Effect of direct addition of nanocrystalline complex pyrochlores
on mechanical behaviour of austenitic ODS SS316Lsteels


Abstract

Advanced nuclear reactors require cladding materials with high creep, corrosion and swelling resistance. Fuel pins inside the cladding tube undergo intense irradiation at high temperatures. Life period of fuel pins largely depends on the temperature and irradiation levels to which they are exposed. Due to high breeding ratio of the fuels used in the fuel pins, the materials undergo void swelling, irradiation creep and irradiation embrittlement. Hence, the cladding tube materials essentially should have high swelling resistance and creep strength for increased life span.Structural materials for core nuclear components have evolved continuously to increase the fuel performance. The first-generationmaterial, austenitic stainless steel can withstand only up to 50 displacements per atom (dpa). For the doses above 50dpa, ferritic/martensitic steels are used. Ferritic/martensitic steels exhibit high swelling resistance than austenitic steels but their poor thermal creep strength at operating temperatures limits their usage. To overcome this problem, new emerging materials, namely, oxide dispersion strengthened (ODS) steels have been developed.

These ODS steels are processed mainly through powder metallurgy route.The properties of ODS steels primarily depends on microstructure, particularly the size and number density of clusters formed in these steels. Commonly used oxides for ODS steels are Y2O3, TiO2, ZrO2 and HfO2. The nature, type, amount of oxides and other alloying elements also affect the properties of these steels. In presence of Ti, Y2O3is known to react with Ti to form Y2Ti2O7 during consolidation.These finely dispersed, nanometre sizedparticles restrict dislocation motion, thereby improving mechanical properties at high temperatures, while simultaneously generating a large number of particle-matrix interfaces, which act as a sink for radiation induced defects.

In this study,the effect of nano dispersoids in austenitic steel (SS316L) has been studied in two different methods. First, by direct synthesis of Y2Ti2O7 and Y2(TiZrHfMo)2O7through reverse co-precipitation. Second, adding Y2O3, Ti, to form in-situ oxide. Powders are milled through horizontal attritor in argon atmosphere and The milled powders are consolidated by spark plasma sintering. The microstructure, hardness and compressive behaviour of the sintered pellets were characterized using different techniques. The high temperature mechanical properties of ODS steel with direct addition of Y2Ti2O7,Y2(TiZrHfMo)2O7are compared with in-situ formed Y2Ti2O7. The direct addition oxides (Y2Ti2O7 and Y2(TiZrHfMo)2O7) have better room temperature and high temperature properties compare to in-situ formed oxide. These oxides were characterized using Scanning Transmission Electron Microscope–high angle angular dark field technique (STEM-HAADF) and atom probe tomography (APT). The dispersoid size, grain size, number density and stoichiometric of direct addition oxides are important factors that influence the properties of direct addition oxides.