4:15 PM - SF07.07.06
Pesting Resistance in High Mo Containing Alloys
Alexander Kauffmann1,Daniel Schliephake1,Susanne Obert1,Aditya Tirunilai1,Frauke Hinrichs1,Bronislava Gorr1,Martin Heilmaier1
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.
Among others, Mo-based alloys have attracted particular research interest . 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.
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.
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 . The common and differentiating features of the two developments are highlighted to provide further insights into research opportunities.
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