July 17-21, 2016
Recently, refractory metals and their alloys have received increasing attention for the purpose of substituting Ni-based single crystal superalloys. Compared to commonly used materials, refractory metals have significantly higher melting points while their mechanical properties seem adequate at high temperatures. The main challenge for their application is, that refractory metals show low oxidation resistances at elevated temperatures. Aluminum diffusion layers are promising coatings to suppress harmful oxidation by forming a protective oxide layer. This study deals with the application of such aluminum reservoir layers and with the investigation of their protective properties. Four different unalloyed refractory metals, molybdenum, tantalum, tungsten and niobium, were used as substrate materials. The coatings were manufactured via a pack cementation process carried out for 8 h at 1000°C. Homogeneous intermetallic layers with thicknesses up to 49 µm and high aluminum contents were characterized using optical microscope, EPMA, and XRD analysis.
The oxidation resistance of the samples was investigated using thermogravimetric analysis. The experiments were carried out at 1300°C for up to 100 h in synthetic air. Via mass change curves the oxidation kinetics were analyzed as well as the formed oxide layers using, again, optical microscopy, EPMA, and XRD analysis. It was found that an additional application of a halogen treatment can significantly reduce the oxidative attack of the substrate and support the formation of a continuous protective Al2O3 layer. Furthermore, the effect of varying the amount of halogen on oxide layer formation is shown. The Al2O3 growth mechanism and aluminum depletion of the underlying reservoir layer in the different refractory metals were investigated by comparing uncoated, coated, and additional halogen-treated samples.
A.S. Ulrich and M.C. Galetz, "Improving the oxidation resistance of refractory metals via aluminum diffusion coatings and halogen effect" 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/55