The oxide scales for the entire series were found to be Ti oxide top layers and Ti/Si duplex oxide scales underneath. While grazing incidence X-ray diffraction (XRD) has confirmed the rutile structure of Ti oxide, atom probe tomography indicates that Ti may not exist only as Ti (+IV) in the oxide scale due to significant off-stoichiometry of the Ti oxide. The valance state of Ti in the oxide scale is verified at 800  as well as 1200°C though X-ray photoelectron spectroscopy. Ti consistently appears in the Ti (+IV) state at both temperatures as well as at different depths of the duplex scale indicating only the presence of rutile TiO2-x. Energy dispersive X-ray spectroscopy of the internal oxidation region of Mo-20Si-52.8Ti (at.%, eutectic) and Mo-21Si-34Ti (at.%, eutectoid) oxidized at 1200°C was carried out. Both alloys are composed of a body centered cubic (BCC) solid solution and a hexagonal or tetragonal silicide phase, respectively. Both alloys exhibit apparent Ti depletion and Si enrichment in the BCC phase in the internal oxidation region. The internal oxidation region is practically non-existent at 800°C and the oxide scale is slightly different, showing more phasespecific oxidation of the BCC phase. This is crucial for Mo-Si-Ti alloys at high Mo:Ti ratio as they pest catastrophically at 800°C. A pre-oxidation treatment at T > 1000°C was used on these pesting susceptible alloys. This showed improved pesting resistance with negligible weight loss for some alloys, even after 100h of cyclic oxidation at 800°C.

[2]     S. Obert, A. Kauffmann and M. Heilmaier, "Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti alloys," Acta Materialia, vol. 184, pp. 132 - 142, 2020.
[3]     S. Obert, A. Kauffmann, S. Seils, S. Schellert, M. Weber, B. Gorr, H.-. J. Christ and M. Heilmaier, "On the chemical and microstructural requirements for the pesting-resistance of Mo–Si–Ti alloys," Journal of Materials Research and Technology, vol. 9, no. 4, pp. 8556 - 8567, 2020