Refractory metals and alloys have come into the focus of research due to their high melting points. However, refractory metal alloys often suffer from poor oxidation resistance at intermediate or elevated temperatures [1] . Significant progress was recently made in the development of oxidation-resistant Mo-Si-Ti alloys [2–4]. Since a large amount of Ti leads to a surprisingly protective TiO2/SiO2 scale in Mo-Si-Ti alloys, preventing such alloys from catastrophic oxidation (“pesting”) at these intermediate temperatures (around 800 °C), similar or even superior results might be achieved through alloying with elements that are considered passivating, like Cr. Mo-13.5Si-54.3Cr (at.%) was synthesized via arc melting. In the as-cast state, the alloy solidifies as single-phase σ phase (Cr6Fe7 prototype) with dendritic microstructure. A heat treatment leads to the decomposition of the σ phase into a fine lamellar bcc (W prototype) and (Cr,Mo)3Si (Cr3Si prototype) microstructure as previously reported by Rudy and Nowotny for similar alloy compositions [5]. The heat treated material was subjected to cyclic oxidation at 800, 1100 and 1200 °C for up to 100 h to address the following fundamental questions:
•    How do fine-lamellar Mo-Si-Cr alloys compare to similar Mo-Si-Ti alloys with respect to oxidation resistance, especially in the pesting regime with potential MoO3 evaporation and beyond 1100 °C where CrO3 starts to evaporate?
•    How does solute partitioning of Cr in Mo-Si-Cr compare to Ti in Mo-Si-Cr in view of the continuous phase fields of (Cr,Mo)ss and (Cr,Mo)3Si?

A protective, thin α-Cr2O3 layer was formed on all samples tested. In addition to the Cr2O3 layer, the formation of an internal Si enriched oxide layer is found for 1200 °C. Scale thicknesses after 100 h were below 1, 5 and 18 μm, at 800, 1100 and 1200°C, respectively. Up to 100 h, neither nitridation nor spallation was observed. The oxidation behavior is discussed with respect to other pesting and non-pesting-resistant alloys of Mo, Cr and Ti base.

References
[1] J.H. Perepezko, M. Krüger, M. Heilmaier, Mo-Silicide Alloys for High-Temperature Structural Applications, Matls. Perf. Charact. 10 (2021). https://doi.org/10.1520/MPC20200183 .
[2] S. Obert, A. Kauffmann, M. Heilmaier, Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti alloys, Acta Mater. 184 (2020) 132–142. https://doi.org/10.1016/j.actamat.2019.11.045 .
[3] S. Obert, et al., On the chemical and microstructural requirements for the pesting-resistance of
Mo–Si–Ti alloys, J. Mater. Res. Technol. 9 (2020) 8556–8567.
https://doi.org/10.1016/j.jmrt.2020.06.002 .
[4] D. Schliephake, et al., Constitution, oxidation and creep of eutectic and eutectoid Mo-Si-Ti
alloys, Intermetallics 104 (2019) 133–142. https://doi.org/10.1016/j.intermet.2018.10.028 .
[5] E. Rudy, H. Nowotny, Eine sigma-Phase im System Chrom-Molybdän-Silicium, Monatshefte für
Chemie/Chemical Monthly (1974) 156–168.