On the point-defect annealing mechanism for stage III recovery in irradiated or quenched tungsten

1. 1. In previously published FIM studies (2,3) we have found no overwhelming evidence for isolated thermally-converted SIAs, as required in a two-interstitial model, at a concentration above 5×10^-6 at.fr. at T_i {reversed tilde equals} 430 K for an electron irradiations to dose of ∼1×10²⁰ cm^-2. The FIM microstructure of the electron-irradiated tungsten (both R = 15 and R = 5×10⁴) consisted of immobile isolated monovacancies and a second defect which possesses a xomplex contrast-patetrn that extends over five to fifteen successive atomic layers. The complex contrast-patterns were interpreted in terms of a SIA(s) trapped at impurity atom(s) to form a cluster, Thus, we are led to the conclusions that Stage III recovery of electron-irradiated W must involve the migration of monovacancies to SIAs trapped at impurity atoms. 2. 2. The published electron-irradiation recovery studies, on W, indicate the complete absence of a Stage IV; and that the recovery of the Stage III point defect could not be separated from the annealing of the quenched-in vacancies. This constitutes very strong indirect evidence that the vacancy migrates in Stage III of electron-irradiated W. 3. 3. The behavior of vacancies in quenched W is such that all the vacancies recover in a single major isochronal recovery stage centered at temperatures between 800 to 1200 K. It is incorrect to call this recovery behavior Stage IV even though it lies somewhat above the Stage III (∼500 to 1000 K) of electron or neutron-irradiated W. The reason why Stage III recovery of quenched W often lies above Stage III of irradiated W is simply that the average number of jumps made by a vacancy, in Stage III of quenched W, before it is annihilated is greater than in the case of irradiated W. That is, the temperature range of Stage III is a sensitive function of the sink structure of the specimen. 4. 4. The best value of the activation enthalpy of migration of a monovacancy (Δh^m_lv) is 1.78 ± 0.1 eV, as determined from isothermal-annealing experiments on quenched W. 5. 5. The best value of the activation enthalpy of formation of a monovacancy (Δh^f_lV) is 3.6 ± 0.2 eV, as determined from resistivity measurements and positron annihilation studies. 6. 6. The sum of Δh^f_lv + Δh^m_lv is 5.4 ± 0.3 eV and this value is in agreement with the activation energy for self-diffusion by a monovacancy mechanism (Q^SD_lv) as determined by tracer self-diffusion experiments on tungsten single-crystals; this best value of Q^SD_lv is 5.4₅ eV. 7. 7. The activation enthalpy for recovery of neutron (thermal or fast) or electron-irradiated W has been consistently found to be 1.7 - 1.8 eV; this value is essentially a constant independent of the irradiation flux or dose, and the initial impurity content of the W. We associate this activation enthalpy with Δh^m_lv; i.e., Stage III recovery of irradiated W is governed by the migration of the monovacancies to SIAs trapped at impurity atoms. 8. 8. FIM experiments have provided evidence for the trapping of SIAs at Re atoms, in W-3 at .% Re alloys, up to at least as high as 390 K. This suggests that in neutron irradiations with a thermal-neutron component to the dose the SIAs are trapped at the Re atoms which are produced as a result of transmutation reactions.