Modeling injected interstitial effects on void swelling in self-ion irradiation experiments

Although accelerator-based ion irradiation has been widely accepted to simulate neutron damage, neutron-atypical features need to be carefully investigated In this study, we have shown that Coulomb force drag by ion beams can introduce significant amounts of carbon, nitrogen, and oxygen into target materials even under ultra-high vacuum conditions The resulting compositional and microstructural changes dramatically suppress void swelling By applying a beam-filtering technique, introduction of vacuum contaminants is greatly minimized and the true swelling resistance of the alloys is revealed and matches neutron behavior closely These findings are a significant step toward developing standardized procedures for emulating neutron damage

Beam-contamination-induced compositional alteration and its neutron-atypical consequences in ion simulation of neutron-induced void swelling

Abstract Heavy ion irradiations at high dose rates are often used to simulate slow and expensive neutron irradiation experiments. However, many differences in the resultant modes of damage arise due to unique aspects of heavy ion irradiation. One such difference was recently shown in pure iron to manifest itself as a double peak in void swelling, with both peaks located away from the region of highest displacement damage. In other cases involving a variety of ferritic alloys there is often only a single peak in swelling vs. depth that is located very near the ion-incident surface. We show that these behaviors arise due to a combination of two separate effects: 1) suppression of void swelling due to injected interstitials, and 2) preferential sinking of interstitials to the ion-incident surface, which are very sensitive to the irradiation temperature and displacement rate. Care should therefore be used in collection and interpretation of data from the depth range outside the Bragg peak of ion irradiation experiments, as it is shown to be more complex than previously envisioned.

In radiation effects on materials utilizing self-ion irradiations, it is necessary to calculate the local displacement damage level and ion injection profile because of the short distance that self-ions travel in a material and because of the strong variation of displacement rate with depth in a specimen. The most frequently used tool for this is the software package called Stopping and Range of Ions in Matter (SRIM). A SRIM-calculated depth-dependent dose level is usually determined under the implicit assumption that the target does not undergo any significant changes in volume during the process, in particular SRIM ignores the effect of sputtering, injected ions, and void swelling on the redistribution of the dose and injected ion profiles. This approach become increasingly invalid as the ion fluence reaches ever higher levels, especially for low energy ion irradiations. The original surface is not maintained due to sputter-induced erosion, while within the irradiated region of the specimen, injected ions are adding material, and if void swelling is occurring, it is creating empty space. An iterative mathematical treatment of SRIM outputs to produce corrected dose and injected ion profiles based on these phenomenon and without regard to diffusion is presented along with examples of differences between SRIM-calculated values and corrected values over a range of typical ion energies. The intent is to provide the reader with a convenient tool for more accurately calculating dose and injected ion profiles for heavy-ion irradiations.

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Modification of SRIM-calculated dose and injected ion profiles due to sputtering, injected ion buildup and void swelling

Swelling resistance of advanced austenitic alloy A709 and its comparison with 316 stainless steel at high damage levels

Ion bombardment is employed to simulate the neutron damage that will occur in fusion reactor structural materials. These experiments usually involve specimens whose thickness is large compared to the ion range. The correct interpretation of such experiments requires inclusion of the impact of swelling- induced stresses, which arise from the rigidity of the non-swelling region beyond the ion range. An initial low level of swelling is accommodated by elastic compression of the voided region, giving rise to anisotropic stresses whose deviatoric components lead to plastic deformation by various irradiation creep mechanisms. Swelling generates the stresses which lead to creep, which in turn limits the magnitude of stress. The hydrostatic stress along the ion range eventually saturates at a level which reduces the swelling rate from the anticipated stress-free value. The swelling is fully anisotropic, two-thirds of the deformation having been diverted from directions normal to the ion beam. Crystalline anisotropy further modifies the relative rates of creep processes, and radiation-hardening of the lattice imposes an additional fluence dependence. Stress reversal at denuded zones leads to second-order stress effects. The use of step height measurements allows observation and measurement of these phenomena. Estimates of the stress levels and swelling rates canmore » be made using swelling and creep coupling coefficients derived from fission reactor data. A significant reduction in swelling rate is predicted by this analysis. (auth)« less

F.A. Garner

Papers

Stress effects in ion bombardment experiments