The High Temperature Oxidation Behavior of Eutectic High-Temperature Alloy Mo-20Si-52.8Ti in Dry and Wet Atmospheres

Mo-silicide-based alloys containing high concentrations of Ti are a new class of high-temperature materials which offer a promising combination of properties for structural applications such as gas turbines. These alloys possess high solidus temperatures, high creep resistance, and excellent phase stability. However, they suffer under inadequate oxidation properties as well as the lack of plasticity at room temperature. Recently developed alloys are able to overcome the major obstacle of inadequate oxidation resistance in air and offer reliable oxidation resistance not only at high temperatures up to 1200°C but also at the otherwise critical temperature of 800°C. It is well-known that many high temperature materials that exhibit high oxidation protectiveness in dry oxidation environments fail if water vapor is present in the surrounding. Since water vapor is present in many combustion environments at approximately 10%, it is indespensible to explore the corrosion resistance of new Ti-containing Mo-silicide-based materials in atmospheres containing water vapor. In this contribution, the effect of water vapor on the oxidation resistance of the alloy Mo-20Si-52.8Ti (at. %) at 1200°C will be presented. During exposure to dry air, the typical oxide layer formed on Ti-rich Mo-Ti-Si alloys consists of a duplex oxide scale TiO₂/SiO₂ – which is responsible for the good oxidation resistance – and an outer TiO₂ layer. To explore whether the outer TiO₂ layer can diminish the detrimental effect of water vapor, this scale was removed which causes a dramatic acceleration of the scale growth kinetics in the wet environment. It is discussed that water vapor may increase the concentration of oxygen vacancies in titania leading to faster inward diffusion of oxygen. Though the growth of TiO₂ is accelerated in presence of water vapor, no notable pore formation was observed in titania. Besides, TiO₂ does not form OH groups and thus seems to serve as a moderate barrier against water vapor. On the contrary, substantial pore formation was identified in SiO₂ particularly during exposure to the wet conditions, whereby the highest concentration of pores was observed at the interface oxide/substrate. Further, the diffusion of water in silica is substantially higher than that of oxygen obviously accounting for the higher oxidation rates in wet atmospheres.