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Effect of helium on void swelling was studied in high-purity α-iron, irradiated using energetic selfions to 157 displacements per atom (dpa) at 773 K, with and without helium co-implantation up to 17 atomic parts-per-million (appm) He/dpa. Helium is known to enhance cavity formation in metals in irradiation environments, leading to early void swelling onset. In this study, microstructure characterization by transmission electron microscopy revealed compelling evidence of dramatic swelling reduction by helium co-implantation, achieved primarily by cavity size reduction. A comprehensive understanding of helium induced cavity microstructure development is discussed using sink strength ratios of dislocations and cavities. [ABSTRACT FROM AUTHOR]

Abstract: The first walls of future fusion reactors will be exposed to 14MeV neutron irradiation. In order to gain insight into their radiation resistance, model materials (a high purity iron and a Fe–5.40wt.% Cr alloy) have been ion-irradiated in the JANNuS facility to high doses with and without helium. The materials were self-ion-irradiated at 500°C up to ∼100 displacements per atom (dpa) with gas implantation rates equal to 0, 2.5 and 25appm Hedpa−1. He and Cr effects on the radiation damage formation have been studied at the microscopic scale by transmission electron microscopy. In pure Fe, a coarse dislocation network was observed after irradiation, irrespective of helium implantation rate. Concerning the V-containing features (nanobubbles or cavities), few large cavities were observed in pure iron irradiated without helium. The main effect of helium implantation was to increase their density and diminish their size. The main effect of chromium was to reduce the swelling for all the He implantation rates. Furthermore, in both materials, the cavity formation was mainly heterogeneous. [Copyright &y& Elsevier]

In order to study the effects of 14MeV neutron irradiation on materials used in the first walls of future fusion reactors, high-purity iron was ion-irradiated with and without helium in the JANNuS facility. Thin foils of high-purity iron were dual-beam irradiated in situ in a transmission electron microscope using 1MeV Fe+ and 15keV He+ ions. Several important results regarding dislocation loops and helium bubbles were obtained. For example, it was demonstrated that dislocation loops with a 0 〈010〉 type Burgers vectors are glissile and can move and eliminate at the surface of the thin foil at 500°C. A comparison of irradiations with and without helium showed that helium atoms reduce the mobility of dislocation loops in pure iron irradiated at 500°C. Also, we demonstrated that the heterogeneous formation of bubbles inside dislocation loops found previously is also present for helium implantation rates of ∼80 atomic parts per million (appm) He/displacements per atom (dpa). [ABSTRACT FROM AUTHOR]

• In-situ irradiations performed on ultra-high purity Fe14wt.%Cr alloy at 500 °C. • Helium (He) co-injection drastically modifies dislocation loop microstructures. • Disappearance of a/2<111> type dislocation loops occurred due to He. • High cavity density by He may cause preferential loss of glissile a/2<111> clusters. • Cr reduces loop mobilities to stabilize a/2<111> loops, concurring literature. Single and dual-beam self-ion irradiations were performed at 500°C on ultra-high purity Fe14%Cr alloy to ∼0.33 displacements-per-atom (dpa), and 0 or 3030 atomic-parts-per-million (appm) helium/dpa, respectively. Using transmission electron microscopy, we reveal that helium can drastically modify the dislocation loop Burgers vector in Fe-Cr alloys. Helium co-implantation caused complete disappearance of a/2<111> type dislocation loops, and the microstructure consisted of only a<100> loops. Conversely, a/2<111> type loops were predominant without He co-implantation. The total loop density remained largely unaffected. The results strikingly contrast literature asserting that helium stabilizes a/2<111> type loops in bcc Fe alloys, based on low temperature irradiations. Collectively analyzing the results with literature suggest that the small positive interaction between helium and self-interstitial atoms (SIA) in Fe predicted by atomistic simulations maybe insufficient to holistically explain the dislocation loop microstructure development in presence of helium. Helium-SIA positive binding inadvertently implies elevated a/2<111> loop fraction and higher loop densities that the present results contradict. Helium induced high cavity density causing a preferential loss of highly glissile <111> clusters, leaving the matrix saturated with <100> type clusters is proposed as a potential mechanism. Further, the in-situ irradiations combined with Burgers vector analysis strengthened the evidence of Cr-induced dislocation loop mobility reduction that appears to stabilize the a/2<111> type loops and causes higher loop densities in Fe-Cr alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: In order to understand the evolution of materials under irradiation conditions similar to those in future fusion reactors, we have irradiated high purity iron in situ in a transmission electron microscope with 1MeV Fe+ ions while simultaneously implanting 15keV He+ ions, at 500°C. Once ∼1dpa/500appm He were reached, helium bubbles and large dislocation loops were observed. The study reveals that helium bubbles nucleated heterogeneously: a majority of them were observed inside the large dislocation loops. [Copyright &y& Elsevier]