Direct observation of the primary state of damage of ion-irradiated tungsten I. Three-dimensional spatial distribution of vacancies

Abstract

The results of an extensive field-ion microscope (FIM) investigation of the primary state of damage of ion-irradiated tungsten are presented. Two-pass zone-refined single crystals of tungsten were irradiated in situ, at ≤ 15 K, with a magnetically analysed beam of various ions at a background pressure of (5–10) × 10⁻¹⁰ Torr in the absence of the imaging electric field. The value of the standard fluence was small enough (5 × 10¹² cm⁻²) to guarantee that each depleted zone (DZ) detected was associated with a single projectile ion. After an irradiation each specimen was examined on an atom-by-atom basis employing the pulse field-evaporation technique. The two main experimental programmes were: (1) the determination of the effect of the mass of the projectile ion (M ₁) on the three-dimensional spatial distribution of vacancies in DZs, in specimens which had been irradiated with 30 keV W⁺, Mo⁺, Kr⁺ Cu ⁺, Cr⁺, or Ar⁺ ions; and (2) the characterization of the effect of the initial energy of the projectile ion (E ₁) on the vacancy structure of DZs created by 15, 30, 45, 60 or 70 keV Kr ions. Three-dimensional visualizations are presented of a number of the DZs detected, based on the use of the OR TEP program. The average number of vacancies (〈v〉) per DZ is 174 ± 48 for E ₁ = 30 keV, independent of M ₁, for the range of ion masses employed. The value of the average vacancy concentration per DZ decreases from ≈ 16 to 2 at.% as M ₁, is decreased from ∼184 a.m.u. (W) to ∼40 a.m.u. (Ar) for E ₁ = 30 keV. For the Kr ion irradiations the value of 〈v〉 increases linearly as E ₁ is increased from 15 to 70 keV. Many other detailed physical properties of the DZs are presented.