Effect of the free surface on near-surface void swelling in self-ion irradiated single crystal pure iron considering the carbon effect

Abstract

The influence of ion-incident free surfaces on void swelling in near-surface regions was systematically studied using self-ion irradiation of single crystal pure iron. The key interest was to evaluate the effect of free surfaces not only on the formation of void-denuded zones but also on the swelling in the region immediately adjacent to the void-denuded zone. The irradiation matrix included beam energies varying from 1.0 to 5.0 MeV, peak displacements-per-atom levels of 50 and 100 dpa, and irradiation temperatures at 425 °C (at 6 × 10−3 peak dpa/sec, 475 °C (at 1.2 × 10−3 peak dpa/sec) and 525 °C (at 6 × 10−3 peak dpa/sec). The observed denuded depth Δx obtained from transmission electron microscopy was modified to incorporate the sputtering effect. The derived activation energy governing the denuding process is Ex4=1.65±0.03eV, which is significantly higher than the vacancy migration energy EVm known for Fe to be 0.67 eV. This difference is speculated to be due to the strong effect of relatively small amounts of dissolved carbon at 103 appm, which reduces the effective vacancy mobility and thereby increases the effective migration energy. Based on the effective vacancy migration energy extrapolated from a previous first-principle calculation for Fe containing various carbon levels, rate theory calculation was used to model the surface influence and the results support the speculation of carbon influence. For the region immediately adjacent to the void-denuded zone, swelling is locally suppressed, rising to reach the bulk swelling at a depth increment essentially equal to the denuded width. This suppression effect causes shifting of swelling vs. local dpa curves obtained from different peak dpa irradiations. Such shifting is proposed in the present study as a method to define the boundary of the surface-induced suppressed swelling zone beyond which the swelling can be considered to be fully free of surface influence.