Термодинамическое моделирование процесса окисления доэвтектического сплава Moss-Mo3Si, легированного иттрием

Abstract

In order to study the effect of yttrium additives on the oxidation of molybdenum silicide alloys, thermodynamic modeling of the interaction in the Mo-Mo3Si-Y5Si3 system with dry and moist air was performed in the temperature range 25-2000 °C. To predict the composition of oxidation products and the sequence of the formation of phase components, the dependences on the temperature and consumption of oxidants – water vapor and oxygen of the air – are obtained. The calculations were performed using the HSC Chemistry 6.12 software, into the database of which the calculated missing thermochemical characteristics of Y2Si2O7, Y2SiO5 silicates and yttrium molybdates Y2Mo3O12, Y2MoO6 were entered. It is shown that, under equilibrium conditions, the oxidation process with dry and moist air proceeds almost equally, since the interaction of the components of the alloy with oxygen is thermodynamically preferable than with water vapor. According to the obtained thermodynamic models, the oxidation process of the Mo-5 wt. % Si alloy of the hypoeutectic composition doped with yttrium can be represented as a sequence of the following chemical transformations: firstly Mo and Y silicides oxidize forming Y2O3, SiO2 and metallic Mo, then molybdenum is oxidized to MoO2 and Y2O3 interacts with SiO2 with the formation of silicates Y2SiO5 and Y2Si2O7. As a result of the complete oxidation of the alloy, MoO3 and Y2Mo3O12 are added to the condensed product, and molybdenum oxide (MoO3)n vapor appears in the gas phase. Based on the results of a complete thermodynamic analysis, the possibility of the formation of silicates and yttrium molybdate during the oxidation of the hypoeutectic alloy Mo-5Si-3Y (wt. %) was established. This can increase its oxidation resistance due to the formation of a protective film limiting the diffusion of oxygen into the alloy, which, of course, requires experimental confirmation.