Microstructural evolution of ODS steels under neutron irradiation

Yamashita, Shinichiro   ; Akasaka, Naoaki

ODS steels have high resistance to radiation damage and superior long-term thermal-mechanical strengths at high temperature, offering a promise of high performance fuel cladding tube for advanced sodium-cooled fast reactor and being developed intensively in JNC. This report covered microstructural evolutions of ODS steels (1DS and 1DK developed in 1989 and M93, F94, F95 in 1997, respectively) irradiated in JOYO to evaluate the irradiation properties of several type of ODS steels. The new findings obtained were as the followings.1) Dislocation loops and cavities formed under neutron irradiation, depending on irradiation temperature and sink site density prior to irradiation such as dislocation density, total area of interface between precipitate and matrix, total area of grain boundary. 2) Comparison of microstructural evolution between F94 and F95, each of which were mechanically alloyed under different inert gases, indicated that argon is less significant for cavity formation (including its nucleation and growth) than helium. 3) Neither alpha prime nor shigma phases were formed in all of the ODS steels dealt in this study. In the case of high tungsten ODS steels (1DS, 1DK), FeW type of Laves phase precipitated preferentially at grain boundaries and elevated temperatures under irradiation. As for carbide, MC (M=Cr) was a major precipitate of the martensitic ODS steel (M93) before and after irradiation. In ferritic ODS steels (F94, F95), formation of several types of carbides coupled with Ti, W, Cr, Fe were recognized before and after irradiation, but volume of each carbides was very low, being reflected by a carbon content of the pre-irradiated ferritic ODS steels. 4) Complex oxide composed of Y and Ti elements was the majority of all oxide dispersoids. Within neutron irradiation up to 21 dpa, no significant change of individual oxide dispersoid was recognized, but its recoil resolution was implied from the statistical assessment.