Study of radiation-induced amorphization of M23C6 in RAFM steels under iron irradiations

Abstract

The mechanical properties of reduced activation ferritic/martensitic steels (RAFMs) such as creep, thermal aging, and irradiation resistance are improved by the formation of MX and M23C6 particles. However, it is well known that these particles are unstable under irradiation. The purpose of the present study is to clarify the details of the radiation-induced amorphization (RIA) of M23C6 in F82H steels. The instability behavior of M23C6 under a wide range of irradiation temperatures from R.T. to 773 K and various ion beam energies (2.8, 3.7, 6.4 and 10.5 MeV) was systematically investigated. Two types of materials, F82H and its model alloy (F8), were utilized in this study. TEM observation and selected area electron diffraction analysis before and after irradiation were conducted to evaluate the occurrence of RIA, and the chemical composition was analyzed to reveal the possible relation between atom displacement/diffusion and RIA. In the case of the iron irradiation at the temperature below 598 K, a bilayer contrast of the particle, consisting of an amorphous-rim phase and inner crystalline core, was observed. Similar particle survived in the irradiated F82H at the temperature of 623 K, the RIA was not observed in the F8 specimen. Consequently, the critical temperature of RIA was estimated as 623–673 K. Since the melting temperature (Tm) of M23C6 is approximately 2173 K, the critical temperature corresponds to ∼0.29 Tm. From the STEM-EDS analysis, the Fe/Cr ratio of the particle decreased with increasing irradiation temperature in the range of 573–623 K. This behavior was qualitatively explained by the formation of the amorphous phase. It is clarified that the occurrence of the RIA of M23C6 particle can be evaluated by the variation of Fe/Cr ratio.