Microstructural and chemical constitution of the oxide scale formed on a pesting-resistant Mo-Si-Ti alloy

Abstract

The oxidation behaviour of the pesting-resistant eutectic alloy Mo-20Si-52.8Ti was investigated by addressing the microstructural and chemical constitution by grazing incidence X-ray diffraction GIXRD and atom probe tomography APT of oxide scales formed at 800 and 1200 °C in order to understand their protective character. The duplex Si-Ti-oxide scale formed at 800 °C in air, i.e. within the so-called “pesting regime”, is considerably slow growing with a scale thickness of (11 ± 3) μm after 1000 h of exposure. It is composed of a Ti(+IV)-oxide with the chemical composition of approximately TiO2.1 confirmed by APT and SiO2 islands, which are most likely amorphous. The oxide scale formed at 1200 °C, a potential application temperature, is characterised by a top single-phase scale and an underlying duplex scale with a considerably increased thickness of (68 ± 11) μm in total. It consists of rutile TiO2 and some cristobalite SiO2 as determined by GIXRD. APT analysis of the top Ti-oxide scale confirms the stoichiometry TiO2.2. The underlying duplex scale comprising Ti- and Si-oxide is characterised by strongly off-stoichiometric compounds, namely TiO1.4 and SiO1.6. Initial linear oxide scale growth kinetics are found at 800 °C, which change to cubic kinetics for exposure times longer than 100 h. At higher temperatures of 1100 and 1200 °C, diffusion-controlled, almost parabolic kinetics prevail.