Oxidation Behavior of W-Cr-Nb Ternary Alloys and W-Cr-Zr Ternary Alloys

Researchers have investigated the oxidation behavior of W-Cr-Nb ternary alloys and W-Cr-Zr ternary alloys. Suresh et al. reported the oxidation behavior of W-Cr-Nb ternary alloys at 1000°C for 15 hours (30 cycles). The microstructure results of different W-Cr-Nb alloys showed that W-Cr-Nb ternary alloys have high density and small amount of porosity, and their true density is 97.5-98.5% of their theoretical density.W-Cr-Nb ternary alloys are mainly composed of W-rich solid solution (WSS) and Cr₂Nb phases.

With the increase of Cr content, the WSS grain size gradually decreases, the Cr₂Nb content gradually increases, and the distribution of Cr₂Nb at the grain boundaries is more uniform. In addition, a small amount of porosity was observed on the surface of (W_0.7Cr_0.3)₉₀-Nb₁₀ and (W_0.5Cr_0.5)₉₀-Nb₁₀ samples. However, the surface of the (W_0.4Cr_0.6)₉₀-Nb₁₀ sample is very homogeneous and compact with almost no porosity. This indicates that the density of the alloy is related to the Cr content.

The W-Cr-Nb ternary alloy has lower Δ m/S compared to the W-Cr binary alloy and pure W. The Cr₂Nb second phase can effectively reduce the oxidation rate of the W-CrNb ternary alloy. Cross-sectional images and appearance of oxidized W-Cr-Nb alloy samples. It can be seen that the (W_0.7Cr_0.3)₉₀-Nb₁₀ and (W_0.5Cr_0.5)₉₀-Nb₁₀ alloys are more severely chalked and obvious oxidation expansion cracks can be seen at the edges of the samples.

However, the oxidized (W_0.4Cr_0.6)₉₀-Nb₁₀ sample showed no obvious surface defects and the Δ m/S of the alloy was nearly 0.5 times higher than that of (W_0.7Cr_0.3)₉₀-Nb₁₀ and (W_0.5Cr_0.5)₉₀-Nb₁₀ alloys. The cross-sectional SEM results show that the main components of the oxide layer are Nb₂O₅-3WO₃ and Cr₂WO₆. Cr₂WO₆ is mainly present in the bright region of the oxide layer and its content increases with the increase of Cr/Cr₂Nb content in the alloy. Therefore, increasing the Cr/Cr₂Nb content is beneficial to prolong the oxidation life of the W-Cr-Nb ternary alloy.

Tan et al. have prepared W-Cr-Zr thin film alloys on W surfaces by magnetron sputtering technique. The alloy samples had a columnar crystal structure on the surface with a uniform composition and a cross-sectional thickness of about 4 μm, typical of magnetron sputtering techniques. The cross-sectional microstructure images of the W-Cr-Zr alloy oxidized at 1000°C for 75 min to 10 h show. A large amount of white phase is observed inside the sample, which indicates that the interior of the sample has been oxidized. Furthermore, the W-12.5Cr-0.6Zr and W-12.1Cr-1.7Zr alloys have higher Δm/S values and thicker oxide layers with 345 nm and 360 nm oxide thicknesses, respectively. however, the W-10.5Cr-1.3Zr alloy has an oxide layer thickness of only 230 nm.

The results show that the oxidation of W-10.5Cr-1.3Zr alloy is very severe with a large number of longitudinal cracks, and W-11.2Cr-1.7Zr alloy has the best oxidation resistance with the lowest Δm/S. XRD analysis shows that the oxide layer consists mainly of Cr₂WO₆ and WO₃ with small amounts of Cr₂O₃ and ZrO₂ phases. The outermost oxide layer mainly consists of Cr₂WO₆ and WO₃ with a thickness of 2 µm, and the middle layer mainly consists of Cr₂O₃ and ZrO₂ particles with a thickness of 1.5 µm. ZrO₂ particles not only serve as nucleation sites for the initial oxide, but also act as diffusion barriers to inhibit the diffusion of Cr cations, maintain the oxide film structure, and retard the oxidation of the alloy.

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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