Oxidation Behavior of W-Cr Binary Alloys

Cr is a traditional antioxidant beneficial element that is widely used in the oxidation protection process of metals. In addition, it has the same body-centered cubic structure as the tungsten element. Therefore, the oxidation behavior of W-Cr binary alloys has been studied extensively by related scholars. Telu et al. prepared W_0.7Cr_0.3, W_0.5Cr_0.5 and W_0.4Cr_0.6 alloys by mechanical alloying (MA) and observed that the two phases (Wss, Crss) have significantly different brightness.

(Photo credit: Jan Cizek / coatings)

Researchers investigated on the increase in mass per unit area (Δm/S) of the alloy in air at 1000°C and 1200°C. The Δm/S of the alloy was 0.2 to 0.5 times that of pure W during the first 5 hours of exposure at 1000°C. The relatively high Δm/S value of the W_0.7Cr_0.3 alloy compared to other binary alloys is a limitation to improve the oxidation resistance of pure W. Similar conclusions were obtained in the oxidation experiments at 1200°C. This is due to the relatively low content of Cr elements in the W_0.7Cr_0.3 alloy, which cannot form a dense Cr₂O₃ protective film on its surface during oxidation.

However, the mass gain of the W_0.5Cr_0.5 alloy was the lowest in the above experiments. This indicates that there is an upper limit to the increase of Cr element content for improving the oxidation resistance of the W-Cr binary alloys. A more uniform and fine Cr-rich phase was observed on the surface of the alloy. This facilitates the formation of a continuous and dense oxide film on the surface of the alloy at the initial stage of oxidation. The EDS and XRD results show that the oxide layer consists of Cr₂O₃, Cr₂WO₆ and porous WO₃ layers from the inside to the outside.

In the initial stage of oxidation, W and Cr are oxidized simultaneously on the alloy surface, and Cr₂O₃ and WO₃ grow along the alloy surface. In addition, the volume expansion of WO₃ increases the porosity and the thickness of the oxide layer as the reaction proceeds. Due to the relatively low oxygen partial pressure in the oxide layer, the Cr element is greatly oxidized. Finally, a continuous Cr₂O₃ layer was formed at the bottom of the oxide layer. Furthermore, similar conclusions were obtained by Hou et al. researchers when they studied the oxidation behavior of W-Cr (10, 20 wt% Cr) alloys.

(Photo credit: Jan Cizek / coatings)

Reference: Fu T, Cui K, Zhang Y, et al. Oxidation protection of tungsten alloys for nuclear fusion applications: A comprehensive review[J]. Journal of Alloys and Compounds, 2021, 884: 161057.

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