Stability of the Y–Ti–O oxides in reactor materials under neutron irradiation at high temperatures

Abstract

The paper presents the results of electron microscopic studies of ferrite-martensitic steel samples hardened with Y-O oxides, EP-450 DUO in the initial state and after neutron irradiation in the BN-600 reactor at 1000°C to a damaging dose of 77.5 dpa. These studies showed that the main types of oxide phases were Y2(Si, Ti)2O7 and Y2(Si, Ti)O5. These precipitates at sizes less than 10-20 nm were semi-coherent with a ferritic matrix of steel EP-450 DUO with the ratio (110)malrix//(221)palticle. Some of the Y-Ti-O oxides in the initial state were Y2Ti2O7-type with some deviations from the stoichiometric composition.However, after neutron irradiation under BN-600 conditions at temperature ~ 1000°C, oxide particles could not be described by the indicated stoichiometry. Besides, after irradiation, silicon and aluminum were found in the oxide's composition. In the case of taking these elements into account during the construction of a triple composition diagram, it was shown that the oxide phases had Y2(Ti, Si, Al)2O7 and Y2(Ti, Si, Al)O5 types. It was established that in samples of EP-450 DUO steel in the initial state with oxide particles up to 20 nm in size, the yttrium content is generally lower than the titanium concentration. The titanium and yttrium concentrations corresponded to the stoichiometric composition Y2Ti2O7 (1:1) with a further increase in the average diameter of these phases. After irradiation, the situation changed somewhat: the yttrium content in most oxide phases exceeds the total concentration of titanium, silicon, and aluminum.The paper also presents the analysis of porosity and evolution of grain structure in EP-450 DUO steel after neutron irradiation.