Radiation-Induced Swelling and Precipitation in Fe ++ Ion-Irradiated Ferritic/Martensitic Steels

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 0.007 %/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 0.038 %/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 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.