The role of Cr diffusion in the oxidation resistance of W-Cr-Pd alloys at high temperatures

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 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 studied 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. 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 the grain boundaries, where Pd islands segregate. This observation assess that the 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. Moreover, W and Cr have a wide miscibility gap. Upon increasing of the temperature we found that the grain boundaries serve as diffusion "highways", along which rapid transportation of Cr takes place, resulting in mutual dissolution of Cr and W.