated with the solute elements910. Table 1 shows the binding energies of solute atoms to point defects, and it clearly indicates that all the combinations of solute elements and point defects are attractive, including Re in Molybdenum (Mo) crystals that also causes RIP11.…”

“…Our previous studies1016 investigate stability and mobility of Re and Os interstitials; these solutes form a mixed dumbbell with a W atom and have three-dimensional (3D) motion instead of one-dimensional (1D) one that self-interstitial atoms (SIAs) have. We formulated a hypothesis that the change of migration dimension might be related to the suppression of radiation effects.…”