The dependence on displacement rate and temperature of near-surface void-denuding in self-ion irradiated pure polycrystalline and single-crystal iron

Abstract

Pure iron has been irradiated with Fe²⁺ ions in a series of studies to identify neutron-atypical physical processes that influence the depth dependence of void swelling, focusing especially on suppression effects arising from injected interstitials and surface proximity. One paper in this series examined the surface influence in single-crystal Fe irradiated to 50 and 100 peak dpa over a range of temperature (425–525 °C) and ion energy (1.0, 2.5, 3.5, 5.0 MeV) while keeping the peak damage rate at 1.2 × 10^–3 dpa/s, although the surface dpa rate was lower but increasing with decreasing ion energy, providing a small range of surface dpa rate. The observed denuded width $\Delta x$ was modified to incorporate sputtering loss. The activation energy governing the denuding process was found to be $E_{\Delta x^4}$=1.65$\pm 0.03eV$, higher than the vacancy migration energy $E_V^m$ known to be 0.67 eV. This difference was attributed to the effect of dissolved carbon (103 appm) which reduces the effective vacancy mobility and thereby increases the effective migration energy.

Since the previous study involved a factor of only 2.83 in near-surface dpa rate it is important to confirm that the dependence of $E_{\Delta x^4}$ on dpa rate is maintained over a larger range of damage rates. In this study polycrystalline Fe with 140 appm carbon was irradiated with 5 MeV Fe²⁺ ions to 50, 100 and 150 dpa over a range of peak dpa rates (2.0 × 10^–4, 1.2 × 10^–3, 6.0 × 10^–3 dpa/sec) and temperatures (425, 475, 525 °C). At very high dpa levels sputtering and void growth lead to loss of voids via shrinkage, limiting the upper dose level where this technique can be applied. It was found that the single-crystal and polycrystal specimens yielded essentially identical behavior with $E_{\Delta x^4}$ = 1.65 eV, validating the application of this activation energy over a wider range of dpa, dpa rate, temperature, and crystal form.