On the mechanism of oxidation resistance of W-Cr-Pd alloys at high temperatures
July 17-21, 2016
While studying activation sintering of tungsten, Evans  and Ito and Furusawa  revealed that W-Cr-Pd alloys exhibit unexpected oxidation resistance at elevated temperatures. The role of palladium in stimulating oxidation resistance in W-Cr alloys is still not fully understood. Evans  suggested that a Pd inhibits inward diffusion of oxygen. Dzykovich et al.  suggested that the Pd accumulates at grain boundaries and acts as easy diffusion channels for Cr. Lee and Simkovich  added that Pd may act as a Cr reservoir. Applying oxidation tests and diffusion experiments we aim to study the kinetics of oxidation of W-Cr-Pd alloys, the microstructure of the oxides formed on the external surface and to understand the mechanisms of protection of the alloy from oxidation by Cr and Pd. Pure W gain weight due to oxidization at a rate of 100 mg/cm2hr at 1000˚C. W-29wt%Cr oxidizes at an order of magnitude lower rate and W-29Cr-1Pd oxidizes at two orders of magnitude lower rate. Porosity significantly increases the oxidation rates. As previously observed, at 800˚C these alloys form a relatively dense scale that consists of an inner layer of Cr2O3, an intermediate layer of Cr2WO6 and an external layer of WO3. At 1200˚C only Cr2WO6 layer is found, since the Cr2O3 and WO3 evaporate. W and W-Pd alloys were coated with Cr layers and undergone diffusion experiments. An extraordinary affinity between the Cr and Pd was revealed, manifested by extremely fast inward diffusion of Cr along grain boundaries, where Pd islands segregate, and even outward diffusion of Pd to the Cr coating. This observation assess that Pd accumulated at grain boundaries provides easy diffusion channels for Cr and it imparts an order of magnitude improvement of the oxidation resistance of W alloys.
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2. H. Ito and M. Furusawa, J. Jap. Soc. Powder and Powder Metallrgy, 16 (1969) 145.
3. I. Ya Dzykovich, V. V. Panichkina, V. V. Skorokhod and L. I. Shaiderman, Sov. Powder Metall. Met. Ceram., 15 (1976) 151.
4. D. Lee and G. Simkovich, Oxid. Metals, 31 (1989) 265; 34 (1990) 13.
R. Z. Shneck,, "On the mechanism of oxidation resistance of W-Cr-Pd alloys at high temperatures" 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/40