Pesting Resistance in High Mo Containing Alloys

Refractory metal based alloys exhibit intrinsically high creep resistance at anticipated application temperatures beyond 1100 °C due to high solidus temperatures. However, apart from ductility issues at low temperatures, oxidation resistance at elevated temperatures is often observed a fundamental problem for the further development of this group of alloys.<br/><br/>Among others, Mo-based alloys have attracted particular research interest [1]. However, the disordered, body centered cubic solid solution, rich in Mo and required to obtain reasonable toughness and low brittle-to-ductile transition temperatures, usually forms volatile MoO₃ already at temperatures below 1000 °C. The progress of oxidation occurs rather fast and typically leads to a complete disintegration of the part within short periods of time. Thus, this so-called pesting behavior is a catastrophic type of oxidation.<br/><br/>In the present study, we provide recent results on the development of pesting-resistant high Mo containing alloys specifically addressing this fundamental problem when oxidation resistance is considered [2-6]. The alloys with this surprising and not expected property are Ti-rich, two-phase or three-phase alloys from the Mo-Si-Ti system. All of the identified alloys feature: (i) Fine-scale microstructures of the solid solution and one or two silicides originating from eutectic reactions, solid state transformations or combination thereof. (ii) Surprisingly adherent and passivating TiO₂/SiO₂ duplex and/or TiO₂ top layers after oxidation in laboratory air that limit the oxidation progress and the contribution of evaporation at temperatures of 800 up to 1200 °C. (iii) Low densities in conjunction with reasonable creep resistance. A holistic overview is provided about the nature of the TiO₂ which is typically not considered a passivating oxide and the microstructural and chemical requirements of pesting resistance.<br/><br/>Apart from Mo-Si-Ti, further alloys were developed also featuring fine-scaled microstructures of Mo-rich solid solution and silicide phases. Indeed, another alloy from the Mo-Si-Cr system exhibiting pesting resistance was identified but significantly differentiating in the mechanism to suppress MoO₃ formation at 800 °C [7]. The common and differentiating features of the two developments are highlighted to provide further insights into research opportunities.<br/><br/>[1] J.H. Perepezko, M. Krüger, M. Heilmaier, Mo-Silicide Alloys for High-Temperature Structural Applications, <i>Materials Performance and Characterization, </i>10, 22-145, 2021.<br/>[2] D. Schliephake, A. Kauffmann, X. Cong, C. Gombola, M. Azim, B. Gorr, H.-J. Christ, M. Heilmaier, Constitution, oxidation and creep of eutectic and eutectoid Mo-Si-Ti alloys, <i>Intermetallics, </i>104, 133-142, 2019.<br/>[3] S. Obert, A. Kauffmann, M. Heilmaier, Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti alloys, <i>Acta Materialia, </i>184, 132-142, 2020.<br/>[4] S. Obert, A. Kauffmann, S. Seils, S. Schellert, M. Weber, B. Gorr, H.-J. Christ, M. Heilmaier, On the chemical and microstructural requirements for the pesting-resistance of Mo-Si-Ti alloys, <i>Journal of Materials Research and Technology, </i>9, 8556-8567, 2020.<br/>[5] M. Weber, B. Gorr, H.-J. Christ, S. Obert, A. Kauffmann, M. Heilmaier, Effect of Water Vapor on the Oxidation Behavior of the Eutectic High-Temperature Alloy Mo-20Si-52.8Ti, <i>Advanced Engineering Materials, </i> 22, 2000219, 2020.<br/>[6] S. Obert, A. Kauffmann, S. Seils, T. Boll, S. Kauffmann-Weiss, H. Chen, R. Anton, M. Heilmaier, Microstructural and Chemical Constitution of the Oxide Scale formed on a Pesting-Resistant Mo-Si-Ti Alloy, <i>Corrosion Science, </i>178, 109081, 2021.<br/>[7] F. Hinrichs, A. Kauffmann, A.S. Tirunilai, D. Schliephake, B. Beichert, G. Winkens, K. Beck, A. Ulrich, M. Galetz, Z. Long, H. Thota, Y. Eggeler, A. Pundt, M. Heilmaier, A novel nitridation- and pesting-resistant Cr-Si-Mo alloy, <i>Corrosion Science, </i>submitted, 2022.