July 17-21, 2016
To increase efficiency by higher combustion temperatures of aircraft engines and energy generation, new high temperature materials are inevitable. Mo-Si-B alloys for example satisfy several requirements such as good oxidation and creep resistance. Recently, novel Ti-rich Mo-Si-B alloys have shown an increased creep resistance compared to Ti-free Mo-Si-B alloys by the formation of Ti-silicide precipitates during processing. However, due to the formation of a duplex SiO2 – TiO2 oxide layer, where fast inwards diffusion of oxygen takes place, the oxidation resistance is poor.
In this study we show that oxidation resistance of Mo-Si-B-Ti alloys can be enhanced drastically at temperatures ranging from 800 to 1200°C for several hundreds of hours by pack-cementation application of a borosilica based coating. The Mo-12.5Si-8.5B-27.5Ti (in at.%) substrate was produced by repetitive arc-melting of high-purity metals, Si and B in a high-purity argon atmosphere. After homogenization treatment at 1600°C for 100h slices of this alloy were prepared for pack-cementation. The pack-cementation was done in an atmosphere of high-purity argon at 1000°C for 40h, followed by a conditioning step at 1400°C for 10h in air. The resulting layer consists of an outer borosilica layer followed by an inner MoSi2 and Mo5Si3 layer. To study the oxidation behavior, both isothermal and cyclic oxidation tests were carried out. After an initial mass loss during the first hours of oxidation, a steady state is reached for tests up to 1000 hours. To demonstrate the high stability of the outer borosilicate layer SEM cross-sections were prepared after different times of oxidation.
Camelia Gombola, Daniel Schliephake, Martin Heilmaier, and John H. Perepezko, "Enhanced oxidation resistance of Ti-rich Mo-Si-B alloys by pack-cementation process" in "Beyond Nickel-Based Superalloys II", Chair: Dr Howard J. Stone, University of Cambridge, United Kingdom Co-Chairs: Prof Bernard P. Bewlay, General Electric Global Research, USA Prof Lesley A. Cornish, University of the Witwatersrand, South Africa Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/superalloys_ii/22