The point-defect structure in stage II of ion or electron-irradiated tungsten as studied by field-ion microscopy

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Abstract
A search had been performed for the thermally converted self-interstitial atom (SIA), which is required in a two-interstitial model, in both ion- or electron-irradiated tungsten employing the field-ion microscope (FIM) technique. FIM specimens were prepared from four-pass zone-refined tungsten [resistivity ratio R(R = ρ 273 K/ρ 4.2 K) of 4 × 104 to 5 × 104 where ρ represents the resistivity un corrected for the specimen size effect] were irradiated in-situ at ∼10−9 Torr with 20 or 30 keV W+ ions to doses varying from 1.2 × 1012 to 1.1 × 1013 ion cm−2 at temperatures in the range 273 to 558 K (Stage II to bottom of Stage III). A total of ∼ 105 atomic sites, in high index planes, were examined for SIAs at 20 K by the pulsed field-evaporation technique and no definite contrast patterns for isolated SIAs were detected. In addition, four-pass zone-refined single-crystals [R = (4 to 5) × 104] and Materials Research Corporation VP-grade (R = 15) polycrystalline specimens were irradiated simultaneously, in bulk form, with 2.35 MeV electrons at a flux of ∼200 MA cm−2 to a dose of ∼1 × 1020 electron cm−2 at × 430 K. The specimens were subsequently examined by the pulsed field-evaporation technique at ∼20 K; these experiments showed that the electron-irradiated structure consisted of immobile isolated monovacancies and a second defect which possesses a complex contrast-pattern that extends over five to fifteen successive atomic layers. The complex contrast patterns were interpreted as having been most likely caused by impurity atom-SIA clusters. Thus, it was concluded that there is no definite evidence for isolated thermally converted SIAs, at a concentration above 5 × 10−6 at. fr. at 430 K, for tungsten irradiated to a dose of ∼1 × 1020 electron cm−2.