Radiation-induced swelling and precipitation in Fe++ ion-irradiated ferritic/martensitic steels

• Published in: Journal of nuclear materials 2021-11, Vol.555, p.153137, Article 153137
• Main Authors: Lee, Myeongkyu, Kim, Geon, Jung, Yunsong, Ahn, Sangjoon
• Format: Article
• Language: English
• Subjects: Alloying; Alloys; Carbides; Chemical composition; Chemical precipitation; Chromium; Depletion; Dual phase steels; Evolution; fast reactor cladding material; Ferritic stainless steels; Ferritic/martensitic steels; Free surfaces; Helium; ion irradiation; M2X precipitation; Martensitic stainless steels; Metallurgy; Nucleation; Radiation; Radiation damage; Radiation effects; Steel; Swelling; swelling resistance
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2021.153137

•Void swelling was studied in self-ion irradiated HT.9, Gr.92, FC92-B, and FC92-N.•FC92-N showed the greatest swelling resistance of 1.76% at 240 dpa and 2.17% 318 dpa.•M2X evolved only in FC92 series, in which bimodal swelling profiles were observed.•RIP and outward Cr sinking to the surface synergistically formed a low-alloyed zone.•RIP modified swelling-depth profiles, and in turn, determined double peak swelling. The radiation responses of two newly developed ferritic/martensitic steels, FC92-B and -N, were tested in comparison to reference alloys HT9 and Gr.92. Ion irradiations on the steels were performed up to 480 dpa at 475°C using 3.5-MeV Fe++ ions with a helium pre-implantation of 1 appm/dpa. Void swelling and M2X precipitation were characterized using FE-TEM and EDS. Swelling resistance was the greatest in FC92-N, which showed suppressed void nucleation and growth. The swelling rate in FC92-N was determined as 0.005 %/dpa, indicating that FC92-N did not reach the steady-state swelling regime with void nucleation behavior. The least swelling-resistant alloy was HT9 with a swelling rate of 0.048 %/dpa. Cr-rich carbide, M2X, was observed in only 9Cr-FC92 series; however, its formation did not depend on radiation damage. This exceptional M2X evolution in FC92 series may be attributed to B and N alloying, which resulted in suppressed M23C6 carbide formation during metallurgical production and sequentially high C contents in the alloy solution of FC92 series. A narrower range (800 nm) of M2X evolution compared to that of cavity formation (1,000 nm) indicates that radiation-induced precipitation (RIP) is more sensitive to the injected interstitial effect. Precipitation-induced Cr depletion and preferential interstitial outward sinking to the free surface synergistically modified local chemical composition before void evolution and led to double-peak swelling by locally forming a low-alloyed zone. This study provides the first experimental evidence that RIP modifies the swelling–depth profiles and in turn, determines double-peak swelling in ion-irradiated steels. [Display omitted]