Kinetics of oxide scale growth on a (Ti, Mo)₅Si₃ based oxidation resistant Mo-Ti-Si alloy at 900-1300^∘C

Abstract

High-temperature oxidation behaviour of Mo-40Ti-30Si (at.%) alloy was investigated in the temperature regime of 900-1300∘C in air. Isothermal weight change data recorded up to 100 h of exposure revealed parabolic weight gain kinetics at all the tested temperatures. The protective oxide scale composed with SiO2 (silica) and TiO2 (titania) forming a duplex oxide microstructure consisting of TiO2 particles embedded in the continuous SiO2 matrix. The oxide scale showed parabolic growth kinetics, and the activation energies for the scale growth were found to be 72.2 kJ/mol in 900-1200∘C and 324.9 kJ/mol in 1200-1300∘C. The kinetics of the protective scale growth on the alloy surface was mainly controlled by the growth of the silica film and the inward diffusion of oxygen through the duplex oxide layer.

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

“…Therefore, the phase composition is regarded to be comparable to the studies made by Majumdar et al in Ref. [12] on an Mo-30Si-40Ti alloy, which comprises a (Ti,Mo)5Si3 matrix, but with an increased Mo content of 26 at%. The oxidation behaviour was reported to obey parabolic weight gain and oxide scale growth kinetics in the temperature range of 900 up to 1300 °C [12].…”

“…The parabolic oxidation rate constant was found to be about half an order of magnitude lower than in the eutectic alloy at 1200 °C in Ref. [12] (compare with Table 2). The corresponding activation energies were determined to be 72 and 325 kJ/mol in the temperature range between 900 to 1200 °C and 1200 to 1300 °C, respectively [12].…”

“…1). The authors concluded that the formation of volatile MoO3 is suppressed in (Ti,Mo)5Si3 [12]. The parabolic oxidation rate constant was found to be about half an order of magnitude lower than in the eutectic alloy at 1200 °C in Ref.…”

“…[12] on an Mo-30Si-40Ti alloy, which comprises a (Ti,Mo)5Si3 matrix, but with an increased Mo content of 26 at%. The oxidation behaviour was reported to obey parabolic weight gain and oxide scale growth kinetics in the temperature range of 900 up to 1300 °C [12]. Thereby, the formation of an outer TiO2 scale and an underlying SiO2-TiO2 duplex scale was observed [12] like for the investigated Mo-Si-Ti alloys from the present study (the eutectic alloy Mo-20Si-52.8Ti, as well as the other derivatives from the Mo-Si-Ti system listed in App.…”

“…While both M5Si3 silicides (with "M" being either Ti or Mo or a combination of both), hexagonal (Ti,Mo)5Si3 and tetragonal (Mo,Ti)5Si3, exhibit an adequate oxidation resistance in the pesting regime [12,13], it is the pesting-critical MoSS phase which is responsible for the dominance of MoO3 evaporation [11]. Continuous pathways of the Ti-oxide are present in the duplex scale connecting the alloy substrate with the outer TiO2 oxide scale among all investigated Mo-Si-Ti alloys [10].…”