Oxidation Behavior of W-Cr-Y Ternary Alloys

Calvo et al. prepared W-Cr-Y ternary alloys with Cr content of 8 and 10 wt% and Y content of 0.3, 0.5 and 1 wt% by HIP technique and studied their oxidation behavior. The microstructure pictures of the HIPed alloy surface showed a fine surface structure and homogeneous composition. EDS analysis showed that most of the bright gray areas were Cr-rich phases, while the dark gray discontinuous areas were W-rich phase. The W-Cr-Y alloy has a lower grain size of 0.5-1 µm compared to the W-15Cr alloy. This suggests that the addition of Y can act as a grain growth inhibitor. In addition, nanoscale black Y₂O₃ particles were observed at the grain boundaries, which were caused by the decomposition of unstable oxides during the HIPing process.

In addition, Y-O-rich nanoparticles were observed on the surface of the alloy with a Y content of 1 wt%. Due to the relatively high Y content in the alloy, part of the Y was not oxidized and enriched on the surface of the alloy. The cross section of the oxidized alloy consists of a thin Cr₂O₃ layer, a Cr₂WO₆ layer, and a WO₃ layer. A large number of pores were observed at the interface between the WO₃ and Cr₂WO₆ layers, which were mainly due to the compressive stresses generated during the oxidation process and the volume expansion of the oxides. In addition, a large number of cracks and pores were also observed on the surface of the oxidized W-8Cr-0.5Y alloy. Therefore, it is difficult to form a continuous dense protective film on the surface of the alloy when the Cr content is low. Y₂W₃O₁₂ particles were observed on the surfaces of the other two oxidized alloys.

The particle distribution on the surface of the W-Cr-Y ternary alloy is relatively dispersed. When the Y content was 1 wt%, the coating surface was almost completely covered by the Cr₂O₃ layer, and only a small amount of Y₂W₃O₁₂ phase was observed. the highest oxidation rate was observed for W-10Cr-0.3Y alloy, W-10Cr-0.5Y and W-8Cr-0.5Y alloys showed similar oxidation behavior, while the lowest oxidation rate was observed for W-10Cr-1Y alloy. Therefore, Y₂W₃O₁₂ particles can promote the formation of Cr₂O₃ protective film and Y elements can significantly improve the oxidation resistance of W-Cr alloys.

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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