Characterization of oxide dispersion strengthened steels for nuclear applications

Abstract

Oxide dispersion strengthened (ODS) steel is considered as one of the most promising candidate materials for structural components of Generation IV nuclear systems or fusion systems owing to its superior creep strength and irradiation resistance. Nano-sized oxide particles dispersed in the matrix act as pinning points for dislocation motion and grain boundary migration, leading to very high mechanical strength and thermal stability. Oxide particles also contribute to better irradiation resistance by attracting irradiation induced defects such as vacancies and interstitials at the oxide/matrix interface. Therefore, the understanding and controlling of the dispersed oxide particles, such as the phase identification, the size distribution, thermal stability, and orientation relationship with the matrix, are of great importance to the application of ODS steel. In this study two types of ODS steels with extinctive different matrix, 12Cr ferritic ODS steel and 316L stainless steel based austenitic ODS steels, were prepared by mechanical alloying (MA), hot isostatic pressing (HIP) and hot rolling processes. Oxygen content was controlled by a hydrogen reduction process prior to consolidation. To understand and optimized the microstructure and high temperature mechanical strength, the oxide particle phases, size distribution, thermal stability of oxide particles, and orientation relationship of oxide particles with matrix were intensively investigated. First, oxide particles in the 12Cr ferritic ODS steel and austenitic ODS steel were characterized, including the oxide phase identification, size distribution, and formation mechanism. Fine YTaO4 particles (~9 nm) and coarse Cr2O3 particles (~200 nm) were identified in the ferritic ODS steel specimens, and fine Y2Ti2O7 (~7 nm) and coarse Cr2O3 particles (~200 nm) were present in the austenitic ODS steel specimens, indicating the Y2O3 particles were alloyed with minor alloying elements (such as Ti or Ta) to form complex oxide particles. Second, it is widely known that oxygen content has significant influence on the type and size distribution of oxide particles in both ferritic and austenitic ODS steels. The formation of coarse Cr-O particles in ferritic ODS steel, which seems to contribute little to strength but are prone to initiate cracks, can be suppressed by reducing the oxygen content. Fine YTaO4 particles in the ferritic ODS steel, on the other hand, depend less on the oxygen content. Therefore, reduction in oxygen content is considered an effective method to further improve the mechanical properties of ODS steels, especially the ductility. Third, coherency of oxide particles with the matrix is a very important factor to the microstructure stability (Zener pinning) and dispersion strengthening. The orientation relationship and interface structure of oxide particles with the matrix were investigated by diffraction contrast techniques and HRTEM. No common orientation relationship between YTaO4 (monoclinic) particles and ferrite matrix was found. However, lattice continuity across the YTaO4/matrix interface was observed along the {110} close packed plane of the matrix, like (051)O//(011)M. In austenitic ODS steel, it is considered that over 95% of the Y2Ti2O7 particles (3-10 nm in diameter) are semi-coherent with the austenitic matrix, with a specific common orientation relationship, (220)Y2Ti2O7 //(200)Matrix and [116]Y2Ti2O7 //[011]Matrix. Fourth, YTaO4 oxide particle coarsening was found during the fabrication process (for instance, hot rolling). Also, phase transformation from YTaO4 to Y3TaO7 was observed after the isothermal annealing at 1250 ˚C for 100 hr. High thermal stability of ultra-fine grain structure was confirmed in the austenitic ODS steel, owing to the strong pinning effect of semi-coherent Y2Ti2O7 particles with the austenite matrix. The ultra-fine grain structure remained stable after annealing at 1150 ˚C for 100 hr. This study gives an overall yet detailed investigation on the microstructure, correlation between microstructure and property, and thermal stability of the ODS steels, which is expected to provide a better understanding of the superior properties of the developed ODS steels.