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Ion irradiation effects on high purity bcc Fe and model FeCr alloys

Abstract : FeCr binary alloys are a simple representative of the reduced activation ferritic/martensitic (F-M) steels, which are currently the most promising candidates as structural materials for the sodium cooled fast reactors (SFR) and future fusion systems. However, the impact of Cr on the evolution of the irradiated microstructure in these materials is not well understood in these materials. Moreover, particularly for fusion applications, the radiation damage scenario is expected to be complicated further by the presence of large quantities of He produced by the nuclear transmutation (~ 10 appm He/dpa). Within this context, an elaborate ion irradiation study was performed at 500 °C on a wide variety of high purity FeCr alloys (with Cr content ranging from ~ 3 wt.% to 14 wt.%) and a bcc Fe, to probe in detail the influence of Cr and He on the evolution of microstructure. The irradiations were performed using Fe self-ions, in single beam mode and in dual beam mode (damage by Fe ions and co-implantation of He), to separate ballistic damage effect from the impact of simultaneous He injection. Three different dose ranges were studied: high dose (157 dpa, 17 appm He/dpa for the dual beam case), intermediate dose (45 dpa, 57 appm He/dpa for dual beam case) and in-situ low dose (0.33 dpa, 3030 appm He/dpa for the dual beam case). The experiments were performed at the JANNuS triple beam facility and dual beam in situ irradiation facility at CEA-Saclay and CSNSM, Orsay respectively. The microstructure was principally characterized by conventional TEM, APT and EDS in STEM mode. The main results are as follows: 1) A comparison of the cavity microstructure in high dose irradiated Fe revealed strong swelling reduction by the addition of He. It was achieved by a drastic reduction in cavity sizes and an increased number density. This behaviour was observed all along the damage depth, upto the damage peak. 2) Cavity microstrusture was also studied in the dual beam high dose irradiated FeCr alloys, and the results were compared to bcc Fe. The analysis was performed at an intermediate depth 300 – 400 nm below the surface (to avoid injected interstitial effect and surface effects), corresponding to 128 dpa, 13 appm He/dpa. TEM study revealed that the addition of small quantities of Cr, as low as 3wt.%, is highly efficient in strongly reducing void swelling. It was achieved by a drastic reduction of cavity sizes. For instance, average cavity size in Fe3%Cr was 0.9 nm as opposed to 6.8 nm in bcc Fe. Furthermore, the variation of void swelling as a function of Cr content is non-monotonic, with alocal maxima around 9 - 10wt.%Cr. 3) Coupling of conventional TEM, STEM/EDS and APT analysis on low and intermediate dose irradiated FeCr alloys revealed the presence of Cr enriched zones on the habit plane of the dislocation loops. This is expected to be due to radiation induced segregation (RIS) of Cr close to the core of the loops. As the loop grows under irradiation, the segregated areas are probably prevented from re-dissolution by impurity elements such as C. When imaged by TEM using classical diffraction contrast imaging techniques, these enriched zones produce displacement fringe contrast on the loop plane. A quantitative estimate of this enrichment was deduced by STEM/EDSand APT. The Cr content in these areas was between 23 - 35 at.% measured by EDS and 22 ± 2 at.% obtained by APT, whichis well below the Cr content of the Cr-rich α’ phase.
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Arunodaya Bhattacharya. Ion irradiation effects on high purity bcc Fe and model FeCr alloys. Other [cond-mat.other]. Université Paris Sud - Paris XI, 2014. English. ⟨NNT : 2014PA112398⟩. ⟨tel-01252243⟩

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