The volume change of migration of the stage I self-interstitial atom in ion-irradiated tungsten

Abstract

A detailed study has been performed of the effect of the electric field (∼4.75 VA⁻¹) required for field ionization of helium gas atoms, in the field ion microscope (FIM), on the isochronal recovery spectrum of ion (30 keV W⁺) irradiated tungsten. All the irradiations were performed in-situ under ultra-high vacuum conditions (−9 torr) to a constant dose of 5 · 10¹² ion cm⁻². The FIM specimens used were prepared from oriented single crystals of high purity tungsten with a resistivity ratio of 5 · 10⁴. The experimental procedure involved a comparison of the isochronal recovery spectra in the range 18 to 120 K of specimens irradiated at 18 K in the absence of both the electric field and the imaging gas with identical specimens which were irradiated at 50 K and then cooled to 18 K in the absence of both the electric field and the imaging gas. The results of these recovery experiments demonstrated that the field of ∼4.75 VA⁻¹ has only a minimal effect on the annealing kinetics of self-interstitial atoms (SIA's) and that the volume change of migration of the Stage I SIA is less than 0.02 atomic volume. This result strengthens our contention that SIA long-range migration occurs at ∼ 38 K and that Stage I recovery is certainly completed by ∼45 K for tungsten. This result is in contradistinction to other investigations which have assigned SIA long-range migration in tungsten to the 70 to 100 K range.